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From the Department of Neurological Sciences (Drs. Cruccu, Iannetti, Mascia, Romaniello, Truini, Galeotti, and Manfredi), University La Sapienza, Roma, Italy; Neuromed Institute (Drs. Cruccu and Manfredi), Pozzilli, Italy; Inter-University Center for the Neurophysiology of Pain (CIND) (Dr. Leandri), Genoa, Italy; and the Center for Sensory-Motor Interaction (Dr. Romaniello), Aalborg University, Denmark.
Address correspondence and reprint requests to Dr. Giorgio Cruccu, Dipartimento Scienze Neurologiche, Università La Sapienza, Viale Università 30, 00185 Roma, Italy; e-mail: cruccu{at}uniroma1.it
BACKGROUND: In patients with trigeminal neuralgia, results of clinical examination of sensory function are normal. Reflex and evoked potential studies have already provided information on large-afferent (non-nociceptive) function. Using laser-evoked potentials (LEP), the authors sought information on small-afferent (nociceptive) function.
METHODS: The brain potentials evoked by CO2–laser pulses directed to the perioral and supraorbital regions were studied in 67 patients with idiopathic or symptomatic trigeminal neuralgia and 30 normal subjects. Of the 67 patients, 49 were receiving carbamazepine.
RESULTS: All patients with symptomatic and 51% of those with idiopathic trigeminal neuralgia had frankly abnormal LEP on the painful side. The mean latency was significantly higher and mean amplitude lower on the painful than the nonpainful side. However, even on the nonpainful side, the mean latency was significantly longer than that of the age-matched controls. The nonpainful-side latency correlated significantly with the carbamazepine dose.
CONCLUSIONS: LEP detect severe impairment of the nociceptive afferent system 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 effect may result from inhibition of nociceptive transmission in the cingulate gyrus.
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 G. Cruccu, E. Pennisi, A. Truini, G. D. Iannetti, A. Romaniello, D. Le Pera, L. De Armas, M. Leandri, M. Manfredi, and M. Valeriani Unmyelinated trigeminal pathways as assessed by laser stimuli in humans Brain, October 1, 2003; 126(10): 2246 - 2256. [Abstract] [Full Text] [PDF]
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