Objective: To assess changes in motor functionality of FMR1 premutation carriers at risk of developing Fragile X Associated Tremor/Ataxia Syndrome(FXTAS), through traditional and complexity-based measures of postural sway.

Background: FXTAS is a neurodegenerative movement disorder characterized by balance issues and cognitive decline, affecting up to 45% of FMR1 premutation carriers over 50years [1]. However, recent reports also show higher risk of motor symptoms in younger premtuation carriers than has been previously recognised, distinct from those associated with FXTAS. This study employs multiscale entropy analysis (MSE) to assess the ‘complexity’ of postural sway fluctuations in younger carriers to, where ‘complexity’ has been linked with ability of the postural control systems to cope with unexpected stressors [2].

Methods: Sway data of 12 female premutation carriers and 15 controls were recorded under four conditions; eyes open (EO), eyes closed, and two dual-task conditions. While standing on a force platform subjects completed a Sustained Attention Response task (SART) and a working memory N-Back task. MSE analysis was applied and a Complexity Index (Ci) and traditional sway parameters (area, velocity, path length) were calculated for anterior-posterior (AP) and mediolateral (ML) sway.

Results: While carrier group’s Ci was consistent across tasks (p>0.05), the control group’s Ci increased during the SART and N-back tasks (AP:p<0.01; ML:p<0.001, for both tasks). The carrier group’s Ci was significantly lower than that of controls during both dual-tasks (SART:p=0.038; N-back:p=0.004). Most traditional parameters did not differ between groups across conditions except that Carriers did exhibit longer path length during the N-back task (p=0.03). Velocity increased for both groups during the dual task but did not differ significantly from the EO condition (p>0.05). [Figure1]

Conclusions: Lower sway complexity in carriers suggests inefficient division of attention and capacity interference during dual-tasks, consequently, diminished capacity to adapt to cognitive stressors and degradation of motor functioning. These results suggest that MSE offers a complementary and, possibly, more sensitive approach to detect the effects of the premutation on postural control. Therefore, Ci may be a useful clinical marker of functional decline in premutation carriers, and a tool for monitoring progression towards disease onset.

References: 1.Jacquemont, S., et al. (2004). “Penetrance of the fragile X–associated tremor/ataxia syndrome in a premutation carrier population.” Jama 291(4): 460-469. 2. Manor, B. and L. A. Lipsitz (2013). “Physiologic complexity and aging: Implications for physical function and rehabilitation.” Progress in Neuro-Psychopharmacology and Biological Psychiatry 45: 287-293.

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MDS Abstracts - https://www.mdsabstracts.org/abstract/postural-sway-complexity-as-a-measure-of-motor-functionality-in-fragile-x-premutation-carriers/