What is optic neuritis?
Optic Neuritis (ON) is an inflammation, with accompanying demyelination of the Optic Nerve (Cranial Nerve II) serving the retina of the eye.
Optic Neuritis is one of the most frequently presenting symptoms of multiple sclerosis, although there are other causes. Indeed, ON is the most common symptom at onset of MS and presents unilaterally (in one eye only) in 70% of cases. The majority of all cases of ON, MS-related or not, are in fact caused by demyelination.
Who is affected?
Optic neuritis can occur in children or adults and may involve either one or both optic nerves. Optic neuritis typically affects young adults ranging from 18-45 years of age, with a mean age of 30-35 years. As with MS, women are about twice as likely as men to present with ON and the prevalence in Caucasians is higher than in other racial groups.
What are symptoms of optic neuritis?
The major symptom of optic neuritis is loss of vision, from a small area of blurring to complete or partial blindness that occurs over the course of a day to two weeks and may worse by heat and exercise. This vision loss is usually not permanent but can be in some cases. Dyschromatopsia (reduced color vision) occurs in 100% of optic neuritis cases. Typically this is reported as a reduction in color vividness, particularly in the reds. If the inflammation is not visible on the back of the retina, then dyschromatopsia is the most sensitive clinical diagnostic measure of optic neuritis, loss of contrasts, and washed out or less vivid vision than normal. Almost everyone who develops optic neuritis experiences eye pain that is worsened by eye movement. Pain associated with optic neuritis usually peaks within one week and then goes away within several days.
Optic neuritis usually affects one eye, although it may occur in both eyes simultaneously. In cases where only one eye is affected, patients may be unaware of subtle visual loss or changes in the color vision until the doctor asks them to close or cover the healthy eye.
A complete medical examination, including chemical analysis of the blood can help rule out related diseases.
An Ishihara color chart and/or Farnsworth-Munsell hue tests are often used to evaluate the degree of dyschromatopsia (reduced color vision). down the optic nerve, are also used as a diagnostic test for optic neuritis.
Visual evoked potential tests, which detect the speed of nerve transmissions
A person with a first episode of optic neuritis usually undergoes an MRI of the brain to look for central nervous system lesions. MRI with gadolinium enhancement may show an enlarged, enhancing optic nerve. MRI may also help diagnose multiple sclerosis by demonstrating typical abnormalities in the brain.
What causes optic neuritis?
Inflammation of the optic nerve causes loss of vision because of the swelling and destruction of the protective myelin sheath that covers and insulates the optic nerve. Direct damage to the nerve tracts (axons) may also play a role in nerve destruction.
Optic neuritis most commonly develops due to an autoimmune disorder that may be triggered by a viral infection. In some people, signs and symptoms of optic neuritis may be an indication of multiple sclerosis, a disease in which the immune system attacks the myelin sheath covering nerve fibers in the brain and spinal cord, resulting in inflammation and damage to nerve cells in the brain and spinal cord. Demyelinating optic neuritis is another term for this eye condition. In optic neuritis resulting from demyelinating disease, particularly multiple sclerosis, there may be recurrences of optic neuritis over time.
In approximately 15%-20% of multiple sclerosis cases, 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 the episode of optic neuritis. On MRI scanning, almost half of the patients with optic neuritis (and no history or clinical evidence of multiple sclerosis) have abnormal brain white matter changes consistent with multiple sclerosis. In patients undergoing MRI scans of the brain at the time of the initial attack of optic neuritis, the finding of brain lesions on MRI images increased the risk of development of later multiple sclerosis threefold versus people with normal MRI scans. However, almost half of patients with any brain lesions on MRI at the time of the initial episode of optic neuritis will not have developed multiple sclerosis 10 years later.
Another autoimmune condition that causes optic neuritis is neuromyelitis optica. In this unusual condition, inflammation occurs in the optic nerve and spinal cord but usually not within the brain as often as in multiple sclerosis. Optic neuritis arising from neuromyelitis optica is more severe than optic neuritis associated with multiple sclerosis.
There are many causes of optic neuritis unassociated with multiple sclerosis, including:
Infections: Bacterial infections, including Lyme disease, tuberculosis, cat scratch fever, toxoplasmosis and syphilis, or viral infections such as HIV (human immunodeficiency virus), hepatitis B, and herpes zoster can cause optic neuritis. Bacterial meningitis, encephalitis, and sinusitis (all especially in children) may cause optic neuritis or optic nerve damage.
Cranial arteritis: This is an inflammation of the lining of the arteries within the skull. 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 60-90 years of age. This condition is also known as temporal arteritis or giant cell arteritis (GCA).
Diabetes: Diabetes is a condition in which the body cannot make or properly use insulin, a hormone that regulates the amount of sugar in the blood and other tissues. People with diabetes are at an increased risk of developing disorders of the optic nerve, including optic neuritis, most likely from decreased blood flow to the optic nerve.
Sarcoidosis, pernicious anemia, Grave’s disease, bee stings, and vaccinations may cause vision loss from optic nerve swelling.
Autoimmune diseases may cause inflammation of blood vessels (vasculitis) nourishing the optic nerve.
Drugs. Some drugs have been associated with the development of optic neuritis. These include ethambutol (Myambutol) and certain antibiotics.
Toxins: Lead, methyl alcohol, quinine, and arsenic may cause vision loss and other symptoms that may mimic optic neuritis.
Radiation therapy to the head is an uncommon cause of optic neuritis.
Leber’s hereditary optic neuropathy, an inherited form of vision loss that affects mostly males in their 20s or 30s, may cause optic neuritis.
It is important to mention that for a lot of cases, the basic cause of optic neuritis is often unknown.
What is the treatment for optic neuritis?
If a definite cause (such as infection or underlying other disease) is determined, appropriate therapy for that cause can be instituted.
Optic neuritis in demyelinating disease has a favorable prognosis without treatment for an individual attack, but over a period of years significant visual loss is the rule since permanent damage results from recurrent attacks.
Ideally, treatment is directed toward the underlying cause. Systemic corticosteroids are helpful in retrobulbar neuritis by shortening the course of the disease. Their value in influencing the final outcome is under investigation.
Treatment with steroid medications (cortisone medications such as prednisone and methylprednisolone, may speed up vision recovery. Although treatment with steroids have little effect on the final visual outcome in patients with optic neuritis, patients treated initially with intravenous (IV) steroids have about one-half the risk of developing multiple sclerosis in two years as untreated patients do. This effect disappears by the third year of follow up. In addition, patients treated with IV steroids have fewer repeated attacks of optic neuritis than untreated patients.
In papillitis with minimal evidence of collagen vascular disease or other signs of multiple sclerosis, pulsed intravenous methylprednisone therapy continued over days to weeks may be indicated as the visual loss is often more extensive.
When optic neuritis is associated with MRI lesions suggestive of multiple sclerosis (MS), immunomodulator or immunosuppressive therapy may be prescribed to reduce the incidence of future attacks.