[Aggravation of hypoxemia in supine position in myotonic dystrophy]

Myotonic dystrophy (MyD) involves a variety of systems. Respiratory disorders are common, namely elevation of diaphragm, alveolar hypoventilation, aspiration pneumonia and sleep apnea. We evaluated respiratory involvement. The subjects were 11 patients with MyD. Also 6 patients with limb girdle muscular dystrophy (LG) were examined to be compared with MyD. Both groups had the similar activities of daily living. All of them never complained of dyspnea. Arterial blood gas studies were performed in supine position and standing position. A new evidence was found that hypoxemia was aggravated and alveolar-arterial oxygen pressure difference was increased in supine position in MyD. Next, pulmonary function tests were done in supine position and sitting position. Functional residual capacity (FRC) were more reduced in supine position in MyD compared with LG. The value to subtract closing capacity from FRC was negative in supine position in MyD, showing closing phenomenon. We propose the mechanism of the aggravation of hypoxemia may be the following. The reduction of FRC caused by respiratory muscle involvement brings out the closing phenomenon. Abnormal uneven distribution of ventilation-perfusion ratio happens and then hypoxemia is worsened in supine position in MyD.