Electroencephalography Unit Health Article

Definition

The electroencephalography (EEG) unit is used to record the electrical output of the brain to produce a record called an electroencephalogram.

Purpose

Measurements taken by an electroencephalography unit are used to diagnose and classify disorders of the brain including seizures, encephalopathy, encephalitis, some psychiatric diseases, and brain death.

Description

The EEG unit includes a set of electrodes, amplifiers and filters, and a recording unit. Generally, surface electrodes are used for EEG recording, using 21 electrodes placed according to standard positions on the subject's scalp. However, implantable electrodes are also used for particular applications. For example, sphenoidal electrodes, which are fine wires inserted under the zygomatic arch, or cheekbone, are used when a patient is suspected of having temporal seizures. Depth electrodes, or subdural strip electrodes, are surgically implanted into the brain and are used to localize a seizure focus in preparation for epilepsy surgery.

The electrical signals from the brain are very weak, typically on the order of 10 to 100 microvolts. As a result, the amplifiers are used to increase the signal for display or recording, and the filters are used to eliminate artifacts and electrical noise produced by other sources such as muscles. The recording unit can be a paper and pen arrangement, although digital display is now standard.

Operation

The first step in performing an EEG is correct placement of the electrodes. The International Federation of Societies for EEG and Clinical Neurophysiology (IFSECN) has established a system of placement called the 10-20 Electrode Placement System. This system places the 21 electrodes at scalp positions that are at 10% and 20% of the head circumference, thus taking into account differing head size. Each electrode position is identified with a key letter that specifies the region of the brain and a positioning number. For example, prefrontal electrodes are labeled Fp, while central electrodes are C and occipital are O. Even numbers are on the right side of the head and odd numbers are on the left, with lower numbers anterior and higher numbers posterior. Midline electrodes are labeled with a "z" rather than a number.

Finding the exact point of placement of the electrodes involves measuring distances from various head landmarks, such as the nasion (the midpoint of the frontonasal suture) and the inion (the posterior occipital protuberance), and from one preauricular point (in front of the ear) to another. The electrodes are placed either at the intersection of these lines or at 10% or 20% of these various measurements about the head. For example, Cz is placed at the intersection of the nasion-inion line and the line that connects the preauricular points, while Fpz is 10% above the nasion and Oz is 10% above the inion. Standardized placement ensures that the results will be consistent and more easily compared to normal tracings.

Electrodes are applied to the head using electrode gel, which acts as a malleable extension of the electrode. Often, precleaning the scalp electrode site with a water-based, conductive agent is suggested for good contact. For long-term recordings, electrodes can be even more securely anchored using collodion (a mixture of pyroxylin, ether, and alcohol). This mixture quickly dries to a clear, tenacious film using compressed air.

Once the electrodes are attached, the machine is calibrated in two phases, through square-wave calibration and biological calibration. Square-wave calibration involves sending a standard pulse and altering the time constants of the low-frequency filters (LFF) and high-frequency filters (HFF) such that the resulting wave has the desired square-wave shape. Standard settings for LFF is 1 Hertz (Hz) and for HFF is 70 Hz. (Frequencies below 1 Hz can be artifacts due to sweat or other sources. The EEG typically does not have components above 70 Hz.) Biological calibration involves sending one channel (the signal from a pair of electrodes) through all the amplifier inputs and comparing the signal with the displayed response; the two are adjusted until they are identical. Other adjustments that may be needed are damping the pen and setting sensitivity, which is usually started at 7 microvolts per millimeter and adjusted depending on the amplitude of the signal.

Generally, recordings are taken for at least 20 minutes to obtain an artifact-free result. The EEG unit records a series of waveforms that are not intrinsically normal or abnormal but must be interpreted based on the patient's age, awake-sleep state, and topographical location of the wave. Information can also be determined by looking for missing right-to-left symmetry. Further tests include the response of the subject to stimulation (such as visual flash or hyperventilation) or state changes (drowsiness or sleep). Although not always abnormal, the