Electroencephalogram EEG

An electroencephalogram (EEG) is a recording of brain activity. An ambulatory electroencephalogram (EEG) is a neurodiagnostic test that measures and records the electrical activity in your brain. Unlike an EEG, an ambulatory EEG allows an extended recording. The patient is able to move around and is not required to stay in one place.

• EEG is the abbreviation for electroencephalography.  The electroencephalograph is a machine that translates the electrical activity of the brain into a series of wavy lines (a graph) on a computer called the EEG record.
• An EEG measures the electrical activity of the brain, sometimes referred to as brain waves.  This test is performed to see how the different parts of your brain function.  It records a graph of your brain waves.
• Digital analysis is a procedure that can give additional information about any problems that may be found.
• Analysis and examination of the data obtained allows your doctor to see one of the many ways that your brain functions.
• EEG is not a treatment of any kind.  No electricity is transferred to your brain.  The EEG only detects activity in the brain.
• If you have a seizure during the test, you should behave as you normally would during a seizure.  Family and friends should follow your usual first aid or emergency procedures.
• It can tell us what may be causing your episodes and help with deciding the best treatment for you.
• The doctor can see seizure activity as well as sleep stages during your EEG.

Why need an EEG?

1. Classification of seizure type in members who have epilepsy (routine EEG is equivocal) — only ictal recordings can reliably be used to classify seizure type (or types) which is important in selecting appropriate anti-epileptic drug therapy; or
2. Diagnosis of a seizure disorder (epilepsy) — members who have episodes suggestive of epilepsy when history, examination, and routine EEG do not resolve the diagnostic uncertainties (routine EEG should be negative with provocative measures); or
3. Localization of the epileptogenic region of the brain during pre-surgical evaluation — to identify appropriate surgical candidates.

The main use of an EEG is to detect and investigate epilepsy, a condition that causes repeated seizures. An EEG will help your doctor identify the type of epilepsy you have, what may be triggering your seizures, and how best to treat you.

Less often, an EEG may be used to investigate other problems, such as dementia, head injuries, brain tumours, encephalitis (brain inflammation) and sleep disorders, such as obstructive sleep apnoea.

Different types of EEG recordings

Outpatient short EEG (no video)

This is usually referred to as ‘routine EEG.’ It is the oldest, cheapest, and the ‘default’ way to obtain an EEG. The limitations of routine EEG are well known and obvious, and have to do with low sensitivity (due to a short-time sample). For the diagnosis of seizures, the yield of a single routine EEG is between 30 and 50%, and increases with repeated EEGs, possibly up to 90% by the fourth EEG. Certainly, some patients with epilepsy will lack interictal epileptiform discharges despite repeated EEGs. Specificity of routine EEG (for epilepsy) is very high in theory, but is low in practice because of the (under-reported) problem of over-reading. Nonetheless, despite obvious limitations, routine EEG is inexpensive and simple, and can be sufficient, even if normal, in most clinical situations, that is, when patients respond to treatment.

Outpatient short EEG with video (Video Telemetry)

Virtually, all EEG machines nowadays have a (digital) video recorder, so video should probably be added to any routine EEG, in case a clinical event is captured. If the purpose is mainly to capture the event in question for diagnosis, EEGvideo can be short-term and have a high diagnostic yield. Appropriate to such situations would be, for example, patients with generalized epilepsies of the Lennox–Gastaut type with multiple daily seizures, and other patients with daily events that are strongly suspected to be psychogenic, especially when combined with activation procedures.

Ambulatory EEG

Ambulatory electroencephalography (AEEG) monitoring is a relatively recent technology that allows prolonged electroencephalographic (EEG) recording in the home setting. Its ability to record continuously for up to 72 hours increases the chance of recording an ictal event or interictal epileptiform discharges. AEEG is a less expensive alternative to inpatient monitoring, with costs that are 51-65% lower than a 24-hour inpatient admission for video/EEG monitoring.

The Gold Standard: Prolonged EEG-Video Monitoring

For the epilepsy specialist, this is the gold standard and the starting point to care for patients whose seizures do not respond to basic treatment. Because it is both prolonged and with video, this combines an increase in yield of capturing interictal discharges and, even more important, the ability to record the episodes in question. In most cases, EEG-video monitoring will allow us to answer the following questions:

• Are the events epileptic or not?
• If not epileptic, what are they?
• If epilepsy, what type?
• If focal, where is the likely focus?

A common misconception is that prolonged EEG-Video monitoring is performed inpatient. Thanks to the latest technologies, advanced storage and data warehousing facilities has made it possible, cheaper, and convenient to perform outpatient.

Sleep EEG or sleep-deprived EEG

A sleep EEG is carried out while you’re asleep. It may be used if a routine EEG doesn’t give enough information, or to test for sleep disorders. In some cases, you may be asked to stay awake the night before the test to help ensure you can sleep while it’s carried out. This is called a sleep-deprived EEG.

Preparation for an EEG / AEEG

Placement of the EEG wires for monitoring

EEG wires will be attached to your head with a special glue so that the electrodes will stay attached for several days.  Sometimes, the electrodes can cause some itching to occur and you can take medication to help the itching.  Do not scratch your head with the electrodes in place.  Benadryl 25 mg to 50 mg can be used for itching.  This can be obtained over the counter at your local pharmacy.

Food

Please do not eat potato chips or other snack foods or chew gum, since this will interfere with the EEG – it generates a lot of “noise” on the graph which makes it impossible to detect anything else.

Clothing

You should wear comfortable clothing while your ambulatory EEG is being performed.  Sweat pants and a loose fitting top with buttons down the front are suggested.  Tight fitting sleeves and pull over tops will not be permitted.  Do not attempt to pull a shirt or other clothing over your head during the ambulatory EEG.  The electrodes may become dislodged and the quality of the recording will be affected.

What do I need to do before my test?

• Assemble enough comfortable, appropriate clothes to wear.  Most patients wear street clothes or a sweat suit during the day and warm pajamas and socks at night.  Remember that the tops should button and be loose fitting.

• Bathe and wash your hair well.  Do not leave any hair products in your hair and remove any braids or hair extensions.  This will facilitate comfortable placement of the electrodes

What do Results tell?

Confirmation of clinical suspicion of epilepsy

A clinical suspicion of epilepsy can be confirmed by recording a seizure on AEEG. This is most likely to occur when the patient is experiencing daily or almost daily spells. Studies looking at the diagnostic yield of AEEG indicate that 6-15% of AEEGs record seizures.

Higher yields have been reported from 16-channel AEEG with computer-assisted seizure detection than from older 4- or 8-channel systems without seizure-detection algorithms. A 2001 study in which 502 patients were evaluated with computer-assisted 16-channel AEEG demonstrated that 8.5% of patients had a seizure during the recording period (mean, 28.5 h).

In patients with intractable epilepsy, AEEG has been used to localize seizure onset as part of presurgical evaluation. However, inpatient video/EEG monitoring remains the standard for presurgical evaluation.

Evaluation of interictal epileptiform activity

Detection of interictal epileptiform abnormalities in the absence of recorded seizures can provide supporting evidence for a clinical diagnosis of epilepsy.

Studies have demonstrated that 34.9% of patients with known seizures had a positive AEEG, whereas 15.3% of 216 patients in whom the diagnosis of seizures was considered (ie, patients with episodic alterations of behavior, perception, sensation, or motor functioning) had interictal epileptiform abnormalities on 4-channel AEEG. When a 16-channel recorder was used, 38% of patients who were referred for AEEG had some type of epileptiform abnormality.

AEEG is highly specific; spikes were found on overnight AEEG in only 0.7% of asymptomatic adults without a history of migraine or a family history of epilepsy. In patients with a history of migraine headaches and those with a family history of epilepsy, the incidence of spikes on AEEG was 12.5% and 13.3%, respectively.

Some patients in whom epilepsy is suspected have a normal routine or sleep-deprived EEG. In these patients, AEEG can increase the chance of detecting an epileptiform abnormality. Of patients who previously had normal or nondiagnostic routine EEG, 12-25% have epileptiform activity on AEEG.

A study comparing the usefulness of sleep-deprived EEG and computer-assisted 16-channel AEEG in patients with suspected epilepsy (but a nondiagnostic initial routine EEG) found that sleep-deprived EEG improved detection of epileptiform discharges by 24%, whereas AEEG improved detection by 33%. Of the 46 patients studied, 15% had actual seizures recorded on AEEG, and none had seizures during the sleep-deprived recording.

Patients may have epilepsy without interictal epileptiform abnormalities on EEG, but this occurs in fewer than 20% of patients. In a study using a 4-channel recording system, 3 patients had only seizures recorded without interictal abnormalities. AEEG with 16 or more channels increases the probability that interictal epileptiform abnormalities will be found.

Documentation of seizures of which patients are unaware

For a patient to have seizures and yet be unaware of them is not uncommon. Brief alterations of awareness occur in both absence and complex partial seizures. AEEG is helpful at identifying seizures that are unrecognized or unreported by the patient.

Absence seizures may be so brief that the patient is unaware of them. A study using AEEG to evaluate absence seizures in pediatric patients found that most paroxysms of generalized spike and wave discharges were asymptomatic.

Patients with complex partial epilepsy are often amnestic for their seizures. The sequelae of a nocturnal generalized convulsive seizure, if present at all, may be so subtle (eg, fatigue, muscle soreness) that the patient is unsure whether a seizure actually occurred.

A study of patients in an epilepsy monitoring unit found that 63% of all seizures were unrecognized by the patients. This difficulty in identifying the occurrence of seizures impedes seizure diagnosis and assessment of treatment adequacy.

Evaluation of response to therapy

Because a significant number of patients are unaware of their seizures, responses to treatment are frequently difficult to gauge. Patients with mental retardation or other forms of encephalopathy may be unable to report seizures accurately. In such cases, AEEG can have a significant impact on clinical management.

AEEG is particularly useful in quantitating response to the treatment of absence seizures. If untreated, such seizures typically occur numerous times per day; adequate treatment usually normalizes the EEG.

Evaluation of nocturnal or sleep-related events

Certain diagnoses are difficult to confirm with the typical 20-minute outpatient EEG. The interictal epileptiform discharges of benign rolandic epilepsy, for example, are highly activated by sleep and may not always be achieved adequately in a laboratory. Continuous spike and wave activity during slow-wave sleep is another entity that may demonstrate a relatively normal EEG during waking hours and a strikingly abnormal EEG during deep sleep.

Because of its capacity to record an entire night of sleep, AEEG is invaluable in assessing these clinical situations. Another advantage is that children can be monitored at home.

If a nonepileptic sleep disorder is suspected, PSG is the preferred study because of the added information from monitoring electromyography (EMG), eye movements, electrocardiography (ECG), and respiration.

The history may not differentiate clearly between a sleep disorder and epilepsy. AEEG may record frequent arousals (suggesting sleep apnea) or decreased rapid eye movement (REM) sleep latency (suggesting narcolepsy). In a study of 500 patients who had AEEG, narcolepsy was suggested in 6 patients, including 3 in whom narcolepsy had not been suspected.

Evaluation of Suspected Pseudoseizures

Pseudoseizures, also known as psychogenic seizures or nonepileptic events, are clinical events in which patients perceive altered movement, emotion, sensation, or experience similar to those due to epilepsy but without an electrographic ictal correlate.

Pseudoseizures are surprisingly frequent, occurring in as many as 20% of patients at epilepsy referral centers and in 5-20% of outpatient populations. Some patients have both pseudoseizures and epileptic seizures; coincident events occur in an estimated 10-60% of epilepsy patients.

AEEG can be a useful screening tool in identifying patients who have nonepileptic paroxysmal events. In one study, 36% of patients had event marker activations without associated electrographic changes.

Potential problems exist in using AEEG to definitively diagnose nonepileptic seizures. A 24-hour recording without associated video does not allow evaluation of clinical stereotypy, which is valuable when evaluating patients with unusual seizure manifestations and minimal EEG changes. Scalp EEG may not show electrographic ictal abnormality during some frontal lobe seizures or may show only subtle abnormalities that would be difficult to interpret without associated video.

Seizures and nonepileptic seizures may be associated with movement and muscle artifact that may obscure the underlying EEG. Although AEEG may be a useful initial screening tool for nonepileptic events, inpatient video/EEG monitoring remains the criterion standard in evaluating nonepileptic seizures.

Evaluation of Syncope

AEEG may be helpful in evaluating syncope or near-syncope if an ECG lead replaces 1 of the EEG channels. If cardiogenic syncope is suspected, a Holter monitor or prolonged cardiac event monitor may be more useful clinically. Although arrhythmias have been diagnosed with continuous ambulatory EEG/ECG recording, a study of epileptiform abnormalities in AEEG found that only 1 of 67 patients with syncope, near-syncope, or episodic dizziness had an epileptiform abnormality.

What should I do if I have a seizure during the test?

• Most recorders have an “event” button to press if you have any seizures or different symptoms during the test.
• When the button is pressed, it marks the time on the EEG recording. The doctors can then compare what you feel or what is seen by others to what the EEG shows at the same time.
• If you are not able to press the button during a seizure, someone else can do it for you.
• Newer recorders also have built-in programs to identify epilepsy waves and seizures. Some can even record a video of what happened when the button was pushed.