Elusive Barriers: The Challenges of Diagnosing Subaortic Membranes

Subvalvular aortic stenosis (SAS) is a relatively uncommon cause of left ventricular outflow tract (LVOT) obstruction, constituting only 8-20% of cases. Among the etiologies, subaortic membranes (SAoM) are the most prevalent, manifesting in various anatomical forms, including thin discrete membranes, fibromuscular ridges, and diffuse tunnel-like narrowings. While transthoracic echocardiography (TTE) is the primary diagnostic tool, it often presents challenges, particularly in cases where the membrane is not readily visible, and needs further imaging with transesophageal echocardiogram (TEE) or cardiac magnetic resonance imaging (CMR). This case series explores 2 diagnostically challenging instances of SAoM, highlighting the importance of multimodal imaging and the nuances of interpreting these findings. The first case is of a 19-year-old female with congenital aortic stenosis and ESRD presented with worsening dyspnea; initial TTE, TEE, and CMR failed to identify a subaortic membrane, but intra-procedural 3D TEE revealed an oval-shaped membrane, redirecting management from balloon angioplasty to surgical excision. The second is of a 62-year-old female with prior diagnosis of hypertrophic obstructive cardiomyopathy (HOCM) and progressive dyspnea was found on TEE to have a SAoM, contradicting her prior diagnosis; medical therapy was adjusted, and she was referred for surgery. These cases underscore the diagnostic challenges of SAoM, often evading detection on initial TTE and CMR, necessitating advanced techniques like 3D TEE. Misdiagnosis, as seen with HOCM, can lead to years of inappropriate treatment. In conclusion, accurate and early differentiation through expert interpretation of multimodal imaging, particularly TEE, is crucial for guiding proper management and avoiding unnecessary interventions.