Giant panda sign: an adult case of single large-scale mitochondrial DNA deletion syndrome.

A 38-year-old man presented with long-standing bilateral ptosis and new-onset paroxysmal limb weakness, which improved transiently after steroid treatment. He developed lethargy after self-discontinuation of medication. Brain MRI revealed the classic giant panda sign in the midbrain. Elevated plasma lactate and lactate peak on MRS were detected. Genetic testing identified a 7424 bp large-scale mitochondrial DNA deletion, confirming the diagnosis of single large-scale mitochondrial DNA deletion syndrome. Although the giant panda sign is classically associated with Wilson's disease, this case highlights that this characteristic imaging finding warrants mitochondrial genetic screening in adults with ophthalmoplegia and hyperlactatemia.