The most basic description of Moebius syndrome is a congenital facial palsy with impairment of ocular abduction. The facial nerve (cranial nerve VII) and abducens nerve (CN VI) are most frequently involved, but other cranial nerves may be involved as well. Other variable features include orofacial dysmorphism and limb malformations. Mental retardation has been reported in a subset of patients. Most cases of Moebius syndrome are sporadic, but familial occurrence has been reported (Verzijl et al., 2003). The definition of and diagnostic criteria for Moebius syndrome have been controversial and problematic. The syndrome has most frequently been confused with hereditary congenital facial paresis (HCFP; see 601471), which is restricted to involvement of the facial nerve and no other abnormalities. Verzijl et al. (2003) and Verzijl et al. (2005) concluded that HCFP and Moebius syndrome are distinct disorders, and that Moebius syndrome is a complex developmental disorder of the brainstem. Moebius syndrome was defined at the Moebius Syndrome Foundation Research Conference in 2007 as congenital, nonprogressive facial weakness with limited abduction of one or both eyes. Additional features can include hearing loss and other cranial nerve dysfunction, as well as motor, orofacial, musculoskeletal, neurodevelopmental, and social problems (summary by Webb et al., 2012). Kumar (1990) provided a review of Moebius syndrome, which was critiqued by Lipson et al. (1990). Briegel (2006) provided a review of Moebius sequence with special emphasis on neuropsychiatric findings.

Acromesomelic dysplasia-2A (AMD2A), or Grebe chondrodysplasia, is an autosomal recessive disorder characterized by severe abnormality of the limbs and limb joints. The severity of limb shortening progresses in a proximal-distal gradient, with the hands and feet being most affected. The fingers and toes lack articulation and appear as skin appendages. In contrast, axial skeletal structures and the craniofacial skeleton are not affected. Heterozygous individuals are of average stature and have mild skeletal abnormalities (summary by Thomas et al., 1997). Because Grebe syndrome exhibits increasing severity in a proximal-distal gradient, it is classified as a form of acromesomelic dysplasia (Costa et al., 1998). For discussion of the genetic heterogeneity of acromesomelic dysplasia, see AMD1 (602875).

The Al-Awadi/Raas-Rothschild/Schinzel phocomelia syndrome (AARRS) is a rare autosomal recessive disorder characterized by severe malformations of upper and lower limbs with severely hypoplastic pelvis and abnormal genitalia. The disorder is believed to represent a defect of dorsoventral patterning and outgrowth of limbs (summary by Kantaputra et al., 2010). Overlapping limb reduction syndromes, less severe in nature, that are also caused by homozygous mutation in the WNT7A gene include Fuhrmann syndrome (228930), characterized by fibular aplasia or hypoplasia, femoral bowing, and poly-, syn-, and oligodactyly, and Santos syndrome (228930), characterized by fibular agenesis/hypoplasia, oligodactylous clubfeet, and anonychia/nail hypoplasia. Al-Qattan et al. (2013) stated that AARRS and Fuhrmann syndrome can be differentiated by the following features, which are seen only in AARRS: complete aplasia of 1 or both lower limbs, and absent elbow with radiohumeral synostosis. In addition, the number of digits per hand is 1 to 3 in AARRS, whereas there are 4 to 5 digits in Fuhrmann syndrome. 'Phocomelia' refers to an intercalary limb defect with the hand or foot being directly attached to the humerus or femur (absent zeugopod) or directly attached to the trunk (absent stylopod and zeugopod). AlQattan et al. (2013) stated that the limb defect observed in Schinzel phocomelia syndrome represents 'true' phocomelia, whereas the limb defect in AARRS is an 'apparent' phocomelia, in which there is absent ulna with radiohumeral synostosis. The authors described 3 radiologic features that define 'apparent' phocomelia: a single arm/forearm bone that appears too long to be the humerus alone; a thicker cortex at the area of the radiohumeral synostosis, with or without slight angulation at the site of synostosis; and the apparently single bone resembling the humerus proximally and the radius distally. The authors also noted that phocomelia is not a feature of the allelic disorder Fuhrmann syndrome (228930). Other distinguishing features of Schinzel phocomelia syndrome include normal nails and dorsal hand skin; distoproximal gradient of lower limb defects, without a resultant stick-like appearance; and a characteristic large cranial defect. AlQattan et al. (2013) concluded that Schinzel phocomelia syndrome and AARRS are distinct phenotypes.

The TP63-related disorders comprise six overlapping phenotypes: Ankyloblepharon-ectodermal defects-cleft lip/palate (AEC) syndrome (which includes Rapp-Hodgkin syndrome). Acro-dermo-ungual-lacrimal-tooth (ADULT) syndrome. Ectrodactyly, ectodermal dysplasia, cleft lip/palate syndrome 3 (EEC3). Limb-mammary syndrome. Split-hand/foot malformation type 4 (SHFM4). Isolated cleft lip/cleft palate (orofacial cleft 8). Individuals typically have varying combinations of ectodermal dysplasia (hypohidrosis, nail dysplasia, sparse hair, tooth abnormalities), cleft lip/palate, split-hand/foot malformation/syndactyly, lacrimal duct obstruction, hypopigmentation, hypoplastic breasts and/or nipples, and hypospadias. Findings associated with a single phenotype include ankyloblepharon filiforme adnatum (tissue strands that completely or partially fuse the upper and lower eyelids), skin erosions especially on the scalp associated with areas of scarring, and alopecia, trismus, and excessive freckling.

Fuhrmann syndrome is an autosomal recessive limb reduction disorder characterized by severe bowing of the femora and aplasia or hypoplasia of the fibulae and ulnae. The radius may be shortened and bowed. Patients also exhibit variable poly- and/or oligodactyly, including absence or coalescence of tarsal bones, absence of various metatarsals, hypoplasia and aplasia of toes, clinodactyly, hypoplasia of fingers and fingernails, and postaxial polydactyly. Hypoplasia of the pelvis and congenital dislocation of the hip have also been observed (Fuhrmann et al., 1980; Pfeiffer et al., 1988). Overlapping limb reduction syndromes that are also caused by homozygous mutation in the WNT7A gene include Al-Awadi/Raas-Rothschild syndrome (AARRS; 276820), consisting of absence of ulna and fibula with severe limb deficiency, and Santos syndrome (228930), consisting of fibular agenesis/hypoplasia, oligodactylous clubfeet, and anonychia/nail hypoplasia. Al-Qattan et al. (2013) stated that AARRS and Fuhrmann syndrome can be differentiated by the following features, which are seen only in AARRS: complete aplasia of 1 or both lower limbs, and absent elbow with radiohumeral synostosis. In addition, the number of digits per hand is 1 to 3 in AARRS, whereas there are 4 to 5 digits in Fuhrmann syndrome. AlQattan et al. (2013) also noted that phocomelia is not a feature of Fuhrmann syndrome.

ROR2-related Robinow syndrome is characterized by distinctive craniofacial features, skeletal abnormalities, and other anomalies. Craniofacial features include macrocephaly, broad prominent forehead, low-set ears, ocular hypertelorism, prominent eyes, midface hypoplasia, short upturned nose with depressed nasal bridge and flared nostrils, large and triangular mouth with exposed incisors and upper gums, gum hypertrophy, misaligned teeth, ankyloglossia, and micrognathia. Skeletal abnormalities include short stature, mesomelic or acromesomelic limb shortening, hemivertebrae with fusion of thoracic vertebrae, and brachydactyly. Other common features include micropenis with or without cryptorchidism in males and reduced clitoral size and hypoplasia of the labia majora in females, renal tract abnormalities, and nail hypoplasia or dystrophy. The disorder is recognizable at birth or in early childhood.