Objective: To compare somatosensory temporal discrimination threshold (STDT) in both body sides previously and 4 months after unilateral high intensity focused ultrasound (HIFU) subthalamotomy of the most affected side, in a cohort of markedly asymmetric PD.

Background: Somatosensory temporal discrimination threshold (STDT) is the ability to identify two successive stimuli as separated in time. STDT in healthy individuals is about 30-70msec, and in PD rises to 100-120msec. Levodopa improves STDT in PD, but not DBS of the subthalamic nucleus (STN), which may worsen this value because of local electric field noise.

Method: 20 markedly asymmetric PD patients were recruited. Basal clinical and demographic characteristics were collected. STDT was determined by a 10msec ascending step paradigm, with paired stimuli delivered over the index finger, starting with a simultaneous pair. The first of three consecutive interstimulus intervals at which patients recognized the pair of stimuli as separate in time was considered the STDT. To avoid a possible learning effect bias, random catch trials (simultaneous pairs) were delivered in between of these three stimuli. STDT was tested with and without tremor on the most affected side (MAS) and without tremor on the less affected side (LAS) in basal circumstances, and it was retested 4 months after the procedure in both body sides. Underlying polyneuropathy was ruled out by means of electroneurography of both median nerves.

Results: Among the 20 patients, 14 (70%) were men. Mean age was 55.95 (SD 9.87) years. Mean disease duration was 7.3 (SD 1.93) years. Mean basal UPDRS-III OFF medication was 38.26 (SD 6.76). Basal STDT in the MAS with tremor was 100.6msec and without tremor 96.11msec, with no differences between both states (t paired test; p=0.74). Four months after HIFU it was 72.78msec (t paired test; p=0.008 comparing to tremor state and 0.009 comparing to non-tremor state). Basal TDT in the LAS was 82.71msec and 4 months afterwards it was 74.08msec (t paired test; p= 0.2).

Conclusion: In our PD cohort, STDT significatively improved after unilateral HIFU subthalamotomy in the treated side, whereas it remained unchanged in the non-treated side. Tremor did not seem to interfere with the outcome. These results support the idea that, unlike STN-DBS, STN-HIFU may not induce local electrical noise which could interfere with STDT.

References: 1. Lacruz F, Artieda J, Pastor MA, Obeso JA. The anatomical basis of somaesthetic temporal discrimination in humans. J Neurol Neurosurg Psychiatry. 1991 Dec;54(12):1077–81. 2. Artieda J, Pastor MA, Lacruz F, Obeso JA. Temporal discrimination is abnormal in Parkinson’s disease. Brain. 1992 Feb;115 Pt 1:199–210. 3. Conte A, Modugno N, Lena F, Dispenza S, Gandolfi B, Iezzi E, et al. Subthalamic nucleus stimulation and somatosensory temporal discrimination in Parkinson’s disease. Brain. 2010 Sep;133(9):2656–63. 4. Martínez-Fernández R, Rodríguez-Rojas R, Del Álamo M, Hernández-Fernández F, Pineda-Pardo JA, Dileone M, et al. Focused ultrasound subthalamotomy in patients with asymmetric Parkinson’s disease: a pilot study. Lancet Neurol. 2018;17(1):54–63.

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MDS Abstracts - https://www.mdsabstracts.org/abstract/somatosensory-temporal-discrimination-threshold-improves-after-unilateral-hifu-subthalamotomy-in-asymmetrical-pd/