SENSORY DEFICITS OF A NERVE ROOT-LESION CAN BE OBJECTIVELY DOCUMENTEDBY SOMATOSENSORY-EVOKED POTENTIALS ELICITED BY PAINFUL INFRARED-LASERSTIMULATIONS - A CASE-STUDY

Somatosensory evoked potentials (SEPs) in response to painful laser st imuli were measured in a patient with a unilateral sensory deficit due to radiculopathy at cervical levels C7 and C8. Laser evoked potential s (LEPs) were compared with SEPs using standard electrical stimulation of median and ulnar nerves at the wrist and mechanical stimulation of the fingertips by means of a mechanical stimulator. Early and late ul nar and median nerve SEPs were normal. Mechanical stimulation resulted in w shaped early SEPs from all five fingertips with some degree of a bnormality at the fourth and fifth digits of the affected hand. Late L EPs were completely absent for stimulations at affected dermatomes and normal in the unaffected control dermatomes. The border between skin areas with normal or absent LEPs was very sharp and fitted the dermato mes of intact C6 and damaged C7 and C8 nerve roots. It is suggested th at pain dermatomes are narrower than tactile dermatomes because thin f ibres of the nociceptive system, activated by laser stimuli, probably do not overlap between adjacent spinal segments to the same extent as thick fibres of the mechanoreceptive system, activated by standard ele ctrical or mechanical stimulation.