An abnormal EEG can't mean anything
What do you do with an EEG?
Examination and education of the patient are carried out by a neurologist. A routine EEG requires up to 21 electrodes, which are usually incorporated into a kind of hood. This facilitates placement and adhesion to the patient's head. The electrodes are coated with a contact gel, attached to the patient's scalp according to a standardized scheme and connected via cables.
During the actual measurement, which cannot be felt, the patient should be as relaxed and calm as possible and keep their eyes closed. The attending doctor gives brief instructions from time to time, for example opening your eyes or solving a simple arithmetic problem. This leads to a change in brain activity and is recorded in the EEG. The entire measurement usually takes no longer than 20 to 30 minutes. The doctor then removes the electrode cover.
A sleep EEG is usually carried out in a special inpatient sleep laboratory. As with a routine EEG, the doctor puts an electrode cap on the patient. The patient should then go to bed in the evening as usual and not take any medication or alcohol. The brain activity is now measured and recorded over the entire duration of sleep. The doctor often also records eye movements, muscle activity and heart rate.
The long-term EEG measures brain activity over 24 or 48 hours. For this purpose, the patient receives a portable recorder that is attached to the body. During the long-term recording, the patient logs all incidents.
To provoke an epileptic seizure, the neurologist uses three different methods: hyperventilation (increased breathing), photostimulation and sleep deprivation. For hyperventilation, the doctor asks the patient to inhale and exhale as deeply as possible for three to five minutes.
During photostimulation, the patient is exposed to bright flashes of light. The doctor derives an EEG directly during both hyperventilation and photostimulation.
For sleep deprivation, on the other hand, the patient should remain awake for the entire night. For this he is usually admitted as an inpatient. Caffeinated drinks are not allowed. The EEG is then taken the next day.
EEG: evaluation and diagnosis
The neurologist assesses the EEG according to the shape, frequency and amplitude of the waves. A generally slowed down basic rhythm in adult, awake patients occurs, for example, with poisoning, coma or encephalitis. A so-called focal finding, i.e. a locally limited change in brain activity, on the other hand, speaks for tumors or brain damage from injuries (traumatic brain injury). Focal findings are often also clarified with an MRI.
Diagnosing epilepsy is more difficult because the EEG between attacks is often normal and the typical brain waveforms typical of epilepsy do not appear until an attack occurs.
Physiological waves in the EEG
Depending on the level of alertness, a distinction is made between different patterns (grapho elements) in the electroencephalogram that have no disease value:
- Alpha waves (eight to twelve Hertz): alert, relaxed adult with closed eyes
- Beta waves (13 to 30 Hertz): awake adult with open eyes and mental activity
- Theta waves (four to seven Hertz): severe tiredness when falling asleep
- Delta waves (0.5 to three Hertz): deep sleep
If the patient opens their eyes or concentrates on a task, there is a change from alpha-EEG to beta-EEG. This is known as the Berger effect or the arousal reaction.
Instead of the pattern mentioned, newborns and children show slow and rather irregular waves. The EEG is only fully developed towards the end of puberty, so that the typical grapho elements become visible.
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