ABSENCE OF EXERCISE-INDUCED MRI ENHANCEMENT OF SKELETAL-MUSCLE IN MCARDLES DISEASE

To assess the role of glycogenolysis in mediating exercise-induced increases in muscle water as monitored by changes in muscle proton relaxation times on magnetic resonance imaging (MRI) and cross-sectional area (CSA), five patients with myophosphorylase deficiency (MPD) were compared with seven controls. Absolute and relative work loads were matched during ischemic handgrip and graded cycling, respectively. Relaxation times of active muscle did not increase after handgrip in MPD (T1: 1 +/- 14%, P > 0.1; T2: 4 +/- 4%, P > 0.1) but did in controls (T1: 59 +/- 30%, P < 0.005; T2: 26 +/- 9%, P < 0.005). The volume of exercised muscles, estimated by CSA, increased in both groups after handgrip (controls: 13.8 +/- 3.5%, n = 7, P < 0.0001; MPD: 7.5 +/- 1.5%, n = 4, P < 0.005), but the change was greater in controls (P < 0.02). Ischemic handgrip in controls resulted in a large increase in finger flexor signal intensity (SI) on short tau-inversion recovery images (25 +/- 7%, n = 3; P < 0.005 compared with preexercise) and a further increase with subsequent reflow (43 +/- 11%, n = 3; P < 0.001 compared with rest); in MPD, SI did not increase. The ratio of active to inactive muscle SI did not increase from rest to maximal cycle exercise in MPD (0 +/- 20%, n = 2, P > 0.1) but did in normals (73 +/- 36%, n = 3; P < 0.001). Lack of exercise-mediated change in muscle proton MRI in MPD and attenuated increases in muscle CSA imply an important role for glycogenolysis in mediating these effects and suggest lack of normal muscle water accumulation during exercise in MPD.