Nager syndrome is the prototype for a group of disorders collectively referred to as the acrofacial dysostoses (AFDs), which are characterized by malformation of the craniofacial skeleton and the limbs. The major facial features of Nager syndrome include downslanted palpebral fissures, midface retrusion, and micrognathia, the latter of which often requires the placement of a tracheostomy in early childhood. Limb defects typically involve the anterior (radial) elements of the upper limbs and manifest as small or absent thumbs, triphalangeal thumbs, radial hypoplasia or aplasia, and radioulnar synostosis. Phocomelia of the upper limbs and, occasionally, lower-limb defects have also been reported. The presence of anterior upper-limb defects and the typical lack of lower-limb involvement distinguishes Nager syndrome from Miller syndrome (263750), another rare AFD; however, distinguishing Nager syndrome from other AFDs, including Miller syndrome, can be challenging (summary by Bernier et al., 2012).

NSDHL-related disorders include CHILD (congenital hemidysplasia with ichthyosiform nevus and limb defects) syndrome, an X-linked disorder that is usually male lethal during gestation and thus predominantly affects females; and CK syndrome, an X-linked disorder that affects males. CHILD syndrome is characterized by unilateral distribution of ichthyosiform skin lesions and ipsilateral limb defects that range from shortening of the metacarpals and phalanges to absence of the entire limb. Intellect is usually normal. The ichthyosiform skin lesions are usually present at birth or in the first weeks of life; new lesions can develop in later life. Onychodystrophy and periungual hyperkeratosis are common. Heart, lung, and kidney malformations can also occur. CK syndrome is characterized by mild-to-severe cognitive impairment and behavior problems (aggression, attention-deficit/hyperactivity disorder [ADHD], and irritability). All reported affected males have developed seizures in infancy and have cerebral cortical malformations and microcephaly. All have distinctive facial features, a thin habitus, and relatively long, thin fingers and toes. Some have scoliosis and kyphosis. Strabismus is common. Optic atrophy is also reported.

Jawad syndrome (JWDS) is an autosomal recessive disorder characterized by congenital microcephaly, moderately to severely impaired intellectual development, and digital malformations including phalangeal joint swelling, clinodactyly, polydactyly, syndactyly, and total absence of nails (summary by Qvist et al., 2011).

Cousin syndrome is an autosomal recessive complex cranial, cervical, auricular, and skeletal malformation syndrome with scapular and pelvic hypoplasia (summary by Lausch et al., 2008).

A very rare congenital limb malformation with characteristics of postaxial syndactyly of hands and feet, associated with metacarpal and metatarsal fusion of fourth and fifth digits. So far, less than ten reports have been described in the literature. Soft tissue syndactyly (involving the third and fourth fingers and the second and third toes) may be present. The locus associated with SD5 maps to 2q31-q32. Mutations in the HOXD13 gene may be causative. The condition is inherited as an autosomal dominant trait.

Any autosomal recessive non-syndromic intellectual disability in which the cause of the disease is a mutation in the NSUN2 gene.

Syndrome with the association of toe syndactyly, facial dysmorphism including telecanthus and a broad nasal tip, urogenital malformations and anal atresia. Around ten cases have been reported so far. The syndrome is caused by mutations in the FAM58A gene (located on the X chromosome) encoding a protein of unknown function.

Ectodermal dysplasia-syndactyly syndrome (EDSS) is characterized by sparse to absent scalp hair, eyebrows, and eyelashes, hypoplastic nails, tooth enamel hypoplasia, conical-shaped teeth, palmoplantar keratoderma, and partial cutaneous syndactyly (summary by Raza et al., 2015). Genetic Heterogeneity of Ectodermal Dysplasia-Syndactyly Syndrome Ectodermal dysplasia-syndactyly syndrome-2 (EDSS2; 613576) maps to chromosome 7p21-p14.

Classic Joubert syndrome (JS) is characterized by three primary findings: A distinctive cerebellar and brain stem malformation called the molar tooth sign (MTS). Hypotonia. Developmental delays. Often these findings are accompanied by episodic tachypnea or apnea and/or atypical eye movements. In general, the breathing abnormalities improve with age, truncal ataxia develops over time, and acquisition of gross motor milestones is delayed. Cognitive abilities are variable, ranging from severe intellectual disability to normal. Additional findings can include retinal dystrophy, renal disease, ocular colobomas, occipital encephalocele, hepatic fibrosis, polydactyly, oral hamartomas, and endocrine abnormalities. Both intra- and interfamilial variation are seen.

SALL1-related Townes-Brocks syndrome (SALL1-TBS) is characterized by the triad of imperforate anus or anal stenosis, dysplastic ears (overfolded superior helices and preauricular tags; frequently associated with sensorineural and/or conductive hearing impairment), and thumb malformations (duplication of the thumb [preaxial polydactyly], triphalangeal thumbs, and, rarely, hypoplasia of the thumbs) without hypoplasia of the radius. Impaired kidney function, including end-stage kidney disease (ESKD), may occur with or without structural abnormalities (mild malrotation, ectopia, horseshoe kidney, renal hypoplasia, polycystic kidneys, vesicoureteral reflux). Foot malformations (flat feet, overlapping toes) and genitourinary malformations are common. Congenital heart disease occurs in 15% of affected individuals. Developmental delay and/or learning difficulties occur in approximately 15% of affected individuals. Rare features include growth deficiency, Duane anomaly, iris coloboma, and Chiari I malformation.

Feingold syndrome 1 (referred to as FS1 in this GeneReview) is characterized by digital anomalies (shortening of the 2nd and 5th middle phalanx of the hand, clinodactyly of the 5th finger, syndactyly of toes 2-3 and/or 4-5, thumb hypoplasia), microcephaly, facial dysmorphism (short palpebral fissures and micrognathia), gastrointestinal atresias (primarily esophageal and/or duodenal), and mild-to-moderate learning disability.

Split-foot malformation with mesoaxial polydactyly (SFMMP) is characterized by a split-foot defect and nail abnormalities of the hands, as well as hearing loss in some patients (Spielmann et al., 2016).

Synpolydactyly (SPD), or syndactyly type II, is defined as a connection between the middle and ring fingers and fourth and fifth toes, variably associated with postaxial polydactyly in the same digits. Minor local anomalies and various metacarpal or metatarsal abnormalities may be present (summary by Merlob and Grunebaum, 1986). In some families with SPD, the foot anomalies are characterized by preaxial as well as postaxial polydactyly, and appear to be fully penetrant. The more severe features of classic SPD, involving 3/4 synpolydactyly in the hands and 4/5 synpolydactyly in the feet, also occur, but at reduced penetrance. This foot phenotype is not seen in patients with classic SPD due to HOXD13 polyalanine tract expansions (Goodman et al., 1998). Malik (2012) reviewed the syndactylies, noting that the extreme phenotypic heterogeneity observed in SPD families consists of approximately 18 clinical variants that can be 'lumped' into 3 categories: typical SPD features, minor variants, and unusual phenotypes. Genetic Heterogeneity of Synpolydactyly See also SPD2 (608180), caused by mutation in the fibulin-1 gene (FBLN1; 135820) on chromosome 22q13, and SPD3 (610234), which has been mapped to chromosome 14q11.2-q12.