Hydroxychloroquine Cardiotoxicity Confirmed by Electron Microscopy: The Myocarditis Mimic

Hydroxychloroquine (HCQ) is widely used for autoimmune disease, but rare cardiotoxicity can mimic inflammatory cardiomyopathy.

A 79-year-old woman on 10 years of HCQ presented with fatigue, dizziness, and exertional dyspnea and was found to have low cardiac output (1.75 L/min) and cardiac index (1.01 L/min/m²) physiology. Cardiac magnetic resonance imaging showed diffuse, patchy midmyocardial and subepicardial late gadolinium enhancement, whereas cardiac positron emission tomography demonstrated diffuse fluorodeoxyglucose uptake sparing the subendocardium, raising concern for myocarditis. Endomyocardial biopsy revealed cardiomyocyte vacuolization, and electron microscopy demonstrated membranous whorls and curvilinear inclusion bodies consistent with HCQ-induced lysosomal dysfunction. HCQ was discontinued with symptomatic improvement at 1 month.

HCQ cardiotoxicity may resemble myocarditis on imaging. Biopsy with electron microscopy confirms the diagnosis and guides management.

Maintain suspicion for HCQ toxicity despite inflammatory imaging patterns. Early tissue diagnosis and drug discontinuation may prevent progression and support recovery.