Commentary

MADD is a heterogeneous metabolic disorder that can induce limb-girdle weakness with onset ranging from infantile to adult age. It is caused by defects in fatty acid oxidation in the respiratory chain. Multiple genes can be involved including electron transfer flavoprotein (ETF), ETF dehydrogenase, and riboflavin transporter and metabolizing (riboflavin kinase and flavin adenine dinucleotide synthase) genes. The essential vitamin riboflavin is taken up from the blood by transporters and metabolized into flavin mononucleotide and flavin adenine dinucleotide. These serve as cofactors for dehydrogenases. Not all forms of MADD are responsive to riboflavin treatment, and genetic testing may confirm MADD in riboflavin-insensitive patients. Cardiomyopathy may occur irrespective of age of disease onset. In this patient, serum cTnT was increased without proven cardiac or renal dysfunction. The suspected source of cTnT, which is normally only expressed in cardiac muscle, was skeletal muscle, consistent with previous reports on a number of neuromuscular disorders. Subclinical cardiac damage as the source for cTnT remains possible in these cases, but a more likely scenario is reexpression of cTnT in skeletal muscle upon damage. The hs-cTnT assay discriminates cTnT from troponin family members using 2 independent antibodies. To assess reexpression in skeletal muscle, sequencing of cTnT cDNA or protein is preferred above using single antibodies. This has shown reexpression of cTnTmRNAand protein in skeletal muscle from Pompe patients without cardiac involvement. cTnI is consistently not increased in neuromuscular disease without cardiac involvement, which may be caused by a lack of reexpression of cTnI in skeletal muscle. Release of proteins such as CK and cTnT from diseased muscle is likely due to leaky muscle fibers. This case stresses that results from the hs-cTnT assay should be interpreted with caution in patients with neuromuscular disease.