Habenula as a Marker of Thalamic Asymmetrical Skew and a Landmark for the Vim-Vc Boundary in MRgFUS Thalamotomy.

BACKGROUND

Traditional targeting based on the anterior commissure-posterior commissure (AC-PC) line in MR-guided focused ultrasound (MRgFUS) thalamotomy does not account for individual thalamic asymmetry, potentially leading to suboptimal outcomes or adverse effects.

OBJECTIVE

To investigate whether habenular torsion, an anatomical marker of thalamic asymmetry, predicts clinical outcomes and sensory side effects following ventral intermediate nucleus (Vim) thalamotomy.

METHODS

We retrospectively analyzed 157 patients (essential tremor: 121; Parkinson's disease: 36) who underwent unilateral MRgFUS thalamotomy. Angular metrics, including the Torsion Index (inter-hemispheric habenular angular difference) and the Lesion-Habenula difference (based on a lesion axis originating from the midline-habenula intersection), were measured on axial MRI 2.0 mm dorsal to the AC-PC line. Tremor improvement (CRST) and ventralis caudalis (Vc)-related sensory symptoms were evaluated immediately and at the 1-year follow-up.

RESULTS

Tremor improved by over 70% immediately after treatment, with no significant difference between torsion groups (p = 0.490), indicating that torsion did not compromise efficacy. However, Anterior Torsion (Torsion Index < 0) was significantly associated with a higher incidence of both immediate (43.2% vs. 14.5%, p < 0.001) and persistent (25.0% vs. 5.1%, p = 0.001) Vc symptoms. ROC analysis identified the Torsion Index (AUC 0.72-0.75) and Lesion-Habenula difference (AUC 0.76-0.84) as robust predictors for sensory side effects, significantly outperforming the standard AP ratio (AUC 0.47-0.50).

CONCLUSION

Habenular torsion is a critical anatomical biomarker for predicting post-operative sensory complications. Incorporating the Torsion Index into preoperative planning allows for a personalized, anatomy-driven targeting approach, potentially enhancing the safety and efficacy of stereotactic thalamotomy.