Balance impairment in myotonic dystrophy type 1: Dynamic posturography suggests the coexistence of a proprioceptive and vestibular deficit

Abstract

Falls are frequent in Myotonic Dystrophy type 1 (DM1), but the pathophysiology of the balance impairment needs further exploration in this disease. The current work aims to provide a richer understanding of DM1 imbalance. Standing balance in 16 patients and 40 controls was tested in two posturographic tests (EquiTest™). In the Sensory Organization Test (SOT), standstill balance was challenged by combining visual (eyes open vs. closed) and environmental conditions (fixed vs. sway-tuned platform and/or visual surround). In the "react" test, reflexes induced by sudden shifts in the support base were studied. Oscillations of the body centre of mass (COM) were measured. In the SOT, COM sway was larger in patients than controls in any condition, including firm support with eyes open (quiet standing). On sway-tuned support, COM oscillations when standing with closed eyes were larger in patients than controls even after taking into account the oscillations with eyes open. In the "react" paradigm, balance reflexes were delayed in patients. Results in both experimental paradigms (i.e., SOT and react test) are consistent with leg muscle weakness. This, however, is not a sufficient explanation. The SOT test highlighted that patients rely on vision more than controls to maintain static balance. Consistently enough, evidence is provided that an impairment of proprioceptive and vestibular systems contributes to falls in DM1. Rehabilitation programs targeted at reweighting sensory systems may be designed to improve safe mobility in DM1.

Keywords: balance; falls; myotonic dystrophy; neurological rehabilitation; posturography.

FIGURE 1

Experimental setup of the sensory organization test (EquiTest system). Two support and three visual conditions (figure’s rows and columns, respectively) are combined to obtain the six balance tasks of the sensory organization test. In the sway-referenced condition, the platform and/or the screen move in the sagittal plane after the whole-body center of mass oscillation. Keeping balance is more and more challenging from conditions 1 (eyes open, fixed) to 6 (eyes open with sway-referenced screen and sway-referenced platform). Three 20-s trials are completed in each of the six tasks.

FIGURE 2

Sensory organization test. Upper row: natural logarithm (ln) of the normalized amplitude of the sway of the centre of mass (COM) when standing on the firm (A) and the sway-referenced platforms (B). Lower row: main effects of the regression model. The ln of the normalized COM sway amplitude is shown for the two groups of participants [(C) Group], the two support conditions [(D) Support], and the three visual conditions [(E) Vision]. Even if significant, the model’s interactions did not alter this pattern of significance (see main text and Supplementary material). CNT, controls; PTS, patients; EO, eyes open; EC, eyes closed; SRS, sway referenced screen; FP, firm platform; SRP, sway-referenced platform. Mean and 95% confidence interval are plotted; *a significant difference.

FIGURE 3

Motor control test. Upper row: the immediacy of the reflexes elicited by backward (A) and forward (B) platforms’ displacements. Lower row: main effects and interaction from the regression model. The immediacy of the force reflexes is shown for the two groups of participants [(C) Group] and the two directions of the platforms’ shifts [(D) Direction]. The amplitude of the perturbation also affected the reflexes’ immediacy. However, this is not shown because the interaction between direction and amplitude [(E) Direction × Amplitude] showed that the perturbation’s amplitude only affected the immediacy of the reflexes evoked by backward shifts. CNT, controls; PTS, patients. Mean and 95% confidence interval are plotted; *Significant difference at p < 0.05. The difference between the reflexes’ immediacy evoked by backward and forward shifts is significant for each of the three perturbations’ amplitudes (not marked with * for graphical reasons).

FIGURE 4

Visual dependence in DM1. Results from the SOT test. The abscissa gives the subjects sway (natural logarithm, ln, of the normalized COM oscillations) with eyes open, both in controls (open circles) and patients (filled circles). The ordinate gives the sway amplitude in different conditions (see panel labeling). Linear regression lines calculated separately for controls (CNT) and patients (PTS) are also shown. Regression predicts that standing with closed eyes (A) and with sway-referenced surround (B) does not differ between patients and controls on firm support. By contrast, given the same performance with the eyes open on the sway referenced platform, patients perform worse than controls with eyes closed (C) and with the sway referenced surround (D). This finding points out “visual dependence” (i.e., the excessive reliance on visual information in keeping balance) and suggests difficulty using the vestibular or the proprioceptive information for regulating balance, at least in the most challenging conditions. EO, eyes open; EC, eyes closed; SRS, sway-referenced surrounding screen.