Glycogenosis type V (McArdle's diseass) mimicking atypical myositis

A 13-year-old girl was referred to our clinic because of a positive rheumat oid factor test, muscle pain and weakness. Laboratory evaluation revealed a n increased ESR, hypergammaglobulinaemia, antinuclear antibodies, circulati ng immune complexes, complement consumption and elevated serum creatine kin ase (CK) activity. A needle biopsy of the dolent muscle showed normal routi ne histology. Immunohistochemistry disclosed single lymphocytes and a weak myocytic HLA class I expression. The diagnosis of myositis was considered a nd corticosteroids were initiated, leading to an increase of complement lev els and a decrease of CK-activity and ESR. She subjectively felt stronger b ut still reported exercise intolerance and metabolic myopathy was considere d. Myophosphorylase activity was completely lacking, establishing the diagn osis of McArdle's disease. CK level was found to be elevated in an obese 4- year-old brother too, who refused extensive walking but reported no muscle pain. Myophosphorylase deficiency was demonstrated by histochemistry and by biochemical analysis of his muscle. The female case illustrates that in ch ildren with the clinical picture of inflammatory myopathy and serological b ut not clinical response to therapy underlying metabolic muscle disorders s hould be excluded. Since the pathogenesis of polymyositis remains unclear, we speculate that inflammatory changes observed in the muscles may have bee n initiated by muscular damage resulting from the underlying metabolic dise ase. The serological changes remained unexplained and may contribute to a s o far undeterminable connective tissue disease.