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Electroencephalography

, medical expert
Last reviewed: 11.04.2020
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Electroencephalography (EEG) is the recording of electrical waves characterized by a certain rhythm. When analyzing the EEG, attention is paid to the basal rhythm, symmetry of brain electrical activity, spike activity, response to functional tests. The diagnosis is based on the clinical picture. The first human EEG was registered by the German psychiatrist Hans Berger in 1929.

Electroencephalography is a method of studying the brain with the help of recording the difference in electrical potentials that arise in the process of its vital activity. Recording electrodes are located in certain areas of the head so that all major parts of the brain are represented on the record. The resulting record - the electroencephalogram (EEG) - is the total electrical activity of many millions of neurons, represented mainly by the potentials of dendrites and bodies of nerve cells: excitatory and inhibitory postsynaptic potentials and in part - the action potentials of bodies of neurons and axons. Thus, the EEG reflects the functional activity of the brain. The presence of a regular rhythm on the EEG indicates that the neurons synchronize their activity. Normally this synchronization is determined mainly by the rhythmic activity of pacemakers (rhythm drivers) of nonspecific thalamus nuclei and their thalamocortical projections.

Since the level of functional activity is determined by nonspecific median structures (reticular formation of the trunk and forebrain), these systems determine the rhythm, appearance, overall organization and dynamics of the EEG. Symmetric and diffuse organization of the bonds of nonspecific median structures with the cortex determines the bilateral symmetry and the relative uniformity of the EEG for the entire brain.

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Purpose of electroencephalography

The main goal of the use of electroencephalography in clinical psychiatry is the detection or exclusion of signs of organic brain damage (epilepsy, tumors and brain injuries, cerebral circulation and metabolic disorders, neurodegenerative diseases) for differential diagnosis and clarification of the nature of clinical symptoms. In biological psychiatry, the EEG is widely used for an objective assessment of the functional state of certain brain structures and systems, for the study of neurophysiological mechanisms of mental disorders, and the effects of psychotropic drugs.

Indications for electroencephalography

  • Differential diagnosis of neuroinfections with volume lesions of the central nervous system.
  • Assessment of the severity of CNS damage in neuroinfections and infectious encephalopathies.
  • Clarification of the localization of the pathological process in encephalitis.

Preparation for the study of electroencephalography

Before the study, the patient should refrain from consuming beverages containing caffeine, taking sleeping pills and sedatives. For 24-48 h before electroencephalography (EEG) the patient stops taking anticonvulsants, tranquilizers, barbiturates and other sedatives.

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Method for the study of electroencephalography

Before the examination, the patient is informed about the EEG technique and its painlessness, because the emotional state significantly influences the results of the study. EEG is carried out in the morning before eating in the position lying on the back or half-asleep in a chair in a relaxed state.

Electrodes on the scalp are in accordance with the International Scheme.

First, when the patient's eyes are closed, the background (basal) EEG is recorded, then the recording is performed against a background of various functional tests (activation - for eye opening, photostimulation and hyperventilation). Photostimulation is performed using a stroboscopic light source, blinking at a frequency of 1-25 per second. When tested for hyperventilation, the patient is asked to breathe quickly and deeply for 3 minutes. Functional tests can reveal pathological activity, in other situations not detectable (including the focus of seizure activity), and provoke a convulsive attack in the patient, which is possible after the study, therefore, it is necessary to pay special attention to a patient who has some forms of pathological activity .

Position of electrodes

To assess the functional state of the main sensory, motor and associative zones of the cerebral cortex and their subcortical projections on the scalp, a significant number of electrodes (usually from 16 to 21) are established for EEG evaluation.

In order to make it possible to compare the EEG in different patients, the electrodes have a 10-20% standard international system. At the same time, the bridge of the nose, the occipital mound and external ear canals serve as the reference points for the electrode installation. The length of the longitudinal semicircle between the bridge of the nose and the occipital mound, and also the transverse semicircle between the external auditory canals is divided in the ratio of 10%, 20%, 20%, 20%, 20%, 10%. The electrodes are installed at the intersections of the meridians drawn through these points. The frontal-pole electrodes (Fp1, Fpz and Fp2) are closest to the forehead (at a distance of 10% from the bridge of the nose), and frontal (F3, Fz and F4) and frontal (F7 and F8 ). Then - central (SZ, Cz and C4) and temporal (T3 and T4). Further - parietal (P3, Pz and P4), posterior (T5 and T6) and occipital (01, Oz and 02) electrodes, respectively.

Odd figures denote electrodes located on the left hemisphere, even - electrodes located on the right hemisphere, and index z - electrodes located along the middle line. The reference electrodes on the earlobes are denoted as A1 and A2, and on the mastoid processes - as M1 and M2.

Typically, electrodes for recording EEG are metal discs with a contact rod and a plastic casing (bridged electrodes) or concave "cups" with a diameter of about 1 cm with a special silver chloride (Ag-AgCI) coating to prevent their polarization.

To reduce the resistance between the electrode and the patient's skin, special tampons moistened with NaCl solution (1-5%) are put on the disc electrodes. Cup electrodes are filled with electrically conductive gel. The hair under the electrodes is pushed apart, and the skin is degreased with alcohol. Electrodes are fixed to the head with a helmet made of rubber bands or special adhesive compounds and thin flexible wires are attached to the input device of the electroencephalograph.

At the present time, special helmet-hats are made of elastic fabric, in which the electrodes are mounted on a system of 10-20%, and wires from them in the form of a thin multicore cable are connected to an electroencephalograph by means of a multi-pin connector, which simplifies and speeds up the process of installing the electrodes.

Registration of brain electrical activity

The amplitude of the EEG potentials does not normally exceed 100 μV, therefore the equipment for EEG recording includes powerful amplifiers, as well as bandpass and barrier filters for isolating low-amplitude oscillations of brain biopotentials against a background of various physical and physiological disturbances-artifacts. In addition, electroencephalographic devices contain devices for photo- and photostimulation (less often for video and electrostimulation), which are used in studying the so-called "induced brain activity" (evoked potentials), and modern EEG complexes are also computer tools for analysis and visual graphic display (topographic mapping) of various EEG parameters, as well as a video system for monitoring the patient.

Functional load

In many cases, functional loads are used to detect hidden disorders of brain activity.

Types of functional loads:

  • rhythmic photostimulation with different frequencies of light flashes (including those synchronized with EEG waves);
  • phonostimulation (tones, clicks);
  • hyperventilation;
  • sleep deprivation;
  • continuous recording of EEG and other physiological parameters during sleep (polysomnography) or during the day (EEG monitoring);
  • registration of EEG in the performance of various perceptive-cognitive tasks;
  • pharmacological tests.

Contraindications to electroencephalography

  • Violation of vital functions.
  • Convulsive status.
  • Psychomotor agitation.

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Interpretation of the results of electroencephalography

The main rhythms that are allocated to the EEG include α, β, δ, θ-rhythms.

  • α-Rhythm - the basic cortical rhythm of EEG-dormancy (with a frequency of 8-12 Hz) is recorded when the patient is awake and closed eyes. It is maximum expressed in the occipital-parietal regions, has a regular character and disappears with afferent stimuli.
  • β-Rhythm (13-30 Hz) is usually associated with anxiety, depression, sedation, and is better recorded over the frontal region.
  • θ-Rhythm with a frequency of 4-7 Hz and amplitude of 25-35 μV is the normal component of adult EEG and dominates in childhood. Normally in adults, 9-vibrations are recorded in a state of natural sleep.
  • The δ-Rhythm with a frequency of 0.5-3 Hz and different amplitudes is normally recorded in the state of natural sleep, wakefulness is met only at a small amplitude and in a small amount (not more than 15%) with the presence of α-rhythm in 50%. Pathological consider 8-oscillations, exceeding the amplitude of 40 μV and occupying more than 15% of the total time. The appearance of the 5-rhythm in the first place indicates signs of a violation of the functional state of the brain. In patients with intracranial foci lesions on the EEG reveal slow waves over the corresponding region. The development of encephalopathy (hepatic) causes changes in the EEG, the severity of which is proportional to the degree of impairment of consciousness, in the form of generalized diffuse slow-wave electrical activity. The extreme expression of the pathological electrical activity of the brain is the absence of any oscillations (a straight line), which indicates the death of the brain. When detecting brain death, you should be prepared to provide moral support to the patient's relatives.

Visual analysis of EEG

To informative parameters of evaluation of the functional state of the brain both in the visual and in computer analysis of the EEG include the amplitude-frequency and spatial characteristics of the bioelectric activity of the brain.

Indicators of visual analysis of EEG:

  • amplitude;
  • average frequency;
  • index - the time occupied by a particular rhythm (in%);
  • degree of generalization of the basic rhythmic and phasic components of the EEG;
  • the focus localization is the greatest in amplitude and index of the basic rhythmic and phasic components of the EEG.

Alpha rhythm

Under standard conditions of registration (the state of motionless quiet wakefulness with closed eyes), the EEG of a healthy person is a set of rhythmic components that differ in frequency, amplitude, cortical topography and functional reactivity.

The main component of the EEG under standard conditions is normal α-rhythm [regular rhythmic activity with waves of a quasisinusoidal frequency of 8-13 Hz and characteristic amplitude modulations (α-spindle)], maximally represented in posterior (occipital and parietal) leads. Suppression of the α-rhythm occurs when opening and eye movements, visual stimulation, orientation reaction.

In the α-frequency range (8-13 Hz), several more types of α-like rhythmic activity are distinguished, which are detected less often in the occipital α-rhythm.

  • μ-Rhythm (Rolandic, central, arcuate rhythm) is a sensorimotor analogue of the occipital α-rhythm, which is recorded mainly in the central leads (above the central or Roland furrow). Sometimes it has a specific arcuate shape of the waves. The inhibition of rhythm occurs with tactile and proprioceptive stimulation, as well as with real or imaginary movement.
  • κ-Rhythm (Kennedy waves) is recorded in the temporal leads. It occurs in a situation of high level of visual attention when suppressing the occipital α-rhythm.

Other rhythms. There are also θ- (4-8 Hz), σ- (0,5-4 Hz), β- (above 14 Hz) and γ- (above 40 Hz) rhythms, as well as a number of other rhythmic and aperiodic (phasic) components EEG.

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Factors affecting the outcome

In the process of registration, the moments of the patient's motor activity are noted, as this affects the EEG and may be the reason for its incorrect interpretation.

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Electroencephalogram in mental pathology

Deviations of the EEG from the norm in mental disorders, as a rule, do not have a pronounced nosological specificity (with the exception of epilepsy ) and most often come down to several basic types.

The main types of EEG changes in mental disorders: deceleration and desynchronization of the EEG, flattening and disruption of the normal spatial structure of the EEG, the emergence of "pathological" waveforms.

  • EEG slowdown - decrease in frequency and / or inhibition of α-rhythm and increased content of θ- and σ-activity (for example, in dementia of elderly age, in zones with impaired cerebral circulation or in brain tumors).
  • Desynchronization of the EEG is manifested in the form of oppression of the α-rhythm and an increase in the content of β-activity (for example, in arachnoiditis, increased intracranial pressure, migraine, cerebrovascular disorders: cerebral atherosclerosis, stenosis of the cerebral arteries).
  • The "compaction" of the EEG includes general inhibition of the EEG amplitude and a low content of high-frequency activity [for example, in atrophic processes, in the expansion of subarachnoid spaces (external hydrocephalus), over superficial brain tumor or in the region of subdural hematoma].
  • Disruption of the normal spatial structure of the EEG. For example, rough interhemispheric asymmetry of EEG at local cortical tumors; smoothing interzonal differences in the EEG due to oppression of the occipital α-rhythm in anxiety disorders or in the generalization of α-frequency activity due to almost identical expression of α- and μ-rhythms, which is often detected in depression; displacement of the focus of β-activity from the anterior to posterior leads in vertebrobasillar insufficiency.
  • The appearance of "pathological" wave forms (primarily high-amplitude acute waves, peaks, complexes [eg, peak-wave in epilepsy) !. Sometimes such "epileptiform" EEG activity is absent in conventional surface leads, but it can be recorded from the nasopharyngeal electrode, which is injected through the nose to the base of the skull. It allows to reveal deep epileptic activity.

It should be noted that the listed features of changes in the visually determined and quantitative characteristics of the EEG for various neuropsychic diseases are mainly attributed to the κ-background EEG recorded under standard EEG recording conditions. This kind of EEG-examination is possible for most patients.

Interpretation of EEG disturbances is usually given in terms of reduced functional state of the cerebral cortex, a deficiency in cortical inhibition, increased excitability of stem structures, cortical-stem irrigation (irritation), the presence of EEG signs of a lowered threshold of convulsive readiness, indicating (if possible) the localization of these disorders or source pathological activity (in cortical areas and / or in subcortical nuclei (deep anteropic, limbic, diencephalic or lower barbar structures)).

This interpretation is based mainly on data on EEG changes in the sleep-wake cycle, on the reflection of established local organic brain lesions in the EEG picture, and cerebral blood flow disorders in the neurological and neurosurgical clinic, and on the results of numerous neurophysiological and psychophysiological studies (including data about EEG connection with the level of wakefulness and attention, with the action of stress factors, with hypoxia, etc.) and on the extensive empirical experience of the clinical electroencephalic cillograph.

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Complications

When carrying out functional tests, there may be a convulsive attack, which must be registered and ready to provide first aid to the patient.

The use of various functional tests, of course, increases the information content of the EEG survey. But increases the time required for EEG recording and analysis, leads to patient fatigue, and may also be associated with the risk of provoking convulsive attacks (for example, in hyperventilation or rhythmic photostimulation). In this regard, it is not always possible to use these methods in patients with epilepsy, the elderly or young children.

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Alternative methods

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Spectral analysis

As the main method of automatic computer analysis of EEG, spectral analysis based on Fourier transform is used, the representation of the native EEG picture as a set of a set of sinusoidal oscillations that differ in frequency and amplitude.

The main output parameters of the spectral analysis are:

  • average amplitude;
  • average and modal (most frequent) frequencies of EEG rhythms;
  • spectral power of EEG rhythms (integral index corresponding to the area under the EEG curve and depending both on the amplitude and on the index of the corresponding rhythm).

Spectral analysis of EEG is usually performed on short (2-4 sec) fragments of recording (epochs of analysis). Averaging the EEG power spectra for several dozen individual epochs with the calculation of the statistical parameter (spectral density) gives an idea of the most characteristic picture of the EEG for a given patient.

By comparing the power spectra (or the spectral density, the EEG coherence index is obtained in different leads, which reflects the similarity of the biopotential oscillations in different regions of the cerebral cortex.) This index has a certain diagnostic value: for example, increased coherence in the α-frequency band (especially when desynchronization EEG) are detected with the active joint participation of the relevant departments of the cerebral cortex in the performed activity.In contrast, the increased coherence in the 5-rhythm band reflects with izhennoe functional state of the brain (e.g., superficial tumors).

Periodometric analysis

Periodometric analysis (period-analysis, or amplitude-interval analysis) is used less often when the periods between the characteristic points of EEG waves (wave vertices or zero-line intersections) and the peak amplitude of the waves (peaks) are measured.

Period-analysis of EEG allows you to determine the average and extreme values of the amplitude of EEG waves, the average periods of waves and their dispersion, accurately (by the sum of all periods of waves of a given frequency range) to measure the EEG rhythm index.

Compared to Fourier analysis, the period-analysis of EEG has greater resistance to interference, since its results are much less dependent on the contribution of single high-amplitude artifacts (for example, interference from patient movements). However, it is used less often for spectral analysis, in particular, because standard criteria for detection thresholds of EEG wave peaks have not been developed.

Other non-linear methods of EEG analysis

Other nonlinear EEG methods are described, based, for example, on the calculation of the probability of the appearance of successive EEG waves belonging to different frequency ranges, or on the determination of temporal relationships between certain characteristic fragments of EEG | EEG patterns (eg, α-rhythm spindles) | in different leads. Although the experimental results showed the informativeness of the results of such types of EEG analysis in regard to the diagnosis of certain functional states of the brain, in practice, these methods are practically not used.

Quantitative electroencephalography makes it possible to determine the localization of foci of pathological activity in epilepsy and various neurological and vascular disorders more accurately than in the case of visual analysis of the EEG, to detect violations of amplitude-frequency characteristics and spatial organization of the EEG, in a number of mental disorders, to quantify the effect of therapy including psychopharmacotherapy ) on the functional state of the brain, as well as to perform automatic diagnosis of certain disorders and / or functional states of a healthy person by comparing individual EEG with databases of normative EEG data (age norm, different types of pathology, etc.). All these advantages allow to significantly reduce the time of preparation of the conclusion based on the results of the EEG survey, increase the probability of detecting abnormalities of the EEG from normal.

The results of quantitative EEG analysis can be given both in digital form (in the form of tables for subsequent statistical analysis) and also as a visual color map, which can be conveniently compared with CT, magnetic resonance imaging (MRI) and positron emission tomography PET), as well as with estimates of local cerebral blood flow and neuropsychological testing data. Thus, it is possible to directly compare the structural and functional disorders of brain activity.

An important step in the development of quantitative EEG was the creation of software for determining the intracerebral location of equivalent dipolar sources of the most high-amplitude EEG components (eg, epileptiform activity). The latest achievement in this area is the development of programs combining MRI and EEG maps of the patient's brain, taking into account the individual shape of the skull and the topography of the brain structures.

When interpreting the results of visual analysis or EEG mapping, it is necessary to take into account age-related (both evolutionary and involutional) changes in the amplitude-frequency parameters and spatial organization of the EEG, as well as changes in the EEG against the background of medication that naturally arise in patients in connection with treatment. For this reason, the EEG record is usually performed before or after temporary discontinuation of treatment.

Polysomnography

Electrophysiological study of sleep, or  polysomnography  - one of the areas of quantitative EEG.

The aim of the method is to objectively assess the duration and quality of night sleep, to identify sleep structure disorders [in particular, the duration and latent period of different phases of sleep, especially the sleep phase with fast eye movements], cardiovascular (cardiac and conduction disorders) and respiratory apnea) disorders during sleep.

Methodology of research

Physiological parameters of sleep (night or daytime):

  • EEG in one or two leads (most often C3 or C4);
  • data of the electrooculogram;
  • data of electromyogram;
  • frequency and depth of breathing;
  • general motor activity of the patient.

All these indicators are necessary to identify the stages of sleep according to generally accepted standard criteria. Slow-wave sleep is determined by the presence of EEG sleepy spindles and σ-activity, and the phase of sleep with rapid eye movements - desynchronization of the EEG, the appearance of rapid eye movements and a deep decrease in muscle tone.

In addition, an electrocardiogram (ECG) is often recorded. HELL. Skin temperature and oxygenation of the blood (using an ear photo-oximeter). All these indicators allow you to assess vegetative disorders during sleep.

Interpretation of results

Reducing the latency of the sleep phase with rapid eye movements (less than 70 min) and early (at 4-5 h) morning awakening - established biological signs of depressive and manic states. In this regard, polysomyography makes it possible to differentiate depression and depressive pseudodementia in elderly patients. In addition, this method objectively identifies insomnia, narcolepsy, somnambulism, as well as nightmares, panic attacks, apneas and epileptic seizures that occur during sleep.

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