SIRENOMELIA

SIRENOMELIA (symmelia, symphus), a developmental abnormality in which there appears to be a fusion of the lower extremities into a single formation resembling a turnip or the tail of fish. The fusion itself may be more or less complete. In sympus bipus, two legs can be distinguished, while in sympus monopus only one is present. If a shapeless mass resembling a stump forms in place of the fused extremities, it is referred to as sympus apus (see Amelia). Details are established by X-ray. Sirenomelias arise due to irregularities in the development of the trunk; they are frequently associated with other defects of the perineum, urinary bladder, as well as with conditions of abdominal fissuring (see Gastroschisis) and underdevelopment of the pubic bones. The latter leads to the absence of the angulus pubis, the fusion of the inferior rami of the pubic bones, and the outward rotation of the greater trochanters. The lower extremities also undergo strong rotation as a result, with the thighs turning outward and coming close together and fusing with their soft parts, while the lower legs and feet rotate outward so much that the heels and popliteal fossae face forward and the patellae face backward. The abnormality is generally rare.