Objective: In this study, we focused on the transient sensation of being tilted along the vertical axis during MRI-guided focused ultrasound (MRgFUS) for tremor. We investigated this report in a retrospective cohort of thirteen patients, and on a prospective cohort of seventeen patients with essential tremor.

Background: The nucleus ventralis intermedius (VIM) of the thalamus is the target for treatment of pharmacological-resistant tremor with MRgFUS. VIM provides the target for fibres from the deep cerebellar nuclei, from the vestibular system, and from some kinesthetic neurons. Transient adverse effects have been frequently described in patients undergoing this procedure. Between them, vestibular symptoms such as spinning, dizziness, and tilting/rising and falling have been reported.

Method: We compared demographic data between patients with and without the reported vertical tilting, and MRgFUS related parameters comparing sonication with and without the tilting sensation (i.e. semi-inverted posture). In addition, we investigated whether a spatial organization within the VIM was conceivable, using a neuroanatomical 3D model of the VIM and adjacent structures based on the Schaltenbrand-Wahren atlas.

Results: We identified four out of thirteen patients in the retrospective cohort who reported a misperception of verticality, whereas most of the patients in the prospective cohort reported such a sensation (fourteen out of seventeen patients). No significant differences in the MRgFUS parameters have been identified as a possible cause for this phenomenon. Mapping the sonication target has led us to build a map of the perception of verticality within the VIM and adjacent structures.

Conclusion: Our findings highlight how the VIM is a relevant hub along the vestibular network and how the vestibular network is spatially organized, with the VIM having a central role in the perception of verticality. This may help explaining observations of misperception of verticality in some neurological diseases. In addition, our findings confirmed how focused ultrasound techniques are a valuable tool to explore brain functions of superficial and deep structures.

References: Ohye C, Narabayashi H. Physiological study of presumed ventralis intermedius neurons in the human thalamus. J Neurosurg. 1979 Mar;50(3):290-7. doi: 10.3171/jns.1979.50.3.0290.
Tasker RR, Organ LW, Hawrylyshyn P. Sensory organization of the human thalamus. Appl Neurophysiol. 1976-1977; 39 (3-4): 139-53. doi: 10.1159/000102487
Jameel A, Gedroyc W, Nandi D, Jones B, Kirmi O, Molloy S, Tai Y, Charlesworth G, Bain P. Double lesion MRgFUS treatment of essential tremor targeting the thalamus and posterior sub-thalamic area: preliminary study with two year follow-up. Br J Neurosurg. 2021 Aug 12:1-10. doi: 10.1080/02688697.2021.1958150. Epub ahead of print. PMID: 34382881.

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MDS Abstracts - https://www.mdsabstracts.org/abstract/illusion-of-semi-inverted-posture-during-high-frequency-ultrasound-stimulation-thalamotomy-for-tremor/