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Home > D. General pathology > Genetic and developmental anomalies > sirenomelias


Thursday 2 December 2004

Definition: Sirenomelia or Symelia is the fusion of the lower extremities,2 and it has been classified into three types:

- 1) sirenomelia apus: no feet, only one tibia and one femur
- 2) sirenomelia unipus: one foot, two femora, two tibiae, two fibulae
- 3) sirenomelia dipus: two feet and two fused legs (giving the appearance of a flipper). This latter type is also called “mermaid syndrome”.

The sirenomelia sequence is a late defect in blastogenesis leading to deficient development of midaxial and/or caudal structures. It is regarded as a part of the caudal dysplasia sequence.

Sirenomelia represents a severe developmental field defect of the posterior axis caudal blastema, resulting in partial or complete fusion of the lower limb buds. It is always associated with agenesis or hypoplasia of diverse organs.

Its frequency is 1.5-4.2 per 100,000 births.

Some authors consider sirenomelia to be an extreme form of caudal regression syndrome (CRS), but they can be two distinct entities.

Maternal diabetes mellitus is considered to be an important predisposing factor for both caudal regression syndrome and sirenomelia.


- fusion of the lower extremities

  • partial or total femoral fusion (common femur)
  • common tibia without fibula
  • rudimentary foot with oligodactyly
  • limb reduction defects

- incomplete development of the bony pelvis

- caudal regression syndrome (CRS)

- single large umbilical artery


- sirenomelia dipus
- sirenomelia unipus
- sirenomelia apus


- caudal regression syndrome

  • agenesis of the urinary system
  • agenesis of the genital system
  • anorectal atresia

- single large umbilical artery
- complete situs inversus (8719727) (see Case 10041)
- cardiovascular malformation (9018422)
- renal agenesis (9018422)
- ectrodactyly (case 10516)


- sirenomelia in monoamniotic twins (16572916)

Case records

- Case 10041: sirenomelia apus with situs inversus
- Case 10516: sirenomelia apus with ectrodactyly and vertebral anomalies
- Case 12623: Sirenomelia dipus


- Stocker JT, Heifetz SA. Sirenomelia. A morphological study of 33 cases and review of the literature. Perspect Pediatr Pathol. 1987;10:7-50. PMID: 3588246

- Kjaer KW, Keeling JW, Opitz JM, Gilbert-Barness E, Hartling U, Hansen BF, Kjaer I. Sirenomelia sequence according to the distance between the first sacral vertebra and the ilia. Am J Med Genet A. 2003 Aug 1;120(4):503-8. PMID: 12884429