From the Guidelines
The presence of a small nasal bone, normal nuchal translucency, and intrauterine growth restriction (IUGR) indicates a need for further evaluation to assess the risk of trisomy 21, and definitive genetic testing is strongly recommended. A small or absent nasal bone is associated with an increased risk of trisomy 21, as approximately 60-70% of fetuses with Down syndrome have this feature 1. However, normal nuchal translucency is reassuring, as increased NT is typically a marker for chromosomal abnormalities. The presence of IUGR is concerning but not specific to trisomy 21, as it can occur with various chromosomal abnormalities or non-genetic conditions.
Given these mixed findings, the following options are recommended:
- Non-invasive prenatal testing (NIPT), which analyzes cell-free fetal DNA in maternal blood with high accuracy
- Diagnostic procedures such as amniocentesis or chorionic villus sampling (CVS) which provide definitive results These findings alone cannot confirm or rule out trisomy 21, and the overall risk assessment should incorporate maternal age, gestational age, and other screening results. A consultation with a maternal-fetal medicine specialist or genetic counselor would be beneficial to discuss testing options and interpret results in the context of the complete clinical picture 1.
Some key points to consider:
- The positive likelihood ratio (LR) of an absent or hypoplastic nasal bone is 23, suggesting a high risk, although there is substantial heterogeneity in the studies included in this meta-analysis 1
- The presence of a thickened nuchal fold is one of the most specific markers for trisomy 21, with a positive LR ranging between 11 and 23 1
- The decision to perform a detailed examination should be made through a shared decision-making framework, incorporating patient preferences, values, and availability of resources 1
Overall, a comprehensive approach is necessary to evaluate the risk of trisomy 21 in the presence of a small nasal bone, normal nuchal translucency, and IUGR, and to provide appropriate counseling and management options for the patient.
From the Research
Trisomy 21 Screening
The presence of a small nasal bone, normal nuchal translucency, and intrauterine growth restriction (IUGR) can be indicative of an increased risk for trisomy 21.
- A small nasal bone is associated with an increased risk of trisomy 21, as studies have shown that the nasal bone is absent in approximately 70% of fetuses with trisomy 21 2, 3, 4, 5, 6.
- Normal nuchal translucency is a good sign, as increased nuchal translucency thickness is a marker for trisomy 21 2, 4, 5, 6.
- IUGR can be a complication of trisomy 21, but it is not a definitive indicator of the condition.
Detection Rates and False-Positive Rates
- The detection rate for trisomy 21 using a combination of maternal age, fetal nuchal translucency, and maternal serum biochemical markers is approximately 90% for a false-positive rate of 5% 2, 6.
- The inclusion of nasal bone assessment in the screening test can increase the detection rate to 97% for a false-positive rate of 5% 6.
- A two-stage screening strategy, which includes assessment of the nasal bone only in cases with an intermediate risk, can achieve a detection rate of 93% for a false-positive rate of 2.4% 5.
Nasal Bone Assessment
- The nasal bone is absent in approximately 70% of fetuses with trisomy 21 2, 3, 4, 5, 6.
- The presence or absence of the nasal bone is independent of the level of maternal serum biochemical markers and fetal nuchal translucency thickness 6.
- Assessment of the fetal nasal bone can improve the performance of first-trimester screening for trisomy 21 5.