Correct Answer: A. Amniotic band syndrome
Amniotic band syndrome (ABS) is the most common cause of congenital finger/limb absence in live births. The pathophysiology involves rupture of the amniotic membrane (usually in the first trimester), creating fibrous strands that entangle fetal parts. These bands cause mechanical constriction, leading to amputation of digits, limbs, or facial structures distal to the band. The key discriminating feature is that ABS produces asymmetric, random distribution of defects—a child may have absent fingers on one hand but normal digits on the other, or facial clefts combined with limb defects. This randomness reflects the unpredictable nature of amniotic rupture and strand formation. Diagnosis is clinical: presence of congenital amputations, constriction rings, or syndactyly in an otherwise normal infant. Imaging (ultrasound in utero or postnatal radiographs) may show the characteristic amputation stumps. The incidence is approximately 1 in 1,200–15,000 live births in India, making it a common differential for congenital limb defects. Management is supportive; surgical reconstruction (e.g., digit restoration, scar release) may be considered later. The prognosis is generally good because internal organs are rarely affected—the defect is purely mechanical and external.
Why the other options are wrong
B. Polyhydramnios — Polyhydramnios is an excess of amniotic fluid and is a maternal/fetal condition affecting fluid volume, not a cause of congenital digit loss. While polyhydramnios may be associated with some fetal anomalies (e.g., GI atresia, cardiac defects), it does not directly cause amputation. This is an NBE trap confusing a fluid abnormality with a structural defect. C. Congenital constriction ring — Congenital constriction rings are part of the amniotic band syndrome spectrum, not a separate diagnosis. However, they differ in presentation: rings cause circumferential constriction with lymphedema/tissue swelling distal to the ring, whereas ABS typically causes frank amputation. The question specifies 'absent finger,' which is amputation—the hallmark of ABS, not simple constriction. D. Arthrogryposis multiplex — Arthrogryposis multiplex congenita (AMC) is a syndrome of multiple joint contractures due to intrauterine immobility or neuromuscular pathology. It presents with fixed joint deformities, not digit amputation. AMC affects multiple joints symmetrically and is associated with internal anomalies; ABS is asymmetric and external-only, making it the correct answer for isolated finger absence.
High-Yield Facts
- Amniotic band syndrome is the most common cause of congenital digit/limb amputation in live births (1 in 1,200–15,000).
- Pathophysiology: rupture of amniotic membrane → fibrous strands entangle fetal parts → mechanical amputation distal to the band.
- Key feature: asymmetric, random distribution of defects (e.g., absent fingers on one hand, normal on the other).
- Congenital constriction rings are part of the ABS spectrum but cause circumferential constriction with distal edema, not frank amputation.
- Diagnosis: clinical recognition of congenital amputations, constriction rings, or syndactyly; prenatal ultrasound may show amniotic bands.
- Prognosis: generally excellent because internal organs are spared; defect is purely mechanical and external.
Mnemonics
ABS = Amputation + Band + Strands Amputation (digit/limb loss), Band (fibrous amniotic strands), Strand (mechanical entanglement). Use this when you see congenital finger/limb absence—think of the amniotic membrane rupture creating bands that 'cut off' the developing limb. Random ≠ Syndromic Amniotic band syndrome produces random, asymmetric defects (one hand affected, other normal). Syndromic conditions (arthrogryposis, congenital heart disease) are symmetric and bilateral. If the defect is lopsided, think ABS.
NBE Trap
NBE pairs "congenital constriction ring" with amniotic band syndrome to trap students who confuse the two. While rings are part of the ABS spectrum, the question specifies "absent finger"—frank amputation, not constriction—making ABS the correct answer, not the ring alone.
Clinical Pearl
In Indian pediatric practice, amniotic band syndrome is often diagnosed at birth when parents notice a missing finger or limb. Early recognition allows counseling on prognosis (excellent) and planning for surgical reconstruction (digit restoration, scar release) in the first 2–3 years of life, improving functional and cosmetic outcomes.
_Reference: OP Ghai Essentials of Pediatrics Ch. 3 (Congenital Anomalies); DC Dutta Textbook of Obstetrics Ch. 24 (Congenital Anomalies)_