Optic neuritis usually affects one eye. Symptoms might include:

• Pain. Most people who develop optic neuritis have eye pain that's worsened by eye movement. Sometimes the pain feels like a dull ache behind the eye.
• Vision loss in one eye. Most people have at least some temporary reduction in vision, but the extent of loss varies. Noticeable vision loss usually develops over hours or days and improves over several weeks to months. Vision loss is permanent in some cases.
• Visual field loss. Side vision loss can occur in any pattern.
• Loss of color vision. Optic neuritis often affects color perception. You might notice that colors appear less vivid than normal.
• Flashing lights. Some people with optic neuritis report seeing flashing or flickering lights with eye movements.

The exact cause of optic neuritis is unknown. It's believed to develop when the immune system mistakenly targets the substance covering your optic nerve (myelin), resulting in inflammation and damage to the myelin.

Normally, the myelin helps electrical impulses travel quickly from the eye to the brain, where they're converted into visual information. Optic neuritis disrupts this process, affecting vision.

The following autoimmune conditions often are associated with optic neuritis:

• Multiple sclerosis. Multiple sclerosis is a disease in which your autoimmune system attacks the myelin sheath covering nerve fibers in your brain and spinal cord. In people with optic neuritis, the risk of developing multiple sclerosis following one episode of optic neuritis is about 50 percent over a lifetime.

Your risk of developing multiple sclerosis after optic neuritis increases further if an MRI scan shows lesions on your brain.

• Neuromyelitis optica. In this condition, inflammation recurs in the optic nerve and spinal cord. Neuromyelitis optica has similarities to multiple sclerosis, but neuromyelitis optica doesn't cause damage to the nerves in the brain as often as multiple sclerosis does.

Other factors that have been linked to the development of optic neuritis include:

• Infections. Bacterial infections, including Lyme disease, cat-scratch fever and syphilis, or viruses, such as measles, mumps and herpes, can cause optic neuritis.
• Other diseases. Diseases such as sarcoidosis and lupus can cause recurrent optic neuritis.
• Drugs. Some drugs have been associated with the development of optic neuritis. They include quinine and some antibiotics.

Risk factors for developing optic neuritis include:

• Age. Optic neuritis most often affects adults ages 20 to 40.
• Sex. Women are much more likely to develop optic neuritis than men are.
• Race. In the United States, optic neuritis occurs more frequently in whites than it does in blacks.
• Genetic mutations. Certain genetic mutations might increase your risk of developing optic neuritis or multiple sclerosis.

Complications arising from optic neuritis may include:

• Optic nerve damage. Most people have some permanent optic nerve damage after an episode of optic neuritis, but the damage might not cause symptoms.
• Decreased visual acuity. Most people regain normal or near normal vision within several months, but a partial loss of color discrimination might persist. For some people, vision loss persists after the optic neuritis has improved.
• Side effects of treatment. Steroid medications used to treat optic neuritis subdue your immune system, which causes your body to become more susceptible to infections. Other side effects include mood changes and weight gain