Undiagnosed Maternal Myotonic Dystrophy Type 1 Revealed by Congenital Myotonic Dystrophy in the Neonate

Polyhydramnios and fetal morphological anomalies, such as micrognathia and clubfoot, are findings that often suggest fetal hereditary syndromes. However, these findings may also be associated with undiagnosed neuromuscular disorders in the mother, particularly myotonic dystrophy type 1 (DM1). Maternal hypogammaglobulinemia and neonatal findings in prior pregnancies may represent overlooked diagnostic clues. We report a case in which a postnatal diagnosis of congenital myotonic dystrophy (CDM1) in the neonate led to suspicion of undiagnosed maternal DM1, with retrospective recognition of suggestive findings in the first child. A gravida 2, para 1 pregnant woman in her 30s with a history of cesarean section was under follow-up at our hospital. Her first child had exhibited mild hypotonia, low Apgar scores, slow feeding, and hypogammaglobulinemia at birth. The mother was also found to have hypogammaglobulinemia, but no definitive diagnosis was reached at that time. In the current pregnancy, polyhydramnios (amniotic fluid index (AFI) 27.8 cm) was identified at 33 weeks of gestation, and fetal ultrasonography revealed micrognathia and clubfoot. Severe polyhydramnios (AFI 40 cm) with frequent uterine contractions prompted cesarean section at 36 weeks and 5 days. The infant (birth weight 2601 g; Apgar scores 1/4) had no spontaneous breathing and exhibited marked hypotonia, requiring immediate intubation and NICU admission. Investigation prompted by the infant’s severe hypotonia revealed grip myotonia in the mother during a detailed maternal interview. Genetic testing of the infant identified approximately 1,500 CTG repeats in the DMPK gene, confirming CDM1. Maternal genetic testing has not yet been performed. When polyhydramnios is accompanied by fetal anomalies, such as micrognathia and clubfoot, the possibility of an undiagnosed maternal neuromuscular disorder should be considered. Neonatal hypogammaglobulinemia may reflect maternal hypogammaglobulinemia associated with DM1 rather than an intrinsic neonatal immune defect, and subtle findings in prior pregnancies may represent missed diagnostic opportunities. A thorough maternal history, combined with attention to nonspecific maternal clinical features, may contribute to earlier diagnosis, more accurate prenatal counseling, and improved perinatal outcomes.