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Overview
Refractive errors are the most common eye
disorders. Most people have a refractive error,
but it is usually too small to affect their vision
significantly. For the eye to see clearly, light
rays must be bent, or refracted, by the cornea and
the lens and
focused on the retina. The
retina receives the light rays and converts them
to neural signals that are transmitted by the
optic nerve to the brain, where the signals are translated
into images.
Like a camera, the human eye must be properly
focused to see an image clearly. If light does not
bend or refract correctly and focus directly on
the retina, the result is blurred vision, or a
refractive error.
The four most common refractive errors are
-
myopia,
or nearsightedness,
-
hyperopia,
or farsightedness,
-
astigmatism,
and
-
presbyopia.
It is possible to have more than one refractive
error, such as having both myopia and astigmatism.
The loss of visual acuity that results from a
refractive error is measured by numbers. For
example, 20/20 generally describes seeing clearly
at 20 feet away what normally should be seen at
that distance. People with 20/80 vision must be as
close as 20 feet to clearly see objects that those
with normal vision can see from a distance of 80
feet.
Myopia
A myopic, or nearsighted person has difficulty
seeing distant objects clearly, such as road
signs, blackboards in a classroom, or scoreboards
at a sporting event. Myopia varies in its
severity. Extremely nearsighted people may only
have clear vision up to a few inches from their
eyes; mildly nearsighted people may not even be
aware of the disorder.
Myopia develops during childhood. The child’s
eyeball grows too long, which interferes with the
light-focusing mechanism. Rays of light from
distant objects focus in front of the retina
rather than on the retina, resulting in blurry
vision. This can also occur if the cornea is
curved too acutely.
Incidence and Prevalence
Myopia is fairly common worldwide. Approximately
25% of the U.S. population is myopic to some
degree, affecting men and women equally. There is
greater prevalence of this disorder among children
from higher socioeconomic groups, although the
reasons for this remain unclear. Incidence is much
higher in certain Asian countries. Taiwan
experiences the world’s highest rate. In some
nations, such as Singapore, the incidence has
rapidly increased over the past decade. In 1990,
one in five grade school students there was
diagnosed with myopia, and by 2000 that number had
risen to one in three.
Risk Factors
While the exact cause of myopia remains unknown,
it is believed that an interaction of heredity and
environment may play a role. It tends to run in
families. When both parents are nearsighted, their
children have a greater than average chance of
developing myopia. Environmental factors such as
reading in dim light or doing excessive amounts of
close work may contribute to myopia.
Signs and Symptoms
Symptoms of myopia usually are first noticed in
childhood. Myopic children may hold their books
very close to their face or be unable to read the
blackboard at school. They may squint and complain
of headaches and eyestrain. These signs often
alert a teacher or parent that the child is having
vision problems and that an eye exam is needed.
Diagnosis
A diagnosis of myopia can be made only after a basic eye examination
performed by an ophthalmologist
or an optometrist.
The initial diagnosis of myopia is usually made by
having the patient read letters from an eye chart.
Other tests are performed to determine the degree
of myopia.
Treatment
Treatment for nearsightedness is most commonly
prescription eyeglasses
or contact lenses.
Wearing the glasses or contact lenses allows the
patient to experience normal or at least
dramatically improved, vision.
Myopia often worsens progressively during
adolescence, and the eyeglass or contact lens
prescription may need to be adjusted periodically.
However, myopia tends to stabilize once the
patient is in their twenties.
A number of surgical procedures are used to
correct refractive errors, including radial keratotomy, photorefractive
keratotomy, clear lens replacement
therapy, and LASIK.
These procedures reshape the central area of the
cornea. In many cases, correction makes eyeglasses
unnecessary. Not every patient is a good candidate
for surgical correction. Surgery is not
recommended for people under the age of 18 because
their eyes are still growing.
Prevention
There is no way to prevent myopia. Avoiding
eyestrain, reading and studying in a well-lit
room, and taking breaks from working in front of a
computer screen or doing close work may help. Eye
health can be supported by the following:
-
Eating a healthy diet that is rich in vitamins A
and C
-
Protecting the eyes from excessive amounts of
ultraviolet light by wearing sunglasses when
outside
-
Drinking adequate fluids to prevent eye dryness
-
Protecting the eyes when working with hazardous
or caustic substances
-
Having regular eye exams, especially if at high
risk for eye disease (e.g., diabetic)
Hyperopia
Hyperopia, or farsightedness, is a disorder where
distant objects are usually seen clearly, but
close ones do not come into proper focus and are
blurred. This occurs when the eyeball is too short
or the cornea is
too flat, and light rays entering the eye focus
behind the retina rather than directly on it.
Hyperopia is often present at birth, but
sometimes, vision normalizes as the eyeball
lengthens during the growth process. Young adults
and children with mild to moderate cases of
farsightedness often see close objects clearly
because the lens is
able to adjust or change its shape. This process
is called accommodation. As a person ages,
the ability to accommodate often lessens and
eyeglasses or contact lenses may be needed.
Farsightedness is a risk factor for closed-angle
glaucoma.
Therefore, patients with hyperopia should discuss
glaucoma testing with their eye care practitioner.
Incidence and Prevalence
Approximately one-quarter of the U.S. population
is hyperopic, and incidence increases with age. At
least half of all persons over the age of 65 have
some degree of hyperopia. Men and women appear to
be affected equally.
Risk Factors
Because it is often present at birth, there may be
a hereditary risk factor. Age is also a risk
factor for hyperopia.
Causes
Hyperopia results when the eyeball is too short or
the cornea is too flat and the lens is unable to
adjust its shape to see close objects clearly (the
process called accommodation).
Signs and Symptoms
Most children and young adults with mild to
moderate hyperopia do not experience symptoms
because of the lens’s ability to accommodate.
Older persons and those with more severe hyperopia
may experience these symptoms:
-
Achy feeling in eyes
-
Blurred vision of close objects
-
Eye strain
-
Headache
Diagnosis
A diagnosis of hyperopia can be made only after a basic eye examination
performed by an ophthalmologist
or an optometrist.
Treatment
Treatment for farsightedness is prescription eyeglasses
or contact lenses.
Wearing the glasses or contact lenses allows the
patient to experience normal, or dramatically
improved, vision. Many farsighted people wear
their glasses only when doing close work, such as
reading or sewing.
A number of surgical procedures are used to
correct refractive errors, including photorefractive
keratotomy, LASIK, and clear lens replacement
therapy. Some of these procedures are
appropriate for hyperopia, and in many cases,
surgical correction can make eyeglasses
unnecessary. Not every patient is a good candidate
for surgical correction. Surgery is not
recommended for people under the age of 18 because
their eyes are still growing.
Prevention
There is no way to prevent hyperopia. Eye health
can be supported by the following:
-
Eating a healthy diet that is rich in vitamins A
and C
-
Protecting the eyes from excessive amounts of
ultraviolet light by wearing sunglasses when
outside
-
Drinking adequate fluids to prevent eye dryness
-
Protecting the eyes when working with hazardous
or caustic substances
-
Having regular eye exams
Astigmatism
Astigmatism causes blurry vision when looking at
objects that are close (e.g., reading a letter)
and objects that are far away (e.g., reading a
road sign). Vision with astigmatism is not
necessarily uniformly blurry; some distances are
more out of focus than others.
The cornea is
normally smooth and uniformly curved on all sides.
In astigmatism, the cornea is irregularly curved.
Instead of being round, it may be shaped more like
a football. This irregular shape causes light to
scatter and to bend, or refract, improperly as it
passes through the cornea. Instead of focusing
directly on the retina, some light rays focus in
front of and some focus behind the retina. These
multiple focal points distort vision. In some
cases, an irregularly curved lens
produces minor degrees of astigmatism.
Astigmatism is usually present at birth and often
goes unnoticed until the child begins school or
starts to read. The condition may worsen over
time, but generally remains the same.
Incidence and Prevalence
Some experts believe that most people have some
degree of astigmatism. Some people experience no
symptoms, and others have severely impaired
vision. Astigmatism often occurs with
nearsightedness or farsightedness, or both.
Risk Factors
Because astigmatism is often present at birth,
many believe that there is a hereditary risk
factor. It also may be development as the child’s
eye grows.
Astigmatism can be acquired through an injury
that causes the lens or the cornea to become
distorted. This includes trauma and scarring, as
well as surgical procedures involving those
structures.
A condition called keratoconus may be a
risk factor for astigmatism. In keratoconus, the
cornea becomes thin and cone-shaped. This
condition may be inherited or result from chronic
eye rubbing. It is more common in women and
usually occurs in early adolescence
Diabetes
also may lead to astigmatism because high blood
sugar levels can cause changes in the shape of the
lens.
Signs and Symptoms
People with mild astigmatism may experience
headaches, eyestrain, fatigue, or blurred vision
at certain distances. Severe astigmatism produces
distorted images and blurring at all distances.
Diagnosis
A diagnosis of astigmatism can be made only after
a basic eye examination
performed by an ophthalmologist
or an optometrist.
A variety of tests may be used to diagnose
astigmatism. One that is commonly used is the
astigmatic dial, which shows a series of lines
radiating from a center. People with astigmatism
usually see some lines more clearly than others.
Treatment
Almost all degrees of astigmatism can be corrected
with prescription eyeglasses
or contact lenses.
Wearing the glasses or contact lenses allows the
patient to experience normal, or dramatically
improved, vision.
A number of surgical procedures
are used to correct refractive errors. Some forms
of astigmatism are surgically correctable. Surgery
is not recommended for people under the age of 18
because their eyes are still growing.
Prevention
There is no way to prevent astigmatism.
Presbyopia
A person with presbyopia has difficulty focusing
on close objects. It is similar to hyperopia,
but the underlying cause is different.
In presbyopia, the crystalline lens
has become less flexible and the eye muscles that
support the lens and allow it to accommodate have
weakened. The crystalline lens is composed like an
onion. The nucleus is surrounded by the cortex,
and the cortex is surrounded by the lens capsule.
The lens is an elastic structure that changes
shape, or accommodates, to focus on objects at
various distances. When focusing on distant
objects, the lens becomes flatter. When focusing
on close objects, it becomes rounder.
Symptoms may seem to occur suddenly, but
presbyopia develops over several years, as the
lens capsule loses its ability to expand as a
person ages. The result is that while distant
objects are seen clearly, close objects are
blurred because the lens cannot resume the shape
that it needs to bring them into focus.
Presbyopia can complicate other vision conditions.
For example, mildly farsighted people suddenly may
find that they need glasses. Prior to becoming
presbyopic, their eyes may have been able to
accommodate, avoiding the need for corrective
lenses. With the onset of presbyopia, they no
longer see close objects clearly.
Incidence and Prevalence
Virtually everyone experiences some degree of
presbyopia when they reach middle age. Most people
begin to notice presbyopia in their early to
mid-forties, although symptoms occasionally become
noticeable during the late thirties. The ability
to focus continues to decrease until about the age
of 55.
Risk Factors
Presbyopia is a normal part of ageing.
Signs and Symptoms
The first symptom of presbyopia is usually the
momentary blurring of distant objects that occurs
after doing close work. For example, if a man is
reading and then looks up and across the room at
his wife, her image is blurry for a few moments.
This occurs because the crystalline lens can no
longer rapidly change focus (accommodate) from
near to far. As time goes on, it takes longer to
refocus. Other symptoms include:
-
Blurry vision of close objects
-
Difficulty reading small print
-
Eye fatigue, especially when reading in poor
lighting or at the end of the day
-
Eyestrain and headache when doing close work
-
Holding reading material at arm’s length to see
it clearly
Symptoms can worsen in the morning or when the
eyes are tired. Poor lighting may exacerbate
symptoms.
Diagnosis
A diagnosis of presbyopia can be made only after a basic eye examination
performed by an ophthalmologist
or an optometrist.
Treatment
Treatment depends on the patient’s age, lifestyle,
occupation, and the presence of other eye
conditions. If focusing on close objects is the
problem, a pair of reading glasses may correct
vision satisfactorily. Those who have difficulty
focusing on both near and distant objects may need bifocals or
two pairs of glasses.
Monovision is another treatment option in which
the physician corrects one eye to see distant
objects and one to see near objects. This can be
done with either contact lenses, refractive surgery,
or implanted intraocular
lenses.
Between the ages of 45 and 65, presbyopia almost
always continues to cause changes in the eye,
necessitating new prescriptions for eyeglasses or
contact lenses. It is advisable to have an eye
exam every 2 to 3 years. More frequent visits may
be needed if changes in vision occur more rapidly.
Prevention
Presbyopia cannot be prevented |
