EEG Applications

One of the major roles of EEG is as an aid to diagnose epilepsy. Abnormal patterns such as spikes, sharp waves and/or spike and wave complexes can be seen. The type of activity and the area of the brain that it is recorded from will assist the physician in prescribing the correct medication for that type of epilepsy.

Patients with epilepsy that can not be controlled by medication will often have surgery in order to remove the damaged tissue. The EEG plays an important role in localising this tissue. Special electrodes can be inserted through the cortex or alternatively a grid of electrodes placed directly on the surface of the cortex. These recordings, often called Long Term Monitoring for Epilepsy (LTME), can be carried out for periods ranging from 24 hours to 1 week. The EEG recorded will indicate which areas of the brain should be surgically removed.

EEG studies can also be used in patients who are deeply unconscious, to distinguish between brain death and possible reversible conditions.

Electrocerebral inactivity (ECI) or electrocerebral silence (ECS) is defined as no EEG activity over 2 µ V in amplitude when recording from electrodes on the scalp, that are 10 cm or more apart.

Using the 10/20 International System of electrode placement, the average distance between electrodes in an adult is 6 to 6.5 cm. Activity recorded using these distances and at a normal display sensitivity may suggest ECS. However if the same activity was recorded using longer inter-electrode distances, some activity might be seen. Therefore some double distance electrode linkages are recommended for example FP1-C3, F3-P3, C3-O1 etc.

Display sensitivities of a minimum of 2 µ V/mm are required. However digital EEG systems have the added advantage of having sensitivity values of 1.5 and 1 µV/mm. This 50-100 % increase in sensitivity will allow a more confident assessment of the presence or absence of a 2 µV signal.

The EEG is also used to investigate other conditions that may affect brain function such as strokes, brain injuries, liver and kidney disease and dementia.

EEG Activity

EEG activity can be broken down into 4 distinct frequency bands:

Beta activity > 13 Hz
Alpha activity 8 Hz-13 Hz
Theta activity 4 Hz-7 Hz
Delta activity < 4 Hz

Beta activity is a normal activity present when the eyes are open or closed. It tends to be seen in the channels recorded from the centre or front of the head. Some drugs will increase the amount of beta activity in the EEG.

Beta activity

Alpha activity is also a normal activity when present in waking adults. It is mainly seen in the channels recorded from the back of the head. It is fairly symmetrical and has an amplitude of 40 µV to 100 µV. It is only seen when the eyes are closed and should disappear or reduce in amplitude when the eyes are open.

Alpha activity

Theta activity can be classed as both a normal and abnormal activity depending on the age and state of the patient. In adults it is normal if the patient is drowsy. However it can also indicate brain dysfunction if it is seen in a patient who is alert and awake. In younger patients, theta activity may be the main activity seen in channels recorded from the back and central areas of the head.

Theta activity

Delta activity is only normal in an adult patient if they are in a moderate to deep sleep. If it is seen at any other time it would indicate brain dysfunction.

Abnormal activity may be seen in all or some channels depending on the underlying brain problem.

Delta activity

There are a number of other waveforms which tend to be a little more specific to certain conditions. For example spike and wave activity indicates a seizure disorder and may be seen in the EEG even if the patient is not having an epileptic seizure. Other epileptic conditions may be diagnosed if spikes or sharp waves are seen.

Spike and wave activity

Triphasic waves are sometimes seen if the patient has severe liver or kidney disease that is affecting brain function.

These are just brief descriptions of some of the simpler waveforms that may be seen in any one EEG recording. Combinations of any of the above patterns are possible which can make interpretation of the record difficult. Abnormal activity is not always specific to any condition and may suggest a few different diagnoses.

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