Abstract
Pompe disease is a rare metabolic disorder that is characterized by the deficiency of the acid aglucosidase. As a result, glycogen accumulates in several tissues including motor neurons, skeletal, cardiac, and smooth muscles. The course of the disease varies according to the type of mutations, and the clinical phenotype can be affected by the enzyme levels. Late-onset Pompe disease (LOPD) is a challenging issue for clinicians as it has a milder phenotype with later onset of symptoms and slower disease progression. One of the most important differentials in the diagnosis of LOPD is inflammatory myositis as both diseases have some common clinical and laboratory features. Herein, we presented a 30-year-old female patient initially diagnosed as polymyositis and treated with immunosuppressive therapy without a benefit on her symptoms and later diagnosed as LOPD.