Introduction

Although trigeminal neuralgia is relatively uncommon - it occurs in about 4 per 100,000 people - it is a most painful complaint, and one that responds to treatment. It occurs more often in women than men, usually aged 50 to 70. Attacks of knife-like pain - sudden, sharp, shock-like or burning - are unilateral (more often on the right side) along the distribution of the trigeminal nerve. Most often it is seen in the maxillary and mandibular division, or just the maxillary branch; very occasionally, the ophthalmic division is involved.

Attacks can be triggered by such stimuli as touch, movement, wind exposure, eating, tooth cleaning, etc. The clusters of pain last from a few seconds to several minutes. Sometimes there are tic-like muscle cramps - hence the name tic doloureux. Spontaneous remissions lasting months or even years are common, but the condition is often progressive; attacks become more frequent, so that the patient develops persistent pain, with disruption of daily life.

Differential diagnosis

Trigeminal neuralgia is, by definition, idiopathic. However, there are a number of other conditions associated with facial pain from which it must be differentiated. Local causes of pain, such as otitis media, sinusitis, herpes zoster, and acute glaucoma, can usually be excluded by a careful examination. If there are no local findings, the history of the pain can be helpful. Persistent pain can be caused by giant cell arteritis, postherpetic neuralgia, cranial nerve compression, and optic neuritis. Absence of a trigger for the pain suggests cluster headache (Horton syndrome). Finally, other neuralgias (including glossopharyngeal neuralgia, Raeder paratrigeminal neuralgia, and diabetic neuritis) must be considered; if there is any neurological deficit, magnetic resonance imaging (MRI) should be done. It should be noted that 2-4% of patients with trigeminal neuralgia also have multiple sclerosis.

Treatment

Carbamazepine is usually so effective initially that lack of response in a particular case suggests the diagnosis may be in doubt. The starting dose is 100 mg twice daily, increasing to three times a day. Increases of 100 mg/day are given until pain relief is achieved, or the maximum of 1,200 mg /day is reached. After the pain has been controlled for 6-8 weeks the drug can be gradually reduced or withdrawn. Carbamazepine blood levels should be measured if any drugs that affect cytochrome P450 enzyme activity are being used concomitantly, and blood cell counts and liver function tests are advised for patients given the drug for long periods.

Second-line drugs used to treat the condition include phenytoin and baclofen. Phenytoin is given in doses of 300-600 mg daily, or may be used intravenously for patients in an acute crisis. Baclofen, a gamma-aminobutyric acid (GABA) agonist, given in divided doses of 50-60 mg/day, is effective in patients who have become unresponsive to carbamazepine.

All three of these drugs can cause drowsiness and other mild side effects. However, they also produce occasionally severe adverse effects - blood dyscrasias, systemic lupus erythematosis and hepatotoxicity in the case of carbamazepine and phenytoin, and hallucinations, palpitations and hyperglycemia with baclofen.

Other drugs tried with varying degrees of success in the treatment of trigeminal neuralgia include pimozide (neuroleptic agent), lamotrigine (anticonvulsant), tizanidine (carbamazepine-like agent), valproate sodium (anticonvulsant), and capsaicin cream (topical analgesic).

If medical treatment is unsuccessful, surgery can be considered. Peripheral approaches include cryotherapy and alcohol injections around the branches of the nerve. Although initially effective, recurrence of pain is frequent; repeated blocks run the risk of causing permanent facial anesthesia. Alternatively, percutaneous destruction of the trigeminal ganglion can be done, using radio-frequency lesions (thermal rhizotomy), glycerol injections or balloon micro-compression. Thermal rhizotomy is usually the most successful of the less invasive procedures, but carries the greatest risk of producing facial numbness. A more certain approach is open microvascular decompression, but it involves craniotomy, and the accompanying operative risks.

Comment

Trigeminal neuralgia, although rare, is the most likely cause of unilateral episodic facial pain. While new specific drugs for the condition are not a high priority for pharmaceutical research, drugs developed in the past for other uses (e.g. anticoagulants, neuroleptics) can provide successful medical treatment. It is hoped that this pattern will continue, obviating the necessity for surgery except in the most resistant cases.