OPTIC NEURITIS

Optic neuritis is a condition characterized by optic nerve inflammation and often is associated with multiple sclerosis.

The condition occurs due to demyelination of optic nerve fibers that show as lesions. Myelin is a dielectric (electrically insulating) material composed of lipid and protein that forms a sheath around the axon portion of nerves. It is indispensable for the proper functioning of the nervous system. Demyelation is a loss of myelin sheath integrity, which causes interruptions in the circuitry of nerves. In the eye, demyelation causes acute vision loss and pain.

DIAGNOSIS

Your doctor will check for evidence of a relative afferent pupillary defect (also known as a Marcus Gunn Pupil). This is determined by shining a bright light alternately into the left and right eyes. A normal response would be equal constriction of both pupils, no matter at which eye the light is shone. However, when the test is performed in an eye with a relative afferent pupillary defect, light directed in the affected eye will cause only mild constriction of both pupils. However, light shone in the unaffected eye will cause a normal constriction of both pupils. Thus, light shone in the affected eye will produce less pupillary constriction than light shone in the unaffected eye, which is a clinical sign of optic neuritis.

CAUSES OF OPTIC NERVE INFLAMMATION

This condition usually develops due to an autoimmune disorder that may be triggered by a viral infection. In some people, signs and symptoms of optic nerve inflammation may be an indication of multiple sclerosis, a condition resulting in inflammation and damage to nerves in your brain and spinal cord. Other causes include Lyme disease, tuberculosis, syphilis, systemic lupus erythematosus, diabetes, and viral agents such as HIV, hepatitis B virus, herpes virus, and cytomegalovirus. Optic nerve inflammation also can occur as a complication of radiation therapy.

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RISK FACTORS

The following represent major risks factors associated with optic nerve inflammation.

• Age. Optic neuritis typically occurs in people 10-50 years of age.
• Gender. Females are twice as likely to have an episode as males.
• Race. Whites of northern European descent are affected 8 times more frequently than blacks and Asians. Whites of Mediterranean ancestry are at intermediate risk. African blacks and Asians are rarely affected.
• Genetic Mutations. Certain genetic mutations can increase the risk of developing optic nerve inflammation or multiple sclerosis.

SYMPTOMS

This condition presents with a classic triad of symptoms.

• Vision loss. Loss of central vision is the primary symptom reported in more than 90% of patients who have acute optic neuritis. The extent of vision loss associated with optic nerve inflammation varies. Some people experience severe difficulty in visual acuity. Others do not notice any changes in their vision. Patients that have normal visual acuity sometimes complain of a loss of peripheral vision to one side. Although vision loss usually is temporary, it can be permanent in some cases. This particularly is true if inflammation is not adequately controlled or is left untreated and leads to optic nerve damage.
• Loss of color vision (dyschromatopsia). Optic nerve inflammation often affects the perception of colors. You may notice that the colors of objects, particularly red ones, temporarily appear faded or less vivid than normal. Patients may not be aware of the color vision loss until the doctor asks them to close or cover the healthy eye.
• Eye pain. Most patients with optic nerve inflammation experience mild eye pain located above or behind the eyeball. This pain can be intensified by eye movement and usually is accompanied by vision loss. It usually peaks within one week and then goes away within several days.

The initial incidence of optic neuritis affects one eye in 70% of cases and both eyes in 30%. The average age of onset is when a person is in their thirties, although the condition can occur anytime between first and seventh decades of life.

In addition to the triad of symptoms listed above, patients often report a reduced perception of light intensity and Uhthoff's symptom, which is vision loss induced by exercise or increased body temperature.

Vision loss may be subtle or profound. Complete vision loss can be caused by a single demyelated lesion. In some cases, vision may be 20/20 with the only symptoms being blurred vision on exertion or other isolated symptoms. Vision loss most commonly occurs over days, but in rare instances can occur in hours.

Fortunately, vision recovers to normal or near normal in over 90% of patients within six months. No available treatment improves those chances. Reported residual visual abnormalities after an incident include: contrast sensitivity, color vision, visual field loss, and decreased sensitivity to light brightness.

Although direct imaging of the optic nerves by MRI is reserved for atypical cases, an MRI of the brain yields information that can change management and yield information about a patient's future risk for development of multiple sclerosis. An abnormal baseline brain MRI is a strong predictor of multiple sclerosis after diagnosis of optic neuritis in adults.

Complications arising from optic nerve inflammation may include:

• Optic nerve damage. Most people experience some permanent optic nerve damage following an episode of optic neuritis, but it often is not severe enough to be noticeable to patients.
• Decreased visual acuity. Vision loss may persist after optic nerve inflammation has improved. Up to 10 percent of people with a history of optic neuritis have some degree of long-term vision loss.
• Side effects of treatment. Steroid medications used to treat optic nerve inflammation depress the immune system, which can cause your body to become more susceptible to infection. Long-term use of steroids may also cause thinning of your bones (osteoporosis) and can cause elevations in intraocular pressure in susceptible individuals, which itself can cause optic nerve damage.

TREATMENT

Choice of treatment is dependent upon whether the focus is on managing an acute episode or if it is prophylactic and geared toward preventing future attacks.

Treatment of Acute Episodes

For patients with a magnetic resonance imaging (MRI) scan that is positive for multiple sclerosis, acute optic neuritis typically is treated with corticosteroids. Intravenous (IV) therapy with methylprednisolone therapy is recommended. The IV methylprednisolone is infused slowly over six hours and administered daily for three days. This is followed by oral prednisone for 11 days. Every patient must be fully informed of the potential side effects and risks associated with corticosteroids before treatment is administered.

Treatment with oral prednisone is contraindicated in patients with idiopathic optic neuritis (unknown cause).

Prophylaxis Therapy

Long-term prophylaxis therapy is designed to prevent future acute episodes from occurring. Four medications: Interferon beta-1a (Avonex and Rebif), Interferon beta-1b (Betaseron) and Glatiramer acetate (Copaxone) are prescribed to delay the onset of clinical multiple sclerosis in patients with optic neuritis and a positive MRI. Each of these medications effectively reduce the number of relapses by approximately 30%.

These represent general recommendations for treatment. However, each patient brings unique challenges. Based upon their comprehensive examination and diagnosis, your doctor will make treatment decisions that are designed to manage your specific case.

REFERENCES:

Bonhomme GR, Waldman AT, Balcer LJ, Daniels AB, Tennekoon GI, Forman S, Galetta SL, and Liu GT. Pediatric optic neuritis: brain MRI abnormalities and risk of multiple sclerosis. Neurology. 2009;72(10):881-885.

Chan JW. Optic Nerve Disorders: Diagnosis and Management. New York: Springer Science+Business Media, 2007:1-30.

Optic Neuritis Study Group. The clinical profile of optic neuritis. Experience of the Optic Neuritis Treatment Trial. Archives of Ophthalmology. 1991;109:1673–1678.

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