Managing Myocardial Bridge and Right Ventricular Outflow Tract Obstruction in an Adolescent With Hypertrophic Cardiomyopathy

Abstract

We report a case of symptomatic myocardial bridge in an adolescent with dynamic right ventricular outflow tract obstruction and history of congenital pulmonary valve stenosis as well as hypertrophic cardiomyopathy. Definitive treatment was surgical infundibular myectomy and coronary unroofing, resulting in improvement in right ventricular outflow tract gradient and ischemic symptoms.

Introduction

Hypertrophic cardiomyopathy (HCM) most commonly affects the left ventricle but can also involve the right ventricle (RV), either secondary to left ventricular hypertrophy or more rarely, isolated right ventricular outflow tract (RVOT) obstruction. RV involvement is associated with more severe disease including an increased risk of sudden cardiac death and ventricular arrhythmias. ¹

Myocardial bridges are relatively common and typically benign but can cause symptoms, particularly in children with underlying congenital heart disease. ² In pediatric patients with HCM, myocardial bridging is associated with poor outcomes. ³ The combination of myocardial bridge with isolated RVOT obstruction is a rare phenomenon, and few cases have been reported to date. Both conditions are associated with worse prognosis in HCM and guidelines for management are unclear.

We present a case of an adolescent with myocardial bridge and RV HCM and the key role that functional testing with coronary angiography played in determining the best management strategy.

Case Report

A 15-year-old male with a history of mild congenital pulmonary valve stenosis and obesity presented to the emergency department with several months of exertional dyspnea, chest pain, and dizziness while playing sports and grade 4/6 harsh systolic murmur best heard at left upper sternal border. He was admitted due to chest pain with a significant troponin elevation to 418 pg/mL, which down trended to 365 pg/mL at the time of discharge after 24 hours of observation. N-terminal pro-B-type natriuretic peptide level was low at 14.8 pg/mL. Echocardiogram showed mild to moderate pulmonary valve stenosis (peak gradient 35-40 mm Hg), dynamic subvalvar (infundibular) stenosis, and moderate RV hypertrophy (Figure 1).

Figure 1.

Echocardiographic image in the parasternal short axis view, demonstrating color Doppler flow acceleration beginning in the subvalvular region and extending into the main pulmonary artery across the pulmonary valve. PA, pulmonary artery; RVOT, right ventricular outflow tract.

Cardiac magnetic resonance imaging (MRI) was scheduled for further workup. It showed asymmetric thickening of the interventricular septum due to prominent RV trabeculations and infundibular thickening that caused a dynamic RVOT obstruction, 3.2 m/s at rest and 5 m/s with stress (Figure 2). It also revealed a long and deep myocardial bridge of the proximal and mid left anterior descending artery (LAD), which was further characterized by a coronary computed tomography angiogram for a length of 50 mm and depth of 8.5 mm (Figure 3).

Figure 2.

Cardiac magnetic resonance imaging (MRI) in diastole (A) and systole (B) showing asymmetric thickening of the interventricular septum leading to dynamic right ventricular outflow tract (RVOT) obstruction. PA, pulmonary artery; RV, right ventricle.

Figure 3.

Coronary computed tomography (CT) angiogram revealing a long and deep myocardial bridge of the proximal and mid left anterior descending artery (LAD) for a length of 50 mm and depth of 8.5 mm.

Genetic testing revealed pathogenic MYH7 mutation. A preoperative stress test was positive for chest pain; EKG changes were uninterpretable due to baseline abnormalities indicating ischemia. He underwent coronary angiography with intravascular ultrasound and instantaneous wave-free ratio (iFR) testing that revealed flow restriction at rest (0.82), which increased with dobutamine administration (0.72). There was a significant myocardial bridge of the proximal to mid-LAD with obliteration of the lumen in systole, which was the likely cause of his ischemic symptoms (Figure 4).

Figure 4.

Coronary catherization showing myocardial bridge of the left anterior descending artery (LAD). A, An image in diastole. (B) Obliteration of the lumen of the myocardial bridge in systole.

Despite activity restriction and medical management with verapamil and nebivolol, he had a progression of exertional symptoms. He then underwent surgical repair with unroofing of the myocardial bridge and resection of the infundibular muscle bundle that was causing the RVOT obstruction. After the surgery, he had an echocardiogram that showed stable mild pulmonary valve stenosis and normal biventricular function. He had no chest pain on postoperative stress test and no recurrence of symptoms six months after surgery, confirmed on stress echocardiogram without right- or left ventricular outflow obstruction. He has a repeat cardiac MRI scheduled.

Discussion

Recognition of RV HCM phenotype is often subtle on echocardiography and is underdiagnosed. ⁴ Genetic testing for HCM can be helpful in cases like this where the diagnosis is unclear. This patient had an MYH7 gene mutation (beta-myosin gene), one of the most common genes implicated in HCM, which helped confirm the diagnosis of HCM in this atypical case. ⁵ Another unique aspect of this patient's presentation was the myocardial bridge. In this case, it was challenging to ascertain how much of the patient's ischemic symptoms were being driven by the myocardial bridge versus dynamic RVOT obstruction. Myocardial bridging is defined by dynamic extravascular compression which makes functional assessment challenging. The use of iFR with dobutamine administration provides a more reliable functional assessment of myocardial bridging, as compared to conventional fractional flow reserve. ⁶ With dobutamine provocation, the patient's iFR was 0.72, which was below the cutoff value (<0.76) for identifying myocardial bridging associated with stress-induced myocardial ischemia. ⁶

The decision to perform coronary unroofing was based on guideline recommendation for myotomy for patients with symptomatic long/deep myocardial bridge who fail medical therapy as well as the catheterization results, which demonstrated ischemia at rest and with dobutamine administration. ⁷ RV involvement in HCM is associated with worse prognosis and thus more aggressive intervention with surgical myectomy is recommended. ⁸ Thus, our patient underwent simultaneous myotomy and resection of the infundibulum.

In conclusion, isolated RVOT obstruction is a rare phenotype of HCM. Myocardial bridge should be included in the differential for chest pain in an adolescent patient, especially in those with structural/congenital heart disease. The combination of both myocardial bridge and dynamic RVOT obstruction in HCM can result in early and more severe clinical presentation, and treatment of both conditions may be required to improve symptoms.