Myocardial Bridge in Dextrocardia with Situs Inversus

It is ambiguous whether the myocardial bridge (MB) over the coronary arteries and their branches is present in or absent from dextrocardia with situs inversus. Two cases of dextrocardia with situs inversus were accidentally found in adult Thai hearts received for researches from the Department of Anatomy, Faculty of Medicine, Chiang Mai University. They were found in 43and 78-year-old Thai men. One case of dextrocardia with situs inversus was found in gross anatomy practice of Chiang Mai University. It was found in a 62-year-old Thai man. Therefore, the authors investigated whether the MB over the left and right coronary arteries and their branches was present in or absent from three cases of dextrocardia with situs inversus. The MB was found over the distal left coronary artery in one case of dextrocardia with situs inversus.

their branches is present in or absent from dextrocardia with situs inversus.
Two cases of dextrocardia with situs inversus were accidentally found in about 330 adult Thai hearts received for researches from the Department of Anatomy, Faculty of Medicine, Chiang Mai University from 2001 to 2015. In addition, one case of dextrocardia with situs inversus was found in gross anatomy practice of Chiang Mai University in 2018. Therefore, the authors investigated whether the MB over the coronary arteries and their branches was present in or absent from three cases of dextrocardia with situs inversus. The MB was found over the distal left coronary artery in one case of dextrocardia with situs inversus.

Materials and Methods
Thai cadavers were treated by injection of a mixture of 26% methanol, 14% glycerin, 3% phenol, 14% formalin, 0.34 M potassium nitrate, and 14 mM arsenic oxide through the femoral artery [6]. For researches, the authors received about 330 adult Thai hearts from the Department of Anatomy, Faculty of Medicine, Chiang Mai University from 2001 to 2015. Two cases of dextrocardia with situs inversus were accidentally found within the adult Thai hearts. In addition, one case of dextrocardia with situs inversus was found in gross anatomy practice of Chiang Mai University in 2018. The hearts were preserved in 10% formalin solution.

Introduction
Dextrocardia with situs inversus is a very rare condition. Its incidence is one or two in 10,000 [1][2][3]. Usually, the subject with situs inversus viscerum totalis survives enough long life without congenital defects. It is well known that the myocardial bridge (MB) over the coronary arteries and their branches is frequently present in the normal human heart [4,5]. However, it is ambiguous whether the MB over the coronary arteries and the right ventricle. The small ramus intermedius artery (diameter 1.2 mm) distributed the anterior wall of the right ventricle.
The left coronary artery (diameter 5.2 mm) arose from the left aortic sinus and coursed through the left atrioventricular groove (Figure 1). The left coronary artery ramified first the sinoatrial nodal artery and secondly the conus branch. The fifth branch was the acute marginal branch and ran along the margin of the left ventricle. The posterior interventricular artery of the left coronary artery coursed downward through the posterior interventricular groove. No MB was found over the left and right coronary arteries and their branches in the first case of dextrocardia with situs inversus.
The coronary circulation of the first case was classified as left dominant, because the posterior interventricular artery was supplied by the left coronary artery [7].

The second case (cadaver number 165/56)
The right aortic sinus gave rise to the right coronary artery. The right coronary artery (diameter 8.9 mm) was trifurcated into the anterior interventricular artery, the circumflex artery, and the ramus intermedius artery To examine the presence of the MB, fatty tissue was removed from the heart and arterial blood vessels were prepared by careful dissection with special reference to the presence of the MB.

Results
Three cases of dextrocardia with situs inversus were found in a 43-year-old man (cadaver number 41/47) who died of hepatoma, a 78-year-old man (cadaver number 165/56) who died of senility, and a 62-year-old man (cadaver number 11/60) who died of sepsis.

The first case (cadaver number 41/47)
The right aortic sinus gave rise to the right coronary artery. The right coronary artery (diameter 7.0 mm) was trifurcated into the anterior interventricular artery (or anterior descending artery), the circumflex artery, and the ramus intermedius artery (Figure 1). The anterior interventricular artery (diameter 5.1 mm) coursed through the anterior interventricular groove and gave off the diagonal branches to the anterior wall of the right ventricle. The circumflex artery (diameter 4.5 mm) coursed through the right atrioventricular groove and gave off the marginal branches to the lateral wall of

The third case (cadaver number 11/60)
The right aortic sinus gave rise to the right coronary artery. The right coronary artery (diameter 5.0 mm) was trifurcated into the anterior interventricular artery, the circumflex artery, and the ramus intermedius artery (Figure 3). The anterior interventricular artery (diameter 4.4 mm) coursed through the anterior interventricular groove and gave off the diagonal branches to the anterior wall of the right ventricle. The circumflex artery (diameter 4.5 mm) coursed through the right atrioventricular groove and the posterior interventricular groove and gave off the marginal branches to the posterior wall of the right ventricle. Thereafter, the circumflex artery gave off the posterior interventricular artery. The ramus intermedius artery (diameter 2.5 mm) distributed the anterior wall of the right ventricle.
The left coronary artery (diameter 3.5 mm) arose from the left aortic sinus and coursed through the left atrioventricular groove (Figure 3). The left coronary artery ramified first the conus branch and secondly the acute marginal branch. The acute marginal branch ran along the margin of the left ventricle. The left coronary ( Figure 2). The anterior interventricular artery (diameter 4.2 mm) ran through the anterior interventricular groove and gave off the diagonal branches to the anterior wall of the right ventricle. The circumflex artery (diameter 3.9 mm) coursed through the right atrioventricular groove and gave off the marginal branches to the lateral wall of the right ventricle. The ramus intermedius artery (diameter 2.4 mm) distributed the anterior wall of the right ventricle. from 33 to 72 years and reported that severe atherosclerotic lesions were seen in almost all of the anterior interventricular artery. However, as it is recognized that the MB is an anatomical variant, they did not examine whether there are the presence or absence of the MB in dextrocardia with situs inversus [2,8,9].
There are a few reports [10,11] on the coronary arteries of dextrocardia with situs inversus. Mano, et al. [10] reported one case of situs inversus viscerum totalis who was a 78-year-old Japanese man. Coronary vessel morphology was clearly inverted. The right-side left coronary artery divided into the anterior interventricular and the circumflex arteries. Conversely, the left-side right coronary artery ran dorsally and rightward along the atrioventricular groove to the crux and gave off the posterior interventricular artery. Roongruangchai, et al. [11] reported one case of situs inversus viscerum totalis who was an 87-year-old Thai woman. The right coronary artery divided into the anterior interventricular and the circumflex arteries. The left coronary artery ran along the left atrioventricular groove and gave off the posterior interventricular artery. The coronary circulation of two cases was classified as left dominant, artery ramified the sinoatrial nodal artery. The myocardial bridge (1.4 cm in length) was found over the distal left coronary artery (Figure 3c). The angle between the directions of the myocardial bundles and of the left coronary artery was about 60°.
The coronary circulation of the third case was classified as right dominant, because the posterior interventricular artery was supplied by the right coronary artery.
In three cases, the left and right coronary arteries formed a mirror picture of the normal pattern. The MB was found over the distal left coronary artery in the third case.

Discussion
There are pathological reports [2,8,9] on dextrocardia with situs inversus. Merklin, et al. [2] examined pathologically 111 cases of situs inversus viscerum and reported that a high number of patients with situs inversus totalis were afflicted with cardiac defects such as tetralogy of Fallot, interatrial septal defect, interventricular septal defect, and so on. Hynes, et al. [9] studied clinically and pathologically the coronary arteries in six cases of dextrocardia with situs inversus, ranging in age