This study tests a new treatment to help with droopy eyelids (ptosis) and eye movement problems (squint) in children and young people with genetically confirmed mitochondrial conditions - using red-light.

We use a torch to shine a red light through the closed eyelid for 3 minutes a day. The study will run for 18 months.

We believe that this treatment could help strengthen the muscle in the eyelid to make it easier to open the eyes, and could also help some children with squint.

Inclusion: Age 3 to less than 18 years Genetically confirmed diagnosis of primary mitochondrial disease (known pathogenic mutation(s) in mitochondrial or nuclear DNA known to cause primary mitochondrial disease) Ptosis

Exclusion: Genetic diagnosis not consistent with primary mitochondrial disease. No genetic diagnosis Unable to cooperate with light treatment or ophthalmological assessments.