Screening for Down’s syndrome FAQ’s Dr Sheila Balakrishnan MD DNB FRCOG FICOG Prof in OBGYN Medical College Trivandrum

Do you think screening for Down’s is necessary and if so for what age group?

What is meant by ‘apriori risk’?

A woman’s risk of having a fetus with trisomy 21 depends on her age, gestational age and her history of chromosomal defects In general we mean the age related risk at term

Age Midtrimester Term 35 1 in 384 1 in 250 40 1 in 106 1 in 70

Best method of screening as of today?

First trimester screening by the combined test- NT PAPP A and β hCG- is added to the apriori risk

What is NT?

USS term for the physiologic collection of fluid underneath the nuchal skin extending for a variable distance over the fetal head and back at 11-14 weeks For unclear reasons this collection is unduly sensitive to fetal problems

Who described NT and in which year?

Kypros Nicolaides in 1992

What are the requirements of a good NT?

The maximum thickness of the subcutaneous translucent area between the skin and soft tissue that overlies the fetal spine in the sagittal plane

11 weeks – 13 weeks 6 days CRL 45- 84 mm Mid sagittal view Neutral position of head Magnification (>70%) Widest part of translucency to be measured Frontal process of maxillary bone should not be seen Amnion must be seen separate Biggest of 3 measurements

What other markers can be added to NT in the scan to increase detection rate?

Increases detection rate to 95% and decreases false pos rate to 2% Nasal bone (fallacy – nasal skin) Ductus venosus (absent or reversed flow) Tricuspid regurgitation Increases detection rate to 95% and decreases false pos rate to 2% FMF angle >85° (b/w the upper surface of upper palate and frontal bone in midsagittal view) The more the markers the more error can occur

73% Downs absent nasal bone 0.5% normal

A 11-14 weeks scan reports an NT of 1.2 mm. What does it imply?

Not useful and action should not be taken on basis of absolute values usually All values are to be expressed as MOM Special software available Standardisation is by the FMF which provides certification

Are there any absolute cut offs for NT and the other tests?

NT >3.5 mm falls above 95th centile for any week bw 11-14 vweeks PAPP A <0.5 MOM(think of IUGR) β hCG >2.5 MOM These values justify invasive testing in themselves

What is the detection rate with the combined test?

85-90% for a false pos rate of 5% Amniocentesis rate is about 5% Adding nasal bone may increase the detection rate to 90% If NT alone – 77%

Other methods available?

Second trimester screening by triple and quadruple test Integrated test Sequential test Contingent test In all these amniocentesis rate is higher (about 8%)

Method First trimester screening Triple test Quadruple test Integrated test Sequential test Components NT, β hCG and MSAFP MSAFP, β hCG and uE3 MSAFP, β hCG, uE3 & Inhibin First trimester screen & Quadruple test Detection rate 85% 70% 95% 90%

What is the contingent test?

FTS is offered and depending upon results women are divided into three groups High risk- invasive procedure Low risk- reassurance Intermediate risk- offer second trimester serum screening and invasive testing

What is the genetic sonogram?

Soft tissue markers of aneuploidy Nuchal skin fold thickness Absent or hypoplastic nasal bone (7) Ventriculomegaly Aberrant right subclavian artery Renal pyelectasis Choroid plexus cysts Hyperechoic bowel Short femur Echigenic focus in heart Structural anomalies like duodenal atresia

What will you recommend in the foll situations 1 Choroid plexus cysts 2 Increased NSFT in second trimester scan

Presence of one such marker should promote a detailed search for other markers and if more than one is present, invasive testing can be recommended One marker like NSFT, nasal bone or a structural anomaly may be an indication for amniocentesis

Which structural anomalies are definite indications for karyotyping?

Duodenal atresia Omphalocoele AV canal defects

Thick NT and normal KT, what next ?

Cardiac anomalies Structural defects like omphalocoele Predictor of TTTS in multiple pregnancies Inspite of everything a fetal loss of 4% is reported- COUNSEL

11-13 week scan Uterine artery PI FMF Certificate of competence in measurement of uterine artery PI The Fetal Medicine Foundation has now established software for the calculation of risk for preeclampsia. The software is provided free of charge to those who comply with the FMF regulation of NT screening and have demonstrated competence in the Doppler assessment of the uterine arteries at the 11-13 weeks scan.

FMF screening for PE Established software for prediction of PE Uterine artery PI + maternal history, BP, Papp A and PlGF Risk calculated for early and late PE Detects 90% of early onset PE and 45% late PE for a false pos rate of 10%

Thank you