Cleidocranial dysplasia (CCD) spectrum disorder is a skeletal dysplasia that represents a clinical continuum ranging from classic CCD (triad of delayed closure of the cranial sutures, hypoplastic or aplastic clavicles, and dental abnormalities), to mild CCD, to isolated dental anomalies without other skeletal features. Individuals with classic CCD spectrum disorder typically have abnormally large, wide-open fontanelles at birth that may remain open throughout life. Clavicular hypoplasia can result in narrow, sloping shoulders that can be opposed at the midline. Moderate short stature may be observed, with most affected individuals being shorter than their unaffected sibs. Dental anomalies may include delayed eruption of secondary dentition, failure to shed the primary teeth, and supernumerary teeth. Individuals with CCD spectrum disorder are at increased risk of developing recurrent sinus infections, recurrent ear infections leading to conductive hearing loss, and upper airway obstruction. Intelligence is typically normal.

Spondylolisthesis is defined as forward slipping of a vertebral body on the one below it. Spondylolysis is defined as a defect in the pars interarticularis without vertebral slipping (summary by Wiltse et al., 1975).

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.

Pycnodysostosis is characterized by short-limbed short stature, typical facial appearance (convex nasal ridge and small jaw with obtuse mandibular angle), osteosclerosis with increased bone fragility, acroosteolysis of the distal phalanges, delayed closure of the cranial sutures, and dysplasia of the clavicle. In affected individuals, the facial features become more prominent with age, likely due to progressive acroosteolysis of the facial bones, but can usually be appreciated from early childhood, particularly the small jaw and convex nasal ridge. Additional features include dental and nail anomalies. Intelligence is typically normal with mild psychomotor difficulties reported in some individuals.

Aspartylglucosaminuria is a lysosomal storage disorder characterized by developmental delay, intellectual disability, behavioral manifestations (hyperactivity in young children, anxiety and restlessness in adolescence, and apathy in adulthood), recurrent infections, musculoskeletal features, and characteristic craniofacial features (prominent supraorbital ridges, hypertelorism, periorbital fullness, short nose with broad nasal bridge, thick vermilion of the upper and lower lips, and macroglossia) that become more prominent with age. Additional neurologic manifestations can include seizures, poor balance and coordination, and progressive cerebral atrophy in adulthood. Macrocephaly is common. Musculoskeletal features include lordosis, scoliosis, and arthritis in adolescents and young adults; vertebral dysplasia and/or rib cage abnormalities; and progressive muscle wasting, joint contractures, bursitis, and osteoporosis in adulthood. Skin manifestations (facial seborrhea, rosacea, and angiofibromas), gastrointestinal manifestations, neutropenia, and thrombocytopenia occur in some individuals. The clinical manifestations of aspartylglucosaminuria worsen with age, and adults have progressive psychomotor decline.

Arthrosis, i.e., of degenerative joint disease, affecting the cervical vertebral column.

Spondyloepiphyseal dysplasia, Reardon type is an extremely rare type of spondyloepiphyseal dysplasia (see this term) described in several members of a single family to date and characterized by short stature, vertebral and femoral abnormalities, cervical instability and neurologic manifestations secondary to anomalies of the odontoid process.

The X-linked otopalatodigital (X-OPD) spectrum disorders, characterized primarily by skeletal dysplasia, include the following: Otopalatodigital syndrome type 1 (OPD1). Otopalatodigital syndrome type 2 (OPD2). Frontometaphyseal dysplasia type 1 (FMD1). Melnick-Needles syndrome (MNS). Terminal osseous dysplasia with pigmentary skin defects (TODPD). In OPD1, most manifestations are present at birth; females can present with severity similar to affected males, although some have only mild manifestations. In OPD2, females are less severely affected than related affected males. Most males with OPD2 die during the first year of life, usually from thoracic hypoplasia resulting in pulmonary insufficiency. Males who live beyond the first year of life are usually developmentally delayed and require respiratory support and assistance with feeding. In FMD1, females are less severely affected than related affected males. Males do not experience a progressive skeletal dysplasia but may have joint contractures and hand and foot malformations. Progressive scoliosis is observed in both affected males and females. In MNS, wide phenotypic variability is observed; some individuals are diagnosed in adulthood, while others require respiratory support and have reduced longevity. MNS in males results in perinatal lethality in all recorded cases. TODPD, seen only in females, is characterized by a skeletal dysplasia that is most prominent in the digits, pigmentary defects of the skin, and recurrent digital fibromata.

X-linked syndromic intellectual developmental disorder-37 (MRXS37) is a developmental disorder showing phenotypic variability and variable severity. Male mutation carriers tend to be more severely affected than female mutation carriers, some of whom may even be asymptomatic. In general, the disorder is characterized by global developmental delay with delayed walking, speech delay, impaired intellectual development that ranges from borderline low to moderate, and behavioral abnormalities, such as autism and sleeping difficulties. Many patients are able to attend mainstream schools with assistance and work under supervision. Additional more variable features include sensorineural hearing loss, ocular anomalies, feeding difficulties, dysmorphic facial features, inguinal and umbilical hernia, genitourinary defects, congenital heart defects, musculoskeletal anomalies, and endocrine dysfunction, such as hypogonadism or hyperparathyroidism (Shepherdson et al., 2024).