Corticosteroid-induced myopathy involving respiratory muscles in patients with chronic obstructive pulmonary disease or asthma.

We made observations on two patients with asthma and one with COPD who developed steroid-induced myopathy during prolonged treatment with high doses of corticosteroids. On admission, quadriceps force was on the average reduced to 31% of predicted (range 16 to 46% of predicted, nondominant leg), and urinary excretion of creatine in 24 h averaged 687 mg (range 275 to 1,045 mg/24 hr). Respiratory muscle involvement was evidenced by reductions in PImax and PEmax, being 38% (range 36 to 39) and 48% of predicted (range 36 to 68), respectively. Tapering of treatment with corticosteroids resulted in important recovery of quadriceps force and respiratory muscle force. In all three patients, a correlation between muscle forces and steroid dose was present during reduction of the dose. After 6 months quadriceps force averaged 62% of predicted (range 31 to 85), and PImax and PEmax reached 74% (range 52 to 92) and 92% of predicted (range 80 to 106), respectively, after 3 months. Consequently, respiratory muscle force appeared to recover faster than quadriceps force. The implications of these observations for patients treated with the usual doses of corticosteroids for shorter periods require further investigation.