EEG rhythms – designation and interpretation
EEG rhythms are diagnosed electrical vibrations in the brain. Various degrees of wakefulness are accompanied by changes in the frequency spectrum of EEG signals.
Depending on the amplitude, waveform, topography, frequency range and type of reaction, the rhythms of electroencephalography are distinguished.
The main EEG rhythms are denoted by Greek letters:
- ἄλφ α (alpha);
- β ήτ α (beta);
- γάμμ α (gamma);
- δέλτ α (delta);
- θήτ α (theta);
- κά ππα (kappa);
- μυ (mu);
- σίγμ α (sigma).
How does electroencephalography work?
Transmission of signals in the human nervous system is carried out both chemical (using neurotransmitters ) and electrical (action potentials) paths. A single action potential or membrane tension of one neuron is too weak to be captured by non-invasive diagnostic methods. However, electrodes can capture the summation of synchronously acting action potentials and make fluctuations in electrical activity visible.
There is a definite connection between the mental state of a person and EEG waves. Abnormalities or unusual brain waves can indicate pathology. A neurologist deals with the analysis and description of such waves.
The electrodes measure the activity of those parts of the cerebral cortex that have a high density of nerve cells. However, EEG measures not only the electrical potential of nerve cells in the brain, but also the muscles of the head and skin. Accordingly, the basic EEG rhythms do not reflect the exact activity of the neurons. EEG rhythms and their relationship with the functional state of the brain is a subject of controversy in the scientific community.
Delta EEG rhythms have a low frequency of 0.1 to <4 Hz. Delta waves are typical functional waves of deep dreamless sleep phases. In infants, the delta rhythm is also present upon awakening.
Theta wave is a slow rhythm in the frequency range from 4 to <8 Hz. They are more common during sleepiness and in a state of drowsiness. EEG rhythms and their characteristics depend on the patient’s age. While awake, they are present in babies, but their presence in adults may indicate dysfunction or brain damage.
The normal alpha rhythm on the EEG has the following features:
- frequency 8-12 Hz: the lower limit of the normal alpha rhythm in adults and children over 8 years old is 8 Hz;
- location: occipital regions;
- morphology: rhythmic and regular;
- amplitude: usually 20-100 mV;
- reactivity: appears when the eyes are closed and disappears when they are opened.
A normal beta EEG has the following characteristics:
- Frequency (by definition) more than 13 Hz.
- Location: diffuse distribution.
- Morphology: Usually rhythmic and symmetrical.
- Amplitude: Range 5-20mV.
Reactivity: Beta activity increases during the first and second stages of sleep, and decreases in the deepest phases. Central beta activity can be reactive to voluntary movement and proprioceptive stimuli.
A gamma wave is a signal in the frequency range above 30 Hz. This rhythm occurs with a strong concentration of attention, during study or meditation. Recent studies have shown that the emergence of gamma rhythms is necessary for the integration of various stimuli.
It should be noted that gamma rhythms are not visible on the EEG strip with the naked eye.
An electroencephalogram is used:
- identifying epileptiform or epileptic seizures;
- diagnostics of sleep disorders of various etiologies;
- detecting morphological and functional changes in the brain (brain tumors or circulatory disorders);
- identification of diseases of the central nervous system of unclear etiology (encephalitis, increased intracranial pressure and brain atrophy);
- ascertaining the death of the brain.
Electroencephalography is a simple diagnostic tool for epilepsy. However, an imperceptible EEG does not rule out epilepsy, encephalitis, or organic disease. Localized brain changes are currently being diagnosed using modern imaging techniques.
If the brain waves cannot be measured, this is called a null EEG. Zero activity means total brain death. The death of the brain is a prerequisite for the removal of organs from a dead person and their transplantation.
The electroencephalograph lost its relevance when modern imaging techniques such as computed tomography or magnetic resonance imaging were developed. These studies usually reflect brain damage better. Because electroencephalography can be performed simply and without complications, and is particularly useful for detecting epilepsy, it continues to be used in clinical practice. EEG is also used to detect age-related disorders of brain maturity and sleep rhythm disturbances.
For a more accurate diagnosis of epilepsy, additional (provocative) methods are sometimes used. These include hyperventilation, photostimulation, and 24-hour sleep deprivation.