Recovery from brain-stem lesions involving the nociceptive pathways: comparison of clinical findings with laser-evoked potentials

Abstract

Dissociated sensory impairment in brain-stem disorders suggests a lateral 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 deficits, because electrical stimuli predominantly activate large myelinated fibers that project into the medial lemniscal system. In contrast, laser-evoked potentials (LEPs), in response to brief radiant heat pulses, stimulate nociceptive afferents of the superficial skin and allow evaluation of thin fiber and spinothalamic tract function. We describe the recovery of deficits in pain sensitivity in five patients with isolated lateral brain-stem lesions that could be successfully monitored by LEP recordings in the acute stage and after intervals ranging from 7 months to 4 years. Upon first examination, LEPs were abnormal on the affected body side in all five cases of lateral medullary syndrome, irrespective of whether the etiology was vascular or inflammatory. The degree of recovery of pain sensitivity upon reexamination was reflected by the extent of normalization of the LEP. A control patient with vascular pontine lacunar stroke had normal LEPs on both sides, suggesting preserved spinothalamic conduction. The peak-to-peak amplitude of the main LEP component (N250-P400) correlated significantly with clinical pain sensitivity scored by standardized sensory testing (r = 0.76, p < 0.01). In contrast, early and late SEPs, after standard electrical median or tibial nerve stimulation, were normal in all patients, consistent with their intact mechanosensitivity. In conclusion, LEP studies allow the status of nociceptive function to be objectively and reliably documented on repeated examinations and therefore provide a useful supplement to multimodal sensory assessment in brain-stem disorders.