Objective: Here, we describe how sPPI from the leg is modulated in different balance conditions in patients with Parkinson’s disease (PD) and progressive supranuclear palsy (PSP). In addition, we collected data on sway on a force platform for all patients, focusing on antero-posterior and medio-lateral sway paths, RMS, sway area, and sway velocity.

Background: Somatosensory prepulse inhibition (sPPI) is a paired-pulse neurophysiological paradigm in which a somatosensory stimulus induces significant changes in the response to a subsequent suprathreshold stimulus. Measuring changes in the R2 area of the blink reflex is a reliable method to quantify sPPI. Interestingly, a modulation of inhibition according to posture has been described, suggesting that sPPI may be modulated in different postural conditions.

Method: Twenty-five PD and five PSP patients were recruited. sPPI was tested in four different conditions (lying down, standing on hard surface, standing on soft surface, and standing on hard surface during tandem stance). The intensity for evoking a blink reflex was set at 3 times the motor threshold for R2, while the peripheral stimulus was delivered over the lateral aspect of the right ankle at twice the sensory threshold.

Results: Both PD and PSP patients showed a lack of sPPI in the supine condition. In addition, both groups showed a lack of modulation of sPPI according to posture. PD patients showed an inhibition of 6% while standing on hard surface, 6% while standing on soft surface, and 7% while tandem standing. PSP patients showed similar results. No significant correlations between sPPI and sway parameters is noted in both groups.

Conclusion: sPPI in PD and PSP is not differently modulated according to the posture. A possible explanation may lie in an abnormal integration of tactile, vestibular, and proprioceptive inputs that contribute to posture and balance control while standing on different surface and under different circumstances. As sPPI is thought to be a measure of peduncolopontine nucleus (PPN) activity, the lack of sPPI modulation may reflect a dysfunction of PPN activity, which has been previously described both in PD and PSP patients. Further studies are needed to investigate whether this finding is present in early stages of the disease process or if different disease subtypes showed different patterns of modulation.

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MDS Abstracts - https://www.mdsabstracts.org/abstract/modulatory-effect-of-posture-on-prepulse-inhibition-in-parkinsons-disease-and-atypical-parkinsonism/