RECOVERY FROM BRAIN-STEM LESIONS INVOLVING THE NOCICEPTIVE PATHWAYS -COMPARISON OF CLINICAL FINDINGS WITH LASER-EVOKED POTENTIALS

Dissociated sensory impairment in brain-stem disorders suggests a late ral lesion involving the spinothalamic tract. Evoked potential studies of the somatosensory system with standard electrical stimulation (SEP ) generally fail to establish objective correlates of such sensory def icits, because electrical stimuli predominantly activate large myelina ted fibers that project into the medial lemniscal system. In contrast, laser-evoked potentials (LEPs), in response to brief radiant heat pul ses, stimulate nociceptive afferents of the superficial skin and allow evaluation of thin fiber and spinothalamic tract function. We describ e the recovery of deficits in pain sensitivity in five patients with i solated lateral brain-stem lesions that could be successfully monitore d by LEP recordings in the acute stage and after intervals ranging fro m 7 months to 4 years. Upon first examination, LEPs were abnormal on t he affected body side in all five cases of lateral medullary syndrome, irrespective of whether the etiology was vascular or inflammatory. Th e degree of recovery of pain sensitivity upon reexamination was reflec ted by the extent of normalization of the LEP. A control patient with vascular pontine lacunar stroke had normal LEPs on both sides, suggest ing preserved spinothalamic conduction. The peak-to-peak amplitude of the main LEP component (N250-P400) correlated significantly with clini cal pain sensitivity scored by standardized sensory testing (r = 0.76, p < 0.01). In contrast, early and late SEPs, after standard electrica l median or tibial nerve stimulation, were normal in all patients, con sistent with their intact mechanosensitivity. In conclusion, LEP studi es allow the status of nociceptive function to be objectively and reli ably documented on repeated examinations and therefore provide a usefu l supplement to multimodal sensory assessment in brain-stem disorders.