Nasal Bone length at the time of N T SCan

What may go wrong in imaging and estimating Nasal bone length while performin N T scan ??  Normally , the median nasal bone lengths in healthy foetus was  1.4 mm (range, 1.1-1.9), 1.7 mm (range, 1.1-2.5), and 2.1 mm (range, 1.5-2.6) at gestational age of 11, 12, and 13 weeks respectively. Fetal nasal bone examination has recently been used as an additional screening tool in first-trimester Down syndrome screening.

But what are the practical difficulties in measuring Nasal Bone length at N T scan??

Point A)  Since nasal bone is a small, bifid structure, obtaining an appropriate image is somewhat problematic. It can be easily missed if the image is not exactly in the midsagittal view, or the nasal bone is parallel to the ultrasound beam. Point 2:-- Since nasal bone is a small, bifid structure, obtaining an appropriate image is somewhat problematic. . Point 3:--Variance in nasal bone echogenicity and discrimination between echoes of nasal skin and bone lead to difficulties in nasal bone measurement even by an experienced sonographer. Point 4: Subject variation, maternal obesity, for example, has an effect on the quality of an ultrasound image, resulting in a higher chance of inaccuracy of nasal bone assessment, which leads to significant differences in measurement between examiners. Point 5 :--Using high resolution ultrasound equipment is necessary to achieve good ultrasound images.

 Point 6:- Further , there can be  intraobserver variability of fetal nasal bone length measurement at 11–14 weeks of gestation, but interobserver variability varied among pairs of examiners, ranging from moderate to excellent. However, the overall interobserver variability was good with an ICC of 0.749 and only a small mean difference and good agreement. Proper training and standardization of the measurement technique with strict adherence.  Identification of fetal nasal bone is subjective. Additionally, technical and methodological errors may occur.

For  example, including or excluding the relatively hypoechogenic ends of nasal bone  measuring segments of the zygomatic bone instead of the nasal bone or including the tip of the nose in the nasal bone length measurement are distinct ppossibilities. . Methodological difficulties in nasal bone examination in the first trimester have been reported in the FASTER trial by Malone et al., in which well-trained sonographers who had little previous experience in evaluation of the nasal bone were able to depict the nasal bone in only 75.9% of 6316 fetuses (Malone FD, Ball RH, Nyberg DA, Comstock CH, Saade G, Berkowitz RL, Dugoff L, Craigo SD, Carr SR, Wolfe HM, Tripp T, D’Alton ME. First-trimester nasal bone evaluation for aneuploidy in the general population. Obstet Gynecol. 2004;104:1222–1228. doi: 10.1097/01.AOG.0000143255.46196.7a. [PubMed] [CrossRef] [Google Scholar)  Principle: What is taken granted??  Absence of the nasal bone can be used as a marker for Down syndrome in the first trimester of pregnancy but in few cases of DS it may be small or un-imagable e. .   Nasal bone assessment can be successfully achieved in ultrasound examinations in most cases, but not all during NT scan if the machine is good and person concerned has adequate training usually fortnight on NT Scan and patient is not too obese. TVS will however maximize the result. .. Within the group of cases (where NT Scan is done)  nasal bone  may be absent and that may amount to about   in 10 of 1000 (1.0%) unaffected cases(normal cases) , 10 of 15 (66.7%) Down syndrome cases and 5 of 12 (41.7%) cases with other pathological conditions. Nasal bone length usually on an average measures from 2.48 mm at a crown–rump length of 45 mm ( to 3.12 mm at a crown–rump length of 84 mm.) .

 What happens in DS?? Ans: It is either not present or the nasal bone length the nasal bone was present was less than the median measurement of unaffected cases.

WARNING:--But one should not use Nasal measurement in isolation instead  , the combination of nasal bone with maternal age, nuchal translucency, free beta‐human chorionic gonadotropin (hCG) and pregnancy associated plasma protein‐A (PAPP‐A)  should achieve  a detection rate of 95% with a false‐positive rate of 2.9%. At a fixed 1% false‐positive rate, the detection rate I some studies was  91%.

Take home message: --Absence of the nasal bone can be used as a marker for Down syndrome in the first trimester of pregnancy. Inclusion of the nasal bone in the current first‐trimester screening protocol along with nuchal translucency, free beta‐hCG and PAPP‐A can achieve high detection at a very low false‐positive rate but  a large datasets are needed  I our country to confirm whether the measurement of nasal bone length provides additional benefits beyond the assessment of the presence or absence of the nasal bone.