Down Syndrome Trisomy 21 (Mongolism) – Causes, Karyotype, Symptoms, Antenatal Diagnosis
Down syndrome, trisomy 21 or the older term mongolism is a condition in which a person is born with certain distinctive features : flat face, short neck, and a degree of mental delay (mental retardation). Although Down syndrome cannot be treated, most patients can lead a normal life. With the proper care and help they need, children with Down syndrome can have a spectacular growth and development and can become healthy and happy adults.
Down syndrome was described in 1861 by Seguin in 1866 by Langdon-Down who specify the clinical features of this syndrome and in 1959, Lejeune specifies the chromosomal etiology.
Frequency of Down syndrome is 1.5 per 1000 infants and sex distribution is 3 to 2 for male gender. It is considered that in the occurrence of Down syndrome is involved advanced maternal age, especially over the age of 35. At mothers aged 28 years it is recorded an increased frequency of births with Down syndrome, but this corresponds to a maximum number of births at this age. The risk increases with maternal age, as follows:
- Under the age of 30 years, the risk of having a baby with Down syndrome is less than 0.1%;
- Between 30 and 40 years, the risk of having a baby with Down syndrome is less than 1%;
- Over 40 years, the risk of having a baby with Down syndrome is more than 1%;
- Over 45 years, the risk of having a baby with Down syndrome is 3,3%;
- Over the age of 50 years, the risk of having a baby with Down syndrome is about 15%.
Down Syndrome Causes And Karyotype
- Free and homogeneous trisomy 21 in 92.5% of cases of Down syndrome. These cases are generally cases with de novo appearance, in which is involved maternal age. Karyotype is 47XX+21 or 47XY+21 and the cause is represented by a chromosomal non-disjunction of maternal origin (90%) or a chromosomal non-disjunction of paternal origin (10%).
- Mosaic trisomy 21 in 2.5% of cases of Down syndrome. These are sporadic cases, showing karyotype 47XX+21 / 46XX or 47XY+21 / 46XY. Phenotypic manifestations in this type of Down syndrome are more attenuated.
- Trisomy 21 with translocation in 5% of cases of Down syndrome. These are cases with de novo appearance, in which is involved a transmission of a paternal translocation. The karyotype is 46XX or 46XY with a translocation between a supernumerary chromosome 21 and, most commonly, a chromosome of group D (pair of chromosomes 13, 14 and 15).
- Partial trisomy 21, very rare and is represented by cases of Down syndrome where is an excess of genetic material represented by an extra chromosome 21 which has deletions on q arms.
The Clinical Presentation Of Down Syndrome (signs and symptoms)
- Short stature : the child usually have slow growth rate, and in adulthood their height is lower than average;
- Low muscle tone : a child suffering from Down syndrome may have less muscle strength than other children of the same age;
- Short neck, thick with fat and excess skin : usually this feature becomes less obvious as the child grows;
- Short and stocky limbs, some children may have a wider space between the thumb and second finger of the foot;
- One fold in the central part of the palm : it is called the simian line.
Down Syndrome Facial features:
- Ears with modified form : usually small and with a low placement ;
- Abnormal mouth and tongue: mouth is often open, exfoliative glossitis, tongue with scrotal appearance (in adolescents and adults), pseudomacroglossia;
- Flattened nasal bridge : flat nose portion located between the two eyes (nasal bridge) is frequently clogged;
- Brushfield’s spots : colored spots on the iris, these spots are not affecing the sight;
- Malformation of the teeth: baby teeth may grow later and in an unusual way, agenesis of lateral incisors.
- Cardiac: atrioventricular septal defect, ventricular septal defect, interatrial septal defect, patent ductus arteriosus;
- Gastrointestinal: duodenal stenosis, anal imperforation, celiac disease, Meckel diverticulm, omphalocele and Hirschprung disease;
- Genitourinary: micropenis, cryptorchidism, renal malformations and hypospadias;
- Eye: cataracts, astigmatism, myopia, strabismus and glaucoma;
- Endocrine: Hashimoto’s thyroiditis, hypothyroidism, hyperthyroidism;
- Hematologic: acute lymphocytic leukemia, acute non-lymphoblastic leukemia,
- Immunological: various immune deficiency;
- Metabolic: hyperuricemia, diabetes, obesity;
- Sterility in males patients with Down syndrome;
- Alzheimer’s disease.