Anatomical variants are those variations in normal anatomy that could be misinterpreted as pathological conditions. Many anatomical variants occur as remnants of embryological development and fetal circulation, commonly visualized in the atria. Anatomical variants can be differentiated from artifacts (or errors in interpretation) as they persist despite sonographic changes in transducer frequency, gain, compression, or depth; and are seen in multiple image planes. Ultrasound artifacts usually occur due to a violation of the assumptions inherent to all ultrasound systems. Fundamentally, ultrasound imaging assumes that sound travels in a straight line, travels directly back from a reflector, and travels at exactly 1540 m/s through soft tissue. Additionally, it is assumed that the ultrasound beam is very thin, reflections are entirely from structures within the main axis of the beam, and the intensity of reflections is related only to the tissue characteristics of the reflector.¹ Artifacts can be distinguished from anatomical variants as they tend to cross known anatomical planes and boundaries and usually disappear with alternative imaging planes or sensitivity changes such as changes to the Doppler baseline or the pulse repetition frequency. Thus it is vital to be knowledgeable about the common anatomical variations and ultrasound imaging artifacts to ensure accurate echocardiographic interpretation and to avoid unnecessary interventions.²

In the fourth week of gestation, the atria and the sinus venosus evolve and merge with the embryological heart. Initially, the sinus venosus receives venous blood from left and right sinus horns (Fig. 6-1A and B). Soon thereafter, the veins to the left sinus horn are obliterated and the remnants become the coronary sinus. The right sinus horn enlarges to create the smooth-walled part of the right atrium (RA), which displaces the trabeculated tissue of the primitive RA into the periphery and into the right atrial appendage (RAA), resulting in the prominent pectinate muscles that are characteristic of the atrial appendages. Right and left venous valves mark the junction of the original right sinus horn and the primitive RA. The left venous valve disappears as it fuses with the developing atrial septum. The right venous valve of the right sinus venosus horn develops inferiorly into (1) the valve of the inferior vena cava (IVC), or the eustachian valve, which directs fetal blood flow from the IVC across the foramen ovale; and (2) the valve to the coronary sinus, or the thebesian valve (Fig. 6-2). Superiorly, the convergence of the smooth and trabeculated tissue of the RA results in the crista terminalis. Concurrently, the atrial septum forms with migration of the septum primum toward the endocardial cushion. The septum secundum forms through invagination of the atrial walls and migrates to cover the fenestrations formed in the septum primum. Eventually septum primum and septum secundum fuse, leaving the foramen ovale as the only residual interatrial communication. Incomplete coverage of the septum primum fenestrations leads to the formation of a secundum atrial septal defect (ASD). Persistent separation of the septum primum and septum secundum without atrial septal tissue deficiency results in patent foramen ovale.⁴ Congenital abnormalities of the interatrial septum will be further discussed in Chapter 19.

In the left atrium (LA), the smooth tissue of the pulmonary veins is incorporated into the wall of the left atrium and similarly displaces the primitive atrial trabeculated tissue almost entirely into the left atrial appendage (LAA). The ridge of tissue at the junction of the left superior pulmonary vein and the trabeculated left atrial appendage is called the ligament of Marshall or, more colloquially, the coumadin ridge (because this structure was initially misinterpreted as a thrombus requiring anticoagulation). Another variant occurs during the formation of the coronary sinus. Normally, the distal end of the left common cardinal vein degenerates, and the proximal portion connects via the left brachiocephalic vein to the right brachiocephalic vein, forming the superior vena cava (SVC). The left posterior cardinal vein also degenerates, and the remnants of the left sinus horn, receiving venous drainage from the heart, become the coronary sinus. Failure of the left posterior cardinal vein to resorb results in a persistent left superior vena cava (PLSVC) that drains into and dilates the coronary sinus (see Chapter 19).

The anatomical variants are best classified by primary location, although some variant structures may be present in more than one cardiac chamber (Table 6-1).

Right Atrium

CRISTA TERMINALIS

The crista terminalis is a vertical ridge of smooth myocardium located at the junction of the SVC and the RA, forming a structure that appears to protrude longitudinally into the RA. This structure is often visualized in the midesophageal (ME) bicaval view and should not be misinterpreted as thrombus or tumor (Fig. 6-3). Of note, the crista terminalis is thought to be a location where atrial tachydysrhythmias originate due to the high density of adrenergic nerve fibers, and thus may be a site for ablation therapy.⁵

EUSTACHIAN VALVE

The Eustachian globally valve is an embryological remnant that usually regresses in adulthood, but can be seen in about 25% of individuals as a prominent crescent-shaped tissue at the posterior aspect of the IVC. It can be found in multiple views, including the ME bicaval view and right ventricular (RV) inflow-outflow views (Fig. 6-4). It may even appear to extend from the IVC to the border of the fossa ovalis and thus appear to bisect the right atrium. However, in distinction to triatriatum dexter, the eustachian valve is distinguished by a lack of flow disturbance on color flow Doppler examination.⁶ Although the eustachian valve is of no physiological consequence, it may be confused with an intracardiac thrombus, cause turbulent atrial blood flow, complicate IVC cannulation, or serve as a site for endocarditis or thrombus formation.⁷

THEBESIAN VALVE

Called the “gatekeeper of the coronary sinus,”⁸ the Thebesian valve is a structure that can be seen as a thin piece of tissue guarding the entrance to the coronary sinus. Present in up to 80% of individuals, it can be visualized in the ME four-chamber view with the probe slightly advanced, or in the ME modified bicaval view inferior to the left atrium in the atrioventricular groove (Fig. 6-5). Although it is morphologically varied, thebesian valves are classified according to their shape as semilunar, fenestrated, biconcave, or bandlike, according to their composition as membranous, fibromuscular, fibrous, or muscular and the extent to which the valve covers the coronary sinus ostium. The valve serves to prevent retrograde flow into the coronary sinus during atrial contraction and is inconsequential unless it inhibits cannulation of the coronary sinus for a retrograde cardioplegia catheter or biventricular pacing wire placement, as some variants can occlude more than 50% of the coronary sinus ostium.⁸

CHIARI NETWORK

The Chiari network is a remnant of sinus venosus–derived structures that is seen as a thin, fenestrated, mobile, membranous structure within the RA. It is most highly associated with the IVC opening; however, the primary site of origin can vary to include the RA wall, interatrial septum, or the coronary sinus.⁹ It should be distinguished from thrombus or vegetation as it can be seen moving in the RA in multiple imaging views (Fig. 6-6). The structure can be further delineated with the use of 3D imaging and may appear as a “spider web moving in the wind.” It has little clinical significance except that it may cause of entrapment of right-heart catheters and can complicate atrial septal device occluder placement. It has also been associated with a patent foramen ovale, interatrial septal aneurysm, and paradoxical embolization.⁶

CORONARY SINUS (CS) AND PERSISTENT LEFT SUPERIOR VENA CAVA (PLSVC)

The coronary sinus courses in the atrioventricular groove superior to the mitral valve annulus¹⁰ and is normally less than 1 cm wide and approximately 3 cm long. Echocardiographically, it can be visualized in (1) the ME four-chamber view as an echolucency in the RA, just superior to the tricuspid valve (Fig. 6-7); (2) the modified ME bicaval view as it curves around the left atrium in the atrioventricular groove (see Fig. 6-5); or (3) the ME two-chamber view (Fig. 6-8) in the posterior atrioventricular groove. The coronary sinus is a useful structure to identify in order to assist with the placement of coronary sinus catheters for retrograde cardioplegia delivery and pacing wires for biventricular pacing. The entry of the CS may take an acute course (best appreciated with 3D echocardiography), which can make advancement of the catheter and wires difficult. One must also be able to recognize the normal coronary sinus to identify occasionally coronary sinus injury from retrograde catheters¹¹ or identification of an inferior sinus venosus defect. Coronary sinus dilation can result from atrial hypertension, tricuspid regurgitation, or a PLSVC that drains into the coronary sinus.^12,13 Diagnosis of a PLSVC (Fig. 6-9) is suggested by a dilated coronary sinus (>1.1 cm). The PLSVC can be seen between the left upper pulmonary vein and the left atrial appendage (LAA) at the ME level (Fig. 6-10A). Injection of agitated saline into a left upper extremity vein results in opacification of the coronary sinus from the PLSVC flow (see Fig. 6-10B), confirming the diagnosis.

PATENT FORAMEN OVALE

The normal foramen ovale is an embryological remnant, which appears as a thin slice of tissue bound by thicker ridges of tissue. Up to 30% of the population may have a probe patent foramen ovale (PFO), but it may be greater than 50% in patients younger than 55 years of age who have had a stroke as a consequence of right-to-left intracardiac shunting (Fig. 6-11).¹⁴ Transesophageal echocardiography (TEE) evaluation of the foramen ovale^15,16 should include 2D/3D assessment for interatrial movement and color flow Doppler assessment, optimized for measurement of lower-velocity flow. Injection of agitated saline (“bubble study”) along with a Valsalva maneuver is used to provoke right-to-left shunting. After a Valsalva maneuver produces a decrease in RA volume, agitated saline is injected and the Valsalva released (when the microbubbles are first seen to enter the RA) in order to transiently increase RA pressure over LA pressure. Admixture of agitated saline with small quantities of blood has been reported to improve the acoustic signal of the microbubbles. The bubble study is positive if bubbles appear in the left atrium within three to six cardiac cycles (Fig. 6-12).⁴

ATRIAL SEPTAL ANEURYSM

An atrial septal aneurysm is characterized by an undulating atrial septum that moves between atria during the cardiac cycle (Fig. 6-13).¹⁷ It has been defined as being more than 1.5 cm in size and/or extending into either atrium by 1.5 cm or more (Fig. 6-14), but variable grading systems exist, largely based on the extent of excursion into the left and right atrium (see Appendix F). Atrial septal aneurysms have been associated with PFO and Chiari network and may predispose to thrombus formation, resulting in potential paradoxical embolism and stroke.¹⁷ Percutaneously inserted closure devices for PFOs may be efficacious in patients with paradoxical emboli.¹⁸ An atrial septal aneurysm may also impede efforts of wire passage into the superior vena cava in preparation for femoral venous cannulation.

LIPOMATOUS HYPERTROPHYOFTHE ATRIAL SEPTUM

The interatrial septum may be markedly thickened, mimicking an infiltrative or pathological process. However, this benign finding can be seen to involve primarily the superior and inferior portions of the interatrial septum, sparing the fossa ovalis, and thus leading to the “dumbbell-like” (Fig. 6-15) appearance. The echogenic fat may also involve the right atrial wall, a finding that is associated with coronary artery disease.¹⁹ The prevalence of lipomatous hypertrophy is estimated to be between 1% and 8%. It usually occurs in older, obese people, and there may be a higher incidence in women.

TRABECULATIONSAND PECTINATE MUSCLES

Muscle bands can exist on all endocardial surfaces of the heart, known echocardiographically as trabeculations. In the right and left atria these muscle bands are known as pectinate muscles, which course across the anterior endocardial surfaces, including both appendages. Pectinate muscles are more prominent in the RA than in the LA (Fig. 6-16) and more apparent in the LAA than the RAA. Prominent pectinate muscles can be distinguished from a mass or thrombus by their uniform texture and density, as well as movement that is in synchrony with cardiac tissue. In distinction, thrombus is often asynchronous with cardiac motion and is associated with arrhythmias such as atrial fibrillation or low-flow states due to obstructive valvular disorders, such as mitral stenosis. Of note, trabeculations can be particularly prominent in the RV, and RV hypertrophy can accentuate these trabeculations making measurements of RV wall thickness difficult.

RIGHT ATRIAL APPENDAGE (RAA)

The RAA is most commonly seen in an ME bicaval view where the crista terminalis separates the SVC and RAA. Occasionally, the prominent trabeculations or pectinate muscles can also be seen. The RAA can also appear as an echo-free space anterior to the ascending aorta and near the right ventricular outflow tract in the ME aortic valve long-axis view. The RAA should not be forgotten as a site for thrombus formation during conditions such as atrial fibrillation. Although the risk of systemic emboli is low from the RAA, it is theoretically possible with a patent foramen ovale (PFO). An abnormally large RAA may also trap devices (such as a PA catheter) during their placement.

Left Atrium

LIGAMENTOF MARSHALL

The atrial tissue between the left upper pulmonary vein (LUPV) and the LAA is known as the ligament of Marshall (LOM). LOM is the embryological vestige of the left common cardinal vein. It has many appearances, including that resembling a “Q-tip” (Figs. 6-17 and 6-18) and has historically been misinterpreted as a thrombus, leading to its common reference as the “Warfarin” or “coumadin” ridge. The LOM contains muscle bundles (Marshall bundles), has been identified as a source for paroxysmal atrial fibrillation, and is an important landmark to the electrophysiologist.²⁰

LEFT ATRIAL APPENDAGE (LAA)

The LAA is best seen in an ME two-chamber view where it is separated from the left superior pulmonary vein by the ligament of Marshall (Figs. 6-18 and 6-19). It can be heavily trabeculated with pectinate muscles and is associated with thrombus formation during low-flow states. Pectinate muscles are distinguished from thrombus by similar density and texture to other LAA tissue, as well as their synchronous movements with surrounding cardiac tissue. Due to the posterior location of the LAA, TEE is superior to transthoracic echocardiography (TTE) for examination of the LAA to identify thrombus as a potential source for cardiac embolism in patients with a history of transient ischemic attack (TIA) or stroke (Fig. 6-20A). In addition to 2D imaging, color flow Doppler, pulsed-wave Doppler, tissue Doppler, power Doppler, contrast echo, and 3D imaging can enhance the diagnostic process for thrombi.^21,22 A pulsed-wave Doppler sample at the mouth of the LAA with a velocity greater than 40 cm/s in the LAA decreases the likelihood of a thrombus (Fig. 6-20B). TEE is also used to assist with the guidance of LAA closure devices or LAA ligation to decrease the potential for thromboembolism.²³ Thirty to fifty percent of people also have a bilobed or multilobed left atrial appendage (Fig. 6-21).