Objective: To analyze the neuronal firing patterns in the posterior subthalamic area (PSA) in a Holmes’ tremor (HT) patient who underwent DBS implantation procedure.

Background: The PSA includes the caudal zona incerta (cZI) and the prelemniscal radiation (RaprL), which contains the cerebello-thalamic (DRTT) and pallido-thalamic tracts. PSA has been proposed as an effective DBS target for the treatment of different types of tremor. Up to date, the neurophysiological properties of this area have not been described.

Method: Single-unit microrecording (MER) exploration was performed on a 50-year-old patient with HT who underwent DBS surgery. The surgical target and trajectory, aimed to cross both thalamic VIM and PSA were planned using a CT-MRI fusion-based technique. The patient’s MRI-CT images were coregistered and normalized using a subcortical atlas to match the neuronal signal with its corresponding anatomical position. MERs were analyzed offline: mean firing rate, interspike interval (ISI), and tonicity score (TS) were calculated. Lastly, the DBS contact position was correlated to the patient’s clinical outcome.

Results: Twelve single units from the PSA were analyzed. All of them showed a paused-type discharge (TS>1). Tract length from VIM to inferoposterior DRTT’s limit was 8 mm. Recordings from the cZI showed a mean firing rate of 30.6±6.5 Hz, with an ISI of 33.7±63.3 ms; similar to previous descriptions of ZI neuronal activity. DRTT was found 6mm after the VIM limit and presented a mean firing rate of 33.1±21.1 Hz, with an ISI of 39.3±75.4 ms, which increased to 110.6±15.2 Hz with a simple proprioceptive stimulus (p<0.0005). Stability of PSA stimulation parameters was achieved at 2 months post-surgery: 3.5 mA, 70 μs, 120 Hz. DBS active contact was placed overlapping proprioceptive responding neurons. At 7 months post-surgery, the patient improved TETRAS-ADL and performance scores from 42 to 14/48, and 22 to 16/64 respectively.

Conclusion: Our results show unique firing characteristics of PSA which are easily distinguished from VIM. A remarkable change in firing pattern upon simple proprioceptive stimulus at the more distal segment of PSA was observed. The positive response of those fibers may confer the therapeutic effect of PSA-DBS in this case. Whether these firing characteristics represent PSA normal properties or are just a result of the lesion responsible for the HT should be elucidated with further studies.

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MDS Abstracts - https://www.mdsabstracts.org/abstract/neurophysiological-analysis-of-the-posterior-subthalamic-area-in-a-patient-with-holmes-tremor/