What are anomalous muscle bundles of the right ventricle, also known as double-chambered right ventricle?
As will be seen, these malformations really are anomalous infundibular muscle bundles.
Statistics
In the Congenital Cardiac Pathology Database of Boston Children’s Hospital, the diagnosis of anomalous muscle bundles of the right ventricle was made in 33 postmortem patients.
The diagnosis of double-chambered right ventricle was made in 17 of our postmortem congenital heart patients.
These two diagnoses were made in a total of 50 of 3216 autopsied patients with congenital heart disease (1.55%).
Gender: males, 9; females, 10; males/females, 0.9. The gender was not known by us in one consultation. No gender predominance was found.
Age at death: mean, 5.2 years; standard deviation, ±13.1 years; minimum, 1 day; maximum, 55 years. The age at death was not known by us in one consultation.
Terminology
I have used the terminology of Lev, Edwards, and the American school. But other designations are used by some of our friends and colleagues. Hence, a few words of explanation are given here to facilitate widespread comprehension.
When introduced, trabecula septomarginalis meant the moderator band: trabecula (little beam) septomarginalis (from the septum to the acute margin of the right ventricle). But since trabecula septomarginalis is Latin, some of our colleagues translated it into English: septomarginal trabeculation . We think that’s fine; we have no quarrel with English.
But the problem is: the meaning of this term has changed. Now, some of our colleagues use septomarginal trabeculation to mean both septal band and moderator band.
As will be seen, in dealing with anomalous muscle bundles of the right ventricle, it is helpful to be able to designate the septal band and the moderator band separately and clearly, and to avoid the confusion in usage that has now occurred to the meaning of septomarginal trabeculation.
Findings
The normal morphologically and segmentally right ventricle ( Fig. 19.1A ) and left ventricle ( Fig. 19.1B ) are presented diagrammatically in this figure.
A random sample of 20 autopsied cases of anomalous muscle bundles (AMB) of the right ventricle (RV) revealed the following:
We did not encounter a case of AMB of the right RV in which this anomaly occurred as an isolated malformation; it was always associated with other anomalies.
AMB of the RV can be associated not only with malformation of the proximal infundibulum, that is, the obstructive moderator band type ( Fig. 19.1A ). It can also be associated with malformation of the distal infundibulum, that is, the obstructive parietal band type ( Fig. 19.1A ).
C83-162 was a 2½-year-old boy with double-outlet right ventricle (DORV) {S,D,D}. He had an incomplete bilateral conus, that is, a complete subpulmonary conus, and an incomplete subaortic conus. There was a “tongue” of fibrous tissue between the aortic valve and the tricuspid valve. The left ventricular pressure measured at cardiac catheterization was 132/13 mm Hg, which was 46 mm Hg suprasystemic. The ventricular septal defect (VSD) was obstructive. Inferior atrioventricular endocardial cushion tissue produced the obstruction. The VSD was of the atrioventricular endocardial cushion type. The pulmonary outflow tract was also stenotic; the pulmonary outflow tract gradient was 40 mm Hg and the pulmonary artery pressure was 41/28 mm Hg, with a mean pulmonary arterial pressure of 32 mm Hg. Thus, despite the pulmonary outflow tract stenosis, there was mild pulmonary arterial hypertension.
In this patient, anomalous distal infundibular muscle bundles of the parietal band type (component 4, Fig. 19.1A ) separated the RV sinus (component 2, Fig. 19.1A ) from the infundibular outflow tract (components 3 and 4, Fig. 19.1A ).
In this study of 20 autopsied cases of AMB of the RV, this was the only case in which the distal, subarterial, parietal band part of the infundibulum (component 4, Fig. 19.1A ) was involved (5%). In all other patients (95%), it was the proximal part of the infundibulum—the septal band and the moderator band—that was involved.
In this patient, there were three sites of obstruction:
- 1.
at the VSD, obstructing the left ventricle (LV) outflow tract;
- 2.
between the RV sinus and the infundibulum, caused by the distal infundibular muscle bundles—the parietal band type of AMB of the RV; and
- 3.
the pulmonary outflow tract.
It will be recalled that this patient had DORV {S,D,D} with a bilateral conus (subaortic and subpulmonary), but that the muscular subaortic conus was incomplete, with a “tongue” of fibrous tissue connecting the aortic and tricuspid valves. We think that this conal anatomy represents an early stage in the normal demuscularization or involution of the subaortic infundibular free wall musculature, known as apoptosis. Normally, demuscularization or involution of the subaortic infundibular free wall happens more extensively, permitting normal aortic-mitral and aortic-tricuspid fibrous continuity with normally related great arteries.
DORV was present in 3 of these 20 patients with AMB of the RV (15%).
C89-176 was another fascinating patient with AMB of the RV and DORV. This patient had DORV {S,D,D} and a subpulmonary infundibulum. There was no subaortic infundibular free wall, which permitted aortic valve–to–tricuspid valve direct fibrous continuity.
This heart was like the original Taussig-Bing heart, , except that there was no subaortic infundibular free wall in the present AMB case. The VSD was subpulmonary. The double-chambered RV (DCRV) muscle bundles between the RV sinus to the right and the infundibulum to the left were very restrictive. The proximal os infundibuli had one slit-like opening and two barely probe patent ostia. There was subaortic stenosis between the infundibular septum anteriorly and the tricuspid leaflets posteriorly. In other words, the infundibular septum was much too close to the tricuspid valve, producing significant subaortic outflow tract stenosis. The result was hypoplasia of the ascending aorta, tubular hypoplasia of the aortic arch, and a juxtaductal coarctation of the aorta. Thus, DCRV was part of complex congenital heart disease.
Tetralogy of Fallot (TOF) occurred in 4 of these 20 patients (20%) with AMB of the RV. Three patients had the usual solitus segmental anatomy, {S,D,S}. But one patient had inverted TOF with inverted segmental anatomy, {I,L,I}, and also with dextrocardia. All had a high takeoff of the moderator band from the septal band, that is, the proximal infundibular or moderator band type of AMB of the RV ( Fig. 19.1A ).
Multiple congenital anomalies occurred in 4 patients with AMB of the RV (20%), three of which were noteworthy: the polysplenia syndrome in 1, trisomy 13 in 1 patient with TOF {S,D,S}, and parachute mitral valve with no congenital mitral stenosis in 1.
Congenital absence of tricuspid valve leaflets, also known as congenitally unguarded tricuspid orifice, occurred in one patient with AMB of the RV (5%).
C73-311, a 4-week-old girl with normal segmental anatomy, that is, {S,D,S}, had congenital absence of the tricuspid valve leaflets, hypoplasia of the RV sinus, and Uhl’s disease of the RV sinus free wall. The RV sinus did not communicate with the infundibulum. This is a rare case of double-chambered RV with complete separation of the RV sinus and the proximal infundibular chamber. In other words, there is atresia of the proximal os infundibuli pulmonalis (atresia of the proximal ostium leading into the subpulmonary infundibulum).
There was aortic-mitral fibrous continuity, and the left ventricular–to–aortic outflow tract was normal.
There was no right ventricular–to–pulmonary artery outflow tract, as above. But there was an outflow tract from the LV, through the subaortic VSD, into the subpulmonary infundibulum and through a normal appearing pulmonary valve. The main pulmonary artery and branches were mildly hypoplastic.
The functional part of this heart was similar to a Holmes heart: a single LV {S,D,S}. The differences are as follows: the RV sinus has appeared, but the tricuspid leaflets have not appeared; the RV free wall myocardium is absent or very deficient (Uhl’s disease); and the RV sinus does not communicate with the proximal part of the infundibulum, which was not identified (absent). The LV opens into the distal or subarterial part of the infundibulum, as it does in single LV with an infundibular outlet chamber and normally related great arteries {S,D,S}—the Holmes heart.
These observations suggest that more attention needs to be focused on anomalies of the proximal part of the infundibulum, which is often malformed in AMB of the RV. The distal part of the infundibulum is involved in abnormally related great arteries.
A79-105, a 3-month-old girl, had congenital absence of the pulmonary valve leaflets. She had pulmonary annular stenosis. She had marked dilation of the main pulmonary artery, right pulmonary artery, and left pulmonary artery. There was associated anterior flattening of the trachea and bronchi. She also had a VSD of the infundibuloventral type that was subaortic in location. Her anomalous muscle bundle of the right ventricle was a high takeoff of a prominent moderator band from the septal band.
This patient did not have TOF. This negative finding is mentioned specifically because absence of pulmonary leaflets has been associated with TOF. Rarely, we have seen absence of pulmonary valve leaflets in TGA {S,D,S}.
Pulmonary atresia with intact ventricular septum occurred in two patients (A71-60 and A70-6). Extremely severe pulmonary valvar stenosis with an intact ventricular septum occurred in one additional patient (A82-61). All three cases had high, prominent moderator bands.
Truncus arteriosus type A2 occurred in a 7-day-old girl (A88-44). Type A means that a VSD is present. Type 2 means that there is no aortopulmonary septal remnant present. AMB of the RV was present, namely, a high takeoff of the moderator band from the septal band. This patient also had a completely common atrioventricular canal, type A of Rastelli, an interrupted inferior vena cava, a small right ventricle, a thick myxomatous bicuspid truncal valve, and absence of the right superior vena cava.
In addition, this patient with truncus arteriosus also had multiple congenital anomalies with a normal karyotype: dysmorphic facies with a broad flat nasal bridge, right microtia, right hemifacial microsomia, anteverted nares, retrognathia, pectus excavatum, short sternum, webbed neck, two-vessel umbilical cord, 13 ribs bilaterally, bifid upper thoracic vertebrae, umbilical hernia, clinodactyly, and hypoplasia of the left lobe of the thymus.
TGA {S,D,D} with a large muscular VSD and a patent ductus arteriosus were found in a 4-month-old girl (A90-137). This patient had typical AMB of the RV, namely, a high takeoff of the moderator band from the septal band. There was hypoplasia of the aortic arch and aortic isthmus. Was this aortic hypoplasia secondary to her AMB of the RV? We were not sure.
Severe aortic valvar stenosis was found in a 55-year-old man (MR 15). He had a congenitally bicuspid aortic valve with marked underdevelopment of the right coronary–left coronary commissure. He also had typical AMB of the RV with a high takeoff of the moderator band from the septal band. We thought that his right ventricular AMB was of no clinical importance.
Discussion
Accurately speaking, anomalous muscle bundles of the right ventricle is really anomalous muscle bundles of the infundibulum.
There are two main anatomic types:
- 1.
anomalous muscles of the inferior infundibulum, involving a high takeoff of the moderator band from the septal band ( Fig. 19.1A ); and
- 2.
anomalous muscle bundles of the superior infundibulum, involving the distal conal septum and the parietal band ( Fig. 19.1A ).
In a patient with solitus or D-loop ventricles, anomalous infundibular muscles, both inferiorly and superiorly, tend to separate the right ventricular sinus (component 2) from the outflow tract (components 3 and 4) ( Fig. 19.1A ). Consequently, anomalous infundibular muscle bundles at the junction of the RV sinus (component 2) and the proximal infundibulum (component 3) and the distal infundibulum (component 4) can produce stenosis or rarely atresia at these component junctions ( Fig. 19.1A ). The result can be obstruction at approximately the mid-RV, halfway between the tricuspid valve and the semilunar valve, creating the appearance of a double-chambered RV.
Conclusions
The salient findings of this study are as follows:
- 1.
Anomalous muscle bundles of the right ventricle are really anomalous muscle bundles of the infundibulum.
- 2.
The inferior infundibulum (septal and moderator bands), the superior infundibulum (the infundibular septum and the parietal band), or both, may be involved in anomalous muscle bundles of the right ventricle.
- 3.
Anomalous muscle bundles of the right ventricle usually involves the inferior infundibulum, typically resulting in a high takeoff of the moderator band from the septal band (95% in this study). Involvement of the distal or subarterial infundibulum (the parietal band type) was infrequent (5%).
- 4.
The physiologic consequences of anomalous muscle bundles of the right ventricle were variable. They could result in stenosis in approximately the mid-RV, or rarely in atresia, or occasionally in no demonstrable obstruction.
- 5.
The obstruction, whether demonstrable or not, typically occurs at the junction of the right ventricular sinus (component 2) and the proximal infundibulum (component 3) ( Fig. 19.1A ). Typically, anomalous muscle bundles of the RV may produce stenosis at the proximal ostium of the infundibulum.
- 6.
We did not encounter anomalous muscle bundles of the right ventricle as an isolated anomaly.
- 7.
Anomalous muscle bundles of the right ventricle typically occurs with many different anatomic types of congenital heart disease, and with multiple noncardiovascular malformations ( Table 19.1 ).
