Clinical Reasoning: A 59-Year-Old Man With Progressive Proximal Weakness Since Childhood

A 59-year-old man presented with slowly progressive weakness. During childhood, he had difficulty climbing stairs. At age 30 years, he noted difficulty getting out of a chair or standing up from the floor without using his arms. At age 48 years, he started having 3-4 falls per year. He denied orthopnea and dyspnea. He had well-controlled hypothyroidism and hyperlipidemia. Medications included levothyroxine and niacin, and he denied the use of statins, fibrates, steroids, and alcohol intake. His parents (unrelated), sister, and 2 sons (aged 16 and 18 years) had no weakness. His maternal grandfather had leg weakness with difficulty climbing stairs around age 30 years and was using a wheelchair by age 60 years.