HOW THE EYE WORKS
Before we talk about what can go wrong, it is important to understand how the eye works when it is functioning properly. The eye is like a camera. When you take a picture, the lens in the front of the camera allows light through and focuses the light on the film that covers the back and side walls of the camera. When the light hits the film, a picture is taken. The eye works in much the same way. The front part of the eye, including the pupil, cornea, and lens, are clear and allow light to pass through them. The light also passes through a large space in the center of the eye called the vitreous cavity. The vitreous cavity is filled with a clear, gel-like substance called the vitreous or vitreous gel. The light is then focused on a thin layer of tissue called the retina. The retina is like the film in your camera, and is the seeing tissue of the eye. When the focused light hits the retina, a picture is taken. The messages about this picture are sent to the brain through the optic nerve. This is how we see.
The most serious retinal problems that require surgery are caused by difficulties with the vitreous. The vitreous is the clear gel-like substance that fills the central cavity of the eye. The vitreous is attached to the retina. It is most strongly attached to the retina at the optic nerve, macular area, and in a belt-like circle at the front part of the eye called the vitreous base.
Posterior vitreous detachment (PVD)
As a person ages, the thick vitreous gel becomes less like a gel and more like fluid, and small pockets of fluid form within the gel of the vitreous. As the eyeball moves, the liquefied vitreous moves around inside the vitreous cavity. Because of the movement of this fluid, the vitreous begins to pull on the retina, and with time, the vitreous can pull free and can separate from the retina and from the optic nerve in the back, or the posterior, part of the eye. This is called a posterior vitreous detachment (PVD). This type of detachment usually eventually happens in most people, and only infrequently causes a problem. By age seventy, 70% of patients have had a PVD, and by age one hundred, nearly everyone has.
Flashes and floaters
When a person develops a posterior vitreous detachment, flashes of light or large spots in the vision may occur. The flashes of light are caused by tugging of the vitreous where it attached to the retina. As the vitreous liquefies and pulls away from the retina, the vitreous becomes somewhat more condensed and stringy, forming strands. These strands and strings can be seen as spots, small circles, or irregular fine threads in your vision, and are called floaters.
Vitreous changes are most commonly caused by aging, but can also be caused by previous inflammation in the eye, nearsightedness, trauma, or previous surgery. If you have floaters, you should be examined to be certain there is no other serious retinal problem such as a retinal tear or detachment.
Retinal Tear and Vitreous Hemorrhage
There are many areas where the vitreous is very strongly attached to the retina. If the vitreous pulls away from the retina in an area where the retina is weak, the retina may tear. One condition which weakens the retina is called lattice degeneration. This condition is present in ~8% of people. It is inherited in an autosomal dominant fashion - that is, if you have it, your mother or father has it. When lattice degeneration is present, it indicates that the retina is thin and may be more susceptible to a tear of the retina than in an area without lattice degeneration.
If the vitreous is firmly attached to an area of the retina or if the retina is weak, as the vitreous pulls away, it can tear the retina. If the tear occurs across a blood vessel, there will be bleeding into the vitreous. This is called a vitreous hemorrhage.
When there is a little bleeding, red blood cells floating and moving in the vitreous cavity create the sensation of walking through a swarm of flies. If more bleeding occurs in the vitreous, it looks like a spider web or a swirling mass of black or red lines. With a lot of bleeding, vision may reduce significantly or even become very dark. When a retinal tear occurs, it is a potentially serious problem. If a vitreous hemorrhage also occurs, it is even more serious.
The retina can tear immediately following a posterior vitreous detachment, or weeks later. If a tear has not developed within eight weeks after a posterior vitreous detachment, it is much less likely that the retina will do so. Anyone who develops a sudden onset of new floaters or flashing lights of any kind should have a complete retinal evaluation. These symptoms may indicate that a retinal tear has occurred. A retinal tear can result in a retinal detachment.
Treatment of a Retinal Tear
If a tear in the retina has occurred, laser treatment, cryotherapy, or both may be used to seal the retinal tear in order to prevent a retinal detachment. The laser provides a beam of light that turns to heat when it hits the retina. Laser light is directed through a special contact lens. Cryotherapy (also called simply "cryo") involves freezing the part of the retina which needs treatment. This is done with a cryo probe which is placed on the outside of the eye wall. Both of these treatments seal the retina to the back of the eye wall by forming a scar. The scar, which takes approximately ten days to heal, forms a bond which seals the retina around the tear and prevents a detachment. Both laser surgery and cryotherapy are performed in the office. Patients are able to return to full activity without restrictions in a short period of time. Vision may be blurred for several days following these therapies. If cryotherapy is used to treat a retinal tear, the eye may also be red for several weeks.
Why is a retinal tear considered a serious problem? When a tear of the retina occurs, the liquid of the vitreous cavity may pass through the tear and get under the retina. The liquid can collect under the retina and lift it up off the back of the eye, similar to peeling wallpaper off a wall. Little by little, the liquid from the vitreous passes through the retinal tear and settles under the retina, separating it from the back of the eye wall. This separation of the retina is called a retinal detachment. Vision is lost wherever the retina becomes detached. Because most tears are located in the peripheral, or the side, of the retina, the retinal detachment first results in a loss of side vision. The patient may notice a dark shadow or veil coming from one side, either above or below. In most cases, after a retinal detachment begins, the entire retina will eventually detach and all useful vision will be lost, without treatment.
Who gets a retinal detachment?
Each year in the United States, approximately one out of ten thousand people develop a retinal detachment. Certain people have a greater chance of a retinal detachment occurring than others: Those with high myopia (nearsightedness); those with a family history of retinal detachment; or those who have once had a retinal detachment in the other eye. Patients who have lattice degeneration or other degenerative changes are also at increased risk. Patients who have had cataract surgery have a 1 - 2% chance of developing a retinal detachment as well. A person in any of these high-risk groups should be examined immediately if they note sudden flashing lights, new floaters, or a loss of peripheral vision.
Scleral buckling surgery for retinal detachment
If the retina has become detached, and the detachment is too large for laser treatment or cryotherapy, surgery is necessary to re-attach the retina. Without some type of retinal re-attachment surgery, vision will be completely lost. There are two types of surgery for a retinal detachment; one is called scleral buckling surgery, the other is called pneumatic retinopexy.
The traditional surgery for retinal detachment is scleral buckling surgery. This procedure is generally performed in the operating room under local anesthesia with sedation, though in some cases general anesthesia is used. The surgeon first treats the retinal tear with cryotherapy, with the cryo probe placed on the outside part of the eye as the surgeon looks into the eye. The cryo probe is then placed in the cryo position and treats the tear. A piece of silicone plastic or sponge is then sewn onto the outside of the eye wall (the sclera) over the site of the retinal tear. This pushes the sclera in towards the retinal tear, and holds the retina against the sclera until the scarring from the cryotherapy seals the tear. The surgery is called scleral buckling because the sclera is "buckled" (pushed) by the silicone. This silicone buckle is left on the eye permanently.
The silicone may also be placed all around the outside circumference of the eye; this is called an encircling or band scleral buckle. The purpose of an encircling scleral buckle is to lessen the pulling of the vitreous on the retina. During the surgery, the surgeon may drain the fluid from beneath the retina through a tiny slit made in the sclera, and then make a small puncture into the space under the retina. The fluid under the retina then drains out through the slit in the sclera.
The surgeon may occasionally place a gas bubble into the vitreous cavity. When the surgery is complete, the patient is positioned so that the gas bubble rises and pushes the retinal tear against the scleral buckle, to help keep the tear closed. In most cases, there is an 80 - 90% of successfully re-attaching the retina with a single surgery. However, successful re-attachment does not necessarily mean restored vision. The return of good vision after such surgery depends upon if, and for how long, the macula (the central portion of the vision) was detached prior to the surgery. If the macula was detached, a full return to normal vision is rare. However, vision usually improves. The best vision may not occur for many months after surgery. In some cases, it may take up to two years for maximum visual improvement. Even if the macula was still attached before surgery, in some cases, and even if the surgery results in successful retinal re-attachment, some vision may be lost. If the first retinal detachment operation fails, a second surgery is usually possible.
Complications of scleral buckling surgery include bleeding under the retina; cataract formation; glaucoma; retinal re-detachment; proliferative vitreoretinopathy; vitreous hemorrhage; drooping of the upper lid; infection; and double vision (strabismus). Although any one of these can result in the need for more surgery or in a total loss of vision, these complications are very infrequent. Retinal re-detachment is the commonly-occurring problem. If this should occur, your surgeon will discuss the chances that repeat surgery will successfully re-attach the retina. In approximately 10% of retinal detachment ca
ses, proliferative vitreoretinopathy (scar tissue formation on the retina with re-detachment) occurs. Surgical success in patients who have a re-detached retina from proliferative vitreoretinopathy is ~50%.
Pneumatic retinopexy for retinal detachment
Another type of surgery which can be done for retinal detachment is called pneumatic retinopexy. This is performed on an outpatient basis. Local anesthesia is used. Cryotherapy or laser treatment is used to seal the retinal tear. Instead of placing the scleral buckle on the outside of the eye, the surgeon uses a needle to inject a gas bubble inside the vitreous cavity. The patient is then instructed to hold the head in a specific position so that this gas bubble pushes the detached retina against the back wall of the eye to keep the retinal tear sealed. This positioning also needs to be maintained for various lengths of time until the retinal tear is sealed against the back eye wall. Your surgeon will tell you how long special positioning is required. Anti-?? eye drops are also used following this procedure. The gas bubble in the vitreous cavity of the eye expands for several days, taking two to six weeks to disappear. During this time, plane travel or travel to a high altitude must be avoided because high altitudes can result in gas expansion, and an increased pressure on the eye. Again, your surgeon will advise you when it is safe to travel. It is important for patients with a gas bubble NOT to lie face-up, as the gas bubble may come to rest against the lens of the eye, and cause a cataract. The chances of successful retinal re-attachment with pneumatic retinopexy is slightly than that of scleral buckling surgery, and is in the 80 - 85% range. Also, pneumatic retinopexy cannot be used or is ineffective for every retinal detachment. Your surgeon will discuss with you the feasibility of pneumatic retinopexy in your case. With pneumatic retinopexy, hospitalization, anesthesia, and the cutting done for the scleral buckling surgery are all avoided. The complications of this method are similar to those scleral buckling surgery, with the most common one for this also recurrence of the retinal detachment. If the retina becomes detached again, scleral buckling surgery or vitrectomy is usually performed to re-attach it.
Vitreous surgery for retinal detachment
Occasionally, a retinal detachment is so complicated and severe that it cannot be treated with either standard scleral buckling surgery or pneumatic retinopexy alone. In some cases, vitreous surgery may be necessary to re-attach the retina. This surgery is also performed in the hospital, usually under local anesthesia with sedation, or possibly general anesthesia. The vitreous is removed, and this procedure is therefore called a vitrectomy. The surgeon uses a fiber-optic light to illuminate the inside of the eye, and other instruments in the eye such as forceps, scissors, and a cutter for the surgery. The vitreous is replaced with either clear fluid compatible with the eye, or with air, completely filling the eye cavity. The lack of vitreous does not affect the functioning of the eye. Vitrectomy is required for retinal re-attachment in a variety of conditions. For example, scar tissue may grow on the vitreous or on the surface of the retina which will then pull on the retina and detach it. Something is occasionally in the vitreous, such as blood, that prevents the passage of light through the eye to the retina; in this case, the blood (vitreous hemorrhage) needs to be removed to allow treatment of the retinal tear of detachment, and to allow successful retinal re-attachment.
Vitrectomy can be combined with the placement of a scleral buckle. Air, gas or silicone is occasionally placed in the vitreous cavity to hold the retina in place. After this procedure, it may be important for the patient to maintain a certain position of the head, which is often face-down (prone) positioning. Eventually the air or gas is absorbed by the body, and is replaced by fluid produced by the eye. If silicone oil has been used, it must usually be removed at a later time with another surgical procedure. Vitreous surgery typically lasts one to two hours, with severe and/or difficult cases requiring more time.
If there is a combined vitreous hemorrhage and retinal detachment, vitrectomy must be performed to remove the blood so the surgeon can see the retina. A scleral buckle is also placed around the eye in these cases. Because of the combination of retinal detachment and vitreous hemorrhage, the eye is at high risk for development of proliferative vitreoretinopathy. In cases of severe vitreous hemorrhage with a retinal detachment, ultrasonography will be required to help make the diagnosis of a retinal detachment beneath the hemorrhage. Ultrasonography is a harmless and painless test. Like the sonar in a submarine, sound waves are sent into the eye, travel through the hemorrhage and bounce off the retina. The returning sound waves make an image on the monitor which allows the doctor see if the retina is attached or detached.
Scleral buckling surgery fails ~5% of the time because excessive scar tissue grows on the surface of the retina. This scar tissue is bad for the eye, and is called proliferative vitreoretinopathy (PVR). If PVR occurs, retinal re-detachment usually occurs within four to eight weeks after the initial surgery. The scar tissue puckers the retina into stiff folds, like wrinkled aluminum foil. The only way to unfold and re-attach the retina is to cut away the vitreous and remove the scar tissue with vitrectomy surgery, then re-attach the retina. The lens of the eye almost always has to be removed during this procedure. If an artificial lens implant from cataract surgery is present, however, it can usually be left alone. After the vitreous and the scar tissue have been removed, an encircling scleral buckle is placed around the eye. The eye is then filled with air so the retina is pushed against the back of the eye wall, against the buckle. Once the retina is pushed into place, laser is applied to seal the retinal tears and form a strong attachment between the retina and the back of the eye wall. At this point, the surgeon will either replace the air with a long-acting gas or with silicone oil. While the gas is in the eye, vision is always poor. The gas keeps the retina pushed against the eye wall long enough for the laser burns to heal and take hold.
The chances of successful re-attachment with vitrectomy for PVR is ~3 out of 4, and there is an approximately 50% chance of regaining good vision. Reading vision rarely returns. It should be clearly understood that the purpose of PVR surgery is to give the patient an eye with some vision, and would serve as a "spare" if the other eye ever lost vision.
Giant retinal tear
Just as the vitreous pulling away can cause small retinal tears, it can also cause very large retinal tears. If the retinal tear is very great (one quarter or more of the retina), it is called a giant retinal tear. Such a tear is usually so large that the retina folds over on itself like a piece of paper folded in half.
Vitreous surgery, scleral buckling, and laser may be used to treat giant retinal tears associated with retinal detachment. The lens is usually removed to make the surgery successful, with the goal of this surgery to unfold the retina, put it back in place and seal it into proper position. Perfluorocarbons, which are fluids heavier than water, are usually used to unfold a giant retinal tear and to re-attach the retina. Silicone oil may also be used. Many patients will see well after giant retinal tear surgery, but some will not. Visual recovery is usually sufficient to ambulate. The risk of developing scar tissue (PVR) is higher in giant tear cases. The success rate is this surgery is fair.
Retinal detachments represent a relatively rare yet sight-threatening condition. With modern advances in scleral buckling, pneumatic retinopexy, and vitreous surgery, retinal re-attachment is highly likely in patients with retinal detachment. However, proliferative vitreoretinopathy remains a significant cause of permanent visual loss in retinal detachment patients. Ongoing research into reducing the risk of PVR formation has been fruitless to this point.