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.


CONCLUSIONS
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.
Department of Neurological Sciences, "La Sapienza" University, Viale Universita, 30, 00185 Rome, Italy; Department of Neurological Sciences, IRCCS San Raffaele, Rome, Italy.
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Truini
Department of Neurological Sciences, "La Sapienza" University, Viale Universita, 30, 00185 Rome, Italy. Electronic address: cruccu@uniroma1.it.
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Cruccu
D000328:Adult; D000368:Aged; D000369:Aged, 80 and over; D005071:Evoked Potentials / Q000502:physiology / Q000528:radiation effects*; D005260:Female; D006801:Humans; D007834:Lasers; D008297:Male; D046529:Microscopy, Electron, Transmission / Q000379:methods; D008875:Middle Aged; D009413:Nerve Fibers, Myelinated / Q000473:pathology / Q000502:physiology / Q000528:radiation effects* / Q000648:ultrastructure*; D036421:Nerve Fibers, Unmyelinated / Q000502:physiology / Q000528:radiation effects*; D012189:Retrospective Studies; D020433:Trigeminal Nerve Diseases / Q000473:pathology* / Q000503:physiopathology*