Lethal infantile mitochondrial myopathy is a rare mitochondrial oxidative phosphorylation disorder characterized by progressive generalized hypotonia, progressive external ophthalmoplegia and severe lactic acidosis, which results in early fatality (days to months after birth). Patients may present with lethargy and areflexia and may associate additional features, such as cardiomyopathy, renal dysfunction, liver involvement and seizures.

A myocardial disorder in which the heart muscle is structurally and functionally abnormal, in the absence of coronary artery disease, hypertension, valvular disease and congenital heart disease sufficient to cause the observed myocardial abnormality.