Decoding of the results of electroencephalography

EEG analysis is performed during the recording and finally after its completion. At the time of recording, the presence of artifacts (focusing of the field of the network current, mechanical artifacts of the electrodes movement, electromyogram, electrocardiogram, etc.) is evaluated, take measures to eliminate them. The frequency and amplitude of the EEG are evaluated, characteristic graph elements are identified, their spatial and temporal distribution is determined. The analysis is completed by physiological and pathophysiological interpretation of the results and the formulation of a diagnostic conclusion with clinical-electroencephalographic correlation.

The main medical document on EEG is a clinical-electroencephalographic conclusion written by a specialist on the basis of analysis of the "raw" EEG. Conclusion on the EEG should be formulated in accordance with certain rules and consist of three parts:

1. Description of the main types of activity and graph elements;
2. a summary of the description and its pathophysiological interpretation;
3. correlating the results of the previous two parts with clinical data. The basic descriptive term in the EEG is "activity", which determines any sequence of waves (alpha activity, activity of acute waves, etc.).

• The frequency is determined by the number of oscillations per second; it is recorded with a corresponding number and expressed in Hz (Hz). The average frequency of the activity is given in the description. Usually take 4-5 EEG segments with a duration of 1 s and calculate the number of waves on each of them.
• Amplitude - the range of oscillations of the electric potential on the EEG; measured from the peak of the preceding wave to the peak of the next wave in the opposite phase, are expressed in microvolts (μV). A calibration signal is used to measure the amplitude. So, if a calibration signal corresponding to a voltage of 50 μV has a height of 10 mm on the record, then, accordingly, 1 mm of the pen deviation will mean 5 μV. To characterize the activity amplitude in the description of the EEG, the most typical maximum values are found, excluding pop-ups.
• The phase determines the current state of the process and indicates the direction of the vector of its changes. Some phenomena on the EEG are estimated by the number of phases they contain. A single-phase oscillation is called an oscillation in one direction from an isoelectric line with a return to the initial level, a two-phase one is such a vibration when, after the completion of one phase, the curve goes over the initial level, deviates in the opposite direction and returns to the isoelectric line. Polyphase means vibrations containing three phases or more. In a more narrow sense, the term "polyphase wave" defines a sequence of a- and slow (usually 5) waves.

[1], [2], [3], [4], [5], [6], [7]

Rhythms of an electroencephalogram of an adult waking person

By the notion of "rhythm" on the EEG is meant a certain type of electrical activity, corresponding to a certain state of the brain and associated with certain cerebral mechanisms. When describing the rhythm, its frequency, typical for a certain state and region of the brain, amplitude and some characteristic features of its changes in time with changes in the functional activity of the brain is indicated.

1. Alpha (a) rhythm : frequency 8-13 Hz, amplitude up to 100 μV. It is registered in 85-95% of healthy adults. It is best expressed in the occipital parts. The greatest amplitude of a-rhythm is in a state of calm relaxed wakefulness with closed eyes. In addition to changes related to the functional state of the brain, in most cases spontaneous changes in the amplitude of the a rhythm are observed, which are manifested in alternating growth and decrease with the formation of characteristic spindles, lasting 2-8 seconds. With an increase in the level of functional activity of the brain (intense attention, fear), the amplitude of the a-rhythm decreases. EEG appears high-frequency low-amplitude irregular activity, reflecting desynchronization of neuronal activity. With short-term, sudden external stimulation (especially a flash of light), this desynchronization arises abruptly, and in the event that the stimulation is not of an emotiogenic nature, a-rhythm is quickly restored (0.5-2 s). This phenomenon is called "activation reaction", "orientation reaction", "a-rhythm fading reaction", "desynchronization reaction".
2. Beta-rhythm : frequency 14-40 Hz, amplitude up to 25 μV. Beta-rhythm is best recorded in the area of the central gyri, but extends to the posterior central and frontal gyrus. In norm it is very weakly expressed and in most cases has an amplitude of 5-15 μV. The beta-rhythm is associated with somatic sensory and motor cortical mechanisms and gives a fading response to motor activation or tactile stimulation. Activity with a frequency of 40-70 Hz and an amplitude of 5-7 μV is sometimes called у-rhythm, it has no clinical significance.
3. Mu-rhythm : frequency 8-13 Hz, amplitude up to 50 μV. Parameters of the mu-rhythm are similar to those of the normal a-rhythm, but the mu-rhythm differs from the latter in physiological properties and topography. Visually, the mu-rhythm is observed only in 5-15% of subjects in the Rolandic region. The amplitude of the mu-rhythm (in rare cases) increases with motor activation or somatosensory stimulation. At routine analysis the mu-rhythm does not have clinical significance.

Types of activity pathological for an adult waking person

• Theta activity : frequency 4-7 Hz, the amplitude of pathological theta activity> 40 μV and more often than the amplitude of normal brain rhythms, reaching 300 μV or more at some pathological conditions.
• Delta activity : frequency of 0.5-3 Hz, amplitude is the same as in theta-activity.

Theta and delta vibrations may be present in a small amount on the EEG of an adult waking person and normally, but their amplitude does not exceed that of a rhythm. Pathological EEG is considered, which contains theta and delta vibrations with an amplitude> 40 microvolts and occupy more than 15% of the total recording time.

Epileptiform activity is a phenomenon typically observed in the EEG of epileptic patients. They arise as a result of highly synchronized paroxysmal depolarization shifts in large populations of neurons, accompanied by the generation of action potentials. As a result of this, high-amplitude acute potentials arise which have corresponding names.

• Spike (English spike - peak, peak) - negative potential of an acute form, duration less than 70 ms, amplitude> 50 microvolts (sometimes up to hundreds or even thousands of microvolts).
• The sharp wave differs from the spike by stretching in time: its duration is 70-200 ms.
• Sharp waves and spikes can be combined with slow waves, forming stereotyped complexes. A spike-slow wave is a complex of spike and slow wave. The frequency of the spike-slow wave complexes is 2.5-6 Hz, and the period, respectively, is 160-250 ms. An acute-slow wave is a complex of an acute wave and the slow wave following it, the period of the complex is 500-1300 ms.

An important characteristic of spikes and sharp waves is their sudden appearance and disappearance and a clear difference from background activity, which they exceed in amplitude. Acute phenomena with corresponding parameters, vaguely different from background activity, are not designated as acute waves or spikes.

Combinations of the phenomena described are indicated by some additional terms.

• Flash is a term for a group of waves with a sudden appearance and disappearance, clearly differing from the background activity by frequency, shape, and / or amplitude.
• Discharge is a burst of epileptiform activity.
• The pattern of epileptic seizure is a discharge of epileptiform activity, typically coinciding with a clinical epileptic seizure. The detection of such phenomena, even if it is not possible to clearly assess clinically the patient's state of consciousness, is also characterized as a "pattern of epileptic seizure".
• Gypsarhythm (Greek "high-amplitude rhythm") - continuous generalized high-amplitude (> 150 microvolts) slow hypersynchronous activity with acute waves, spikes, spike-slow wave complexes, polyspike-slow wave, synchronous and asynchronous. An important diagnostic sign of West syndromes and Lennox-Gastaut syndromes.
• Periodic complexes are high-amplitude bursts of activity, characterized by a constant shape for the patient. The most important criteria for their recognition are: close to a constant interval between complexes; continuous presence throughout the entire recording, provided the level of functional activity of the brain is constant; intra-individual stability of form (stereotype). Most often they are represented by a group of high-amplitude slow waves, sharp waves, combined with high-amplitude, pointed delta or theta-oscillations, sometimes reminiscent of epileptiform complexes, an acute-slow wave. The intervals between the complexes range from 0.5-2 to tens of seconds. Generalized bilaterally synchronous periodic complexes are always combined with deep disorders of consciousness and indicate a severe brain damage. If they are not caused by pharmacological or toxic factors (alcohol abstinence, overdose or sudden withdrawal of psychotropic and hypnosedate drugs, hepatopathy, carbon monoxide poisoning), then, as a rule, they are the consequence of severe metabolic, hypoxic, prion or viral encephalopathy. If intoxications or metabolic disturbances are excluded, then periodic complexes with high reliability indicate a diagnosis of panencephalitis or prion disease.

Variants of the normal electroencephalogram of an adult waking person

EEG is substantially uniform for the entire brain and symmetrical. Functional and morphological heterogeneity of the cortex determines the features of electrical activity in different regions of the brain. Spatial change of types EEG of separate areas of a brain occurs gradually.

The majority (85-90%) of healthy adults with closed eyes at rest on EEG recorded dominant a rhythm with maximum amplitude in the occipital areas.

In 10-15% of healthy subjects, the amplitude of oscillations on the EEG does not exceed 25 μV, in all leads a high-frequency low-amplitude activity is recorded. Such EEG are called low-amplitude ones. Low-amplitude EEGs indicate the predominance of desynchronizing influences in the brain and are a variant of the norm.

In some healthy subjects, instead of a-rhythm, the activity of 14-18 Hz is detected with an amplitude of about 50 μV in the occipital regions, and, like a normal alpha-rhythm, the amplitude decreases in the direction of the anterior. Such activity is called "fast a-variant".

Very rarely (0.2% of cases) on the EEG with closed eyes in the occipital regions, regular, close to sinusoidal, slow waves with a frequency of 2.5-6 Hz and an amplitude of 50-80 μV are recorded. This rhythm has all the other topographic and physiological characteristics of the alpha rhythm and is called the "slow alpha" variant. Without being associated with any organic pathology, it is considered as a borderline between the norm and pathology and may indicate the dysfunction of diencephalic nonspecific brain systems.

Changes in the electroencephalogram in the wakefulness-sleep cycle

• Active wakefulness (with mental load, visual tracking, training and other situations requiring increased mental activity) is characterized by desynchronization of neuronal activity, EEG is dominated by low-amplitude high-frequency activity.
• Relaxed wakefulness is the condition of the subject, resting in a comfortable armchair or on a bed with relaxed muscles and closed eyes, not occupied with any special physical or mental activity. In most healthy adults, a regular alpha rhythm is recorded in this condition on the EEG.
• The first stage of sleep is equivalent to drowsiness. On the EEG, the disappearance of the alpha-rhythm and the appearance of single and group low-amplitude delta and theta vibrations and low-amplitude high-frequency activity are observed. External stimuli cause outbreaks of alpha rhythm. The duration of the stage is 1-7 min. By the end of this stage, slow oscillations with an amplitude of <75 μV appear. At the same time, there may appear "vertex acute transition potentials" in the form of single or group monophasic surface-negative acute waves with a maximum in the crown region, the amplitude usually not more than 200 μV; they are considered a normal physiological phenomenon. The first stage is also characterized by slow eye movements.
• The second stage of sleep is characterized by the appearance of sleeping spindles and K-complexes. Sleepy spindles are bursts of activity of 11-15 Hz frequency prevailing in the central leads. The duration of the spindles is 0.5-3 s, the amplitude is approximately 50 μV. They are associated with median subcortical mechanisms. The K-complex is a burst of activity, typically consisting of a two-phase high-amplitude wave with an initial negative phase, sometimes accompanied by a spindle. Its amplitude is maximal in the region of the vertex, duration is not less than 0.5 s. K-complexes arise spontaneously or in response to sensory stimuli. At this stage, episodes of flash of polyphase high-amplitude slow waves are also observed. Slow eye movements are absent.
• The third stage of sleep: the spindles gradually disappear and delta and theta waves appear with an amplitude of more than 75 microvolts in the amount of 20-50% of the time of the analysis period. At this stage, it is often difficult to differentiate K complexes from delta waves. Sleepy spindles can completely disappear.
• The fourth stage of sleep is characterized by waves of frequency <2 Hz and more than 75 μV, occupying more than 50% of the time of the analysis period.
• During sleep, the person occasionally experiences episodes of desynchronization on the EEG - a so-called sleep with rapid eye movements. During these periods, polymorphic activity with a predominance of high frequencies is recorded. These periods on the EEG correspond to the experience of the dream, the fall of the muscle tone with the appearance of rapid movements of the eyeballs and sometimes rapid limb movements. The emergence of this stage of sleep is associated with the work of the regulatory mechanism at the level of the bridge of the brain, its violation indicates the dysfunction of these parts of the brain, which is of great diagnostic significance.

[8], [9], [10], [11], [12], [13]

Age-related changes in the electroencephalogram

The EEG of a premature baby aged up to 24-27 weeks of gestation is represented by flashes of slow delta and theta activity, occasionally combining with acute waves, lasting 2-20 seconds, against a background of low-amplitude (up to 20-25 μV) activity.

In children 28-32 weeks of gestation, delta- and theta-activity with an amplitude of up to 100-150 μV becomes more regular, although it may also include outbreaks of a higher-amplitude theta activity, alternating with flattening periods.

In children older than 32 weeks gestation on the EEG, functional states begin to be traced. In a calm sleep, an intermittent high-amplitude (up to 200 μV and higher) delta activity is observed, combined with theta-oscillations and acute waves and alternating with periods of relatively low-amplitude activity.

The full-term newborn on the EEG clearly distinguishes between wakefulness with open eyes (irregular activity at a frequency of 4-5 Hz and amplitude of 50 μV), active sleep (constant low-amplitude activity of 4-7 Hz with superposition of faster low-amplitude oscillations) and a calm sleep characterized by outbreaks high-amplitude delta activity in combination with spindles of faster high-amplitude waves alternating with low-amplitude periods.

In healthy premature infants and full-term newborns, during the first month of life, alternating activity is observed during restful sleep. On EEG of newborns there are physiological acute potentials, characterized by multifocality, sporadic appearance, irregularity of following. Their amplitude usually does not exceed 100-110 μV, the frequency of occurrence on the average is 5 per hour, the main amount of them is confined to a calm sleep. Normal also consider relatively regularly occurring acute potentials in the frontal leads, which do not exceed 150 μV in amplitude. A normal EEG of a mature newborn is characterized by a response in the form of EEG flattening to external stimuli.

During the first month of life of the mature child, the alternating EEG of quiet sleep disappears, sleep spindles appear in the second month, the dominant activity in the occipital leads arrives, reaching a frequency of 4-7 Hz at the age of 3 months.

During the 4th-6th months of life, the number of theta waves on the EEG gradually increases, and the delta wave decreases, so by the end of the 6th month, the rhythm at a frequency of 5-7 Hz dominates the EEG. From the 7th to the 12th month of life, an alpha rhythm is formed with a gradual decrease in the number of theta and delta waves. By 12 months, oscillations dominate, which can be characterized as a slow alpha rhythm (7-8.5 Hz). From 1 year to 7-8 years, the process of gradual replacement of slow rhythms with faster fluctuations (alpha and beta bands) continues. After 8 years, the alpha-rhythm dominates the EEG. The final formation of the EEG occurs by 16-18 years.

Boundary values of the frequency of the dominant rhythm in children

Age, years	Frequency Hz
1	> 5
3	> 6
5	> 7
8	> 8

On the EEG of healthy children, there may be excessive diffuse slow waves, flashes of rhythmic slow oscillations, discharges of epileptiform activity, so that from the point of view of the traditional estimation of the age norm, even among healthy individuals under the age of 21, only 70-80 can be referred to "normal" % EEG.

From 3-4 and up to 12 years, the share of EEG with excess slow waves increases (from 3 to 16%), and then this index decreases quite rapidly.

The reaction to hyperventilation in the form of the appearance of high-amplitude slow waves at the age of 9-11 years is more pronounced than in the younger group. It is not excluded, however, that this is due to the less precise performance of the sample by younger children.

Representation of some EEG variants in a healthy population depending on the age

Activity type	1-15 years old	16-21 year
Slow diffuse activity with an amplitude of more than 50 μV, recorded more than 30% of the recording time	14%	5%
Slow rhythmic activity in the rear leads	25%	0.5%
Epileptiform activity, bursts of rhythmic slow waves	15%	5%
"Normal" EEG options	68%	77%

The aforementioned relative stability of EEG characteristics of an adult person remains approximately up to 50 years. Since this period, the EEG spectrum has been restructured, resulting in a decrease in the amplitude and relative amount of alpha-rhythm and an increase in the number of beta and delta waves. The dominant frequency after 60-70 years has a tendency to decrease. At this age, in practically healthy individuals, theta and delta waves visible in visual analysis also appear.

[14], [15], [16], [17], [18]