Amniotic Band Sequence

Definition

ABS is a set of congenital malformations caused by amniotic bands that adhere, entangle, and disrupt developing fetal parts. The distribution of involvement is variable and can range from minor constriction rings and lymphedema of the digits to multiple, unusually complex anomalies of craniofacial, thoracic, abdominal, and extremity structures.

Prevalence and Epidemiology

The estimated incidence of ABS ranges from 1 : 1200 to 1 : 15,000 live births. ABS affects both genders equally. African Americans are affected 1.76 times more frequently than Caucasians.

Etiology, Pathophysiology, and Embryology

Although the etiology of ABS is unclear, several theories have been proposed to explain the occurrence of the associated anomalies. Two major theories have been suggested: the intrinsic theory, proposed by Streeter in 1930, and the extrinsic theory, proposed by Torpin in 1965.

• 1.

  The intrinsic theory proposes that during embryogenesis, at the time of germ disk and amniotic cavity formation, a disruption initiates a series of events leading to multiple anomalies. Streeter proposed that amniotic bands are the result, not the cause, of the pathologic process. The precise cause of a germline defect leading to ABS is unknown and could be multifactorial. Various causative factors have been proposed, including teratogens and a vascular insult during early development. The intrinsic theory is often used to explain the major anomalies seen in ABS, such as craniofacial defects, body wall abnormalities, and internal organ malformations.

• 2.

  The extrinsic theory is the most widely accepted explanation for ABS. Torpin studied the placenta and fetal membranes in affected cases and surmised that primary rupture of the amnion early in gestation caused the disorder, allowing the fetus to pass from the amniotic cavity to the extraembryonic coelom through the defect. The subsequent contact of the fetus with loose “sticky” mesoderm on the chorionic surface of the amnion would lead to entanglement of fetal parts, causing mechanical disruption, vascular disruption, or both. Swallowing the bands could cause asymmetric clefts on the face.

The actual cause of amnion rupture is variable. It has been observed following maternal abdominal trauma, after amniocentesis, and in fetuses with hereditary collagen defects (i.e., Ehlers-Danlos syndrome and osteogenesis imperfecta). ABS has also been associated with maternal fever, in addition to maternal consumption of lysergic acid diethylamide, methadone, and misoprostol.

Since the early work of Streeter and Torpin, other authors have attempted to determine the etiology of ABS. In 1992 Moerman et al. published their study of fetopathologic evaluation of 18 cases of ABS. The authors concluded that the malformations observed were caused by three distinct lesions: (1) constrictive tissue bands, (2) amniotic adhesions, and (3) complex anomaly patterns (limb–body wall complex). In the series by Moerman et al., four cases had constrictive bands involving the fetal limbs, resulting in annular deformations. No internal malformations or complex anomalies were noted in this group with constrictive bands. In contrast, cases involving broad amniotic adhesions resulted in more disfiguring malformations. The investigators concluded that constrictive bands were morphologically and pathogenetically different from adhesive amniotic bands, which are associated with more severe craniofacial defects such as encephalocele and facial clefts.