This information is provided by the National Institutes of Health (NIH) Genetic and Rare Diseases Information Center (GARD).
Mitochondrial complex I deficiency is a type of mitochondrial disease. Mitochondria are specialized compartments in cells that create more than 90% of the energy needed by the body. In mitochondrial diseases, the mitochondria don’t work correctly resulting in less energy in the cell, cell injury and cell death. Complex I is the first step in a chain reaction in mitochondria leading to energy production. Signs and symptoms of complex I deficiency vary widely in nature and severity, ranging from lethal neonatal disease to adult-onset neurodegenerative disorders. Features may include macrocephaly (large head) with progressive leukodystrophy, encephalopathy, hypertrophic cardiomyopathy, myopathy, liver disease, Leigh syndrome, Leber hereditary optic neuropathy, and some forms of Parkinson disease. The disease is caused by mutations in any of many genes and the inheritance pattern depends on the responsible gene. Treatment is only helpful in some case but may include metabolic therapies such as riboflavin, thiamine, biotin, co-enzyme Q10, carnitine, and a ketogenic diet (a special high-fat, low-carbohydrate diet). 
For more information, visit GARD.