Pediatr Cardiol 12:83-88, 1991

Pediatric Cardiology 9 Springer-Vertag New York Inc. 1991

A n t e r o l a t e r a l M u s c l e B u n d l e o f t h e L e f t V e n t r i c l e in A t r i o v e n t r i c u l a r Defect: Left Ventricular Outflow Tract and Subaortic Stenosis

Septal

H.A. Yvonne Draulans-No~ ~ and Arnold C.G. Wenink 2 ~Department of Pediatric Cardiology, University Hospital Leiden, Leiden: and -'Department of Anatomy and Embryology, University of Leiden, The Netherlands S U M M A R Y . The anatomy of the left ventricular outflow tract ( L V O T ) in 77 hearts with atrioventricular septal defect ( A V S D ) , 36 with a separate A V orifice and 41 with a c o m m o n AV-orifice, were investigated. In all specimens, an anterolateral muscle bundle of the left ventricle was identified b e t w e e n the superior bridging leaflet and the left coronary aortic cusp. It displaced the a t t a c h m e n t of the superior bridging leaflet, resulting in its clockwise rotation. The muscle bundle frequently bulged into the L V O T , but was never prominent enough to have caused significant subaortic stenosis. M e a s u r e m e n t of the L V O T aortic ratio was possible in 54 hearts and ranged f r o m 3 6 - 1 0 0 % . In 23 cases (43%), there was mild to m o d e r a t e subaortic narrowing with a ratio ranging f r o m 53-88%. In six cases (11%), unequivocal subaortic stenosis was present, mainly in A V S D with separate A V orifices (five of six) and iatrogenic in one case with surgically corrected complete defect. A decreased ratio was mainly due to decreased anteroposterior width of the septum in the subaortic area, with anterior displacement of the superior bridging leaflet in cases with dense septal attachment of the superior bridging leaflet (i.e., in A V S D with separate A V orifices, type A complete defect with small ventricular septal defect, or surgically corrected complete defect). Significant subaortic stenosis was caused by hypertrophy of the ventricular septum in the subaortic area with anteroseptal twist in four cases, by anomalous chordal insertion of the superior bridging leaflet in one case, and iatrogenic in one case after surgical correction with left A V valve replacement in a type C complete defect. O t h e r additional obstructive forces were an anomalous papillary muscle, a small left ventricle, and an aneurysm of the m e m b r a n o u s septum. K E Y WORDS: Atrioventricular septai defect - - Anterolateral muscle bundle - - Subaortic stenosis - - Left ventricular outflow tract

The anterolateral muscle bundle of the left ventricle, first described by Moulaert [5], is a horizontal muscle bundle in the left ventricular outflow tract (LVOT), o b s e r v e d in 40% of normal hearts. The bundle lies b e t w e e n the left c o r o n a r y aortic cusp and the aortic leaflet of the mitral valve. Moulaert suggested that it is a muscular remnant of the bulboventricular f o l d - - f o r which we now prefer the term " p r i m a r y f o l d " [ 8 ] - - a n d that it can be very

prominent, causing obstruction of the L V O T . The dual p u r p o s e of this study was to assess the presence of an anterolateral muscle bundle in hearts with atrioventricular septal defect (AVSD) and its role as a potential cause of subaortic stenosis. In addition, associated anomalies, which m a y further obstruct the L V O T , were assessed.

Materials and Methods Address offprint requests to: Dr. Draulans-Noe, Department of

Pediatric Cardiology, University Hospital Leiden, Leiden, The Netherlands.

Seventy-seven hearts with AVSD were examined, taken from the pathological collection of the Department of Anatomy and Embryology of the University of Leiden (Table 1). Hearts with

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Pediatric Cardiology Vol. 12. No. 2, 1991

T a b l e 1. Data on sex, age, and surgical correction in 77 hearts with A V S D

T y p e of defect

Separate A V orifices C o m m o n AV orifice Septally attached S B L (type A) Free-floating S B L (type C) Type C + DORV Total

Total

36 41 26 9 6 77

Data on sex and age

31 39 24 9 6 70

Sex

Age (years)

M

F

1

1-16

16

14 1l 7 2 2 25

17 28 17 7 4 45 (64%)

12 25 19 3 3 37 (53%)

13 13 5 5 3 26

6 1 1 0 1 7 (10%)

Surgery correction

AV valve replacement

17 14 10 3 1

2 2 0 I 1

SBL, superior bridging leaflet; D O R V , double-outlet right ventricle.

complicated discordant ventriculoarterial connection ( " t r a n s p o sition"), single outlet, univentricular A V connection, or e x t r e m e hypoplasia of the left ventricle were excluded. There were 41 hearts in which the bridging leaflets were not fused in the plane of the s e p t u m , resulting in a c o m m o n AV orifice, and 36 with a tongue of valve tissue connecting the bridging leaflets to each other, resulting in separate right and left AV orifice [6]. The complete defects were separated into (a) 26 hearts with a septally attached superior bridging leaflet (Rastelli type A) and (b) 15 with a free-floating superior bridging leaflet (Rastelli type C), the latter including six c a s e s with a true double-outlet right ventricle. Surgical correction had been performed in 31 cases, with r e p l a c e m e n t of the left A V valve in four.

Methods The a n a t o m y o f the L V O T was studied in all cases, especially the incidence of an anterolateral muscle bundle, its significance as a c a u s e of subaortic stenosis, and the presence of additional potentially obstructive lesions. Internal d i a m e t e r s of the L V O T and the aortic orifice were m e a s u r e d , using calibrated H e g a r probes in increments of 1 m m in diameter. L V O T d i m e n s i o n s were e x p r e s s e d as a percentage of the aortic root diameter.

Results

Anterolateral Muscle Bundle, Attachment of the Superior Bridging Leaflet, and Shape and Boundaries of the L VOT In all specimens an usually broad anterolateral muscle bundle could be identified in the LVOT, displacing the free-wall and aortic attachment of the superior bridging leaflet (Figs. IB and 3B-E). Contrary to the normal heart, the free-wall attachment of the superior bridging leaflet in AVSD runs from the lateral commissure along the anterolateral muscle bundle up to the aortic valve at the level of the commissure between the right and noncoronary

aortic cusp. The displaced aortic line of attachment then continues along the noncoronary cusp to the ventricular septum. In the absence of an AV seprum, the septal line of attachment is displaced and prolonged, as it continues along the rim of the defective interventricular septum. The abnormal line of attachment of the superior bridging leaflet, with displacement of the free-wall, aortic, and septal attachment, results in a clockwise rotation of the valve leaflet, such that it occupies a position perpendicular to the ventricular septum, instead of parallel to it (Fig. 2). "Scooping out" of the ventricular septum in AVSD results in apical displacement of the superior bridging leaflet with relative elongation of the LVOT. On the other hand, the decreased anteroposterior width of the septum in the subaortic area (with decreased distance between the superior bridging leaflet and the parietal wall) results in narrowing of the LVOT. The subaortic part of the ventricular septum frequently shows an anteroseptal twist [4]. Thus, the presence of the anterolateral muscle bundle, the malposition of the superior bridging leaflet with its prolonged free-wall and septal line of attachment, absence of an AV septum, scooping out of the ventricular septum, and the decreased width of the septum in the subaortic area all influence the size, shape, and boundaries of the LVOT in AVSD. The LVOT in AVSD is bordered medially by the subaortic septum, laterally by the anterolateral muscle bundle, and inferiorly by the proximal part of the superior bridging leaflet (Fig. 2). In cases with a septally attached superior bridging leaflet, the proximal part of the leaflet is fixed and relatively immobile, being attached on three sides, with a prolonged septal and free-wall attachment. When there is an anteroseptal twist of the septum in the subaortic area, it will form part of the " r o o f " of the LVOT.

Draulans-Noe and Wenink: Anterolateral Muscle Bundle in Hearts with AVSD

85

Fig. 1. LVOT and AV valve attachment. (A) Normal heart: The LVOT gives wide access to the aortic orifice. AO, aortic attachment of the aortic mitral leaflet; AVS, atrioventricular septum; FW, free-wall attachment; S, septal attachment. (B) AVSD: The LVOT is bounded laterally by the anterolateral muscle bundle (ALM), inferiorly by the superior bridging leaflet, and medially by the subaortic part of the ventricular septum.

Fig. 2. Short-axis cross-section of AVSD showing the clockwise rotation of the superior bridging leaflet (SBL), resulting in its position being perpendicular to the ventricular septum.

Table 2. Potentially obstructive lesions of the LVOT in 77 hearts with AVSD Type of defect

Total

AST

ALM bulge

Anomalous PM

Anomalous chords

Left AV valve replacement

Separate AV orifices Common AV orifice Total

36 41 77

19 23 42 (55%)

13 14 27 (35%)

8 23 31 (40%)

4 4 8 (10%)

2 2 4 (5%)

AST, anteroseptal twist; ALM, anterolateral muscle bundle; PM, papillary muscle.

Potentially Obstructive Lesions of the L VOT and Measurement of the LVOT/Aortic Area P o t e n t i a l l y o b s t r u c t i v e l e s i o n s o f t h e L V O T in A V S D w e r e o b s e r v e d in t h e m a j o r i t y o f t h e s p e c i m e n s ( T a b l e 2). T h e a n t e r o l a t e r a l m u s c l e b u n d l e o f t h e left v e n tricle, f o u n d in all s p e c i m e n s , w a s u s u a l l y fiat a n d b r o a d (Fig. 3D a n d E), b u t v a r y i n g d e g r e e s o f bulg-

ing w e r e o b s e r v e d in a p p r o x i m a t e l y o n e t h i r d o f c a s e s . B u l g i n g a p p e a r e d t o b e m o r e p r o n o u n c e d in A V S D w i t h c o m m o n A V o r i f i c e s (Fig. 3C) a s c o m p a r e d to c a s e s w i t h s e p a r a t e o r i f i c e s , b u t n e v e r to s u c h a n e x t e n t t h a t it w o u l d h a v e c a u s e d o u t f l o w tract obstruction without the presence of additional obstructive factors. A n a n t e r o s e p t a l t w i s t o f t h e s e p t u m in t h e s u b a o r t i c a r e a w a s f o u n d in a p p r o x i m a t e l y h a l f o f t h e

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Fig. 3. Potentially obstructive lesions of the LVOT in AVSD. (A) Hypertrophy and anteroseptal twist of the septum in the subaortic area (*), causing significant subaortic stenosis in a 41year-old woman with surgically corrected AVSD with separate orifices. 1BL, inferior bridging leaflet; SBL, superior bridging leaflet. Note the subvalvar fibrous shelf (arrow) (B) Anomalous chordal insertion (*) of the superior bridging leaflet in surgically corrected AVSD with separate orifices. (C) Anterolateral muscle

bundle (**) bulging into the LVOT in type A complete defect. (O) Hypertrophy of an anomalous papillary muscle (*) in type C complete defect. (E) Aneurysm of the membranous septum (*) in type A complete defect. The anterolateral muscle bundle (**) is clearly depicted in C-E. The width of the septum in the subaortic area is decreased, particularly in complete defects as seen in C -

s p e c i m e n s . W h e n c o m b i n e d with h y p e r t r o p h y o f the s e p t u m in this a r e a , this r e s u l t e d in s u b a o r t i c s t e n o s i s (Fig. 3A). A n a n o m a l o u s p a p i l l a r y m u s c l e o f the L V O T w a s s e e n in 25 c a s e s , b u t o n l y in o n e

case w i t h t y p e C c o m p l e t e defect was it large e n o u g h to p o s s i b l y h a v e c a u s e d i a t r o g e n i c o b s t r u c t i o n a f t e r c o r r e c t i o n (Fig. 3D). A n o m a l o u s c h o r d s in the o u t f l o w t r a c t a r i s i n g f r o m the s u p e r i o r bridg-

E.

Draulans-No~ and Wenink: Anterolateral Muscle Bundle in Hearts with AVSD

Table 3. Measurement of LVOT/aortic (Ao) ratio in 54 hearts with AVSD Type of defect Separate AV orifices Common AV orifice Total

No.

Range (%)

Mean (%)

Aortic stenosis (LVOT/Ao = 50%)

25

36-100

79

5 (20%)

29 54

55-100 36-100

91 85

1 (iatrogenic) 6 (11%)

87

let (type A), and in two of l0 cases with complete defect and free-floating superior bridging leaflet (type C). Although the septum in the subaortic area tends to be narrower in the complete than in the partial defects, this did not result in LVOT narrowing when a large ventricular septal defect or a patch in surgically corrected patients forms part of the medial border of the LVOT.

Discussion

ing leaflet (Fig. 3B) or from an anomalous papillary muscle were seen in eight cases, resulting in significant subaortic stenosis in one. An aneurysm of the membranous septum bulging into the LVOT (Fig. 3E) was seen in one case with a complete defect, without causing significant obstruction. In all hearts with prosthetic replacement of the left AV valve by a porcine Carpentier-Edwards valve, the struts of the valve projected into the outflow tract, with significant iatrogenic stenosis in one case. Measurement of the outflow tract/aortic ratio was possible in 54 hearts, and ranged from 36-100% (Table 3). In six cases, unequivocal subaortic stenosis was present with an outflow tract/aortic ratio -