Gamma Knife Radiosurgery for the Management of Trigeminal Neuralgia

Trigeminal neuralgia has been described as one of the worst pains known to humanity. A plethora of medical and surgical therapeutic options are available as a part of the treatment modality with varying degrees of success. The aim of any therapy is to achieve long-lasting pain relief without much facial hypoesthesia. Success depends on appropriate patient selection and technically sound execution of the treatment. Whenever the etiology is found to be a compressing vessel at the nerve root entry zone, the most effective treatment of choice is microvascular decompression. Radiosurgery (RS) has shown benefit in patients with severe disabling pain refractory to medical and routine surgical options, particularly in the elderly age group. The initial freedom from pain following RS as quoted in several studies and in our experience is seen in around 65–80% of cases and this percentage drops down to around 40% at five years follow-up.[1] In our experience of over 25 years in gamma knife radiosurgery (GKRS), almost 66% of the cases show a significant improvement in symptoms at day 1 of GKRS.[2] Atypical neuralgia has a long latency period. Not all patients respond the same way. There are several technical nuances that we should be familiar with to achieve the best possible results with GKRS. Strategic placement of the shot on the involved nerve is very crucial for effective results. Two main sites of RS used commonly are the root entry zone (REZ) and the retrogasserian target along the fifth nerve.[3] REZ as the preferred target is advocated as it contains predominantly the oligodendrocytes which are more radiosensitive than the Schwann cells. The MRI sequences available now help locate the REZ precisely, 0–3 mm from the pons. We have an inclination for the retrogasserian target as it has shown to have better efficacy with a lesser incidence of facial hypoesthesia. Keratitis was almost exclusively found in the patients where REZ was used as the target. The optimal position for the retrogasserian target is 0–8 mm from the point of emergence of the trigeminal nerve from the pons, in the cisternal space. In patients being referred for secondary gamma knife post-medication or microvascular decompression, the nerve is hardly visible in routine sequences because of atrophy or perineural fibrosis. Thin cuts multiple slice three-dimensional (3D) constructive interference in steady-state (CISS) imaging is especially useful in these cases. Post-treatment focal gadolinium enhancement on the trigeminal nerve in T1-weighted MR sequences may be done to confirm the accuracy of radiation delivery. Newer studies show that diffusion tensor imaging (DTI) done at six months post-radiosurgery is helpful in identifying patients with long-lasting pain relief from non-responders.[4] Higher doses in the range of 40 to 45 Gy at 50% isodose line have shown to be most efficacious, limiting 10 mm3 of brainstem exposure to a maximum dose of 12–15 Gy.[5] Beyond this range, the pain relief remains similar with an increased complication rate. What is more important is to understand the concept of biologically effective dose (BED) being delivered to the target which is influenced by the gamma knife calibration rate and overall treatment time.[6] Gamma knife calibration dose rate of less than 2 Gy per minute (which happens with older sources) results in the longer beam-on time and lesser pain relief with a more likelihood of recurrence of symptoms with the same prescription dose. Many surgeons would like to increase the length of the nerve being irradiated by using sector blocking.[7] It leads to increased overall time and reduced BED and is therefore not efficacious. There exists a narrow therapeutic window for the BED where maximum pain relief with minimum complications can be achieved. Targeting BED in the range 1820–1962.5 Gy has been shown to give maximal long-lasting pain relief.[6] Likewise, the use of two shots instead of one to irradiate the nerve with the intention of covering the maximal area is not of many benefits. The Flickinger effect states that increasing the volume of the irradiated nerve leads to increased toxicity without any advantage in efficacy.[8] We routinely use a single 4 mm shot as an isocenter for the GKRS. Above-mentioned technicalities cannot undermine the importance of appropriate patient selection and the timing for GKRS from the onset of symptoms. GKRS is most effective when used as a primary modality of treatment than as a salvage therapy. In our experience, patients (six till date) who were offered repeat GKRS after the first failed gamma knife therapy did not fare well.[2] Opting for early-on GKRS within three years of disease onset has been shown to give longer-lasting pain relief.[9] To conclude, improvising on the finer aspects of GKRS for trigeminal neuralgia in terms of patient selection and dose delivery techniques can improvise the safety and efficacy of this minimally invasive procedure. Financial support and sponsorship Nil. Conflicts of interest There are no conflicts of interest.