The Marshall Complex in Atrial Fibrillation: Anatomy, Mechanisms, and Ablation Strategies

Embryology

Embryologically, the LoM and the VoM originate from the left anterior cardinal vein, which forms part of the embryonic LSVC system.^3,4,5 During normal cardiovascular development, venous return progressively shifts to the right-sided venous system, leading to involution of the embryonic LSVC. As a result of this regression, the venous lumen is largely obliterated, leaving behind a fibrous remnant known as the LoM. In contrast, a small residual venous channel persists as the VoM.^2,3,4,5 Incomplete regression of the embryonic LSVC results in a persistent LSVC, a congenital venous anomaly observed in approximately 0.3–0.5% of the general population and up to 12% of patients with congenital heart disease.⁵ The LoM, therefore, represents a regressed venous structure rather than an actively developed myocardial component.

Gross anatomy

In adults, the LoM lies subepicardially along the left lateral atrial ridge, creating a distinct elevation between the LAA and the LPVs [Figure 1].^3,6 Anatomical studies have shown that the LoM is variable in length, typically short (20–30 mm), and contains multiple muscle bundles, known as Marshall bundles, which are separated by fibro-fatty tissue.^2,7,8 Marshall bundles connect the left atrium to the coronary sinus, forming potential epicardial conduction pathways.

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Figure 1:

Schematic diagram showing the LoM, VoM, and related left atrial venous structures, including the left atrium (LA), left superior pulmonary vein (LSPV), left inferior pulmonary vein (LIPV), left atrial appendage (LAA), left ventricle (LV), coronary sinus (CS), Right superior vena cava (RSVC), and inferior vena cava (IVC). (Illustration not drawn to scale. Colors used in this illustration are not representative). (Adapted from Constans and Asirvatham)

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Microscopic structure and innervation

Histological analyses demonstrate that the LoM contains small veins, adipose tissue, collagen fibers, myocardial tracts, and abundant neural elements.^2,7,10 Autonomic innervation of the LoM is composed of both sympathetic and parasympathetic fibers, with a predominantly parasympathetic profile and regionally variable distribution.^2,10,11,12 This pattern allows the LoM to mediate complex neurocardiac interactions. Experimental studies in canine models have shown that electrical stimulation of the LoM region can induce atrial ectopy through either adrenergic or vagal activation.^4,6,9 Thus, the LoM serves as both an electrical and autonomic conduit between the left atrium and the extracardiac nervous system.

Together, the LoM, the VoM, the associated muscle bundles, and their autonomic nerves act as a single functional unit. This group of structures is often referred to as the Marshall complex when it is being considered for ablation procedures.¹³ The term highlights how different elements in this region work together to influence the atrial arrhythmias.

Electrophysiological role

The LoM’s unique muscular and neural components make it a potent trigger and substrate for atrial tachyarrhythmias. Intracardiac mapping frequently identifies rapid, repetitive electrical activity originating from the LoM during AF.^12,13 The connection between the coronary sinus musculature and the left atrial myocardium via Marshall bundles enables conduction that can bypass standard ablation lines, such as those around the pulmonary veins or mitral isthmus.^3,13

Hwang C et al. emphasized that these bundles may sustain focal discharges or reentrant circuits, particularly in patients with persistent AF.^2,13,14 Furthermore, the LoM’s slow conduction properties have been demonstrated in optical mapping studies and clinical mapping studies, showing that it can maintain reentry when other atrial connections are blocked.^13,14,15 Clinically, a substantial proportion of left atrial tachycardias (AT) following AF ablation involve the LoM region.^13,14 The LoM also serves as a pathway bridging electrical gaps across the mitral isthmus, which may prevent durable mitral block if not addressed.¹³ Hence, many operators target the Marshall complex during substrate modification or when mitral flutters recur after initial isolation.^14,16

Role in atrial fibrillation

As AF progresses, structural remodeling of the left atrium appears to increase the functional importance of the Marshall complex. Fibrosis and conduction slowing along the left atrial ridge and mitral isthmus may reduce effective endocardial conduction, making epicardial pathways more relevant for wavefront propagation.^2,10,13 This may explain why Marshall complex involvement is more commonly observed in persistent AF and in AT occurring after prior ablation, rather than in early paroxysmal AF.^7,13,14 In this setting, the LoM may contribute to AF maintenance by supporting organized reentry or stabilizing fibrillatory conduction, rather than acting as a primary trigger.^13,16,17 These observations support a selective, substrate-based approach to target the Marshall complex, particularly in patients with advanced AF. Ablation of these signals may help improve procedural success in persistent AF.^13,16,18

Mechanism-based selection for Marshall complex targeting

Clinical benefit from Marshall complex ablation appears highly phenotype-dependent. Patients with persistent AF, peri-mitral flutter, or failure to achieve durable mitral isthmus block are more likely to harbor functionally relevant epicardial Marshall bundle conduction or autonomic involvement, and therefore derive greater benefit from targeted intervention.^12,13,14,16 In contrast, patients with paroxysmal AF dominated by pulmonary vein triggers may have limited incremental benefit from routine Marshall complex targeting.^13,19 A mechanism-based approach to patient selection may optimize procedural efficiency while minimizing unnecessary ablation.

Ablation strategies

Endocardial radiofrequency (RF) ablation can sometimes eliminate Marshall complex potentials by targeting the left atrial ridge or mitral isthmus near the ligament’s insertion.^13,18 However, conventional RF ablation may be limited by the LoM’s epicardial course and variable anatomy. When endocardial lesions fail to interrupt conduction, an epicardial approach through the coronary sinus is considered.^13,18 These challenges have led to the development of chemical ablation methods, particularly retrograde ethanol infusion into the VoM, which can affect both muscular and neural elements.^15,16

VoM ethanol infusion (VoM-EI)

Valderrabano et al. first described Ethanol infusion into the VoM as a method to achieve targeted ablation of the Marshall complex.¹⁶ The technique involves cannulating the coronary sinus, advancing a micro-balloon catheter into the VoM branch, inflating it to occlude flow, and injecting small doses of 98% ethanol (typically 4-12 mL in divided aliquots, depending on the anatomy).^15,16 The ethanol permeates the LoM and neighboring tissue, creating a lesion that electrically isolates the region and is associated with autonomic modulation.^14,15,20

This approach offers several advantages: it ensures consistent lesion formation, facilitates bidirectional mitral isthmus block, and ablates local ganglionated plexuses.^21,22 In clinical practice, VoM-EI is often performed before or during mitral isthmus ablation, particularly in persistent AF.¹⁴ Voltage mapping post-procedure confirms a low-voltage scar along the LoM tract, consistent with myocardial injury and presumed autonomic disruption.^17,20

Clinical outcomes and evidence

Several studies and randomized trials have evaluated the impact of targeting the Marshall complex. In the VENUS trial, adjunctive VoM-EI with standard catheter ablation improved arrhythmia-free survival in persistent AF patients [Table 1].¹⁶ Subsequent meta-analyses have confirmed these findings, showing higher long-term freedom from AF and AT when VoM-EI is added to conventional ablation.^7,18,23 Moreover, recent findings by Kong et al. suggest that VoM-EI was associated with a lower rate of chronic reconnections at the pulmonary veins and mitral isthmus.²⁴

Critical insights and limitations

Although the accumulated evidence supports the arrhythmogenic role of the LoM and the benefit of VoM-EI, existing studies show several limitations. Most available data are derived from single-center studies and employ heterogeneous ablation strategies, limiting direct comparisons across studies.^11,13,21 Additionally, long-term durability beyond one to two years remains insufficiently studied.^16,17,18

Safety profile of VOM-EI

VoM-EI is generally safe, with complication rates comparable to standard ablation procedures.^16,21,22 Appropriate imaging, careful balloon inflation, and slow ethanol delivery are essential to minimize risks. The most frequent issues include pericardial effusion, vein dissection, or local ethanol leakage leading to pericarditis.²³ Severe complications, such as tamponade, have been reported in approximately 1–2% of cases.^22,23 Technical failures, primarily due to an absent or insufficient VoM, are reported.^16,23 With growing operator experience, procedural success rates may increase significantly.