Heterotaxy (HTX) is usually stratified in right (RI) and left isomerism (LI).

We analyzed 61 fetal specimens with HTX divided in 5 groups: classic LI [CLI: bronchopulmonary LI, bilaterally absent pectinate muscles (PM), polysplenia, n=22], non-classic LI (NCLI: one discordant feature, n=13), classic RI (CRI: bronchopulmonary RI, bilateral extent of PM to the crux, asplenia, n=12), non-classic RI (NCRI: one discordant feature, n=6, totally discordant features, n=8).

Non-classic patterns were found in 44% (33% RI, 37% LI). Among those, the status of the spleen was highly variable (NCLI with asplenia 38%, NCRI with single spleen 66%), while the bronchial status was almost always concordant. Intestinal malrotation was present in 83% CRI, 64% CLI, 50% NCLI, 50% NCRI.

Extent of PM was highly variable in NCLI (bilaterally left 46%, normal 46%, bilaterally right 8%), and bilaterally right in 83% of NCRI only. Interrupted inferior caval vein was seen in LI only; total extracardiac anomalous pulmonary venous connection (APVC) was seen only in RI, ipsilateral APVC only in LI. A common atrioventricular (AV) junction was constant in RI vs. 66% in LI, with a complete AV canal in 100% RI, 75% LI. Functionally univentricular hearts were found in 67% RI, 46% LI, right in 62.5%. Ventriculo-arterial (VA) connections were abnormal in 100% RI, 59% LI, double outlet right ventricle (DORV) being the most frequent type in all categories. Pulmonary atresia and stenosis were frequent in RI (77%), rare in LI. Aortic arch hypoplasia and coarctation were found almost exclusively in LI (57%). Right-sided aortic arch was found in 67% RI, 6% LI.

HTX and isomerism are not synonymous. Extent of PM is not always symmetrical (15%) and cannot therefore be considered the hallmark of HTX. Because HTX includes both abnormal symmetry and random organization of organs, each anatomic feature should be analysed individually rather than trying to categorize.