Almost all children with congenital myotonic dystrophy are born to mothers with myotonic dystrophy and display a substantial increase in the size of the gene defect. The structure of the affected gene suggests that it is a serine threonine protein kinase. The messenger RNA of the gene is not properly transported to the cytoplasm when the defect is present. This protein kinase may have a role in the normal function of skeletal muscle sodium channels. Microscopic observation reveals changes in skeletal and heart muscle.
Heart muscle shows fibrosis of the conducting system, myocyte hypertrophy and fatty infiltration.
Diagnosis Standard evaluation Evaluation of a patient for myotonic dystrophy includes neurological examination, EMG assessment and obtaining serum for genetic testing. These are described in detail below. At later stages the myotonic dystrophy may resemble limb-girdle atrophy, polymyositis or dermatomyositis. Neurological examination Examination is often diagnostic because the findings of myotonic dystrophy are uniquely characteristic. The key diagnostic feature is a marked transient increase in muscle tone elicited by percussion or use of a muscle.