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 people.
• Visual field loss. Side vision loss can occur in any pattern, such as central vision loss or peripheral vision loss.
• 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.

Eye conditions can be serious. Some can lead to permanent vision loss, and some are associated with other serious medical problems. Contact your doctor if:

• You develop new symptoms, such as eye pain or a change in your vision.
• Your symptoms worsen or don't improve with treatment.
• You have unusual symptoms, including vision loss in both eyes, double vision, and numbness or weakness in one or more limbs, which can indicate a neurological disorder.

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, 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. In people with optic neuritis, the risk of developing multiple sclerosis after one episode of optic neuritis is about 50% 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 affects 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. Still, neuromyelitis optica is more severe than MS, often resulting in a diminished recovery after an attack compared with MS.
• Myelin oligodendrocyte glycoprotein (MOG) antibody disorder. This condition can cause inflammation to the optic nerve, spinal cord or brain. Similar to MS and neuromyelitis optica, recurrent attacks of inflammation can occur. Recovery from MOG attacks is usually better than recovery from neuromyelitis optica.

When symptoms of optic neuritis are more complex, other associated causes need to be considered, including:

• 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, Behcet's disease and lupus can cause recurrent optic neuritis.
• Drugs and toxins. Some drugs and toxins have been associated with the development of optic neuritis. Ethambutol, used to treat tuberculosis, and methanol, a common ingredient in antifreeze, paints and solvents, are associated with optic neuritis.

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. Optic neuritis occurs more often in white people.
• 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 permanent 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.
• 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.

You're likely to see an ophthalmologist for a diagnosis, which is generally based on your medical history and an exam. The ophthalmologist likely will perform the following eye tests:

• A routine eye exam. Your eye doctor will check your vision and your ability to perceive colors and measure your side (peripheral) vision.
• Ophthalmoscopy. During this examination, your doctor shines a bright light into your eye and examines the structures at the back of your eye. This eye test evaluates the optic disk, where the optic nerve enters the retina in your eye. The optic disk becomes swollen in about one-third of people with optic neuritis.
• Pupillary light reaction test. Your doctor may move a flashlight in front of your eyes to see how your pupils respond when they're exposed to bright light. If you have optic neuritis, your pupils won't constrict as much as pupils in healthy eyes would when exposed to light.

Other tests to diagnose optic neuritis might include:

• Magnetic resonance imaging (MRI). An MRI scan uses a magnetic field and pulses of radio wave energy to make pictures of your body. During an MRI to check for optic neuritis, you might receive an injection of a contrast solution to make the optic nerve and other parts of your brain more visible on the images.

  An MRI is important to determine whether there are damaged areas (lesions) in your brain. Such lesions indicate a high risk of developing multiple sclerosis. An MRI can also rule out other causes of visual loss, such as a tumor.

• Blood tests. A blood test is available to check for infections or specific antibodies. Neuromyelitis optica is linked to an antibody that causes severe optic neuritis. People with severe optic neuritis may undergo this test to determine whether they're likely to develop neuromyelitis optica. For atypical cases of optic neuritis, blood may also be tested for MOG antibodies.
• Optical coherence tomography (OCT). This test measures the thickness of the eye's retinal nerve fiber layer, which is often thinner from optic neuritis.
• Visual field test. This test measures the peripheral vision of each eye to determine if there is any vision loss. Optic neuritis can cause any pattern of visual field loss.
• Visual evoked response. During this test, you sit before a screen on which an alternating checkerboard pattern is displayed. Attached to your head are wires with small patches to record your brain's responses to what you see on the screen. This type of test tells your doctor if the electrical signals to your brain are slower than normal as a result of optic nerve damage.

Your doctor is likely to ask you to return for follow-up exams two to four weeks after your symptoms begin to confirm the diagnosis of optic neuritis.

Optic neuritis usually improves on its own. In some cases, steroid medications are used to reduce inflammation in the optic nerve. Possible side effects from steroid treatment include weight gain, mood changes, facial flushing, stomach upset and insomnia.

Steroid treatment is usually given by vein (intravenously). Intravenous steroid therapy quickens vision recovery, but it doesn't appear to affect the amount of vision you'll recover for typical optic neuritis.

When steroid therapy fails and severe vision loss persists, a treatment called plasma exchange therapy might help some people recover their vision. Studies haven't yet confirmed that plasma exchange therapy is effective for optic neuritis.

Preventing multiple sclerosis (MS)

If you have optic neuritis, and you have two or more brain lesions evident on MRI scans, you might benefit from multiple sclerosis medications, such as interferon beta-1a or interferon beta-1b, that may delay or help prevent MS. These injectable medications are used for people at high risk of developing MS. Possible side effects include depression, injection site irritation and flu-like symptoms.

Prognosis

Most people regain close to normal vision within six months after an optic neuritis episode.

People whose optic neuritis returns have a greater risk of developing MS, neuromyelitis optica or MOG antibody associated disorder. Optic neuritis can recur in people without underlying conditions, and those people generally have a better long-term prognosis for their vision than do people with MS or neuromyelitis optica.

If you have signs and symptoms of optic neuritis, you'll likely see your family doctor or a doctor who specializes in diagnosing and treating eye diseases (ophthalmologist or neuro-ophthalmologist).

Here's some information to help you get ready for your appointment.

What you can do

Make a list of:

• Your symptoms, especially vision changes
• Key personal information, including any recent stressors, major life changes, and family and personal medical history, including recent infections and other conditions you have
• All medications, vitamins and other supplements you take, including doses
• Questions to ask your doctor

Take a family member or friend along, if possible, to help you remember the information you're given.

For optic neuritis, questions to ask your doctor include:

• What is likely causing my symptoms?
• Are there other possible causes?
• What tests do I need?
• What treatments do you recommend?
• What are the possible side effects of the medications you're recommending?
• How long will it take for my eyesight to improve?
• Does this put me at greater risk of developing multiple sclerosis, and, if so, what can I do to prevent it?
• I have other health conditions. How can I best manage these conditions together?
• Do you have brochures or other printed material I can have? What websites do you recommend?

What to expect from your doctor

Your doctor is likely to ask you a number of questions, such as:

• How would you describe your symptoms?
• How significantly has your vision decreased?
• Do colors look less vivid?
• Have your symptoms changed over time?
• Does anything seem to improve or worsen your symptoms?
• Have you noticed problems with movement and coordination or numbness or weakness in your arms and legs?