Standing postural control in adults with muscular dysfunction caused by Glycogenosis type II.

INTRODUCTIONGlycogenosis type II or Pompe disease (PD) is a rare autosomal recessive myopathy caused by a deficiency of acid alpha glucosidase, which normally degrades lysosomal glycogen [1]. The abnormal buildup of glycogen occurs in a number of systems mainly impairing the musculoskeletal one with proximal and lower limbs muscles involvement. These dysfunctions may cause deterioration of daily activities such as walking or upright standing. With the aim to quantify the influence of PD on the quiet upright stance, the postural stability was compared between patients and healthy volunteers. METHODSFive male patients with late-onset PD (age 29,6±12,4; cm 182,3±10,7; kg 82,6±15,8) were compared with a group of 6 controls (30,8±11,2 yrs; 181±4,6 cm; 79,7±8,9 kg). The patients were evaluated 2 days before the enzyme replacement therapy with Myozyme. Each subject was asked to stand on a force platform to perform a sequence of quiet stances with 3 feet positions: 1) parallel feet with 20 cm mediolateral-interfoot distance (ML-IFD); 2) parallel feet with 10 cm ML-IFD; 3) feet 30° extra-rotated with the heels together. Passing from 1 to 3 the base of support reduced, decreasing the postural stability. For each feet position the postural task was performed for 50 s either with open-eyes (OE) or closed-eyes (CE). We computed the Center of Pressure (COP) and two types of assessment tools were used: stability-related parameters including the total Sway Path (tSP), which accounts for the entire length of postural sway, the Sway Path in antero-posterior (SP AP) and medio-lateral (SP ML) directions, the Root Mean Square (RMS AP and RMS ML) that gives a measure of the variability of postural sway, and the 95% confidence ellipse area (AREA); the parameters to evaluate the structure of postural control included the Mean Power Frequency (MPF AP and MPF ML), which represents the mean frequency of the power spectrum, and the Fractal Dimension (FD) and Approximate Entropy (ApEn) which quantify the level of geometrical and temporal complexity of COP trajectory, respectively. Preliminary tests for normality (Shapiro-Wilk test) and for equality of sample variances (Levene’s test) were performed. Since few dataset showed non-normality distribution and unequal variances, each parameter was modelled by a ANOVA with Groups as between-subjects factor, and feet positions and vision conditions as within-subjects with repeated measures. Post hoc t-tests with Bonferroni correction were computed for local differences across the conditions. To consider the few deviations from the ANOVA test assumptions we used the Mann-Whitney nonparametric test to compare the groups for each experimental condition.RESULTSThe patients showed a significant reduction of the upright standing stability with significant increase of stability-related parameters with respect to the control group (table 1). These differences were larger during the trials with eyes closed while a less extent influence was exerted by changes in feet position. On the contrary the structural COP parameters remained unchanged with respect to the control group. The same scheme was replicated by using nonparametric test.Table 1. Summary of results of ANOVA - Values in bold indicate significant differences (p < 0.05)Parameters Groups Feet position Vision condition Groups X Feet position Groups X Vision condition Groups X Feet position X Vision conditiontSP 0.029 0.004 <0.001 0.652 0.005 0.469SP AP 0.014 0.181 <0.001 0.528 0.008 0.570SP ML 0.140 <0.001 <0.001 0.957 0.046 0.187AREA 0.003 <0.001 0.009 0.660 0.019 0.009RMS AP 0.026 0.132 0.019 0.543 0.002 0.119RMS ML <0.001 <0.001 0.449 0.747 0.569 0.046FD 0.641 0.081 <0.001 0.523 0.080 0.075ApEn AP 0.769 0.818 0.003 0.967 0.515 0.638ApEn ML 0.160 0.146 0.074 0.245 0.018 0.06MPF AP 0.576 0.587 0.004 0.918 0.567 0.691MPF ML 0.206 0.227 0.258 0.381 0.020 0.150DISCUSSIONThe effect of PD on the musculo-skeletal system influences the stability of quiet upright stance. The absence of changes in the postural control mechanism indicates that the central nervous system does not contribute significantly to the motor deterioration. The further worsening observed with closed eyes might indicate a functional reduction of proprioceptive and/or vestibular sensory channels which not compensate for the vision lacking.REFERENCES[1] Hagemans MLC et al. Brain 128: 671-677, 2005.