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Epilepsy
Which Electroencephalogram Patterns Are Commonly Misread as Epileptiform?
a report by
Steve Winesett
1
and Selim R Benbadis
2
1. Assistant Professor of Neurosurgery; 2. Professor of Neurology, University of South Florida
The accurate diagnosis of epilepsy is difficult. Studies in tertiary epilepsy missing an abnormality or inexperience in reading EEGs and, in particular,
centres have shown that 30% of adult patients referred for intractable paediatric EEGs.
‘epilepsy’ have non-epileptic events, chiefly psychogenic non-epileptic spells
(PNES).
1
Likewise, in paediatric epilepsy centres 15–39% have non-epileptic Sharp activity in order to be epileptiform needs to stand out significantly
paroxysmal events.
2,3
The electroencephalogram (EEG) can be both helpful from the background (>50%) and disrupt it. This is usually in the form of a
and misleading in diagnosis. In children, up to 6.8% will have true spike (<70ms) or sharp wave (70–200ms) with a slow wave that disrupts the
epileptiform discharges without clinical epilepsy when photic stimulation is background. Unfortunately, there is an over-reliance on phase reversals to
used.
4,5
In adults, the percentage is much lower, probably less than 2%.
6,7
distinguish sharp activity. This is often the source of misreading, because
Studies from the early years of EEG showed higher percentages, usually phase reversals are normal phenomena and when superimposed on the
around 2–4%, because they included 6 and 14Hz positive spikes or six-per- underlying rhythms can give the appearance of a spike and slow wave.
second waves as epileptiform.
8
These are widely recognised today as non- Usually, a review of the background will help to distinguish this
epileptogenic. Accuracy in reading EEGs is critical in both identifying patients superimposition of rhythms from true pathological sharp activity.
who have an increased risk of having epilepsy and avoiding the misdiagnosis
of epilepsy in patients without epilepsy. The state of wakefulness or sleep is also important in the determination of
pathogenicity. In general, sharp transients seen only during drowsiness or
Multiple papers have revealed that many patients with non-epileptic light sleep are less likely to be associated with epilepsy. In a study of sleep
disorders such as syncope and PNES have the incorrect diagnosis of epilepsy EEGs carried out on ‘normal’ patients during polysomnograms, there was an
perpetuated by the misreading of benign EEG patterns.
9–11
One-third of the incidence of frontal sharp transients in 68%, temporal sharp transients in
patients later found to have PNES have had previous EEGs that were 37% and spike and wave discharges (predominantly frontal) in 13%. The
interpreted as epileptiform that contributed to the misdiagnosis.
9
When the criteria for the sharp transients were not stated clearly, but they emphasise
studies were obtained and reviewed carefully, most of the misread patterns the presence of sharply contoured waveforms in drowsiness and light sleep
were simple fluctuations of sharply contoured background rhythms or where most of these were found.
16
In contrast, studies of ‘awake’ EEGs
fragmented alpha activity. Other patterns in the studies included wicket show that an incidence has been found of 0.5% in Royal Air Force crews
spikes, hyperventilation-induced slowing and hypnagogic hypersynchrony. (58% of these were during photic stimulation, one of which later developed
The consequences of misreading EEGs are many. First, it may delay arriving epilepsy)
6
and 2% in US Air Force personnel (included drowsy state).
7
at the correct diagnosis. Once a patient is ‘labelled’ with a diagnosis, it is
difficult to undo it. It requires obtaining the original EEG and reinterpreting Training programmes correctly emphasise training in recognising many
it. No amount of normal EEGs can undo an abnormal EEG unless this is common variants that have caused misreading in the past. A recent study
done, and it is often difficult in clinical practice. Acceptance of the diagnosis showed that these patterns have not been the cause of recent misreading
of PNES is difficult when patients have been told that their previous EEG was in adults, but rather it is nameless rhythms such as sharply contoured and
‘abnormal’. This may contribute to the usual delay in diagnosis of PNES of anteriorly displaced alpha rhythms that cause the most confusion in adult
seven years.
12
Unfortunately, the longer the delay in arriving at the diagnosis patients.
9
There has been no systematic evaluation of misreading errors in
of PNES, the worse the prognosis.
13
Conservative reading of EEGs is paediatric patients. Paediatric EEGs require electroencephalographers to
important in avoiding misdiagnosis, which wastes society’s and the patient’s correlate variations in both rhythms and voltage with the patient’s age,
healthcare resources and delays proper treatment. which creates more opportunities for misreading. Here we will briefly discuss
the commonly defined variants that have caused confusion and then discuss
The reasons for misinterpretation are unclear and complex. One factor the nameless alphoid rhythms that appear to be the source of confusion
appears to be the overemphasis on ‘phase reversals’. Many experts feel that in adult patients.
the overemphasis on these and ‘sharp activity’ causes many problems.
14,15
There is a common misconception that sharp activity that points towards In adult patients, most of the misread patterns are temporal lobe sharp
each other, i.e. phase reversals, is pathogenic. The basic principles of wave or spike mimics. Temporal lobe sharp wave activity has a high
polarity and localisation make it clear that this is not true; it is only indicative correlation with clinical seizures. A study of EEGs from one centre showed
of localisation of a negative discharge, much of which is totally normal. anterior temporal spikes to be correlated with epilepsy in 80%. Spikes from
Phase reversals are not one of the criteria used to determine whether a the mid-temporal and posterior temporal leads were less likely to be
discharge is epileptiform. Strict criteria need to be applied to determine associated with seizures but greater than 50%.
17
As a result, separating
whether a discharge is of epileptic significance. Other factors include trying these temporal lobe sharp and spike wave mimics is critical and the source
too hard to find an abnormality because the patient had a ‘seizure’, fear of of many misreading errors.
© TOUCH BRIEFINGS 2008 101
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