The exact cause of optic neuritis is unknown. However, optic neuritis is believed to develop when the immune system mistakenly targets the substance (myelin) covering your optic nerve, resulting in inflammation and damage to the myelin. Normally, the myelin helps electrical impulses travel quickly along the optic nerve, from the eye to the brain. In the brain, those electrical impulses are converted into visual information. Optic neuritis disrupts this process, affecting vision. It's not certain what causes your immune system to target the myelin.
The following autoimmune conditions are often 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. The presence of brain lesions on MRI images is also associated with an increased risk. People with optic neuritis and abnormal MRI scans are much more likely to develop multiple sclerosis compared with people with normal MRI scans.
- Neuromyelitis optica. Another autoimmune condition that may cause optic neuritis is neuromyelitis optica. In this condition, inflammation occurs 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. Optic neuritis arising from neuromyelitis optica tends to be more severe than optic neuritis associated with multiple sclerosis.
Other autoimmune conditions, such as sarcoidosis and systemic lupus erythematosus, have also been associated with optic neuritis.
Not all potential causes of optic neuritis are autoimmune diseases. 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. In addition, some infections may lead to a type of optic neuritis called neuroretinitis, which isn't believed to be associated with the risk of developing multiple sclerosis.
- Cranial arteritis. This is an inflammation of the lining of the arteries in your head. Inflamed cranial arteries can block blood flow to your eyes and brain, which may cause permanent vision loss or a stroke. Cranial arteritis is most likely to occur in adults ages 70 to 80.
- Drugs. Some drugs have been associated with the development of optic neuritis. One of these drugs is ethambutol (Myambutol), which is used to treat tuberculosis.
- Radiation therapy. Radiation to your head is an uncommon cause of optic neuritis.
In addition, any process resulting in inflammation or compression of the optic nerve, including tumors, nutritional deficiencies or toxins, can interfere with the nerve's ability to conduct electrical impulses. This may cause vision loss and other symptoms that may mimic optic neuritis.