Orthopnoea is commonly attributed to heart failure but can be caused by diaphragm weakness, which, when severe, is often associated with hypercapnic respiratory failure. Bilateral diaphragm weakness is generally due to systemic nerve or muscle disease and usually occurs in the setting of severe generalized muscle weakness, but the diaphragm can be the initial or only muscle involved. Here, we report the case of a 39-year-old female who presented with slowly progressive orthopnoea and daytime somnolence. Pulmonary function studies and polysomnogram confirmed bilateral diaphragm weakness complicated by nocturnal hypoventilation and she was subsequently diagnosed with adult-onset Pompe's disease, a rare metabolic myopathy.
Keywords: Hypercapnia; Pompe's disease; myopathy; orthopnoea.