Constriction ring syndrome (CRS) is a congenital disorder with unknown cause. Because of the unknown cause there are many different, and sometimes incorrect names. It is a malformation due to intrauterine bands or rings that give deep grooves in, most commonly, distal extremities like fingers and toes. In rare cases the constriction ring can form around other parts of the fetus and cause amputation or even intrauterine death. The anatomy proximal to the site of constriction (or amputation) is developmentally normal. CRS can be associated with other malformations with club foot being most common. The precise configuration of the bands, lymphedema, and character of the amputations are not predictable and vary with each individual patient. Also more than one extremity is usually affected, and it is rare for only one ring to present as an isolated malformation with no other manifestation of this syndrome. Signs and symptoms
The constriction of appendages by amniotic bands may result in:
Constriction rings around the digits, arms, and legs
Swelling of the extremities distal to the point of constriction (congenital lymphedema)
Amputation of digits, arms, and legs (congenital amputation)

Cause
There are three different theories to the cause of the constriction ring syndrome. The first is the intrinsic theory, which was proposed by Streeter in 1930, implicates an anomaly in germ plasm resulting in the defects. This theory is reinforced by the clinical presentation of the constriction rings with other internal visceral and systematic anomalies.
Differential diagnosis
The differential diagnosis includes:
Symbrachydactyly
Congenital amputations
Hypoplasias of hand, digit, thumb
Adams–Oliver syndrome
ADAM complexADAM Complex; CRS is sometimes mislabeled as ADAM complex. ADAM is an abbreviation for Amniotic Deformity, Adhesions Mutilations. CRS is the malformation due to a constriction ring around mostly a limb. ADAM-complex is the association of limb defects (caused by constriction rings) and certain craniofacial clefts"Adams–Oliver syndrome is often mislabeled as CRS and consists of cutis aplasia of the scalp in which a longitudinal defect can vary in size and can often be associated with full-thickness skullcap loss. The distal digital or toe hypoplasia-aplasia is often confused with CRS. Constriction rings with or without edema are not present. The digital or toe hypoplasia-aplasia usually contains diminutive nails or nail folds". Classification
The constriction ring syndrome is a complex collection of asymmetric congenital anomalies, in which no two cases are exactly alike. This is why a classification is difficult to make. The most widely used classification system was proposed by Patterson. This classification system is based on the severity of the syndrome and is useful because, the different types require different treatments. Other clinicians have amended this scheme by separating the depth of the ring into mild, moderate, severe, and amputation and by further defining the presence or absence of lymphedema or soft tissue loss distal to the ring.