Ultrastructure of the small vessels in the myocardium in a patient with fatal systemic capillary leak syndrome.

A 29-year-old Japanese woman was admitted to our hospital with fever, cardiogenic shock, and cardiac arrest and died 18 h after admission. The patient was diagnosed with systemic capillary leak syndrome associated with coronavirus disease 2019. Electron microscopy of the biopsied right-ventricular myocardium revealed extensive interstitial leakage of blood cells and plasma, damaged capillaries, and reticular vessel drainage into the Thebesian vein. These findings indicate that severe capillary leak and lumen occlusion due to damaged capillaries are the main features of systemic capillary leak syndrome.