Most patients will be asked to carry out a period of deep breathing for approximately 3 minutes. This may produce some abnormal activity which would not be seen while the patient is relaxed. The physiological effect of deep breathing is to increase the amount of carbon dioxide (CO2) being removed from the bloodstream. This fall in CO2 produces a fall in blood pressure and at the same time blood vessels in the brain become constricted. This reduces blood flow and the delivery of oxygen and glucose to the brain. This in turn may produce some abnormal brain activity not seen in the resting record.
Photic stimulation is also carried out. A strobe lamp is placed 30 cm from the patient's eyes. Brief flashes of light (2 - 5 seconds in duration) at a number of different flash frequencies are delivered to the patient with both eyes open and eyes closed. A continuous flash with increasing and decreasing flash frequencies is sometimes used.
Some patients who are sensitive to flashing lights may show abnormal activity in the EEG.
Throughout the test the recordist is constantly annotating the record with any patient movements, or tasks that they are carrying out.Other signals may also be recorded in conjunction with the EEG such as heart rate (ECG), respiration, eye movements (EOG), and muscle activity (EMG).
EEG AnalysisThe EEG reports consists of a number of different sections. The recordist may prepare a report describing the type of activity seen in the record together with changes produced by deep breathing and photic stimulation. They will also comment on the patient's state during the recording. The physician will then interpret these changes with regard to the medical problem being investigated.
With an increase in the number of long recordings being carried out, many departments make use of detection algorithms such as spike and seizure detection. Although it is still necessary for the clinician to review the complete record, such programmes will mark and highlight sections of interest. The most efficient method of implementing these algorithms is for the detection to be carried out on-line.
Other methods of analysing EEG data include Power Spectrum Analysis. A Fast Fourier Transform (FFT) is performed on sections of EEG data to determine the power content of the four main frequency bands. The resulting waveforms can be displayed as a brain map which will show the scalp distribution of the power within each frequency band.
The amplitude of the different waveforms at a single point can also be displayed in a similar format.This type of display provides a more objective analysis of the EEG activity compared to a subjective visual analysis by a physician.
Video monitoringSimultaneous video monitoring of the patient during the EEG recording is becoming more popular. It allows the physician to closely correlate EEG waveforms with the patients activity and may help produce a more accurate diagnosis.
Domestic video recorders and cameras can be connected to an EEG machine using a time code generator. This records an accurate time signal onto the videotape. When the videotape and EEG are reviewed together the two signals are accurately synchronised together.Video monitoring is always used for Long Term Monitoring recordings as the patient is unattended. The patient may also have an event button connected to the EEG machine so that times when the patient thought they were having an epileptic attack can be easily identified.
Sleep studiesThe EEG is frequently used in the investigation of sleep disorders especially sleep apnoea. EEG activity together with other physiological signals such as heart rate, airflow, respiration, oxygen saturation and limb movement are measured simultaneously. These recordings are usually carried out overnight although some sleep studies can be carried out in the department during the day under strictly controlled conditions.
The EEG record can be broken down into epochs which are normally of 30 seconds duration. Using the EEG activity, each epoch is classified into one of 5 sleep stages. This is displayed visually as a Sleep Histogram.
Respiration and airflow are used to look for periods of apnoea which occur when the patient stops breathing. These are then correlated with the sleep stage in which they occurred and the level the oxygen saturation fell to during the apnoea.