Parasitic conjoined twins with omphalocele and tetralogy of Fallot.

Human parasitic twins are very rare. Here we report a unique case of a partial twin attached to the host in the midline at the forehead, chin, chest, and epigastrium. The parasite lacked thoracic organs and major neural tube derivatives. However, it had small peripheral nerves and ganglia within perirenal and pericolonic connective tissue. Also present were a well-developed small intestine, colon, and appendix with normal submucosal and myenteric plexuses. These findings may represent either the initial presence of a neural tube that later regressed or migration of autosite neural crest cells. The parasite had a mature, functioning kidney with its ureter opening to skin and complete absence of urinary bladder or genital organs. This raises questions about the embryological development of the ureteric bud, which is an outgrowth of the mesonephric duct. The host had tetralogy of Fallot and omphalocele containing the parasitic kidney and bowel. Parasitic twinning occurs at 3 weeks of gestation, tetralogy of Fallot at 3-7 weeks, and omphalocele at 6-10 weeks. A single noxa acting at 3 weeks could have caused sequential malformations that initially seem unrelated.