MELAS syndrome: Mitochondrial encephalopathy, lactic acidosis, and stroke-like episodes syndrome, a rare form of dementia caused by mutations in the genetic material (DNA) in the mitochondria. Most DNA is in the chromosomes in the cell nucleus, but another important cell structure that carries DNA is the mitochondrion. Much of the DNA in the mitochondrion is used to manufacture proteins that help to produce energy. As a result of the disturbed function of their cells' mitochondria, patients with MELAS syndrome develop brain dysfunction (encephalopathy), with seizures and headaches, as well as muscle disease, with a buildup of lactic acid in the blood (lactic acidosis), temporary local paralysis (stroke-like episodes), and abnormal thinking (dementia). MELAS syndrome is diagnosed via muscle biopsy that shows characteristic ragged red fibers. Brain biopsy shows stroke-like changes. MELAS syndrome can affect people at different times of life, but most patients show symptoms before age 20. Patients are treated according to which areas of the body are affected at a particular time. There is no known cure for MELAS, which is progressive and fatal.