Syndactyly

Two or more digits fused together, either by skin only (cutaneous, simple) or by bone (osseous, complex). Syndactyly is one of the most common limb anomalies and ranges from a benign familial cutaneous web between fingers 3 and 4 to the diagnostic mitten hand of Apert syndrome. The bedside task is to characterize the fusion, decide whether it is isolated or syndromic, and identify the small set of high-yield syndromes whose syndactyly is essentially diagnostic.

Three questions drive the work-up:

1. Which digits are fused? The classic syndrome maps are specific (Apert: digits 2-3-4 of hand into a mitten; Smith-Lemli-Opitz: toes 2-3).
2. Simple (cutaneous) or complex (osseous)? Simple syndactyly is more often isolated; complex syndactyly raises suspicion for syndromic disease.
3. Hands, feet, or both? Unilateral or bilateral? Bilateral symmetric involvement of both hands and feet suggests a defined syndrome; unilateral hand-and-chest involvement suggests Poland (vascular disruption, not genetic).

• Simple cutaneous: soft-tissue web only, distal phalanges separate. Most common; often isolated.
• Complex osseous: bones of adjacent digits share or fuse; surgical reconstruction more involved.
• Complete vs incomplete: complete extends to the tips; incomplete spares the distal portion.
• Complicated: extra bones or accessory phalanges between fused digits.

Isolated cutaneous syndactyly

• Familial syndactyly, AD (often GJA1, LMBR1, HOXD13; multiple loci): cutaneous web between fingers 3 and 4 (most common) or toes 2 and 3. Bilateral, symmetric, no other anomalies, positive family history. Surgical release is cosmetic/functional.
• Poland sequence: unilateral cutaneous syndactyly + ipsilateral pectoralis major hypoplasia or absence + brachydactyly + chest wall deformity. Sporadic; thought to be vascular disruption of the subclavian artery in utero. (No condition leaf to link.)

Syndactyly + craniosynostosis (the FGFR group)

• Apert syndrome (FGFR2): the prototypic syndactyly syndrome. Bilateral symmetric bony syndactyly of digits 2-3-4 of the hand into a single mitten, with the thumb often free or partially involved. Feet show similar fusion. Combined with bicoronal craniosynostosis and midface hypoplasia.
• Pfeiffer syndrome (FGFR1, FGFR2): craniosynostosis + broad medially deviated thumbs and great toes + variable mild cutaneous syndactyly.
• Saethre-Chotzen syndrome (TWIST1): coronal synostosis + ptosis + low frontal hairline + mild cutaneous 2-3 finger syndactyly.
• Carpenter syndrome (RAB23, AR): craniosynostosis + postaxial polydactyly + preaxial polysyndactyly of feet + obesity + cardiac defects + ID. (No condition leaf to link.)

Syndactyly with other distinctive features

• Smith-Lemli-Opitz syndrome (DHCR7, AR; defective 7-dehydrocholesterol reductase): 2-3 toe syndactyly is the classic tell, combined with microcephaly + ptosis + anteverted nares + cleft palate + hypospadias + low cholesterol + elevated 7-DHC. The toe syndactyly is so characteristic that any dysmorphic child with it should have a cholesterol and 7-DHC sent.
• Greig cephalopolysyndactyly syndrome (GLI3, AD): preaxial and postaxial polydactyly + cutaneous syndactyly + macrocephaly + frontal bossing + hypertelorism.
• Fraser syndrome (FRAS1, FREM2, GRIP1, AR): cryptophthalmos (skin covering the eyes) + cutaneous syndactyly + laryngeal stenosis + renal agenesis + ambiguous genitalia. (No condition leaf to link.)
• Oculodentodigital dysplasia (GJA1, AD): syndactyly type III (4-5 finger) + microphthalmia + small teeth with enamel hypoplasia + thin nose. (No condition leaf to link.)
• Trisomy 21: clinodactyly more common than syndactyly, but 2-3 toe syndactyly seen.

The work-up follows the syndactyly pattern and what else is on exam.

1. Detailed limb exam: which digits, which limbs, simple vs complex, bilateral vs unilateral, position of thumb and great toe.
2. Radiographs of hands and feet: distinguish cutaneous from osseous fusion; surgical planning.
3. Three-generation family history: AD familial syndactyly is common; ask specifically about "webbed fingers" in adults.
4. Targeted exams: head shape (craniosynostosis), eyes (cryptophthalmos in Fraser, ptosis in SLO and Saethre-Chotzen), genitalia (hypospadias in SLO), pectoralis (Poland).
5. Cholesterol and 7-dehydrocholesterol if Smith-Lemli-Opitz features (especially 2-3 toe syndactyly + dysmorphic).
6. Echocardiogram: Carpenter, Fraser, broader syndromic work-ups.
7. Renal ultrasound: Fraser, syndromic syndactyly with multiple anomalies.
8. CT head with 3D reconstruction if craniosynostosis suspected.
9. Targeted gene testing: FGFR2 for Apert/Pfeiffer/Crouzon, TWIST1 for Saethre-Chotzen, GLI3 for Greig, DHCR7 for SLO, RAB23 for Carpenter.
10. Chromosomal microarray for syndromic syndactyly with multiple anomalies.
11. Surgical release is staged in early childhood, prioritizing the thumb-index web for grip function.