Suppression head impulse test paradigm (SHIMP) characteristics in people with Parkinson’s disease compared to healthy controls

The suppression head impulse test paradigm (SHIMP) is a newly described indicator of vestibular function which yields two measures: vestibulo-ocular reflex (VOR) gain and a saccadic response. It is an alternative and complementary test to the head impulse test paradigm (HIMP). Parkinson’s disease (PD) has known saccadic and central vestibular pathway dysfunction. This paper is the first description of SHIMP VOR gain and saccade characteristic in this population. This prospective observational study measured the SHIMP VOR gain and saccade characteristics in 39 participants with idiopathic PD and compared this to 40 healthy controls (HC). The effect of group, demographic variables and SHIMP characteristics were evaluated. SHIMP VOR gains were not significantly different between groups ( p = 0.10). Compared to HC, the PD group mean SHIMP peak saccade velocity was significantly reduced by an average of 77.07°/sec ( p < 0.001), and SHIMP saccade response latency was longer, with an average delay of 23.5 ms ( p = 0.003). SHIMP saccade peak velocity was also associated with both head impulse velocity ( p = 0.002) and SHIMP VOR gain ( p = 0.004) variables, but there was no significant influence of these variables when SHIMP saccade peak velocity was considered as a predictor of PD ( p = 0.52–0.91). VOR gains were unaffected by PD. PD-specific saccadic dysfunction, namely reduced peak saccade velocities and prolonged response latencies, were observed in the SHIMP-induced saccade responses. VOR gain using slow phase eye velocity is preferred as the indicator of vestibular function in the SHIMPs paradigm as non-vestibular factors affected saccade peak velocity.