Nuchal Translucency

 

How to measure accurately?? Exact technique??  

Measurement of the nuchal latency requires careful attention to technique.

Nuchal lucency is measured on a sagittal image through the fetal neck. Besides use of correct technique and at least one week training is essential. The fetus must be in mid sagittal imaging plane (the vertebral column should be facing the bottom of the screen); following structures must be seen to confirm correct mid sagittal position:

 

What to be imaged?

two tiny parallel echogenic lines at the nape of neck.

tip of the nose

nasal bone .

hard palate 

diencephalon 

magnification so that only fetal head and upper thorax included in the image: enabling 1 mm changes in measurement possible

 

What should be the ideal position of foetus in screen?

Fetal head should not be extended or flexed

fetus should be floating free of the uterine wall i.e. amniotic fluid should be seen between its back of foetus and the uterus; this is to not mistakenly measure distance to amniotic membrane or uterine wall

the "+" calipers should be used for measurement

only the lucency is measured (again differing from nuchal thickness) the calipers are put inside the hyperechoic edges

the widest part of the translucency should be measured

 

What should be the ideal position of foetus in screen?(contd)

 

Magnification so that only fetal head and upper thorax included in the image:

fetal head should not be extended or flexed fetus should be floating free of the uterine wall the "+" callipers should be used the callipers are put inside the hyperechoic edges
the widest part of the translucency should be measured. 
It should be measured between 11.3-13.6 weeks and
 it should be less than 3 mm,

 

 Nuchal translucency cannot be adequately assessed if there is: unfavourable fetal lie

unfavourable gestational age: CRL <45 or >84 mm,Interpretation,The rate of aneuploidy when the nuchal translucency is <2 mm is less than 1%.,Correlation with serum markers

To increase the clinical accuracy of nuchal lucency, it can be correlated with serum markers such as:

maternal B-HCG,alpha-fetoprotein (AFP).pregnancy-associated plasma protein A (PAPP-A),oestriol/estriol,The combination of nuchal translucency thickness, PAPP-A, and hCG detects 87% of cases of trisomy 21 at 11 weeks, 85% at 12 weeks, and 82% at 13 weeks, with a 5% false positive rate 

 

 

Immunoassays for Down’s syndrome Screening Tests.

 

 

Why Prenatal screening is so   important ??

 

It’s said 1 in 300 pregnancies have some chromosomal disorder. Of these commonest are trisomy 21, 18 and 13. Trisomy 21 being the commonest.

T21 has some ultrasound markers and is also association with some structural anomalies but almost 30-40% fetuses with T21 may look absolutely normal on a routine anomaly scan, hence the need for screening.T18 and T13 are lethal and have quite a few stigmata on ultrasound if looked at carefully. But these soft markers are often missed in routine anomaly scans, hence the need for screening.

Screening options:-

1) Double marker with NT scan, 2)-triple marker, 
3)quadruple marker, 
4)genetic sonogram  & 
5)NIPT

Till date the first is the most sensitive (96% in good hands and labs) and cost effective method of screening. The rest can be added as per needed.

NIPT though it promises to be a path breaker still needs to get better with results. Have a quite a few limitations (Cost, less sensitive for T18, 13 and culture failure rate) still. Can be judiciously offered to those who have only high risk serum biochemistry but no usg markers.

 

How to screen for Down syndrome / assess other karyotyping disorder in multiple gestations

Like all other NT Measurements in twin gestation also the NT in mm is expressed as MoM (median of mean), so also one can perform PAPP-A (pregnancy associated plasma protein-A); free –beta- hCG.

Detection of SPR(screen positive rate) :Compared to singleton in twins

1.67 MoM – for unconj. Oestriol, 1.84=MoM-for hCG, 2.13-for MS-AFP; Pseudo risk appearance of biochemical reports.

Mid trimester- reports are fallacious- Use multiple markers. However serum anylate values may be more informative.

 

 

 

 

 

Under what conditions NT can’t be assessed ??

 

  Nuchal translucency cannot be adequately assessed if there is:  a) unfavourable fetal lie & b)

unfavourable gestational age: CRL <45 or >84 mm,

 

Interpretation about possible chromosomal abnormalities

may be more accurate if we take into account of report of double markers.

 

 The rate of aneuploidy when the nuchal translucency is <2 mm is less than 1%.but not Zero.  To increase the clinical accuracy of nuchal lucency regarding possibility of associated chromosomal abnomality of foetus , it can be correlated with serum markers such as: A) Maternal free  B-HCG,  & B)  pregnancy-associated plasma protein A (PAPP-A)

 

informative is NT  in picking up chromosomal abnormality of foetus ??

 

 

The combination of nuchal translucency thickness, PAPP-A, and hCG detects 87% of cases of trisomy 21 at 11 weeks, ,   Point 2:   82% at 13 weeks, with a 5% false positive rate 

 

 What will be the further work up à if NT is high ??

If abnormal and screening test results show increased risk of: less than 1 in 300, further work-up may carried out based on patient's desire after counseling and which includes: chorionic villus sampling or amniocentesis. Later   fetal echocardiography should always be done ass Foetal echo is a noninvasive teat ,Treatment and prognosis should be discussed.

 

When does difficulty arises in imaging N T?

Low quality machine, Improper training and poor skill of the sonologist, iincorrect technique, fetal neck skin thickening due to first-trimester hydrops fetalis

Amniotic membrane lying behind fetal neck, chorio-amniotic separation

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