Leigh's disease: A rare, inherited disorder characterized by degeneration of the central nervous system. Leigh's disease can be caused by mutations in mitochondrial DNA or by deficiencies of an enzyme called pyruvate dehydrogenase. Symptoms usually begin between the ages of 3 months and 2 years and progress rapidly. Early symptoms may include poor sucking ability and loss of head control and motor skills, loss of appetite, vomiting, irritability, continuous crying, and seizures. As the symptoms progress, weakness, lack of muscle tone, and episodes of lactic acidosis, which can lead to impairment of respiratory and kidney function, may occur. The prognosis is poor.