N T Scan

 

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What is NT & why  Bottom of Form

Nuchal translucency is raised in some cases? What does such increased NT signify??

Q.1: When to measure-The optimum time?? Nuchal translucency is a finding during a specific period in the late first trimester and early second trimester (11.3-13.6 weeks) and should not be confused with nuchal thickness which is measured in the second trimester. What period of gestation

tin NT should ideally be done?? Assessment

Values obtained when CRL is between 45-84 mm (11.3- 13.6 weeks) may be used for combined first trimester screening.

the lucent region is generally not septated

The thickness rather than the appearance (morphology) is considered to be directly related to the incidence of chromosomal and other anomalies

a value of less than ~2.2-2.8 mm in thickness is not associated with increased risk, however it is maternal age dependent and needs to be matched to exact gestational age and crown rump length (CRL)

 

Q.2: What is normal thickness of translucency?? Ans: a value of less than ~2.2-2.8 mm in thickness is not associated with increased risk; however it is maternal age dependent and needs to be matched to exact gestational age and crown rump length (CRL)

 

 

Q.3: If increased nuchal then what congenital abnormalities are possible???  Ans: If above 2.8 mm then it  is thought to be related to dilate lymphatic channels and is considered a nonspecific sign of more generalized fetal abnormality.

Thickening of the nuchal translucency can be associated with a number of anomalies, including:  1) aneuploidy, 2)  trisomies (including Down syndrome),  3) Turner syndrome, AND 4) non-aneuploidy structural defects and syndromes, congenital diaphragmatic herniation, congenital heart disease, omphalocele ,, Smith-Lemli-Ovitz syndromeand  VACTERL association

 

 

Q.4: How to measure accurately?? Exact technique??   Ans: Measurement of the nuchal latency requires careful attention to technique.

Nuchal lucency is measured on a sagittal image through the fetal neck. Technique

Use of correct technique 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:

two tiny parallel echogenic lines 

tip of the nose

nasal bone (if not absent)

hard palate 

diencephalon 

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

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

Q.5: When 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,

 

Q.6: 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 free  B-HCG,  -fetoprotein (AFP).   pregnancy-associated plasma protein A (PAPP-A),oestriol/estriol,   Point 1:  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 

 

Q.7:  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 counselling and which includes:

amniocentesis and/or chorionic villus sampling fetal echocardiography,Treatment and prognosis should be discussed.

As the second trimester approaches, the region of nuchal translucency might either:  A)    if chromosomally normal, a large proportion of fetuses will have a normal outcome B) But if  spontaneous regression does not however mean a normal karyotype

evolve into nuchal oedema,cystic hygroma

Q.8,Differential diagnosis

Incorrect technique, fetal neck skin thickening due to first-trimester hydrops fetalis

amniotic membrane lying behind fetal neck, chorio-amniotic separation