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EEG, and the authors suggested that SD-EEG is the most cost-effective protocol for
investigation of new epilepsy12.

Prolonged inter-ictal sampling using long-term monitoring also increases yield by about 20%,
and is now widely available through 24-hour ambulatory multi-channel digital EEG
recording.

What are the roles of EEG in diagnosis of epilepsy?

EEG is performed in patients with possible or known seizure disorders to assist accurate
diagnosis and provide information about epilepsy type. EEG findings contribute to the multi-
axial diagnosis of epilepsy, by establishing whether the seizure disorder is focal or
generalised, idiopathic or symptomatic, or part of a specific epilepsy syndrome. In a newly
presenting case of suspected epilepsy, the physician will ask:

 Does the patient have a partial or generalised seizure disorder?
 Is there evidence of photosensitivity, if the subject is likely to have idiopathic

generalised epilepsy?
 Are there EEG features indicative of an epileptic syndrome?

Although conceptual division of partial and generalised seizures/epilepsy types is
fundamental in epilepsy characterisation, there is overlap in both clinical and electrographic
manifestations of focal and generalised seizure disorders. Rapid propagation or generalisation
of epileptiform activity related to a symptomatic focus can mimic idiopathic generalised
epilepsy13; localised discharges and regional accentuation of generalised spike-wave
discharge are widely recognised in idiopathic generalised epileptic syndromes14. In some
individuals, there may be co-existence of a partial and a generalised seizure disorder15. In
most instances, the clinician will be reasonably certain about seizure type, based on accounts
provided by the patient and witness, but when the history is not clear – as with an un-
witnessed ‘blackout’ or brief impairment of awareness, EEG can help to distinguish between
a complex partial seizure with focal ED, and an absence type seizure with generalised ED.

Syndromic findings
Relatively specific EEG abnormalities are found in certain epilepsy syndromes, many of
which present in childhood or adolescence. In some individuals, the true epilepsy syndrome
may not be apparent at initial assessment, necessitating regular electro-clinical appraisal. For
example, juvenile myoclonic epilepsy would be the likely diagnosis in an intellectually
normal teenager presenting with myoclonic jerks; if that patient went on to develop refractory
epilepsy and cognitive decline, the syndromic diagnosis would be revised to a progressive
myoclonic epilepsy.

Epilepsy syndromes presenting in neonatal periods and infancy include benign idiopathic
neonatal seizures, in which the EEG shows trace pointu alternans in 75%; early myoclonic
epilepsy and infantile epileptic encephalopathy (Otahara syndrome) with burst suppression
in the EEG; West syndrome in which infantile spasms are associated with hypsarrhythmia;
and Dravet syndrome or severe myoclonic epilepsy of infancy in which generalised spike-
wave and photosensitivity are reported.

Idiopathic generalised epilepsies (IGE)

EEG features in IGE comprise generalised spike or polyspike and slow wave discharges at
35 Hz, normal background cortical rhythms, and a relatively high occurrence of
photosensitivity16. In childhood absence epilepsy (CAE), the characteristic finding is bilateral
synchronous 3 Hz spike-wave, usually lasting between 5 and 10 seconds, and accompanying