The Curious Case of the Cardiac Stowaway: When a Fibroma Crashed the Hypertrophic Cardiomyopathy Party.

Description of Clinical Presentation: A 43-year-old female with 10-year history of mild asymmetric left ventricular hypertrophy (LVH), left ventricular outflow tract (LVOT) obstruction, mitral regurgitation (MR), and presumed diagnosis of hypertrophic cardiomyopathy (HCM) was referred for outpatient cardiology consultation after previously receiving care elsewhere.

Diagnostic Techniques and Their Most Important Findings: Previous diagnostic imaging included a transthoracic echocardiogram (TTE) with mild basal interventricular septal thickening up to 1.2 cm, a peak inducible gradient with Valsalva maneuver of 31 mmHg, mitral valve (MV) chordal systolic anterior motion (SAM), mild-to-moderate MV regurgitation, and an inability to rule out a subaortic membrane.

Cardiac magnetic resonance (CMR) was ordered as the initial diagnostic test upon re-establishing care. Findings included maximum septal thickening up to 2.2 cm, hyperdynamic LV function (EF 70%), severe anterior mitral leaflet SAM, flow acceleration within the LVOT, and moderate MR (regurgitant volume 24 ml, regurgitant fraction 27%). There was an unexpected finding of a focal, dense, well-circumscribed mass within the maximumly thickened basal septum that was further characterized with late gadolinium enhancement (LGE) and myocardial mapping (T1, T2, and calculated ECV%) sequences.

The LGE pattern appeared atypical for HCM. The native pre-contrast T1 signal in the area of enhancement was 900-950 ms, whereas the calculated ECV% was 70-90%. The native pre-contrast T2 signal was mildly elevated to 55-59 ms in surrounding myocardial segments. Ultimately, a diagnosis of benign cardiac fibroma (rather than HCM) with hemodynamic consequences of severe SAM, LVOT obstruction, and moderate MR was made. She was then referred to our Cardio-Oncologist and surgical colleagues for evaluation and further management.



Learning Points from this Case: Benign cardiac fibroma is a rare HCM phenocopy difficult to recognize using TTE alone, highlighting the importance of CMR in the workup of patients with a clinical diagnosis of HCM.

Atypical LVH patterns and the lack of typical echocardiographic findings of HCM (SAM, LVOT obstruction, papillary muscle anomalies, elongated MV leaflets, myocardial clefts) should raise the level of suspicion for an HCM phenocopy.

The CMR protocol for myocardial hypertrophy should include steady-state free-precession (SSFP) cine sequences using conventional and non-traditional imaging planes, late gadolinium enhancement (LGE) sequences, and parametric mapping consisting of pre- and post-contrast T1 sequences and pre-contrast T2 sequences.

When an unexpected mass is identified, tissue characterization is essential to distinguish benign cardiac fibromas from other cardiac masses. Findings associated with fibromas include lower-than-expected T1 values (800-1000 ms), normal or mildly elevated peripheral T2 values, and exceedingly high ECV% (70-100%).

Dense, well-circumscribed LGE in the area of focal hypertrophy.
Pre-contrast T1 map showing low-normal native T1 values (900-950 ms) in the area of hypertrophy.
Pre-contrast T2 map showing normal native T2 values in the area of hypertrophy with mildly increase signal in the periphery.