Focal Spikes Versus Sharp Waves: Do They Differ?

Conventional EEG interpretation distinguishes spikes (20-70 ms) versus sharp waves (70-200 ms), but the rationale for this distinction is unclear. This preliminary study endeavors to correlate discharge duration with some clinical outcomes.

We measured spike and sharp wave duration for up to 10 discharges in 100 patients referred for routine, inpatient, or ambulatory EEGs. Excluded were generalized spikes, spike waves, polyspikes, seizures, or lateralized periodic discharges (LPDs).

We measured 882 interictal discharges in 100 patients, comprising structural, genetic, autoimmune, and unknown etiologies. Epileptiform discharges, mainly temporal, were unilateral in 64 and bilateral or multifocal in 36 patients. Each record presented 3 to 10 discharges, with 74% having 10 or more. Mean discharge duration was 71.9 ± 31.4, range 15 to 200 ms. Most patients (87%) had mixed sharps and spikes, with 6% having only sharps and 7% having only spikes. Discharge durations within an individual patient were highly variable. Mean discharge duration and seizure frequency were poorly correlated ( r = -0.023, P = 0.82), as were discharge duration and number of antiseizure medications ( r = -0.027, P = 0.80).

In our series, discharge duration did not correlate with seizure frequency or number of antiseizures medicines. Only 13% of patients had exclusive spikes or sharp waves, suggesting that duration of an individual discharge is not a defining characteristic of that person's epilepsy. Although this study is small and preliminary, it suggests that the distinction between spike and sharp waves at 70 ms may not be clinically relevant and perhaps all epileptiform discharges could be called spikes.