Laser evoked potentials in patients with trigeminal disease: the absence of Adelta potentials does not unmask C-fibre potentials.

OBJECTIVE: Although laser stimuli activate both Adelta- and C-fibres, the corresponding laser evoked potentials (LEPs) remain restricted to the Adelta-fibre input. Previous studies found C-LEPs after limb stimulation only in subjects with block or clinical impairment of Adelta-fibres. In this study, we aimed at verifying whether in the trigeminal territory the impairment of Adelta-fibres unmasks the C-LEP. METHODS: By collecting retrospectively LEPs recorded in 370 patients, we analyzed the results from 150 trigeminal divisions with absent Adelta-LEPs. RESULTS: We found signals that were consistent with the C-fibre input in three patients only. In most patients with absent Adelta-LEPs, however, laser stimuli still elicited the Adelta-conveyed pinprick sensation. CONCLUSIONS: The preserved pinprick sensation suggests that the Adelta-fibre volley, though weakened, reached the cortex. The C-LEP absence may be explained according to the first come first served hypothesis: the evoked potential related to an afferent volley reaching the cortex shortly after a preceding input (i.e. a C-fibre volley coming after an Adelta-fibre) will be suppressed. SIGNIFICANCE: In clinical studies using the standard laser pulses to evoke the Adelta-LEPs, the finding of absent signals does not indicate a concomitant impairment of C-fibres.