Histological effects of trigeminal nerve radiosurgery in a primate model: Implications for trigeminal neuralgia radiosurgery

OBJECTIVE: Stereotactic radiosurgical treatment of the proximal trigeminal nerve is used to relieve the pain of trigeminal neuralgia. The mechanism of the radiosurgical effect is not understood. METHODS: Two adult baboons underwent stereotactic magnetic resonance imagin g-guided radiosurgery, using a gamma knife. A single,4-mm isocenter was tar geted to each proximal trigeminal nerve, just anterior to the pens, to deli ver a maximal dose of 80 or inn Gy (total of four nerves). A nonirradiated baboon brain and nerves served as control specimens. Six months after treat ment, magnetic resonance imaging was again performed and the brains and ner ves were studied using light and electron microscopy. RESULTS: Magnetic resonance imaging indicated a 4-mm-diameter area of contr ast enhancement at the target site in each nerve. All irradiated nerves exh ibited axonal degeneration and mild edema at the target, with remnants of s ome myelinated axons. Large and small myelinated and unmyelinated fibers we re affected. No inflammation was observed. Nerve necrosis was identified af ter 100-Cy treatment. The trigeminal ganglion appeared normal. CONCLUSION: Radiosurgery at 80 Gy causes focal axonal degeneration of the t rigeminal nerve. At higher doses, partial nerve necrosis is observed. We th ink that these effects influence the physiological features of trigeminal n euralgia.