Congenital Abnormalities And Their Identification Flashcards
What is a screening test?
What makes a good screening test?
Available for all women
Gives a measure of risk of fetus being affected by particular disorder
A higher risk pt is offered a diagnostic test
E.g. Risk of down syndrome- 1 in 50 pregnancies
Cheap
High sensitivity- does not miss affected individual
High specificity- not many false +ves
Safe
What is a diagnostic test?
Whats an acceptable diagnostic test?
Performed in women w/ high risk to confirm or refute the possibility .
Implications of being affected by the condition should be serieous to warrant the test.m
Not always offered as 1st line tx
Expensive
Or
Sign complications
High risk- impact minimized.
Whats sensitivity?
Proportion of subjects w/ condition classified by test as screen +ve.
Whats the negative predicting value?
NPV- is the probability that the subject who is screen -ve will not have the condition.
Whats specificity?
Ppl w/o condition who are screened as negative
Whats the screen +ve rate?
Proportion of subjects who are classified as high risk by the test.
Whats the Positive Predictive Value?
PPV- probability that proportion of people who’s screen +ve will have the condition.
In practise, PPV is low, most screen +ves do not have the condition…. Rate is similar to FPR
What is the false positive rate?
FPR- ppl classified as high risk who do not have the condition.
What is the Maternal blood testing?
Methods of prenatal testing for congenital abnormalitites
As a screening test:
Neural tube defects- a- fetoprotein( AFP)- product of fetal liver. When fetus has open neural tube defect (NTD) /gastrochisis- maternal levels are raised. Or indicate higher risk of 3rd trimester complications
USS is more accurate
Chromosomal: some maternal markers altered;
B-hCG, pregnancy associated plasma protein A (PAPP-A) , AFP, oestriol, inhibin.
Assc w/ maternal age + USS measurments.
Maternal blood testing as a diagnostic test, what does it do?
Just fetal gender still - few fetal cells in maternal blood will revolusionise prenatal diagnosis in future
Why is the USS used?
To:
1. Confirm gestation- transvaginal scan, pregnancy site and exlude multiple pregnancy.
2. Cornerstone for screening for trisomies
A. Nuchal translucency - space b/w skin & soft tissue overlying cervical spine betwwen 11-14 weeks is measured. The larger it is the higher the risk.
Esp- structural- cardiac abnormalities, exopthalmos !!
3. To aid other diagnostic tests- amniocentesis and chorionic villus sampling (CVS) are performed under USS vision.
4. As a diagnostic test. 18-21 weeks- anomaly scan for struct. Abn. Congenital malformations of all organs & systems are detectable. 25% abn detectable in 11-14 week. Cardiac.. 20 weeks.
Whats fetal magnetic resonance imaging?
MRI- scan for fetus in utero- diagnosis of intracrania lesions
Better at differentiating diff types of tissue.
Whats thw 3-D/4-D USS?
Reconstructs 3-D images, tertiary referral centres.
Whats amniocentesis?
Diagnostic test- removal of amniotic fluid- fine-gauge needle under USS guidance.
Safe from 15 weeks
Allows for prenatal diagnosis of chromosomal abn, some infx- cytomegalovirus(CMV) & toxoplasmosis & inherited disorders- sickle cell anaemia, thalassaemia, CF.
What % of women miscarry after amniocentesis?
1%
Whats chorionic villus sampling?
Diagnostic test- biopsy of the tropgoblast, fine-gauge needle through the abdo wall or cervix and into the placenta, after 11 weeks.
Its used ro diagnose chromosomal abn & autosomal dominant and recessive conditions.
What are the advantages of Chorionic villus sampling?
Diagnosis faster than amniocentesis, allows abnormal fetus to be identified at a time for abortion- if requested- under general anaesthesia.
Why are miscarry rates for CVS higher?
Miscarriage rates are higher- cz its performed earler when spontaneous miscarriage is still common + cz its a more diff procedure.
How do we diagnose the abn after sampling?
FISH- flurescence in situ and PCR- poly,erase chain rct ysed to diagnos abn in less than 48hrs.
What are preimplantation genetic disorders?
In vitro fertilization(IVF) allows cells from a developing embryo to be removed for analysis before the embryo is transferred ro the uterus.
Expensive techniques- ethical dilemmas.
Has been used in sex-linked disorders, trisomies, both autoaomal dominant and recessive disorders.
Even if couples are fertile.
What are some chromosomal abnormalities?
6/1000 live births. Most- trisomies.
Down’s syndrome-
Nuchal scan- aneuploidy, pregn dates.
Trisomy 18- Edwards syndrome
Trisomy 13- patau’s syndrome - also common w/ ⬆️ maternal age.
Affected fetus dies in utero or shortly after birth.
Sex chromosome abn- klinefelter’s syndrome-47XXY- males- normal intelect, small testes, infertile.
Turner’s syndrome- X0- single X chro osome- female, infertile , normal intelect.
What is downs syndrome?
trisomy 21- usually random non-dysfuntional meiosis.
Common w/ advanced maternal age.
Mental retardation, characteristic faces and often (50%) congenital cardiac disease.
What are some RFs for Downs?
Hx- ⬆️ maternal age, px affected baby ⬆️ risk by 1%, balanced parental translocation- rare
USS- thickened nuchal translucency, struct. Abn., abscent or shortened nasal bone, tricuspid regurgitation.
Blood tests: Low PAPP-A (1st trimester) ⬆️⬆️ b-hCG (1st/2nd trimester) ⬇️AFP (1st/2nd trimester) ⬇️ oestriol -2nd ⬆️ inhibin -2nd
Screening test- 75% sensitivity 3% Fositive rate
Whats the combined test,
Screening test + PAPP-A + b-hCG bloods, + nuchal translucency by USS measurments in 11-13 + 6 weeks
Whats the triple test?
Blood test at 16 weeks- AFP, hCG and oestriol
Less accurate.
No nuchal translucency- USS- less esxpensive.
What are some congenital abnormalities?
2% of pregnancies Can be: Structural deformities- diaphragmatic hernia Chromosomal: esp trisomies- downs Inherited diseases- Cystic fibrosis Or result od intrauterine infx- rubella Drug exposure- antiepileptics
25% perinatal deaths + disability in later life
Counselling must be non-directive
