Pulmonary Dysfunction and Skeletal Muscle Changes in Horses with RAO

Background: Chronic pulmonary diseases (recurrent airway obstruction [RAO]) have been reported to alter skeletal muscle cells in humans. The purpose of this study was to evaluate a potential relationship between pulmonary and muscle variables in horses with a clinical diagnosis of RAO. Muscle biopsies from healthy horses and from horses with RAO were investigated and the relationship between the severity of lung disease and the degree of muscular changes was determined. Hypothesis: We hypothesized that chronic pulmonary disease can lead to changes of the skeletal muscle in horses. Animals: Fifteen healthy horses (control) and 50 horses with RAO were examined. Methods: In a prospective clinical trial, a complete lung examination was performed in all horses. In all horses, muscle enzyme activity at rest and after exercise and muscle biopsies from the M. gluteus medius were examined. Results: None of the horses had clinical or histologic signs of primary or neurogenic myopathies. According to the clinical, endoscopic, and radiographic findings and with a scoring system, the horses with RAO were grouped according to the severity of pulmonary findings (15 horses mild, 24 horses moderate, 11 horses severe RAO). Pathologic changes of the skeletal muscle (fiber atrophy or fiber hypertrophy, myofibrillar degeneration, hyperplasia of mitochondria, and ragged‐red‐like fibers) were identified in most horses with RAO but in only a few individual control horses. In addition, a marked depletion of muscle glycogen storage was evident in the RAO horses but not in the control group. Other pathologic changes of skeletal muscle such as centralized nuclei and regenerating fibers were rare, but were more frequent in horses with lung diseases than in the control group. The degree of muscle cell changes was also graded with a scoring system and correlated with the severity of pulmonary disease (r= 0.55). Conclusion: Chronic pulmonary disease in horses is associated with structural changes in skeletal muscle. Clinical Importance: Because chronic pulmonary disease may affect muscles, early and effective therapy may prevent these changes. This finding could be of clinical importance but requires further studies.