Causes
The optic nerve comprises axons that emerge from the retina of the eye and carry visual information to the primary visual nuclei, most of which is relayed to the occipital cortex of the brain to be processed into vision. Inflammation of the optic nerve causes loss of vision usually because of 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 most common etiology is multiple sclerosis. Up to 50% of patients with MS will develop an episode of optic neuritis, and 20-30% 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 of optic neuritis include infection (e.g. syphilis, Lyme disease, herpes zoster), autoimmune disorders (e.g. lupus), inflammatory bowel disease, drug induced (e.g. chloramphenicol, ethambutol) vasculitis, and diabetes
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