The symptoms of mitochondrial myopathies include muscle weakness or exercise intolerance, heart failure or rhythm disturbances, dementia, movement disorders, stroke-like episodes, deafness, blindness, droopy eyelids, limited mobility of the eyes, vomiting, and seizures. The prognosis for these disorders ranges in severity from progressive weakness to death. Most mitochondrial myopathies occur before the age of 20, and often begin with exercise intolerance or muscle weakness. During physical activity, muscles may become easily fatigued or weak. Muscle cramping is rare, but may occur. Nausea, headache, and breathlessness are also associated with these disorders.