Of Pompe and Circumstance: A Case Review of Late Onset Pompe Disease in a 52-Year-Old Male.

Late-onset Pompe disease (LOPD) is a rare glycogen storage disorder caused by deficiency of the enzyme lysosomal acid alpha-glucosidase (GAA). It can mimic inflammatory myopathies, leading to diagnostic delays. We present a case of LOPD in a 52-year-old man who came to us with a diagnosis of dermatomyositis for which he had been undergoing treatment for over a decade. Although he was on seemingly adequate immunosuppression, the patient continued to have progressive proximal muscular weakness during the first 4 years of our care. It was not until his sister was diagnosed with LOPD that we pursued biochemical and genetic testing that established the diagnosis by demonstrating reduced lysosomal acid alpha-glucosidase activity and pathogenic variants in the GAA gene. The diagnosis of LOPD can be a clinical challenge given its rarity, as well as its similarities to other more common neuromuscular mimics. However, treatment is very different than myositis and is accomplished through enzyme replacement therapy, with the goal of preventing progression of the disease.