Deciphering electroencephalography results

EEG analysis is performed during the recording and finally upon its completion. During the recording, the presence of artifacts (induction of network current fields, mechanical artifacts of electrode movement, electromyogram, electrocardiogram, etc.) is assessed, and measures are taken to eliminate them. The frequency and amplitude of the EEG are assessed, characteristic graphic elements are identified, and their spatial and temporal distribution is determined. The analysis is completed by physiological and pathophysiological interpretation of the results and formulation of a diagnostic conclusion with clinical-electroencephalographic correlation.

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

1. description of the main types of activity and graphic elements;
2. summary of the description and its pathophysiological interpretation;
3. correlation of the results of the previous two parts with clinical data. The basic descriptive term in EEG is "activity", which defines any sequence of waves (alpha activity, sharp wave activity, etc.).

• Frequency is defined as the number of oscillations per second; it is written down as a corresponding number and expressed in hertz (Hz). The description provides the average frequency of the activity being assessed. Usually, 4-5 EEG segments of 1 second duration are taken and the number of waves in each of them is calculated.
• Amplitude is the range of electric potential oscillations on the EEG; it is measured from the peak of the preceding wave to the peak of the following wave in the opposite phase, expressed in microvolts (μV). A calibration signal is used to measure the amplitude. Thus, if the calibration signal corresponding to a voltage of 50 μV has a height of 10 mm on the recording, then, accordingly, 1 mm of pen deflection will mean 5 μV. To characterize the amplitude of activity in the description of the EEG, its most typically occurring maximum values are taken, excluding outliers.
• The phase determines the current state of the process and indicates the direction of the vector of its changes. Some EEG phenomena are assessed 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 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. Polyphasic are oscillations containing three or more phases. In a narrower sense, the term "polyphasic wave" defines a sequence of a- and slow (usually 5) waves.

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Electroencephalogram rhythms of an adult awake person

The term "rhythm" in EEG refers to a certain type of electrical activity corresponding to a certain state of the brain and associated with certain cerebral mechanisms. When describing a rhythm, its frequency, typical for a certain state and area of the brain, amplitude, and some characteristic features of its changes over time with changes in the functional activity of the brain are 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 regions. The a-rhythm has the greatest amplitude in a state of calm relaxed wakefulness with closed eyes. In addition to changes associated with the functional state of the brain, spontaneous changes in the amplitude of the a-rhythm are observed in most cases, expressed in an alternating increase and decrease with the formation of characteristic "spindles" lasting 2-8 s. With an increase in the level of functional activity of the brain (intense attention, fear), the amplitude of the a-rhythm decreases. High-frequency low-amplitude irregular activity appears on the EEG, reflecting the desynchronization of neuronal activity. With a short-term, sudden external stimulation (especially a flash of light), this desynchronization occurs sharply, and if the stimulation is not of an emotiogenic nature, the a-rhythm is restored fairly quickly (in 0.5-2 s). This phenomenon is called the "activation reaction", "orienting reaction", "reaction of a-rhythm extinction", "desynchronization reaction".
2. Beta rhythm: frequency 14-40 Hz, amplitude up to 25 μV. The beta rhythm is best recorded in the area of the central convolutions, but also extends to the posterior central and frontal convolutions. Normally, it is expressed very weakly 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 an extinction reaction to motor activation or tactile stimulation. Activity with a frequency of 40-70 Hz and an amplitude of 5-7 μV is sometimes called the y-rhythm, it has no clinical significance.
3. Mu rhythm: frequency 8-13 Hz, amplitude up to 50 μV. The 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. In routine analysis, the mu rhythm has no clinical significance.

Types of activity that are pathological for an adult awake person

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

Theta and delta oscillations may be present in small quantities in the EEG of an adult awake person and in the norm, but their amplitude does not exceed that of the a-rhythm. An EEG containing theta and delta oscillations with an amplitude of >40 μV and occupying more than 15% of the total recording time is considered pathological.

Epileptiform activity is a phenomenon typically observed in the EEG of patients with epilepsy. It results from highly synchronized paroxysmal depolarization shifts in large populations of neurons, accompanied by the generation of action potentials. This results in high-amplitude, acute potentials, which have corresponding names.

• Spike (English spike - point, peak) is a negative potential of a sharp form, lasting less than 70 ms, with an amplitude of >50 μV (sometimes up to hundreds or even thousands of μV).
• A sharp wave differs from a spike in that it is extended in time: its duration is 70-200 ms.
• Sharp waves and spikes can be combined with slow waves, forming stereotypical complexes. Spike-slow wave is a complex of a spike and a slow wave. The frequency of spike-slow wave complexes is 2.5-6 Hz, and the period, respectively, is 160-250 ms. Sharp-slow wave is a complex of a sharp wave and a 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 their clear distinction from the background activity, which they exceed in amplitude. Sharp phenomena with corresponding parameters that are not clearly distinguishable from the background activity are not designated as sharp waves or spikes.

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

• Burst is a term used to describe a group of waves with sudden onset and cessation that are clearly distinct from background activity in frequency, shape, and/or amplitude.
• A discharge is a burst of epileptiform activity.
• An epileptic seizure pattern is a discharge of epileptiform activity that typically coincides with a clinical epileptic seizure. The detection of such phenomena, even if the patient's state of consciousness cannot be clearly assessed clinically, is also characterized as an "epileptic seizure pattern".
• Hypsarrhythmia (Greek: "high-amplitude rhythm") is a continuous generalized high-amplitude (>150 μV) slow hypersynchronous activity with sharp waves, spikes, spike-slow wave complexes, polyspike-slow wave, synchronous and asynchronous. An important diagnostic feature of West and Lennox-Gastaut syndromes.
• Periodic complexes are high-amplitude bursts of activity characterized by a constant form for a given patient. The most important criteria for their recognition are: a nearly constant interval between complexes; continuous presence throughout the entire recording, provided that the level of functional brain activity is constant; intra-individual stability of form (stereotypicality). Most often, they are represented by a group of high-amplitude slow waves, sharp waves, combined with high-amplitude, sharpened delta or theta oscillations, sometimes resembling epileptiform complexes of a sharp-slow wave. Intervals between complexes range from 0.5-2 to tens of seconds. Generalized bilaterally synchronous periodic complexes are always combined with profound disturbances of consciousness and indicate severe brain damage. If they are not caused by pharmacological or toxic factors (alcohol withdrawal, overdose or sudden withdrawal of psychotropic and hypnosedative drugs, hepatopathy, carbon monoxide poisoning), then, as a rule, they are a consequence of severe metabolic, hypoxic, prion or viral encephalopathy. If intoxication or metabolic disorders are excluded, then periodic complexes with high reliability indicate a diagnosis of panencephalitis or prion disease.

Variants of a normal electroencephalogram in an awake adult

EEG is largely uniform for the entire brain and symmetrical. The functional and morphological heterogeneity of the cortex determines the characteristics of the electrical activity of different areas of the brain. The spatial change of EEG types in individual areas of the brain occurs gradually.

In the majority (85-90%) of healthy adults, with eyes closed at rest, the EEG records a dominant a-rhythm with maximum amplitude in the occipital regions.

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

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

Very rarely (0.2% of cases) on the EEG with closed eyes in the occipital areas 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". Not being associated with any organic pathology, it is considered as a borderline between the norm and pathology and may indicate dysfunction of the diencephalic non-specific systems of the brain.

Changes in the electroencephalogram during the sleep-wake cycle

• Active wakefulness (during mental stress, visual tracking, learning and other situations requiring increased mental activity) is characterized by desynchronization of neuronal activity; low-amplitude high-frequency activity predominates on the EEG.
• Relaxed wakefulness is the state of the subject resting in a comfortable chair or bed with relaxed muscles and closed eyes, not engaged in any special physical or mental activity. In most healthy adults, a regular alpha rhythm is recorded on the EEG in this state.
• The first stage of sleep is equivalent to drowsiness. The EEG shows the disappearance of the alpha rhythm and the appearance of single and group low-amplitude delta and theta oscillations and low-amplitude high-frequency activity. External stimuli cause bursts of the alpha rhythm. The stage lasts 1-7 min. By the end of this stage, slow oscillations with an amplitude of <75 μV appear. At the same time, "vertex sharp transient potentials" may appear in the form of single or group monophasic superficially negative sharp waves with a maximum in the crown area, with an amplitude usually not exceeding 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 sleep spindles and K-complexes. Sleep spindles are bursts of activity with a frequency of 11-15 Hz, predominant 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 biphasic high-amplitude wave with an initial negative phase, sometimes accompanied by a spindle. Its amplitude is maximum in the crown area, the duration is not less than 0.5 s. K-complexes occur spontaneously or in response to sensory stimuli. At this stage, bursts of polyphasic high-amplitude slow waves are also episodically observed. Slow eye movements are absent.
• Stage 3 sleep: spindles gradually disappear and delta and theta waves with an amplitude greater than 75 μV appear in quantities of 20 to 50% of the analysis period. At this stage, it is often difficult to differentiate K-complexes from delta waves. Sleep spindles may disappear completely.
• Stage IV sleep is characterized by waves with a frequency of <2 Hz and more than 75 μV, occupying more than 50% of the time of the analysis epoch.
• During sleep, a person occasionally experiences periods of desynchronization on the EEG - the 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 dreaming, a drop in muscle tone with the appearance of rapid movements of the eyeballs and sometimes rapid movements of the limbs. The occurrence of this stage of sleep is associated with the work of the regulatory mechanism at the level of the pons, its disruption indicates dysfunction of these parts of the brain, which is of great diagnostic importance.

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Age-related changes in the electroencephalogram

The EEG of a premature baby up to 24-27 weeks of gestation is represented by bursts of slow delta and theta activity, episodically combined with sharp waves, lasting 2-20 s, against the 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 bursts of higher-amplitude theta activity interspersed with periods of flattening.

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

In a full-term newborn, the EEG clearly distinguishes between wakefulness with open eyes (irregular activity with a frequency of 4-5 Hz and an amplitude of 50 μV), active sleep (constant low-amplitude activity of 4-7 Hz with superimposed faster low-amplitude oscillations) and quiet sleep, characterized by bursts of high-amplitude delta activity in combination with spindles of faster high-amplitude waves interspersed with low-amplitude periods.

In healthy premature and full-term newborns, alternating activity is observed during quiet sleep during the first month of life. The EEG of newborns contains physiological acute potentials characterized by multifocality, sporadic occurrence, and irregularity of occurrence. Their amplitude usually does not exceed 100-110 μV, the frequency of occurrence is on average 5 per hour, their main number is confined to quiet sleep. Relatively regularly occurring acute potentials in the frontal leads, not exceeding 150 μV in amplitude, are also considered normal. The normal EEG of a mature newborn is characterized by the presence of a response in the form of EEG flattening to external stimuli.

During the first month of life of a mature child, the alternating EEG of quiet sleep disappears; in the second month, sleep spindles appear, organized dominant activity in the occipital leads, 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 delta waves decrease, so that by the end of the 6th month, the EEG is dominated by a rhythm with a frequency of 5-7 Hz. From the 7th to the 12th month of life, the alpha rhythm is formed with a gradual decrease in the number of theta and delta waves. By 12 months, oscillations that can be characterized as a slow alpha rhythm (7-8.5 Hz) dominate. From 1 year to 7-8 years, the process of gradual displacement of slow rhythms by faster oscillations (alpha and beta range) continues. After 8 years, the alpha rhythm dominates on the EEG. The final formation of the EEG occurs by 16-18 years.

Limit values of the dominant rhythm frequency in children

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

The EEG of healthy children may contain excessive diffuse slow waves, bursts of rhythmic slow oscillations, and epileptiform activity discharges, so that from the point of view of the traditional assessment of age norms, even in obviously healthy individuals under the age of 21, only 70-80% of the EEG can be classified as “normal”.

From 3-4 to 12 years of age, the proportion of EEG with excessive slow waves increases (from 3 to 16%), and then this indicator decreases quite quickly.

The response to hyperventilation in the form of high-amplitude slow waves at the age of 9-11 years is more pronounced than in the younger group. It is possible, however, that this is due to the less precise performance of the test by younger children.

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

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

The already mentioned relative stability of the EEG characteristics of an adult is maintained until approximately 50 years of age. From this period onwards, a restructuring of the EEG spectrum is observed, expressed in a decrease in the amplitude and relative amount of the alpha rhythm and an increase in the amount of beta and delta waves. The dominant frequency after 60-70 years tends to decrease. At this age, theta and delta waves, visible during visual analysis, also appear in practically healthy individuals.

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