This is a photomicrograph of an endomyocardial biopsy showing abnormal cardiomyocytes.
Storage disease / Cardiotoxicity
Diffuse myocyte vacuolization necessitates a differential diagnosis of a storage disease, underlying mitochondrial disease, and cardiotoxicity. Electron microscopy is a useful tool in this differential.
Storage disease (Fabry) vs cardiotoxicity (hydroxychloroquine).
Two primary considerations for myocyte vacuolization are Fabry disease and hydroxychloroquine cardiotoxicity. They have a similar appearance on light microscopy with diffuse, irregular sarcolemmic vacuolization. Upon ultrastructural examination, both have concentric, laminated electron-dense structures called myeloid figures. However, hydroxychloroquine toxicity will have myelinoid figures and curvilinear bodies (right image), the latter of which are absent in Fabry Disease.
Additional diseases can fall into the differential diagnosis, with unique ultrastructural findings. The SCVP has additional resources on interpretation of electron microscopy.