Anatomically corrected malposition of the great arteries is defined as malposition of the great arteries in which both great arteries nevertheless arise above the morphologically appropriate ventricles, aorta above the morphologically left ventricle (LV), and pulmonary artery above the morphologically right ventricle (RV). Such cases have been regarded as errors of observation (Lochte, 1898), as inexplicable variations of nature (Geipel, 1903), as embryologically impossible and hence nonexistent (Van Mierop and Wiglesworth, 1963), and as probably nonexistent (Van Praagh and Van Praagh, 1964).
But shortly thereafter, we encountered three autopsied cases that proved that anatomically corrected malposition of the great arteries (ACMGA) does indeed exist. The anomaly now known as ACMGA was first described in 1895 by Théremin (his case 47). In 1939, Harris and Farber named this malformation “anatomically corrected transposition,” but they did not present any case of this rare anomaly.
Why anatomically correct transposition ? you may wonder. Before 1971, transposition of the great arteries was defined, very broadly, as any anomaly of the great arteries between the great arteries themselves, and/or between the great arteries and the ventricles, or the ventricular septum, or the atrioventricular (AV) valves. So, before about 1971, virtually any positional anomaly of the great arteries was called a transposition of the great arteries; hence Harris and Farber’s anatomically corrected transposition of the great arteries.
In 1971, we proposed that transposition of the great arteries (TGA) be used in its literal and original sense: trans, “across” and positio, “a placing” (Latin), the ventricular septum being the frame of reference. This is what Farre meant when he introduced the term transposition of the aorta and pulmonary artery in 1814. Both great arteries are placed across the ventricular septum, and so both arise above the wrong ventricles. By 1970, this very broad usage of transposition was becoming obsolete. Clear, accurate terminology was needed, not only for TGA, but also for double-outlet RV (DORV), for double-outlet LV (DOLV), and for ACMGA. In ACMGA, TGA is not present, by definition. In ACMGA, neither great artery is transposed, accurately speaking.
What Causes Anatomically Corrected Malposition of the Great Arteries?
The answer is surprisingly simple. The ventricles loop in one direction, say to the right; and the subarterial infundibulum and the great arteries twist in the opposite direction, to the left. Opposite directions of torsion of the ventricular segment, relative to the infundibular and the great arterial segments, result in ACMGA. (What developmentally causes these opposite morphogenetic movements is unknown at the present time.)
However, at the level of cardiac anatomy, it may safely be said that cardiac geometry facilitates diagnostic understanding. Fig. 25.1 summarizes some normal cardiovascular anatomic geometry and other important diagnostic concepts.
Now let us consider a case of ACMGA ( Fig. 25.2 ). This patient was 2 years and 20 days of age. She died during a surgical attempt to relieve her pulmonary outflow tract stenosis. Autopsy revealed dextrocardia, situs solitus of the viscera and atria with a normal spleen, left-sided juxtaposition of the atrial appendages (both to the left of the great arteries), tricuspid atresia, a large secundum type of atrial septal defect, a ventricular D-loop (right-handed morphologically RV to the right of a left-handed morphologically LV), a small infundibuloventricular type of ventricular septal defect (VSD) between the infundibulum above and the ventricular septum below, a bilateral infundibulum (subaortic and subpulmonary), infundibular and valvar pulmonary outflow tract stenosis with a thickened, bicuspid pulmonary valve, and L-malposition of the great arteries (L-malposed aortic valve to the left of the malposed pulmonary valve). The malposed aortic valve was located above the LV, and the malposed pulmonary valve was above the RV. So, ACMGA was present. The diagnosis was ACMGA {S,D,L} with a bilateral infundibulum (subaortic and subpulmonary), tricuspid atresia, and pulmonary outflow tract stenosis. There was a probe-patent ductus arteriosus and a right aortic arch.
Cardiac geometry makes this diagnosis much more precise. The following measurements are projected on the horizontal plane and are expressed relative to the Z axis (the anteroposterior line):
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Atrial septal angle ≈ 60 degrees left (normal ≈ 30 degrees left; see Fig. 25.2 )
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Ventricular septal angle ≈ 60 degrees right (normal ≈ 35 to 40 degrees left)
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AV septal angle ≈ 120 degrees a
aThe AV septal angle = the difference between the atrial and the ventricular septal angles.
(normal ≈ 5 to 10 degrees)
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Infundibular septal angle ≈ 65 degrees right (normal ≈ 65 degrees right)
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Infundibuloventricular septal angle b
bInfundibuloventricular septal angle ˜ the difference between the infundibular and the ventricular septal angles.
≈ 5 degrees 1 (normal ≈ 100 degrees)
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Rotation at the semilunar valves ≈ 20 degrees left (normal ≈ 150 degrees right)
Because all of these geometric measurements can now be made noninvasively, my task is to show why they are diagnostically and surgically helpful. The abnormally large atrial septal angle is related to left-sided juxtaposition of the atrial appendages (Lt JAA). Lt JAA in turn indicates that the right atrial appendage (RAA) is still in its early embryonic location to the left of the great arteries, which is normal at an embryonic age of less than 24 days of age in utero. But between 24 and 27 days of age, the RAA moves to the right of the great arteries—to its normal, definitive postnatal location. In somewhat greater detail, ventricular loop formation is a “two-step dance.” With normal D-loop formation, first you loop to the right (step 1). Then you swing to the left (step 2)—from dextrocardia, through mesocardia, to levocardia. If step 2 is not performed well, one is left with dextrocardia (as in this patient) or mesocardia. And the great arteries are not carried far enough to the left. So the great arteries remain to the right of both atrial appendages, resulting in Lt JAA. So Lt JAA indicates subnormal ventricular D-looping.
If this left-to-right shift in the location of the RAA fails to occur, this strongly suggests that there is a major malformation at the ventricular level. Left-sided juxtaposition of the RAA is strongly associated with right-heart pathologic conditions, and this patient has tricuspid atresia and a diminutive RV (see Fig. 25.2D–E ).
The angle between the atrial septum and the ventricular septum (the AV septal angle) is enormous (120 degrees), documenting that there is a major malalignment between the atria and the ventricles. This ventriculoatrial malalignment placed the ventricular septum immediately beneath the expected site of the tricuspid valve, apparently resulting in tricuspid atresia . The very right-sided location of the ventricular septum (60 degrees right) and the large LV correlate with dextrocardia (see Fig. 25.2A, C ). The L-malposition of the aortic valve (20 degrees left) plus the malformed ventricular D-loop support the diagnosis of anatomically corrected malposition (ACM).
Thus, cardiovascular geometry provides a new level of spatial diagnostic precision that can be used clinically by noninvasive diagnostic imaging.
ACMGA also occurs in patients with L-loop ventricles (ventricular inversion), as the next case illustrates. This baby boy ( Fig. 25.3 ) died at 4 weeks and 5 days of age from congestive heart failure. Autopsy revealed levocardia (left-sided heart), situs solitus of the viscera and atria, a ventricular L-loop (morphologically RV left-sided, levo- or L-, and left-handed, and morphologically LV right-sided and right-handed), a bilateral infundibulum (subaortic and subpulmonary, with no semilunar–AV valvar fibrous continuity), and with D-malposition of the great arteries, malposed aortic valve to the right, dextro- or D-, of the malposed pulmonary valve. The malposed aortic valve was above the morphologically LV, and the malposed pulmonary valve was above the morphologically right ventricle. Hence, anatomically corrected malposition of the great arteries (ACMGA) was present. The diagnosis was ACMGA {S,L,D} with a bilateral infundibulum and a slit-like infundibuloventricular type of VSD (between the infundibular septum above and the ventricular septum below) (see Fig. 25.3 ).
