Abstract
Amniotic band sequence (ABS) refers to a set of highly variable congenital anomalies involving multiple structures, all of which are caused by amniotic bands that adhere, entangle, and disrupt developing fetal parts. Findings can range from minor constriction rings and lymphedema of the digits to multiple, complex anomalies of craniofacial, thoracic, abdominal, and extremity structures. On ultrasound (US), constriction rings of the fingers and toes are the most common finding. Other findings include limb amputations and asymmetric body wall, thoracic, or craniofacial defects. Although typically difficult to visualize on routine US examination, in some cases, the amniotic band may be seen entangling the fetus and restricting movement. Magnetic resonance imaging (MRI) is a useful adjunct as it can provide more comprehensive images of the fetal anomalies. Although the precise etiology of ABS is unclear, two major theories exist: the intrinsic theory, which proposes that a disruption at the time of germ disk and amniotic cavity formation initiates a series of events leading to multiple anomalies; and the extrinsic theory, which proposes that a rupture of the amnion early in gestation allows contact of the fetus with mesoderm on the chorionic surface of the amnion, leading to the entanglement of fetal parts and subsequent disruption. Options for prenatal intervention include fetoscopic surgery to remove the amniotic bands. However, fetuses with more severe defects of the craniofacial region or with limb–body wall complex should not be considered candidates for this intervention, owing to poor prognosis. Delivery should occur at a tertiary care center where multispecialty care is available.
Keywords
limb–body wall complex, body wall defect, constriction rings, amniotic band sequence
Introduction
Amniotic band sequence (ABS) refers to a broad group of highly variable congenital anomalies involving multiple fetal structures. Embryologically normal fetal structures become entangled in amniotic bands, leading to constriction, disruption, and malformation defects. The term sequence describes this condition as a heterogeneous pattern of malformations, which cannot be traced back to a single factor.
Disorder
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.
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.
Manifestations of Disease
Clinical Presentation
The clinical spectrum of ABS is highly variable. Amniotic bands most commonly affect the fetal extremities, with a greater affinity for the distal segments. Approximately 80% of ABS cases involve the fetal hands and feet, with the second, third, and fourth fingers most commonly affected. Constriction rings and digital and limb amputations are common. Defects can range in severity from mild constriction to complete intrauterine limb amputation and fetal demise.
In general, the anomalies observed in ABS can be described as disruptions, deformations, and malformations. Disruptions refer to breakdown or alteration of otherwise normally developed tissue. The classic disruptive features in ABS include constriction bands, amputations, and acrosyndactyly. Deformation is the end result of abnormal forces acting on the developing fetus. For example, oligohydramnios can lead to scoliosis, talipes equinovarus, and joint contractures by limiting fetal movement and exerting direct pressure on developing structures. An insult during organogenesis leads to abnormal development or malformation. Classic malformations seen in ABS include limb–body wall complex, internal organ abnormalities, and craniofacial lesions such as encephalocele.
Imaging Technique and Findings
Ultrasound.
ABS is associated with various features on prenatal ultrasound (US). Constriction rings of the fingers and toes are the most common findings. ABS should be suspected in cases with limb amputations and in the presence of unusual, asymmetric body wall, thoracic, or craniofacial defects. In some instances, the amniotic band may be seen entangling the fetus, restricting movement within the amniotic cavity. Amniotic bands can be extremely difficult to visualize on routine US examination; in most cases, they are never seen. Inubashiri et al. reported that three-dimensional and four-dimensional US can help to visualize more clearly the relationship of amniotic bands on the fetus ( Figs. 98.1–98.3 ).
