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Progressive myoclonic epilepsies (PME). The individual disorders which manifest as PME
share neurophysiological features – generalised spike-wave discharge, photo-sensitivity,
‘giant’ SEPs, facilitation of MEPs by afferent stimulation, and abnormalities of background
cerebral activity, typically an excess of slow activity. The background abnormalities are
usually progressive, with marked changes occurring in syndromes with dementia or
significant cognitive decline, such as Lafora body disease. Relatively specific findings
include vertex sharp waves in sialidosis, occipital spikes in Lafora body, and giant VEPs in
Batten’s disease (late infantile neurolipofuscinosis).

Partial epilepsy syndromes. Mesial temporal lobe epilepsy with unilateral hippocampal
sclerosis21 shows anterior/mid temporal inter-ictal spikes, which are ipsilateral or
predominant over the pathological temporal lobe in 60% of cases, and a typical rhythmic 57
Hz ictal discharge accompanying seizures in 80%. There may also be post-ictal ipsilateral
slow activity and potentiation of spikes, both of which are reliable lateralising findings.

Several familial partial epilepsies have now been described. There is variation in phenotypic
and EEG expression between families; overall, familial partial seizure disorders tend to be
more benign than lesional partial epilepsy. In familial temporal lobe epilepsy, focal temporal
spikes are seen in only around 20% of clinically affected cases. EEG findings are a defining
feature of familial partial epilepsy with variable foci, which manifests as temporal and
extratemporal seizures; the EEG focus is usually congruent with seizure type in individual
cases. In autosomal dominant frontal lobe epilepsy, inter-ictal EEG is often normal, and ictal
EEG unhelpful or non-localising.

Routine inter-ictal EEG and management of epilepsy

An important question for a patient presenting with a first unprovoked epileptic seizure is
risk of seizure recurrence. Here EEG is helpful, since risk is higher when the initial EEG
shows unequivocal ED. If so, treatment should be offered after the first tonic-clonic seizure.
In a systematic review22, the pooled risk of recurrence at two years was 27% if the EEG was
normal, 37% if there was non-specific abnormality, and 58% if epileptiform activity was
present. The inherent risk of recurrent seizures is also dependent on the cause of the first
attack and declines with time: probably because of these factors, the EEG is more likely to
show abnormality if recorded within 48 hours of the seizure8.

Inter-ictal EEG does not provide a reliable index of severity or control in seizure disorders.
Reduction in the amount of epileptiform activity shows only a weak association with reduced
seizure frequency, and antiepileptic drugs (AEDs) have either little or variable effect on
discharge frequency. Hence routine EEG has limited value for assessment of treatment effect,
except IGE when persistent epileptiform activity or a photoparoxysmal response usually
indicates sub-optimal therapy in patients taking sodium valproate or lamotrigine. In general,
‘treating the EEG’, i.e. to abolish spikes, is unnecessary, although there is emerging evidence
that suppression of inter-ictal discharges which cause transient cognitive impairment can
improve school performance in some children.

A common reason for EEG referral is prediction of likelihood of seizure relapse in the patient
who has been seizure free for a number of years and wishes to come off medication. However,
the usefulness of EEG recording is uncertain. Relative risk of relapse if the EEG is abnormal
ranges from 0.8 to 6.47 in published studies23. Some of these studies include both children
and adults, or a range of seizure types and epilepsy syndromes, and it is not clear which
aspects of EEG may be important – viz demonstration of non-specific abnormality versus
ED, prior or persisting abnormality, or de novo appearance of ED during the course of or
following drug withdrawal. Overall, EEG appears to be more helpful in prediction of seizure
relapse in children, and otherwise for identification of epilepsies that carry relatively high