Muscle blood flow (MBF) and capillary diffusion capacity (CDC) were determined in 8 patients with Duchenne muscular dystrophy and 5 age-matched control males by measuring the simultaneous clearance of 133Xe and 131I–/sup>. MBF was significantly decreased in older patients with severe dystrophy: 35.1 ± 5.8 ml/100g/min in 4 advanced Duchenne patients and 45.8 ± 6.5 ml/100 g/min in controls (p < 0.005). MBF in 4 early cases did not differ from controls (44.0 ± 7.9 ml/100 g/min). There was no significant difference in CDC in Duchenne patients and controls. No structural abnormalities in muscle microvasculature were found by light microscopy. In the electron microscope, the Duchenne basement membranes had duplicated or multiple layers of electron-dense material in 63.9 of muscle capillaries while only 6.6 of the control capillaries showed this. These results do not support an ischemic etiology of Duchenne muscular dystr
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