Trigeminal neuralgia (TN), tic douloureux, is characterized by paroxysmal lancinating attacks of severe facial pain. TN has an incidence of approximately 4/100,000 with a large majority of cases occurring spontaneously. Both genders experience TN with a slight female predominance, and the diagnosis is most common over the age of 50. Classic TN is characterized by abrupt onset and termination of unilateral brief electric shock-like pain. Pain is often limited to the distribution of one or two (commonly the second and third) divisions of the trigeminal nerve. Trivial stimuli, including washing, shaving, smoking, talking and/or brushing the teeth (trigger factors) can evoke the pain. Some areas in the nasolabial fold and/or chin may be particularly susceptible to stimulation (trigger areas). In individual patients, pain attacks are stereotyped, recurring with the same intensity and distribution. Most TN patients are symptom-free between attacks and clinical examination is usually normal. Attacks of TN occur in clusters and remissions can last for months.
The cause of TN pain attacks is unknown. Compression of the trigeminal nerve by benign tumors and vascular anomalies may play a role in the development of clinical symptoms. Studies of surgical biopsy specimens from TN patients who had presumed vascular compression, demonstrate evidence of inflammation, demyelination, and close apposition of axons (leading to the possibility of ephaptic transmission between fibers). The “ignition hypothesis” of Devor, proposes that a trigeminal nerve injury induces physiologic changes that lead to a population of hyperexcitable and functionally linked trigeminal primary sensory neurons. The discharge of any individual neuron in this group can quickly spreads to activate the entire population resulting in a sudden synchronous discharge and a sudden jolt of pain characteristic of a TN pain attack.
The diagnosis of TN is based primarily on a history of characteristic paroxysmal pain attacks. The White and Sweet criteria are still commonly used worldwide (Table 1). In the majority of TN patients, the clinical examination, imaging studies, and laboratory tests are unremarkable (“classic TN”). In a smaller group, TN is secondary to another disease process affecting the trigeminal system (“symptomatic TN”). Because a significant percentage of patients have symptomatic TN resulting from another disease process, diagnostic brain imaging studies should be part of the initial evaluation of any patient with TN symptoms. MRI is better than CT to visualize the trigeminal (Gasserian) ganglion and the cerebello-pontine angle. Special attention should be paid to MS plaques, tumor and subtle vascular anomalies that may be the source of root compression.
Carbamazepine is the first choice for treatment of TN. Both controlled and uncontrolled studies confirm its clinical efficacy. Carbamazepine monotherapy provides initial symptom control in as many as 80% of TN patients. Of those initially responding to the drug, approximately 75% will continue to have long-term control of pain attacks. Controlled studies demonstrate that baclofen, and lamotrigine are superior to the placebo for treatments of TN. In the experience of many clinicians, baclofen is just as effective as carbamazepine and often better tolerated. Baclofen could be an alternate choice for initial drug therapy. Other medications that may be effective include oral gabapentin, clonazepam, oxcarbazepine, topiramate and phenytoin. If a patient is not satisfied with single medication therapy adding another oral medication may offer additional benefits. Intravenous lidocaine or phenytoin may be effective for some severe refractory cases of TN. However, these treatments carry additional risks and require close cardiovascular monitoring. Opioid analgesics have not been proven effective for TN and should be avoided.
Posterior fossa exploration and microvascular decompression (MVD) is assumed to directly treat the cause of TN. However, this is a complex and invasive therapy with a possibility of death. With the availability of other less invasive procedures, MVD is infrequently used and is only reserved for younger and healthier patients. Several studies demonstrated trigeminal radiofrequency rhizotomy successfully controls symptoms in over 85% of TN cases.
The technique is minimally invasive. To heat the Gasserian ganglion, a radiofrequency needle is inserted through the foramen ovale under the guidance of fluoroscopy. The procedure can be finished in less than 30 minutes in experienced hands. A few patients experience sensory loss and dysesthesia (analgesia dolorosa) in the distribution of the damaged trigeminal fibers with this procedure. Stereotactic radiosurgery employs computerized stereotaxic methods to concentrate ionizing radiation on the trigeminal root entry zone. Several studies have demonstrated the high clinical efficacy and the relative safety of this new technique. It is currently recommended as a first line noninvasive surgical technique in many pain centers, especially for frail or elderly patients.