English physician John Down first characterized Down
syndrome as a distinct form of mental disability in 1862 due to
his perception that children with Down syndrome shared physical
facial similarities (epicanthal folds) with those of Mongolian race.
-Incidence:-
- In general population 1:660.
- It is the most common Autosomal abnormalities.
- It has equal sex distribution.
1-Complete Trisomy 21 (non disjunction):-
-Incidence:- 95%.
-Due to non-disjunction of chromosome 21 during meiotic division (
(i.e failure of a chromosome 21 pair to separate) so an ovum with
24 chromosomes when fertilized by a sperm carrying 23
chromosomes lead to formation of a fertilized ovum with
47 chromosome.
-It occur during oogenesis more than spermatgenesis.
-The risk increases with age of the pregnant mother especially
over 40years as the primary oocytes of the mother have satyed in
the prophase for a long time ( 40 years or more).
-Karyotyping:-
47,xx+21(female down).
47,xy+21(male down).
(+ means that an extra chromosome is present).
(Non disjunction in mother)
2-Mosaic Down´s syndrome:-
-Incidence:- 1%.
-It is the best type.
-Due to non-disjunction of chromosome 21 occurig early in the
division of zygote (i.e after zygote formation) which results in
formation of two cell lines (normal & trisomic).
-Karyotyping:-
some cells as non-disjunction type & the other are normal.
-Clinical manifestaions are less than complete triosomy.
3-Translocation Down Syndrome:-
-Incidence:- 4%.
-Due to unbalanced translocation of chromosome 21 to another
chromosome in group D (usually chromosome 21) or to another
chromosome in group g resulting in phenotype same as trisomy 21
Down syndrome but genotype is 46 chromosome.
-It is the most serious type.
-Robertsonian translocations is the most common type.
-Clinical manifestaions:-
1-conatant features:-
- Physical growth retardation.
- Mild to moderate degrees of mental retardation.
- Hypotonia.
- Brachycephaly(flat occipit).
- silky hair.
- Microcephaly.
- Wide anterior fontanel.
- Delayed closure of posterior fontanel.
- upward slanting eyes.
- Medial epicanthal folds(extra skin folds at the medial
- corners of the eyes).
- narrow palpebral fissures.
- Brushfield spots iris(speckled irises).
- hypertelorism.
- cataract.
- small.
- low set.
- Backward displacment.
- Short.
- webbed.
- small.
- depressed nasal bridge.
- Congenital HD(50%)
- Most common(Av canal,VSD).
- Protruded (small oral cavity).
- fissured.
unilateral or bilateral absence of one rib.
10-Abdomen:-
- Duodenal atresia.
- Imperforate anus.
- Umbilical hernia.
- Congenital mega colon.
- Distended abdomen(Due to hypotonia).
- Diastasis recti.
- Brachydactyle(short & stocky).
- Clinodactyly i.e incurved little fingers(due to rudimentary or absent middle phalanx of little finger as it the last phalanx to develop).
- Simian creases:- may be partial or complete.
- Ridges hypoplasia.
- Syndal gap (i.e wide space between the big toe & second toe).
- syndal line(i.e deep planter crease).
- Hypothyrodism(15%).
- cryptorchidism.
- Complications:-
- Accidents due to mental retardation.
- Heart failure due to Congenital HD.
- Recurrent Infections (chest,Otitis media,sinusitis) due to decrease immunity,hypotonia,aspiration.
- They are liable to leukemia "20 times more than general population.
- GIT complications.
- Arrythmia which is the cause of sudden death.
-Investigations:-
(A)Prenatal diagnosis:-
- Amniocentesis
- chrionic villous sampling
- Karyotyping
- Triple test
- Karyotyping:- is dianostic.
- For Cvs:-Echo,ECG.
- For blood:CBC to exclude leukemia.
- Baruim for GIT anomalies.
- Radiology for U.T anomalies.
- Thyroid function tests (T3,T4,TSH).
-Treatment:-
- Treatment of congenital anomalies & associated disorders.
- Social,educational& behavioral therapy in special institutes.
*References:-
(1)Langman’s Medical Embryology 9th ed,ch1,p.g11.
(2)Priciples of histology vol1,staff members ,faculty of medicine,mansoura university,ch3,p.g72,73.
(3)Richard E Behrman,Nelson Textbook of Pediatrics,17th ed,Ch70,
(http://www.elsevier.com).
(4)Down syndrome from Wikipedia,here
Return to list of medical syndromes here
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