The FLNB disorders include a spectrum of phenotypes ranging from mild to severe. At the mild end are spondylocarpotarsal synostosis (SCT) syndrome and Larsen syndrome; at the severe end are the phenotypic continuum of atelosteogenesis types I (AOI) and III (AOIII) and Piepkorn osteochondrodysplasia (POCD). SCT syndrome is characterized by postnatal disproportionate short stature, scoliosis and lordosis, clubfeet, hearing loss, dental enamel hypoplasia, carpal and tarsal synostosis, and vertebral fusions. Larsen syndrome is characterized by congenital dislocations of the hip, knee, and elbow; clubfeet (equinovarus or equinovalgus foot deformities); scoliosis and cervical kyphosis, which can be associated with a cervical myelopathy; short, broad, spatulate distal phalanges; distinctive craniofacies (prominent forehead, depressed nasal bridge, malar flattening, and widely spaced eyes); vertebral anomalies; and supernumerary carpal and tarsal bone ossification centers. Individuals with SCT syndrome and Larsen syndrome can have midline cleft palate and hearing loss. AOI and AOIII are characterized by severe short-limbed dwarfism; dislocated hips, knees, and elbows; and clubfeet. AOI is lethal in the perinatal period. In individuals with AOIII, survival beyond the neonatal period is possible with intensive and invasive respiratory support. Piepkorn osteochondrodysplasia (POCD) is a perinatal-lethal micromelic dwarfism characterized by flipper-like limbs (polysyndactyly with complete syndactyly of all fingers and toes, hypoplastic or absent first digits, and duplicated intermediate and distal phalanges), macrobrachycephaly, prominant forehead, hypertelorism, and exophthalmos. Occasional features include cleft palate, omphalocele, and cardiac and genitourinary anomalies. The radiographic features at mid-gestation are characteristic.

ESCO2 spectrum disorder is characterized by mild-to-severe prenatal growth restriction, limb malformations (which can include bilateral symmetric tetraphocomelia or hypomelia caused by mesomelic shortening), hand anomalies (including oligodactyly, thumb aplasia or hypoplasia, and syndactyly), elbow and knee flexion contractures (involving elbows, wrists, knees, ankles, and feet [talipes equinovarus]), and craniofacial abnormalities (which can include bilateral cleft lip and/or cleft palate, micrognathia, widely spaced eyes, exophthalmos, downslanted palpebral fissures, malar flattening, and underdeveloped ala nasi), ear malformation, and corneal opacities. Intellectual disability (ranging from mild to severe) is common. Early mortality is common among severely affected pregnancies and newborns; mildly affected individuals may survive to adulthood.

Severely hypoplastic and triangular-shaped tibiae and absence of the fibulae.Two sporadic cases have been described. Moderate mesomelia of the upper limbs, proximal widening of the ulnas, pelvic anomalies and marked bilateral glenoid hypoplasia also reported.

Acromesomelic dysplasia-2B (AMD2B) is characterized by normal head and trunk, hypoplastic/dysplastic or absent fibulae, and severe hypoplastic/dysplastic hand/feet abnormalities. Mental development is normal (summary by Szczaluba et al., 2005).

The brachydactyly type C (BDC) phenotype includes brachymesophalangy of fingers 2, 3, and 5. The fourth finger is usually unaffected and thus appears as the longest finger of the hand. Shortening of metacarpal 1 and hyperphalangy in fingers 2 and 3 may occur and can be considered relatively characteristic signs. BDC can be highly variable, ranging from severely affected hands with very short fingers to mildly affected cases with only moderate brachydactyly, most often affecting the middle and proximal phalanges of fingers 2 and 3 (summary by Lehmann et al., 2006).

CHST3-related skeletal dysplasia is characterized by short stature of prenatal onset, joint dislocations (knees, hips, radial heads), clubfeet, and limitation of range of motion that can involve all large joints. Kyphosis and occasionally scoliosis with slight shortening of the trunk develop in childhood. Minor heart valve dysplasia has been described in several persons. Intellect and vision are normal.

Yuan-Harel-Lupski syndrome is a complex neurodevelopmental disorder characterized by global developmental delay and early-onset peripheral neuropathy. The disorder comprises features of both demyelinating Charcot-Marie-Tooth disease type 1A (CMT1A; 118220), which results from duplication of the PMP22 gene on 17p12, and Potocki-Lupski syndrome (PTLS; 610883), which results from duplication of a slightly proximal region on 17p11.2 that includes the RAI1 gene. These 2 loci are about 2.5 Mb apart. The resultant YUHAL phenotype may be more severe in comparison to the individual contributions of each gene, with particularly early onset of peripheral neuropathy and features of both central and peripheral nervous system involvement (summary by Yuan et al., 2015).

Harel-Yoon syndrome is a syndromic neurodevelopmental disorder characterized by delayed psychomotor development, intellectual disability, truncal hypotonia, spasticity, and peripheral neuropathy. Other more variable features such as optic atrophy may also occur. Laboratory studies in some patients show evidence of mitochondrial dysfunction (summary by Harel et al., 2016).

Ehlers-Danlos syndrome spondylodysplastic type 1 (EDSSPD1) is characterized by short stature, developmental anomalies of the forearm bones and elbow, and bowing of extremities, in addition to the classic stigmata of Ehlers-Danlos syndrome, including joint laxity, skin hyperextensibility, and poor wound healing. Significant developmental delay is not a consistent feature (Guo et al., 2013). Genetic Heterogeneity of Ehlers-Danlos Syndrome, Spondylodysplastic Type See EDSSPD2 (615349), caused by mutation in the B3GALT6 gene (615291), and EDSSPD3 (612350), caused by mutation in the SLC39A13 gene (608735).

Distal arthrogryposis type 2B2 (DA2B2) is characterized by congenital contractures of the distal limb joints and facial dysmorphism. Marked inter- and intrafamilial variability has been reported (summary by Daly et al., 2014). For a phenotypic description and a discussion of genetic heterogeneity of distal arthrogryposis, see 108120.

X-linked multiple congenital anomalies-neurodevelopmental syndrome (MCAND) is an X-linked recessive congenital multisystemic disorder characterized by poor growth, global developmental delay with impaired intellectual development, and variable abnormalities of the cardiac, skeletal, and genitourinary systems. Most affected individuals also have hypotonia and dysmorphic craniofacial features. Brain imaging typically shows enlarged ventricles and thin corpus callosum; some have microcephaly, whereas others have hydrocephalus. The severity of the disorder is highly variable, ranging from death in early infancy to survival into the second or third decade. Pathogenetically, the disorder results from disrupted gene expression and signaling during embryogenesis, thus affecting multiple systems (summary by Tripolszki et al., 2021 and Beck et al., 2021). Beck et al. (2021) referred to the disorder as LINKED syndrome (LINKage-specific deubiquitylation deficiency-induced Embryonic Defects).

Autosomal recessive spastic paraplegia-86 (SPG86) is a complex neurologic disorder characterized by global developmental delay apparent from early childhood combined with early-onset progressive spasticity mainly affecting the lower limbs, but also affecting the upper limbs. Affected individuals have hyperreflexia, extensor plantar responses, pyramidal signs, and difficulty walking or inability to walk. Some may have joint contractures and foot or ankle deformities. Patients with SPG86 have impaired intellectual development with poor or absent speech, often with behavioral abnormalities. Brain imaging shows thin corpus callosum and white matter abnormalities. Rare patients may have seizures. The disorder is thus a complicated form of SPG (summary by Yahia et al., 2021, Miyake et al., 2022). For a discussion of genetic heterogeneity of autosomal recessive spastic paraplegia, see SPG5A (270800).

Neurodevelopmental disorder with microcephaly and movement abnormalities (NEDMIM) is an autosomal recessive disorder characterized by global developmental delay, impaired intellectual development with poor or absent speech, and delayed walking with an abnormal gait. Affected individuals may show hypotonia or hypertonia with spasticity, ataxia, and choreoathetoid movements. Most patients have microcephaly and short stature. Ophthalmic features, behavioral abnormalities, and nonspecific dysmorphic features are commonly observed. Additional more variable features include seizures, brain imaging abnormalities, and skeletal defects (Serey-Gaut et al., 2023).