added to the corrective prescription, that accom-
modates the limited ability of the lens to focus on
near objects (such as when reading).
Eyeglasses
Eyeglasses are plastic resin or polycarbon, and less
commonly glass, lenses ground to the thicknesses
and shapes necessary to achieve the desired
refractive specifications. Because eyeglasses are
external to the eye, they can correct for a broad
range of refractive errors and are the most com-
mon means of refractive correction. Eyeglasses
also can contain tints and dyes that change their
color; some have additives that provide protection
from ultraviolet light. About 85 percent of people
who have refractive errors of vision wear eye-
glasses to correct them.
Bifocal and trifocal eyeglasses have a clear shift
(sometimes visible as a line on the lens) to the
presbyopic correction; a progressive lens transi-
tions to the presbyopic correction. Reading glasses
such as those available without an eye care practi-
tioner’s prescription, are magnifying lenses that
enlarge close objects, requiring the lens to make
less of an adjustment to bring them into focus.
How well reading glasses work depends on
whether there are refractive errors that remain
uncorrected. With aging, most people develop at
least a small degree of astigmatism, which can
result in blurred or distorted images not related to
presbyopia.
The primary risk of wearing eyeglasses is trau-
matic injury due to a blow that strikes the glasses.
The energy of such a blow concentrates initially at
the contact points on the NOSE. The frame may
break, causing lacerations to the face. Of more sig-
nificant consequence is a blow that breaks the
lens, which can result in vision-threatening injury
to the eye. Polycarbonate lenses have the highest
inherent shatter resistance; plastic resin and glass
lenses should have shatter-resistant coatings or
additives. People who engage in physical activities
such as ball sports should wear polycarbonate eye-
glasses or custom protective eyewear.
Contact LensesContact lenses fit directly onto the eye, covering
the cornea. There are two basic kinds of contact
lenses in use today: gas permeable (hard) and
hydrophilic (soft). Gas-permeable contact lenses
float on a layer of tears over the center of the
cornea and often are the contact lens of choice to
correct for moderate to significant astigmatism as
well as KERATOCONUS, a condition in which the
cornea’s center bulges outward. Gas-permeable
lenses also can correct for mild to moderate
myopic and hyperopic refractive errors. Made of
rigid polymers of fluorocarbon and polymethyl
methacrylate, gas-permeable lenses allow oxygen
molecules to pass through but do not absorb mois-
ture from the eye. Hydrophilic contact lenses
cover the entire cornea and can correct for mild to
moderate myopia and hyperopia. Soft and flexi-
ble, hydrophilic lenses contain a high percentage
of water and draw additional moisture from the
tears to remain hydrated. A special kind of
hydrophilic lens, the toric lens, is necessary to cor-
rect for astigmatism. A toric lens has varying
thicknesses that compensate for corneal irregulari-
ties to correct refraction.
Contact lenses can incorporate correction for
moderate presbyopia, though this tends to be a
less satisfactory approach than eyeglasses. There
are two methods for accommodating presbyopia
with contact lenses: progressive or bifocal lenses
and monovision. Progressive or bifocal contacts
function much the same as progressive or bifocal
eyeglasses, with the lower portion of the lens con-
taining the presbyopic correction. Because contact
lenses shift position on the eye with blinking and
when the wearer alters the angle of the head
(such as when lying down), the presbyopic correc-
tion may not remain in an effective position.
Monovision takes the approach of modifying the
BRAIN’s interpretation of visual signals. One eye,
usually the dominant eye, wears a contact lens
with the refractive correction. The other eye wears
a contact lens with the presbyopic correction. The
brain learns to distinguish which signals to inter-
pret, accepting those from the dominant eye dur-
ing normal visual activities and those from the
other eye when reading or doing close-focus
work.
The primary risks of wearing contact lenses are
damage to the cornea and INFECTION. Even
hydrophilic lenses can irritate the cornea andcorrective lenses 83