Amniotic Band Syndrome

Etiology

There is no known clear etiology of ABS. However, it is thought that rupture of amnion early in pregnancy results in the development of multiple loose strands (amniotic bands) that adhere to or entangle the fetus.[1][3] Constriction of otherwise normally developing organs results in constriction rings, some severe cases leading to vascular disruption and could potentially result in amputation of the involved anatomic structure. Adherence, even without constriction, can have adverse mechanical effects that result in malformation or deformation. The cause of the rupture of amnion is unknown in most cases.

However, in some cases, iatrogenic invasive procedures like chorionic villus sampling can lead to ABS-like clinical presentation. The aforementioned etiological reasoning does not explain all ABS findings, as in some cases, the affected anatomic structures are internal visceral organs and have histologically intact amniotic lining. In these cases, disruption of fetal blood flow unrelated to bands is thought to be the primary cause. This vascular insult could result from misoprostol exposure in the first trimester and or chorionic villus sampling before 10 weeks of gestation. For a few cases, neither the amniotic band nor vascular disruption is thought to result from a genetic mutation. Almost all ABS cases in twins have been seen in monozygotic twins, suggesting a genetic correlation.[4]

Epidemiology

The estimated incidence of ABS ranges between 1 in 1000 to 1 in 15000 live births and 1 in 70 stillbirths.[5] Both male and female newborns are equally affected. There is no known inheritance pattern. Almost all cases are sporadic, but few examples of familial ABS have been reported, primarily with monozygotic twin gestation.

Pathophysiology

Although not completely understood, as the embryo grows between the amnion and the chorion, the amnion eventually obliterates the extracolonic space. The incomplete obliteration of the extracolonic space may render the amnion fragile and susceptible to rupture. The ruptured amnion could attach itself to body parts, leading to clinical features of ABS. Theories of defective germ plasma and vascular disruption have been proposed to explain the pathophysiology, especially in internal organ cases.

History and Physical

Clinical presentation can be classified into 4 major categories:

• Constrictive rings
• Limb defects
• Neural or spine defects
• Craniofacial defects

Constriction rings and limb or digital amputation are the most common findings, present in at least 80 percent of cases. The amniotic bands may be present, confined to the skin or soft tissue, and extend deep into the tissue. If constriction results in amputation in utero, then the amputated part is usually resorbed and not visible following birth. Limb defects include constriction ring of an extremity with swelling of the distal part, absence of distal portions of 1 or more fingers and toes, especially the central digits, contractures, or fractures. Lower extremities are affected more commonly than the upper, and the thumb is usually spared due to the protection of the palm in utero. While atypical, craniofacial abnormalities, such as encephalocele, facial clefts, and cleft lip/cleft palate, may be present. Spinal defects and scoliosis have also been known to occur with ABS.

Evaluation

Due to the extensive clinical presentation and heterogeneous etiology, diagnosing ABS could be challenging, especially prenatally. If constriction, amputation, or deformation of major anatomic structures are present, ABS can be suspected prenatally as early as the first trimester. Distal limb edema can help make a diagnosis of ABS prenatally. In very few cases, a strand of amniotic fibrous tissue could be seen attached to tissues and restricting the free movement of the fetus in utero. Postnatally, ABS should be suspected in newborns with visible constrictions, amputations, non-midline, and unusually located craniofacial or body wall defects. Investigating the fresh fetal membranes and placenta is important in cases where amniotic bands are absent in newborns.

Treatment / Management

There are no set guidelines for the management of ABS when diagnosed prenatally. Regardless of the therapeutic approach chosen, all patients should receive counseling about fetal abnormalities detected and the possibility of other hidden abnormalities. For chromosomal microarray, amniocentesis indicates if the results affect pregnancy decision-making. Consulting with concerned subspecialists is ideally recommended. In cases of lethal anomalies, post-natal palliative care should also be a topic of discussion. The in-utero intervention of amniotic band lysis can potentially slow the progression of constriction effects and restore normal flow to the downstream structures. There is a hypothesis that fetal limb recovery is more likely after fetal intervention than postnatal recovery because of the plasticity of tissue healing during fetal life. Although fetoscopic intervention may restore blood flow and save the limb, plastic surgery may still be necessary after birth. However, the efficacy of this intervention is unknown, as there are no set criteria for the selection of candidates and a lack of clinical studies.[6] There are no standard guidelines for the management of pregnancy complicated with fetal ABS. Follow-up, ultrasound, and intervention are individually tailored and depend on the severity of ABS complications.[7] Postnatal management includes a thorough physical exam and, if needed, imaging studies to clearly describe the extent of ABS. Relief of constriction rings postnatally may help salvage some limb function by relieving venous pressure. Vascular comprise diagnosed postnatally may require urgent surgical intervention.(B2)

Differential Diagnosis

Differential diagnosis of amniotic bands divides into 2 main groups.

1. Uterus and placental anomalies:

• Synechiae and septa—Uterine synechiae are intrauterine adhesions typically resulting from a previous intrauterine surgical site or infection. Uterine septa are congenital malformations. Synechiae and septa do not restrict fetal movement and are not associated with fetal anomalies like ABS. 
• Residual gestational sac - A residual gestational sac left after the death of the fetus in case of twin pregnancy. 
• Circumvallate placenta - Circumvallate placenta refers to a placenta with an unusually small chorionic plate but with the growth of extracranial placental tissue, which also does not restrict fetal movement and does not cause ABS.

     2. Limb Amputations - there are other causes of congenital absence of limbs:

• Syndromes and teratogens—Numerous syndromes have associated congenital limb anomalies. An example is absent thumb and radius syndrome. Some of the limb anomalies could also be due to teratogenic substances.

Prognosis

The prognosis of a newborn affected by ABS depends on the extent of the defects, from minor cosmetic defects to the lethal involvement of vital organs. 

Complications

ABS carries correlations with miscarriages, preterm, and stillbirths.