Definition: Mitochondrial diseases are a group of inherited conditions that affect the ability of the mitochondria to produce enough energy for the body to function properly. The signs and symptoms of these diseases can vary greatly from person to person and so can the severity. Symptoms may include poor growth, loss of muscle coordination, muscle weakness and pain, seizures, vision and/or hearing loss, gastrointestinal issues, and organ failure. The signs and symptoms of mitochondrial diseases may present at birth or later in life.