This is the commonest of the mitochondrial diseases. Ninety five percent of patients carry one of three point mutations in their mitochondrial DNA. In the majority this is inherited from the mother but sometimes the mutation arises for the first time in the patient. Although both men and women can have the mutation, more men go on to have symptoms.

The disease primarily affects the optic nerve. This is the large nerve that leaves the back of each eye to carry visual information to the brain. Patients usually first notice problems with their vision in their twenties or thirties. The first symptoms are of blurring of central vision and loss of colour vision. Often this starts in just one eye, but in the vast majority the other eye will also be affected within six months. The eyes are not usually painful. Eventually vision may be limited to being able to make out rough shapes or to count fingers only.

No specific treatment exists but there is thought to be an increased risk of patients with LHON developing blindness if they also smoke and drink excess alcohol. The main thrust of treatment is therefore to identify those family members who carry the mutation to advise them to avoid these extra risks. For patients who already have symptoms treatment involves the provision of visual aids.