Breach rhythm: the EEG related to skull defects.

From 33 patients with skull defects 89 EEGs were recorded, most with X ray control of the electrode sites. In 10 patients records were made shortly before and about 10 days after bone replacement. The remaining cases had EEGs either with or without bone replacement. The amplitude of alpha and frontal fast rhythms might be increased over or near unilateral posterior and frontal defects respectively. This enhancement was by a factor of less than 3. Since the electrode involved might not be in the defect but on adjacent bone this does not seem to be an effect of greater proximity to the generators. In 21 cases with defects involving or near to electrodes C3(/) and T3(4) sharply focal mu-like rhythms at 6--11 c/sec, usually with faster components, were seen. They formed two groups, at C3(4), responsive to fist clenching and other stimuli (not to eye opening) and at T3(4), unresponsive to any stimulus. In both groups the waves often had spike-like negative phases, but true spikes and also random slow waves with the same restricted focus and responsiveness were sometimes seen. Because of this complexity we prefer the term breach rhythm to mu or mu-like. Bone (or acrylic) replacement abolished central breach rhythm in 3 cases, but not in others and it might or might not restore the symmetry of alpha rhythm or fast rhythm, but burr holes, saw cuts etc., always remain after craniotomy. It is argued that, with the possible exception of 2 patients, the breach rhythms described in this series do not represent enhanced normal mu rhythm. Breach rhythm, even when very spike-like, appears to have little relationship to epilepsy and is not an indicator of recurrence of a tumour.