• A Truini and G Cruccu.
• Department of Neurological Sciences, "La Sapienza" University, Viale Universita, 30, 00185 Rome, Italy.
• Clin Neurophysiol. 2008 Aug 1;119(8):1905-8.

ObjectiveAlthough 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.MethodsBy collecting retrospectively LEPs recorded in 370 patients, we analyzed the results from 150 trigeminal divisions with absent Adelta-LEPs.ResultsWe 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.ConclusionsThe 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.SignificanceIn 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.

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