What causes optic neuritis?
The precise cause of optic neuritis is unknown, but most cases are thought to be a type of autoimmune disorder. The immune system is generally used by the body to fight infection by creating a reaction that combats bacteria, viruses, fungi, and other foreign proteins. In autoimmune diseases, this reaction is mistakenly directed against a normal part of the body, creating inflammation and potential damage. In the case of optic neuritis, the optic nerve becomes swollen and its function is impaired. Inflammation and destruction of the protective myelin sheath that coats and insulates the optic nerve, plus direct damage to the nerve axons results in loss of vision, which may be temporary or permanent.
Optic neuritis is frequently an indication of multiple sclerosis (MS), a disease in which the immune system attacks the myelin sheath covering nerve fibers in the brain and spinal cord, resulting in inflammation and nerve damage. Optic neuritis typically resolves initially but often recurs.
In 15%-20% of people who eventually develop multiple sclerosis, optic neuritis is their first symptom. The risk of developing multiple sclerosis following one episode of optic neuritis is approximately 50% within 15 years of any first episode of optic neuritis. Various studies have shown that approximately 50% of patients who have optic neuritis for the first time will have magnetic resonance imaging (MRI) brain and/or spinal cord abnormalities consistent with MS at the time of the initial optic neuritis.
Neuromyelitis optica is another autoimmune condition in which demyelination occurs both in the spinal cord and both optic nerves but often sparing the brain. In this infrequent condition, the patient can experience weakness or paralysis in the limbs and/or bladder and bowel dysfunction, as well as bilateral loss of vision.
Certain bacterial infections (including Lyme disease, cat scratch fever, and syphilis, as well as viral infections such as measles, mumps, herpes simplex, and herpes zoster) can cause optic neuritis.
Other diseases (including sarcoidosis and systemic autoimmune disorders such as lupus) can cause optic neuritis.
Some drugs (including quinine, tetracycline, linezolid antibiotics, amiodarone, ethambutol, and isoniazid) have been associated with the development of optic neuritis. The phosphodiesterase type 5 inhibitors, such as Viagra, may cause damage to the optic nerve from inadequate blood supply (ischemia), rather than primary inflammation.
Approximately one-half of initial cases of optic neuritis have an unprovable cause and are described as idiopathic. These cases are presumed to be an inflammatory reaction developing 1 week to 1 month following an upper respiratory viral infection.
There are a variety of conditions that can affect the optic nerve, causing symptoms like optic neuritis. These include various optic neuropathies due to infection, trauma, hereditary conditions, toxic or nutritional problems, excessive use of nicotine or alcohol, compressive lesions of the optic nerves, and vascular diseases, including arteritic optic neuropathy, diabetes, and glaucoma. Treatment of these optic neuropathies is directed at the underlying disease.