Reliability of fetal thymus measurement in prediction of 22q11.2 deletion: a retrospective study using four-dimensional spatiotemporal image correlation volumes

Objectives To evaluate the thymicthoracic ratio (TT-ratio) method in assessment of the fetal thymus in normal fetuses and in those with cardiac abnormalities, in the presence or absence of 22q11.2 deletion. Method Database records were reviewed for cases of conotruncal and arch abnormalities found on fetal echocardiography between January 2007 and September 2011. The 22q11.2 deletion status was retrieved and cases in which this was not known were excluded from the analysis, as were fetuses with aneuploidy or other genetic disorders. An additional 55 normal fetuses were analyzed as a control group. The TT-ratio was measured retrospectively using stored spatiotemporal image correlation (STIC) volume datasets. Results Sixty-nine fetuses with relevant cardiac diagnoses were identified and, of these, 18 (26%) had 22q11.2 deletion. The mean gestational age at diagnosis was 22 weeks. Significant pairwise differences, but also overlap, were observed between all three groups (i.e. fetuses with heart defects with and without the 22q11.2 deletion and controls). The mean TT-ratio was 0.44 in our normal control group and was significantly smaller in fetuses with 22q11.2 deletion, corresponding to previously published data. However, the mean TT-ratio in the group with conotruncal anomalies but without the 22q11.2 deletion was also smaller than that in controls, in contrast to previously published data. The TT-ratio was above the normal mean, regardless of fetal karyotype, in all cases of interrupted aortic arch. Conclusion The TT-ratio method is a feasible and potentially useful tool during detailed fetal heart assessment. However, the absolute measurement is not reliable for prediction of 22q11.2 deletion and the obtained results should therefore be interpreted with caution. Fetal karyotyping should be recommended in cases with conotruncal heart abnormalities, irrespective of the TT-ratio.