Right Atrial Isomerism – Preponderance in Asian Fetuses. Using the Stomach-distance ratio as a Possible Diagnostic Tool for Prediction of Right Atrial Isomerism

INTRODUCTION: To present the characteristics and spectrum of associated anomalies in right- and left-sided isomerism in our local population and to assess the possibility of using stomach-distance ratio (SDR) of less than 0.34 as a diagnostic tool to predict right atrial isomerism. MATERIALS AND METHODS: This was a retrospective study of fetuses in our department over a period of 8 years with postnatally confirmed prenatal diagnosis of atrial isomerism. RESULTS: In 22 cases, atrial isomerism was confirmed by post-mortem or postnatal echocardiography. Eighteen (81.8%) fetuses had right isomerism. Their main abnormal ultrasound findings were pulmonary stenosis or atresia (n = 9), atrioventricular septal defect (n = 10), right-sided stomach (n = 9), transposition of great arteries (n = 6), dextrocardia (n = 8), single ventricle (n = 4), juxtaposition of inferior vena cava and descending aorta (n = 5), ventricular septal defect (n = 2), interrupted inferior vena cava with azygous drainage (n = 2) and double outlet right ventricle (n = 3). Four (18.2%) fetuses had left isomerism. Their abnormal ultrasound findings were dextrocardia (n = 3), right-sided stomach (n = 3), atrioventricular septal defect (n = 2), double outlet ventricle (n = 2), ventricular septal defect (n = 1), pulmonary stenosis (n = 2) and interrupted inferior vena cava with azygous drainage (n = 1). 66.7% (12/18) of cases with right isomerism had SDR of less than 0.34 compared to 0% (0/4) of the cases with left isomerism (P = 0.02). CONCLUSION: Our study suggests an Asian predilection towards right isomerism compared to Western populations. We postulate that there may be racial differences in the expression of these 2 forms of isomerism. The ultrasound findings of complex heart disease and abnormal arrangement of great vessels in abdominal cavity, though important, are varied and non-specific evidence for either form of fetal atrial isomerism. There is a possibility of using the SDR <0.34 (representing stomach proximity to the fetal spine) as a possible diagnostic tool to predict right-sided atrial isomerism.