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J Thorac Cardiovasc Surg 1997;114:9-15
© 1997 Mosby, Inc.

SURGERY FOR CONGENITAL HEART DISEASE

THE PARACHUTE-LIKE ASYMMETRIC MITRAL VALVE AND ITS TWO PAPILLARY MUSCLES

Supported by The Netherlands Heart Foundation, grant 93.057, and by a Collaborative Research Grant from the Association of European Pediatric Cardiologists.

Received for publication August 8, 1996 revisions requested Dec. 5, 1996; revisions received Dec. 30, 1996 accepted for publication Dec. 31, 1996. Address for reprints: A. C. G. Wenink, MD, PhD, Department of Anatomy, P.O. Box 9602, 2300 RC Leiden, The Netherlands.

Abstract

Objectives: The morphologic features of parachute-like asymmetric mitral valves are described to discriminate this anomaly from parachute mitral valves. Background: Mitral valves with unifocal attachment of chords have been called "parachute valves," independent of the number of papillary muscles. Therefore the anomaly involving two papillary muscles has not received separate attention. Methods: The gross anatomy of 29 mitral valves with focalized attachment of chords was studied. Results: In 28 of the autopsy specimens asymmetric mitral valves with two papillary muscles were present, and one of the muscles was elongated, located higher in the left ventricle with its tip reaching to the anulus, and attached at both its base and lateral side to the left ventricular wall. The valve leaflets could be directly attached to this abnormal muscle that received few chords or, in three hearts, no chords at all, resulting in an oblique and eccentric orifice. Because of the focalized attachment of chords to one of the two papillary muscles, we call this malformation "parachute-like asymmetric mitral valve." We found only one "true parachute mitral valve," that is, one having a single papillary muscle that received all chords. Conclusions: The morphologic features of asymmetric mitral valves are essentially different from those of true parachute valves. A distinction between these two anomalies will contribute to recognition by the pediatric cardiologist and surgeon

The parachute mitral valve is a congenital cardiac malformation that causes stenosis of the mitral valve and is often found in combination with other obstructive anomalies of the left side of the heart. ¹ Parachute mitral valves were originally defined as having all chords converging to insert into one major papillary muscle. Valves with two papillary muscles that were immediately adjacent, thus yielding the effect of a single papillary muscle, were included in the parachute group. ^1,2 However, in a later study "parachute mitral valves" and "mitral valves with one single papillary muscle" were discriminated from each other, ³ although the differences in morphologic features between these two mitral valve anomalies were not explained, which made this use of terms rather confusing.

In 1974, Rosenquist ² described mitral valves with two papillary muscles but with all chords being inserted into one of them. Despite different morphologic features, the term parachute mitral valve was also used for this mitral valve anomaly. In a later description, the cases with two papillary muscles, of which one could be hypoplastic, were discriminated from the cases with one papillary muscle. The parachute valves with one papillary muscle were claimed to represent fusion of the two papillary muscles. ⁴ Although this term might be rather dynamic it was not meant to explain the developmental background of this malformation, which is still largely unknown.

To set new light on this confusing area of morphology and terminology of parachute mitral valves we studied hearts in our autopsy collection with unifocal attachment of chords. We decided to discriminate the mitral valves with two papillary muscles from the true parachute valves by calling them "parachute-like asymmetric mitral valves."

Material and methods

Morphology.
Abnormal mitral valves were found in 129 autopsy specimens. Twenty-nine of these hearts had focalized attachment of chords to one papillary muscle. Twenty-eight of these hearts had two papillary muscles and an asymmetric distribution of chords, that is, all chords or the majority of them were attached to one of the two muscles. We called this malformation "parachute-like asymmetric mitral valve." Only one heart had a "true parachute," that is, the arrangement in which one single papillary muscle received all chords. The mean age at death of the children from whom the hearts were obtained was 12 months and ranged from 1 day (born at 30 weeks of amenorrhea) to 15 years. In none of the hearts was the parachute or parachutelike asymmetric mitral valve an isolated cardiovascular abnormality (Table I). Hearts with an atrioventricular septal defect and a parachute-like morphology ⁵ and hearts with a hypoplastic left ventricle were not included in this study.

View larger version (149K): [in this window] [in a new window]   Fig. 1. A schematic drawing (a) and a photograph (b) of a normal mitral valve from a newborn child. The orifice is indicated by dots (a) and by a thick arrow (b). A, Anterolateral papillary muscle; Ao, aorta; P, posteromedial papillary muscle.

At the site of each commissure the following values were determined (Fig. 2): the length of the (1 on Fig. 2) leaflets and (2) chords that were nearest to the midline of the aortic leaflet; the length of the papillary muscle from (3) the base to the tip and from (4) the base to the attachment of the chords; and the distance from the apical endocardium to (5) the base or (6) the tip of the papillary muscle. The diameters of the mitral and tricuspid valve anuli (7) and the diameter of the orifice of the mitral valve at the free rim of the valve leaflets were determined with a Hegar probe. The left ventricular length (8) was measured from the aortic orifice to the apical endocardium.

View larger version (30K): [in this window] [in a new window]   Fig. 2. Schematic drawing of a mitral valve in which the parts that were measured are indicated by arrows (numbers identified in the Material and methods section).

Results

Morphology.
The 28 asymmetric mitral valves were characterized by one normal and one abnormal papillary muscle. Twenty-six of these had an abnormal anterolateral papillary muscle. Only in two cases was the posteromedial papillary muscle abnormal. We observed a spectrum of asymmetry of the mitral valve because one valvulopapillary muscle complex was slightly to severely abnormal (Fig. 3). We subdivided the 28 hearts into three groups by grade (Table I).

View larger version (882K): [in this window] [in a new window]   Fig. 3. The spectrum of parachute-like asymmetric mitral valves. a through c, Grade I asymmetric mitral valves demonstrating elongated anterolateral papillary muscles (cases 2 and 4). d through f, Asymmetric grade II mitral valves with only a few chords attached to the elongated anterolateral papillary muscle (cases 11 and 16). g through i, Asymmetric grade III mitral valves with the valve leaflet directly attached to the anterolateral papillary muscle (cases 24 and 25). The mitral valve orifices are indicated by dots in the drawings and by thick arrows on the photomicrographs. Thin arrows indicate chordal attachments and arrowheads indicate direct attachment of the valve leaflet to the anterolateral papillary muscle. A, Anterolateral papillary muscle; Ao, aorta; P, posteromedial papillary muscle.

Grade II
(Fig. 3, d through f). Nineteen abnormal hearts had more obvious asymmetry of the mitral valves. In two of these hearts the posteromedial papillary muscle was abnormal. The elongated papillary muscle and especially the tip was located close to the mitral valve anulus. In addition, the abnormal papillary muscle was attached at its lateral side to the left ventricular free wall. At the abnormal commissure the valve leaflets were short and directly attached to the tip of the papillary muscle and only a few short chords or almost no chords were present. This remarkable attachment of chords and leaflets caused an asymmetric location of the valve orifice, leading to an oblique position in the left ventricle. Moreover, the attachment of the leaflets and the short chords prevented opening of the orifice at the abnormal commissure (Fig. 3, f). Nine of these mitral valves had a smaller orifice diameter compared with the annular diameter.

Grade III
(Fig. 3, g through i). Three hearts were characterized by the absence of chords to one papillary muscle. As in grades I and II, two papillary muscles were present, one of them elongated and displaced to the base of the heart: the tip of the abnormal papillary muscle was located near the mitral valve anulus and the papillary muscle was attached at its lateral side to the left ventricular wall. The valve leaflets were directly attached to the abnormal papillary muscle and their edges were partly attached to each other at this site. Therefore opening of the commissure was not possible on the abnormal side, and the orifice was oblique and had an eccentric position (Fig. 3, h). All three mitral valves with complete unifocal attachment of chords had a stenotic orifice, that is, a smaller orifice diameter compared with the diameter of the anulus.

Only one heart had a true parachute mitral valve, that is, only one papillary muscle to which all chords were attached (Fig. 4). The valvulopapillary muscle complex was located somewhat posteriorly. The orifice was located between the chords, just above the papillary muscle, the valve tissue formed a funnel, and no cleft was present. Because of this centrally located valve orifice, the parachute valve was strikingly different from the asymmetric valves with their characteristic oblique orifice.

View larger version (140K): [in this window] [in a new window]   Fig. 4. A schematic drawing (a) and a photograph (b) of the single parachute mitral valve with one papillary muscle to which all chords are attached. The mitral valve orifice is indicated by dots (a) and by a thick arrow (b). Ao, Aorta.

In this paper we describe two types of mitral valve malformations that are both characterized by focalized attachment of chords to one papillary muscle. The essential difference is the number of papillary muscles: one or two. The morphologic features of mitral valves with two papillary muscles, however, do not fit with the original description of parachute valves. ¹ We use the term parachute-like asymmetric mitral valve to discriminate this malformation from the "true parachute valves" with one papillary muscle.

Parachute-like asymmetric mitral valve.
The main characteristics of asymmetric mitral valves are unequal distribution of chords and one normal and one elongated papillary muscle that is displaced toward the mitral valve anulus.

We have observed a spectrum of asymmetry in which especially grade III hearts have a parachute-like appearance because the abnormal papillary muscle receives no chords. In these hearts the abnormal papillary muscle is attached to the ventricular wall in such a way that it might be interpreted as a trabecula ⁶ or might not be recognized at all. ¹ However, its location and the attachment of the valve leaflets make this myocardial structure a papillary muscle. The spectrum of asymmetry we found supports this. The commissure can still be defined as the junction between two leaflets, and the oblique and eccentric orifice is located between the two papillary muscles ⁷ but is not a valvular "cleft" inasmuch as its closure will not restore normality, ⁸ nor will it be opened by pulling of the tensor apparatus. ⁹ In most hearts the anterolateral papillary muscle was abnormal. This has also been found by others. ^4,10

Long papillary muscles, attached at their lateral side to the ventricular wall, have been described in normal adult human hearts ¹¹ and in adult rodent hearts ¹² and are normally present in early fetal hearts as well. ¹³ Papillary muscle development normally results in symmetry, ^14-16 and the presently described asymmetry seems to be the result of abnormal development of one papillary muscle and tensor apparatus.

Clinical implications.
Stenosis is typical for the parachute valve ^17,18 and in our study all the abnormal mitral valves, even those with mild asymmetry (grade I), were also stenotic compared with the values reported by Rosenquist. ² Stenosis at the orifice level was only present in grade II and III hearts but an oblique, eccentric orifice did not result in a smaller orifice per se.

The presence and location of papillary muscles can be visualized by two-dimensional echocardiography, ^19,20 especially when the blood flow is also studied by Doppler interrogation. ²¹ Anomalous insertion of the papillary muscles into the valve leaflets, a morphologic condition that is comparable with the malformed papillary muscle in asymmetric mitral valves, can only be identified by nonstandard cross-sectional planes. ²²

In addition to mitral stenosis, other malformations were present in our cases. An abnormal aortic arch often coexists with parachute mitral valves, ^1,2,19,23 and this was present in many of our specimens; however, Shone's complex ¹ was not found. When other malformations are present, restoration of the mitral valve seems to be of a lower priority and is not even done for parachute mitral valves ³; additionally, this type of stenosis is not easy to repair. True parachute mitral valves are not candidates for balloon angioplasty and only relatively poor results were achieved in children who had disproportionate chordal attachments to one of the papillary muscles. ²⁴ Subdividing the papillary muscle into anterior and posterior portions to widen the orifice has proved to be successful in some parachute valves. ^18,25 Whether this technique can also be used for asymmetric mitral valves seems to be questionable.

Acknowledgments

We gratefully thank E. A. van der Velde, MSc, for his help with the statistical analysis and J. H. Lens for the photographic work.

Footnotes

From the Department of Anatomy, Leiden University, Leiden, The Netherlands,^a and the Serviço de Cardiologia Pediátrica, Hospital de Santa Marta, Lisboa, Portugal.^b

References

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