Sunday, April 10, 2011

Nuchal Translucency

Nuchal translucency is the sonographic appearance of fluid between skin and soft tissue at the back of the fetal neck. Nuchal translucency thickness (NT) is measured at the end of the first trimester, between gestational weeks 11 and 14. The measurement for a normal NT is generally thought to be less than 3 mm. However, this fixed value ignores the normal increase in NT with gestation. More recently, the delta NT has been proposed as a way of accounting for this change in NT with gestation. Delta NT is the difference between the measured NT and the normal median NT at the measured crown–rump length (CRL). The normal median NT can be calculated from the formula:

Log10NT = −0.3599 + 0.0127 CRL − 0.000058 CRL2

A cut-off of 1.5 mm has been proposed for the delta-NT.

Nuchal translucency thickness, when combined with maternal age, is a noninvasive screen for Down syndrome. Combining nuchal translucency measurements with maternal age allows identification of 75% of fetuses with Down syndrome and approximately 70% of fetuses with other chromosome abnormalities. By combining nuchal translucency, maternal age, and maternal serum biochemistry, the detection rate for fetuses with Down syndrome is increased to 89%, with a false-positive rate of 5%.

Fetuses without chromosomal abnormalities also have an increased risk of major anomalies (most commonly cardiac) when the nuchal translucency is thicker than normal.

Nuchal translucency should be measured by properly trained sonographers, otherwise its diagnostic value basically disappears. Detailed guidelines exist for proper measurement of the nuchal translucency:
  • Technique: Either transabdominal or transvaginal
  • Crown–rump length: Should be between 45 mm and 84 mm.
  • Magnification: The fetus should occupy 75% of the image to ensure that each increment in the distance between calipers will be approximately 0.1 mm.
  • Amnion: The fetal skin must be clearly separated from the amnion. This can be achieved by waiting for or encouraging fetal movement (asking the mother to cough or tapping on her lower abdomen).
  • Position of fetal head: The fetal head should be in the neutral position. Hyperextension can increase the measurement by as much as 0.6 mm, and hyperflexion can decrease it by as much as 0.4 mm.
  • Plane: True sagittal scan of the fetus is required. The nasal bone and chin should be visible.
  • Measurement: Finally, we can make the measurement (see figure).
    • The + calipers should be used.
    • The horizontal crossbars of the calipers should not be in the black space.
    • A vertical line connecting the two calipers should be perpendicular to the long axis of fetus.
    • Three measurements must be taken and the highest used for risk stratification.

References

  • Fong KW, Toi A, Salem S, Hornberger LK, Chitayat D, Keating SJ, McAuliffe F, Johnson JA. Detection of fetal structural abnormalities with US during early pregnancy. Radiographics. 2004 Jan-Feb;24(1):157-74.
  • Maymon R, Tercanli S, Dreazen E, Sartorius G, Holzgreve W, Herman A. Comparison of pregnancy outcome of euploid fetuses with increased nuchal translucency (NT) expressed in NT MoM or delta-NT. Ultrasound Obstet Gynecol. 2004 May;23(5):477-81.
  • Snijders RJ, Noble P, Sebire N, Souka A, Nicolaides KH. UK multicentre project on assessment of risk of trisomy 21 by maternal age and fetal nuchal-translucency thickness at 10-14 weeks of gestation. Fetal Medicine Foundation First Trimester Screening Group.Lancet. 1998 Aug 1;352(9125):343-6.
  • Spencer K, Bindra R, Nix AB, Heath V, Nicolaides KH. Delta-NT or NT MoM: which is the most appropriate method for calculating accurate patient-specific risks for trisomy 21 in the first trimester? Ultrasound Obstet Gynecol. 2003 Aug;22(2):142-8.

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