Wide iliac angle and short limbs. The best marker for Trisomy 21?

Abstract

Aim: • To create a standard protocol for measuring the posterior angle of the iliac crests. • To construct a score based upon the iliac angle and fetal proximal long bone lengths. • To compare results between fetuses affected by Trisomy 21 and those unaffected. Methods: Fetal morphology and biometry including biparietal diameter, humeral length and femur length were assessed between 15 and 23 weeks gestation according to standard protocol. A symmetrical image of the transverse plane of the iliac wings demonstrating the widest angle and the longest length of the iliac bones was recorded. This view was best obtained with the ultrasound beam perpendicular to the long axis of the fetal spine and perpendicular to the skin line, imaged from posteriorly. It can also be obtained from an anterior aspect. The iliac angle was measured from the internal angle at the intersection of tangential lines drawn along the posterior iliac crests.Results: A total of 2998 fetuses, 30 with Trisomy 21 and 2968 unaffected fetuses (902 had amniocentesis) were assessed. There was a significant difference (16.78° (95% CI 16.694; 16.846)) between the mean iliac angle for Trisomy 21 fetuses (94.33 ± 7.2°) and unaffected fetuses (77.55 ± 8.86°). The total of 90° was selected as cut‐off. The 22 of 30 (83%) Trisomy 21 fetuses and 228 of 2968 (7.7%) unaffected had iliac angle greater than or equal to 90° (sensitivity 83%, specificity 92.3%). A previous study in our unit had shown that short proximal long bones occurred in 41 of 71 (58%) Trisomy 21 fetuses and 609 of 9022 (6.7%) of a low risk unaffected group (unpublished data). Iliac angle greater than or equal to 90° scored one. Short proximal long bones also scored one. A positive index was a score of 2. The 14 of 30 (46.7%) Trisomy 21 fetuses and 29 of 902 (3.2%) known unaffected fetuses had a score of 2. The likelihood ratio for positive limb–iliac index is 14 (46.7/3.2).Conclusions: In screening for Trisomy 21, the prevalence and likelihood ratio of fetal anomaly (29%, 96), nuchal skin fold (39%, 11), echogenic bowel (20%, 2), renal pelvic dilatation (20%, 4.2), echogenic intracardiac focus (35%, 2.6) and short long bones (58%, 1.9) are known for our population. A positive iliac angle/limb length score has a prevalence of 47% and likelihood ratio of 14. In our population, this combination surpasses the performance of other markers.