Optic neuritis describes any condition that causes inflammation of the optic nerve; it may be associated with demyelinating diseases, or infectious or inflammatory processes.

It is also known as optic papillitis (when the head of the optic nerve is involved), neuroretinitis (when there is a combined involvement of the optic disc and surrounding retina in the macular area) and retrobulbar neuritis (when the posterior part of the nerve is involved). Prelaminar optic neuritis describes involvement of the non-myelinated axons in the retina. It is most often associated with multiple sclerosis, and it may lead to complete or partial loss of vision in one or both eyes. Other causes include:

1. Leber's hereditary optic neuropathy
2. Parainfectious optic neuritis (associated with viral infections such as measles, mumps, chickenpox, whooping cough and glandular fever)
3. Infectious optic neuritis (sinus related or associated with cat-scratch fever, tuberculosis, Lyme disease and cryptococcal meningitis in AIDS patients
4. Autoimmune causes (sarcoidosis, systemic lupus erythematosus, polyarteritis nodosa, MOG antibody disease, granulomatosis with polyangiitis)
5. Diabetes mellitus
6. Low phosphorus levels
7. Hyperkalaemia

Partial, transient vision loss (lasting less than one hour) can be an indication of early onset multiple sclerosis.