Differences and similarities between atypical facial pain and trigeminal neuropathic pain.

Objective: To investigate contribution of neuropathic mechanisms to clinically diagnosed atypical facial pain (AFP) using neurophysiologic and thermal quantitative sensory testing (QST) and comparing findings in AFP with those in definite trigeminal neuropathic pain (TNP).

Methods: Twenty patients with AFP and 12 patients with TNP participated after thorough clinical diagnostic workup. All patients underwent blink reflex (BR) recordings, habituation of the BR, and (except one patient with TNP) thermal QST. The results were compared with the reference values of our laboratory for normality.

Results: Of the patients with AFP, 75% showed abnormal findings. The BR responses were abnormal in three (15%) AFP patients (in two patients, the findings were compatible with a peripheral neuropathy and in one with a brainstem lesion), and in seven (58%) TNP patients. Seven (35%) patients with AFP and four (33%) with TNP showed increased excitability of the BR in the form of deficient habituation. Thermal QST indicated abnormal small fiber function in 11 (55%) patients with AFP and in all patients with TNP tested. QST showed thermal hypoesthesia in 45% and warm allodynia in 10% of patients with AFP. In TNP, all findings indicated thermal hypoesthesia. Abnormalities in BR and thermal QST were less frequent in AFP than TNP, but when present, type and pattern of findings were similar in both conditions.

Conclusions: Clinical diagnosis of atypical facial pain represents a heterogeneous entity and seems to form a continuum regarding the level and extent of neuropathic involvement. Without detailed neurophysiologic and quantitative sensory examinations, neuropathic cause of chronic orofacial pain may be overlooked.