Tangled up in bursts

Abstract

An 18-year-old lady with well controlled hyperthyroidism presented after her first generalised tonic-clonic seizure. There was no family history of epilepsy and her neurological examination was unremarkable. An EEG was performed. A single EEG epoch is shown in Fig. 1A and B. Viewing the EEG, which of the two paroxysmal bursts is more strongly associated with an epileptic predisposition?A)The first burst (red asterisk).B)The second burst (black asterisk).C)Both are equally associated with an epileptic predisposition.D)Both are benign variants and neither are associated with an epileptic predisposition. B The single EEG epoch shows two paroxysmal bursts of spike-and-wave activity. At first glance they may perhaps appear broadly similar, however electroencephalographers should be able to distinguish between them and note their different clinical implications. The frequency of the second burst of generalised spike-wave complexes is approximately 3 Hz with easily discernible surface-negative spikes. This is the archetypal interictal signature of idiopathic generalised epilepsy [[1]Seneviratne U. Cook M.J. D'Souza W.J. Electroencephalography in the diagnosis of genetic generalized epilepsy syndromes.Front Neurol. 2017; 8: 499Crossref PubMed Scopus (54) Google Scholar], and hence such a burst strongly supports generalised seizure predisposition in our patient. By contrast, the frequency of the first burst of spike-wave complexes is approximately 6 Hz and the spike components are smaller and more difficult to distinguish. This EEG variant was first described nearly 70 years ago as the “phantom” spike-and-wave and it has an incidence of 1–2.5% of all EEG recordings [[2]Schomer D.L. Lopes da Silva F.H. Niedermeyer’s Electroencephalography. 6th ed. Wolters Kluwer Health, Philadelphia2011Google Scholar]. Hughes characterised such 6 Hz spike-and-wave bursts along two ends of a spectrum: FOLD seen mainly in Females with Occipital location, Low amplitude and during Drowsiness and WHAM seen mainly in Males with Anterior location, High amplitude and during Wakefulness [[3]Hughes J.R. Two forms of the 6/sec spike and wave complex.Electroencephalogr Clin Neurophysiol. 1980; 48: 535-550Abstract Full Text PDF PubMed Scopus (47) Google Scholar]. The current prevailing consensus is that 6 Hz spike-and-wave bursts should not be regarded as unequivocal delineators of epileptic predisposition but rather should be considered as electrographic patterns of uncertain clinical significance, especially given their occurrence in normal subjects and the consistent lack of clinical symptoms during the bursts [2Schomer D.L. Lopes da Silva F.H. Niedermeyer’s Electroencephalography. 6th ed. Wolters Kluwer Health, Philadelphia2011Google Scholar, 4Ebersole J.S. Current Practice of Clinical Electroencephalography. 4th ed. Wolters Kluwer Health, Philadelphia2014Google Scholar]. In cases of WHAM (particularly if they occur with a frequency less than 6 Hz or if they persist into slow-wave sleep) a greater likelihood of association with a history of seizures is described by some authors, though careful interpretation remains advisable [2Schomer D.L. Lopes da Silva F.H. Niedermeyer’s Electroencephalography. 6th ed. Wolters Kluwer Health, Philadelphia2011Google Scholar, 4Ebersole J.S. Current Practice of Clinical Electroencephalography. 4th ed. Wolters Kluwer Health, Philadelphia2014Google Scholar]. Our unusual EEG epoch (Fig. 1) is notable for fortuitously demonstrating two distinct bursts in very close proximity, each with different clinical implications. This co-occurrence on a single EEG page provides an excellent visual contrast between typical 6 Hz “phantom” and 3 Hz generalised epileptic discharges. It is important for interpreting clinicians to avoid the pitfall of mistaking the former for the latter. None (all authors). No funding was provided in relation to this manuscript.