Electroencephalography (EEG) is often helpful in the diagnosis and classification of epilepsy.10-27 However, it is essential to understand the scope and limitations of the technique when requesting an EEG examination and subsequently evaluating an expert report on the recording.2 Non-specific EEG abnormalities are relatively common, especially in the elderly, patients with migraine, psychotic illness and psychotropic medication. Non-specific abnormalities should not be interpreted as supporting a diagnosis of epilepsy.
A normal EEG does not exclude a diagnosis of epilepsy. A single routine EEG recording will show definite epileptiform abnormalities in 29-38% of adults who have epilepsy. With repeat recordings this rises to 69-77%.10-28-31 The sensitivity is improved by performing an EEG soon after a seizure, and by recordings with sleep or following sleep deprivation.10-32-33
Incidental epileptiform abnormalities are found in 0.5% of healthy young adults, but are more likely in people with learning disability and psychiatric disorders, patients with previous neurological insult (e.g. head injury, meningitis, stroke, cerebral palsy) and patients who have undergone neurosurgery.3436
In a patient in whom the clinical history suggests an epileptic seizure but is not conclusive, the prevalence of epilepsy will be high. The finding of epileptiform abnormalities is specific, and the diagnostic value of the test is good. In a patient in whom the history is typical of some other disorder, such as syncope, the prevalence of epilepsy will be low, and any epileptiform abnormalities are more likely to be incidental. The test should not be performed in this circumstance.
EEG can aid classification of epileptic seizures and epilepsy syndromes. The finding or not of a photoparoxysmal response can allow appropriate advice to be given.37 If performed within the first few weeks after a first seizure, EEG has prognostic value; patients with epileptiform abnormalities are more likely to have a second attack.38
When clinical information and standard investigations do not allow a confident diagnosis, referral for the recording of attacks should be considered. The attacks should usually be occurring at least once a week. If non-epileptic attack disorder is a possibility, then monitoring in patients with less frequent attacks may be worthwhile, as they are often easily provoked.39-46 The best method is inpatient video EEG recording.47 Twenty four hour ambulatory EEG recording has the advantage of recording attacks in the patient's usual setting, but does not usually allow correlation of EEG and video data. Home video recordings can be useful, but rarely capture the onset of attacks.47-48 These investigations should include single channel electrocardiography recording.49
Electrocardiography (ECG) should be performed in the assessment of all patients with altered consciousness, particularly those in older age-groups, when disorders of cardiac rhythm may simulate epilepsy. Twenty four hour ambulatory ECG and other cardiovascular tests may also be helpful.49
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