What is the Retina?
The Retina is a thin film of tissue that coats the inside of the eye. If you were to stick a sharp stick into the white part of your eye, it would first pass through the conjunctiva, a clear protective layer that turns red if you rub your eye (infections of the conjunctiva are called conjunctivitis, or pink eye), then sclera, the white part, then choroid, a pigmented layer (the iris is part of the choroid), then retina, then finally the vitreous cavity. The retina has blood vessels running through it that provide nutrients, like oxygen, but they only bring in enough goods for the inner two thirds. The outer third of the retina gets its oxygen and nutrients from blood vessels in the choroid. A retinal detachment occurs when the retina separates from the choroid. For example, if a break forms in the retina, fluid from the vitreous goes underneath the retina, and peels it off of the choroid (the layer just beneath the retina). Now, since one third of the retina gets nutrients from the choroid, if you separate the retina away from the choroid...... You guessed it... If you separate the retina from the choroid, then it dies. Yes, two thirds of the retinal layer is still alive, but that ain't enough to see with, and the retina isn't very good at regenerating.
But having a retinal detachment doesn't necessarily mean that you will go completely blind, if only an area of peripheral retina was detached. As you read this print, light rays originating from the words are striking your retina in a highly specialized area called the Macula. Still looking at the screen, you can also see, in your peripheral vision, your mouse, and a bunch of junk cluttering up your desk. The macula has the highest acuity (and the best color vision), but it only covers a very narrow range, and it only works in relatively bright light. Using your peripheral vision, you can't read the writing on your mouse, but using your macula, you can. On the other hand, if you are trying to navigate around the house, your peripheral vision is what keeps you from bashing into door frames and furniture. If you only had your maculas, walking in the the house would be like trying to map out the inside of a dark room using only a narrow beamed flash-light. Another nice feature about the peripheral retina is that it is much more sensitive to light than the macula. In fact, in very dim light, your maculas are hardly working at all. Pilots are trained to use their peripheral vision when flying at night for this reason.
Floaters and Flashes
When people reach middle age, the vitreous gel inside the eye may begin to clump together making small particles that appear to float across the field of vision. These "floaters" are more apparent when staring at light colored backgrounds such as a blue sky or white wall. Posterior vitreous detachment can cause a more rapid onset of floaters and is more common in people who have had a history of:
- Cataract surgery
- YAG laser surgery
- Inflammation inside the eye
Flashes appear as flashing lights or lightning streaks as vitreous gel pulls on the retina. Flashes are more common as people age, but the increase in frequency of flashes should prompt you to see your Optometrist. A sudden increase in flashes and/or floaters could indicate the presence of a retinal tear or retinal detachment. Prompt scheduling of an evaluation is very important in these situations.
What is the Vitreous?
Anatomically the eye is similar to a basketball, with Vitreous filling the eye much the same way that air fills a ball. Vitreous is also known as Vitreous Humor, and, contrary to what you might have thought, it is NOT a liquid. Vitreous is really a sticky, gooey gel with several firm attachments to the Retina. By and large the Vitreous is clear, but you may actually have seen your vitreous floating around! There is one area of the vitreous in particular that is thickened and opaque; normally this little "attachment" rests on the head of the Optic Nerve. Since the Optic Nerve Head is our natural blind spot, you can't see this bit of Vitreous. Eventually, however, this little attachment separates from the optic nerve head, and you CAN see it floating about on the inside of your eye. Often this spot looks like a circle, or spider web, or fly.... there seem to be many insect-like configurations, but other times you will only notice a dark spot floating in your vision. Usually floaters are harmless, but occasionally they signal the beginning of a retinal detachment, bleeding, or infection.
ADDITIONAL COMMONLY ASKED QUESTIONS
I've got this thing floating around in my vision in one eye. I notice it when I read, or look up at the sky, and it looks like a bug. What's the deal?
"Floaters" can be a sign of serious problems, such as hemorrhage inside the eye, or retinal detachment, but in the vast majority of cases, floaters are harmless. The vitreous is normally a clear, jelly like substance that fills the center of the eye, but it has a few "condensed" areas which are opaque. One of these condensations rests on top of the optic nerve. As I reviewed in the anatomy section, even though the optic nerve carries all the visual impulses to the brain, the little round hole where the optic nerves exits the eye is actually a blind spot in our vision. As long as that vitreous condensation stays nestled up against the optic nerve, you can't see it; it's hidden in our blind spot.
Over time the vitreous slowly liquefies, and one day, without warning, that little condensation (the optic nerve is only 1.5 millimeters in diameter here) peels off, or "detaches" from the optic nerve, and starts floating inside the vitreous cavity. Now the little opacity, which is often shaped like a ring, or a bug, casts a shadow on the retina, and we see something floating around. Most of the time we don't notice it, but when looking at a bright background, there it is, annoying us. These types of floaters never go away, but after a couple of weeks, our brains just ignore them (most of the time).
Unfortunately, floaters can also arise from retinal detachments, bleeding inside the eye, infections, or inflammation inside the eye, so if you have a new floater, please see your eye doctor for a dilated examination.