Non-arteritic Anterior Ischemic Optic Neuropathy
Sometimes people joke that neuro-ophthalmology is the specialty of determining what things aren't.
So, first of all, we are not discussing Giant Cell Arteritis, which is why the name of this condition begins with "non-arteritic."
Some people say it is like a stroke of the eye. That is a handy shortcut to roughly explain what this disease means, but it is not entirely accurate and I feel like it is important to be clear about what is happening to my patients.
First of all, we are not talking about the brain, it is the optic nerve of the eye, so it is not a stroke. A stroke is a brain disease.
Second of all, the mechanism is different. Most strokes occur from clots. Maybe a clot forms in the heart or in the neck, then it dislodges and goes up to the brain and does damage.
NAION is different because it is due to poor circulation.
This image shows the circulation of the eye:
The optic nerve is the yellow structure that connects into the back of the eye. Notice that it is surrounded by multiple tiny blood vessels. Because there are multiple blood vessels and they are relatively small, it it unlikely that a clot could do damage to the optic nerve. The blood vessels are too small for a clot to get through, and since there are so many, a random and tiny clot would not likely do much damage.
However, the smallness of the blood vessels causes a different problem. If the blood is coming from the carotid artery inside the head and traveling forward along the nerve sometimes there is just not adequate circulation to get the blood all the way down to the end of the nerve and the tip of the nerve gets damaged.
I like to use a plumbing analogy. Imagine a lawn with a sprinkler system:
What would happen if the irrigation line was partially gummed up, and/or there was a drop in the water pressure? Probably the sprinklers would still work and most of the lawn would be OK, but there could be a spot down at the end of the line that doesn't get any water and the grass would die in that patch.
This is like the optic nerve in NAION. Most of the nerve gets blood but the tip of the nerve, the end of the line, might not get enough blood and it gets damaged.
So, in NAION there are two kinds of risk factors. Those risks that cause reduced blood flow, and those risks that cause drop in blood pressure.
Let's start with hardening of the arteries. (Most people know these risk factors.) Over time, high blood pressure, diabetes, cholesterol and tobacco use cause arteriosclerosis.
There is another risk factor unique to the optic nerve. We call it "Disc at Risk." This is a normal anatomic variant that means someone was born with a small optic nerve (disc).
Here are two examples of normal, healthy optic nerves:
The nerve exits through an opening in the back of the eye. Whether it is a small opening or a large opening, there are about 1,000,000 nerve fibers that need to fit through that opening. When it is small the nerve fibers are more crowded together. That crowding is visible by looking at the cup (a dimple in the center of the nerve that looks like a bright spot.) You can imagine if the nerve fibers are crowded together, those blood vessels are, too. So if a patient has that "Disc at Risk" it can contribute to the inadequate circulation.
The other category of risk factors are those things that cause low blood pressure. Some people just have a naturally low pressure. Some patients are on too much medication and their pressure gets low. Some patients have complication of surgery and during anesthesia the blood pressure goes too low. The low blood pressure is the trigger or the moment that the disease occurs.
You will notice that chronic high blood pressure is a risk factor that predisposes to this problem and low blood pressure can be the trigger that causes the event. Blood pressure is tricky. We don't want it high or low, but just right.
I usually recommend my patients avoid any blood pressure lowering medication before bed. Many people have a natural dip in their blood pressure during the night, and if you are asleep you won't feel any symptoms, but when you wake up there could be some damage done.
I should also mention obstructive sleep apnea. This is when people snore and stop breathing during the night. The connection is not for certain. Possibly sleep apnea patients have fluctuations in blood pressure, possibly they have seriously low drops in oxygen, possibly it is just bad for circulation. Whatever the reason, we know that a lot of NAION patients have OSA, too.
So you can see that it is almost never one cause. There is a confluence of risk factors that conspire together to cause the reduced circulation and damage the nerve.
Once the nerve is damaged it swells. Swelling is a natural response to lots of forms of damage. But in this case that swelling could cause additional problems. We already discussed that a crowded nerve has worse circulation. The additional swelling only makes it more crowded, which makes it possible to get even more damage and even more swelling and even worse disease in a vicious cycle. The swelling basically causes the nerve to choke itself!
There is not much we can do about this. Once it happens it has to run its course. Researchers have tried medications, eye drops, ocular injections, even surgery. The nerve is delicate tissue. Anything we try to do to it just makes things worse.
It takes about two months for the swelling to run its course and work itself out. Once the swelling is gone the nerve is probably not going to get any worse. It might improve a bit, but unfortunately, I would only expect just a bit of improvement. The nerve is just not very good at healing itself.
We are all basically symmetric. If a patient has the risk factors for this to happen in one eye. The risk factors are all there for this to happen in the other eye. If no intervention is done there is about a 20% chance that this could happen later in the other eye. That means 1 out of 5.
Having vision loss in one eye is really a big deal. Having vision loss in both eyes is life changing.
This is why it is so important to identify the risk factors and treat all of them.
This is where obstructive sleep apnea is really important. If patients have moderate or severe sleep apnea and receive treatment, their risk of second eye involvement goes way down.
All the other risk factors can be important, too.
So, in summary NAION is nerve damage due to insufficient circulation. Multiple risk factors come together to cause the damage. The disease doesn't have any proven treatment. The main focus is to identify risk factors and treat those to prevent any further disease.