Title

Deceptive Diagnosis: Subaortic Stenosis Masquerading as Hypertrophic Cardiomyopathy

Authors

Gina DiMattia, DO; Ariana Ramirez, MD; Rosco Gore, MD

Introduction

Hypertrophic cardiomyopathy (HCM) is a genetic disorder caused by mutations in sarcomere genes that is characterized by left ventricular hypertrophy (LVH) which can progress to left ventricular outflow tract (LVOT) obstruction. Subaortic valvular membrane, a congenital anomaly, has been identified as a rare cause of LVH and LVOT obstruction in adults. We present a case of LVOT obstruction and aortic regurgitation secondary to subaortic valvular membrane mimicking HCM.

Case Presentation

A 65-year-old female presented to the cardiology clinic for routine follow up for abnormal findings on an echocardiogram. She was asymptomatic and exercising regularly. Her cardiac history included asymmetric septal hypertrophy (septum 1.9cm, posterior wall 0.7cm), mild aortic regurgitation, and paroxysmal supraventricular tachycardia treated with metoprolol succinate. Her baseline echocardiogram demonstrated a LVOT peak velocity of 2.8m/sec and possible systolic anterior motion of the mitral valve concerning for HCM. She underwent cardiac magnetic resonance imaging (CMR) to evaluate her presumptive HCM which did not demonstrate late gadolinium enhancement (LGE) or systolic anterior motion of the mitral valve (SAM), however a possible subaortic membrane causing turbulent flow was noted. She was referred for transesophageal echocardiogram (TEE) to evaluate for the presence of a subaortic membrane. The TEE identified a subaortic membrane with significant flow acceleration across the LVOT with a subsequent echocardiogram demonstrating a decreased left ventricular cavity with systolic anterior motion of the mitral valve, but with a flow pattern consistent with fixed obstruction. She was also noted to have severe sub-valvular aortic stenosis with a Vmax of 4.9 meters per second and a mean gradient of 52 millimeters of mercury (mmHg). Due to her significantly thickened septum in an asymmetric pattern, she was referred to genetic counseling. Genetic lab markers returned negative for heritable HCM. An exercise treadmill test was performed to confirm her asymptomatic status. The patient exercised on a full Bruce protocol for 3:20 stopping due to a 23mmHg decline in her systolic pressure. She was diagnosed with severe sub-valvular aortic stenosis with high-risk features. She was referred to cardiothoracic surgery and underwent surgical membrane resection. Post-op TEE showed a small, residual membrane with a peak LVOT velocity of 2.3m/sec. At clinical follow-up, she has remained asymptomatic.

Discussion

This case demonstrates a difficult diagnosis of sub-valvular aortic stenosis in an adult patient with LVH and LVOT obstruction. Subaortic membranes are most often identified in children and adolescents and rarely progress to adulthood without prior diagnosis. Sub-valvular aortic stenosis frequently causes LVH from longstanding LVOT obstruction, and as such can be misdiagnosed as HCM. Furthermore, aortic regurgitation is a known consequence of subaortic valvular membranes and persists even after surgical removal. This case highlights the importance of comprehensive multimodality imaging, including CMR and TEE, in the evaluation of LVOT obstruction.

References

1. Barekatain A, Fanari Z, Hammami S, Qureshi W. Subvalvular Aortic Stenosis. Del Med J. 2015 Nov;87(11):346-8. PMID: 26731888. 2. Farhan MUN, Akhtar H, Al Sudani H. Subaortic Membrane Co-existing With Hypertrophic Cardiomyopathy: A Clinically Challenging Diagnosis. Cureus. 2022 Sep 25;14(9):e29580. doi: 10.7759/cureus.29580. PMID: 36312612; PMCID: PMC9595250.