Nuchal translucency

Nuchal translucency is the normal fluid filled subcutaneous space identified at the back of the fetal neck during in the late first trimester and early second trimester (11.3-13.6 weeks).

It should not be confused with the nuchal fold, which is seen in the second trimester.  

Pathology

Increased nuchal translucency is thought to be related to dilated lymphatic channels and is considered a nonspecific sign of more generalised fetal abnormality. Measurement of the nuchal translucency requires careful attention to technique.

Associations

Thickening of the nuchal translucency can be associated with a number of anomalies, including:

• aneuploidy
  • trisomies (including Down syndrome)
  • Turner syndrome
• non-aneuploidy structural defects and syndromes
  • congenital diaphragmatic herniation
  • congenital heart disease
  • omphalocoele
  • skeletal dysplasias
  • Smith-Lemli-Opitz syndrome
  • VACTERL association

Radiographic features

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 and the uterus; this is to not mistakenly measure the distance to the 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

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)

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 ⁸.

Further work up

If abnormal and screening test results show increased risk of less than 1 in 300, further workup may be carried out based on patient's desire after counselling and which includes:

• amniocentesis and/or chorionic villus sampling
• fetal echocardiography

Treatment and prognosis

As the second trimester approaches, the region of nuchal translucency might either:

• regress
  • if chromosomally normal, a large proportion of fetuses will have a normal outcome
  • spontaneous regression does not, however, mean a normal karyotype
• evolve into
  • nuchal oedema
  • cystic hygroma

Differential diagnosis

• incorrect technique
• fetal neck skin thickening due to first-trimester hydrops fetalis
• amniotic membrane lying behind fetal neck
• chorio-amniotic separation