Understanding a Stroke of the Optic Nerve
(Anterior Ischemic Optic Neuropathy)
Laura K. Windsor, O.D.
Richard L. Windsor, O.D.
AION: Anterior Ischemic Optic Neuropathy
Anterior Ischemic Optic Neuropathy (AION) is a potentially visually devastating disease that occurs in the middle aged and the elderly. This condition is often referred to as a stroke of the optic nerve, and it usually begins suddenly with little warning in one eye, but frequently progresses to the other eye over time. Vision loss often includes both the loss of visual field and visual acuity which can vary from being nearly normal to severely impaired. The unexpected sudden visual acuity and visual field loss makes AION a particularly overwhelming disease for many patients.
The optic nerve contains approximately 1.2 million nerve fibers. All of the visual images we see are transmitted through this nerve to our brain. A stroke of the optic nerve is caused by ischemia or poor circulation to the blood vessels that supply the front or anterior portion of the optic nerve. Without nutrients and oxygen being supplied in adequate amounts, a portion of the nerve tissue will be lost. A stroke of the optic nerve is not related to a stroke occurring in the brain; therefore it is not accompanied by any weakness or paralysis.
The “Arteritic” Form of AION
There are two different mechanisms responsible for causing the vision loss from AION. The first and most dangerous form is the “arteritic” form caused by a disease called Giant Cell Arteritis (GCA). This disease affects those over the age of 55 and is three times more common in women than men. About 80% of GCA patients report feeling ill for some time prior to the episode of vision loss. Common signs and symptoms may include general fatigue, weight loss, fever, temple pain, neck pain, pain on chewing, scalp tenderness when combing their hair, headache, anemia and achy joints. Unfortunately, there is also a variant of GCA that produces none of the above mentioned symptoms, and the patients generally are in good health. A very important visual symptom in an arteritic AION is a transient blurring or loss of vision that precedes the permanent vision loss.
GCA is a dangerous disease that can be fatal. When AION is diagnosed, immediate blood work is performed to look for signs indicating Giant Cell Arteritis and biopsies of the arteries of the temple region of the head must be performed to confirm the diagnosis of GCA. Fluorescein angiography, where a dye is injected and a series of rapid photographs are taken, may help confirm the blockage of blood flow in an arteritic AION.
The “Non-Arteritic” Form of AION
The second form is the “Non-Arteritic” Anterior Ischemic Optic Neuropathy (NAAION). It is the most common form of AION and generally has a better visual outcome than the arteritic form. Studies have indicated that NAAION can affect a patient at any age with about 10% of patients being under the age of 45. Both males and females are affected equally. NAAION is caused by an acute impairment to the circulation of the arteries supplying the optic nerve. This transient poor circulation occurs due to a temporary fall in blood pressure to the point that there is not enough blood pressure to fully perfuse the vessels supplying the optic nerve. There are many mechanisms for this fall in blood pressure, and it can be caused by a variety of different factors.
Some of the common diseases which can put a patient at higher risk for NAAION are Diabetes mellitus, Rheumatoid Arthritis, Herpes Zoster, Anemia, Sickle Cell Trait, Syphilis, Behcet’s, Antiphospholipid Antibody Syndrome, and Polyarteritis nodosa. Other systemic conditions that may increase a patient’s risk for developing a NAAION are systemic low blood pressure, gastro-intestinal ulcers, severe high blood pressure, atherosclerosis, arteriosclerosis, internal carotid artery disease, cardiac valvular disease, vasculitis, collagen vascular diseases, defective cardiovascular autoregulation, migraine and other vasospastic diseases, massive or recurrent hemorrhages, type A personality, and sleep apnea.
It is believed that a drop in blood pressure at night while we sleep is an important precipitating factor for the development of a NAAION. Seventy five percent of patients with NAAION report the vision loss upon waking in the morning leading researchers to believe that some ischemia must be occurring during sleep. This is not believed to be the only cause, but when accompanied by one or more of the systemic risk factors listed above, it plays an important role in the development and/or the progression of the vision loss. It is theorized that new highly potent anti-hypertensive drugs may reduce blood pressure too low at night and thus increase the risk of AION.
Medical Treatments
Currently, there are limited treatments that can be offered to directly treat the optic nerve. Many doctors place the patient on drugs that lower the intraocular pressure, and that may provide some neuro-protection to the nerve. Lowering intraocular pressure may improve the blood flow to and through the optic nerve. After a unilateral episode of AION, a baby aspirin may reduce the risk of experiencing an AION in the other eye. Smoking should be discontinued, and while high blood pressure is a risk factor, the sudden lowering of blood pressure should be avoided. Do not, however, make sudden changes in your medications without first consulting your physician.
Diagnosis and treatment of the systemic conditions that cause AION may lessen the risk for vision loss. In patients with Giant Cell Arteritis, early detection and the treatment of the GCA with steroids may not only save the patient’s remaining vision, but may save their life.
Vision Loss
Visual acuity after an AION may range from 20/20 to no light perception. Vision may worsen slightly within the first few weeks. The arteritic form of AION (GCA) usually causes a much worse or higher degree of vision loss than the non-arteritic form (NAAION). Visual acuity is directly related to the location and amount of the optic nerve affected by the stroke. Some patients may have a profound loss in one eye while the other eye still has 20/25 vision. However, loss of side vision often complicates the loss of vision.
Visual Field Loss
Visual field testing is essential in evaluating patients with AION. Visual field loss is variable and depends on the extent of the poor circulation and the area of the optic nerve involved. Patients experience loss of peripheral vision, usually first in the lower field of view. In severe cases, tunnel vision may result. These field defects range in their severity and can be either relative, where one can still perceive motion, or could be absolute, where no motion or light can be seen in the area of the loss. Visual field defects may cause the patient to be unsure in travel, causing one to trip or bump into objects and have difficulty walking down stairs.
Loss of Contrast Sensitivity
Contrast sensitivity is the ability of the eye to detect subtle changes in shades. Our world is not all black and white, but rather it is one of infinite shades. We depend on our ability to see contrast changes to read, and to even judge depth. After an AION, patients often can no longer decipher the subtle changes in vision. The use of increased task lighting and amber filters can aid contrast loss.
Prognosis
Recent studies have suggested that 40% of patients may experience some degree of improvement in central vision or visual acuity weeks to months after the vision or visual field loss. In some, the loss of side vision may not improve, but patients learn to adapt to the field loss and become less aware of the loss over time.
However, there may be improvement in some; about 20% to 25% of the patients with an AION in one eye will develop an AION in the other eye within three years. Only about 5% of patients will have further episodes in one eye, and a small number of patients may have their vision worsen over time. Further vision loss may be associated with sudden lowering of blood pressure or smoking.
Light and Glare
AION patients may experience problems with light and glare which is often related to the loss of contrast. Because of the loss of nerve function, amber sun filters are usually more effective in controlling glare without dimming the vision. Grey sun lenses may cause an additional dimming or decrease in vision in some AION patients. Inside, the use of light amber and yellow filters have been helpful to many patients.
The Emotional Toll
The sudden and unexpected visual acuity and visual field loss, combined with the impact on daily activities, including working and driving often create great emotional stress on the patients and their families. Unlike many other eye diseases, this loss is unexpected, sudden and can occur at a much earlier age than diseases like macular degeneration. It often strikes people in their fifties who are still employed.
Low Vision Rehabilitation
Low vision rehabilitation centers around four main areas when dealing with AION patients. The low vision specialist should address magnification, improvement in contrast, control of glare, and management of the visual field loss. Patients with mild vision loss in one eye may not require low vision aids other than appropriate filters and. When both eyes are impaired, rehabilitation can include special reading eyewear or magnifiers and amber sun filters.
In cases with severe vision loss, closed circuit television magnification systems are very helpful, because they provide both magnification and increased contrast.
The Jordy system has been very helpful for our patients with profound vision loss. It can be used in a stand when connected to a television monitor as a video magnifier or head worn as a portable system with a battery pack to view objects at near, far and intermediate distances.
Visual field loss can sometimes be aided with the use of visual field awareness systems. Such systems include the Gottlieb Visual Fields Awareness System, the new Peli Lens and custom press-on prism systems.
Light amber filters for inside wear and darker amber filters for outside wear are often helpful in improving contrast for AION patients no matter the level of vision loss. Grey filters should be avoided, because they are often too dark for many situations.
Driving Issues
Patients with severe vision loss in only one eye may be able to continue to drive. In most cases of a bilateral AION, the combination of visual acuity loss, visual field impairment and loss of contrast sensitivity prevents these patients from returning to driving.
Websites
International Foundation for Optic Nerve Disease
University of Iowa Department of Ophthalmology