Who invented the electroencephalogram?
Electroencephalography (electroencephalogram, EEG)
Electroencephalography is an examination method used in neurology. With the help of this procedure it is possible to get a quick overview of the brain activity. Today it has the most important place in the diagnosis of epileptic diseases and sleep disorders. It is also used to monitor patients in the intensive care unit (e.g. coma patients) or during operations.
In the course of this examination, several electrodes register voltage fluctuations on the surface of the skull. These electrical signals are caused by the activity of nerve cells in the outermost layer of the brain - the so-called cerebral cortex. The result of this examination is called an electroencephalogram, or EEG for short. The examination is completely painless and poses no risk.
Electroencephalography is the oldest method with which functional processes in the brain (in Greek "encephalon") can be recorded. It was developed in 1929 by the German neurologist Hans Berger. Since then, the importance of the EEG in neurology has changed significantly due to the introduction of further diagnostic procedures. Before the introduction of computed tomography (CT) it was also used for tumor diagnostics.
How is an EEG recorded?
In order to be able to record an EEG, several small metal electrodes are placed on the patient's head. Due to the uniform placement of the electrodes, the activity of the most important areas of the cerebral cortex can be recorded. Furthermore, different EEGs can be easily compared with one another.
Before the measurement
21 electrodes are usually used for a routine EEG. Placement and wiring of all electrodes can take time. So that the numerous electrodes stick to the patient's head without any problems, they are usually incorporated into a hood.
The electrodes are coated with a contact gel to enable a better measurement. This can easily be removed after the examination. Ideally, the patient's hair should be freshly washed and dry for the exam. The use of hairsprays and hair gels should be avoided.
For some questions it may be necessary for the patient not to sleep the night before the EEG recording (sleep deprivation). As a rule, you will be hospitalized overnight and kept awake through occupation. The EEG is then carried out the next day.
During the measurement
After the electrodes have been connected to the EEG device via cables, the measurement is carried out. The examination can be done sitting or lying down and usually lasts between 15 and 30 minutes. The examination room should be as quiet and dark as possible. The patient is looked after by one person during the entire recording or receives brief instructions from him in between (e.g. to open his eyes).
During the examination, it is important to be as relaxed and calm as possible. In order not to react to optical stimuli, the eyes remain closed during the entire examination. The eyes are only opened briefly when instructed by the attending medical staff.
In the course of most EEG examinations, so-called provocation measures are carried out. This is understood to mean actions that stimulate the brain cells in a more special way. The patient is asked, for example, to inhale and exhale more intensely for several minutes (hyperventilation). A flickering light (stroposcope) with different frequencies is also used for excitation. Sleep deprivation also serves this purpose. The aim of this maneuver is to intensify EEG changes with a disease value and to make them more recognizable. These tests are particularly important for the detection of epilepsy.
What is actually being measured?
The brain consists of billions of individual nerve cells that are connected to one another via countless contact points (so-called synapses). At these points, activity can be transferred from one cell to another.
The processes that take place during this transmission lead to slight electrical fluctuations; this is referred to as a "synapse potential". The synapse potential of a single synapse is so low that it cannot be measured using skin electrodes. When a region of the cerebral cortex is activated, however, thousands of synapses are stimulated at the same time, which creates a measurable total potential. The measured voltages are nevertheless very low, mostly only between 10 and 100 microvolts. In order for these small voltage fluctuations to be measured, there must be good contact between the electrode and the scalp. For this purpose, the electrodes are coated with special gels.
In order to show the potential differences of individual brain regions, the electrodes are interconnected. Nowadays, an EEG is mostly recorded and stored digitally using a computer. The records on paper used in the past are rarely used today. The computer-aided evaluation has the advantage for the doctor that one and the same EEG section can be assessed in any connection, thus improving the informative value of the examination.
When is electroencephalography done?
The diagnosis of epilepsy, the diagnosis of sleep disorders and the monitoring of patients in intensive care units as well as during surgical interventions are among the most important areas of application of electroencephalography today.
The EEG can also provide important diagnostic clues for other brain disorders. For example, severe liver or kidney diseases lead to changes in brain activity. Poisoning can also make itself felt in the EEG. A local disturbance in the area of the cerebral cortex, e.g. caused by tumors, vascular damage or injuries, can also be detected in the EEG. However, it is not possible to draw any conclusions about the type of damage. In these cases, further clarification (e.g. by an MRI) is necessary. Mental illnesses can also lead to typical changes in the EEG.
What does an EEG look like?
The result of the examination, the electroencephalogram, consists of several parallel graphs of the voltage fluctuations (traces). Each of these tracks represents an interconnection between certain electrodes and can therefore be assigned to a certain cerebral cortex region.
Several typical wave patterns can be distinguished in the EEG traces. Changes or an atypical occurrence of these patterns can provide indications of damage or changes in the brain.
With the help of computer programs, the EEGs created are evaluated by a doctor. The measured fluctuations in the individual tracks are analyzed for their frequency, height (amplitude) and shape (steepness). The doctors also pay attention to characteristic wave patterns as well as their propagation and localization.
The most important wave patterns are designated with Greek letters and differentiated based on their frequency, i.e. the number of vibrations per second (unit: Hertz):
Beta waves (13-30 Hertz)
These fast EEG waves are typically found in awake adults when the eyes are open, i.e. when the brain has to process sensory impressions. Even if one is mentally active, e.g. thinking about something, this wave pattern can be recorded.
Alpha waves (8-13 Hertz)
Alpha waves are found in most people when they are relaxed, awake and with their eyes closed. One therefore speaks of an alpha basic rhythm.
Theta waves (4-8 Hertz)
These waves typically occur in adults when they are very tired or in early stages of sleep.
Delta waves (up to 4 Hertz)
This very slow wave pattern is found in healthy people in deep sleep.
There are differences between an EEG from a child and an adult. EEGs from healthy people can also differ significantly from one another. Normal EEGs have a considerable number of variants. Their assessment therefore requires a great deal of skill and experience.
Special electroencephalography procedures
Since you can only cover a few minutes with a conventional EEG, it is possible to carry out a so-called long-term EEG over several hours or days. For this purpose, the electrodes are usually fixed to the scalp with special adhesives. Equipped with portable recorders, patients can move around relatively freely.
In addition to the registration of the brain activity by a long-term EEG, a video recording can take place at the same time. For this, the patients are admitted to the hospital. The combination of video and EEG enables doctors to make better statements about the type of disorder. This rather time-consuming examination is particularly helpful for epilepsy diagnostics.
A special feature are EEG procedures in which the electrodes are not positioned on the scalp but in the patient's skull. One then speaks of an electrocorticogram ("Cortex" is the Latin name for cortex, in this case: the cerebral cortex). The electrodes are located in close proximity to the cerebral cortex, which means that a very precise measurement can be carried out. The placement of the correspondingly small electrodes takes place through a surgical procedure. The electrodes are either inserted through natural openings in the bony skull or inserted through artificially created bone windows. The electrodes can register the brain activity of the corresponding area for several weeks. These special procedures are mainly used to precisely localize pathological areas of the brain in advance of an epilepsy operation.
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Dr. Christoph Weiss
M. Stör, W. Wagner, K. Pfadenhauer, K. Scheglmann (eds.): Neuromonitoring. Darmstadt: Steinkopff Verlag 1999
Karlzeile, Eduard Auff, Lüder Deecke (ed.): Clinical Neurology I. Vienna: Faculty Verlag 2000
Marianne Springer-Kremser, Henriette Löffler-Stastka, Martin Kopeinig-Kreissl (eds.): Mental functions in health and illness. Vienna: Facultas 2005
Alois Ebner, Günther Deuschl (eds.): EEG. Stuttgart: Thieme Verlag 2006
Marco Mumenthaler, Heinrich Mattle (ed.): Neurology. Stuttgart: Thieme Verlag 2008
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