Ambulatory electroencephalography (AEEG) monitoring allows a prolonged electroencephalographic (EEG) recording of the electrical current potential or brain activity through the skull. The procedure for an EEG involves placing a series of electrodes, with at least four recording channels on the patient. A very low electrical current is sent through the electrodes and the baseline brain energy is recorded on a diagnostic machine. Electrical activity is recorded and analyzed through an audio amplifier system. Patients are then exposed to a variety of external stimuli, including bright or flashing light, noise or certain drugs, or are asked to open and close their eyes, or to change breathing patterns. The electrodes transmit the resulting changes in brain wave patterns. Variations in wave characteristics correlate with neurological conditions and are used to diagnose specific medical conditions. With identification and classification of brain waves, the analysis of data provides information useful in mapping the brain and various areas involved with body function in relation to disease status. Since movement and nervousness can change brain wave patterns, patients usually recline in a chair or on a bed during the test, which takes up to an hour. Testing for certain disorders may also require an EEG during sleep. An AEEG has the ability to record continuously for up to 72 hours which increases the opportunity of recording an ictal event (during a seizure), or interictal (between seizures) epileptiform discharge (4). This method of recording offers the ability to gather data on a long term, outpatient basis.
In the past decade, computer technology has enabled portable recording of up to 36 channels with sampling rates of up to 400 Hz. AEEG'S can be transmitted by telephone, in which the electrical brain activity is recorded and transmitted to an offsite center for analysis and reporting. The AEEG can also be transmitted by radio or wire in the diagnosis of complex seizure variants which require inpatient monitoring, but do not require the patient to be bed bound. Virtually all contemporary EEG recordings use digital recording methods. There are few, if any, paper analog EEG recordings carried out in current medical practice. There is a distinction between digital recording and digital analysis of EEG data.
Digital recording uses a digital EEG recorder (machine); but it still involves visual analysis of the wave forms. It is digital to the extent that an analog, close-ended paper recorder is not used at the time of wave form (data) capture. This type of reading-by-eye represents the typical EEG interpretation in most clinical situations.
Digital analysis requires the use of quantitative analytical techniques. Data selection, quantitative software processing and dipole source analysis are some of the techniques utilized.
EEG video monitoring (VEEG) is the simultaneous recording of the EEG and video monitoring of patient behavior. This allows for correlation of ictal and interictal electrical events with demonstrated or recorded seizure symptomology. The combined image of EEG tracings and visible behavior helps the physician diagnose the epilepsy and identify affected areas of the brain. Intensive closed circuit TV and EEG monitoring of this type also helps distinguish between true epileptic seizures caused by electrical discharge and non-epileptic seizures caused by psychological factors.
Various ictal (during a seizure) and interictal (between seizure) EEG patterns correspond to specific seizure types and types of epilepsy. While the EEG is almost always abnormal during a seizure, it may be normal between seizures. Thus, lack of interictal EEG abnormalities does not exclude a diagnosis of epilepsy. However, at some time, most epilepsy patients have abnormal EEG discharges. In contrast, some persons with EEG'S that show epilepsy-like activity never have seizures. Thus physicians interpret EEG results within the context of other information they are gathering. Apart from the patient history and the neurological exam, the EEG is the most influential tool in the diagnosis of seizures and epilepsy. (5)