The most common etiology is multiple sclerosis. Up to 50% of patients with MS will develop an episode of optic neuritis, and 20% of the time optic neuritis is the presenting sign of MS . The presence of demyelinating white matter lesions on brain MRI at the time of presentation of optic neuritis is the strongest predictor for developing clinically definite MS. Almost half of the patients with optic neuritis have white matter lesions consistent with multiple sclerosis. At five years follow-up, the overall risk of developing MS is 30%, with or without MRI lesions. Patients with a normal MRI still develop MS (16%), but at a lower rate compared to those patients with three or more MRI lesions (51%). From the other perspective, however, almost half (44%) of patients with any demyelinating lesions on MRI at presentation will not have developed MS ten years later.
Some other causes include viral-bacterial infections (e.g. herpes zoster), autoimmune disorders (e.g. lupus), chloramphenicol and the inflammation of vessels (vasculitis) nourishing the optic nerve. Ethambutol, an antitubercular drug, can also cause optic neuritis.
On medical examination the head of the optic nerve can easily be visualised by an ophthalmoscope; however frequently there is no abnormal appearance of the nerve head in optic neuritis, though it may be swollen in some patients. In many cases, only one eye is affected and patients may not be aware of the loss of color vision until the doctor asks them to close or cover the healthy eye.
Paradoxically it has been demonstrated that oral administration of corticosteroids in this situation may lead to more recurrent attacks than in non-treated patients (though oral steroids are generally prescribed after the intravenous course, to wean the patient off the medication). This effect of corticosteroids seems to be limited to optic neuritis and has not been observed in other diseases treated with corticosteroids.
Very occasionally, if there is concomitant increased intracranial pressure the sheath around the optic nerve may be cut to decrease the pressure.
When optic neuritis is associated with MRI lesions suggestive of multiple sclerosis (MS) then general immunosuppressive therapy for MS is most often prescribed (IV methylprednisolone may shorten attacks; oral prednisone may increase relapse rate).