Syndactyly

► [Osseous and/or cutaneous fusion between two adjacent digits]

Syndactyly is a relatively common differentiation defect, with an estimated incidence ranging from 1 in 2,000 to 1 in 3,000 births (Temtamy and McKusick 1978). The defect is twice as frequent in males as in females, and is 10 times as common in Whites as in Blacks. Fusion may be limited to the soft tissues between two adjacent digits (simple, or cutaneous, syn-

Fused 4th And 5th Digit
Fig. 6.27. Simple syndactyly in a 1-year-old male baby. Soft tissue fusion occurs at the distal ends of digits 4 and 5

dactyly) (Fig. 6.27), or it may involve the bones, soft tissues, and neurovascular structures (complex syndactyly) (Fig. 6.28). Syndactyly is termed partial when it involves the proximal segments of the digits, and complete when it extends towards the tips for the full length of the digits. Fusion involving only the distal portions of the digits is sometimes termed acrosyndactyly.

Five phenotypically different types of hand syn-dactyly, with or without foot involvement, have been recognized (Temtamy and McKusick 1978). All types are inherited as autosomal dominant traits, and uniformity of type is recognized within a given pedigree. These genetic types of syndactyly should be differentiated from syndactyly associated with congenital constricting bands, a nonmendelian condition. In the common type I syndactyly (zygodactyly, OMIM 185900),there is usually complete or partial webbing, e.g., cutaneous fusion, between the 3rd and 4th fingers, sometimes associated with bone fusion at the level of their distal phalanges. In the feet, syndactyly is usually between the 2nd and 3rd toes (Fig. 6.29 a,b). The hands and feet may be involved either in isolation or in combination. The isolated forms include, in order of declining frequency, syndactyly of toes 2 and 3, syndactyly of fingers 3 and 4, and syndactyly of toes 4 and 5. The overall incidence of type I syndactyly (without other associated limb anomalies, Poland complex, or amniotic bands) has been estimated at 3 per 10,000 newborn infants (Castilla et al. 1980). Linkage analysis has provided evidence that the syndactyly type I locus is situated on 2q34-q36 (Bosse et al. 2000). In type II syndactyly (synpoly-dactyly, OMIM 186000) there is usually syndactyly of

Syndactyly

Fig. 6.28. Complex syndactyly in a woman patient. Fusion between digits 4 and 5 involves both soft tissues and bones. Syndactyly in this case is termed 'partial,' since it involves only the proximal portions of the digits (proximal phalanges). Additional findings include proximal deficiency of the 4th metacarpal and carpal coalition between lunate-triquetrum

Fig. 6.29 a, b. Type I syndactyly in a male infant. Note complete (a left hand) and partial (b right hand) webbing between fingers 3 and 4. Soft tissue fusion between the 2nd and 3rd toes was a further manifestation in this child

Fig. 6.28. Complex syndactyly in a woman patient. Fusion between digits 4 and 5 involves both soft tissues and bones. Syndactyly in this case is termed 'partial,' since it involves only the proximal portions of the digits (proximal phalanges). Additional findings include proximal deficiency of the 4th metacarpal and carpal coalition between lunate-triquetrum the 3rd and 4th fingers, associated with duplication of finger 3 or 4 in the web (Fig. 6.30 a-c). The feet usually show syndactyly of the 4th and 5th toes, with duplication of the 5th toe in the web. Aplasia/hy-poplasia of the middle phalanges of the toes can be an associated finding. This phenotype is caused by mutation in the HOXD13 gene, which maps to 2q31-q32 (Sarfarazi et al. 1995; Muragaki et al. 1996). An excess of affected males has been a consistent feature in several kindreds (Thomsen 1927; Cross et al. 1968). Instances of male-to-male transmission have been described (Camera et al. 1995). Significant variability in the clinical expression of the defect within the same family is recognized, with individuals presenting typical features of synpolydactyly, others exhibiting both pre- and postaxial polydactyly or postaxial polydactyly type A (OMIM 174200), and still others manifesting a severe phenotype consistent with homozygosity (Akarsu et al. 1995). The clinical manifestations of the homozygous phenotype include very small hands and feet with short digits, complete soft tissue syndactyly involving all four limbs, complex polydactyly, distortion of the tubular bones in the hands and feet, and abnormal carpo-

tarsal bones (Akarsu et al. 1995). In type III syndactyly (ring and little finger syndactyly, OMIM 186100) there is usually complete and bilateral soft tissue syndactyly between the 4th and 5th fingers. Occasionally, osseous fusion of the distal phalanges occurs. A short, rudimentary or absent 5th middle phalanx is part of the phenotype. The feet are not involved. The

Zygodactyly Genetics

Fig. 6.30 a-c. Type II syndactyly (synpoly-dactyly). a In a 19-year-old female. Distal syndactyly between fingers 3 and 4 with attempted duplication of digit 4. b, c In a 16-year-old girl. Note soft tissue syndactyly between fingers 3 and 4, with duplication of the 4th digit in the web. The extra-digit is only partially formed and is fused with the 4th (b, left hand). On the right hand (c) a more complex malformation is apparent: forked 3rd metacarpal with duplication of the 3rd finger, and proximal and distal synostosis of the extra digit with the 4th. The feet were also affected

Fig. 6.30 a-c. Type II syndactyly (synpoly-dactyly). a In a 19-year-old female. Distal syndactyly between fingers 3 and 4 with attempted duplication of digit 4. b, c In a 16-year-old girl. Note soft tissue syndactyly between fingers 3 and 4, with duplication of the 4th digit in the web. The extra-digit is only partially formed and is fused with the 4th (b, left hand). On the right hand (c) a more complex malformation is apparent: forked 3rd metacarpal with duplication of the 3rd finger, and proximal and distal synostosis of the extra digit with the 4th. The feet were also affected

Incomplete Syndactyly

coexistence of syndactyly type III and spastic paraplegia in the same family over multiple generations has raised the possibility that these two genes are linked (Opjordsmoen et al. 1980). There is linkage evidence that isolated type III syndactyly is determined by a mutation of the gene in 6q22-q24, where the gene for oculodentodigital syndrome (OMIM 164200) was assigned (Gladwin et al. 1997). The trait is inherited as an autosomal dominant, with instances of male-to-male transmission (de Smet et al. 1994). In type IV syndactyly (Haas polysyndactyly, OMIM 186200) there is complete cutaneous syndactyly of all the fingers in both hands, associated with pre- or postaxial hexadactyly (the extra digit being fully developed, with complete metacarpal duplication). Flexion of the fingers may give the hands a peculiar cup-shaped form (Haas 1940). Unlike Apert syndrome, syndactyly type IV involves no bone fusion. Partial cutaneous syndactyly of toes 2 and 3 is possible (Gillessen-Kaesbach and Majewski 1991). Syndactyly type IV with hexadactyly of the feet may be a different and more complex entity in which other lower limb malformations, such as tibial aplasia, can be part of the clinical spectrum (Rambaud-Cousson et al. 1991). In the rare type V syndactyly (OMIM 186300), soft tissue syndactyly occurs in association with metacarpal and metatarsal synostosis (Kemp and Ravn 1932). Soft tissue syndactyly usually affects the 3rd and 4th fingers and the 2nd and 3rd toes, but can be more extensive. The metacarpals and metatarsals most commonly involved are the 4th and 5th (Robinow et al. 1982).

In addition to the isolated familial cases described above, syndactyly has been associated with a large

Fig. 6.31. Apert syndrome. Note soft tissue and osseous syndactyly with a mitten appearance of the hand. (From Offiah et al. 2003)

number of miscellaneous anomalies and with well-defined malformation syndromes. Syndactyly is common in the acrocephalo(poly)syndactyly syndromes, in association with peculiar craniofacial abnormalities. In Apert syndrome (acrocephalosyndactyly type I, OMIM 101200), multiple progressive synostoses involving the distal phalanges (most commonly of the 3rd and 4th fingers) and proximal ends of the metacarpals (4th and 5th) of both hands are typical (Fig. 6.31). With osseous fusion of fingers 2nd to 4th, a single nail may be seen for the central bony mass. Carpal fusion (capitate-hamate), progressive symphalangism, and a peculiar thumb configuration (short and broad distal phalanx, with radial deviation; short, delta-shaped proximal phalanx) are additional findings. Cutaneous syndactyly of the 2nd to 5th fingers and toes is common. Manifestations in the feet include progressive tarsal fusion (cuneiforms mostly involved), toe symphalangism, and short great toe with varus deformity. As a rule, in other types of acrocephalosyndactyly both hand and skull changes are mild. In Saethre-Chotzen syndrome (acrocephalosyndactyly type III, OMIM 101400), partial cutaneous syndactyly of fingers 2 and 3 and toes 3 and 4 is typical. The thumb is normal. In Pfeiffer syndrome (acrocephalosyndactyly type V, OMIM 101600), an autosomal recessive, there is wide variability of the phenotypic expression, with changes ranging from mild to the severity of those approaching the findings in Apert syndrome. In the full-blown syndrome the striking feature is a broad, short thumb and big toe, with triangular/trapezoid proxi mal phalanx resulting in varus deformity. About 10% of patients with syndactyly of the hand have Poland syndrome (Poland anomaly, OMIM 173800), a condition with unilateral aplasia of the sternal head of the pectoralis major muscle, and varying degrees of symbrachydactyly, sometimes associated with more severe reduction deficiency in the ipsilateral upper limb. Although this syndrome is overwhelmingly sporadic and the recurrence risk is negligible, there are several reports of familial occurrence and par-ents-to-child transmission (Fuhrmann et al. 1971; David 1982; David and Winter 1985). It has been suggested that Poland syndrome, Klippel-Feil anomaly (OMIM 148900), Möbius syndrome (OMIM 157900), and Sprengel anomaly (OMIM 184400) are the result of diminished blood flow in the subclavian artery, vertebral artery and/or their branches during the 6th week of fetal development. Thus, Poland anomaly can be regarded as a causally nonspecific developmental field defect in which the pattern of defects depends on the specific area of diminished blood supply.

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