Sirenomelia Sympus Dipus: Autopsy Case Report of a Second-Trimester Fetus With Multiple Congenital Anomalies

Sirenomelia (mermaid syndrome) is an extremely rare and lethal congenital malformation characterized by fusion of the lower limbs, with a global incidence of approximately one in 60,000-100,000 live births. No proven hereditary pattern or genetic basis has been established. Sympus dipus (Type I, Stocker-Heifetz classification) is the variant in which both feet are preserved despite lower-limb fusion. We report the autopsy findings of a 19-week male fetus received at the Department of Anatomical Pathology of Hospital Metropolitano "Dr. Bernardo Sepúlveda" in Monterrey, México. External examination confirmed complete lower-limb fusion with two preserved feet and a lumbosacral myelomeningocele. Internal examination revealed transposition of the great vessels, absent renal arteries from the abdominal aorta, bilateral renal hypoplasia, agenesis of the bladder, ureters, and urethra, Bochdalek diaphragmatic hernia with bilateral pulmonary hypoplasia, bilobed right lung, pyloric and duodenal atresia, rectal atresia, imperforate anus, multiple subcapsular hepatic hematomas, and bilateral cryptorchidism. Radiography confirmed sacral and pelvic agenesis with two femora, tibiae, and feet. The placenta showed a two-vessel umbilical cord. Findings broadly correlate with the existing literature on sirenomelia; however, Bochdalek diaphragmatic hernia and bilobed right lung appear to represent novel associated anomalies not previously reported for sympus dipus.