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

Laser pulses selectively excite mechano-thermal nociceptors and evoke brain potentials that may reveal small-fiber dysfunction. We applied CO2-laser p ulses to the perioral and supraorbital regions and recorded the scalp laser -evoked potentials (LEPs) and reflex responses in the orbicularis oculi, ma sticatory, and neck muscles in 30 controls and 10 patients with facial sens ory disturbances. Low-intensity pulses readily evoked scalp potentials cons isting of a negative component with a latency of 165 ms followed by a posit ive component at 250 ms. In vertex recordings, the amplitude of LEPs exceed ed 30 mu V. although only high-intensity pulses evoked reflex responses, so me 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 an d normal in patients with normal pain sensitivity and abnormal trigeminal r eflexes. Possibly because of the high receptor density in this area and the short conduction distance, laser stimulation of the trigeminal territory y ields low-threshold and large LEPs, which are useful for detecting dysfunct ion in peripheral and central pain pathways. (C) 1999 John Wiley & Sons, In c.