Electroencephalography - what is it? How is electroencephalography performed? Electroencephalography in clinical practice. Rules for registering an electroencephalogram and functional tests
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INTRODUCTION
CONCLUSION
INTRODUCTION
Relevance of the research topic. Currently, there is an increased interest in the study of the rhythmic organization of processes in the body, both under normal and pathological conditions, all over the world. Interest in the problems of chronobiology is due to the fact that rhythms dominate nature and cover all manifestations of life - from the activity of subcellular structures and individual cells to complex forms of behavior of an organism and even populations and ecological systems. Periodicity is an inherent property of matter. The phenomenon of rhythm is universal. Meaning Facts biological rhythms for the vital activity of a living organism accumulated for a long time, but only in last years began their systematic study. Currently, chronobiological studies are one of the main directions in the physiology of human adaptation.
CHAPTER I General representations about the methodological foundations of electroencephalography
Electroencephalography is a method of studying the brain, based on the registration of its electrical potentials. The first publication on the presence of currents in the central nervous system was made by Du Bois Reymond in 1849. In 1875, data on the presence of spontaneous and induced electrical activity in the brain of a dog were obtained independently by R. Caton in England and V. Ya. Danilevsky in Russia. Research by domestic neurophysiologists during the late 19th and early 20th centuries made a significant contribution to the development of the fundamentals of electroencephalography. V. Ya. Danilevsky not only showed the possibility of recording the electrical activity of the brain, but also emphasized its close connection with neurophysiological processes. In 1912, P. Yu. Kaufman revealed the connection between the electrical potentials of the brain and the “internal activity of the brain” and their dependence on changes in brain metabolism, exposure to external stimuli, anesthesia, and an epileptic seizure. A detailed description of the electrical potentials of the dog's brain with the definition of their main parameters was given in 1913 and 1925. V. V. Pravdich-Neminsky.
The Austrian psychiatrist Hans Berger in 1928 was the first to register the electrical potentials of the human brain using scalp needle electrodes (Berger H., 1928, 1932). In his works, the main EEG rhythms and their changes during functional tests and pathological changes in the brain. Big influence the development of the method was influenced by the publications of G.Walter (1936) on the importance of EEG in the diagnosis of brain tumors, as well as the works of F.Gibbs, E.Gibbs, W.G.Lennox (1937), F.Gibbs, E.Gibbs (1952, 1964), which gave detailed electroencephalographic semiotics of epilepsy.
In subsequent years, the work of researchers was devoted not only to the phenomenology of electroencephalography in various diseases and brain conditions, but also to the study of the mechanisms of electrical activity generation. A significant contribution to this area was made by the works of E.D. Adrian, B. Metthews (1934), G. Walter (1950), V. S. Rusinov (1954), V. E. Mayorchik (1957), N. P. Bekhtereva (1960) , L. Novikova (1962), H. Jasper (1954).
Great importance to understand the nature of electrical oscillations of the brain, studies of the neurophysiology of individual neurons using the microelectrode method revealed those structural subunits and mechanisms that make up the total EEG (Kostyuk P.G., Shapovalov A.I., 1964, Eccles J., 1964) .
EEG is a complex oscillatory electrical process that can be recorded when electrodes are placed on the brain or on the surface of the scalp, and is the result of electrical summation and filtering of elementary processes occurring in brain neurons.
Numerous studies show that the electrical potentials of individual brain neurons are closely and fairly accurately quantitatively related to information processes. In order for a neuron to generate an action potential that transmits a message to other neurons or effector organs, it is necessary that its own excitation reaches a certain threshold value.
The level of excitation of a neuron is determined by the sum of excitatory and inhibitory effects exerted on it at a given moment through synapses. If the sum of excitatory influences is greater than the sum of inhibitory ones by a value exceeding the threshold level, the neuron generates a nerve impulse, which then propagates along the axon. The described inhibitory and excitatory processes in the neuron and its processes correspond to a certain form of electrical potentials.
The membrane - the shell of the neuron - has electrical resistance. Due to the energy of metabolism, the concentration positive ions in the extracellular fluid is maintained at a higher level than inside the neuron. As a result, there is a potential difference that can be measured by inserting one microelectrode into the cell, and placing the second one extracellularly. This potential difference is called the resting potential of the nerve cell and is about 60-70 mV, and the internal environment is negatively charged relative to the extracellular space. The presence of a potential difference between the intracellular and extracellular environment is called the polarization of the neuron membrane.
An increase in the potential difference is called hyperpolarization, respectively, and a decrease is called depolarization. The presence of a resting potential is a necessary condition normal functioning neuron and generating electrical activity. When metabolism stops or decreases below an acceptable level, the differences in the concentrations of charged ions on both sides of the membrane are smoothed out, which is the reason for the cessation of electrical activity in the event of clinical or biological brain death. The resting potential is the initial level at which changes occur associated with the processes of excitation and inhibition - spike impulse activity and gradual slower changes in the potential. Spike activity (from the English spike--point) is characteristic of bodies and axons nerve cells and is associated with the non-decremental transmission of excitation from one nerve cell to another, from receptors to the central sections nervous system or from the central nervous system to the executive organs. Spike potentials arise when the neuron membrane reaches a certain critical level of depolarization, at which an electrical breakdown of the membrane occurs and a self-sustaining process of excitation propagation in the nerve fiber begins.
During intracellular registration, the spike has the form of a high-amplitude, short, fast positive peak.
Characteristic features of spikes are their high amplitude (of the order of 50-125 mV), short duration (of the order of 1-2 ms), the confinement of their occurrence to a rather strictly limited electrical state of the neuron membrane (the critical level of depolarization) and the relative stability of the spike amplitude for a given neuron (the law all or nothing).
Gradual electrical responses are mainly inherent in the dendrites in the soma of the neuron and represent postsynaptic potentials (PSPs) that arise in response to the arrival of spike potentials to the neuron along afferent pathways from other nerve cells. Depending on the activity of excitatory or inhibitory synapses, respectively, excitatory postsynaptic potentials (EPSPs) and inhibitory postsynaptic potentials (IPSPs) are distinguished.
EPSP is manifested by a positive deviation of the intracellular potential, and IPSP by a negative one, which is respectively referred to as depolarization and hyperpolarization. These potentials are distinguished by their locality, decremental propagation over very short distances in neighboring areas of the dendrites and soma, relatively low amplitude (from a few to 20–40 mV), and long duration (up to 20–50 ms). Unlike spikes, PSP occur in most cases regardless of the level of membrane polarization and have different amplitude depending on the volume of the afferent message that came to the neuron and its dendrites. All these properties provide the possibility of summation of gradual potentials in time and space, reflecting the integrative activity of a certain neuron (P. G. Kostyuk, A. I. Shapovalov, 1964; Eccles, 1964).
It is the processes of summation of TPSP and EPSP that determine the level of neuron depolarization and, accordingly, the probability of generating a spike by a neuron, i.e., transferring accumulated information to other neurons.
As can be seen, both these processes are closely related: if the level of spike bombardment caused by the arrival of spikes along the afferent fibers to the neuron determines the membrane potential fluctuations, then the level of the membrane potential (gradual reactions) in turn determines the probability of generating a spike by a given neuron.
As follows from the above, spike activity is a much rarer event than gradual fluctuations in somatodendritic potential. An approximate relationship between the temporal distribution of these events can be obtained by comparing the following numbers: spikes are generated by brain neurons at an average frequency of 10 per second; at the same time, for each of the synaptic endings, the kdendrites and the soma receive, respectively, an average of 10 synaptic influences per second. If we take into account that up to several hundreds and thousands of synapses can terminate on the surface of the dendrites and soma of one cortical neuron, then the volume of synaptic bombardment of one neuron, and, accordingly, of gradual reactions, will be several hundreds or thousands per second. Hence, the ratio between the frequency of the spike and gradual response of one neuron is 1-3 orders of magnitude.
The relative rarity of spike activity, the short duration of impulses, which leads to their rapid attenuation due to the large electrical capacitance of the cortex, determine the absence of a significant contribution to the total EEG from spike neuronal activity.
Thus, the electrical activity of the brain reflects the gradual fluctuations of somatodendritic potentials corresponding to EPSP and IPSP.
The connection between the EEG and elementary electrical processes at the level of neurons is non-linear. The concept of statistical display of the activity of multiple neuronal potentials in the total EEG seems to be the most adequate at present. It suggests that the EEG is the result of a complex summation of the electrical potentials of many neurons operating largely independently. Deviations from random distribution events in this model will depend on functional state brain (sleep, wakefulness) and the nature of the processes that cause elemental potentials (spontaneous or evoked activity). In the case of significant temporal synchronization of neuron activity, as is noted in certain functional states of the brain or when a highly synchronized message from an afferent stimulus arrives at cortical neurons, a significant deviation from random distribution will be observed. This can be realized in an increase in the amplitude of the total potentials and an increase in the coherence between elementary and total processes.
As shown above, the electrical activity of individual nerve cells reflects their functional activity in processing and transmitting information. From this we can conclude that the total EEG also in a preformed form reflects the functional activity, but not of individual nerve cells, but of their huge populations, i.e., in other words, the functional activity of the brain. This position, which has received numerous indisputable evidence, seems to be extremely important for EEG analysis, since it provides the key to understanding which brain systems determine the appearance and internal organization of the EEG.
At different levels of the brainstem and in the anterior parts of the limbic system, there are nuclei, the activation of which leads to a global change in the level of functional activity of almost the entire brain. Among these systems, the so-called ascending activating systems are distinguished, located at the level of the reticular formation of the middle and in the preoptic nuclei of the forebrain, and inhibitory or inhibitory, somnogenic systems, located mainly in the nonspecific thalamic nuclei, in the lower parts of the bridge and the medulla oblongata. Common to both of these systems are the reticular organization of their subcortical mechanisms and diffuse, bilateral cortical projections. Such a general organization contributes to the fact that the local activation of a part of the nonspecific subcortical system, due to its network-like structure, leads to the involvement of the entire system in the process and to the almost simultaneous spread of its influences on the entire brain (Fig. 3).
CHAPTER II. The main elements of the central nervous system involved in the generation of electrical activity of the brain
The main elements of the CNS are neurons. A typical neuron consists of three parts: a dendritic tree, a cell body (soma), and an axon. The highly branched body of the dendritic tree has a larger surface area than the rest of it and is its receptive sensory area. Numerous synapses on the body of the dendritic tree provide direct contact between neurons. All parts of the neuron are covered with a shell - a membrane. At rest inner part neuron - protoplasm - has a negative sign in relation to the extracellular space and is approximately 70 mV.
This potential is called the resting potential (RP). It is due to the difference in the concentrations of Na+ ions, prevailing in the extracellular environment, and K+ and Cl- ions, prevailing in the protoplasm of the neuron. If the membrane of a neuron depolarizes from -70 mV to -40 mV, when a certain threshold is reached, the neuron responds with a short impulse, at which the membrane potential shifts to +20 mV, and then back to -70 mV. This neuron response is called an action potential (AP).
Rice. 4. Types of potentials recorded in the central nervous system, their time and amplitude relationships.
The duration of this process is about 1 ms (Fig. 4). One of important properties PD is that it is the main mechanism by which the axons of neurons carry information over considerable distances. The propagation of an impulse along nerve fibers occurs as follows. An action potential that arises in one place of the nerve fiber depolarizes neighboring areas and, without decrement, propagates along the nerve fiber due to the energy of the cell. According to the propagation theory of nerve impulses, this propagating depolarization of local currents is the main factor responsible for the propagation of nerve impulses (Brazier, 1979). In humans, the length of the axon can reach one meter. This length of the axon allows information to be transmitted over considerable distances.
At the distal end, the axon divides into numerous branches that terminate in synapses. The membrane potential generated on the dendrites propagates passively into the soma of the cell, where the summation of discharges from other neurons takes place and the neuronal discharges initiated in the axon are controlled.
A nerve center (NC) is a group of neurons united spatially and organized into a specific functional-morphological structure. In this sense, NCs can be considered: afferent and efferent pathways, subcortical and stem nuclei and ganglia of the reticular formation of the brain stem, functionally and cytoarchitectonically specialized areas of the cerebral cortex. Since neurons in the cortex and nuclei are oriented parallel to each other and radially with respect to the surface, the model of a dipole can be applied to such a system, as well as to an individual neuron, a point source of current, the dimensions of which are much smaller than the distance to the points measurements (Brazier, 1978; Gutman, 1980). When NC is excited, a total dipole-type potential arises with a nonequilibrium charge distribution, which can propagate over long distances due to the potentials of the distant field (Fig. 5) (Egorov, Kuznetsova, 1976; Hosek et al., 1978; Gutman, 1980; Zhadin, 1984 )
Rice. 5. Representation of an excited nerve fiber and nerve center as an electric dipole with field lines in a bulk conductor; design of a three-phase potential characteristic depending on the relative location of the source in relation to the discharge electrode.
The main elements of the CNS that contribute to the generation of EEG and EP.
A. Schematic representation of the processes from generation to derivation of the scalp evoked potential.
B. Response of one neuron in Tractus opticus after electrical stimulation of Chiasma opticum. For comparison, the spontaneous response is depicted in the upper right corner.
C. The response of the same neuron to a flash of light (sequence of PD discharges).
D. Connection of the histogram of neuronal activity with EEG potentials.
It is now recognized that the electrical activity of the brain, recorded on the scalp in the form of EEG and EP, is mainly due to the synchronous occurrence a large number microgenerators under the influence of synaptic processes on the membrane of neurons and passive flow of extracellular currents in the registration area. This activity is a small but significant reflection of the electrical processes in the brain itself and is associated with the structure of the human head (Gutman, 1980; Nunes, 1981; Zhadin, 1984). The brain is surrounded by four main layers of tissue that differ significantly in electrical conductivity and affect the measurement of potentials: cerebrospinal fluid (CSF), dura mater, skull bone, and scalp skin (Fig. 7).
The electrical conductivity values (G) alternate: brain tissue -- G=0.33 Ohm m)-1, CSF with better electrical conductivity -- G=1 (Ohm m)-1, weakly conductive bone above it -- G=0, 04 (Ohm m)-1. The scalp has a relatively good conductivity, almost the same as that of the brain tissue - G=0.28-0.33 (ohm m)-1 (Fender, 1987). Thickness of solid layers meninges, bone and scalp, according to a number of authors, fluctuates, but the average dimensions are respectively: 2, 8, 4 mm with a head curvature radius of 8–9 cm (Blinkov, 1955; Egorov, Kuznetsova, 1976 and others).
Such an electrically conductive structure significantly reduces the density of currents flowing in the scalp. In addition, it smooths out spatial variations in current density, i.e., local inhomogeneities of currents caused by activity in the CNS are slightly reflected on the scalp surface, where the potential pattern contains relatively few high-frequency details (Gutman, 1980).
An important fact is also that the pattern of surface potentials (Fig. 8) is more “smeared” than the distributions of intracerebral potentials that determine this picture (Baumgartner, 1993).
CHAPTER III. Equipment for electroencephalographic studies
From the foregoing, it follows that the EEG is a process due to the activity of a huge number of generators, and, in accordance with this, the field created by them seems to be very heterogeneous throughout the entire space of the brain and varies in time. In this regard, between two points of the brain, as well as between the brain and tissues of the body remote from it, variable potential differences arise, the registration of which is the task of electroencephalography. In clinical electroencephalography, the EEG is taken using electrodes located on the intact scalp and at some extracranial points. With such a registration system, the potentials generated by the brain are significantly distorted due to the influence of the integument of the brain and the peculiarities of the orientation of the electric fields with different relative positions of the discharge electrodes. These changes are partly due to the summation, averaging and attenuation of potentials due to the shunting properties of the media surrounding the brain.
The EEG taken with scalp electrodes is 10-15 times lower than the EEG taken from the cortex. High-frequency components, when passing through the integument of the brain, are weakened much more strongly than slow components (Vorontsov D.S., 1961). In addition, in addition to amplitude and frequency distortions, differences in the orientation of the discharge electrodes also cause changes in the phase of the recorded activity. All these factors must be kept in mind when recording and interpreting the EEG. The difference in electrical potentials on the surface of the intact integuments of the head has a relatively small amplitude, normally not exceeding 100-150 μV. To register such weak potentials, amplifiers with a high gain (of the order of 20,000–100,000) are used. Given that EEG recording is almost always carried out in rooms equipped with industrial alternating current transmission and operation devices that create powerful electromagnetic fields, differential amplifiers are used. They have amplifying properties only in relation to the differential voltage at the two inputs and neutralize the common-mode voltage that equally acts on both inputs. Considering that the head is a bulk conductor, its surface is practically equipotential with respect to the source of noise acting from the outside. Thus, noise is applied to the inputs of the amplifier in the form of a common-mode voltage.
The quantitative characteristic of this feature of a differential amplifier is the common mode rejection ratio (rejection factor), which is defined as the ratio of the common mode signal at the input to its value at the output.
In modern electroencephalographs, the rejection factor reaches 100,000. The use of such amplifiers makes it possible to record EEG in most hospital rooms, provided that no powerful electrical devices such as distribution transformers, X-ray equipment, and physiotherapy devices are operating nearby.
In cases where it is impossible to avoid the proximity of powerful sources of interference, shielded cameras are used. The best shielding method is to sheathe the walls of the chamber in which the subject is located with sheets of metal welded together, followed by autonomous grounding using a wire soldered to the screen and the other end connected to a metal mass buried in the ground to the level of contact with groundwater.
Modern electroencephalographs are multi-channel recording devices that combine from 8 to 24 or more identical amplifying-recording units (channels), thus allowing simultaneous recording of electrical activity from the corresponding number of pairs of electrodes mounted on the head of the subject.
Depending on the form in which the EEG is recorded and presented for analysis to the electroencephalographer, electroencephalographs are divided into traditional paper (pen) and more modern paperless ones.
In the first EEG, after amplification, it is fed to the coils of electromagnetic or thermal-writing galvanometers and written directly onto a paper tape.
Electroencephalographs of the second type convert the EEG into digital form and enter it into a computer, on the screen of which the continuous process of recording the EEG is displayed, which is simultaneously recorded in the computer's memory.
Paper-based electroencephalographs have the advantage of being easy to operate and somewhat less expensive to purchase. Paperless have the advantage of digital recording, with all the ensuing conveniences of recording, archiving and secondary computer processing.
As already mentioned, the EEG records the potential difference between two points on the surface of the subject's head. Accordingly, voltages are applied to each registration channel, taken away by two electrodes: one - to the positive, the other - to the negative input of the amplification channel. Electroencephalography electrodes are metal plates or rods various shapes. Typically, the transverse diameter of a disk-shaped electrode is about 1 cm. Two types of electrodes are most widely used - bridge and cup.
The bridge electrode is a metal rod fixed in a holder. The lower end of the rod in contact with the scalp is covered with hygroscopic material, which is moistened with an isotonic sodium chloride solution before installation. The electrode is attached with a rubber band in such a way that the contact lower end of the metal rod is pressed against the scalp. A lead wire is connected to the opposite end of the rod using a standard clamp or connector. The advantage of such electrodes is the speed and simplicity of their connection, the absence of the need to use a special electrode paste, since the hygroscopic contact material retains for a long time and gradually releases an isotonic sodium chloride solution onto the skin surface. The use of electrodes of this type is preferable when examining contact patients who are able to sit or reclining.
When registering an EEG to control anesthesia and the state of the central nervous system during surgical operations, it is permissible to divert potentials with the help of needle electrodes injected into the integuments of the head. After the discharge, the electrical potentials are fed to the inputs of the amplifying-recording devices. The input box of the electroencephalograph contains 20-40 or more numbered contact sockets, with the help of which an appropriate number of electrodes can be connected to the electroencephalograph. In addition, the box has a socket for a neutral electrode, connected to the instrument ground of the amplifier and therefore indicated by a ground mark or a corresponding letter symbol, such as "Gnd" or "N". Accordingly, the electrode mounted on the body of the subject and connected to this socket is called the ground electrode. It serves to equalize the potentials of the patient's body and the amplifier. The lower the under-electrode impedance of the neutral electrode, the better the potentials are equalized and, accordingly, the less common-mode interference voltage will be applied to the differential inputs. Do not confuse this electrode with instrument ground.
CHAPTER IV. Lead and ECG recording
Before recording the EEG, the operation of the electroencephalograph is checked and calibrated. To do this, the operation mode switch is set to the "calibration" position, the motor of the tape drive mechanism and the galvanometer feathers are turned on, and a calibration signal is supplied from the calibration device to the inputs of the amplifiers. With a properly adjusted differential amplifier, an upper bandwidth above 100 Hz, and a time constant of 0.3 s, the positive and negative calibration signals are perfectly symmetrical in shape and have the same amplitude. The calibration signal has a jump and an exponential fall, the rate of which is determined by the selected time constant. At the upper transmission frequency below 100 Hz, the top of the calibration signal from a pointed one becomes somewhat rounded, and the roundness is the greater, the lower the upper bandwidth of the amplifier (Fig. 13). It is clear that electroencephalographic oscillations themselves will undergo the same changes. Using the re-applying of the calibration signal, the gain level is adjusted for all channels.
Rice. 13. Registration of a calibration rectangular signal at different meanings low and high pass filters.
The top three channels have the same bandwidth for low frequencies; the time constant is 0.3 s. The bottom three channels have the same upper bandwidth limited to 75 Hz. Channels 1 and 4 correspond to the normal mode of EEG recording.
4.1 General methodological principles of the study
For getting correct information in an electroencephalographic study, some general rules must be observed. Since, as already mentioned, the EEG reflects the level of functional activity of the brain and is very sensitive to changes in the level of attention, emotional state, impact external factors, the patient during the study should be in a light and soundproof room. The position of the examined reclining in a comfortable chair is preferable, the muscles are relaxed. The head rests on a special headrest. The need for relaxation, in addition to ensuring maximum rest of the subject, is determined by the fact that muscle tension, especially of the head and neck, is accompanied by the appearance of EMG artifacts in the recording. The patient's eyes should be closed during the study, as this is the most pronounced normal alpha rhythm on the EEG, as well as some pathological phenomena in patients. In addition, at open eyes the subjects, as a rule, move their eyeballs and make blinking movements, which is accompanied by the appearance of oculomotor artifacts on the EEG. Before conducting the study, the patient is explained its essence, they talk about its harmlessness and painlessness, outline the general procedure for the procedure and indicate its approximate duration. To apply light and sound stimuli, photo and phonostimulators are used. For photostimulation, short (about 150 μs) flashes of light, close in spectrum to white, of a sufficiently high intensity (0.1-0.6 J) are usually used. Some photostimulator systems allow you to change the intensity of flashes of light, which, of course, is an additional convenience. In addition to single flashes of light, photostimulators make it possible to present, at will, a series of identical flashes of the desired frequency and duration.
A series of flashes of light of a given frequency is used to study the reaction of rhythm assimilation - the ability of electroencephalographic oscillations to reproduce the rhythm of external stimuli. Normally, the reaction of rhythm assimilation is well expressed at a flicker frequency close to one's own. EEG rhythms. Spreading diffusely and symmetrically, rhythmic assimilation waves have the highest amplitude in the occipital regions.
brain nervous activity electroencephalogram
4.2 Basic principles of EEG analysis
EEG analysis is not a time-determined procedure, but is essentially carried out already in the process of recording. EEG analysis during recording is necessary to control its quality, as well as to develop a research strategy depending on the information received. EEG analysis data during the recording process determine the need and possibility of conducting certain functional tests, as well as their duration and intensity. Thus, the separation of EEG analysis into a separate paragraph is determined not by the isolation of this procedure, but by the specifics of the tasks that are solved in this case.
EEG analysis consists of three interrelated components:
1. Evaluation of the recording quality and differentiation of artifacts from the actual electroencephalographic phenomena.
2. Frequency and amplitude characteristics of the EEG, identification of characteristic graph elements on the EEG (phenomena sharp wave, spike, spike-wave, etc.), determination of the spatial and temporal distribution of these phenomena on the EEG, assessment of the presence and nature of transient phenomena on the EEG, such as flashes , discharges, periods, etc., as well as determining the localization of sources various types potentials in the brain.
3. Physiological and pathophysiological interpretation of data and formulation of a diagnostic conclusion.
Artifacts on the EEG can be divided into two groups according to their origin - physical and physiological. Physical artifacts are caused by violations of the technical rules for EEG registration and are represented by several types of electrographic phenomena. The most common type of artifacts are interference from electric fields created by devices for the transmission and operation of industrial electric current. In the recording, they are quite easily recognized and look like regular oscillations of a regular sinusoidal shape with a frequency of 50 Hz, superimposed on the current EEG or (in its absence) representing the only type of oscillations recorded in the recording.
The reasons for these interferences are as follows:
1. The presence of powerful sources of electromagnetic fields of the mains current, such as distribution transformer stations, X-ray equipment, physiotherapy equipment, etc., in the absence of appropriate shielding of the laboratory premises.
2. Lack of grounding of electroencephalographic equipment and equipment (electroencephalograph, stimulator, metal chair or bed on which the subject is located, etc.).
3. Poor contact between the discharge electrode and the patient's body or between the ground electrode and the patient's body, as well as between these electrodes and the input box of the electroencephalograph.
To highlight on the EEG significant features it is analyzed. As for any oscillatory process, the basic concepts on which the EEG characteristic is based are frequency, amplitude and phase.
The frequency is determined by the number of oscillations per second, it is written with the appropriate number and expressed in hertz (Hz). Since the EEG is a probabilistic process, strictly speaking, waves of different frequencies occur in each section of the recording; therefore, in conclusion, the average frequency of the estimated activity is given. Usually, 4-5 EEG segments are taken with a duration of 1 s and the number of waves on each of them is counted. The average of the obtained data will characterize the frequency of the corresponding activity on the EEG
Amplitude - the range of fluctuations in the electric potential on the EEG, it is measured from the peak of the previous wave to the peak of the next wave in the opposite phase (see Fig. 18); estimate the amplitude in microvolts (µV). A calibration signal is used to measure the amplitude. So, if the calibration signal corresponding to a voltage of 50 μV has a height of 10 mm (10 cells) on the record, then, accordingly, 1 mm (1 cell) of the pen deviation will mean 5 μV. By measuring the amplitude of the EEG wave in millimeters and multiplying it by 5 μV, we obtain the amplitude of this wave. In computerized devices, amplitude values can be obtained automatically.
Phase determines Current state process and indicates the direction of the vector of its changes. Some EEG phenomena are evaluated by the number of phases they contain. Monophasic is an oscillation in one direction from the isoelectric line with a return to the initial level, biphasic is such an oscillation when, after the completion of one phase, the curve passes the initial level, deviates in the opposite direction and returns to the isoelectric line. Polyphase oscillations are those containing three or more phases (Fig. 19). In a narrower sense, the term "polyphase wave" defines the sequence of a- and slow (usually e-) waves.
Rice. 18. Measurement of frequency (I) and amplitude (II) on the EEG. Frequency is measured as the number of waves per unit time (1 s). A is the amplitude.
Rice. 19. Monophasic spike (1), two-phase oscillation (2), three-phase (3), polyphasic (4).
The term “rhythm” on the EEG refers to a certain type of electrical activity corresponding to a certain state of the brain and associated with certain cerebral mechanisms.
Accordingly, when describing the rhythm, its frequency is indicated, which is typical for a certain state and region of the brain, the amplitude and some characteristic features of its changes over time with changes in the functional activity of the brain. In this regard, it seems appropriate, when describing the main EEG rhythms, to associate them with certain human states.
CONCLUSION
Brief summary. The essence of the EEG method.
Electroencephalography is used for all neurological, mental and speech disorders. According to EEG data, it is possible to study the “sleep and wakefulness” cycle, determine the side of the lesion, the location of the lesion, evaluate the effectiveness of the treatment, and monitor the dynamics of the rehabilitation process. The EEG is of great importance in the study of patients with epilepsy, since only the electroencephalogram can reveal the epileptic activity of the brain.
The recorded curve, reflecting the nature of the biocurrents of the brain, is called an electroencephalogram (EEG). The electroencephalogram reflects the total activity of a large number of brain cells and consists of many components. Analysis of the electroencephalogram allows you to identify waves on it that are different in shape, constancy, periods of oscillation and amplitude (voltage).
LIST OF USED LITERATURE
1. Akimov G. A. Transient disorders cerebral circulation. L. Medicine, 1974.p. 168.
2. Bekhtereva N. P., Kambarova D. K., Pozdeev V. K. Sustained pathological state in diseases of the brain. L. Medicine, 1978.p. 240.
3.Boeva E. M. Essays on pathophysiology closed injury brain. M. Medicine, 1968.
4. Boldyreva G. N. The role of diencephalic structures in the organization of the electrical activity of the human brain. In book. Electrophysiological study of stationary brain activity. M. Nauka, 1983.p. 222-223.
5. Boldyreva G. N., Bragina N. N., Dobrokhotova K. A., Vikhert T. M. Reflection in the human EEG of a focal lesion of the thalamic-subtubercular region. In book. The main problems of electrophysiology of the brain. M. Nauka, 1974.p. 246-261.
6. Bronzov I. A., Boldyrev A. I. Electroencephalographic parameters in patients with visceral rheumatism and paroxysms of rheumatic origin. In book. All-Russian conference on the problem of epilepsy M. 1964.p. 93-94
7. Breger M. Electrophysiological study of the thalamus and hippocampus in humans. Physiological Journal of the USSR, 1967, v. 63, N 9, p. 1026-1033.
8. Wayne A. M. Lectures on the neurology of nonspecific brain systems M. 1974.
9. Wayne A. M., Solovieva A. D., Kolosova O. A. Vegetative-vascular dystonia M. Medicine, 1981, p. 316.
10. Verishchagin N. V. Pathology of the vertebrobasilar system and disorders of cerebral circulation M. Medicine, 1980, p. 308.
11. Georgievsky MN Medical and labor examination in neuroses. M. 1957.
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Study of the functional state of the central nervous system by electroencephalography. Formation of the survey protocol. Mapping the electrical activity of the brain. Study of the brain and peripheral circulation rheography method.
term paper, added 02/12/2016
The beginning of the study of the electrical processes of the brain by D. Raymon, who discovered its electrogenic properties. Electroencephalography as a modern non-invasive method for studying the functional state of the brain by recording bioelectrical activity.
presentation, added 09/05/2016
Characterization of the use of the stereotaxic method in neurosurgery for the treatment of severe diseases of the human central nervous system: parkinsonism, dystonia, brain tumors. Descriptions of modern devices for the study of deep structures of the brain.
term paper, added 06/16/2011
The use of an electroencephalogram to study brain function and diagnostic purposes. Ways of assignment of biopotentials. The existence of characteristic rhythmic processes determined by spontaneous electrical activity of the brain. The essence of the method of principal components.
term paper, added 01/17/2015
Main clinical forms craniocerebral injury: concussion, brain contusion of mild, moderate and severe degree, compression of the brain. CT scan brain. Symptoms, treatment, consequences and complications of TBI.
EEG laboratory
should consist of a soundproof, shielded from electromagnetic waves, light-proof room for the patient (chamber) and a control room where an electroencephalograph, stimulating and analyzing equipment are placed
the room for the EEG laboratory should be chosen in the quietest part of the building, away from the roadway, x-ray units, physiotherapy devices and other sources of electromagnetic interference.
General rules for conducting an EEG study
Studies are carried out in the morning, not earlier than two hours after eating, smoking.
On the day of the study, it is not recommended to take medications; barbiturates, tranquilizers, bromides and other drugs that change the functional state of the central nervous system must be canceled in three days.
If it is impossible to cancel drug therapy, a record should be made with the name of the drug, its dose, time and method of application are indicated.
In the room where the subject is located, it is necessary to maintain a temperature of 20-22 C.
During the study, the subject can lie or sit.
The presence of a doctor is necessary, since the use of functional loads can in some cases cause an extended epileptic seizure, a collaptoid state, etc., and, accordingly, have a set of medications to stop the violations that have arisen.
Number of electrodes , superimposed on the convexital surface of the skull should be at least 21. In addition, for monopolar registration, it is necessary to apply a buccal electrode located between the round muscle of the mouth and the masticatory muscle. 2 electrodes are also applied to the edges of the eye sockets to record eye movements and a ground electrode. The location of the electrodes on the head is carried out according to the "ten-twenty" scheme.
6 types of electrodes are used, which differ both in shape and in the way they are fixed on the head:
1) contact overhead non-adhesive electrodes, which are attached to the head with the help of cords of a mesh helmet;
2) adhesive electrodes;
3) basal electrodes;
4) needle electrodes;
5) pial electrodes;
6) multi-electrode needles.
The electrodes must not have their own potential.
The electroencephalographic setup consists of electrodes, connecting wires, an electrode junction box with numbered sockets, a switching device, and a number of registration channels that allow a certain number of independent processes. In doing so, it must be borne in mind that
4-channel electroencephalographs are unsuitable for diagnostic purposes, as they allow to detect only gross changes generalized over the entire convexital surface,
8-12-channels are suitable only for general diagnostic purposes - assessment of the general functional state and detection of gross focal pathology.
Only the presence of 16 or more channels makes it possible to record the bioelectrical activity of the entire convexital surface of the brain simultaneously, which makes it possible to conduct the most delicate studies.
The assignment of biopotentials is necessarily carried out with two electrodes, since their registration requires a closed electrical circuit: the first electrode-amplifier-recording device-amplifier-second electrode. The source of potential fluctuations is the area of brain tissue lying between these two electrodes. Depending on the location of these two electrodes, bipolar and monopolar leads are distinguished.
For topical diagnosis it is necessary a large number of leads that are recorded in various combinations. In order to save time (since the set of these combinations on the selector is a very laborious process), modern electroencephalographs use pre-fixed lead patterns (wiring diagrams, routine programs, etc.).
The most rational for the implementation of topical analysis using electroencephalography are the following principles for constructing wiring diagrams:
the first wiring diagram - bipolar leads with large interelectrode distances, the "ten-twenty" circuit), connecting the electrodes in pairs along the sagittal and frontal lines;
the second - bipolar leads with small interelectrode distances with the connection of electrodes in pairs along sagittal lines;
the third - bipolar leads with small interelectrode distances with the connection of electrodes in pairs along the frontal lines;
the fourth - monopolar leads with indifferent electrodes on the cheek and according to the Goldman method;
the fifth - bipolar leads with small interelectrode distances with the connection of electrodes in pairs along the sagittal lines and registration of eye movements, ECG or galvanic skin response during exercise.
The electroencephalograph channel includes a biopotential amplifier with a high amplification factor, which allows amplifying bioelectric activity from a single microvolt to tens of volts, and a high discrimination factor, which makes it possible to counteract electrical interference in the form of electromagnetic pickups. The amplifying path of the electroencephalograph to the recording device, which has various options. At present, electromagnetic vibrators with various recording methods (ink, pin, jet, needle) are more often used, which allow recording oscillations, depending on the parameters of the recording device, up to 300 Hz.
Since signs of pathology are not always detected in the resting EEG, then, as with other methods functional diagnostics, in clinical electroencephalography apply physical exercise, some of which are mandatory:
load to assess the orienting response
load to assess resistance to external rhythms (rhythmic photostimulation).
Also mandatory is a load that is effective for detecting latent (compensated) pathology, trigger photostimulation - stimulation in the rhythms of the bioelectric activity of the brain itself using a trigger-converter of the wave components of the electroencephalogram in a flash of light. In order to excite the main brain rhythms of delta, theta, etc. (the method of "delaying" the light stimulus is used.
At decoding EEG it is necessary to distinguish artifacts, and when recording EEG, eliminate their causes.
An artifact in electroencephalography is a signal of extracerebral origin that distorts the recording of brain biocurrents.
Artifacts of physical origin include
pickup 50 Hz from mains current
lamp or transistor noise
base line instability
"microphone effect"
interference due to movements on the subject's head
sharp aperiodic movements of feathers (pencils, needles, etc.) that occur when contacts of selector switches are dirty or oxidized
the appearance of amplitude asymmetry, if, when retracted from symmetrical parts of the skull, the interelectrode distances are not the same
phase distortions and errors in the absence of drawing feathers (pencils, etc.) on one line
Artifacts of biological origin include:
flashing
nystagmus
eyelid trembling
screwing up
muscle potentials
electrocardiogram
breath registration
registration of slow bioelectrical activity in persons with metal dentures
galvanic skin reaction that occurs when profuse sweating on the head
General principles of electroencephalography
The advantages of clinical electroencephalography are
objectivity
the possibility of direct registration of indicators of the functional state of the brain, a quantitative assessment of the results obtained
observation in dynamics, which is necessary for the prognosis of the disease
the great advantage of this method is that it is not associated with intervention in the body of the subject.
When prescribing an EEG study, the expert doctor must:
1) clearly set the diagnostic task, indicating the expected localization of the pathological focus and the nature of the pathological process;
2) know in detail the research methodology, its capabilities and limitations;
3) to carry out psychotherapeutic preparation of the patient - to explain the safety of the study, to explain its general course;
4) cancel all drugs that change the functional state of the brain (tranquilizers, neuroleptics, etc.), if the functional state of the patient allows;
5) demand maximum complete description the results obtained, not just the conclusions of the study. To do this, the medical examiner must understand the terminology of clinical electroencephalography. The description of the results obtained should be standardized;
6) the doctor who ordered the study must be sure that EEG study was carried out in accordance with the "Standard Method of Research in Electroencephalography for Use in Clinical Practice and Medical Occupational Examination".
Conducting EEG studies repeatedly, in dynamics, makes it possible to monitor the course of treatment, to dynamically monitor the nature of the course of the disease - its progression or stabilization, to determine the degree of compensation of the pathological process, to determine the prognosis and employment opportunities of the disabled person.
Algorithm for describing an electroencephalogram
1. Passport part: EEG number, study date, last name, first name, patronymic, age, clinical diagnosis.
2. Description of the resting EEG.
2.1. Description of the alpha rhythm.
2.1.1. Expression of the alpha rhythm: absent, expressed by flashes (indicate the duration of the flash and the duration of the intervals between flashes), expressed by the regular component.
2.1.2. Alpha rhythm distribution.
2.1.2.1. To judge the correct distribution of the alpha rhythm, only bipolar leads with small interelectrode distances with leads along the sagittal lines are used. For the correct distribution of the alpha rhythm, its absence is taken for leads from the frontal-pole-frontal electrodes.
2.1.2.2. The area of dominance of the alpha rhythm is indicated on the basis of a comparison of the methods used for deriving bioelectric activity. (The following methods should be used: bipolar leads with connection between the electrodes along the sagittal and frontal lines according to the method of reversed phases over large and small interelectrode distances, monopolar leads with an average Goldman electrode and with the distribution of an indifferent electrode on the cheek).
2.1.3. Symmetry of the alpha rhythm. Alpha-rhythm symmetry is determined by amplitude and frequency in symmetrical areas of the brain on monopolar wiring diagrams for EEG recording using an average electrode according to Goldman or with an indifferent electrode located on the cheek.
2.1.4. The image of the alpha rhythm is spindle-shaped with well-defined spindles, i.e., modulated in amplitude (there is no alpha rhythm at the junctions of the spindles); spindle-shaped with poorly expressed spindles, that is, insufficiently modulated in amplitude (waves with amplitudes of more than 30% of the maximum amplitude of the alpha rhythm are observed at the junctions of the spindles); machine-like or sawtooth, i.e. not modulated in amplitude; paroxysmal - the spindle of the alpha rhythm begins with a maximum amplitude; arched - big difference in half cycles.
2.1.5. Alpha rhythm shape: not distorted, distorted by slow activity, distorted by electromyogram.
2.1.6. The presence of hypersynchronization of alpha rhythm waves (in-phase beats in various areas brain and their number per unit of time (10 s is taken for the analysis epoch))
2.1.7. The frequency of the alpha rhythm, its stability.
2.1.7.1. The frequency of the alpha rhythm is determined on random one-second segments of the EEG throughout the entire recording time and is expressed as medium size(if there is a change in frequency while maintaining the stability of the periods, they indicate a change in the frequencies of the dominant rhythm).
2.1.7.2. Stability is often estimated on the basis of the extreme values of the periods and is expressed as deviations from the main medium frequency. For example, (10ё2) fluctuations / s. or (10ё0, 5) fluctuations/s.
2.1.8. The amplitude of the alpha rhythm. The amplitude of the rhythm is determined on monopolar EEG recording schemes using an average electrode according to Goldman or in a lead with large interelectrode distances in the central-occipital leads. The amplitude of the waves is measured from peak to peak without taking into account the presence of an isoelectric line. 2.1.9. The alpha-rhythm index is determined in the leads with the highest severity of this rhythm, regardless of the method of bioelectrical activity derivation (the epoch of the rhythm index analysis is 10 s.).
2.1.9.1. If the alpha rhythm is expressed by a regular component, then its index is determined on 10 complete EEG frames and the average value is calculated.
2.1.9.2. With uneven distribution of the alpha rhythm, its index is determined during the entire recording of EEG rest.
2.1.10. The absence of an alpha rhythm is always noted first (see 2.1.1).
2.2. Description of dominant and subdominant rhythms.
2.2.1. The dominant activity is described according to the rules for describing the alpha rhythm (see 2.1).
2.2.2. If there is an alpha rhythm, but there is also another frequency component, represented to a lesser extent, then after the description of the alpha rhythm (see 2.1.), it is described according to the same rules as subdominant.
It should be borne in mind that the EEG recording band is divided into a number of ranges: up to 4 Hz (delta rhythm), from 4 to 8 Hz (theta rhythm), from 8 to 13 Hz (alpha rhythm), from 13 to 25 Hz (low frequency beta or beta 1 rhythm), 25 to 35 Hz (high frequency beta or beta 2 rhythm), 35 to 50 Hz (gamma or beta 3 rhythm). In the presence of low-amplitude activity, it is also necessary to indicate the presence of aperiodic (polyrhythmic) activity. For ease of verbal description, flat EEG, low-amplitude slow polymorphic activity (NPMA), polyrhythmic activity, and high-frequency low-amplitude (“double”) activity should be distinguished.
2.3. Description of beta activity (beta rhythm).
2.3.1. In the presence of beta activity, only in the frontal parts of the brain or at the junctions of the spindles of the alpha rhythm, under the condition of symmetrical amplitudes, an asynchronous aperiodic image, with an amplitude of not more than 2-5 μV, beta activity is not described or characterized as a norm.
2.3.2. In the presence of the following phenomena: the distribution of beta activity over the entire convexital surface, the appearance of a focal distribution of beta activity or beta rhythm, asymmetry of more than 50% of the amplitude, the appearance of an alpha-like image of the beta rhythm, an increase in amplitude of more than 5 μV - beta rhythm or beta activity is described according to the relevant rules (see 2.1, 2.4, 2.5).
2.4. Description of generalized (diffuse) activity.
2.4.1. Frequency response of outbreaks and paroxysms.
2.4.2. Amplitude.
2.4.3. The duration of outbreaks and paroxysms in time and the frequency of their occurrence.
2.4.4. An image of generalized activity.
2.4.5. By what rhythm (activity) flashes or paroxysms are distorted.
2.4.6. Topical diagnostics focus or main focus of generalized activity.
2.5. Description focal changes EEG.
2.5.1. Topical diagnosis of the lesion.
2.5.2. Rhythm (activity) of local changes.
2.5.3. Image of local changes: alpha-like image, regular component, paroxysms.
2.5.4. Than local EEG changes are distorted.
2.5.5. Quantitative characteristics of changes: frequency, amplitude, index.
3. Description of the reactive (activation) EEG.
3.1. Single flash of light (approximate load).
3.1.1. The nature of changes in bioelectrical activity: depression of the alpha rhythm, exaltation of the alpha rhythm, other changes in frequency and amplitude (see section of the Manual).
3.1.2. Topical distribution of changes in bioelectrical activity.
3.1.3. The duration of changes in bioelectrical activity.
3.1.4. The rate of extinction of the orienting reaction when using repeated stimuli.
3.1.5. The presence and nature of the evoked responses: negative slow waves, the appearance of a beta rhythm.
3.2. Rhythmic photostimulation (RPS).
3.2.1. Rhythm range.
3.2.2. The nature of the rhythm assimilation reaction (RUR).
3.2.3. The amplitude of the learned rhythm in relation to the background activity: above the background (distinct), below the background (indistinct).
3.2.2.2. The duration of RUR in relation to the time of stimulation: short-term, long-term, long-term with a consequence.
3.2.2.3. Symmetry in the hemispheres.
3.2.3. Topical distribution of RUR.
3.2.4. The emergence of harmonics and their particular characteristics.
3.2.5. The emergence of subharmonics and their frequency response.
3.2.6. The emergence of rhythms that are not multiple of the frequency of light flashes.
3.3. Trigger photostimulation (TFS).
3.3.1. frequency range, excited by TFS.
3.3.2. Topic of the appeared changes.
3.3.3. Quantitative characteristics of changes: frequency, amplitude.
3.3.4. The nature of the excited activity: spontaneous waves, evoked responses.
3.4. Hyperventilation (HV).
3.4.1. The time from the beginning of the load to the appearance of changes in bioelectrical activity.
3.4.2. Topic of changes.
3.4.3. Quantitative characteristics of changes in bioelectrical activity: frequency, amplitude.
3.4.4. Time to return to background activity.
3.5. Pharmacological loads.
3.5.1. Exposure concentration (in mg per 1 kg of patient's body weight).
3.5.2. The time from the start of exposure to the appearance of changes in bioelectrical activity.
3.5.3. The nature of changes in bioelectric activity.
3.5.4. Quantitative characteristics of changes: frequency, amplitude, duration.
4. Conclusion.
4.1. Evaluation of the severity of EEG changes. EEG changes within the normal range, moderate, moderate, significant changes, heavy changes EEG.
4.2. Localization of changes.
4.3. clinical interpretation.
4.4. Assessment of the general functional state of the brain.
The recording electrodes are placed in such a way that all the main parts of the brain are represented on the multichannel recording, denoted by the initial letters of their Latin names. In clinical practice, two main EEG lead systems are used: the international 10-20 lead system and a modified circuit with a reduced number of electrodes. If it is necessary to obtain a more detailed picture of the EEG, the "10-20" scheme is preferable.
Such a lead is called a reference lead when a potential is applied to the "input 1" of the amplifier from an electrode located above the brain, and to "input 2" - from an electrode at a distance from the brain. The electrode located above the brain is most often called active. The electrode removed from the brain tissue is called the reference electrode. As such, use the left (A 1) and right (A 2) earlobes. The active electrode is connected to the “input 1” of the amplifier, the supply of a negative potential shift to which causes the recording pen to deviate upward. The reference electrode is connected to "input 2". In some cases, a lead from two shorted electrodes (AA) located on the earlobes is used as a reference electrode. Since the potential difference between the two electrodes is recorded on the EEG, the position of the point on the curve will be in equally, but in the opposite direction affect the potential changes under each of the pair of electrodes. In the reference lead under the active electrode, an alternating potential of the brain is generated. Under the reference electrode, which is away from the brain, there is a constant potential that does not pass into the AC amplifier and does not affect the recording pattern. The potential difference reflects without distortion the fluctuations in the electrical potential generated by the brain under the active electrode. However, the area of the head between the active and reference electrodes is part of electrical circuit"amplifier-object", and the presence in this area of a sufficiently intense source of potential, located asymmetrically with respect to the electrodes, will significantly affect the readings. Therefore, in the case of a referential assignment, the judgment about the localization of the potential source is not entirely reliable.
Bipolar is called a lead, in which electrodes above the brain are connected to the "input 1" and "input 2" of the amplifier. The position of the EEG recording point on the monitor is equally affected by the potentials under each of the pair of electrodes, and the recorded curve reflects the potential difference of each of the electrodes. Therefore, the judgment of the form of oscillation under each of them on the basis of one bipolar assignment is impossible. At the same time, the analysis of the EEG recorded from several pairs of electrodes in various combinations makes it possible to determine the localization of potential sources that make up the components of a complex total curve obtained with bipolar derivation.
For example, if there is a local source of slow oscillations in the posterior temporal region, when the anterior and posterior temporal electrodes (Ta, Tr) are connected to the amplifier terminals, a recording is obtained containing a slow component corresponding to slow activity in the posterior temporal region (Tr), with superimposed faster oscillations generated by the normal medulla of the anterior temporal region (Ta). To clarify the question of which electrode registers this slow component, pairs of electrodes are switched on two additional channels, in each of which one is represented by an electrode from the original pair, that is, Ta or Tr. and the second corresponds to some non-temporal lead, such as F and O.
It is clear that in the newly formed pair (Tr-O), including the posterior temporal electrode Tr, located above the pathologically altered medulla, there will again be a slow component. In a pair whose inputs are fed with activity from two electrodes placed over a relatively intact brain (Ta-F), a normal EEG will be recorded. Thus, in the case of a local pathological cortical focus, the connection of an electrode located above this focus, paired with any other one, leads to the appearance of a pathological component in the corresponding EEG channels. This allows you to determine the localization of the source of pathological fluctuations.
An additional criterion for determining the localization of the source of the potential of interest on the EEG is the phenomenon of oscillation phase distortion. If three electrodes are connected to the inputs of two channels of the electroencephalograph as follows: electrode 1 - to "input 1", electrode 3 - to "input 2" of amplifier B, and electrode 2 - simultaneously to "input 2" of amplifier A and "input 1" of amplifier B; assume that under electrode 2 there is a positive shift of the electrical potential relative to the potential of the remaining parts of the brain (indicated by the sign "+"), then it is obvious that electricity, due to this potential shift, will have the opposite direction in the circuits of amplifiers A and B, which will be reflected in oppositely directed potential difference shifts - antiphases - on the corresponding EEG records. Thus, the electrical oscillations under electrode 2 in the records on channels A and B will be represented by curves having the same frequencies, amplitudes and shape, but opposite in phase. When switching electrodes through several channels of the electroencephalograph in the form of a chain, antiphase oscillations of the investigated potential will be recorded through those two channels, to the opposite inputs of which one common electrode is connected, standing above the source of this potential.
Rules for registering an electroencephalogram and functional tests
During the examination, the patient should be in a light and soundproof room in a comfortable chair with eyes closed. Observation of the study is carried out directly or with the help of a video camera. During recording, significant events and functional trials are marked with markers.
During the test of opening and closing the eyes, characteristic electrooculogram artifacts appear on the EEG. The resulting changes in the EEG make it possible to identify the degree of contact of the subject, the level of his consciousness and tentatively assess the reactivity of the EEG.
To detect the brain's response to external influences apply single stimuli in the form of a short flash of light, a sound signal. In patients in coma it is permissible to use nociceptive stimuli by pressing the nail on the base of the nail bed index finger sick.
For photostimulation, short (150 μs) flashes of light, close in spectrum to white, of sufficiently high intensity (0.1-0.6 J) are used. Photostimulators make it possible to present a series of flashes used to study the reaction of rhythm assimilation - the ability of electroencephalographic oscillations to reproduce the rhythm of external stimuli. Normally, the rhythm assimilation reaction is well expressed at a flicker frequency close to the intrinsic EEG rhythms. Rhythmic assimilation waves have the highest amplitude in the occipital regions. With photosensitivity epileptic seizures, rhythmic photostimulation reveals a photoparoxysmal response - a generalized discharge of epileptiform activity.
Hyperventilation is carried out mainly to induce epileptiform activity. The subject is asked to breathe deeply rhythmically for 3 minutes. The respiratory rate should be in the range of 16-20 per minute. EEG registration begins at least 1 minute before the onset of hyperventilation and continues throughout the hyperventilation and at least 3 minutes after it ends.
There are many mysteries in the human body, and not all of them are subject to doctors yet. The most complex and confusing of them, perhaps, brain. Various methods of brain research, such as electroencephalography, help doctors lift the veil of secrecy. What is it and what can the patient expect from the procedure?
Who is eligible for an electroencephalography test?
Electroencephalography (EEG) allows you to clarify many diagnoses associated with infections, injuries and brain disorders.
The doctor may refer you for an examination if:
- There is a possibility of epilepsy. The brain waves in this case show a special epileptiform activity, which is expressed in the modified form of the graphs.
- It is required to establish the exact location of the injured part of the brain or tumor.
- There are some genetic diseases.
- There are serious violations of sleep and wakefulness.
- Disrupted work cerebral vessels.
- An assessment of the effectiveness of the treatment is needed.
The electroencephalography method is applicable to both adults and children, it is non-traumatic and painless. A clear picture of the work of brain neurons in its different parts makes it possible to clarify the nature and causes of neurological disorders.
Method of brain research electroencephalography - what is it?
Such an examination is based on the registration of bioelectric waves emitted by the neurons of the cerebral cortex. With the help of electrodes, the activity of nerve cells is captured, amplified, and the device is translated into a graphic form.
The resulting curve characterizes the process of work of different parts of the brain, its functional state. AT normal condition it has a certain shape, and deviations are diagnosed taking into account changes appearance graphic arts.
EEG can be performed in various options. The room for him is isolated from extraneous sounds and light. The procedure usually takes 2-4 hours and is performed in a clinic or laboratory. In some cases, electroencephalography with sleep deprivation requires more time.
The method allows doctors to obtain objective data about the state of the brain, even when the patient is unconscious.
How is an EEG performed?
If a doctor prescribes electroencephalography, what is it for the patient? He will be asked to sit in comfortable position or lie down, put on the head fixing the electrodes a helmet made of elastic material. If the recording is supposed to be long, then a special conductive paste or collodion is applied at the points of contact of the electrodes with the skin. The electrodes do not cause any discomfort.
EEG does not suggest any violation of the integrity of the skin or the introduction medicines(premedication).
Routine recording of brain activity occurs for a patient in a state of passive wakefulness, when he lies quietly or sits with his eyes closed. It's quite difficult, time drags on slowly and you have to fight sleep. The laboratory assistant periodically checks the patient's condition, asks to open his eyes and perform certain tasks.
During the study, the patient should minimize any physical activity, which would interfere. It is good if the laboratory manages to fix neurological manifestations of interest to physicians (convulsions, tics, epileptic seizure). Sometimes an attack in epileptics is provoked purposefully in order to understand its type and origin.
Preparation for the EEG
On the eve of the study, it is worth washing your hair. It is better not to braid your hair and not to use any styling products. Leave hairpins and clips at home, and collect long hair in a ponytail, if necessary.
Metal jewelry should also be left at home: earrings, chains, lip and eyebrow piercings. Before entering the office, disable mobile phone(not only sound, but completely), so as not to interfere with sensitive sensors.
Before the examination, you need to eat so as not to feel hungry. It is advisable to avoid any unrest and strong feelings, but you should not take any sedatives.
You may need a tissue or towel to wipe off any remaining fixative gel.
Samples during the EEG
In order to track the reaction of brain neurons in various situations, and to expand the demonstrative capabilities of the method, the electroencephalography examination includes several tests:
1. Eye opening-closing test. The laboratory assistant makes sure that the patient is conscious, hears him, and follows the instructions. The absence of patterns on the chart at the time of opening the eyes indicates pathology.
2. Test with photostimulation, when flashes of bright light are directed into the eyes of the patient during recording. Thus, epileptimorphic activity is revealed.
3. A test with hyperventilation, when the subject breathes deeply voluntarily for several minutes. The frequency of respiratory movements at this time decreases slightly, but the oxygen content in the blood rises and, accordingly, the supply of oxygenated blood to the brain increases.
4. Sleep deprivation, when the patient is immersed in a short sleep with the help of sedatives or stay in the hospital for daily observation. This allows you to obtain important data on the activity of neurons at the time of awakening and falling asleep.
5. Stimulation of mental activity consists in solving simple problems.
6. Stimulation of manual activity, when the patient is asked to perform a task with an object in his hands.
All this gives a more complete picture of the functional state of the brain and notice violations that have a slight external manifestation.
The duration of the electroencephalogram
The time of the procedure may vary depending on the goals set by the doctor and the conditions of a particular laboratory:
- 30 minutes or more if you can quickly register the activity you are looking for;
- 2-4 hours in the standard version, when the patient is examined reclining in a chair;
- 6 or more hours on EEG with daytime sleep deprivation;
- 12-24 hours, when all phases of night sleep are examined.
The scheduled time of the procedure can be changed at the discretion of the doctor and laboratory assistant in any direction, because if there are no characteristic patterns corresponding to the diagnosis, the EEG will have to be repeated, spending extra time and money. And if all the necessary records are obtained, there is no point in tormenting the patient with forced inaction.
What is video monitoring during an EEG?
Sometimes the electroencephalography of the brain is duplicated by a video recording, which records everything that happens during the study with the patient.
Video monitoring is prescribed for patients with epilepsy to correlate how behavior during an attack correlates with brain activity. Timed matching of characteristic waves with the picture can clarify gaps in the diagnosis and help the clinician understand the condition of the subject for more accurate treatment.
The result of electroencephalography
When the patient underwent electroencephalography, the conclusion is handed out along with printouts of all graphs of the wave activity of various parts of the brain. In addition, if video monitoring was also carried out, the recording is saved on a disk or flash drive.
At a consultation with a neurologist, it is better to show all the results so that the doctor can assess the features of the patient's condition. Electroencephalography of the brain is not the basis for the diagnosis, but significantly clarifies the picture of the disease.
To ensure that all the smallest teeth are clearly visible on the graphs, it is recommended to store the printouts flattened in a hard folder.
Encryption from the brain: types of rhythms
When an electroencephalography is passed, which each graph shows, it is extremely difficult to understand on your own. The doctor will make a diagnosis based on the study of changes in the activity of areas of the brain during the study. But if the EEG was prescribed, then the reasons were good, and it would not hurt to consciously approach your results.
So, we have in our hands a printout of such an examination, like electroencephalography. What are these - rhythms and frequencies - and how to determine the limits of the norm? The main indicators that appear in the conclusion:
1. Alpha rhythm. The frequency normally ranges from 8-14 Hz. Between the cerebral hemispheres, a difference of up to 100 μV can be observed. The pathology of the alpha rhythm is characterized by asymmetry between the hemispheres exceeding 30%, the amplitude index is above 90 μV and below 20.
2. Beta rhythm. It is mainly fixed on the anterior leads (in frontal lobes). For most people, a typical frequency is 18-25 Hz with an amplitude of no more than 10 μV. The pathology is indicated by an increase in amplitude over 25 μV and a persistent spread of beta activity to the posterior leads.
3. Delta rhythm and Theta rhythm. Fixed only during sleep. The appearance of these activities during the period of wakefulness signals a malnutrition of the brain tissues.
5. Bioelectric activity (BEA). A normal indicator demonstrates synchrony, rhythm, and the absence of paroxysms. Deviations are manifested in early childhood epilepsy, predisposition to convulsions and depression.
In order for the results of the study to be indicative and informative, it is important to follow the prescribed treatment regimen exactly, without canceling the drugs before the study. Alcohol or energy drinks taken the day before can distort the picture.
What is electroencephalography used for?
For the patient, the benefits of the study are obvious. The doctor can check the correctness of the prescribed therapy and change it if necessary.
In people with epilepsy, when a period of remission is established by observation, the EEG may show seizures that are not superficially observable and still require medical intervention. Or avoid unreasonable social restrictions, specifying the features of the course of the disease.
The study can also contribute to the early diagnosis of neoplasms, vascular pathologies, inflammation and brain degeneration.
