Background. The complex architecture of the right atrium, crista terminalis (CT), and the musculi pectinati (MP) poses enormous challenges in electrophysiology and cardiac conduction. Few studies have been undertaken to substantiate the gross features of MP, in relation to the CT, but there is still scarcity of data regarding this. We tried to reinvestigate the gross arrangement of muscle bundles in the right atrium. Methods. Utilizing 151 human hearts and orientation of MP and its variations and relationship to the CT were investigated along with taenia sagittalis (TS). Patterns of MP were grouped in 6 categories and TS under three groups. Result. A plethora of variations were observed. Analysis of all the specimen revealed that 68 samples (45%) were of type 1 category and 27 (18%) fell into type 2 category. Prominent muscular columns were reported in 12 samples (8%). 83 samples (55%) presented with a single trunk of TS. Multiple trunks of TS were reported in 38 samples (25%). Conclusion. Samples with type 6 MP and type B/type C TS, which have a more complex arrangement of fibers, have a tendency to be damaged during cardiac catheterization. Nonetheless, the area as a whole is extremely significant considering the pragmatic application during various cardiac interventions. 1. Introduction The crista terminalis (CT) is a well-defined fibromuscular ridge formed by the junction of the sinus venosus and primitive right atrium that extends along the posterolateral aspect of the right atrial wall [1]. It originates from the atrial septal wall medially, passes anterior to the orifice of the superior vena cava (SVC), descends posteriorly and laterally, and then turns anteriorly to skirt the right side of the orifice of the inferior vena cava (IVC). It is an important anatomic landmark due to its close association with sinoatrial nodal artery and the origin of the musculi pectinati (MP, pectinate muscles). The MP are muscular ridges that extend anterolaterally from the CT to reach the auricle and may present in a number of variable forms and shapes/sizes. The largest and most prominent MP forming the bridge of the sulcus terminalis internally is called taenia sagittalis (TS), literally meaning sagittal worm [2]. Occasionally, CT and the MP can be prominent, thus mimicking right atrial mass like a pseudomass, tumor, thrombus, or vegetation. An understanding of the anatomy and proper identification helps in avoiding misdiagnosis [1]. The MP with highly trabeculated muscle fibers predispose to arrhythmias. Moreover, prominent muscular columns with velamentous MP
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