NUCHAL TRANSLUCENCY THICKENING AND

THE CHROMOSOMALLY NORMAL FETUS  

 

An increased nuchal translucency in the chromosomally normal fetus is associated with an increased fetal loss rate due to associated structural malformations than would have been expected in unaffected populations:

·         Cardiac

·         Diaphragmatic.

·         Abdominal wall defects.

Resolution of the nuchal translucency is not indicative of a normal karyotype (1). In a group of patients who elected to continue their pregnancy, Panyada and co-workers (2) described spontaneous resolution of this sonographic finding in five of six fetuses with proven trisomy 21.

However, fetuses with ultrasound evidence of isolated nuchal fluid collections and normal karyotype may have normal outcomes. The nuchal translucency may resolve or evolve into a cystic hygroma.

Nuchal translucency >3.5mm (99th centile for CRL = 85 mm) + Normal fetal karyotype + Normal follow up ultrasounds (including fetal echocardiography):

·         Survival rate = 81.8% overall (4).

·         Adverse outcome in 2-18% of cases (4).

·         Chance of livebirths in the absence of a defect  (4):

o       3.5 – 4.4 mm      = 86%

o       4.5 – 5.4 mm      = 77%

o       5.5 – 6.4 mm      = 67%

o       6.5 mm or higher = 31%

·         Undiagnosed congenital malformations (especially cardiac defects).

·         Undiagnosed genetic syndromes (single gene disorders in 8.5%) (5).

·         Developmental problems in children (2.4% if nuchal translucency is above 3.5 mm in the absence of other structural anomalies).

·         Problem – “How normal is a normal karyotype” – Normal karyotype from trophoblasts following CVS is safe and adequate to rule out aneuploidy, however analysis is dependent on the amount of tissue obtained, the adequacy of the sample and the examination of 150-200 chromatin bands. “This may not be sensitive enough to detect small deletions such as 4p-, 5p-, 5q-, 13q-, 14q- and 18p- or small de novo arrangements on fetal chromosomes 13, 18, 21 and 22 in cases with balanced parental translocations that have been associated with cystic hygromata” (6).

Increased nuchal translucency is however associated with an increased frequency of:

·         Chromosomal abnormality.

·         Spontaneous abortion (3).

·         Premature delivery (3).

·         Intrauterine demise.

·         Postnatal death of survivor with defects.

·         Twin-twin transfusion syndrome in monochorionic twins.

 

 

Video clip of increased Nuchal Translucency - Normal Karyotype

 

 

 

 

REFERENCES
  1. Schulman LP, Emerson DS, Felker RE et.al. High frequency of cytogenetic abnormalities in fetuses with cystic hygroma diagnosed in the first trimester. Obstet Gynecol 1992;80:80-82.
  2. Panyada PP, Snijders RJM, Johnson SP et.al. Natural history of trisomy 21 fetuses with increased nuchal translucency thickness. Ultrasound Obstet Gynecol 1995;5:381-383.
  3. Reynders CS, Pauker SP, Benacerraf BR. First trimester isolated nuchal lucency: Significance and outcome. J Ultrasound Med 1997;16:101-105.
  4. Souks AP, Krampl E, Bakalis S et.al. Outcome of pregnancy in chromosomally normal fetuses with increased nuchal translucency in the first trimester. Ultrasound Obstet Gynecol 2001;18:9-17.
  5. Pajkrt E, Mol BW, Blecker OP et.al. Pregnancy outcome and nuchal translucency measurements in fetuses with normal karyotype. Prenat Diagn 1999;19:1104-1108.
  6. Ville Y. Nuchal translucency in the first trimester of pregnancy: ten years on and still a pain in the neck? Ultrasound Obstet Gynecol 2001;18:5-8.