Nuchal translucency

Nuchal translucency

Description

Nuchal translucency refers to the fluid collection behind the fetal neck which is measured between crown rump lengths of 38mm and 84mm , corresponding approximately with 10 to 14 weeks gestation. It is the maximum width of the subcutaneous translucency between the skin and the soft tissue overlying the cervical spine of the fetus. It should not be confused with nuchal (fold) thickness which appears to be a separate entity. Very few cases of increased nuchal translucency progress to raised second trimester nuchal thickness. It is unknown what pathological nuchal translucency represents but several theories have been advanced to explain it. It is suggested that fluid collects behind the neck, representing the endpoint of one of several pathological processes, in a similar fashion to dependent ankle edema later in life. This is partly because of a tendency on the part of the fetus to lie on its back and partly due to laxity in the skin of the neck. Theories of pathogenesis include: i) cardiac failure related to congenital cardiac abnormality or dysfunction; ii) venous congestion due to chest compression, for example due to an intrathoracic lesion; iii) failure of lymphatic drainage due to neuromuscular problems, for example arthrogryposis causing contractures and flexion deformities; iv) abnormal development of lymphatics; or, v) altered connective tissue composition ,for example the increased elasticity of connective tissue in trisomy 21. It seems not to be a marker for fetal or maternal infection in the first trimester. Resolution after 14 weeks gestation may occur for a variety of reasons. The lymphatic system may have developed sufficiently to drain away any excess fluid. Furthermore, changes to the placental circulation will result in drop in peripheral resistance. However, a recent study suggested that fetoplacental vascular resistance per se does not contribute to increased nuchal translucency. It should be noted that nuchal translucency is a feature of both normal and abnormal pregnancies. Further research into the normal anatomical development of the head and neck between the 8th and 14th week will be required to fully understand its’ significance. It may function as a pressure valve. The placenta is undergoing rapid growth with a consequent increase in circulating blood volume. The choroid plexus and other posterior fossa contents are also completing organogenesis at this time and may need to be protected from overperfusion. Nuchal translucency is related to crown rump length CRL and therefore normally increases with gestational age. This must be taken into account when interpreting results as the critical point for separating ’normal’ from ‘abnormal’ will vary as pregnancy progresses. However, nuchal translucency does not vary with maternal age. Significant differences have been found in nuchal translucency in different ethnic groups but too small to be of clinical significance when screening for Down syndrome. There is a range of ‘normal’ values for nuchal translucency, for which centile charts and tables exist. Above 3mm is generally considered to be high and about 5% of the population will fall into this group. Only 1-2% of pregnancies will have a translucency greater than 3.5mm. Increased nuchal translucency is associated with an increase in the risk of chromosomal abnormalities and in fetuses with a normal karyotype, an increase in the risk of other defects, especially cardiac. The greater the measurement, the greater the risk of chromosomal abnormality. At 3mm there is a three-fold increase in the risk of Down syndrome, rising to 36 times the background risk at > 6mm. Conversely, a “normal” measurement correlates with a decreased risk of aneuploidy. Nuchal translucency may also be a predictor of fetal outcome. About 90% of fetuses with nuchal translucency greater than 3mm at 12 weeks are normal at birth with 10% having major abnormalities. In contrast, only 10% of fetuses with a measurement of greater than 6mm at 12 weeks are normal at birth. As well as Down, increased nuchal translucency is associated with other major aneuploides – trisomy 18, 13, Turner’s Syndrome and triploidy. It is also associated with major congenital abnormalities including cardiac anomalies, diaphragmatic hernia, abdominal wall defects and a number of both genetic syndromes and single gene disorders. There is a background risk of aneuploidy / congenital abnormality in every pregnancy based on maternal age, previously affected pregnancies and gestational age. The risk increases with maternal age and decreases with advancing gestation. Tables exist for many of the common abnormalities. Importantly, nuchal translucency does not vary with maternal age. This means these two factors can be used independently to refine the risk for an individual pregnancy. An important application has been in screening for Down Syndrome (trisomy 21) in the first trimester. Nuchal translucency and crown rump length are used in combination with maternal age to refine risk and target diagnostic tests, eg CVS and amniocentesis. There are many studies examining the effectiveness of using nuchal translucency in the first trimester to screen for Down, including large series of unselected pregnancies. Most have used a nuchal translucency of either > 3 mm or > 95th centile in defining high / abnormal results. If amniocentesis is offered to 5% of the population with a calculated risk of 1 in 300 or more, approximately 78% of cases of Down may be detected prenatally. This compares with about 30% if testing is based on maternal age alone and approximately 65% based on maternal serum screening. Even so it should be noted that 30 invasive tests will be required to detect one case of Down Syndrome. Nuchal translucency is a useful screening tool for aneuploldy in twin pregnancies where serum screening is not applicable. It may also have a small predictive value in the twin-twin transfusion syndrome. A major advantage of nuchal translucency measurement is to raise suspicion of an abnormal pregnancy much earlier than with traditional screening methods. Invasive diagnostic procedures may be targeted more effectively and counseling and interventions offered at an earlier gestation. The finding of increased nuchal translucency between 10 and 14 weeks should prompt consideration of karyotyping and, especially in those with normal karyotype, a search for other major congenital abnormalities, including fetal echocardiography.

Diagnosis

Measurement of nuchal translucency must be performed in a standardized and reproducible fashion by operators trained in the technique. Their measurements should be validated before being used within a screening program. The method developed by the Fetal Medicine Foundation in the U.K. is carried out between 10 and 14 weeks, CRL- The transabdominal approach will be satisfactory 95% of the time but transvaginal imaging can be used in other cases if necessary. Their approach includes the following points. -A good sagittal section of the fetus, as for measurement of CRL should be obtained. -The fetus should occupy at least three quarters of the image. -A careful distinction between fetal skin and amnion should be made as both of these appear as a thin membrane at this gestation. Either wait for or induce fetal movement away from the amnion by asking the mother to cough or tapping the abdomen. -Measure the maximum thickness of the subcutaneous echolucency between the skin and the soft tissue overlying the cervical spine. The calipers should be placed on the internal edges of the membranes. -Take more than one measurement and use the mean of two good measurements. Intra- and inter-observer variability has been demonstrated to be 0.54 and 0.62mm respectively in 95% of cases. As it increases with CRL, gestation must be taken into account when determining whether a given nuchal translucency is raised or not. The position of the fetal neck may be of importance. An extended position gives a greater value (by 0.62mm), whilst the flexed position results in a lesser value (by 0.4mm) than the mean nuchal translucency measured in the neutral position. A nuchal cord may also bias results adding a mean of 0.8mm to the measurement. This occurs in about 8% of pregnancies at 10-14 weeks. Color Doppler ultrasound may be useful to lower the false positive rate. A computer program has been developed by the Fetal Medicine Foundation, London, UK. which will calculate the risk based on maternal age, crown rump length and nuchal translucency measurement.

Differential Diagnosis

Nuchal translucency may be part of the spectrum of cystic hygroma. However, a recent study examining the differences between the two using 3-D ultrasound showed greater irregularity, extent and amplitude of membrane in cystic hygroma whereas in simple nuchal translucency the membrane was of a more homogenous linear nature.

Sonographic Features

Increased width of the echolucency between the nuchal skin and the soft tissue overlying the cervical spine.

In general, a measurement greater than 3 mm would be considered abnormal but crown rump length must also be taken into account.

Associated Syndromes 

References

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