Small-fiber dysfunction in trigeminal neuralgia - Carbamazepine effect on laser-evoked potentials

Background: In patients with trigeminal neuralgia, results of clinical exam ination of sensory function are normal. Reflex and evoked potential studies have already provided information on large-afferent (non-nociceptive) func tion. Using laser-evoked potentials (LEP), the authors sought information o n small-afferent (nociceptive) function. Methods: The brain potentials evok ed by CO2-laser pulses directed to the perioral and supraorbital regions we re studied in 67 patients with idiopathic or symptomatic trigeminal neuralg ia and 30 normal subjects. Of the 67 patients, 49 were receiving carbamazep ine. Results: All patients with symptomatic and 51% of those with idiopathi c trigeminal neuralgia had frankly abnormal LEP on the painful side. The me an latency was significantly higher and mean amplitude lower on the painful than the nonpainful side. However, even on the nonpainful side, the mean l atency was significantly longer than that of the age-matched controls. The nonpainful-side latency correlated significantly with the carbamazepine dos e. Conclusions: LEP detect severe impairment of the nociceptive afferent sy stem on the painful side of patients with idiopathic as well as symptomatic trigeminal neuralgia. A dysfunction of small-myelinated afferents may play an important role in the pathophysiology of neuralgic pain. Carbamazepine markedly dampens these brain potentials. The authors propose that this effe ct may result from inhibition of nociceptive transmission in the cingulate gyrus.