OPTIC NEURITIS

inflammation of the optic (second cranial) nerve, the nerve that carries visual data from the retina of the eye to a relay station in the centre of the brain for transmission to a cortical area at the back of the brain. Most instances of optic neuritis occur as a result of multiple sclerosis, a disease of the brain and spinal cord in which hard patches in the nerve-fibre sheaths interfere with the transmission of impulses. Other causes include inflammation of the brain and spinal cord as a result of such infections as smallpox or measles and poisoning by such substances as wood alcohol or lead. Optic neuritis may be centred in the optic disk, the point of entrance of the nerve and of blood vessels into the eyeball (this type is called papillitis), or it may be in the nerve shaft behind the eyeball (retrobulbar, or axial, neuritis). The effect may be in the nerve fibres or only in the sheath enveloping the nerve, and it may extend to adjacent areas of the retina. The chief effects of the inflammation are a blind spot (a scotoma, usually in the centre of the visual field), pain in the eyeball, and headache on the affected side. The optic nerve usually recovers from the inflammation, but there may be some residual degeneration of the nerve fibres.