Trigeminal neuralgia is a debilitating neuropathic condition characterized by recurrent paroxysms of shock-like pain across the distribution of the trigeminal nerve, often brought on by innocuous stimuli. Available therapies presently include symptomatic management using anti-convulsant drugs and invasive surgical procedures to target the pathogenic drivers of disease or ablate the trigeminal nerve. Radiosurgery is a treatment modality that offers a minimally invasive, low-risk alternative that has promising clinical utility in the management of refractory trigeminal neuralgia. Radiosurgical approaches like Gamma Knife (GKS) and Cyberknife (CK) utilize image-guided, algorithm-defined dose contouring to disrupt pathogenic pain signaling by delivering targeted amounts of radiation along different points on the trigeminal nerve. This chapter reviews the structure of radiotherapy paradigms, treatment protocol, major outcomes, associated risks, relative clinical utility, and areas of further research that underlie the use of CK and GKS in managing trigeminal neuralgia.
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Trigeminal neuralgia (TN) is a rare neurological condition characterized by recurrent unilateral paroxysms of electric shock-like pain across the distribution of the trigeminal nerve. The characteristic, stabbing pain of TN presents with maximal intensity at symptom onset and is often brought on by identifiable, innocuous stimuli such as facial touch, brushing, or chewing. Episodes of pain may last up to two minutes, and are followed by a refractory period in which pain symptoms are no longer triggered. Similarly, while the episodes of pain most often occur unilaterally, rare bilateral episodes may occur; however, the pain sensation never crosses to the opposite side. Episodes of trigeminal neuropathic pain occur more frequently along the maxillary (V2) and mandibular (V3) divisions of the trigeminal nerve, though the ophthalmic (V1) division may still be affected.
Epidemiologically, trigeminal neuralgia is a rare disorder. The annual incidence rate is estimated to be between 4-13/100,000/year. (Macdonald, Cockerell, Sander, & Shorvon, 2000; Koopman et al., 2009; Katusic, Beard, Bergstralh, & Kurland, 1990) Incidence rates increase significantly with age in both men and women, with the average of onset being between 50-53 years old. (Maarbjerg, Gozalov, Olesen, & Bendtsen, 2014). Disease rates are found to be significantly higher in women than in men (Katusic et al., 1990). The majority of trigeminal neuralgia cases are found to be idiopathic; however, individual reports of familial trigeminal neuralgia have been reported. (Smyth, Greenough, & Stommel, 2003; Fleetwood, Innes, Hansen, & Steinberg, 2001)
Based on its clinical presentation and the case history, trigeminal neuralgia may be classified as classical trigeminal neuralgia or as classical trigeminal neuralgia with concomitant persistent facial pain (formerly known as atypical trigeminal neuralgia). (Headache Classification Committee of the International Headache Society [IHS], 2013) The presentation of classical TN, as described above, is characterized by bouts of lancinating pain lasting for up to two minutes and occurring along the distributions of V1-V3. However, patients with this form of TN do not feel pain in between their recurrent paroxysms. In contrast, patients with the atypical variant of TN feel a persistent pain along the distribution of V1-V3 in between episodes, albeit of a lesser intensity. In these cases, the pain is qualitatively different and is often characterized as burning or tingling rather than as shock-like. Electrophysiological evidence suggests that while microvascular compression may contribute to the etiology, central sensitization that results in over-activation of nociceptive signaling may produce the characteristic persistent facial pain seen in this atypical form. (Obermann et al., 2007) Diagnostically, however, there remains significant overlap between the typical and atypical variants of classical trigeminal neuralgia. The general criteria for a diagnosis of classical trigeminal neuralgia include a minimum of three episodes of pain occurring without radiation along the distribution of the trigeminal nerve. Characteristically, this pain must fulfill three of the four following criteria: 1) occur in paroxysms; 2) be of severe intensity; 3) be lancinating or stabbing in quality; or, 4) be brought on by innocuous stimuli.