Minimally united ischiopagus twins: infraumbilical union with cloacal anomalies.

In researching the embryology and pathological anatomy of conjoined twins, more than 1,200 cases were reviewed from the literature of the past 100 years; a few additional cases were obtained by personal communications. One hundred twenty-eight were classified as ischiopagus, the typical cases having union in the perineum, the pelvis, and the lower abdominal wall. Eighteen atypical ischiopagus cases, however, were conjoined only in the infraumbilical abdominal wall, none with union in the perineum or the bony pelvis but all with anomalies of the cloaca. Twelve infants survived surgery in nine of the ten cases separated since 1964. The malformations of the cloaca were not immediately life-threatening, but the death of one twin or an associated gastroschisis or ruptured omphalocele in some cases required prompt attention. The invariable involvement of the urachus and/or bladder and the shared distal ileum and colon led to the conclusion that the anomalies in these twins arose from union of the allantois and the caudal portion of the yolk sac.