► [Short middle phalanges]
The skeletal phenotype seen as short middle phalanges has been called type A brachydactyly by Bell (1951). According to the distribution of the defect in the hand, several types have been recognized. These various types, however, are not always distinct entities, and large intrafamilial variability is seen. Brachydactyly syndrome, type A1 (OMIM 112500) is characterized by rudimentary or absent middle phalanges of all digits in the hands and feet and by shortening of the proximal phalanx of the thumb and big toe. Usually, the 2nd and 5th fingers are involved to a greater extent than the others. Synostosis between rudimentary middle phalanges and distal phalanges is common. The phenotype displays a certain degree of intra- and interfamilial variability. For example, shortening of the proximal phalanges also may occur, thus overlapping with the phenotype of brachydactyly type C (OMIM 113100,see next section). Furthermore, severe cases of type A1 brachydactyly can be associated with shortening of the metacarpals, notably the 4th and 5th. Accessory carpal bones in the distal row have also been reported (Hoefnagel and Gerald 1966). Stiff thumb, short stature, and mental retardation have been observed in subjects with this phenotype (Haws and McKusick 1963; Piussan et al. 1983). Brachydactyly type A1 has been shown to be caused by mutations in the Indian Hedgehog (IHH) gene, which is located on chromosome 2 (Yang et al. 2000; Gao et al. 2001). Linkage to chromosome 5 has also been found in a family with mild type A1 brachydactyly and short stature but no other clinical features (this type has been termed A1B) (Armour et al. 2002). Unilateral short middle phalanges in association with syndactyly are seen in the Poland syndrome. Type A1 brachydactyly also occurs in some syndromes, notably Carpenter syndrome (acro-cephalopolysyndactyly type II, OMIM 201000), atelosteogenesis type 2 (OMIM 256050), tricho-rhi-no-phalangeal syndrome type 2 (OMIM 150230), and Smith-Lemli-Opitz syndrome (OMIM 270400) (Fig. 6.5). In the rare brachydactyly syndrome type A2 (OMIM 112600), shortening is confined to the middle phalanx of the index finger and the 2nd toe, all other digits being more or less normal (Temtamy and McKusick 1978; Rasore-Quartino and Camera 1977). The hypoplastic phalanx can be rhomboid or triangular, resulting in radial deviation of the finger. The epiphysis is often lacking, perhaps as a result of early fusion (Lawrence et al. 1989). The defect is in-
Fig. 6.5. Brachymesophalangy type A1 in a 3-year-old boy with Smith-Lemli-Opitz syndrome. Note hypoplasia of the middle phalanges, most prominent in the 2nd and 5th fingers, with clinodactyly of these two fingers. There is a pseudoepiphysis at the base of the 2nd metacarpal. Carpal ossification is retarded
Fig. 6.5. Brachymesophalangy type A1 in a 3-year-old boy with Smith-Lemli-Opitz syndrome. Note hypoplasia of the middle phalanges, most prominent in the 2nd and 5th fingers, with clinodactyly of these two fingers. There is a pseudoepiphysis at the base of the 2nd metacarpal. Carpal ossification is retarded herited as an autosomal dominant trait with high penetrance and variable expressivity. Type A2 bra-chydactyly has been observed in association with microcephaly (OMIM 211369) (Graham 1989) and seems to occur consistently with sclerosteosis (OMIM 269500). A phenotype reminiscent of type A2 brachy-dactyly is seen in the autosomal recessive du Pan syndrome (fibula aplasia and complex brachydactyly, OMIM 228900). In this condition a trapezoid middle phalanx of the index finger with radial deviation is associated with shortening of various metacarpals, small carpals, and bilateral absence of the fibula with tibiotarsal dislocation (du Pan 1924). Bell's type A3 brachydactyly (brachymesophalangy V, OMIM 112700) consists in shortening of the middle phalanx of the 5th finger. This defect is very common, occurring in 0.5-24% of the general population (Poznanski 1984; Sugiura et al. 1962; Garn et al. 1967) and is usually inherited as a mendelian dominant trait with 50-60% penetrance (Temtamy 1966). As in the case of the index finger, the rhomboid or triangular shape of the rudimentary phalanx often results in radial deviation (clinodactyly) (Fig. 6.6). The finger can also be straight, however. Cone-shaped epiphyses at the 5th middle phalanx with early union are com-
mon associated findings. An association with short stature is also well documented (Garn et al. 1972). The condition shows a characteristic sex and racial distribution, being more common in females, Mongoloids and American Indians than in males, Whites and Blacks (Hertzog 1967). Several syndromes are associated with shortened 5th middle phalanx, including Down syndrome (OMIM 190685), oto-palato-digital syndrome, type I (OMIM 311300), Treacher-Collins syndrome (OMIM 154500), de Lange syndrome (OMIM 122470), Goltz-Gorlin syndrome
(OMIM 305600), and Holt-Oram syndrome (OMIM 142900), among others. The uncommon type A4 brachydactyly (brachymesophalangy II, OMIM 112800) features brachymesophalangy affecting mainly the 2nd and 5th digits (Fig. 6.7). In a pedigree studied by Temtamy and McKusick (1978) occasional involvement of the 4th digit was associated with radial deviation of the distal phalanx owing to the abnormal shape of the short middle phalanx. Furthermore, absence of the middle phalanges in the lateral four toes was found. As in brachydactyly types A1 and A3 an association with short stature has been described (Ohzeki et al. 1993). In brachydactyly type A5 (OMIM 112900) absence of the middle phalanges occurs in association with nail dysplasia (Bass 1968; Cuevas-Sosa and Garcia-Segur 1971). The terminal phalanx of the thumb is duplicated. Male-to-male transmission has been observed. Brachydactyly type A6 (Ose-bold-Remondini syndrome, OMIM 112910) is an association of hypoplastic or absent middle phalanges in the hands and feet, mesomelic limb shortening, mildly short stature, and normal intelligence. Additional features include radial deviation of the terminal phalanges of the index fingers, capitate-hamate fusion, and delayed coalescence of bipartite calcanei in infancy. As in brachydactyly type A5, male-to-male transmission has been described (Osebold et al. 1985; Opitz and Gilbert 1985).
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