Amniotic band syndrome (ABS) is an uncommon, congenital fetal abnormality with multiple disfiguring and disabling manifestations. A wide spectrum of clinical deformities are encountered and range from simple ring constrictions to major head, face and internal organ defects. Lower extremity limb malformations are extremely common and consist of asymmetric digital ring constrictions, distal atrophy, congenital intrauterine amputations, and clubfoot. Although debated, early amnion rupture with subsequent entanglement of fetal parts (mostly limbs and appendages) by amniotic strands is the primary theory of pathogenesis.
Amniotic band syndrome is associated with an excessive number of synonyms and acronyms such as congenital constriction band syndrome, Streeter's dysplasia, Simonart's bands, amniotic band disruption complex, congenital annular defects, congenital ring constrictions, ADAM (Amniotic Deformity, Adhesion, Mutilations) complex, TEARS (The Early Amnion Rupture Spectrum) of defects, and fetal disruption complex. The overabundance of synonyms/acronyms used to describe the congenital malformations in ABS attest partly to the confusion surrounding its etiology.
Current literature supports the theory that early amniotic rupture leads to the formation of fibrous strands that entangle limbs and appendages. This sequence affects the development of the embryo and leads to the findings seen in ABS. Therefore, nearly all cases reach their final form before birth with tissue damage healing before birth. The nature and severity of deformities that result appears to be related to the timing and initiating event of amniotic rupture.
Clinical Findings
Amniotic band syndrome is a complex collection of asymmetric congenital anomalies, in which no two cases are exactly alike. However, several characteristic features are relatively consistent findings. Distal ring constrictions and intrauterine amputations, are the most common findings of ABS and are typically seen in the distal aspect of extremities. Multiple extremity involvement is usually expressed with an average of three extremity parts affected.
Incidence
Amniotic band syndrome is not a rare anomaly as first described more than a century and a half ago and appears to be rising. Once believed to have an incidence of 1:100,000, recent literature supports the incidence today as 1:1200 to 1:1500 births. No distinct sex predilection has been determined. Nearly sixty percent of the cases documented have some sort of abnormal gestation history. Prenatal risk factors associated with amniotic band syndrome include prematurity (<37 weeks), low birth weight (<2500 g), maternal illness (during pregnancy), maternal drug exposure and maternal hemorrhage/trauma. Attempted abortion in the first trimester is also a highly associated finding.
Family history seldom reveals any direct inheritance pattern, since the syndrome occurs in no particular association with know genetic or chromosomal disorders. Karotypes are virtually always normal, and the syndrome is almost always sporadic in nature. Infants of young, black women who have been pregnant more than once (<20 years, more than one pregnancy) show the highest prevalence. incidences of malformations seen in the hand are two times as common as accompanied foot deformities.
Pathogenesis
Many theories concerning the pathogenesis of ABS have been proposed and debated. Furthermore, no single pathogenic conclusion has been determined to reconcile the diverse findings seen in ABS. However, much of the controversy still centers around a fundamental question: Is ABS a primary event of an intrinsic (endogenous) fetal anomaly or a condition extrinsic (exogenous) to the fetus that is secondarily involved?
Although a number of experimental models have been developed to reproduce its occurrence, no unified theory exists. It is easy to believe the amniotic band theory proposed when the majority of pregnancies (60%) demonstrate an abnormal pregnancy history. However, this does not explain the remaining 40% occurrence of ABS in mothers with no abnormal prenatal history. One may never reconcile the differences in findings seen in ABS into a single pathogenic scheme. Perhaps, ABS is composed of a combination of causes as stated above.
Characteristic Features
Amniotic band syndrome is a poorly defined clinical entity, owing partly to its debated causes and large number of different names. However, it is routinely characterized by distinct fetal malformations, which should make its diagnosis unmistakable.
The most common triad of clinical manifestations include congenital distal ring constrictions, intrauterine amputations, and acrosyndactyly. These anomalies appear most frequently in the distal aspect of extremities without other organ involvement. It has been found that ABS is the most common cause of a terminal congenital malformation of a limb.
Additional abnormalities encountered routinely with ABS include webbing of the fingers or toes, progressive lymphedema (swelling), clubfoot, stunted growth of the small bones in the fingers and toes and limb length discrepancy. Less common findings include: pseudarthrosis, metatarsus adductus, peripheral nerve palsy, dystrophic nails, postnatal gangrene, cleft lip and palate, skin-tube pedicles, dislocated hip visceral body wall malformations and eccentric craniofacial synostosis defects. Owing to the fact that no two cases are exactly alike, only some of these above-mentioned defects are present in each individual case. Other congenital brain abnormalities, cardiac malformations, short statue, spina bifida, and added miscellaneous conditions reported in the literature probably represent coincidental findings. Fetal death associated with amniotic band strangulation of the umbilical cord has also been reported.
Amniotic bands are more likely to constrict, entangle or amputate fingers or toes that protrude the furthest. In the hand, digital amputations most commonly involve the index, middle, and ring fingers, whereas in the foot, amputations of the big toe are most often noted. Congenital band indentations are usually at multiple levels with or without distal lymphedema (swelling). However, these fibrous band indentations are noted encircling the fingers or toes of the newborn child most frequently. Bands are of variable width and depth, ranging from shallow indentations of the skin and subcutaneous tissue to deep grooves extending down to deep fascia or bone. Proximal bands occur more commonly in the lower extremity and have been associated with neural compression. Osseous abnormalities occurring at or below band indentation such a bony fusion, angular deformities, and discontinuity have infrequently been reported.
If ring constriction is severe, the veins, arteries, lymphatics, and nerves may be comprised. However, vascular insufficiency is seldom symptomatic. Neurological impairment is usually attributed to axontmesis or neurotmesis. This may be caused by direct pressure from the constriction band or attributable in compartment syndrome distal to the band in infants with rapidly progressive swelling.
Distal digits are typically malformed, owing to phalageal hypoplasia or terminal amputation. Acrosyndactyly (fenestrated syndactyly) is frequently associated with distal amputation. This type of syndactyly involves the binding of adjacent digits in a “lassoed” appearance. If acrosyndactyly is present, it invariably is associated with a proximal interdigital sinus or cleft that communicates from dorsal to plantar. The cutaneious syndactyly seen usually does not involve underlying bony fusion.
A strong relationship between ABS and clubfoot exists. A 31.5% incidence of associated clubfoot deformity and ABS can be correlated with 20% occurring bilaterally. In the majority of cases, the clubfoot deformity present is inordinately rigid and unresponsive to conservative modalities.
Limb length discrepancy has also been noted in legs encircled by amniotic bands with an average functional deficit of greater than 2.5 cm. This often results in biomechanical abnormalities and altered gait patterns.
Diagnosis
Ultrasonographic analysis allows for the detection of ABS prenatally by visualization of amniotic sheets or bands attached to the fetus. In the first trimester, it is extremely difficult to detect ABS, especially if the bands are limited to the extremities. However, in the second and third trimester of pregnancy, it is relatively easy to detect the major anomalies of ABS by its characteristic features and restrictions of motion. When characteristic asymmetric fetal anomalies are observed ultrasonographically, regardless of the presence or absence of fibrous membranes, ABS should be considered.
Although visualization of amniotic bands on ultrasonography is helpful in confirming ABS, it is not in itself a diagnostic feature of the ABS. Recently, amniotic sheets or bands have been described as aberrant sheets of tissue, often amnion and chorion, having a free edge within the amniotic fluid. The free edge does not attach to the fetus and have been labeled “innocent amniotic sheet”. These sheets do not restrict fetal motion nor cause any fetal abnormalities.
Additionally, elevated maternal serum alpha-fetoprotein (MASFP) has been associated with ABS. However, elevated MSAFP is not diagnostic for ABS, since it is also elevated in neural tube defects, placental chorioangioma, and congenital nephrosis. MSAFP is now a standard screening test recommended for all pregnancies, though rarely elevated.
Treatment
Several treatment options are available for the lower extremity manifestations of ABS. Shallow grooves or bands require no operative treatment, unless they interfere with circulation or lymphatic drainage. Deep constriction bands often present with swelling distal to the band, extreme pain, and diminished circulation and must be surgically released immediately to prevent risk of gangrene or auto-amputation. Surgical excision of the fibrous band and any necrotic tissue with circumferential Z-plasty or W-plasty are the procedures most commonly employed. Removal of the constriction band may be accomplished in a one or two stage release, usually beginning at three months of age. Early surgical intervention is necessary to prevent progressive lymphedema. Patients treated late in the course of the syndrome often heal very slowly with secondary eczematous skin changes to distal parts.
When deemed necessary, subcutaneous fat and fascial flaps are advanced into the defect to prevent reoccurrence of the deformity. If edema persists after correction of the band, excision of the edematous area (debulking) may be necessary, with direct closure or conversion of the overlying skin to thick, free partial thickness skin grafts. Gross motor and sensory deficits distal to the bands resulting in neuropathy with secondary ulceration and osteomyelitis (bone infection) are best treated with amputation and fitting of a prosthesis. It is fairly uncommon that the underlying bones to be affected, however, if they are then cresentic osteotomies may be indicated.
Desyndactylizations may also need to be performed in conjunction with skin grafting of hypoplastic bones. Due to the high incidence of associated rigid clubfoot deformity, aggressive surgical correction is often required. Limb length discrepancies greater than 6 cm may require leg-lengthening procedures.
Overall, the goal of pedal care is to create a functional foot and to minimize additional problems as the child grows. Parental counseling is recommended to convey there is no known associated risk for subsequent pregnancies.
Conclusion
Amniotic band syndrome is an uncommon fetal malformation with increasing prevalence. It is a constellation of congenital anomalies that lacks a precise definition and satisfactory pathogenic explanation. Multiple asymmetrical limb, craniofacial, visceral, and body wall defects are commonplace. Although a myriad of fetal deformities can ensue, manifestations in the foot such as distal ring constrictions, intrauterine pedal amputations, acrosyndactyly, and clubfoot are encountered routinely. With the aid of ultrasonography, a prenatal diagnosis of ABS can be made by serial observations demonstrating restriction of fetal movement. Appropriate surgical intervention can eliminate potential limp threatening constrictions and provide a remarkably improved quality of life for these patients.
