Methimazole embryofetopathy

Methimazole embryofetopathy (also known as methimazole embryopathy, MMI embryopathy, or fetal methimazole syndrome) is a rare congenital disorder caused by prenatal exposure to methimazole (or its prodrug carbimazole), medications commonly used to treat maternal hyperthyroidism, particularly Graves' disease. Methimazole is a thionamide antithyroid drug that crosses the placenta and can disrupt fetal development, especially when exposure occurs during the first trimester of pregnancy. The condition is classified as a teratogenic embryopathy rather than a genetic disorder. The clinical features of methimazole embryofetopathy are variable but characteristically include aplasia cutis congenita (localized absence of skin, most commonly on the scalp), choanal atresia (blockage of the nasal passages), esophageal atresia, tracheoesophageal fistula, and facial dysmorphism including a broad nasal bridge and upturned nasal tip. Additional features may include nipple anomalies, minor facial anomalies, developmental delay, and other midline defects. Some affected infants may also present with patent ductus arteriosus or other cardiac defects, omphalocele, and growth restriction. The severity of the condition varies widely, ranging from mild skin defects to life-threatening structural malformations. Management of methimazole embryofetopathy is primarily supportive and depends on the specific malformations present. Surgical correction may be required for choanal atresia, esophageal atresia, tracheoesophageal fistula, or cardiac defects. Aplasia cutis congenita typically heals with conservative wound care, though larger defects may require surgical intervention. Prevention is the most important strategy: current guidelines recommend that women requiring antithyroid treatment during the first trimester of pregnancy should preferably use propylthiouracil (PTU) instead of methimazole, as PTU has a lower teratogenic risk during early organogenesis. Close collaboration between endocrinologists and obstetricians is essential for managing hyperthyroidism in pregnancy.

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