To assess relationship of tremor reduction and volume of lesion post Focused Ultrasound (FUS) therapy in Essential Tremor (ET) and Parkinson Disease (PD) patients.

FUS is a non-invasive ablative procedure to treat medically refractory tremor in ET and PD. Sonication from FUS therapy leads to cavitation in ventralis intermediate nucleus of thalamus resulting in tremor improvement. Volume of tissue ablated (VTA) may relate to the degree of tremor reduction of the corresponding upper extremity.

Retrospective analysis of patients that underwent FUS therapy between August 2022 to 2023 at a single center was performed. Tremor was assessed with the Essential Tremor Rating Assessment Scale (TETRAS) rating scales pre- and post-FUS on the day of procedure. First sonication was performed 10 to 11 mm lateral from the wall of the third ventricle, 25% anterior to the posterior commissure in the Anterior Commissure - Posterior Commissure plane. Additional ablations were performed dorsal and anterior to the initial sonication based on the clinical response and side effects. MRI sequences were acquired immediately post-FUS and VTA in mm³ was manually delineated on a T2 or T2 Flair sequences on the 3DSlicer imaging platform.

20 ET and 4 PD patients underwent FUS procedure at our institute during this timeframe. Population contained 87.5% males, mean age of 74 years (±9, n= 24) and disease duration 17.75 years (±14.12). TETRAS and VTA data was not available for all patients. After FUS, TETRAS of treated hand changed from 12.83 (±3.98, n=21) to 2.12 (±1.96, n=21). Average improvement in tremor was 84.71% (±15.18, n=21) in the treated hand. Average VTA ablated was 234.51mm³ (±115.35, n =23). Greater volume correlated significantly to improved tremor scores (r= 0.45, p=0.044, n=20).

Greater improvement in tremor scores correlated with VTA likely indicating greater involvement of thalamic tracts. Comparison with side effect profile will be performed.