While the eyes are responsible for detecting light, the brain is what actually processes information received by the eye and transforms it to what we know as vision.
The Optic Nerve
In order for this information to get from the eye to the brain, it must travel through a nerve called the optic nerve.
You can think of the optic nerve similar to a large cord containing many wires. Each cord is an axon carrying information from a light-detecting cell to the brain.
When the optic nerve is damaged, we lose some of our wires—therefore part of the signal from the eye is no longer reaching the brain, which is then seen as a lack of sight, or visual field defect.
One major cause of damage to the optic nerve is called anterior ischemic optic neuropathy, which occurs when, for whatever reason, not enough blood is reaching the optic nerve. Subsequently, the nerve becomes starved of blood (and thus oxygen and other nutrients) and part of the nerve dies creating a visual field defect.
There are two major categories of anterior ischemic optic neuropathy—AAION and NAION.
AAION—Arteritic Anterior Ischemic Optic Neuropathy
Of the two categories, AAION is the worser. AAION is the result of an autoimmune disease that attacks medium-to-large sized blood vessels called Giant Cell Arteritis or Temporal Arteritis. This disease results in inflammation of the blood vessels.
When blood vessels become inflamed, it limits the amount of blood that can course through them. In some cases, it can even block off the blood supply completely!
When an organ, such as the eye, does not have adequate blood supply, the tissue dies. In AAION, the blood vessels affected are the posterior ciliary arteries.
The posterior ciliary arteries are responsible for the blood supply of the entire optic nerve. Therefore, if these arteries become swollen and inflamed, limiting the blood perfusion to the optic nerve, the optic nerve dies. Whatever axons were in the area of the affected optic nerve also die, and hence vision in that area is permanently lost.
This process results in sudden, painless, normally unilateral (affecting one eye), vision loss.
Other common signs and symptoms associated with AAION, and therefore Giant Cell Arteritis, include pain while combing your hair, headaches (especially in the temporal region), and pain while chewing or eating.
What is important to note here is that while AAION affects one eye initially, the second eye is often affected within 24-72 hours of the initial onset.
If you have sudden, painless vision loss in one eye, it is of upmost importance to get in to see your eye doctor immediately so that proper testing can be conducted in hopes of preventing an AAION from occurring in the other eye.
At your visit, your eye doctor will need to dilate your eyes to take a look at your optic nerve. It is likely at this visit your doctor will want you to perform a visual field test in which stimuli are presented across the visual field and any defects will be detected to quantify your vision loss.
Next, your doctor will need you to get blood work done to check your CRP and ESR levels in addition to a complete blood panel to check for inflammatory markers indicative of Giant Cell Arteritis.
Depending on your blood work, your doctor may refer you for a temporal artery biopsy. This allows doctors to analyze the main artery affected to ensure that you do indeed have Giant Cell Arteritis causing the optic nerve damage, not something else.
The treatment for AAION is a high-dose steroid regimen to decrease the inflammation in the arteries. This will hopefully prevent an AAION from occurring in the non-affected eye.
Unfortunately, even after recovering from an AAION, a patient’s vision typically does not improve much, even with glasses. On the other hand, fortunatelyAAION is rare, it’s brother NAION occurs much more frequently.
NAION—Non-Arteritic Anterior Ischemic Optic Neuropathy
NAION also results from lack of blood flow to the eye, but it is not caused by inflammation. Instead, NAION occurs from narrowed arteries (as in atherosclerosis), a sudden decrease in blood pressure, or other vascular disease as seen in diabetes, hypertension, hypercholesteremia, chronic smokers, sleep apnea patients, those with sickle cell anemia, etc.
In other words, while AAION’s underlying cause is due to inflammation, NAION’s underlying cause is due to a vasculature problem.
The process of NAION is quite similar to AAION. Lack of blood flow to the nerve results in optic nerve death and loss of vision.
The major difference in NAION is that the loss of blood to the optic nerve is often intermittent, or not a complete absence of perfusion. This means that in most cases, some, not all, vision will be restored after the event.
It is still important, however, that you go see your eye doctor immediately if you have sudden painless vision loss, as your doctor will want to rule out AAION.
If you have NAION it is a sign to your doctor that your cardiovascular disease is running a bit unchecked and modification may need to be made to your medications or lifestyle.
In NAION, the most important thing to do is to get the problem causing the decreased blood supply to the nerve under control. Your eye doctor will work with your primary care doctor to ensure this is accomplished efficiently.
There are currently no major treatments for NAION besides treating the underlying disease. In a select number of cases, NAION can occur in the fellow eye, this typically occurs 2-4 weeks after the initial incident.