Absent/Hypoplastic Fetal Nasal Bone and Its Association with Aneuploidies

Absence or hypoplasia of fetal nasal bone (AHNB) in the first or second trimester scans, with increased risk for trisomy 21 has been shown in many studies. In view of reports of ethnic difference in the size of the nasal bone the usefulness of its evaluation in Indian women is desirable. All pregnant women presenting to the Genetic Clinic from Jan 2012 through April 2014 with ultrasound finding of absent or hypoplastic nasal bone, in either first or second trimester of pregnancy were prospectively enrolled in the study, after obtaining informed consent. Nasal bone hypoplasia was defined as nasal bone length less than 2.5th percentile for the period of gestation. The presence of other associated soft markers for aneuploidies along with result of biochemical screening was also recorded. All the patients were counseled for the appropriate invasive testing. Chorionic villi or amniotic fluid cells were analyzed using FISH studies for chromosome no. 13, 18, 21 and sex chromosomes and also cultured for the complete karyotype. The gestational age varied from 11 to 22 weeks, with the majority being between 17 and 20 weeks. The cohort of 92 patients was divided into five groups: Group 1: Isolated hypoplastic nasal bone with low risk in biochemical screening and no additional soft marker—30 cases (32. 6 %); Group 2: Isolated absent nasal bone—25 cases (27.1 %); Group 3: AHNB with additional ultrasound markers—29 cases (31.5 %); Group 4: Absent nasal bone (ANB) with high risk in biochemical screening with no other ultrasound abnormality—7 cases (7.6 %); and Group 5: ANB with additional ultrasound findings and abnormal biochemical screen (1 case). The karyotype in 20 (21.7 %) of 92 cases was abnormal. In group 1 three cases (10 %) had chromosomal abnormalities—47, XXX; mosaic 45, X; 47, XXY. In group 2, three (12 %) cases had trisomy 21. In group 3, 12 (41.3 %) cases had chromosomal abnormality—trisomy 21 (10), derivative 22 (1), triploidy (1). In group 4 trisomy 21was present in two (28.5 %) cases. The one patient in group 5 had a normal fetal karyotype. The present study shows that isolated absent/hypoplastic nasal bone was associated with chromosomal disease in 10–12 % of cases. However, cases where AHNB was associated with ultrasound anomalies, or with high risk biochemical studies yielded abnormal chromosomes in a significantly larger number of patients (41.4 and 28.5 % respectively). It is, therefore, recommended that this group should always be analyzed for chromosomal abnormalities. Recent studies suggest that nasal bone length may be smaller in fetuses in Indian women. If standards derived in Indian women are used, the yield of chromosomal abnormalities may be greater.