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autonomic manifestations) before other cortical regions of relatively higher threshold that
generate focal cortical symptoms (sensory, motor, visual or other). Seizures remain purely
autonomic if ictal neuronal activation of non-autonomic cortical areas fails to reach threshold;
otherwise they consist of autonomic and localisation-related cortical symptoms and signs that
may only rarely occur from onset. This hypothesis may explain why similar autonomic
manifestations may appear from anterior or posterior, right or left brain onsets. As seizures
primarily involve a particular system (the autonomic), PS may be considered as an
electroclinical example of ‘system epilepsy’7.

Syncopal-like attacks may be difficult to explain in individual cases. They may be a distinct
seizure type similar to atonic seizures, but on some occasions they may be due to cardiac
asystole (ictal syncope) generated by the seizure discharge195.

Electroencephalography
Inter-ictal EEG findings show great variability4,6,73,75,77-80,82,83. In about 90% of cases, the EEG
reveals mainly multifocal, high amplitude, sharp slow wave complexes that may appear in any
area, often shifting from one region to another in the same or the contralateral hemisphere in
sequential EEGs of the same child. Occipital spikes predominate but they do not occur in one-
third of patients. Occipital paroxysms in their classical form with fixation off sensitivity (FOS)
are even rarer. Clone-like, repetitive, multifocal spike-wave complexes may be characteristic
features when they occur (19%)4. Brief generalised discharges of slow waves, intermixed with
small spikes, may occur either alone (4%) or more often with focal spikes (15%). A single
routine EEG may be normal in 10% of patients, and a few children have consistently normal
wake EEGs before a diagnostic sleep recording. Sleep typically accentuates the spike
abnormalities, and photosensitivity is practically absent.

As in benign rolandic epilepsy, the frequency, location and persistence of spikes do not
determine the clinical manifestations, the duration, the severity and frequency of seizures or
their prognosis. For instance, spikes may persist for many years after clinical remission or
appear only once despite multiple EEGs. The multifocal potential for epileptogenesis in PS has
also been documented by EEG dipole analysis104 and magnetoencephalography, which have
implicated areas along the parieto-occipital, calcarine and central sulci or in the frontal
lobes102,103,105.

In the few reported ictal EEGs, the discharges consist mainly of unilateral rhythmic slow
activity, usually intermixed with fast rhythms and small spikes. They start in wider more often
in posterior than anterior regions, quickly become diffuse and last for many minutes75,92-94,106.
The first ictal clinical symptoms become apparent long after the onset of the electrical discharge
and present as tachycardia, breathing irregularities, coughing or emesis, which would be
impossible to consider as seizure events without an EEG.

Differential diagnosis
PS is easy to diagnose because of the characteristic clustering of clinical seizure semiology,
which is often supported by inter-ictal EEG findings. The main problem is to recognise emetic
and other autonomic manifestations as seizure events, and not to dismiss them or erroneously
consider them as unrelated to the ictus and as a feature of encephalitis, migraine, syncope or
gastroenteritis, which is the reason for the belated recognition of this common
syndrome4,73,114,115. A most difficult situation that demands experienced evaluation is when a
child is seen at the acute stage of a seizure when symptoms may dramatically accumulate in
succession and the diagnosis of true encephalitis is possible. A history of a previous similar
seizure or full recovery after a few hours of sleep is reassuring and may help to avoid
unnecessary investigations and promote withdrawal of any medication that may have been
initiated6,116.