Subaortic (type 6) muscular band-innocent bystander or pathologic structure?

Intraventricular tendons are structures that was identified more than a hundred years ago. It has been suggested that they represent intracavitary radiations of the bundle of His and that they may be an isolated finding or be associated with structural cardiac abnormalities.Loukas et al divided these structures into five categories and recently a sixth type have been added.Various physiological disturbances have been observed due to the sixth type of tendon, such as ST segment elevation and right bundle branch block. It has been noted that this peculiar structure appears too thick to be called a tendon, thus the term band.This retrospective analysis analyzed the incidence of the thick, subaortic (type 6) muscular band in a cardiovascular clinic.

Introduction

Intraventricular tendons was first observed 115 years ago.1 They are usually thin, fibrous or fibromuscular structures that traverse the left ventricular cavity.2 They may be single or multiple and have no connection to the valvular cusps.2

False tendons can be an isolated finding or can be associated with cardiac abnormalities.3

Loukas et al4 examined 200 formalin fixed, adult hearts and divided false tendons into five categories: In type I the false tendon is located between the posteromedial papillary muscle and the ventricular septum. In type II the false tendon is located between the two papillary muscles. In type III the false tendon is located between the anterolateral papillary muscle and the ventricular septum. In type IV the false tendon is located between the ventricular septum and the free wall. In type V the false tendons appear weblike and have three or more points of insertion.

After careful histological examination Abdulla et al3 suggested that false tendons may be intracavitary radiations of the bundle of His and that their clinical significance may be more relevant than previously believed and merits further study.

Recently, various physiological effects due to a newly observed thick, subaortic muscular band have been described.5–7

The purpose of this study was to describe the incidence of this peculiar new type of muscular band in a cardiovascular clinic.

Methods

This study was a retrospective analysis. A total of 4900 adult echocardiograms, performed between 2004 and 2010 for various clinical indications—ranging from hypertension to cardiac murmurs and dyspnoea—were reviewed and analyzed for the presence of a left ventricular muscular band in the subaortic position.

Results

A total of 376 adult echocardiograms clearly demonstrated the presence of a muscular structure, located between the left ventricular inferior wall and the subaortic interventricular septum (see Figure 1 and 2 and movie clip 1 and 2). In all of these cases a striking local ventricular hypertrophic response was present at the site of septal implantation. In some of these cases the local hypertrophic response even mimics the echocardiographical appearance of hypertrophic cardiomyopathy6 (see movie clip 1).

Figure 1

Parasternal, long-axis echocardiographical image, depicting a thick subaortic, muscular band, marked with X.

Figure 2

Another example of a thick subaortic muscular band, marked with X.

Discussion

A peculiar muscular structure, too thick in appearance to be called a tendon, extending between the subaortic portion of the interventricular septum and the inferior left ventricular wall is identified in this study. A striking feature is a localized area of ventricular hypertrophy at the site of subaortic implantation of the muscular structure.

Among these 376 patients with the muscular subaortic band present, the following physiological effects have been observed: ST-segment elevation of the anterior electrocardiographic leads, mimicking acute myocardial infarction,4 echocardiographical mimicking of hypertropic cardiomyopathy5 and right bundle branch block.6

It has been proposed before that this muscular, subaortic band may represent a sixth type of ventricular false tendon.8 The current clinical implication of the presence of this peculiar intracardiac structure is that it may mimic cardiac disease, such as myocardial infarction or Brugada syndrome in the case of ST segment elevation,5 it may lead to the erroneous diagnosis of hypertrophic cardiomyopathy if the echocardiographer is not vigilant6 and recently it has been described that a third type of right bundle branch block may result due to the presence of such a subaortic muscular band.7