Case Study

Possible Mahaim Fiber in Wolff-Parkinson-White Syndrome

Christopher Wilcox, DO, Internal Medicine Resident, and Robbie Wall, DO, electrophysiologist 

University at Buffalo; Mercy Hospital of Buffalo

Buffalo, New York

Christopher Wilcox, DO, Internal Medicine Resident, and Robbie Wall, DO, electrophysiologist 

University at Buffalo; Mercy Hospital of Buffalo

Buffalo, New York


Normal conduction in the heart involves impulse propagation from the atria to the atrioventricular node (AVN), then down the bundle of His-Purkinje system into the ventricles. The Purkinje system separates into fascicles, with one going to the right ventricle and two to the left ventricle. Accessory pathway is a term that encompasses all aberrant tracts connecting the atria and ventricle other than the AVN pathway. One such well-known pathway is seen in Wolff-Parkinson-White (WPW) syndrome, in which a bundle of fibers exist that directly connect the atria to the ventricle myocardium without passing through the His-Purkinje system. Electrocardiograms of WPW pattern can be examined to help localize the pre-excitation pathway. Several studies have attempted to create an algorithm to use the 12-lead EKG to localize the pathway.1-3 These studies have found success utilizing R/S ratios in leads V1 and aVF, with sensitivity and specify in the 90s. 

Mahaim fibers have come to represent accessory pathways connecting either nodoventricular4 (from the AV node to the ventricle), or fasiculoventricular (stemming from the His bundle to the ventricle myocardium).5 Mahaim fibers are also commonly associated with second accessory pathways in up to 40 percent of cases,6 and have been shown to be unmasked as a secondary pathway after the first has been ablated.7 In some cases, these fibers have been shown to be incidental and not associated with any tachyarrythmia.8 The characteristic feature of Mahaim fibers is the presence of decremental conduction when pacing from the atrial lead.9 This decremental conduction is identified as a prolongation in the time between recorded AV signals during an EP study.

Case Description

The patient is a 47-year-old Caucasian male who reported having possible mitral valve prolapse and occasional palpitations as a child. He recently began experiencing episodes of palpitations with associated dizziness and lightheadedness without chest pain, shortness of breath, or syncope. The patient has a medical history of hyperlipidemia and hypertension. He currently works as a machinist. He reports no tobacco or illicit drug use, and only occasional alcohol and caffeine use (which he stopped completely due to the recent dizziness). There is no reported incidence of arrhythmia or sudden cardiac death (SCD) in his family. His physical exam and laboratory values were within normal limits. EKG demonstrated a WPW pattern (Figure 1).


Access to the left atrium (LA) was gained via transseptal puncture and mapped (Figure 2) using the CARTO system (Biosense Webster, Inc., a Johnson & Johnson company). Initial EP testing revealed left lateral atrioventricular signal consistent with an accessory pathway near the distal CS lead 1-2 in the lateral LA, but was later seen medially at 3-4 and 5-6 (Figure 3). Extensive ablation of this pathway demonstrated separation of atrial (A) and ventricular (V) signals at CS 1-2 with corresponding change in QRS morphology (Figure 3); however, there was no resolution in delta wave. Persistent AV signal fusion and pre-excitation was seen at multiple points in the left atrium, including posterior septal throughout the left lateral position. Subsequently, the RA was reentered and mapped (Figure 4), and an AV signal fusion was noted near the os of the CS. After a brief ablation of this area, there was a separation of the VA on CS 7/8 as well as a corresponding change in QRS, this time with resolution of pre-excitation (Figure 4).


EKG as well as CARTO mapping suggested a left lateral and left septal pathway. With ablation of the LA, there was no resolution in pre-excitation demonstrated by the persistent delta wave. Also, even after separation of AV signals in CS 1,2, there was still fusion in CS 3,4 and 5,6, indicating either only partial pathway ablation or that a second accessory pathway was still present. The CARTO mapping algorithm may have been detecting two pathways, one in lateral LA and one in RA, and averaging their location at the septum. There was partial separation in distal CS as seen in Figure 3; however, the ablation catheter demonstrated persistent AV signal overlap posterior septal and around the left lateral wall in the LA. 

In mapping the RA, an area near the CS was identified and ablated with complete resolution of pre-excitation as well as separation of CS 3,4, 5,6 and 7,8. The extensive ablation time required and continued fusion signal in the left atria was unexplained, which is why a Mahaim fiber was considered. Initially, a Mahaim fiber was suspected possibly connecting into the left posterior fascicle and giving the appearance of a left-sided pathway. Review of data after the EP study did not demonstrate decremental conduction in the RA pathway (Figure 5). As the left atrial pathway was not able to be isolated, we were unable to confirm or refute decremental conduction.

Second pathways are a fairly common phenomena, occurring in 10 percent of patients1 with WPW pattern and up to 40 percent of those with a Mahaim fiber. Interestingly, this patient’s pathway mapped to the LA and was present in a large oblique pattern. After extensive ablation of this pathway, there was an EKG pattern change, but no elimination of pre-excitation. On subsequent mapping of the RA, another potential pathway or pathway origin was identified and ablated in 1-2 minutes. During this ablation, there was AV separation in CS catheter with simultaneous resolution of pre-excitation. This would seem to indicate that there were multiple pathways present in this patient, as the EKG pattern changed (Figure 2) after ablation in the LA and pre-excitation was eliminated after ablation in the RA.

With this conclusion in mind, post-procedure analysis of the EKGs was conducted and no decremental conduction, a key feature of Mahaim pathways, was found. However, the LA pathway was long and oblique, and its role in the patient’s rhythm is unclear because resolution in pre-excitation did not occur until after ablation in the RA. Assuming there were two pathways in this patient, it is difficult to eliminate the possibility that the left atrial pathway was directly connected to the LV as a Mahaim fiber — this pathway was not isolated in the study, as it was ablated first. Its activity may have been difficult to detect due to the pre-excitation pathway that existed in the RA, which was isolated and ablated after the LA pathway. A large percentage of Mahaim fibers have been associated with secondary pathways; therefore, it is reasonable that some would be difficult to detect, especially in the presence of a second pre-excitation that may conduct the predominance of the atrial impulses.

Post-operative analysis and discussion of these possibilities is beneficial, since it is difficult to explore all of the options for complicated and aberrant pathway patterns during the procedure, both for the safety of the patient and the success of the procedure. With increasing access, amount of procedures, and knowledge base in the area of EP studies and ablation techniques, it is likely that such pathways will be encountered. Discussion of their significance, mechanism, and localization is helpful for more efficiently recognizing potential patterns, and therefore, improving outcomes when encountered.

In Memoriam. Sadly, one of the authors of this article passed away suddenly prior to article publication. Dr. Robbie Wall was a clinical electrophysiologist in Buffalo, New York, as well as a veteran of both the U.S. Air Force and National Guard. He was a compassionate and caring physician known for not only his clinical skills, but also his dedication to teaching and mentorship. He is greatly missed by his family, colleagues, and students.

Disclosure: Dr. Wilcox has no conflicts of interest to report regarding the content herein.   


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