"Black Toenail" Sign.

A 11-year-old boy with genetically confirmed MELAS (mitochondrial encephalopathy, lactic acidosis, and stroke-like episodes syndrome with mutation at mt.3243A>G) presented with extreme fatigue. Upon presentation, he had elevated lactic acid of 5.2 mmol/L. His magnetic resonance imaging scan of the brain showed progression of multifocal gyral necrosis and cerebral atrophy compared to the images done 3 years prior to presentation(Fig). He was discharged soon after his fatigue resolved in response to supportive therapy.This image finding of gyral necrosis is known as "black toenail" sign, representing the MELAS-related gyral necrosis that is commonly seen in chronic MELAS patients. Although the etiology of gyral necrosis has not been clearly elucidated, it is assumed that the necrosis is associated with multifactorial etiologies including hypoxic ischemic injury, decreased substrate delivery, decreased toxin removal, and superimposed neuronal excitation leading to energy depletion.