BACKGROUND: Leber's hereditary optic neuropathy (LHON) is a maternally inherited 
optic neuropathy caused by mutations in mitochondrial DNA (mtDNA). It is also 
believed that several epigenetic factors have an influence on the development of 
LHON.
METHODS: A case series was observed.
RESULTS: Three patients who developed bilateral optic neuropathy are presented. 
All patients had a primary LHON mutation in their mtDNA, but also a subnormal 
vitamin B12 serum level at the time of presentation.
CONCLUSIONS: The clinical picture of optic neuropathy associated with vitamin 
B12 deficiency shows similarity to that of LHON. Both involve the nerve fibres 
of the papillomacular bundle. The present case reports suggest that optic 
neuropathy in patients carrying a primary LHON mtDNA mutation may be 
precipitated by vitamin B12 deficiency. Therefore, known carriers should take 
care to have an adequate dietary intake of vitamin B12 and malabsorption 
syndromes like those occurring in familial pernicious anaemia or after gastric 
surgery should be excluded.