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Procedure
Practice 09/15/98 - Coding
Recommendations
Feature
Article 09/15/98:
The Eye - Part 1
The structure and function of the eye are complex and fascinating.
The eye constantly adjusts the amount of light it lets in,
focuses on objects near or far, and produces continuous images
that are instantly transmitted to the brain. This month, we
review the eye's anatomy and discuss several common disorders
such as glaucoma, refractive errors, and cataracts, and their
corresponding surgical procedures.
The Globe
The eyeball (globe) is covered by the conjunctiva,
a single layer of tissue that runs along the inside of the
eyelid and curves to meet the sclera, the tough white
outer layer of the eye. In the center of the eye, the transparent,
dome-shaped membrane called the cornea protects the
eye's surface. By changing shape, this thin, tough, transparent
tissue provides about two-thirds of the eye's focusing power.
After passing through the cornea, light enters the pupil,
the black opening in the middle of the colored area of the
eye, i.e., the iris. The iris controls the amount of
light that enters the eye by opening and closing like the
aperture of a camera lens. The iris allows more light into
the eye when the environment is dark and allows less light
into the eye when the environment is bright. The size of the
pupil is controlled by the pupillary sphincter muscle
which opens and closes the iris.
The uvea, or middle layer of the eyeball, consists
of the choroid in the back of the eye and the iris
in the front of the eye. The choroid is filled with blood
vessels that nourish the retina. Anteriorly, the choroid is
continuous with the iris.
Behind the iris sits the lens. By changing its shape,
the lens focuses light onto the retina. For the eye to focus
on nearby objects, a small muscle called the ciliary muscle
contracts, thickening and strengthening the lens. For the
eye to focus on distant objects, the same muscle relaxes,
making the lens thinner and weaker. Although devoid of blood
vessels and nerves, the lens continually accumulates new fibers
throughout life. The additional bulk restricts elasticity
so that eventually the lens cannot focus on near objects.
The retina is the thin lining at the back of the eye.
It contains the nerves that sense light and the blood supply
that nourishes them. Within its ten layers of cells are rod
cells that perceive light and cone cells that perceive
both light and color. Rods outnumber cones by a ratio of 20:1,
so far less light is needed to register simple black-and-white
outlines of objects than is needed to distinguish colors.
The most sensitive part of the retina is a small area called
the macula which has hundreds of nerve endings close
together. The high density of nerve endings makes the visual
image sharp. The rod and cone cells convert the image into
electrical impulses. The optic nerve, a bundle of more
than 1 million nerve fibers, carries these impulses to the
visual cortex at the back of the brain.
The globe is divided into two fluid-filled segments. The
front (anterior segment) extends from the cornea
to the lens; the back (posterior segment) extends
from the back edges of the lens to the retina. The anterior
segment is filled with a fluid called the aqueous humor
that nourishes its internal structures and cushions impacts
to the eye; the posterior segment contains a gel-like substance
called the vitreous humor. These fluids help the eyeball
maintain its shape. The anterior segment itself is divided
into two chambers. The front (anterior chamber) extends
from the cornea to the iris; the back (posterior chamber)
extends from the iris to the lens. The aqueous humor is produced
in the posterior chamber, passes through the pupil into the
anterior chamber, and then drains out of the eyeball through
the trabecular meshwork near the front of the eye.
From the trabecular meshwork, the aqueous humor flows through
a passageway called Schlemm's canal and out of the
eye through the ocular veins.
Adnexal Structures
The ocular adnexa are the structures around the eye
that protect it from dust, wind, bacteria, and other potentially
injurious substances while allowing it to remain open enough
to catch light rays. The orbit is the circle of bone
made by the eyebrow, cheekbone, and bridge of the nose. It
safely nestles the eye away from direct physical impact. This
bony structure continues around the back of the eye.
After the orbit, the eyelids form the second line
of defense. The lids are thin folds of skin that cover the
eye, protecting it from dust, intense light, and impact. From
birth, the eyelids reflexively shut tightly at the sight of
an oncoming object. When blinked, the lids spread liquid over
the surface of the eyes, and when closed, they help keep the
surface moist. Without such moisture, the normally transparent
cornea can become dried, injured, and opaque. Eyelashes
grow from the edge of the eyelid and provide further protection
from dust and other tiny foreign objects.
The small lacrimal glands behind the upper eyelids
secrete tears. Tears drain from the eyes into the nose through
the two nasolacrimal ducts. Each of these ducts has
openings at the edge of the upper and lower eyelids near the
nose. Tears not only keep the surface of the eye moist, they
also trap and sweep away small particles that enter the eye,
and they are rich in antibodies to prevent infection.
An ophthalmic artery and a retinal artery provide
blood to each eye, and an ophthalmic vein and a retinal
vein drain blood from it. These blood vessels enter and
leave through the back of the eye.
The coordinated actions of six muscles attached to the eyeball
under the conjunctiva move the eye. Each muscle is stimulated
by a specific cranial nerve. The lateral muscles are the superior
rectus, lateral rectus, and inferior rectus; and the medial
muscles are the superior oblique, medial rectus, and inferior
oblique.
Glaucoma
Glaucoma is not a single disease, but rather a heterogeneous
group of disorders characterized by a distinct type of optic
nerve damage caused by the death of retinal ganglion cells.
These diseases involve several tissues in the front and back
of the eye. Commonly, but not always, glaucoma begins with
a defect in the front of the eye. Most types of glaucoma are
associated with defects that interfere with aqueous humor
outflow and, hence, lead to a rise in intraocular pressure.
As a consequence optic nerve function is compromised. The
result is a distinctive optic nerve atrophy, which clinically
is characterized by cupping of the optic nerve.
Usually glaucoma is inherited. Parents may be genetic carriers
without developing the disease. Glaucoma usually occurs after
the age of 40, but abnormal development of the eye may cause
glaucoma in infants and toddlers. Injuries, cataracts, and
bleeding in the eye may also precipitate glaucoma. Infants
with congenital glaucoma have an aversion to light, enlarged
corneas, copious tearing, and big cloudy-looking eyes.
It is estimated that 1 out of every 25 Americans has glaucoma,
but only half of these people are aware of the disease. More
than 62,000 Americans are legally blind due to glaucoma, with
an additional 5,350 losing their sight each year. The disease
can cause extensive damage before the symptoms are noticed.
- Open-angle glaucoma
is the prevalent type of glaucoma in which the drainage
of the aqueous humor is slowed so that fluid and pressure
in the eye build up gradually. The structures that allow
for drainage or absorption of aqueous humor fail, and the
increased fluid pressure in the eyeball pinches the blood
vessels that supply the optic nerve. Starved, the nerve
slowly dies. Peripheral vision decreases, or the affected
person may notice halos around lights. Eventually, the eye
can achieve only tunnel vision. If untreated, blindness
results.
- Narrow- or closed-angle glaucoma
is a mechanical form of the disease caused by contact
of the iris with the trabecular meshwork, resulting in blockage
of the drainage channels that allow fluid to escape from
the eye. It is much rarer than open-angle glaucoma, amounting
to only 5 to 10 percent of total glaucoma cases. Within
days, the eye becomes red, rock hard, and painful enough
to cause nausea and vomiting. The cornea appears hazy, lights
develop halos, and vision is poor. Closed-angle glaucoma
is an obvious eye emergency.
Refractive Disorders
The eye is like a camera, with the cornea and lens comprising
the focussing system and the retina the film on which the
image is photographed. When an eye with no refractive error
is at rest, parallel light rays focus exactly on the macula
in the center of the retina, the area of clearest vision.
Refractive errors occur when light is not focused exactly
on the retina, but either in front of or behind it.
- Hyperopia or hypermetropia,
also known as farsightedness, is a refractive error that
causes parallel rays of light to focus behind the retina
instead of on it. This usually occurs because the eye is
too short from front to back, but may also be due to insufficient
curvature of the cornea. It is the most common of refractive
errors and is present to some degree in about two thirds
of all adults. People with hyperopia have good distant vision
and poor near vision.
- Myopia or nearsightedness
is a condition in which parallel rays of light are
focused in front of the retina, usually because the eye
is too long from front to back. Myopia is present in approximately
2 percent of all adults. While most authorities agree that
it is due to hereditary factors, some physicians believe
that the condition can also be aggravated by too strenuous
use of the eyes during the early school years. People with
myopia usually have good near vision and poor distant vision.
- Astigmatism
is a refractive abnormality in which the surface of the
cornea is irregular instead of spherical. The irregular
corneal surface creates a blurred image on the retina. Astigmatism
may be simple, that is, it may exist by itself; or it may
coexist with nearsightedness (myopic astigmatism)
or farsightedness (hyperopic astigmatism).
Cataracts
Cataracts are the leading cause of vision loss among adults
age 55 and older. Poor vision from cataracts affects 60 percent
of all adults over age 60. However, cataracts can affect all
ages as they also can result from injury, heredity, or medications.
A cataract forms when the natural lens of the eye, responsible
for focussing light and sharpening images, becomes cloudy
and hardens, resulting in a loss of visual function. A cataract
is painless and usually develops gradually over several months
or years. Normally, the onset of a cataract in one or both
eyes may cause decreased night vision, impaired depth perception,
and color distortion. Because all light entering the eye must
pass through the lens, any part of the lens that blocks, distorts,
or diffuses light can cause poor vision. How much vision deteriorates
depends on where the cataract is and how dense (mature) it
is.
In bright light, the pupil constricts, narrowing the cone
of light entering the eye, so that it can't easily pass around
a cataract. Thus, bright lights are especially disturbing
to many people with cataracts, who see scattering of light,
glare, and light halos. A cataract at the back of the lens
(posterior subcapsular cataract)
particularly interferes with vision in bright light. It affects
vision more than other cataracts because the opacity is at
the point where light rays cross.
Surprisingly, a cataract in the central part of the lens
(nuclear cataract) may improve vision at first. The
cataract causes light to be refocused, improving vision for
objects close to the eye. People who have trouble with near
vision may discover that they can read again without glasses.
Once a cataract is surgically removed, it cannot recur. However,
some people develop an opacity in the back portion of the
lens capsule weeks or even years after surgery. This cloudiness
of the lens capsule is called a secondary or after-cataract,
and it causes the same vision problems as cataracts.
Glaucoma Surgery
- Trabeculectomy
is the most common glaucoma operation. In this procedure,
the surgeon removes a small section of the trabecular meshwork.
This allows expansion of the remaining trabecular cells,
which allows the aqueous humor to drain more easily and
reduces the pressure in the eye.
- Argon laser trabeculoplasty
(ALT) was first used as an intermediate step
between medication and more radical surgery, but is now
being used to treat early stages of open-angle glaucoma.
The laser beam is focused on the trabecular meshwork, and
50 to 100 burns over 180° to 360° are placed on the meshwork.
The laser's intense heat causes some areas of the meshwork
to shrink, resulting in adjacent areas stretching open and
permitting the fluid to drain more easily. It is also possible
that the laser stimulates regrowth of trabecular cells.
- Scleral fistulization with
iridectomy/laser iridotomy is performed for
closed-angle glaucoma. The surgeon opens up drainage by
removing part of the iris, or by making a tiny hole in the
iris, either with a scalpel or a laser. In newer procedures,
tiny plastic valves are implanted to permit outflow of liquid.
Refractive Surgery Techniques
Refractive surgery refers to a family of new surgical procedures
designed to produce better eye focus with less dependence
upon glasses or contact lenses for near-sighted and astigmatic
individuals. Procedures for far-sightedness are still undergoing
FDA study.
- Radial keratotomy (RK)
is the original procedure of all incisional refractive surgeries.
It is performed to reduce myopic refractive errors. The
procedure is usually performed on one eye at a time. Under
microscopic guidance, the surgeon measures the thickness
of the cornea with an instrument called an ultrasonic pacchymeter.
A diamond micrometer blade is then set to this thickness
and a series of 4 or 8 incisions are then made with the
blade concentrically around the center of the cornea overlying
the pupil.
- Astigmatic keratotomy
is a procedure often done along with RK on patients with
myopia and astigmatism of the cornea. A series of precise
incisions are placed in the astigmatic portion of the cornea
to flatten it and create a more spherical shape.
- Photorefractive keratectomy
(PRK) is a more recent development in myopic
surgery. This procedure uses an excimer laser system, which
emits non-thermal light, to ablate corneal tissue layer
by layer in a very controlled and precise fashion. Because
no incisions are made, PRK does not weaken the structure
of the cornea.
- Laser-assisted in-situ keratomielusis
(LASIK) is another type of corrective procedure
performed on the cornea to treat myopia. In this procedure,
a very thin flap of cornea is created with an instrument
called a corneal shaper or microkeratome. The surgeon passes
the instrument across 90 percent of the corneal dome, creating
a hinge of corneal tissue that is then reflected back to
expose the cornea's refractive layer. An excimer laser is
then applied to the refractive layer to reshape it. At the
end of the procedure, the corneal flap is replaced over
the treated layer.
Cataract Surgery Techniques
Years ago, cataract surgery was a major operation requiring
general anesthesia and several days of hospitalization. With
modern techniques, cataract surgery is now done on an outpatient
basis under local anesthesia or intravenous sedation. There
are three basic cataract extraction techniques:
- Intracapsular cataract extraction
(ICCE) includes removal of the entire lens
and its surrounding protective capsule. This method is seldom
used because it requires larger incisions and more healing
time than either ECCE or phacoemulsification. ICCE is generally
performed only when a cataract cannot be safely removed
with a less invasive technique, for example, on an eye that
has sustained previous injury or undergone prior surgery.
- Extracapsular cataract extraction
(ECCE) is the most common type of cataract
surgery. The surgeon makes a horizontal incision where the
cornea and sclera meet. Carefully entering the eye through
the incision, the surgeon gently opens the front of the
capsule and removes the hard center, or nucleus, of the
lens. Using a microscopic instrument, the surgeon then suctions
out the soft lens cortex, leaving the capsule in place.
This type of surgery usually needs sutures to close the
wound.
- Phakoemulsification
is a modification of the ECCE. In phacoemulsification,
the cataract nucleus is shattered by an ultrasonic oscillating
probe. The technique was invented by Dr. Charles Kelman
in 1967. After fragmentation, the capsule is gently torn
in a procedure called a capsulorrhexis. The surgical
tear leaves a smooth edge that stretches without further
damage to the capsule. The phaco probe is then inserted
into the eye and the cataract suctioned out through a device
called an IA (irrigation-aspiration) probe. Once all this
material is removed, the eye is ready for the insertion
of the intraocular lens (IOL). IOLs are plastic discs
that replace the natural lens and come in numerous styles,
including foldable and multifocal IOLs. The strength of
the implants is expressed in diopters, a measurement
of refractive power. A diopter is equal to the reciprocal
of the focal length of a lens (in meters). For example,
a 2-diopter lens brings parallel rays of light into a focus
at 1/2 meter.
- No-stitch cataract surgery
uses a vertical corneal incision rather than the traditional
horizontal incision to access the lens. Because the cornea
seals itself when this type of incision is used, no stitches
are needed to close the wound and the risk of infection
and postoperative complications is reduced.
Secondary Cataract Surgery
The treatment for secondary cataracts is YAG
capsulotomy. The YAG laser is a surgical instrument
that emits a short pulsed, high energy light beam that can
be precisely focused by computer to cut, vaporize, or fragment
tissue. With the YAG laser, the opacified posterior capsular
tissue is vaporized with carefully controlled pulses of light.
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Read the operative reports on our procedure
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