Pompe disease misdiagnosed as polymyositis.

Late-onset Pompe disease manifests predominantly in the proximal lower limbs and may be mistaken for an inflammatory myopathy. A 46-year-old man with acromegaly had an 8-year history of progressive weakness. His myopathy was initially attributed to the acromegaly, but severe progression prompted a muscle biopsy, which suggested an inflammatory myopathy. However, his weakness progressed despite treatment for polymyositis. His muscle ultrasound scan pattern was more suggestive of Pompe disease than polymyositis, and Pompe disease was confirmed by genetic and enzymatic testing. Patients with apparent polymyositis, which persists despite treatment, require reconsideration of the diagnosis, with particular attention to treatable genetic causes.