European Journal of Human Genetics (2012) 20, 817–824 & 2012 Macmillan Publishers Limited All rights reserved 1018-4813/12 www.nature.com/ejhg REVIEW Syndactyly: phenotypes, genetics and current classification Sajid Malik*,1 Syndactyly is one of the most common hereditary limb malformations depicting the fusion of certain fingers and/or toes. It may occur as an isolated entity or a component of more than 300 syndromic anomalies. Syndactylies exhibit great inter- and intra-familial clinical variability. Even within a subject, phenotype can be unilateral or bilateral and symmetrical or asymmetrical. At least nine non-syndromic syndactylies with additional sub-types have been characterized. Most of the syndactyly types are inherited as autosomal dominant but two autosomal recessive and an X-linked recessive entity have also been described. Whereas the underlying genes/mutations for types II-1, III, IV, V, and VII have been worked out, the etiology and molecular basis of the other syndactyly types remain unknown. In this communication, based on an overview of well- characterized isolated syndactylies, their cardinal phenotypes, inheritance patterns, and clinical and genetic heterogeneities, a ‘current classification scheme’ is presented. Despite considerable progress in the understanding of syndactyly at clinical and molecular levels, fundamental questions regarding the disturbed developmental mechanisms leading to fused digits, remain to be answered. European Journal of Human Genetics (2012) 20, 817–824; doi:10.1038/ejhg.2012.14; published online 15 February 2012 Keywords: syndactyly; syndactyly classification; clinical heterogeneity; genetic heterogeneity; webbed digits INTRODUCTION penetrances. On the other hand, autosomal recessive syndactylies are Syndactyly (Greek Syn together; Dactylos digit) is a digital malfor- clinically more severe with rather consistent phenotypes. ¼ ¼ mation in which adjacent fingers and/or toes are webbed because they In this communication, first a review of classification schemes fail to separate during limb development. It is one of the most proposed by various authors for syndactyly is given. Then, the ‘current common hereditary limb malformations depicting a prevalence classification’ is presented, which is a revised and extended version of of 3–10 in 10 000 births, though higher estimates ranging from the scheme put forward by Temtamy and McKusick.3 It is supported 10–40/10 000 have been reported.1,2 by snapshots of hallmark features as well as examples of clinical and Clinically syndactyly is one of the most heterogeneous develop- genetic heterogeneity of each syndactyly. Finally, the key questions mental deformities known in the medical literature. A number of in syndactyly research, which remain to be answered, have been combinations are possible in which the adjacent fingers and/or toes highlighted. remain connected by a web. It may be unilateral or bilateral, and symmetrical or asymmetrical. Furthermore, inter- and intra-familial SYNDACTYLY: APPRECIATION AND DEVELOPMENT OF phenotypic variability is quite common. The condition is so variable CLASSIFICATION that the same individual may exhibit asymmetrical phenotypes in the Syndactyly appears in the medical literature under several synonyms. upper and lower, and right and left limbs. Syndactyly can be identified For example, adherent fingers,4 fingers coated with common skin,5 as partial or complete, cutaneous or bony, and involving only the coherence of fingers,6 fingers grown together,3,5 fingers knit together,4 phalanges or further extending up to metacarpal/metatarsal or carpal/ skin fusion, digits in stocking,7 fingers stuck together,8 symphalanginae, tarsal levels, sometimes even proximating the distal end of forearm/ symphalangus syndactylous,9 syndactylia, syndactylous ossification,9 foreleg. On the minimal extreme, a milder phenotype may only be webbed toes,10 and zygodactyly.11 The earliest appreciation of syndac- recognized by the alterations in interphalangeal creases and peculia- tyly as a birth anomaly or burn-trauma can be traced back to a famous rities in dermatoglyphics.3 Andalusian surgeon in the middle ages named Al-Zahrawi Abulcasis Syndactyly may segregate as an isolated clinical phenotype. There (936–1013).12,13 Ambroise Pare (1510–1590) in the sixteenth century are at least nine well-characterized syndactylous entities with subdivi- described syndactyly as fingers stuck together and polydactyly as sions, the majority of which have non-syndromic nature. Most of superfluous fingers, respectively.8,4 Thus, it was established quite these entities segregate in Mendelian dominant fashion. However, two early that webbed fingers are not infrequent, appear in various autosomal recessive and an X-linked recessive type have also been forms, usually without the involvement of other organ systems but described. Generally, autosomal dominant phenotypes are rather less frequently witnessed with extra digits (Bell4 and references therein). severe and demonstrate widely variable expressivity and incomplete Minor webbing types could be easily overlooked; however, severe types 1Human Genetics Program, Department of Animal Sciences, Quaid-i-Azam University Islamabad, Islamabad, Pakistan *Correspondence: Dr S Malik, Human Genetics Program, Department of Animal Sciences, Quaid-i-Azam University Islamabad, 45320 Islamabad, Pakistan. Tel: +92 51 9064 3158; Fax: +92 51 260 1176; E-mail: [email protected] Received 7 October 2011; revised 3 January 2012; accepted 6 January 2012; published online 15 February 2012 Syndactyly: phenotypes, genetics and classification S Malik 818 Table 1 Classification schemes proposed for syndactylies Author Syndactyly type identified Description Anatomical approach Roblot (1906)14 Complete vs partial Based on the extent of webbing; also observed syndromic vs non-syndromic Weidenreich (1923)11 Zygodactyly Identified two types for 2/3 toes webbing; partial vs complete; and common vs rare type Bell (1953)4 A1, A2, B1, C; and subgroups Each type is unique but combination of various types are possible Kelikian (1974)15 One to eight categories Considers cutaneous/bony fusion, number of involved digits, and other digital insults Woolf and Cone (1977)16 Division of type I (Ia, Ib) Type Ia shows 2/3 toes webbing; type Ib has 2/3 toes; and 3/4 fingers involvement Lenz and Majewski (1981)17 Syndactyly type Ia Separated Lueken type from syndactyly type I Descriptive and embryological classification Swanson (1976)18 Simple vs complicated Based on failure of differentiation (separation of digits) Winter and Tickle (1993)19 Pre-, meso-, post-axial, and total Based on normal/abnormal patterns and secondary limb modeling during development; syndactyly types preaxial, mesoaxial, postaxial, and total syndactyly types Stoll et al (1998)20 Radial, central, ulnar, complex types A descriptive system to classify limb defects Clinical, genetic and molecular approach Temtamy and McKusick (1978)3 Five types (I—V) Based on the combination of fused digits and inheritance pattern Goldstein et al (1994)21 Eight types (I—VIII) Extension of Temtamy-McKusick, types VI–VIII were introduced Malik et al (2004, 2005)22,23 Nine types (I—IX) Extension of Temtamy-McKusick, recessive type IX syndactyly was introduced Harpf et al (2005)24 Subgroups in type VII Identified ‘spoon hand type’ and ‘oligodactyly type’ within type VII Malik et al (2005)25 Syndactyly type I extended Four subdivisions proposed for type I syndactyly (I-1 to I-4) Malik et al (2006)26 Syndactyly type II extended Splitting type II syndactyly (SPD1, SPD2, SPD3) Malik and Grzeschik (2008)27 Clinical variants in Type II Identified typical features, minor variants, and unusual phenotypes in type II required surgical corrections. As the number of reports regarding Clinical and genetic approach the deformity grew into the medical and anthropological records, it Temtamy and McKusick3 proposed a classification essentially based on permitted a systematic evaluation of various types. Hence, several the phenotypic presentation (ie, site and nature of digit involvement) attempts were made to classify webbing of digits which, depending as well as the pattern of disease segregation in large families. They upon the approach taken, fall into three categories (summary in identified five discrete and isolated syndactylies, in addition to a few Table 1): unclassified types (Table 1). Since then it has been the most widely used scheme by the geneticists and clinicians. Simple anatomical classification systems The classical approach has been a simple anatomical categorization CURRENT SYNDACTYLY CLASSIFICATION: CLINICAL, depending upon the digits within the web, number of digits involved, GENETIC, AND MOLECULAR APPROACH and also, the extent of webbing. In this context, the systems intro- The Temtamy-McKusick classification has been a well-appreciated duced by Roblot,14 Weidenreich,11 Bell,4 Kelikian,15 and Lenz and scheme for a number of reasons. First, the majority of the reported Majewski17 are worth mentioning (Table 1). Of particular interest is the isolated syndactyly phenotypes in families/subjects can be easily classification system proposed by Bell4 who identified four major types typed with this scheme.28 Second, novel syndactyly phenotypes (A1, A2, B1, and C). However, there were types with overlapping features could be easily accommodated by extending the existing classification and still others that remained unclassified (reviewed in Malik et al28). system.21–23 Third, subgroups could be introduced with equal flexibility as fresh