Down syndrome

Qu'est-ce que Down syndrome?

Down syndromeest l'une des maladies génétiques les plus courantes, et environ 1 sur 700 bébés nés aux États-Unis naissent avec cette syndrome.

Les chercheurs ne sont toujours pas en mesure d'identifier les raisons exactes pour lesquelles le chromosome supplémentaire 21 se développe, et il y a peut-être plusieurs facteurs en jeu. L'âge de la mère est considéré comme un facteur de risque important, bien que plus Down syndrome les bébés naissent de mères plus jeunes, c'est simplement parce que le taux de natalité chez les mères plus jeunes est plus élevé.

Les problèmes de santé associés à Down syndrome comprennent l'hypotonie, les maladies cardiaques et thyroïdiennes, les retards de croissance physique, les retards et troubles intellectuels légers à modérés et les traits du visage très caractéristiques.

L'hypotonie (faible tonus musculaire), le faciès plat, les fentes palpébrales inclinées vers le haut, le retard de développement et le pli palmaire unique sont parmi les plus fréquents. Down syndrome les caractéristiques.

Syndrome Synonymes :
Down syndrome

Quelles sont les causes des changements génétiques Down syndrome?

Down syndromeest une maladie génétique. Elle est causée par l'ajout d'une copie complète ou partielle du chromosome 21. Le syndrome est aussi souvent appelée trisomie 21. 95 % des personnes diagnostiquées avec Down syndrome présent avec la trisomie 21 qui est la forme la plus courante de la maladie génétique. 3 % des personnes diagnostiquées auront le type de translocation, et les 2 % restants présenteront une mosaïque Down syndrome. Il n'y a aucun élément héréditaire à la trisomie 21 et au mosaïcisme. ⅓ des cas de Down syndrome résultant d'une translocation ont une composante héréditaire, représentant environ 1 % de tous les syndrome cas.

Gènes, emplacements et modes d'héritage
PRDX2, 19p13.13
MTHFR, 1p36.22
RCAN1, 21q22.12
NTF3, 12p13.31
VIP, 6q25.2
CTCF, 16q22.1
PRDX6, 1q25.1
CALCA, 11p15.2
KANSL1, 17q21.31
DCAF7
EZH2, 7q36.1
RAD21, 8q24.11
NRAS, 1p13.2
S100B, 21q22.3
GATA1, Xp11.23
SOD1, 21q22.11
STAG2, Xq25
GSTM2, 1p13.3
TP53, 17p13.1
SLC19A1, 21q22.3

Numéro OMIM - 190685 (veuillez consulter la page OMIM pour les informations mises à jour)

Quels sont les principaux symptômes de Down syndrome?

Le principal symptômes de Down syndrome comprennent des traits du visage caractéristiques tels qu'un visage aplati, en particulier sur l'arête du nez, des yeux en forme d'amande, un cou court, de petites oreilles et un faible tonus musculaire.

Les malformations cardiaques congénitales sont un symptôme de Down syndrome, et d'autres problèmes de santé peuvent inclure la perte auditive, l'apnée obstructive du sommeil, les otites et les maladies oculaires.

Les individus avec le syndrome peut être diagnostiqué avec une déficience intellectuelle ou cognitive et des retards de développement.

Traits/caractéristiques cliniques possibles :
Phalange moyenne courte du 5e doigt, Hypothyroïdie, Petite taille, Aile iliaque hypoplasique, Épicanthus, Aplatissement malaire, Déficience auditive de transmission, Sténose duodénale, Anomalie complète du canal auriculo-ventriculaire, Microtie, Hypotonie musculaire, Trouble myéloprolifératif, Déficience intellectuelle, Macroglossie, Laxité articulaire, Fissure palpébrale inclinée, Face plate, Langue saillante, Paume courte, Sporadique, Fosse acétabulaire peu profonde, Atrésie anale, Maladie d'Alzheimer, Mégacôlon aganglionnaire, Leucémie mégacaryocytaire aiguë, Instabilité atlanto-axiale, Brachycéphalie, Paume large, Taches de broussailles, Pli nucal épaissi , Pli palmaire transversal unique

Traits/caractéristiques cliniques possibles :
Phalange moyenne courte du 5e doigt, Hypothyroïdie, Petite taille, Aile iliaque hypoplasique, Épicanthus, Aplatissement malaire, Déficience auditive de transmission, Sténose duodénale, Défaut complet du canal auriculo-ventriculaire, Microtie, Hypotonie musculaire, Trouble myéloprolifératif, Déficience intellectuelle, Macroglossie, Laxité articulaire, Fissure palpébrale inclinée, Face plate, Langue saillante, Paume courte, Sporadique, Fosse acétabulaire peu profonde, Atrésie anale, Maladie d'Alzheimer, Mégacôlon aganglionnaire, Leucémie mégacaryocytaire aiguë, Instabilité atlanto-axiale, Brachycéphalie, Paume large, Taches de broussailles, Pli nucal épaissi , Pli palmaire transversal unique

Comment quelqu'un se fait-il tester pour Down syndrome?

Dépistage et test de Down syndrome peut commencer avant la naissance. Les dépistages prénataux évaluent les chances que le fœtus ait Down syndrome tandis que les tests de diagnostic sont capables de fournir un diagnostic définitif avec un taux de précision très élevé.

Après la naissance, le diagnostic initial de Down syndrome peut commencer par un dépistage par analyse faciale, en passant par le FDNA Telehealth plateforme de télégénétique, qui permet d'identifier les marqueurs clés de la syndrome et souligner la nécessité de tests supplémentaires. Une consultation avec un conseiller génétique puis un généticien suivra. 

Sur la base de cette consultation clinique avec un généticien, les différentes options pour les tests génétiques seront partagées et le consentement sera recherché pour des tests supplémentaires.

Informations médicales sur Down syndrome

Cognitive impairment, muscle hypotonia at birth, and dysmorphic features occur to some extent in all individuals with Down syndrome (DS). In addition, various anomalies of the respiratory, cardiovascular, gastrointestinal, hematological, immune, endocrine, musculoskeletal, renal and genitourinary systems, and sensory organs, are often associated with DS, as well as psychiatric disorders (Arumugam, 2016). The syndrome is characterized by extensive phenotypic variability; most of the mentioned anomalies occur in only a fraction of affected individuals.

In 1866, John Langdon Down first described a set of characteristics of the disorder that is now referred to as DS. In 1959, Jerome Lejeune discovered that an extra copy of chromosome 21 caused the condition.

While intellectual disability is ubiquitous in DS, there is a wide range of variation in cognitive performance (Lot, 2012; Couzens et al., 2011; Tsao and Kindelberger, 2009). Weakness in language abilities has been noted in all children with DS (Dykens et al., 2006; Fidler et al., 2005; Rondal et al., 2003). But even here, there is performance improvement with age as noted in lexical store, comprehension of interpersonal relations, and visual motor processing. Socialization and competence in daily living skills appear to improve through age 30 years in DS (Dressler et al., 2010). Children with DS appear to present memory profiles that are distinct from Williams and fragile-X syndromes in that DS is characterized by good immediate visual memory and rapid phonological retrieval with poor verbal working memory skills (Conners et al., 2011; Edgin et al., 2010b).

A modern understanding of neurocognitive function in DS was established by Nadel (1986), who suggested that intellectual disabilities gradually arose in early childhood from developmental arrest of late-maturing brain structures, including the prefrontal cortex, hippocampus, and cerebellum. Incomplete development of the prefrontal cortex, hippocampus, and cerebellum has a variety of effects on DS cognition (reviewed by Fernandex et al., 2015). The impairment of the prefrontal cortex in DS precludes real-world abilities to creatively troubleshoot and overcome problems.

Consistent with the underdevelopment of the hippocampus, individuals with DS are impaired in verbal and nonverbal assessments of intermediate or long-term memory. Also in accordance with hippocampal abnormalities, teenagers and adults with DS have trouble navigating in real environments when forced to use geometric and layout information (Edgin et al., 2012).

Overt structural pathology in the cerebellum of DS patients causes difficulties with gross motor coordination; however, these difficulties are nuanced (Baxter et al., 2000; Aylward et al., 1997). Older children and adults with DS display atypical patterns of movement and problems with handwriting and other tasks that require use of fine digits (Galli et al., 2010; Latash et al., 2002).

Researchers have charted IQ declines in children with DS that begin around the time toddlers learn to walk (Fernandex et al., 2015). Cognition deteriorates to varying degrees in typically developing DS individuals as they enter advanced age, despite moderate compensation through the posterior-anterior shift (Davis et al., 2008).

Infants and toddlers born with trisomy 21 start life with deficits in mastery motivation, an intrinsic quality that compels very young children to explore and gain control over the surrounding environment (Niccols et al., 2003). Deficits in mastery motivation cripple the emergence of instrumental learning in those with DS (Fidler, 2006; Fidler et al. 2005). Infants with DS take longer than chronological age-matched controls to move from shorter chains of continuous goal-directed behaviors to longer chains and are less happy when performing more complex chain-linking (Ruskin et al., 1994; Dunst 1988). Gradually, motivational issues exacerbate the cognitive disabilities that arise from poor brain development (Cicchetti and Sroufe 1976).

Individuals with DS participate in special education or mainstream schooling. Despite efforts to the contrary, they are exposed to significant cognitive difficulties in these settings that will end in only some limited success and skills achievement. Less-than-favorable learning histories and dependence on caregivers might create the impression of diminishing returns on further educational activities. The result is seclusion from peers and negative self-perceptions that prime feelings of inadequacy and depression (Ali et al., 2012; Capone et al., 2006; Fidler et al., 2005, 2006; Dykens et al., 2002).

Children with DS are predominantly brachycephalic (62.3%), but they can also be hyperbrachycephalic (27.3%), dolichocephalic (7.8%), or mesocephalic (2.6%). In a retrospective analysis of 524 individuals with DS, more than 50% had craniofacial defects such as a downward slant of the eye lids medially (83.9%), ear anomalies (66.9%), palpebronasal (epicanthal) folds (56.9%), and a flat face (50.9%) (Kava et al., 2004). In addition, hypertelorism (increased interocular distance) and a flat nasal bridge appear to be predominant in this population (Rahul et al., 2015).

The oral dysmorphic features common among people with DS are a fissured tongue and a high arched palate (Rahul et al., 2015; Shukla et al., 2014). In addition to these two traits (each with a prevalence of 79%), a recent epidemiological survey of children with DS (n=570) in India noted other oral manifestations such as macroglossia (83%), marginal gingivitis (93%), microdontia (63%), hypodontia (41%), an anterior open bite (23%) and periodontitis (11.5%) (Rahul et al., 2015). In addition, other aberrant oral conditions such as malocclusion (between 3% and 55%), congenital absence of teeth (34%), delayed teeth eruption (10%), angular cheilitis (22%), and ankyloglossia (13%) have also been reported in children with DS (Shukla et al., 2014).

Abnormalities of the musculoskeletal system have frequently been reported in the literature and mostly relate to ligament laxity, a distinctive feature of DS. In the cervical spine region, ligament laxity can give rise to occipitocervical instability and atlantoaxial instability (AAI). About 10-35% of patients with DS are affected by AAI (Cremers et al., 1993; Roy et al., 1990; Alvarez and Rubin, 1986) but only 1-2% present with cervical myelopathy (Hankinson and Anderson, 2010; Pueschel et al., 1987; Pueschel et al., 1984). In addition, os odontoideum (separation of a portion of the dens from the body of the axis [C2]) (Semine et al., 1978), hypoplasia of the atlas (Matsunaga et al., 2007; Taggard et al., 2000), bifid atlantal arches (Menezes, 2008) or ossiculum terminale (congenital non-union of the dens from a terminal ossicle located above the transverse ligament) (Ali et al., 2006) can coexist in DS.

Other musculoskeletal abnormalities include dislocation/subluxation of the patella, deformities such as genu valgum, pes planus, metatarsus primus varus, and scoliosis (Yam et al., 2008; Diamond et al., 1981), all of which have been attributed to ligament laxity (Galli et al., 2014). In addition, brachycephaly, brachydactyly, wide hands, fifth finger clinodactyly, increased web space between the great and second toes, and short stature have been documented as possible morphological changes (Roizen and Patterson, 2003).

The most common reasons for hospitalization of children with DS are respiratory disorders (predominantly because of infection) and congenital heart malformations (Englund et al., 2013; Fitzgerald et al., 2013). Common respiratory problems include upper respiratory tract anomalies, recurrent aspiration, obstructive sleep apnea and recurrent respiratory tract infections (Pandit and Fitzgerald, 2012).

Obstructive sleep apnea is the most common respiratory disorder, occurring in 30-50% of individuals with DS (Lal et al., 2015). It can be associated with a narrowed airway, enlarged tonsils and adenoids, macroglossia, mid-face hypoplasia, delayed development of oromotor function, and micrognathia (Goffinski et al., 2015; Austeng et al., 2014). Children with DS can present with obstructive airway disease caused by macroglossia, a constricted nasopharynx, congenital subglottic stenosis, laryngomalacia ('malacia' denotes any abnormal softening of the tissues), tracheobronchomalacia, and tracheal stenosis (Pravit, 2014; Jacobs et al., 1996).

The congenital cardiac disorders most commonly associated with DS are atrioventricular defects (45%) and ventricular septal defects (VSD) (35%), while abnormalities such as isolated secundum atrial septal defects (8%), isolated tetralogy of Fallot (4%) and isolated patent ductus arteriosus are less frequent (Freeman et al., 1998). The major complication of cardiac anomalies in DS is pulmonary artery hypertension, which can progress to cardiogenic shock and eventually death (de Rubens Figueroa et al., 2003).

Incidence of cardiovascular abnormalities in a DS population was estimated as 42% in a study in the United Kingdom (Irving and Chaudhari, 2012), while the prevalence of congenital cardiac diseases ranged between 40% and 76% depending on the cohort studied (Paladini et al., 2000). The chromosome 21 aneuploidy can result in endocardial cushion defects (complete or incomplete) (Ferencz et al., 1989).

Ear problems such as inner ear dysplasia/hypoplasia, vestibular malformations, lateral semicircular anomalies, and conductive, mixed or neurosensory hearing loss are common in patients with DS (Blaser et al., 2006). Moreover, a high prevalence of chronic otitis media with effusion (60%) has also been reported (Maris et al., 2014).

Vision problems evident in DS include severe refractive errors (50%) and cataracts (15%) (Bull, 2011). In addition, strabismus (47%), nasolacrimal duct obstruction (36%), and nystagmus (16%) have been reported (Stephen et al., 2007). Other ophthalmic conditions such as retinal hemorrhage and macular hypoplasia are rare among affected children (Stephen et al., 2007).

Gastrointestinal defects such as duodenal stenosis/atresia (3.9%), anal stenosis/atresia (1.0%), Hirschsprung disease (0.8%), esophageal atresia with or without tracheoesophageal fistula (0.4%), and pyloric stenosis (0.3%) (Freeman et al., 2009) have been reported (overall frequency 7%). The prevalence of celiac disease in DS is about 5% in Italian patients, manifesting in classical form with diarrhea and vomiting (65%), a silent form (20%), or with atypical symptoms such as short stature/anemia (11%) (Bonamico et al., 2001).

Abnormalities of the immune system have been associated with DS (Kusters et al., 2009). Moreover, increased incidences of leukemia, hypothyroidism, malnutrition (zinc deficiency), celiac disease and diabetes mellitus in children with DS aggravate their immune deficiency (Ram and Chinen, 2011). Leukemia is estimated to be 15 to 20 times more frequent in children with DS (Whitlock, 2006). DS is an independent risk factor for the development of both acute lymphoblastic leukemia (ALL) and acute myeloid leukemia (AML) (Lange, 2000). ALL in DS is characterized by unique clinical features that include heightened sensitivity to methotrexate and an increased propensity to infections (Whitlock, 2006).

Thyroid dysfunction is the most common endocrine abnormality in patients with DS (Iughetti et al., 2014; Hawli et al., 2009). Estimates of its prevalence vary widely, ranging between 3% and 54% in adults (Hawli et al., 2009). In one study, the frequency of hypothyroidism in neonates with DS was estimated to be 28 times higher than expected in the general population (Purdy et al., 2014).

Renal and urinary tract malformations were found in 3.2% of DS patients in the retrospective cohort study by Kupferman et al. (2009). They reported a high risk of cystic dysplastic kidney, renal agenesis, hydronephrosis, anterior urethral obstruction, and anterior urethral obstruction in this population. Other urogenital anomalies such as cryptorchidism (undescended or maldescended testes), bladder exstrophy, posterior urethral valves, hypospadias (urethral opening on the inferior aspect of the penis), testicular microlithiasis, testicular malignancy, and infertility have also been noted in DS (Ebert et al., 2008; Vachon et al., 2006; Mercer et al., 2004). A review of the literature by Mercer et al. (2004) reported renal hypoplasia, glomerular microcysts, and obstructive uropathy as the most common urological abnormalities in DS.

The prevalence of psychiatric disorders in DS ranges between 22.1% and 38% (Dykens et al., 2015). Some risk factors associated with psychiatric disorders in DS are age, gender, serotonin dysfunction, sleep problems, life stressors, hypothyroidism, cardiac surgery, obesity, mosaicism, family genetics, personality, and strength (Dykens, 2007). Recently, high rates of psychosis and depression have been reported among young adults and adolescents with DS (Dykens et al., 2015; Zigman, 2013; Collacott et al., 1992). Other psychopathologies noted in adults with DS include phobias, obsessive-compulsive disorders, anorexia nervosa and other eating disorders, Tourette syndrome, and paraphilias (aberrant sexual desires) (Dykens et al., 2015).

Weisfeld-Adams and colleagues (2016) described a patient with a de novo 2.78-Mb duplication on chromosome 21q22.11 including 16 genes. The patient was born to a 36-year-old mother diagnosed with high risk for Down syndrome on prenatal biochemical screening. At five years of age, some facial features were compatible to features typical to Down syndrome, including a round, flat face with upslanting palpebral fissures, prominent epicanthal folds, flat nasal bridge, and mild macroglossia. Hands were small with bilateral fifth finger clinodactyly and bilateral single transverse palmar creases; the feet showed a wide gap between the first and second toes. The girl had mild developmental delay.

* This information is courtesy of the L M D.
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