Electroencephalography (EEG)

Multiple types of EEG testing can be performed. Depending on your situation, one test may be better than another. Your doctor will choose the best test for you. Types of EEGs include:

Routine EEG

A routine EEG is the most common, often used to initially investigate a possible epilepsy diagnosis or at follow-up visits for people known to have epilepsy. A routine EEG usually records for 20-90 minutes. However, it takes 30-60 minutes for the technologist to place the electrodes before the recording starts. This EEG usually uses standard “activation procedures” that increase the chance of capturing seizure-like discharges or even seizures. The most common are photic stimulation (fast, flashing lights or patterns), hyperventilation (very quick breathing), and sleep deprivation (staying up late the night before). Many labs will also record video and heart rate during the routine EEG.

Other types of EEGs may be done for specific reasons.  

Prolonged EEG

A prolonged EEG lasts several hours and may be done with or without video. A prolonged EEG will allow a longer recording period and increase the likelihood that both sleep and wakefulness can be recorded.

Ambulatory EEG

An ambulatory EEG records an EEG in an outpatient setting, usually in the person’s home. It is typically done for 1 to 3 days, occasionally longer. In most cases, video is also recorded. An ambulatory EEG is often considered if someone has spells that occur every day or two but not frequent enough to capture on a routine EEG.  

An ambulatory EEG can also capture a much longer recording, of both sleep and wakefulness. This is helpful to assess how frequently seizures occur, particularly in cases where seizures are quite subtle. In some cases, ambulatory EEG is done to look for epileptiform discharges to help with decisions about the need for medication or tapering off seizure medications.  

During the ambulatory recording, the video and brain waves are being recorded in the home, but the patient is not being watched in real time. Thus, most healthcare providers are not as comfortable changing seizure medications during an at-home ambulatory EEG.

Long-term Video-EEG Monitoring

Long-term video-EEG monitoring is typically done as an inpatient and usually lasts for several days. There are several reasons to consider long-term video-EEG monitoring:

• Recording typical seizures in people with drug-resistant epilepsy, to see whether epilepsy surgery is an option
• Classification of spells to determine if these are epileptic seizures or some other type of nonepileptic event
• Assessment of seizure frequency, especially if they are subtle or occur only at night, to see if treatment for seizures is effective
• Assessment of frequency of seizure discharges

While ambulatory EEG can also be considered for some of these questions, long-term video-EEG monitoring allows patients to be monitored in real time. Therefore, if a seizure occurs, medical assistance is immediately available. In some cases, particularly in those being evaluated for possible epilepsy surgery, the dose of daily anti-seizure medication is often lowered. If this happens, an intravenous is usually placed in order to administer rescue medication should a bigger seizure occur.  

INVASIVE EEG Monitoring

Unlike the other types of EEGs, where electrodes are simply placed on the scalp, Invasive EEG monitoring places electrodes directly onto or in the brain to record seizures. Electrodes are placed by a neurosurgeon, under anesthesia. Invasive EEG is for people with drug-resistant epilepsy, after long-term scalp video-EEG monitoring, an MRI, and other studies suggest their seizures come from a single focus (one part of the brain where the seizure comes from). The goal of the invasive EEG is to define the boundaries of the seizure focus, as well as important function which may overlap or border this seizure focus.

Several types of invasive EEGs exist. The most common is Stereo-EEG, where electrodes are placed into the brain through tiny holes (called burr holes) in the skull. Another type of invasive EEG, now used less commonly, is called grid and strip recording, where electrodes are placed directly on the brain surface. This type of invasive EEG requires a craniotomy (opening of the skull) and is a much bigger operation.