Letters to the Editor Neurosurgical Forum J Neurosurg Volume 123 • July 2015 289 J Neurosurg 123:289–296, 2015 Trigeminal neuralgia TO THE EDITOR: We are very interested in the arti- cle by Monteith et al. 1 (Monteith SJ, Medel R, Kassell NF, et al: Transcranial magnetic resonance–guided focused ultrasound surgery for trigeminal neuralgia: a cadaveric and laboratory feasibility study. J Neurosurg 118: 319– 328, February 2013). Trigeminal neuralgia often induces severe pain and impairs quality of life, even with multi- modal therapies. The trigeminal pain often results from offending artery compression to the trigeminal nerve. Monteith et al. 1 performed a laboratory investigation in cadaveric specimens to clarify the feasibility of transcra- nial MR-guided focused ultrasound therapy for trigeminal neuralgia. They found that real-time MR thermometry demonstrated the heating effect of focused ultrasound on the trigeminal nerve with 10°C increments in temperature. Moreover, the heating effect may collaterally spread to the internal acoustic canal (IAC). Their study provided solid evidence that MR-guided focused ultrasound surgery (MRgFUS) is capable of in- creasing focal heating of up to 18°C in the trigeminal nerve of a cadaveric specimen at the root entry zone. Im- portantly, MRgFUS did not produce a signifcant heating effect on the skull base and surrounding neural structures in no-pass regions. However, there are some minor con- cerns. First, MRgFUS cannot avoid an off-target effect producing a local temperature effect on adjacent crucial neurovascular structures, such as the brainstem and cra- nial nerves, as the authors observed with the vestibular nerve in the region of the IAC. Their study leads read- ers to ask about the damaging heating effects on vascular structures, which can induce thrombus formation and lead to ischemic injury of the brainstem. Their study delivered results in cadaveric human specimens, but not in living animal models. Therefore, the pain reduction effect still cannot be estimated from their study. We fully agree that in vivo studies are warranted to ensure the safety and ef- fcacy of MRgFUS in treating trigeminal neuralgia. Zhi-Hong Zheng, MD Yi Lin, MD Pin-Shuo Su, MD Peng-Wei Wang, MD Wei-Ting Tsai, MD Dueng-Yuan Hueng, MD, PhD Tri-Service General Hospital, National Defense Medical Center, Taipei, Taiwan DiScLoSurE The authors report no conflict of interest. reference 1. Monteith SJ, Medel R, Kassell NF, Wintermark M, Eames M, Snell J, et al: Transcranial magnetic resonance–guided focused ultrasound surgery for trigeminal neuralgia: a cadaveric and laboratory feasibility study. J Neurosurg 118:319–328, 2013 response We greatly appreciate the thoughtful comments from Dr. Zheng and colleagues. In our study, we investigated the potential use of transcranial MRgFUS treatment for trigeminal neuralgia. As Zheng et al. point out, there is concern regarding the collateral heating of adjacent struc- tures, namely surrounding bone of the acoustic canal. In- deed, it was this concern that prompted specifc design aspects of our investigation and the development of strate- gies to minimize these potential heating effects. From in vivo experimentation in swine 4 and the results of thermal lesioning in the ventral intermediate nucleus of the thala- mus in humans for the treatment of essential tremor, 1 we have learned much in terms of local temperature effects. The lesion size is highly reproducible with an extremely sharp gradient between normal and necrotic tissue that is similar to that in other thermal lesioning modalities, such as radiofrequency, and is probably even more distinct than the gradient produced from ionizing radiation. 4 This sharp lesion drop-off means that there is no damage to surround- ing brain parenchyma. Experience in 30 patients treated for chronic pain and movement disorders, as described by Jeanmonod et al., confrms this sharp drop-off in a le- sion created by FUS. 2,3 No cases of thrombus formation or stroke in the region surrounding the lesion, as determined by diffusion-weighted imaging, were reported. In trigeminal neuralgia, larger vessels near the trigemi- nal nerve would not be directly targeted, and this sharp en- ergy drop-off would be utilized to target the nerve distant to the vascular structure. In addition, the heat-sink effect from rapid vascular fow helps defray heating of the ves- sel. It should also be noted that in the setting of trigeminal neuralgia, a higher “lesional” temperature—as currently utilized in FUS lesioning procedures for pain and move- ment disorders—would not be used since the goal is not to cause necrosis of the entire nerve, but to demyelinate and interrupt the pain fbers only. Concern regarding the IAC is valid and was of particu- lar interest in these experiments. The geometry of the re-