The retina is an extremely thin tissue that lines the inside of the back of the eye. When we look around, light from the objects we are trying to see enters the eye. The light image is focused onto the retina by the cornea and the lens. This light striking the retina causes a complex biochemical change within layers of the retina and this, in turn, stimulates an electrical response in other layers of the retina. Nerve endings within the retina transmit these signals to the brain through the optic nerve. Within specific areas of the brain, this electrical energy is processed to allow us both to see and to understand what we are seeing. The retina has been compared to the film of a camera. However, film, once used, has a permanent image on it. The retina continually renews itself chemically and electrically, allowing us to see millions of different images every day without them being superimposed.
The retina is about the size of a postage stamp. It consists of a central area called the macula and a much larger peripheral retina. The light receptor cells within the retina are of two types, called the rods and the cones. Rods allow us to see in reduced illumination. Cones provide us with sharpness of vision. The peripheral retina allows us to see objects on either side (peripheral vision) and, therefore, provides the vision needed for a person to move about safely. However, because this part of the retina contains a lesser concentration of cones, it does not allow for perception of visual detail. Its larger concentration of rods provides better vision during conditions of darkness.
In contrast, the macula is a smaller, central area of the retina that contains a high concentration of cones. Accordingly, it enables clear central vision to see fine details for such activities as reading or threading a needle. The macula is particularly sensitive to circulatory changes, especially those that occur with aging, such as decreased blood flow. The retina contains a network of branching arteries, which supplies blood that carries the needed oxygen and nutrients to the retina, and accompanying veins, which then carry the blood away together with the waste products of retinal metabolism.
A retinal detachment is a separation of the retina from its attachments to its underlying tissue within the eye. Most retinal detachments are a result of a retinal break, hole, or tear. These retinal breaks may occur when the vitreous gel pulls loose or separates from its attachment to the retina, usually in the peripheral parts of the retina. The vitreous is a clear gel that fills 2/3 of the inside of the eye and occupies the space in front of the retina. As the vitreous gel pulls loose, it will sometimes exert traction on the retina, and if the retina is weak, the retina will tear. Most retinal breaks are not a result of injury. Retinal tears are sometimes accompanied by bleeding if a retinal blood vessel is included in the tear.
Once the retina has torn, liquid from the vitreous gel can then pass through the tear and accumulate behind the retina. The build-up of fluid behind the retina is what separates (detaches) the retina from the back of the eye. As more of the liquid vitreous collects behind the retina, the extent of the retinal detachment can progress and involve the entire retina, leading to a total retinal detachment. A retinal detachment almost always affects only one eye. The second eye, however, must be checked thoroughly for any signs of predisposing factors that may lead to detachment in the future.
Flashing lights and floaters may be the initial symptoms of a retinal detachment or of a retinal tear that precedes the detachment itself. Anyone who is beginning to experience these symptoms should see an eye doctor (ophthalmologist) for a retinal exam. In the exam, drops are used to dilate the patient's pupils to make a more detailed exam easier. The symptoms of flashing lights and floaters may often be unassociated with a tear or detachment and can merely result from a separation of the vitreous gel from the retina. This condition is called a posterior vitreous detachment (PVD). Although a PVD occurs commonly, there are no tears associated with the condition most of the time.
The flashing lights are caused by the vitreous gel pulling on the retina or a looseness of the vitreous, which allows the vitreous gel to bump against the retina. The lights are often described as resembling brief lightning streaks in the outside edges (periphery) of the eye. The floaters are caused by condensations (small solidifications) in the vitreous gel and frequently are described by patients as spots, strands, or little flies. Some patients even want to use a flyswatter to eliminate these pesky floaters. There is no safe treatment to make the floaters disappear. Floaters are usually not associated with tears of the retina.
If the patient experiences a shadow or curtain that affects any part of the vision, this can indicate that a retinal tear has progressed to a detached retina. In this situation, one should immediately consult an eye doctor since time can be critical. The goal for the ophthalmologist is to make the diagnosis and treat the retinal tear or detachment before the central macular area of the retina detaches.
Studies have shown that the incidence of retinal detachments caused by tears in the retina is fairly low, affecting approximately one in 10,000 people each year. Many retinal tears do not progress to retinal detachment. Nevertheless, many risk factors for developing retinal detachments are recognized, including certain diseases of the eyes (discussed below), cataract surgery, and trauma to the eye. Retinal detachments can occur at any age. They occur most commonly in younger adults (25 to 50 years of age) who are highly nearsighted (myopic) and in older people following cataract surgery.
Cataract surgery, especially if the operation has complications, increases the risk of a retinal detachment. Cataracts are areas of cloudiness (opacities) that form in the lens. Following the introduction of extracapsular surgery, a modern method used almost exclusively today for the removal of cataracts, the risk of retinal detachment became far less. In extracapsular cataract surgery, part of the capsule of the lens is left in place so that the vitreous gel is undisturbed. Phacoemulsification is a type of extracapsular cataract surgery that utilizes a very high speed ultrasonic instrument to break up and suck out the clouded lens of the eye. The capsule that is left in the eye may at a later time become cloudy, necessitating opening the capsule by using a laser. Opening the capsule increases the risk of retinal detachment.
In intracapsular cataract surgery, the predominant surgical method used from 1965 to 1990, the entire lens was removed. The capsule at the back of the lens, therefore, was no longer present to hold the vitreous gel in place. Consequently, the vitreous gel moved forward, and the retina was subjected to increased pulling or traction on the retina from the vitreous, which led to tears of the retina. Today, if the capsule is broken, which can be a complication during extracapsular cataract surgery, the vitreous gel similarly can move forward and pull on the retina. This sequence can lead to a retinal tear and a detachment, especially during the first year after surgery.
