Defining transposition: What have we learnt?

Understanding transposition is important for all who hope to effectively treat patients with the condition. The variants of the condition are frequently debated in the literature. We describe an unusual variant of transposition, in which despite the arterial roots being supported by morphologically inappropriate ventricles, the roots themselves were normally related, with the intrapericardial arterial trunks spiraling as they extended into the mediastinum. The specimen was identified following the re-categorization of our archive, and we subsequently conducted a detailed analysis of the underlying morphology. Using the principles of sequential segmental analysis, we compared the morphology with standard examples previously described. We show how it was the recognition of such hearts that promoted that concept that the combination of connections across the atrioventricular and ventriculo-arterial junctions was the essence of transposition. In the most common variant, the arrangements are concordant at the atrioventricular junctions, but discordant at the ventriculo-arterial junctions. We suggest that the overall arrangement of discordant ventriculo-arterial connections is best described simply as "transposition." When the discordant ventriculo-arterial connections are combined with similarly discordant connections at the atrioventricular junctions, the transposition is congenitally corrected. We point out that the use of "d" and "l" as prefixes does not distinguish between transposition and its congenitally corrected variant. For those using segmental notations, the correct description for the rare variant found in the setting of a posteriorly located aortic root with the usual atrial arrangement is transposition (S, D, NR).

Anatomical Studies of Transposition - An Argument for a Unifying Morphological Classification.

In the setting of transposition, recognition of the variability in the morphology of the outflow tracts and the arterial trunks is essential for surgical repair. Presence of a unifying system for classification would minimize the risk of misunderstanding. We examined an archive of 142 unrepaired hearts with transposition, defined as the combination of concordant atrioventricular and discordant ventriculoarterial connections, combined with the clinical records of 727 patients undergoing the arterial switch procedures. In the setting of usual atrial arrangement, we found the aortic root in leftward or posterior location in up to 5% of our cohorts, making the common term "d-transposition" ambiguous. Variability in the relationship of the trunks was commoner in the setting of deficient ventricular septation, especially when the trunks were side-by-side (14% vs 5.5% when the ventricular septum was intact). Bilateral infundibulums were present in 14% of cases, and bilaterally deficient infundibulums in 3.5%. Both of these findings were more common with deficient ventricular septation. Fibrous continuity between the tricuspid and pulmonary valves was not always seen with perimembranous defects, particularly when there were bilateral infundibulums. Fibrous continuity between the leaflets of the tricuspid and mitral valves, in contrast, proved a unifying characteristic of the perimembranous defect. The combination of concordant atrioventricular and discordant ventriculoarterial connections is best described simply as "transposition," with precision provided when details are given of atrial arrangement and the relationships of the arterial trunks.