Laser-evoked cerebral potentials in the assessment of cutaneous pain sensitivity in normal subjects and patients.

Heat stimuli, applied to the skin by non-contact radiation pulses emitted by a CO2-laser, activate simultaneously both A-delta (mean conduction velocity 14 m/s) and C-fibres (0.8 m/s), which terminate in the most superficial skin layers. Correspondingly, brief heat stimuli elicit two pain sensations with mean reaction times of about 500 ms and 1400 ms. Similarly, two evoked potential waveforms were observed in the electroencephalogram: the late components N240/P370 and the ultralate components N1050/P1250. The shape of the two components was reproducible in independent samples of healthy volunteers. In patients with dissociated sensory loss, the laser evoked cerebral potentials are affected, depending on the kind of disturbed nerve and tracts. This is shown in patients with syringomyelia, encephalomyelitis disseminata, myelitis, Brown-Sequard syndrome, Wallenberg syndrome. In cases with hereditary motor and sensory neuropathy type I or with neurosyphilis, ultralate potentials are observed as correlates of delayed pain perception in the affected body areas. The laser evoked cerebral potentials reflected the clinical disorder of pain sensitivity in most cases, whereas somatosensory evoked potentials in response to conventional nerve stimuli failed in objectifying the diagnosis. As such, evoked cerebral potentials in response to laser heat stimuli applied to the hairy skin can be used for an overall examination of the functional integrity of peripheral small fibres, anterolateral tracts and thalamocortical projections.