MELAS (Mitochondrial Encephalomyopathy with Lactic Acidosis and Stroke-like episodes) are a group of disorders with permanent or reversible stroke-like events. Serum and CSF lactate are usually elevated.
On imaging patients have cortical/subcortical signal change associated with basal ganglia change.Signal change may also be seen in the dorsal brain stem and spinal cord. Lesions may cross vascular boundaries and new lesions may appear in different parts of the brain. MRS shows an elevation in lactate in affected parts of the brain, but this is not specific for MELAS. Elevated lactate in normal parts of the brain on conventional MRI is more suggestive of a mitochondrial disease