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Gamma Knife Radiosurgery for Intractable Trigeminal Neuralgia in the Geriatric Population
Trigeminal neuralgia (TGN), also known as tic douloureux, is a disease characterized by unilateral, paroxysmal, lancinating pain in the distribution of the trigeminal nerve. This intermittent pain can be present in either a single division or in multiple divisions of the trigeminal nerve. Maxillary (V2) or mandibular (V3) division pain is much more common than ophthalmic (V1) division pain. Bilateral TGN is exceedingly rare and is often associated with multiple sclerosis.1
The pathophysiology of TGN is controversial. Possible etiologies include injury to the trigeminal nerve (usually by compression from an adjacent blood vessel) or by focal demyelination.2
EPIDEMIOLOGY
TGN is primarily a disease of the geriatric population; the average age of all patients with trigeminal neuralgia in our center is age 65 years. The prevalence of TGN is 0.1 to 0.2 per thousand, affecting women slightly more than men. The incidence increases with age, occurring in up to 20 per 100,000 after age 60 years.3
ASSOCIATED DISEASES/RISK FACTORS
Most cases of TGN are idiopathic. In a series of more than 2000 patients, a tumor involving the trigeminal nerve caused facial pain in approximately 0.8%.4 Two percent of patients with multiple sclerosis will develop TGN.5 Some authors have proposed hypertension as a risk factor for developing TGN.6 A very rare inherited form of TGN has been described in a few families with Charcot-Marie-Tooth disease.6,7
PRESENTATION/IMAGING
The typical patient will present with a complaint of unilateral, paroxysmal, intense facial pain described as “lightening bolts” or “electric shocks” that last from a few seconds to more than ten minutes. The typical patient does not have pain between episodes and may go weeks to months without pain. The disease often progresses from sporadic paroxysms every few days to multiple daily paroxysms that can disable a patient and ruin his/her quality of life. The pain can be triggered by wind or water touching the face, eating, toothbrushing, shaving, or talking. Patients with severe pain may lack oral and facial hygiene, become emaciated, or become socially withdrawn as they attempt to avoid these triggers.2
The physical and neurological examination of a patient with TGN is completely normal. Any objective loss of facial sensation, absence of corneal reflex, other cranial neuropathy (diplopia, facial weakness, decreased hearing), or long tract signs (pronator drift, present Babinski sign) should raise concern that the patient has an associated mass, which requires an imaging study for diagnostic confirmation.
Brain and cranial nerve imaging is also obtained to rule out myelin signal abnormality suggestive of multiple sclerosis. The preferred imaging study is a magnetic resonance imaging (MRI) scan of the brain with and without gadolinium. A computed tomography (CT) scan of the brain with and without contrast should be performed if an MRI is contraindicated, typically due to a pacemaker or other metallic implant.
MEDICAL TREATMENT
Carbamazepine is the first-line medication for treating TGN. Most patients will experience some degree of pain relief with this drug. Periodic blood testing needs to be performed to rule out drug-induced leukopenia or hepatitis. Baclofen, gabapentin, phenytoin, oxcarbazepine, and transdermal lidocaine facial patches are often used in various combinations if patients cannot obtain adequate relief from carbamazepine alone. Unfortunately, many geriatric patients require high doses of multiple medications to control their pain. Consequently, many patients often suffer from side effects such as drowsiness, clouded mental status, ataxia, and sundowning.
Geriatric patients whose TGN is refractory to medications or who cannot tolerate medication side effects are candidates for gamma knife radiosurgery (GKS) treatment.
THE GKS PROCEDURE
Radiosurgery has been used since the 1950s in Europe.8 Gamma knife surgery has been available in the United States since 1987. It consists of a one-session treatment that allows the treating physicians to precisely locate and irradiate the patient’s trigeminal nerve while sparing surrounding brain tissue.
The accurate localization of the trigeminal nerve is achieved by applying a special headframe with built-in three-dimensional coordinates. The frame is attached under local anesthesia to the patient’s head using four pins (Figure 1). Once the patient is in the frame, an MRI is completed to locate the position of the trigeminal nerve. The images are then reviewed on a planning computer, and the precise coordinates of the trigeminal nerve within the three-dimensional frame are determined. Typically, a short length of the nerve is targeted, and a radiation dose is prescribed (Figure 2). The patient’s headframe is then rigidly attached to the gamma knife unit, which emits gamma rays from a radioactive cobalt source (Figure 3). Radiation treatment is typically completed within an hour.
The headframe is removed, and the patient can go home shortly after the procedure is completed. Pain relief usually begins 6 weeks to 6 months after the procedure. Cessation of anticoagulation medications is typically not necessary for this procedure. It has been hypothesized that GKS selectively damages small pain fibers in the trigeminal nerve while sparing larger light touch fibers. This has not been proven.9
CLINICAL OUTCOMES FOR GERIATRIC PATIENTS WITH TGN TREATED WITH GKS
A search of the literature did not reveal any studies dedicated to determining the outcomes of geriatric patients with TGN treated with GKS. We were able to obtain follow-up data on 74 geriatric patients with typical TGN treated with GKS at our institution from October 1999 through June 2003. Average patient age was 68 years (range, 51-94). With an average follow-up of 39 months (range, 20-66), 65% of patients had a positive outcome (defined as greater than 50% improvement in pain). Of these patients with a positive outcome, approximately 75% were completely free of pain, while 25% felt that their pain improved by greater than 50%. A recent study of patients with a median age of 68 years, but not limited to geriatric patients, showed similar results: 70% of patients experienced pain relief at 3-year follow-up.10
COMPLICATIONS/SIDE EFFECTS AFTER GKS
The major side effect of GKS treatment for TGN is facial numbness. In our series, approximately 33% of patients experienced some degree of post-treatment facial numbness. In general, this facial numbness occurred on a small patch of skin and was not reported bothersome by the patient. Of the patients with facial numbness, 75% experienced pain relief and were satisfied with their outcome.
RADIATION-INDUCED TUMOR AFTER RADIOSURGERY
A theoretical complication of radiosurgery is the development of a radiation-induced neoplasm. However, there has been no report in the literature of a radiation-induced tumor following GKS for TGN. There have been more than 200,000 patients treated worldwide with radiosurgery. Four cases of radiation-induced tumors have been reported in patients treated with radiosurgery for large brain arteriovenous malformations or tumors.11 It is theorized that large lesion size and consequent spillover of radiation to the surrounding brain tissue may have contributed to the development of these lesions. The radiation dose given to the surrounding brain in these cases far exceeds the dose typically experienced by the surrounding brain during radiosurgery for TGN. Even though no radiation-induced tumors have ever been reported after GKS for TGN, we mention it to our patients as a theoretical risk.
OPTIONS FOR PATIENTS WHO DO NOT HAVE PAIN RELIEF AFTER GKS
Microvascular decompression of the trigeminal nerve is a major neurosurgical procedure in which a craniotomy is performed under general anesthesia, and the trigeminal nerve is directly explored by the neurosurgeon. Blood vessels that appear to compress the nerve are moved away from the nerve. This treatment provides long-term pain relief in approximately 70% of patients,12 but it is associated with a low incidence of major complications: Catastrophic brain stem stroke or death occurs in 1-2% of patients. Permanent facial paralysis, hearing loss, and postoperative meningitis are potential complications. Additionally, the risks of perioperative myocardial infarction, postoperative pulmonary embolus, and pneumonia following a major surgical procedure are especially concerning in the geriatric population. For this reason, microvascular decompression is typically considered second-line treatment for geriatric patients.
Percutaneous trigeminal rhizotomy is another treatment option. Under intravenous sedation, a long needle electrode is placed into the patient’s cheek through the foramen ovale of the skull base, such that the tip of the electrode resides next to the trigeminal nerve. The tip of the electrode is heated, damaging the nerve. This treatment can be very successful,13 but the degree of facial numbness is typically much greater following rhizotomy as compared with GKS. Furthermore, potential major complications from percutaneous trigeminal rhizotomy include intracranial hemorrhage, stroke, adjacent cranial nerve injury, and postoperative meningitis. Anticoagulation must be halted prior to either microvascular decompression or percutaneous rhizotomy.
CONCLUSIONS
Gamma knife radiosurgery is an excellent treatment option for geriatric patients with trigeminal neuralgia that cannot be managed by medical therapy. In our series, 65% of patients had a favorable outcome. For patients with trigeminal neuralgia refractory to medication, GKS is the least invasive treatment option, does not require general or intravenous anesthesia, and is not associated with potentially catastrophic risks such as stroke, intracranial hemorrhage, or death. In addition, unlike traditional surgery, patients do not need to stop taking anticoagulation medications for this procedure.
The authors report no relevant financial relationships.
