Clinical and electroencephalographic manifestations of vascular lesions of the pons

THE INFLUENCE EXERTED by specific brainstem structures upon the state of alertness and patterns of electrocortical activity in man remains poorly defined, despite the continuing interest this problem has held for neurologists. Much of our information derives from the results of skillfully placed lesions in the brainstem of the cat. Such studies suggest that two functionally antagonistic and anatomically separate pathways within the brainstem reticular formation play a crucial role in modulating both the electrical organization and reactivity of the cerebral cortex and the state of alertness of the animal.1-13 In man, only the analysis of welldefined, stable lesions resulting from vascular disease presents a situation which approaches that found in the experimental animal. The reported incidence of EEG abnormalities associated with vascular lesions of the brainstem varies widely but averages about 30% in most large series,14-lQ although in some, as many as 70% of the tracings have been considered abnormal.20921 The EEG changes are usually slight, consisting of diffuse or bilateral slowing, especially in the temporal regions.16-1**20.21 Paroxysmal discharges and sharp activity, also most conspicuous in the temporal leads;lss20-22 focal abnormalities;14 low-voltage fast records;29-25 and generally low-voltage records have also been observed.1BJss20 In the few patients in whom such data have been recorded, the effect of photic stimulation has usually been regarded as normal,14 while sleep studies have been reported as normal or lacking rapid-eye-movement (REM) patterns.14~~6 In general, normal EEG configurations have been considered compatible with small lesions, laterally and caudally situated in the brainstem, while large, medial and rostral infarcts have been thought to be associated with a relatively high incidence of EEG abnormalities.lO.22.27 Unfortunately, such conclusions are based largely upon case studies lacking necropsy verification. There have been few attempts to correlate behavioral and EEG data with the site of brainstem injury. A review of the literature has yielded only 12 cases, with adequate behavioral and EEG studies, in which there was a major lesion in the pons or lower midbrain without significant abnormality in the cerebral hemispheres or rostral midbrain.21.*3,24,2-2 Analysis of this material together with similar data from 8 additional cases form the basis of the present report (see table). Incomplete accounts of 2 of these cases have been previously published.33.34 EEGs were obtained on Grass Model 4 or 6 machines using either 8 or 16 channels and the International 10-20 system of electrode placement. Bipolar and unipolar montages were used in all cases; in the latter instance, a balanced neck-chest lead served as the common reference.