Assessment of trigeminal small-fiber function: brain and reflex responses evoked by CO2-laser stimulation

Abstract

Laser pulses selectively excite mechano‐thermal nociceptors and evoke brain potentials that may reveal small‐fiber dysfunction. We applied CO2‐laser pulses to the perioral and supraorbital regions and recorded the scalp laser‐evoked potentials (LEPs) and reflex responses in the orbicularis oculi, masticatory, and neck muscles in 30 controls and 10 patients with facial sensory disturbances. Low‐intensity pulses readily evoked scalp potentials consisting of a negative component with a latency of 165 ms followed by a positive component at 250 ms. In vertex recordings, the amplitude of LEPs exceeded 30 μV. Although only high‐intensity pulses evoked reflex responses, some subjects showed—even to low‐intensity pulses—an orbicularis oculi (blink‐like) response that markedly contaminated the scalp recording. Scalp LEPs were abnormal in patients with hypalgesia and normal trigeminal reflexes and normal in patients with normal pain sensitivity and abnormal trigeminal reflexes. Possibly because of the high receptor density in this area and the short conduction distance, laser stimulation of the trigeminal territory yields low‐threshold and large LEPs, which are useful for detecting dysfunction in peripheral and central pain pathways. © 1999 John Wiley & Sons, Inc. Muscle Nerve 22: 508–516, 1999.

Nociceptive withdrawal reflexes evoked by uniform-temperature laser heat stimulation of large skin areas in humans

“…The low intensity needed for eliciting the NWR by electrical stimulation may be due to the synchronized barrage to the spinal neurons caused by the stimuli. Comparing the laser evoked cortical potential following stimulation of the foot, hand and perioral skin showed progressively smaller amplitudes at distal areas (Cruccu et al, 1999). Cruccu et al (1999) speculated that longer afferent conductance routes may temporally disperse the afferent barrage of the spinal nociceptive neurons and thus cause weaker laser evoked cortical potentials.…”

“…Comparing the laser evoked cortical potential following stimulation of the foot, hand and perioral skin showed progressively smaller amplitudes at distal areas (Cruccu et al, 1999). Cruccu et al (1999) speculated that longer afferent conductance routes may temporally disperse the afferent barrage of the spinal nociceptive neurons and thus cause weaker laser evoked cortical potentials. Furthermore, the perception threshold of laser stimulation was found to be lower for stimulation of facial skin compared to stimulation of the skin of the extremities (Romaniello et al, 2002).…”

Neurophysiological assessment of craniofacial pain

“…Iatrogenic damage after dental or maxillo-facial surgery are frequent causes of maxillary and mandibular nerve involvement. A previous study, which investigated LEPs and masseter inhibitory reflex in a patient with a iatrogenic lesion of the inferior alveolar nerve, suggested that LEPs after perioral stimulation are diagnostically more sensitive than the masseter inhibitory reflex [24].…”

“…Trigeminal LEPs are of higher amplitude and are recorded more easily than LEPs after limb stimulation [24].…”

“…A study that correlated clinical, radiological and neurophysiological findings in patients with Wallenberg syndrome [47] suggested that facial pain after medullary infarction is due to lesions of the lower spinal Patients whose MRI showed infarcts in the upper or middle portion of the medulla invariably had abnormal masseter inhibitory reflex responses, suggesting that afferent fibres mediating the masseter inhibitory reflex actually descend as far as the upper portion of the medulla [15]. Previous studies investigating LEPs in patients with neuropathic pain due to a lateral medullary infarction reported the abnormality of both A-related and C-related LEPs [24,39]. Further studies are needed to correlate blink reflex and LEP abnormalities and the presence of pain and neurophysiological abnormalities.…”