Nuchal Translucency

 

Point 1: What is the aim & objectives of estimating NT in late first trimester scan??  Ans: Nuchal Translucency is basically a test which offers us a mathematical calculation (statistical) probality of having aneuploidy of foetus (probality ratio) or Risk stratification: The CRL is used to standardise both biochemistry and nuchal translucency by using data of several thousand normal cases (in retrospect) .We always are concerned about probability of chromosomal abnormality in any foetus.  This is also termed as likelihood ratio.   If mother has one child with Down’s syndrome, then recurrent risk will be 2%. .

Q.2; What is the normal translucency (NT )?  Ans;--Normally NT steadily increase    for the period foetal CRL from 44 mm  to 85 mm .What is normal NT(subcutaneous fluid collection behind the nape of the neck) ??  ( one of the 21 soft markers) .The fluid thickness is  for  a  CRL of 45 mm (  means 11 weeks ) will be  1.9 mm  in 85 percentile  . But the same will be  2.8 mm in 95 percentile   in CRL of  85 mm ( CRL at 13.6  weeks ) .

Point 3: When we should order for NT scan? Ans: The translucency (NT is ) only reliable   from   45  - 84 mm  of CRL. But there is two notes of caution. Caution 1: The sonographer should be specially trained to measure N translucency. Caution 2:  The caliper     of USG machine used should be able   to read upto 0.1 mm. Caution 3: Head should be neither extended nor flexed and the image (area of interest) should occupy the 2/3 rd of  screen of USG.  

Accuracy of measuring Crown Rump length (CRL) is the key:----Inaccurately measuring the CRL can have a serious effect on the risk calculation for Down syndrome screening.. ... For the NT scan / combined first trimester screen, the CRL must be between 45mm and 84mm. Crown Rump length (CRL)

Point 3:  Inaccurately measuring the CRL can have a serious effect on the risk calculation for Down syndrome screening. As mentioned
The CRL is used to standardise both biochemistry and nuchal translucency. If the CRL is over measured the biochemistry will appear to be immature, with the potential to increase the false positive rate. Under measurement of the CRL will potentially have the reverse effect on the biochemistry? Similarly inaccurate measurement of the CRL can adversely affect the accuracy of the nuchal translucency component of the risk calculation. Therefore Foetal medicine Foundation (EMF) warrants a special training   on estimation of NT for a week for all   sonographers  .For the NT scan / combined first trimester screen, the CRL must be between 45mm and 84mm.

It is important to date the pregnancy using the CRL measurement for the combined screening test. An NT scan must take place at a particular  gestation and this  is between 11 weeks and 13 weeks plus six days, to be exact , or when crown rump length (CRL) is between 45  mm (1.8in) and 84  mm (3.3in). So the NT scan will usually happen alongside routine dating scan.

How to measure the CRL accurately: At 12 weeks of gestational age, an "average" nuchal thickness of 2.18  mm has been observed; however, Caution 4: up to 13% of chromosomally normal fetuses present with a nuchal translucency of greater than 2.5mm.

Q,4:- How best to have proper image as recommended by FMF??  Tip 1: : To MAGNIFY the image. Make the fetus as large as possible before freezing the image, ensuring the whole fetus is clearly demonstrated. MIDSAGITTAL view is a absolute necessity. Sonographer should wait patiencely  till a midline sagittal section of the whole fetus is viewed . The image of the head should demonstrate the A) echogenic tip of the nose, B)  the rectangular shape of the palate, C)  and the translucent diencephalon. The head should be in line with the full length of the body, which is displayed along with the full length of the spine.

Tip 2: TO ACHIEVE NEUTRAL POSITION: The fetus should be neither hyperextended nor flexed. There should be fluid visible between the chin and the chest. To ensure the cervical spine is not extended. If the fetus is prone it is helpful to invert the image to ensure there is not hyperextension.

Tip--4: HORIZONTAL The fetus needs to be at 90 degrees to the axis of the ultrasound beam. ie a line drawn between the crown and rump is at 90 degrees to the ultrasound beam.

Tip 5:-CROWN AND RUMP CLEARLY DEFINED The crown is the skin above the parietal bone. The rump is the skin inferior to the tip of the sacrum. There should be a pocket of fluid between the ends of the fetus and the uterine wall.

Use the + calipers.

Tip 6:-HOW  ACCURATELY  THE CALIPERS  BE  PLACED ? To   Place the intersection of the + calipers at the outer margin of the skin borders

 

Q.5:-What we may suspect in NT Scan:-For   any foetal   problem be it anatomic be it genetic,  such abnormalities are usually but not always  is reflected   on NT changes but why fluid collects in nape of the neck of foetus is unclear. WE have already discussed that  as gestation changes   - NT    varies as per CRL. 

 Q.6: All said , all done when to counsel for  confirmatory tets( invasive) tests like CVS /amniocentesis ??  If the risk calculation (statistical risk probability) is above 1: 50 then only CVS procedure for confirmation.

Q.7: How to perform numerical asessmement of chromosomes?? :-The chromosome analysis can be done either by 1) cell culture (traditional karyotyping)-This is one of the method of cytogenetic study. or 2):  by more recently FISH method (Fluorescent  In situ Hybridization).In FISH method  Fluorescent probe is used on the nuclei of amniocytes and if there is evidence of 1) one signal means that the said nucleus is having monosomy ( after using Fluorescent probe), 2) two signals means euploid nucleus and if 3)  some nuclei reveals three florescent signals it means that the said nucleus is having  trisomy..

FISH:- Usually there are five DNA probes(probes with Fluoro Chrome) : This is one of the DNA study. used like 1) LS-1--21,    2) CEP-18,    3)  LS-1-13    , 4 & 5 ) CEP- two Sex chromatins    . The cells which shows two different signals after FISH procedure means normal cells

 i. e diploid chromosome with no alteration in number of total chromosomes. Usually five chromosomes are studied in nuclei like amniocytes. These are Chr 21, 13, 18 and two sex chromosomes. Thus these tests of FISH will detect trisomy or monosomy.