METABOLIC MYOPATHIES II: P.356Lower limb muscle strength and function in a cross-sectional study of patients with glycogen storage disease type IIIa

Abstract

Glycogen storage disease type IIIa (GSDIIIa) is an autosomal recessive disorder caused by mutations in the AGL gene coding for the glycogen debranching enzyme. The symptoms start in childhood with liver involvement and progress at adulthood with skeletal muscle weakness that can lead to ambulation loss. This cross-sectional study focusses on lower limb muscle strength and function evolution with age. Torques were measured at the knee and ankle joints by the Biodex and the MyoAnkle dynamometers, respectively. Muscle function was assessed by the motor function Mmeasure (MFM) scale and by the 6-minute walk distance (6MWD). Relationships between age, torques and functions were assessed by Spearman correlation coefficients (rS). The data of 33 patients (52% males) aged of 29 ± 17 (range 11-68) years old were analysed. Age was correlated with torques of knee extension (rS=-0.38, p < 0.05, n=33), ankle dorsi- (rS=-0.73, p < 0.001, n=31) and plantarflexion (rS=-0.56, p < 0.01, n=31) expressed in percentage of predicted value (%pred). Age was also correlated with function assessed i) by the MFM scores for D1, D2, total and items 4 (ankle dorsiflexion), 28 (heel walk), 30 (run), 31 (hop on one leg) (-0.64 ≤ rS ≤ -0.45, p < 0.01, n=33) and ii) by the 6MWD in absolute value (rS=-0.54, p=0.001, n=33) but not in %pred. The 6MWD (%pred) was correlated with all the muscle torques (%pred) and function parameters (0.50 ≤ rS ≤ 0.81, 0.0001 < p < 0.01, 31 ≤ n ≤ 33). Ankle dorsi- and plantarflexion torques (%pred) were strongly correlated (rS=0.81, p < 0.001, n=31) and also correlated with knee extension and flexion (%pred) (0.48 ≤ rS ≤ 0.70, p < 0.01, n=31). MFM items 4, 28, 30 and 31 were correlated with ankle dorsi- and plantarflexion (0.49 ≤ rS ≤ 0.68, p < 0.01, n=31). A progressive weakness in ankle dorsi- and plantarflexors and in knee extensors develops with age, accounting for the progressive loss of function in lower limbs.