What are cataracts?

Causes and Symptoms

Cataracts are imperfections in the clarity of the eye lens that reduce its ability
to transmit light. They are a very common medical problem, especially among
persons over the age of fifty. Cataracts do not cause pain, redness, tears, or
other discomforts of the eye. The initial symptom is a gradual deterioration of
vision, usually in one eye at a time.
There is no known treatment other than to remove the lens surgically, with or
without the introduction of an intraocular artificial lens. After surgery, neither
the lens nor the cataracts can grow back.

In a discussion of cataracts, it is helpful to review the structure of the human
eye. The eye is often compared to a camera lens. The camera
lens, however, can be moved back and forth slightly to focus on objects at
different distances, whereas the eye lens is squeezed into a thicker shape by
muscular action to change its focus. Both camera and eye have a variable-size
diaphragm to regulate the amount of light that is admitted.

When light enters the eye, it first encounters a transparent, tough outer skin
called the cornea. There are no blood vessels in the cornea, but many
nerve cells make it sensitive to touch or other irritation. Immediately behind the
cornea is the clear aqueous fluid that carries oxygen and nutrients for cell
metabolism. Next comes the colored portion, the iris of the eye, with a
variable-size opening at its center called the pupil. The pupil has no color but
rather looks black. A person looking at his or her own eye in a mirror and then
shining a flashlight on it can see the black pupil quickly shrink in size.

Next comes the lens of the eye, surrounded by an elastic membrane called the lens
capsule. The lens is suspended by ligaments, or short
strands, which are attached to a sphincter muscle. When the muscle contracts, the
lens becomes thicker in the middle, thus increasing its focusing strength. The
transparent lens has no blood vessels, so its metabolism is provided by the
aqueous fluid. Behind the lens is the vitreous fluid, which fills about two-thirds
of the eyeball and maintains its oval shape. At the back of the eye is the retina,
where special visual cells convert light into electrical signals that travel to
the brain via nerve fibers.

The lens of the eye is not simply a homogeneous fluid, but has a unique, internal
structure and growth pattern. It continues to grow larger throughout the life of
the individual. New cells originate at the front surface of the lens, just inside
the capsule enclosure. These cells divide and grow into fibers that migrate toward
the middle of the lens. The protein molecules in the nucleus are less soluble and
more rigid than those in the outer part of the lens, becoming thicker and less
flexible. By the age of forty, in most people, the firm nucleus has enlarged until
the lens has lost much of its elasticity. Even with considerable muscular strain,
the curvature of the lens surface will no longer bulge enough to focus on nearby
objects. The eye loses its power of accommodation, and reading glasses will be
needed.

The mechanism by which cataracts form in the lens is not yet clearly understood.
Like the loss of accommodation, however, it is a normal part of the aging process.
As the tissues in the lens break down, they can clump together and create cloudy
patches that occlude the vision. One proposed biochemical explanation is the
Maillard reaction, in which glucose and protein molecules combine when heated to
form a brown product. The Maillard reaction is responsible for the browning of
bread or cookies during baking. The same process is thought to occur even at body
temperature, though very slowly over a period of years. Some scientists have
theorized that wrinkled skin, hardening of the arteries, and other normal features
of aging may be caused by this biochemical reaction. The biochemistry of aging is
an active area of research, in which the deterioration of the eye lens is only one
example.

The most common symptom of cataracts is a loss of clear vision that cannot be
corrected with eyeglasses; among the indicators of cataract-related vision loss
are reduced night vision, loss of color contrast and intensity, inability to
focus, double vision, and increased sensitivity
to glare. Brighter lighting can partially help to overcome the blockage of light
transmission. However, there is one paradoxical situation reported by some
patients, whose vision becomes worse in bright light. The explanation for this
problem is that brightness causes the pupil to become smaller. If the cataract is
centered right in the middle of the lens, it will block a larger fraction of the
incoming light. In dimmer light, the pupil opening is larger, so light can pass
through the clear periphery of the lens.

By far the most common cataracts are those attributable to normal aging, called
age-related cataracts. Secondary cataracts can also develop due to trauma, toxic
exposure, intraocular disease, or systemic disease. Exposure to x-rays or nuclear
radiation will increase the probability
of cataracts, and the eye lens seems to be particularly sensitive to the effects
of ionizing radiation. Certain medications such as steroids increase the risk of
cataracts. A blow to the eye from a sports injury or an accident can lead to a
cataract. Diabetes mellitus and hypoparathyroidism also increase the
risk of developing cataracts. Intraocular diseases such as chronic uveitis and
retinitis
pigmentosa can also lead to cataract formation. Some studies
have suggested that electric shock, ultraviolet rays, or certain environmental
pollutants may be other causes.

Some babies are born with cataracts. These congenital cataracts are frequently
associated with the mother having had German measles (rubella) during the first three months of pregnancy; they
are also commonly found with a number of genetic disorders, such as
Down syndrome and trisomy 13. Congenital
cataracts are characterized by an infant having cloudy pupils, moving the eye
rapidly, and/or appearing unable to see. For moderate and severe cases, surgery on
the infant’s eye must be done with little delay to prevent permanent
blindness due to amblyopia.

Cataracts are much more prevalent in developing nations as compared to
industrialized countries. It is not clear yet what roles genetics, diet, and life
habits may play in cataract formation. Genetic factors are thought to account for
nearly 50 percent of the variation in the severity of cataracts. The evidence is
not conclusive, and further studies are needed. What causes cataracts is much less
understood than how to treat them surgically.

Treatment and Therapy

When cataracts begin to form in the eye lens, no medication can remove them and
they will not get better on their own. The patient’s vision will continue to
deteriorate as the cataracts develop, although the process may be quite slow.
Fortunately, modern techniques of surgery for cataract removal have a success rate
of better than 95 percent.

When a patient's eye examination reveals the onset of cataracts (as a result of
aging), he or she is referred to an ophthalmologist, who assesses the need for
surgery. If surgery is not needed immediately, the ophthalmologist may recommend
more frequent, semiannual checkups. Reading or other eye-straining activities will
not accelerate cataract growth, but brighter lighting and better eyeglasses will
help the patient to see more clearly. Over the course of several years, the
cataracts slowly darken and increase in size. Eventually, distance vision may
deteriorate markedly in one or both eyes, and the ability of the patient to drive
a car or perform other normal activities becomes seriously impaired. Surgery is
then indicated. Operation on one eye while the other one is still fairly clear is
recommended.

Once the decision has been made to go ahead with surgery, it is necessary for the
patient to have a thorough physical examination. The doctor checks for possible
health problems that could complicate cataract surgery.
Among these are diabetes, high blood pressure,
kidney disease, anemia, and glaucoma (excess pressure in
the eye). Extracting the cataractous lens and implanting an artificial one may be
done at the same time. If the introduction of an artificial intraocular lens is
not possible due to other eye problems, then the lens is removed and the patient's
vision is corrected with eyeglasses or contact lenses. Before proceeding with
surgery, the ophthalmologist must determine what the proper strength of the
implant lens should be, so that light will focus properly on the retina.

On the day of the surgery, an injection is given to make the patient drowsy, and
eye drops are administered to dilate the patient's pupil. Gradually, more eye
drops are administered to produce a large dilation, so that access to the lens is
easier. In the operating room, local anesthetic is injected to keep the eyelids
from closing and to deaden the normally very sensitive surface of the cornea. To
prepare for surgery, a microscope is moved into place above the eye. Making an
incision in the cornea, removing the defective lens, inserting and fastening the
artificial lens, and finally closing the incision with a very fine needle and
thread are all performed by the surgeon while looking through the microscope. Its
magnification and focus controls are operated using foot pedals, so that both of
the surgeon’s hands are free.

A common method of cataract surgery is called extracapsular extraction, in which
the lens is removed while the capsule is left in the eye. The advantage is that
the unbroken back surface of the capsule can prevent leakage of fluid from the
rear of the eye and therefore can decrease the chances for damage to the retina. A
disadvantage is that small fragments of the lens may remain behind, causing a
slight risk of infection or irritation. The recuperation period is about one
month, during which time the patient must avoid strenuous activity to permit
thorough healing.

Another method of cataract surgery is called phacoemulsification. An ultrasonic
probe is used to emulsify, or break up, the lens. The small pieces are then
suctioned out of the capsule while fluid is washed into the opening. The main
advantage of emulsification is that the incisions can be smaller than the incision
for extracapsular extraction, because the lens is brought out in fragments, not as
a whole. This method requires specialized training, however, because surgeons must
learn to operate the microscope, the ultrasonic generator, and the suction
apparatus with their feet while manipulating the probe with their hands. After the
eye lens has been extracted, an artificial intraocular lens (IOL) may be inserted
in its place.

To complete the surgery, the incision in the cornea is closed. During the
recuperation period, the patient is instructed to avoid strenuous exercise and to
protect the eye from any hard contact. Normally, there is little pain, although
some eye irritation should be expected during the healing process. An IOL has a
fixed focal length, with no power of accommodation for different distances. It is
like a box camera that gives a good picture at a set distance, while near and far
objects are somewhat blurry. After the eye has healed thoroughly, the patient is
fitted with prescription glasses for reading and for distance vision,
respectively.

A number of minor complications can develop after cataract surgery. Between 10 and
50 percent of the patients develop a secondary cataract, which is a clouding of
the capsule membrane just behind the implant. This condition is easily corrected
with a laser beam to open the membrane, requiring no surgery. Another potential
complication is astigmatism. The eye is squeezed and
flattened slightly, and the curvature of the surface will differ between the
flattened and the more rounded regions. During surgery, the symmetry of the
corneal surface can be distorted if some sutures are tighter than others.
Astigmatism is relatively easy to correct with prescription glasses. For infants
who undergo cataract removal surgery, infection, inflammation, and bleeding pose
slight risks.

All operations have some risks, and a small percentage of cataract surgeries can
lead to serious complications. Among these are a detached
retina, glaucoma caused by
scar tissue, and hemorrhage into the vitreous fluid in front of the retina.
Fortunately, such problems are rare, and the percentage of successful eye
surgeries continues to improve.

Perspective and Prospects

In the history of medicine, surgery for cataracts has been traced back to Roman times. The method was called “couching.” The physician would insert a needle through the white of the eye into the lens and try to push the lens down out of the line of vision, leaving it in the eyeball. The procedure must have been painful, with a high chance for infection. The complete extraction of a lens from the eye was done for the first time in 1745. A French ophthalmologist named Jacques Daviel was performing a couching operation but was unable to push the lens out of the line of sight. On the spur of the moment, he decided to make a small cut in the cornea, through which he was able to extract the lens. The operation was successful. During the following ten years, he repeated his procedure more than four hundred times with only fifty failures, a much better result than with couching.

A major advance in eye surgery was the
discovery of local anesthesia by Karl Koller in 1884. Together with the famous
psychiatrist Sigmund Freud, Koller had been investigating the psychological
effects of cocaine. He noticed that his tongue became numb from the drug and
wondered if a drop of cocaine solution locally applied to the eyes might work as
an anesthetic. He tried it first on a frog’s eye and then on himself, and the
cocaine made his eye numb. He published a short article, and the news spread to
other physicians. Synthetic substitutes such as novocaine were developed and came
into common use, thereafter making eye surgery virtually painless.

When the lens of the eye is surgically removed, it becomes impossible to focus
light on the retina. A strong replacement lens is needed. For example, the French
painter Claude Monet had cataract surgery in the 1920s, and photographs show him
with the typical thick cataract glasses of that time. Today, contact lenses or
artificial lens implants are much better alternatives to restore good vision.

The recovery period after cataract surgery used to be several weeks of bed rest,
with the head kept absolutely still, because the cut in the cornea had to heal
itself without any stitches. The development of microsurgery made it possible for
the surgeon to see the extremely fine thread and needle that can be used for
closing the cut. With stitches in place, the patient can usually carry on normal
activities within a day after surgery.

In the 1960s, the cryoprobe and the ultrasound probe were developed to replace forceps for removing an eye lens. The size of the required incision was smaller and the healing time correspondingly shorter. In the 1980s, reliable lens implants became available, making near-normal vision possible again. In the future, perhaps drugs can be found that will prevent or delay the onset of cataracts, so that surgery will not be necessary. Further research is needed to obtain a better understanding of biochemical changes in the eye lens that occur with aging.

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