[ABERRANT RIGHT SUBCLAVIAN ARTERY (ARSA)--A NEW ULTRASOUND MARKER FOR CHROMOSOMAL FETAL ABNORMALITIES]

The development of the fetal aorta ends with the formation of the aortic arch which normally branches into three blood vessels: 1) a. brachiocephalica (a. innominata), which divides into the right subclavian artery (RSA) and the right carotid artery; 2) the left carotid artery; and 3) the left subclavian artery. Occasionally, RSA originates as a separate fourth branch of the aortic arch, passing behind the trachea with an oblique course to the right shoulder. This rare variant is called an aberrant right subclavian artery (ARSA) and is observed in approximately 2% of normal individuals. On the other hand, the reported incidence of ARSA varies between 25 and 37% in cases with Down syndrome and other chromosomal abnormalities.

To evaluate the success rate of ultrasound visualization of the fetal RSA between 18 and 23 weeks of gestation and to establish the importance of the prenatal diagnosis of ARSA in the risk assessment for fetal chromosomal abnormalities in the second trimester.

Three experienced sonographers scanned 992 fetuses in MC "Markovs", Sofia between 01.09.2013-01.06.2014 with Voluson 730 Expert (GE Healthcare) ultrasound equipment. Visualization of RSA was successful in 92.7% of cases. Overall, 17 cases with ARSA were diagnosed in the study period. ARSA was an isolated sonographic finding in 13 of them. The remaining 4 cases had additional pathology. In the first case ARSA was associated with a short femurand humerus, short nasal bone and borderline nuchal thickness without any other soft markers or structural abnormalities. Trisomy 21 was diagnosed after amniocentesis and the pregnancy was terminated at patient's request. In the second case ARSA was associated with severe polymalformation syndrome. Trisomy 18 was diagnosed by DNA analysis post abortem. In the third case ARSA was associated with an unilateral cleft lip and cleft palate. Abnormalities of the fetal karyotype and Di George syndrome were excluded by amniocentesis. The fourth case was associated with a single umbilical artery without any structural and chromosomal abnormalities. The pregnancy had a favourable perinatal outcome at term.

Visualization of RSA and prenatal diagnosis of ARSA in the second trimester is relatively easy in experienced hands. The examination slightly prolongs the fetal morphology scan. Since there is an obvious association between ARSA and chromosomal fetal abnormalities, implementation of its sonographic evaluation in the protocol of fetal echocardiography in the second trimester is strongly recommended.