Optic neuritis is the inflammation of the optic nerve that may cause a complete or partial loss of vision. The optic nerve comprises axons that emerge from the retina of the eye and carry visual information to the occipital cortex of the brain to be processed into vision. Inflammation of the optic nerve causes loss of vision usually due to the swelling and destruction of the myelin sheath covering the optic nerve. Direct axonal damage may also play a role in nerve destruction in many cases. The major causes are viral-bacterial infections (e.g. herpes zoster), autoimmune disorders (e.g. lupus) and the inflammation of vessels (vasculitis) nourishing the optic nerve. Optic neuritis can also emerge as an attendant, first, or sole manifestation of multiple sclerosis.

Major symptoms are sudden loss of vision (partial or complete) and occasionally pain on movement of the eyes. Some patients with optic neuritis may lose their color vision, as well. On medical examination the head of the bulging optic nerve can easily be visualised by an ophthalmoscope. 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.

In most cases, visual functions return to normal within a few weeks, but they may also advance to a complete and permanent state of visual loss. Therefore, systemic intravenous treatment with corticosteroids, which may quicken the healing of the optic nerve, prevent complete loss of vision, and delay the onset of other multiple sclerosis symptoms, is strongly recommended. It has been demonstrated that oral administration of corticosteroids in this situation may result in an even worse outcome than that of the 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, the sheath around the optic nerve may be cut off to decrease the pressure and thus increase the pace of healing.