To quantitatively assess standing balance in X-linked Dystonia Parkinsonism (XDP) using a portable force plate and to assess potential utility as a quantitative measure of disease severity.

XDP is a neurogenetic movement disorder found in individuals with Filipino ancestry, involving a combination of parkinsonism and dystonia. These combined features result in an abnormal gait, and as the disease progresses, is associated with postural instability and falls. The main drivers of the postural instability in XDP are not known.

We assessed 15 subjects with XDP from Panay Island, Philippines with varying degrees of disease severity, with another 3 excluded due to inadequate standing balance. Each subject underwent posturography using a portable force plate (Nintendo Wii Balance Board) in 6 conditions, each with three 30s trials: feet apart, feet together and in tandem stance, each assessed with eye open and eyes closed. Disease severity was assessed with the Unified Parkinson’s Disease Rating Scale (UPDRS), Burke-Fahn-Marsden scale (BFM) and the novel MDSP-XDP Rating Scale.

In the feet together stance, path length/mean velocity with both eyes open and closed strongly correlated with dystonia severity measured by the BFM (eyes open r=0.65755,p=0.0201; closed r= 0.78658,p=0.0024) and XDP-MDSP scales (eyes open r= 0.69494,p=0.0121; closed r=0.74810,p=0.0051) but not parkinsonism, as measured by the UPDRS (p>0.45). The MDSP-XDP scale correlation was driven solely by the dystonia component of the scale. Correlations remained highly significant even after adjusting for age and neck and truncal dystonia, which could otherwise result in postural sway due to spasmodic dystonic movements.

Feet together stance postural measures correlated with worsening clinical dystonia but not parkinsonism severity, suggesting that the dystonic component of XDP may preferentially affect standing balance. Therefore, standing posturography could represent a quantitative measure of disease severity. Further study will inform its potential use as an outcome measure in future clinical trials.