A novel case report of isolated cardiac myxedematosus.

Background: Cardiac mucinous deposits are a rare entity only previously described in the setting of scleromyxedema, a disorder characterized by cutaneous and systemic mucin deposits, fibroblastic proliferation, and monoclonal gammopathies. Case summary: A 41-year-old woman was transferred to our hospital after a month-long hospitalization with worsening cardiogenic shock requiring ionotropic support. Cardiac magnetic resonance imaging revealed a left ventricular ejection fraction of 23%, prior right coronary artery infarct, full-thickness late gadolinium enhancement in the left ventricle basilar wall, global abnormal parametric mapping parameters of both native T1, T2, and extracellular volume, and severe biventricular dysfunction concerning for infiltrative cardiomyopathy. Endomyocardial biopsy demonstrated heavy deposits of interstitial mucin, confirmed by electron microscopy; a Congo red stain was negative for amyloid. She was treated with an aggressive decongestive strategy, oral guideline-directed medical therapy, and intravenous immunoglobulin (IVIg); she was discharged home off inotropic support. Subsequently, she had three additional hospitalizations for heart failure exacerbation in a span of 6 months, and her overall prognosis remains guarded. Discussion: We report a first known case of isolated cardiac myxedematosus associated with a severe systolic and diastolic cardiomyopathy. Our patient did not have any clinical evidence of systemic scleromyxedema or paraproteinemia, both of which have been reported in association with cardiac mucin deposits. Mucinosus has been described in patients with systemic lupus erythematous; however, cardiac deposits have not been reported. While IVIg has been used as a treatment in previously reported cases of cardiac scleromyxedema, its clinical benefit remains unclear in isolated cardiac myxedematosus.