MedGen

Down syndrome is a chromosomal condition that is associated with intellectual disability, a characteristic facial appearance, and weak muscle tone (hypotonia) in infancy. All affected individuals experience cognitive delays, but the intellectual disability is usually mild to moderate.

People with Down syndrome often have a characteristic facial appearance that includes a flattened appearance to the face, outside corners of the eyes that point upward (upslanting palpebral fissures), small ears, a short neck, and a tongue that tends to stick out of the mouth. Affected individuals may have a variety of birth defects. Many people with Down syndrome have small hands and feet and a single crease across the palms of the hands. About half of all affected children are born with a heart defect. Digestive abnormalities, such as a blockage of the intestine, are less common.

Individuals with Down syndrome have an increased risk of developing several medical conditions. These include gastroesophageal reflux, which is a backflow of acidic stomach contents into the esophagus, and celiac disease, which is an intolerance of a wheat protein called gluten. About 15 percent of people with Down syndrome have an underactive thyroid gland (hypothyroidism). The thyroid gland is a butterfly-shaped organ in the lower neck that produces hormones. Individuals with Down syndrome also have an increased risk of hearing and vision problems. Additionally, a small percentage of children with Down syndrome develop cancer of blood-forming cells (leukemia).

Delayed development and behavioral problems are often reported in children with Down syndrome. Affected individuals can have growth problems and their speech and language develop later and more slowly than in children without Down syndrome. Additionally, speech may be difficult to understand in individuals with Down syndrome. Behavioral issues can include attention problems, obsessive/compulsive behavior, and stubbornness or tantrums. A small percentage of people with Down syndrome are also diagnosed with developmental conditions called autism spectrum disorders, which affect communication and social interaction.

People with Down syndrome often experience a gradual decline in thinking ability (cognition) as they age, usually starting around age 50. Down syndrome is also associated with an increased risk of developing Alzheimer's disease, a brain disorder that results in a gradual loss of memory, judgment, and ability to function. Approximately half of adults with Down syndrome develop Alzheimer's disease. Although Alzheimer's disease is usually a disorder that occurs in older adults, people with Down syndrome commonly develop this condition earlier, in their fifties or sixties. [from MedlinePlus Genetics]

Noonan syndrome (NS) is characterized by characteristic facies, short stature, congenital heart defect, and developmental delay of variable degree. Other findings can include broad or webbed neck, unusual chest shape with superior pectus carinatum and inferior pectus excavatum, cryptorchidism, varied coagulation defects, lymphatic dysplasias, and ocular abnormalities. Although birth length is usually normal, final adult height approaches the lower limit of normal. Congenital heart disease occurs in 50%-80% of individuals. Pulmonary valve stenosis, often with dysplasia, is the most common heart defect and is found in 20%-50% of individuals. Hypertrophic cardiomyopathy, found in 20%-30% of individuals, may be present at birth or develop in infancy or childhood. Other structural defects include atrial and ventricular septal defects, branch pulmonary artery stenosis, and tetralogy of Fallot. Up to one fourth of affected individuals have mild intellectual disability, and language impairments in general are more common in NS than in the general population. [from GeneReviews]

Prader-Willi syndrome (PWS) is characterized by severe hypotonia, poor appetite, and feeding difficulties in early infancy, followed in early childhood by excessive eating and gradual development of morbid obesity (unless food intake is strictly controlled). Motor milestones and language development are delayed. All individuals have some degree of cognitive impairment. Hypogonadism is present in both males and females and manifests as genital hypoplasia, incomplete pubertal development, and, in most, infertility. Short stature is common (if not treated with growth hormone). A distinctive behavioral phenotype (temper tantrums, stubbornness, manipulative behavior, and obsessive-compulsive characteristics) is common. Characteristic facial features, strabismus, and scoliosis are often present. [from GeneReviews]

Isolated gonadotropin-releasing hormone (GnRH) deficiency (IGD) is characterized by inappropriately low serum concentrations of the gonadotropins LH (luteinizing hormone) and FSH (follicle-stimulating hormone) in the presence of low circulating concentrations of sex steroids. IGD is associated with a normal sense of smell (normosmic IGD) in approximately 40% of affected individuals and an impaired sense of smell (Kallmann syndrome) in approximately 60%. IGD can first become apparent in infancy, adolescence, or adulthood. Infant boys with congenital IGD often have micropenis and cryptorchidism. Adolescents and adults with IGD have clinical evidence of hypogonadism and incomplete sexual maturation on physical examination. Adult males with IGD tend to have prepubertal testicular volume (i.e., <4 mL), absence of secondary sexual features (e.g., facial and axillary hair growth, deepening of the voice), decreased muscle mass, diminished libido, erectile dysfunction, and infertility. Adult females have little or no breast development and primary amenorrhea. Although skeletal maturation is delayed, the rate of linear growth is usually normal except for the absence of a distinct pubertal growth spurt. [from GeneReviews]

Cornelia de Lange syndrome (CdLS) encompasses a spectrum of findings from mild to severe. Severe (classic) CdLS is characterized by distinctive facial features, growth restriction (prenatal onset; <5th centile throughout life), hypertrichosis, and upper-limb reduction defects that range from subtle phalangeal abnormalities to oligodactyly (missing digits). Craniofacial features include synophrys, highly arched and/or thick eyebrows, long eyelashes, short nasal bridge with anteverted nares, small widely spaced teeth, and microcephaly. Individuals with a milder phenotype have less severe growth, cognitive, and limb involvement, but often have facial features consistent with CdLS. Across the CdLS spectrum IQ ranges from below 30 to 102 (mean: 53). Many individuals demonstrate autistic and self-destructive tendencies. Other frequent findings include cardiac septal defects, gastrointestinal dysfunction, hearing loss, myopia, and cryptorchidism or hypoplastic genitalia. [from GeneReviews]

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. [from GeneReviews]

Meckel syndrome, also known as Meckel-Gruber syndrome, is a severe pleiotropic autosomal recessive developmental disorder caused by dysfunction of primary cilia during early embryogenesis. There is extensive clinical variability and controversy as to the minimum diagnostic criteria. Early reports, including that of Opitz and Howe (1969) and Wright et al. (1994), stated that the classic triad of Meckel syndrome comprises (1) cystic renal disease; (2) a central nervous system malformation, most commonly occipital encephalocele; and (3) polydactyly, most often postaxial. However, based on a study of 67 patients, Salonen (1984) concluded that the minimum diagnostic criteria are (1) cystic renal disease; (2) CNS malformation, and (3) hepatic abnormalities, including portal fibrosis or ductal proliferation. In a review of Meckel syndrome, Logan et al. (2011) stated that the classic triad first described by Meckel (1822) included occipital encephalocele, cystic kidneys, and fibrotic changes to the liver. Genetic Heterogeneity of Meckel Syndrome See also MKS2 (603194), caused by mutation in the TMEM216 gene (613277) on chromosome 11q12; MKS3 (607361), caused by mutation in the TMEM67 gene (609884) on chromosome 8q; MKS4 (611134), caused by mutation in the CEP290 gene (610142) on chromosome 12q; MKS5 (611561), caused by mutation in the RPGRIP1L gene (610937) on chromosome 16q12; MKS6 (612284), caused by mutation in the CC2D2A gene (612013) on chromosome 4p15; MKS7 (267010), caused by mutation in the NPHP3 (608002) gene on chromosome 3q22; MKS8 (613885), caused by mutation in the TCTN2 gene (613846) on chromosome 12q24; MKS9 (614209), caused by mutation in the B9D1 gene (614144) on chromosome 17p11; MKS10 (614175), caused by mutation in the B9D2 gene (611951) on chromosome 19q13; MKS11 (615397), caused by mutation in the TMEM231 gene (614949) on chromosome 16q23; MKS12 (616258), caused by mutation in the KIF14 gene (611279) on chromosome 1q32; MKS13 (617562), caused by mutation in the TMEM107 gene (616183) on chromosome 17p13; and MKS14 (619879), caused by mutation in the TXNDC15 gene (617778) on chromosome 5q31. [from OMIM]

Muenke syndrome is characterized by considerable phenotypic variability; features may include coronal synostosis (more often bilateral than unilateral); synostosis of other sutures, all sutures (pan synostosis), or no sutures; or macrocephaly. Bilateral coronal synostosis typically results in brachycephaly, although turribrachycephaly (a "tower-shaped" skull) or a cloverleaf skull can be observed. Unilateral coronal synostosis results in anterior plagiocephaly. Other craniofacial findings typically include temporal bossing, widely spaced eyes, ptosis or mild proptosis, mild midface retrusion, and highly arched palate or cleft lip and palate. Strabismus is common. Other findings can include hearing loss, developmental delay, intellectual disability, behavioral issues, intracranial anomalies, epilepsy, ocular anomalies, brachydactyly, carpal and/or tarsal bone fusions, broad thumbs and great toes, clinodactyly, and radiographic findings of short and broad middle phalanges and/or cone-shaped epiphyses. Of note, some individuals who have the p.Pro250Arg pathogenic variant may have no signs of Muenke syndrome on physical or radiographic examination. [from GeneReviews]

FLNA deficiency is associated with a phenotypic spectrum that includes FLNA-related periventricular nodular heterotopia (Huttenlocher syndrome), congenital heart disease (patent ductus arteriosus, atrial and ventricular septal defects), valvular dystrophy, dilatation and rupture of the thoracic aorta, pulmonary disease (pulmonary hypertension, alveolar hypoplasia, emphysema, asthma, chronic bronchitis), gastrointestinal dysmotility and obstruction, joint hypermobility, and macrothrombocytopenia. [from GeneReviews]

Oral-facial-digital syndrome type I (OFD1) is usually male lethal during gestation and predominantly affects females. OFD1 is characterized by the following: oral features (lobulated tongue, tongue nodules, cleft of the hard or soft palate, accessory gingival frenulae, hypodontia, and other dental abnormalities); facial features (widely spaced eyes, telecanthus, hypoplasia of the alae nasi, median cleft or pseudocleft of the upper lip, micrognathia); digital features (brachydactyly, syndactyly, clinodactyly of the fifth finger, duplicated great toe); polycystic kidney disease; brain MRI findings (intracerebral cysts, agenesis of the corpus callosum, cerebellar agenesis with or without Dandy-Walker malformation); and intellectual disability (in approximately 50% of affected individuals). [from GeneReviews]

Alpha-thalassemia X-linked intellectual disability (ATR-X) syndrome is characterized by distinctive craniofacial features, genital anomalies, hypotonia, and mild-to-profound developmental delay / intellectual disability (DD/ID). Craniofacial abnormalities include small head circumference, telecanthus or widely spaced eyes, short triangular nose, tented upper lip, and thick or everted lower lip with coarsening of the facial features over time. While all affected individuals have a normal 46,XY karyotype, genital anomalies comprise a range from hypospadias and undescended testicles, to severe hypospadias and ambiguous genitalia, to normal-appearing female external genitalia. Alpha-thalassemia, observed in about 75% of affected individuals, is mild and typically does not require treatment. Osteosarcoma has been reported in a few males with germline pathogenic variants. [from GeneReviews]

MED12-related disorders include the phenotypes of FG syndrome type 1 (FGS1), Lujan syndrome (LS), X-linked Ohdo syndrome (XLOS), Hardikar syndrome (HS), and nonspecific intellectual disability (NSID). FGS1 and LS share the clinical findings of cognitive impairment, hypotonia, and abnormalities of the corpus callosum. FGS1 is further characterized by absolute or relative macrocephaly, tall forehead, downslanted palpebral fissures, small and simple ears, constipation and/or anal anomalies, broad thumbs and halluces, and characteristic behavior. LS is further characterized by large head, tall thin body habitus, long thin face, prominent nasal bridge, high narrow palate, and short philtrum. Carrier females in families with FGS1 and LS are typically unaffected. XLOS is characterized by intellectual disability, blepharophimosis, and facial coarsening. HS has been described in females with cleft lip and/or cleft palate, biliary and liver anomalies, intestinal malrotation, pigmentary retinopathy, and coarctation of the aorta. Developmental and cognitive concerns have not been reported in females with HS. Pathogenic variants in MED12 have been reported in an increasing number of males and females with NSID, with affected individuals often having clinical features identified in other MED12-related disorders. [from GeneReviews]

Joubert syndrome is a clinically and genetically heterogeneous group of disorders characterized by hypoplasia of the cerebellar vermis with the characteristic neuroradiologic 'molar tooth sign,' and accompanying neurologic symptoms, including dysregulation of breathing pattern and developmental delay. Other variable features include retinal dystrophy and renal anomalies (Saraiva and Baraitser, 1992; Valente et al., 2005). Genetic Heterogeneity of Joubert Syndrome See also JBTS2 (608091), caused by mutation in the TMEM216 gene (613277) on chromosome 11q13; JBTS3 (608629), caused by mutation in the AHI1 gene (608894) on chromosome 6q23; JBTS4 (609583), caused by mutation in the NPHP1 gene (607100) on chromosome 2q13; JBTS5 (610188), caused by mutation in the CEP290 gene, also called NPHP6 (610142), on chromosome 12q21; JBTS6 (610688), caused by mutation in the TMEM67 gene (609884) on chromosome 8q21; JBTS7 (611560), caused by mutation in the RPGRIP1L gene (610937) on chromosome 16q12; JBTS8 (612291), caused by mutation in the ARL13B (608922) on chromosome 3q11; JBTS9 (612285), caused by mutation in the CC2D2A gene (612013) on chromosome 4p15; JBTS10 (300804), caused by mutation in the CXORF5 gene (300170) on chromosome Xp22; JBTS11 (see 613820), caused by mutation in the TTC21B gene (612014) on chromosome 2q24; JBTS12 (see 200990), caused by mutation in the KIF7 gene (611254) on chromosome 15q26; JBTS13 (614173), caused by mutation in the TCTN1 gene (609863) on chromosome 12q24; JBTS14 (614424), caused by mutation in the TMEM237 gene (614423) on chromosome 2q33; JBTS15 (614464), caused by mutation in the CEP41 gene (610523) on chromosome 7q32; JBTS16 (614465), caused by mutation in the TMEM138 gene (614459) on chromosome 11q; JBTS17 (614615), caused by mutation in the CPLANE1 gene (614571) on chromosome 5p13; JBTS18 (614815), caused by mutation in the TCTN3 gene (613847) on chromosome 10q24; JBTS19 (see 614844), caused by mutation in the ZNF423 gene (604577) on chromosome 16q12; JBTS20 (614970), caused by mutation in the TMEM231 gene (614949) on chromosome 16q23; JBTS21 (615636), caused by mutation in the CSPP1 gene (611654) on chromosome 8q13; JBTS22 (615665), caused by mutation in the PDE6D gene (602676) on chromosome 2q37; JBTS23 (616490), caused by mutation in the KIAA0586 gene (610178) on chromosome 14q23; JBTS24 (616654), caused by mutation in the TCTN2 gene (613846) on chromosome 12q24; JBTS25 (616781), caused by mutation in the CEP104 gene (616690) on chromosome 1p36; JBTS26 (616784), caused by mutation in the KATNIP gene (616650) on chromosome 16p12; JBTS27 (617120), caused by mutation in the B9D1 gene (614144) on chromosome 17p11; JBTS28 (617121), caused by mutation in the MKS1 gene (609883) on chromosome 17q23; JBTS29 (see 617562), caused by mutation in the TMEM107 gene (616183) on chromosome 17p13; JBTS30 (617622), caused by mutation in the ARMC9 gene (617612) on chromosome 2q37; JBTS31 (617761), caused by mutation in the CEP120 gene (613446) on chromosome 5q23; JBTS32 (617757), caused by mutation in the SUFU gene (607035) on chromosome 10q24; JBTS33 (617767), caused by mutation in the PIBF1 gene (607532) on chromosome 13q21; JBTS34 (see 614175), caused by mutation in the B9D2 gene (611951) on chromosome 19q13; JBTS35 (618161), caused by mutation in the ARL3 gene (604695) on chromosome 10q24; JBTS36 (618763), caused by mutation in the FAM149B1 gene (618413) on chromosome 10q22; JBTS37 (619185), caused by mutation in the TOGARAM1 gene (617618) on chromosome 14q21; JBTS38 (619476), caused by mutation in the KIAA0753 gene (617112) on chromosome 17p13; JBTS39 (619562), caused by mutation in the TMEM218 gene (619285) on chromosome 11q24; and JBTS40 (619582), caused by mutation in the IFT74 gene (608040) on chromosome 9p21. [from OMIM]

Alpha-thalassemia X-linked intellectual disability (ATR-X) syndrome is characterized by distinctive craniofacial features, genital anomalies, hypotonia, and mild-to-profound developmental delay / intellectual disability (DD/ID). Craniofacial abnormalities include small head circumference, telecanthus or widely spaced eyes, short triangular nose, tented upper lip, and thick or everted lower lip with coarsening of the facial features over time. While all affected individuals have a normal 46,XY karyotype, genital anomalies comprise a range from hypospadias and undescended testicles, to severe hypospadias and ambiguous genitalia, to normal-appearing female external genitalia. Alpha-thalassemia, observed in about 75% of affected individuals, is mild and typically does not require treatment. Osteosarcoma has been reported in a few males with germline pathogenic variants. [from GeneReviews]

Oculodentodigital dysplasia (ODDD) is characterized by a typical facial appearance and variable involvement of the eyes, dentition, and fingers. Characteristic facial features include a narrow, pinched nose with hypoplastic alae nasi, prominent columella and thin anteverted nares together with a narrow nasal bridge, and prominent epicanthic folds giving the impression of hypertelorism. The teeth are usually small and carious. Typical eye findings include microphthalmia and microcornea. The characteristic digital malformation is complete syndactyly of the fourth and fifth fingers (syndactyly type III) but the third finger may be involved and associated camptodactyly is a common finding (summary by Judisch et al., 1979). Neurologic abnormalities are sometimes associated (Gutmann et al., 1991), and lymphedema has been reported in some patients with ODDD (Brice et al., 2013). See review by De Bock et al. (2013). Genetic Heterogeneity of Oculodentodigital Syndrome An autosomal recessive form of ODDD (257850) is also caused by mutation in the GJA1 gene, but the majority of cases are autosomal dominant. [from OMIM]

EZH2-related overgrowth is a variable overgrowth syndrome characterized by tall stature, macrocephaly, variable intellect (ranging from normal intellect to severe intellectual disability), characteristic facial appearance, and a range of associated clinical features including advanced bone age, poor coordination, soft, doughy skin, camptodactyly of the fingers and/or toes, umbilical hernia, abnormal tone, and hoarse, low cry in infancy. Brain MRI has identified abnormalities in a few individuals with EZH2-related overgrowth. Neuroblastoma occurs at a slightly increased frequency in individuals with a heterozygous EZH2 pathogenic variant, but data are insufficient to determine absolute risk. There is currently no evidence that additional malignancies (including hematologic malignancies) occur with increased frequency, though a few have been reported. [from GeneReviews]

Pitt-Hopkins syndrome (PTHS) is characterized by significant developmental delays with moderate-to-severe intellectual disability and behavioral differences, characteristic facial features, and episodic hyperventilation and/or breath-holding while awake. Speech is significantly delayed and most individuals are nonverbal with receptive language often stronger than expressive language. Other common findings are autism spectrum disorder symptoms, sleep disturbance, stereotypic hand movements, seizures, constipation, and severe myopia. [from GeneReviews]

Fanconi anemia (FA) is characterized by physical abnormalities, bone marrow failure, and increased risk for malignancy. Physical abnormalities, present in approximately 75% of affected individuals, include one or more of the following: short stature, abnormal skin pigmentation, skeletal malformations of the upper and/or lower limbs, microcephaly, and ophthalmic and genitourinary tract anomalies. Progressive bone marrow failure with pancytopenia typically presents in the first decade, often initially with thrombocytopenia or leukopenia. The incidence of acute myeloid leukemia is 13% by age 50 years. Solid tumors – particularly of the head and neck, skin, and genitourinary tract – are more common in individuals with FA. [from GeneReviews]

Cranioectodermal dysplasia (CED) is a ciliopathy with skeletal involvement (narrow thorax, shortened proximal limbs, syndactyly, polydactyly, brachydactyly), ectodermal features (widely spaced hypoplastic teeth, hypodontia, sparse hair, skin laxity, abnormal nails), joint laxity, growth deficiency, and characteristic facial features (frontal bossing, low-set simple ears, high forehead, telecanthus, epicanthal folds, full cheeks, everted lower lip). Most affected children develop nephronophthisis that often leads to end-stage kidney disease in infancy or childhood, a major cause of morbidity and mortality. Hepatic fibrosis and retinal dystrophy are also observed. Dolichocephaly, often secondary to sagittal craniosynostosis, is a primary manifestation that distinguishes CED from most other ciliopathies. Brain malformations and developmental delay may also occur. [from GeneReviews]

SHORT syndrome is a mnemonic for short stature, hyperextensibility, ocular depression (deeply set eyes), Rieger anomaly, and teething delay. It is now recognized that the features most consistently observed in SHORT syndrome are mild intrauterine growth restriction (IUGR); mild to moderate short stature; partial lipodystrophy (evident in the face, and later in the chest and upper extremities, often sparing the buttocks and legs); and a characteristic facial gestalt. Insulin resistance may be evident in mid-childhood or adolescence, although diabetes mellitus typically does not develop until early adulthood. Other frequent features include Axenfeld-Rieger anomaly or related ocular anterior chamber dysgenesis, delayed dentition and other dental issues, and sensorineural hearing loss. [from GeneReviews]