The illness is characterized by paroxysms of unilateral lancinating facial pain involving the jaw, gums, lips, or maxillary region (areas corresponding to branches of the trigeminal nerve). The maxillary and mandibular divisions are affected more frequently than is the ophthalmic division. Minor, repeated contact with a trigger zone often precipitates an attack, setting off fierce pain that usually lasts up to a few minutes. Repeated paroxysms may continue day and night for several weeks. The disease is unilateral and unaccompanied by demonstrable sensory or motor deficits, features that distinguish it from trigeminal pain with other causes, such as tumor.

The condition can be chronic, although spontaneous remissions are not uncommon. Women are more often affected than are men, and the incidence rises with age. The etiology of the condition is unknown. Despite much speculation, no definitive evidence links it to herpes simplex virus. The pathologic lesion found in some electron micrographs appears to be a breakdown of myelin.

Although TN may be a symptom of multiple sclerosis, which should be considered in a young adult with typical V cranial nerve distribution facial pain, it is infrequently the initial or sole manifestation of this disease. Similarly, TN is uncommonly the isolated symptom of a cerebellopontine angle tumor.

Although few conditions absolutely mimic the lancinating pain of TN, pain referable to structures of the face from other causes may be similar. Conditions that should be excluded before diagnosing idiopathic TN include dental disease, temporomandibular joint dysfunction, temporal arteritis, sphenoid sinusitis, and cluster headache. The preeruption pain of herpes zoster, which occurs in the distribution of the ophthalmic division of the trigeminal more frequently than in the distribution of the other two
divisions, and postherpetic neuralgia, which follows the skin eruption by a few weeks, are two other entities to be considered.

Other facial pain syndromes that should be differentiated from TN. Cluster headache is the longest-lasting and most common headache syndrome with autonomic features (15 to 180 minutes) and tends to occur at night. Paroxysmal hemicrania is a female-predominant headache syndrome and can occur up to 100 times per day with exquisite responsiveness to indomethacin. Facial pain related to sinusitis is an important exclusionary diagnosis. Usually there are other symptoms, such as nasal congestion and rhinorrhea. Uncommon etiologies include shortlasting unilateral neuralgiform headache with cranial injection and tearing-related syndromes, whose common feature is significant autonomic symptomatology.

The diagnosis can usually be made on the basis of history and physical examination by excluding other causes of unilateral facial pain (see Table 176-1). It is important to check the history for symptoms of other important etiologies (e.g., purulent nasal discharge, prior history of facial shingles, recent zoster-like rash, visual disturbances, scalp tenderness, arthralgias, jaw claudication, difficulty with chewing, facial numbness, headache, concurrent neurologic deficits elsewhere, history of multiple sclerosis). Physical examination should include a full neurologic exam (that includes a careful check of all cranial nerves) plus assessment of the sinuses, cranial arteries, and TM joints. In TN, the examination should be normal without evidence of sensory loss in the distribution of the trigeminal nerve. Features that suggest secondary causes include age of onset less than 50, bilateral fifth nerve involvement, and, less conclusively, involvement of the first division of the trigeminal nerve and treatment refractoriness.