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A 36-year-old woman was seen in the clinic after congenital myotonic dystrophy was diagnosed in her newborn son. The neonate had profound hypotonia, areflexia, and bradycardia and required mechanical ventilation. Molecular analysis revealed approximately 1800 trinucleotide repeats in the 3' untranslated region of the dystrophia myotonica–protein kinase gene (DMPK) in the infant (normal range, 5 to 35 repeats). The mother had a history of bilateral cataracts, hypothyroidism, and occasional episodes of muscle cramps in the hands. She had swum competitively as a young adult and at presentation could run 5 to 8 km (3 to 5 mi) without difficulty. She had no history of fatigue, weakness, frontal balding, myotonia of lid closure, or developmental delay. There was no family history suggestive of myotonic dystrophy in previous generations. The mother's hands appeared normal on physical examination, and she had a normal handshake (see Section A of the video). Further examination revealed percussion myotonia (sustained muscle contraction) (Section B of the video) and grip-release myotonia (above, and Section C of the video). Molecular analysis of a DNA sample from the mother's peripheral blood revealed 400 to 700 trinucleotide repeats in the 3' untranslated region of DMPK, a finding that was consistent with a diagnosis of myotonic dystrophy type 1. Myotonic dystrophy is an autosomal dominant trinucleotide-repeat disorder in which an increased repeat-element copy number results in increased severity. The age at onset decreases and the severity increases in successive generations, a phenomenon known as anticipation. There is no specific treatment for myotonic dystrophy. The mother was referred to a cardiologist, and the child has been followed by the medical genetics and physical medicine and rehabilitation services. At 17 months of age, he is able to eat by mouth and sits with support but is not yet ambulatory.
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