Aneurysm of the Left Atrial Appendage

Abstract

A 43-year-old woman underwent excision of an aneurysm of the left atrial appendage, which had been causing cerebrovascular embolic episodes. We attribute the aneurysm to congenital dysplasia of the musculi pectinati in the left atrial appendage and of the bands of atrial muscle from which they arise.

In Appendix I, we draw attention to the morphologically similar arrangements of inner and outer bands that emanate from a common transverse interatrial band and yield morphologically similar medial, descending, and ascending palm-leaf arrangements of musculi pectinati. In addition, we observe that the strap-like arrangements of musculi in both atria connect the outer band with the para-annular segment of the inner band. In Appendix II, we briefly review the literature concerning musculi pectinati and related bands.

This report concerns an aneurysm of the left atrial appendage found in association with dysplasia of the musculi pectinati and related atrial muscle bundles. This anomaly is rare and has not been described in recent cardiology textbooks.

Case Report

The patient was a 43-year-old woman, who presented in May of 2000 with dyspnea and palpitations, after having been hospitalized for cerebrovascular embolic episodes on 3 prior occasions since 1995. She had residual motor aphasia and facial nerve palsy. Her heart sounds were normal. No murmurs were detected. She had atrial fibrillation. Chest radiography revealed cardiomegaly (Fig. 1). An echocardiogram was interpreted as revealing a pericardial cyst or loculated effusion, which compressed the left ventricle. Angiocardiography with injection of contrast medium into the pulmonary artery revealed normal pulmonary arteries and veins. Contrast from the left atrial body was filling a large cavity that distorted the left ventricle. Subsequent echocardiographic (including transesophageal) studies (Fig. 2) confirmed an aneurysm of the left atrial appendage. Magnetic resonance imaging also documented the aneurysm.

Fig. 1 Chest radiography reveals cardiomegaly and abnormal left border of the heart.

Fig. 2 Transesophageal echocardiogram confirms aneurysm of the left atrial appendage (LAA) with clots.

Our patient underwent operation on 31 May 2000, in a supine position. After induction of anesthesia, there was spontaneous reversal to sinus rhythm. We performed a left anterolateral thoracotomy through the 3rd intercostal space and divided the sternum transversely, without opening the right pleura. The left pleura and lung were normal. The pericardium was distended and intact. Opening the pericardium revealed a large cucumber-like aneurysm of the left atrial appendage, extending up to the apex of the left ventricle (Fig. 3). The aneurysm was compressing the obtuse border of the heart, which made the lateral wall of the left ventricle concave; the left anterior descending coronary artery followed a course along an abnormal acute anterior margin. The wall of the aneurysm was thin and poorly contractile, with sparse muscular fibers. In the lateral wall of the aneurysm, there was a sharp ridge about 3 cm long that had a narrow projection, resembling a tiny appendage, which contracted with each atrial systole.

Fig. 3 View through left anterolateral thoracotomy reveals a massive aneurysm of the left atrial appendage (LAA), which has slipped out of the pericardial cavity.

LAD = left anterior descending coronary artery;

MPA = main pulmonary artery; RV = right ventricle

The ascending aorta and right atrium were cannulated. After initiation of cardiopulmonary bypass, cardiac fibrillation was induced electrically. The main pulmonary artery was cannulated and vented directly into the venous line, without a pump. ¹ The aneurysm itself was fibrillating and communicated with the left atrial body through a wide neck, about 4 cm in diameter. The neck of the sac was in front of the left pulmonary veins, which were in their normal location. The ligament of Marshall was cephalad to the aneurysm's neck.

A band of muscle crossed the roof of the left atrial body before dividing into palpable inner and outer bands that coursed along the wall of the aneurysm's neck. After we excised the sac at its junction with the body of the left atrium, we observed that the cut edge of the left atrial band in the roof of the left atrium was contracting during atrial systole, which was easily demonstrated by holding the cut end with the forceps. The mouth of the sac was sutured (Fig. 4). Deairing was done through a left atrial vent connected to the venous line. ¹ Weaning the patient from bypass was easy. We created pericardial windows and then closed the chest, save for placement of a single pleural drain. ²

Fig. 4 View after excision of the aneurysm. Observe the distortion of the left ventricle (LV), which has a concave free wall, with the left anterior descending coronary artery (LAD) forming the anterior margin.

LA = left atrium; RV = right ventricle

Adherent to the excised wall of the aneurysm of the left atrial appendage were organized clots, near the small appendage-like projection (Fig. 5). The outer and inner bands that coursed the neck of the sac were easily identified. Arising from the inner band were 2 palm-leaf-like arrangements of sparse musculi pectinati. Another palm-leaf arrangement was seen arising from the outer band. Between the muscle fibers, the atrial wall was thin and devoid of muscle.

Fig. 5 Excised aneurysmal sac with the cut end of the transverse atrial band (arrow) dividing into inner and outer bands. Observe the sparse musculi pectinati, the rudimentary appendage, and clot.

The postoperative course was uneventful. The patient remained in sinus rhythm when seen at follow-up in September 2000. There was no cardiomegaly.

Discussion

This patient had an aneurysm of the left atrial appendage that was confirmed by echocardiographic, angiocardiographic, and operative findings. We suspect that the aneurysm was caused by congenital dysplasia of the musculi pectinati and of the left atrial muscle bundles related to them. Such dysplasia would impair contraction of the appendage during atrial systole.

Review of the Literature

In 1938, Semans and Taussig ³ (quoted by Parmley ⁴) reported saccular dilatation of the left atrium in a 5-year-old girl with dextrorotation of the heart. The appendage was not involved. In 1962, Parmley ⁴ reported dilatation of the left atrial appendage in 2 children. One was an 11-year-old boy who presented with atrial arrhythmia and a history of 2 episodes of systemic embolism. This patient's aneurysmal appen-dage was excised at the Mayo Clinic. A concomitant atrial septal defect was closed by the “well” technique. Parmley's 2nd patient was a 7-year-old girl with an aneurysm of the left atrial appendage associated with a congenital anomaly of the left renal artery and a small left kidney, which was causing hypertension.

In 1963, Williams ⁵ reported a case of thin-walled gross dilatation of a left atrial appendage in a 27-year-old man. Trial clamping of the appendage induced arrhythmia; hence the aneurysm was not excised. However, a subsequent embolic episode 2 years later spurred reoperation, at which time the dilated appendage with thrombi was excised. Williams and colleagues ⁵ refer to a report by Palacio and associates ⁶ concerning a 32-year-old woman with a cerebral embolism and atrial arrhythmia, in whom an aneurysm of the left atrial appendage was excised. They also refer to a patient of Pitts and Potts ⁷ who had a diverticulum of the left atrium; but it is not clear whether the appendage was involved.

Osler ⁸ (quoted by Segall ⁹) refers to a heart in the McGill College Museum that exhibited parchment-like thinning and dilatation of the walls of all the chambers. Unfortunately, the clinical details of this case were lost. The possibility that aneurysm of the left atrial appendage is a “forme fruste” of parchment heart due to dysplasia of the cardiac musculature cannot be excluded.

In 1999, Zhao and co-authors ¹⁰ reported the case of a 27-year-old man with an aneurysm of the left atrial appendage, which was excised in 1996 with-out cardiopulmonary bypass. They preferred a lateral thoracotomy to median sternotomy. Their paper reviews earlier reports, analyzes the clinical features, and discusses the possible causes.

Aneurysm of the left atrial appendage is not, to our knowledge, discussed in recent cardiology textbooks.

Conclusion

Our description (see Appendix I) of the transverse band and the inner and outer bands on either side ¹¹ is similar to that of Keith ^12–14 and Papez, ¹⁵ although it varies in detail (see Appendix II). There is a need for standardizing the description and terminology. The basic pattern of the musculi pectinati that arise from these bands has been described (see Appendices I and II). Attention has been drawn to the morphologic similarity of the musculi pectinati in both atria. However, the arrangement is unique to each heart in the same way that fingerprints (or indeed the mitral ¹⁶ and tricuspid ¹⁷ valves) are unique.

Dysplasia of Musculi Pectinati and Related Bands.

In our patient, the palm-leaf arrangements of musculi pectinati, coursing over the appendage, were discernible in the excised sac. But the musculi pectinati were obviously attenuated. This would have impaired the capacity of the left atrial appendage to empty itself during atrial systole. We were unable to discern a cause for primary dilatation of the atrial appendage or for secondary changes in the musculi.

In the normal left atrial appendage, there is a waist-like constriction of the base caused by the left outer band ¹⁸ and to a lesser extent by the inner band. In our patient, the neck of the sac was wide. This could be due to the failure of lax inner and outer bands to constrict the base of the appendage.

Surgical Approach.

A left anterolateral thoracotomy with a transverse sternotomy in the supine position offered excellent cardiac exposure in our patient. We have found this approach useful for repair of tetralogy of Fallot, ¹⁹ double-outlet right ventricle, ¹⁹ total anomalous pulmonary venous connection, ²⁰ and anomalous origin of the left coronary artery from the pulmonary artery. ²¹

Clinical Relevance.

Knowledge of the atrial muscle bands and the musculi pectinati is applicable to choosing the site of an atriotomy and to Fontan procedures, heart transplantation techniques, electrophysiology, and maze procedures. ^18,22

The classical waist-like constriction of the base of the left atrial appendage is caused by the left inner and outer bands. Digital exploration of the left atrium during closed mitral commissurotomy confirms that the waist varies in size. It is likely that cor triatriatum and cor triatriatum dexter are causally related to excessive atrial infolding brought about by the left and right outer bands. Molecular biological information relating to the atrial musculature merits attention.

Acknowledgment

We are thankful to Prof. R. Alagesan for performing echocardiography and to Prof. P. Thirumalai for cardiac catheterization. We thank R. Sangeetha for secretarial assistance.

Appendix I

Anatomic Observations Pertaining to the Musculi Pectinati and Related Atrial Muscle Bands

The Atria.

Understanding the atria and their musculature is necessary to appreciate the genesis of aneurysms of the left atrial appendage.

The 2 atria present a concave surface behind the transverse sinus (Fig. 1). The superior border is transverse. Inferiorly, the atria are anchored to the atrioventricular annulus. On either side, 2 atrial appendages are seen. The inner and outer surfaces of the appendages meet in a border that is rounded cranially and crenated inferiorly, presenting as a sharp ridge. The outer surface of the right atrial appendage merges with the superior vena cava, the body of the right atrium, and the inferior vena cava. This surface merges with the posterior atrial wall caudal and anterior to the inferior vena cava. The outer surface of the left atrial appendage merges with the atrial body and the left pulmonary veins. Below the left inferior pulmonary vein, it merges with the posterior wall of the left atrium.

Fig. 1 The atria are viewed from the front after division of the great arteries. Observe the transverse atrial muscle band span-ning the roof of both atria. It splits into outer bands (black dots) and inner bands (white dots) on either side that skirt the appendages. The inner bands continue laterally as para-annular bands.

SVC = superior vena cava

Atrial Muscle Bands Related to Musculi Pectinati.

The musculi pectinati in the atria have been taken for granted, ignored, and often divided without regard. We have observed that there is, in our judgment, a purposeful arrangement of musculi pectinati, ^1,2 morphologically identical in both atria. This arrangement is best appreciated in an autopsy specimen of the heart, when one illuminates the atria from within, thereby providing an undistorted view of the musculi pectinati in relation to the muscle bands in the atria. If the specimen is divided, the orientation is disturbed. There is a transverse band of muscle in the roof of both atria (Figs. 1 and 2). This band splits into 2 on either side and embraces the corresponding appendage. Musculi pectinati arise from these inner and outer bands in both atria (Figs. 1 and 2).

Fig. 2 Transillumination outlines the transverse atrial band, splitting into inner and outer bands on either side. Observe medial palm-leaf arrangements of musculi pectinati arising from the inner bands and coursing forward along the inner surfaces of the appendages.

LSPV = left superior pulmonary vein; SVC = superior vena cava

The right outer band skirts the anterior margin of the superior vena caval orifice and descends in front of the right pulmonary veins. Then it winds around the right border of the heart between the orifice of the inferior vena cava and right atrioventricular groove, courses in the posterior wall of the right atrium, and reaches the region of the crux of the heart (Fig. 3). The intercaval segment of this band is usually called the crista terminalis.

Fig. 3 Rear view of transilluminated atria reveals para-annular segments of the inner bands (white dots) and outer bands (black dots) on either side converging at the crux. Observe the straps of musculi pectinati connecting the inner and outer bands.

IVC = inferior vena cava; LA = left atrium; LIPV = left inferior pulmonary vein; LV = left ventricle; RA = right atrium; RV = right ventricle

The right inner band courses along the medial side of the base of the right atrial appendage. This band reaches the atrio-ventricular groove, where it deviates to the right and traverses parallel to the atrioventricular groove as a supra-annular band, up to the crux of the heart (Fig. 3).

The left outer band winds around the cranial margin of the left atrial appendage, courses caudally anterior to the left pulmonary veins, and curves around the left border of the heart between the left inferior pulmonary vein and the left atrioventricular groove. It traverses the posterior wall of the left atrium and also reaches the region of the crux of the heart. The left inner band skirts the base of the left atrial appendage medially, reaches the atrioventricular groove, turns left and courses as a supra-annular atrial band up to the crux of the heart. It is of interest that the base of the left atrial appendage is constricted due to a pincer effect of the inner and outer bands. This constriction possibly increases the velocity of the blood ejected by the left atrial appendage and aids filling of the left ventricle, which is less compliant than the right ventricle.

Palm-Leaf Arrangements of Musculi Pectinati.

Three palm-leaf-like arrangements of the musculi pectinati are seen on either side. ² A medial arrangement is seen coursing anteriorly from the inner band along the inner surface of the corresponding atrial appendage (Fig. 2). It may resemble a coconut or palmyra palm leaf.

An offshoot of the outer band in the right atrium, called the taenia major, descends alongside the rounded and sharp margins of the atrial appendage and gives off branches to either side (Fig. 4A). A similar offshoot from the left outer band descends along the left atrial margin and also gives off branches to either side (Fig. 4B). This descending arrangement on either side resembles a coconut palm leaf.

Fig. 4 Observe a coconut-palm-leaf arrangement of musculi pectinati (arrows) coursing downward from the outer bands (dots), alongside the acute ridges of the appendages of the A) right atrium and B) left atrium.

LPVs = left pulmonary veins; LV = left ventricle; RV = right ventricle; SVC = superior vena cava

The 3rd, ascending, palmyra-palm-leaf arrangement arises from the para-annular segment of the inner band, near the acute or obtuse margin (Fig. 5). The musculi pectinati radiate from this band cranially in the convex anterolateral surface of the body of either atrium and the outer wall of the appendage and interdigitate with the lateral branches of the descending coconut-palm-leaf arrangement in the appendage.

Fig. 5 Observe a palmyra-palm-leaf arrangement (white arrows) of musculi pectinati in the A) right atrium and B) left atrium coursing upward from the corresponding para-annular bands (white dots). Posterolaterally, strap-like musculi pectinati (black arrowheads) connect the para-annular and outer bands.

LAA = left atrial appendage; SVC = superior vena cava

Strap Links.

In addition, linear strap-like musculi pectinati join the inner and outer bands lateral to the ascending palm-leaf arrangement in the lateral and posterior walls of both the atria (Fig. 5).

Similarity in Both Atria.

Thus the arrangements of musculi pectinati in both atria (Fig. 6) are morphologically similar. ² The left atrial arrangement is generally overlooked—especially the ascending and medial arrangements—because the musculi are embedded in the wall like beams concealed in a building. ² These features can be brought out by transillumination.

Fig. 6 Diagrammatic representation of a morphologically similar arrangement of atrial muscle bands and related musculi pectinati in the right and left atria.

IVC = inferior vena cava; LA = left atrium; LIPV = left inferior pulmonary vein; LSPV = left superior pulmonary vein; PLA = palm-leaf arrangement; RA = right atrium; SVC = superior vena cava

Function of Musculi Pectinati.

Obviously the medial and descending palm-leaf arrangements are designed to squeeze the appendage during atrial systole, aided by the ascending arrangement. Angiocardiograms in normal hearts reveal how effectively the appendages empty. During atrial systole, the linear strap-like connections between the inner and outer bands pull the curved segment of the corresponding atrioventricular annulus toward the outer band, which is fixed in front of the pulmonary veins. ^2–5 This movement is like pulling up pants while simultaneously pushing down the legs (atrial blood), which effects the transfer of atrial blood into the ventricle with minimal effort. ^2–5

Coordination of Function of Musculi Pectinati and Related Bands.

A question worth exploring is that of how the contraction of the atrial muscle bands is coordinated with the contraction of the musculi pectinati.

The contraction of the atria is complex. We have observed during surgery and angiocardiography that contraction of the superior vena cava and pulmonary veins precedes the corresponding atrial contraction. ^6,7 When the atria fibrillate, there is fibrillation in the superior vena cava and terminal pulmonary veins. ^7,8 Are the great veins activated by the sinoatrial node in a retrograde manner? Are there caval and pulmonary venous pacemakers? We have also observed that the contraction of the smooth segments of the atrial chambers (the systemic and pulmonary venous sinuses) precedes that of the pectinated segments. ^7,8 What is the myoneurogenic basis for veno-sino-atrial sequential contraction? While weaning patients from cardioplegia, we have observed that asynchrony of the veno-sino-atrial rhythm sometimes precedes re-establishment of the normal sequence. Often, the superior vena cava and pulmonary veins—and the corresponding systemic and pulmonary venous sinuses—start contracting before the pectinated atrial components. This contraction is not evident in the electrocardiogram; P waves appear in the electrocardiogram only when the pectinated atria contract.

The transverse band and its divisions are contractile. The pull of the divided end of the transverse band was demonstrated during surgery in our patient. In supracardiac total anomalous pulmonary venous drainage, the left outer band is not anchored to the pulmonary veins. In this condition, we have observed gross medial displacement of the left atrial appendage during atrial systole.

It is likely that the contraction of the transverse and 2 outer bands precedes the contraction of the pectinated chambers. This contractile ring would provide a fixed base for the musculi, aiding their contraction. The outer and the para-appendageal segments of the inner bands would squeeze the base of the corresponding appendage. The para-annular segments would shorten the curved segment of the atrioventricular annuli, reducing the atrioventricular orifices during late atrial systole in preparation for their complete closure by the leaflets, during ventricular systole.

There is dispute about the existence of internodal pathways of conduction. However, there must be a mechanism for the orderly contraction of the atrial muscle bands and the musculi pectinati. Also, the left atrium contracts later than the right atrium, and this delayed activation has been attributed to the transverse interatrial band. ⁹ Probably this delay is necessary for the left heart to act at the correct time upon the blood propelled by the right ventricle, thereby promoting smooth 1-way circulation. In frogs and reptiles, we have observed that the systemic veins contract before the pulmonary veins. The mechanism of this staggered contraction is interesting and needs to be elucidated.

Appendix II

Earlier Descriptions of Musculi Pectinati and Related Muscle Bands

The right outer band has been labeled by His (quoted by Papez ¹⁰) as the terminal crest, because it forms the boundary between the sinus venosus and the right atrium. In his Hunterian lectures ^3,4,11 in 1904, Keith explained the complex arrangement of muscle bands in the atria. He illustrated the division of the transverse atrial band into the “right taenia terminalis and annular fibres of the auricular atrial canal,” which surround the atrio-ventricular groove. ¹¹ He also described how the left taenia teminalis ⁴ (which corresponds to the left outer band) separates the vestibule of the left atrium (the smooth-walled component) from the pectinated part. The left outer band is described as continuous with the corresponding band of the right side and as passing to the left and looping around the terminal pulmonary veins. ⁴ In amphibians, reptiles, and human embryos, ⁴ Keith attributes to this band the function of closing the orifices of the pulmonary veins. Although complete closure is unlikely, circumferential contraction of the transverse and outer bands still promotes unidirectional forward flow.

Keith ¹¹ describes how the right and left taeniae terminalis form a horseshoe-shaped band anchored to the inferior vena cava, the contraction of which helps to close the venous orifices of the atria. Keith ⁴ refers to the contraction of the atria as the final act of atrial systole. He describes how the base of the atrial appendage moves near the center of the atrium so that the mouth of the appendage lies over the atrioventricular orifice. He attributes this movement to bands arising from the corresponding taenia terminalis. ⁴ He refers to Dr. James Mackenzie's belief that thrombus in the appendage is a result of paralysis of the atrium. ⁴

In 1916, Bachmann ⁹ referred to a distinct band stretching from the right atrium to the base of the left atrial appendage. Probably he included the stem of the descending or medial musculi as part of this band. He credits Lewis, Meakins, and White ¹² for also having described this band ⁹ and approves the appropriateness of their term “inter-auricular band.” Bachmann considered the interatrial band, now also called Bachmann's bundle, to be the most important path of conduction between the 2 atria, and he conducted experiments that involved mechanical damage to the bundle.

In 1920, Papez ¹⁰ documented the musculature of the atria in detail. He states that the interatrial band (so named by Lewis, ¹² who is quoted by Papez) extends to the front of the left atrial appendage and divides to encircle it, producing a sharp constriction that bends the appendage upward. Intermediate pectinate muscles were found to arise from this band. The right posterior crest (the terminal crest of His) is described as yielding 10 posterior pectinate muscles, 2 of which extend into the appendage; the lower muscles were seen to extend to the atrioventricular ring and to the orifice of the coronary sinus.

The right anterior crest is described as giving rise to the right anterior pectinate muscles, which encircle the base of the right atrium and interlace with the pectinated column of the right posterior crest near the orifice of the coronary sinus. The pectinate muscles were found to arise from the anterior crest singly or in 3 radiating clusters. The anterior crest is described by Keith and Flack ¹³ (quoted by Papez ¹⁰) as annular fibers of the auricle and by McMurrick as “ansiform fasciculi.”

The left anterior crest is described as a continuation of the interatrial band that extends to the left along the upper margin of the atrioventricular ring and gives rise to the anterior pectinate muscles, interdigitating with the bundles of the posterior crest on the left side of the atrium.

Papez goes on to describe the left posterior crest as a continuation of the upper part of the interatrial band that passes around the base of the left atrial appendage, in front of the pulmonary veins. The left posterior crest is described as yielding the posterior pectinate muscles. Posteriorly, this crest interdigitates with the anterior crest and forms a triangular area related to the atrio-ventricular ring. Papez ¹⁰ also notes autonomic innervation of the atrial muscle bundles.

In a more recent study, Wang and colleagues ⁵ state that no structure resembling the terminal crest is seen in the left atrium. This is at variance with our observations and with those of Keith and Papez. They also observe that the pectinate muscles are confined more or less to the appendage and do not support the mitral valve. However, transillumination brings out the musculi unseen by the naked eye.

In their description of the terminal crest in the right atrium, Wang and colleagues ⁵ observe that it merges inferiorly into the sub-Eustachian sinus. The interatrial band, they observe, extends from the superior vena cava to the left atrial appendage and divides into upper and lower branches that encircle the narrow mouth of the appendage. The upper band is described as extending to the lateral atrial wall, and the lower band as attached to the mitral ring. These 2 branches were found to interdigitate with other circumferential bundles on the lateral wall and then to continue posteriorly until their insertion into Waterston's groove.

Solomon Victor, FRCP
Vijaya M. Nayak, MS
Department of Cardiac Surgery, The Heart Institute, Chennai, India