Clinical Reasoning: A 54-year-old man with dyspnea and muscle weakness

A 54-year-old man was referred due to a 15-year history of unexplained dyspnea and progressive muscle weakness. The patient had an unremarkable birth and development history. His mother also had dyspnea of unidentified cause and died suddenly at age 70 years. At age 40, a reduced vital capacity (VC) was identified in pulmonary tests during a smoking cessation treatment. He was asymptomatic. At age 45, he developed dyspnea while playing tennis without "second wind" phenomenon or worsening with fasting. At that time, a creatine kinase (CK) level of 490 U/L (normal <200 U/L) was identified during treatment with statins. These were stopped but no resolution was observed. At age 46, he was hospitalized for pneumonia. Chest X-ray only revealed elevation of right diaphragm. Despite resolution of the infection, he developed orthopnea and started sleeping in the sitting position. Due to the acknowledgment of diaphragmatic weakness and the persistence of elevated CK, chest specialists referred him to the neurology department.