Explain Keshan disease caused by Selenium deficiency?

It is a cardiomyopathy (disease of the myocardium, involving heart muscle) that was identified to affect children and women of child bearing age in China. Sudden onset of insufficient heart function is characteristic of the acute form of this disease while in chronic Keshan disease, heart enlargement and insufficiency exist. Intervention trials comprising more than a million subjects in China has demonstrated the protective effect of selenium against Keshan's disease. It is important to note that selenium supplements cannot however reverse cardiac failure if it has occurred.