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interpreted as supporting a diagnosis of epilepsy. Narrowly defined epileptiform
abnormalities are much less common, but still encountered in up to 1% of the healthy
population29,30. The risk of a ‘false positive’ EEG is compounded in patients with DS by the
fact that both non-specific and epileptiform EEG abnormalities may be more common in
patients with DS than in healthy individuals, including those who do not have comorbid
epilepsy31. This is almost certainly because a variety of neurological insults associated with
learning difficulties are common in patients with DS and may be associated with EEG
abnormalities in the absence of epilepsy. Interestingly, patients with borderline personality
disorder (also common in DS) have also been reported to have a high prevalence of non-
specific EEG abnormalities32.

VideoEEG telemetry
VideoEEG (vEEG) telemetry is the gold standard investigation. A good quality video which
captures the onset and evolution of the seizure will on its own often allow a confident
diagnosis. The diagnostic electrographic findings are: for epilepsy, 1) ictal epileptiform
discharges; 2) post-ictal slowing; and in DS, 3) an intact alpha rhythm when the patient is
demonstrably unresponsive3. Again there are some traps: in particular, movement artefact
may obscure or even be mistaken for epileptiform discharges. There are documented cases of
patients having their first ever, and possibly only, DS during telemetry, sometimes as an
elaboration of a simple partial seizure33. This underlines the importance wherever possible of
showing the video to an informant to establish that the seizure is representative of the patient’s
habitual attacks. In addition to the cost of vEEG and its restricted availability there are a
number of important clinical limitations. The technique is of limited use in a patient who has
infrequent seizures. Care must be taken in patients who have multiple seizure types to ensure
that an example of each seizure is seen.

Special mention should be made of simple partial seizures and frontal lobe seizures which
are often not accompanied by any electrographic changes on the ictal scalp EEG34,35. Frontal
lobe seizures in particular may have bizarre behavioural features which are now well known
to specialists but may easily mistaken for DS36. The highly stereotyped nature and very brief
duration of the seizures are helpful features on video. If seizures occur in sleep, as they often
do in frontal lobe epilepsy, the EEG will be helpful, demonstrating seizure onset during
electrographically documented sleep. In DS by contrast (around 50% of patients with DS
report seizures arising from sleep)65 the EEG will reveal that the patient wakes and then has
their seizure37.

A number of studies have demonstrated that placebo methods such as intravenous injection
of saline can be used to provoke a seizure in up to 90% of patients with DS38. Clearly these
studies raise ethical concerns related to the use of placebo. Most recently, however,
McGonigal and colleagues have combined simple suggestion with routine photic and
hyperventilation stimuli, fully disclosing the aims of the procedure to patients39. A total of
60% of patients had a DS provoked in this way compared with 33% in a control group who
received identical activation procedures but without suggestion. These authors estimate they
were able to reduce the need for prolonged telemetry admission in 47% of patients.
Provocation may be of particular value in patients who have infrequent seizures and would
otherwise be unsuitable for telemetry. There is a small risk of false positive results with this
technique (provoking a DS in a patient with epilepsy) and it is therefore critical that an
informant who has witnessed the patient’s seizures is available to confirm that the provoked
seizure resembles their habitual seizures.

Ambulatory EEG monitoring and video recordings obtained by patients’ carers may be very
helpful with the accepted and obvious limitations of lacking video correlation in the first and
usually failing to capture seizure onset in the second40.