Objective: To know the semiological characteristics and physiopathology of instability after thalamotomy with Magnetic Resonance image guided focused ultrasound thalamotomy (MRIg-FUS).

Background: Gait instability is one of the most frequent adverse effects of MRIgFUS thalamotomy. Because the tractographic target is the VIM nucleus of the thalamus and the dentatorubrothalamic pathway, it is assumed that the instability must be related to cerebellar involvement. However, we have observed clinical features that have traditionally been related to vestibular hypofunction.

Method: Prospective study of gait instability in patients with refractory essential tremor treated by MRIgFUS thalamotomy. A neurological examination and a specific vestibular examination including video head impulse test have been performed before and after treatment.

Results: A total of 16 patients were included in this study. The median patient age was 75 years old (70.75-76.5) and 37.50% were women. All patients had been diagnosed with essential tremor. The therapeutic target was the left VIM in 14 patients and the right VIM in 2 patients.  One day after MRIgFUS, 7 patients (43.75%) reported subjective instability. No significant alterations were observed in the basic neurological examination after treatment.

The median HIT score on the right side before treatment was 1.175 (1.03-1.365) and, after treatment, it decreased to 1.02 (0.865-1.12), p=0.03. The median HIT score on the left side before treatment was 1 (0,8575-1,14) and, after treatment, it also decreased to 0,875 (0,825-0,9675) but the differences were not statistically significant (p=0,16). Moreover, saccade velocity decreased bilaterally with statistically significant differences. The median saccade velocity on the right side before treatment was 338 °/s (321,25-360,5) and after treatment was 287 °/s (276-324,5), p= 0,05. On the left side, the median saccade velocity before treatment was 346 °/s (334,25-384,5) and after treatment was 317 °/s (281-337,5), p= 0,002.

Conclusion: The vHIT has traditionally been used to differentiate peripheral vestibular involvement from central nervous system pathologies. However, in this case series, we have found that there is a significant difference in the tests due to a central lesion. These results and the instability post-HIFU presented by the patients could suggest a dysfunction of the vestibular pathway after MRIgFUS thalamotomy.

References: (1) Elias WJ, Lipsman N, Ondo WG, Ghanouni P, Kim YG, Lee W, Schwartz M, Hynynen K, Lozano AM, Shah BB, Huss D, Dallapiazza RF, Gwinn R, Witt J, Ro S, Eisenberg HM, Fishman PS, Gandhi D, Halpern CH, Chuang R, Butts Pauly K, Tierney TS, Hayes MT, Cosgrove GR, Yamaguchi T, Abe K, Taira T, Chang JW. A Randomized Trial of Focused Ultrasound Thalamotomy for Essential Tremor. N Engl J Med. 2016 Aug 25;375(8):730-9.
(2) Jackson LM, Kaufmann TJ, Lehman VT, Lee KH, Miller KJ, Hassan A, Klassen BT. Clinical Characteristics of Patients with Gait Instability after MR-Guided Focused Ultrasound Thalamotomy. Tremor Other Hyperkinet Mov (N Y). 2021 Oct 21;11:41.
(3) De lima AS, Verissimo K, Diniz J, Teixeira ME, Barioni E. Video Head Impulse Test and central nervous system diseases: a integrative review. Audiol Commun Res. 2022;27:e2559

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