The periodicity of lateralized epileptiform discharges

Although the term periodic lateralized epileptiform discharges (PLEDs) was introduced in 1964, there has been little discussion about the definition of periodicity, whether the degree of periodicity has clinical significance, and whether electroencephalographers can accurately assess periodicity. The objectives of this study were to determine if a mathematical descriptor of periodicity could analyse PLEDs and whether such has clinical relevance, to correlate the degree of periodicity with underlying etiology and to assess the accuracy of sight recognition of periodically occurring EEG events.

A 6 year retrospective analysis of EEGs on patients with PLEDs was undertaken. The coefficient of variation of inter-complex intervals was calculated for all records. The relative periodicity of each record was rated by electroencephalographers. Corresponding hospital records were reviewed with regard to clinical and pathological information.

In cases of acute viral encephalitis, discharges were more periodic than in other etiologies (P = 0.013). Age, clinical state, and time of last seizure did not influence periodicity but 40 (70%) of 57 recordings with PLEDs were done within 2 days of a seizure. A history of seizures was present in 42 (89%) of 47 patients. Electroencephalographers' assessment of periodicity correlated well with measured periodicity (r = -0.49, P = 0.0001). However, EEGers were more likely to assess records as periodic if: discharges occurred frequently (r = -0.48, P = 0.0001), or the complexes were visually assessed to have a uniform morphology (r = 0.31, P = 0.02), even though neither of these factors influenced the periodicity of discharges.

PLEDs were objectively measured to be more periodic when they were associated with acute viral encephalitis than with other etiologies. Nonetheless, the considerable overlap of values among the several presenting disorders diminished specificity. Electroencephalographers were able to recognize periodically occurring discharges. However, they were strongly influenced by discharge frequency and their perception of the uniformity of discharge morphology.