Impaired trigeminal nociceptive processing in patients with trigeminal neuralgia

Background: Trigeminal neuralgia (TN) usually leads to paroxysms of short lasting but very severe pain. Between the attacks the patient is usually asymptomatic, but a constant dull background pain may persist in some cases. The mechanisms associated with the development of this chronic pain are not well understood. Objective: To determine trigeminal nociceptive fiber impairment in patients with TN comparing symptomatic and nonsymptomatic sides using the nociceptive blink reflex (nBR) and pain-related evoked potentials (PREP) and to identify possible central mechanisms of pain chronicity. Methods: We investigated 24 patients with TN without and 18 patients with TN with concomitant chronic facial pain. PREP and nBR were investigated following nociception specific electrical stimulation on both sides of the face and in each division of the trigeminal nerve (V1, V2, and V3). Results: We found prolonged PREP and nBR latencies and reduced amplitudes comparing symptomatic and nonsymptomatic sides in all patients with TN. In patients with chronic facial pain, however, PREP amplitudes were larger and latencies shorter compared to patients with TN without facial pain, while nBR results were similar across groups. Conclusion: The data suggest an impairment of the trigeminal nociceptive system due to demyelination and/or axonal dysfunction on the symptomatic side and locate this defect close to the root entry zone in the brainstem. Moreover, central facilitation of trigeminal nociceptive processing was observed in patients with trigeminal neuralgia with concomitant chronic facial pain indicating overactivation of central sensory transmission. This may represent a possible adaptive mechanism for the development of chronic pain.