Page 66 - ILAE_Lectures_2015

Hypsarrhythmia is the EEG pattern most commonly seen in children with West syndrome
(Figure 2). However, up to one-third have other patterns. It comprises a chaotic mix of
asynchronous high amplitude slow waves with intermixed sharp waves and spikes. So-called
atypical or modified hypsarrhythmia includes cases with asymmetries, persistent foci, some
preservation of background rhythms and a degree of synchrony. Some of these, particularly
the first two, are associated with structural brain problems. During non-REM sleep a degree
of synchronisation is common and there may be preserved sleep elements. The EEG is usually
normal in REM sleep. Particularly early on in the disorder, the awake EEG may be normal
while the sleep EEG is abnormal. The ictal EEG, that is the EEG associated with spasms, is
very variable, with at least 11 patterns being described. However, most commonly it shows
a brief period consisting of a generalised high voltage slow wave, episodic, low amplitude
fast activity and then attenuation or flattening of the EEG (electrodecremental response).
For many years West syndrome was recognised to occur in symptomatic and cryptogenic
(probably symptomatic) forms. More recently it has been generally accepted that a true
idiopathic form exists. At least 85% of cases are symptomatic (Table 1). The single
commonest cause is tuberous sclerosis. An increasing number where an aetiology was not
previously apparent have now been found to be genetic in origin, e.g. CDKL5 in girls,
STXBP1.

Figure 2. EEG from a child with Down syndrome and epileptic spasms. The initial EEG
shows symmetrical hypsarrhythmia. The spasm coincides with a large amplitude slow wave
followed by an electrodecremental response.