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Communicated by Garry R. CuttingProximal myotonic myopathy (DM2, PROMM) has not been reported in patients younger than 18 years, and apparent lack of congenital and childhood forms is thought to be one of the distinctive clinical characteristics of this trait. We now describe a 2-year-old boy, the youngest member of a family with a history for myotonia in 2 generations. The patient's 35-year-old mother was diagnosed with DM2 of late juvenile onset. She developed aggravating myotonic symptoms during pregnancy. Remarkably few intrauterine child movements were noticed. After birth the child showed general muscular hypotonia with delayed statomotoric development (sitting and crawling at 13 months, first lifting into standing position at 18 months). Muscle reflexes were normal. In the CL3N58 region ofZNF9, DM2-typical unstable expanded CCTG arrays of about 14.5 kb (about 2,500 repeats) were detected both in the mother and the patient by Southern blotting. Expansion of the DM1-specificDMPKCTG repeat was excluded.