 |
|
|
Cataract surgery may allow you to
see distant and near objects with a reduced need for glasses,
bifocals, or reading glasses. |
|
See Both Near and Far
During cataract surgery the cloudy
natural lens within the eye will be replaced with a clear
artificial intraocular lens (IOL). Single-focus IOLs, new
technology IOLs, and reliable monovision techniques may
allow you to see both near objects and distant objects
without the need for glasses, bifocals, or reading glasses.
Only The Doctor Knows
This article will discuss options
available to most cataract patients and the respective
advantages and disadvantages of each.
Although there are many options to see
near and distant after cataract surgery, only a discussion
with a competent doctor who has completed a comprehensive
examination can determine the most appropriate choice.
Near Vision, Mid-Distance, and Distant Vision
To understand the options for cataract
surgery it is important to understand how the eye works for
focused vision at different distances. When at rest, a
normal eye is focused for distant objects (about 20 feet to
infinity). To see objects closer than 20 feet, the eye needs
to change focus.
When young, the natural crystalline lens
within the eye is able to change shape and thereby change
focus easily and for a wide range of focus. This is called
accommodation.
At about age 10, the natural lens begins
a very slow process of becoming less and less able to
accommodate. This is called presbyopia. Presbyopia normally
does not affect near vision until after age 40. This is when
reading glasses or bifocals become necessary to see near
objects. There is no "cure" for presbyopia.
As a cataract forms the lens becomes
even less able to accommodate, however most people who
develop cataracts have been fully presbyopic for quite some
time and have been using readers and bifocals for years.
A monofocal IOL is an intraocular lens
with a fixed focus for one distance. A cataract doctor may
select monofocal IOLs that are for near focus, for
mid-distance focus, or for distant focus. Only one of these
three can be selected and the focus will not change after
surgery.
Traditional cataract surgery would
provide the focus of both eyes for distant objects and the
patient would use reading glasses to see near objects. The
vast majority of cataract patients elect to have monofocal
IOLs in both eyes set for distance vision and then use
reading glasses to see objects near.
Monofocal IOL Advantages
A major advantage of a monofocal IOL is
that 100% of the light entering the eye and passing through
the IOL reaches the retina, with all of that light focused
the same. A monofocal IOL will provide the very highest
quality of vision, however only for one distance.
Monofocal IOL Limitations
Glasses would be required to see
distances that the monofocal IOL does not focus.
Monofocal IOL Summary
If superior vision quality is a
requirement and using reading glasses is not a concern, then
monofocal IOLs set for distance vision may be the most
appropriate choice.
Methods to See Near and Far
There are three primary methods for a
person to see near and distant after cataract surgery:
Monovision
Multifocal IOL
Accommodating IOL
Monovision is a process of setting the
focus of one eye for near objects and the focus of the other
eye for distant objects. The brain then combines the two
images. There is normally no extra charge for monovision
correction.
Multifocal IOLs include the ReZoom and
ReSTOR. The only accommodating IOL currently approved for
the United States is the Crystalens. The ReZoom, ReSTOR, and
Crystalens are collectively referred to as "premium IOLs".
There is almost always an extra charge for premium IOLs.
Everything is Compromise for Near and Far
Premium IOLs are a significant
improvement on IOLs of only a few years ago, but not every
type of IOL is best for every person. Monovision has been
around for years, but not everyone can contend with its
effect.
All methods to be able to see both near
and far have a trade-off and none will restore vision to
what it was when the patient was in his or her 20s. The
probability of never needing corrective lenses for vision at
all distances is relatively low, however a reduced need for
corrective lenses for a larger range of vision distances is
reasonably probable.
The concept of monovision is very
simple. One eye is corrected for near vision and the other
eye is corrected for distance vision. The brain decides
which eye to use and when.
A monofocal IOL to provide distance
vision would be implanted in the dominant eye. A monofocal
IOL to provide near vision would be implanted in the
non-dominant eye. For many, monovision will reduce or
eliminate the need for readers, bifocals, or trifocals.
Monovision Advantages
The chief advantage of monovision is the
freedom it can provide from reading glasses. After six to
eight weeks the brain makes the vision changes
automatically, without any conscious effort or awareness.
Monovision makes it possible to repeatedly change the range
of focus, without having to constantly remove or add
corrective lenses.
Monovision correction after cataract
surgery uses monofocal IOLs. The very highest vision quality
for each of eye will be achieved because 100% of the light
entering the eye through the monofocal IOL will be focused
for a single distance.
If monovision seems desirable, it may be
achieved with contact lenses prior to cataract surgery to
simulate the effect. In this manner a cataract patient is
able to try monovision before making a decision about
surgery.
Monovision Limitations
Some people cannot stand the monovision
effect of one eye for distant focus and one for near. About
one-third of those who try monovision do not like it.
Monovision may cause a decrease of depth
perception. A pair of distance glasses may be needed when
superior depth perception is required, such as driving at
night.
Blurred vision in the "near" eye may be
noticed when looking at a distant object with the "distant"
eye blocked, and visa-versa.
With both eyes open the patient may
experience a blur effect and
that can present as mild halos and provide less than sharp
vision, however closing one eye or using full correction
glasses will commonly eliminate this.
Vision problems caused by cataracts,
such as hazy vision, color changes, etc., will not be
corrected when trying monovision contacts. A monovision
trial in contacts may provide an idea of its effect, but
will not be exactly the same as when a clear lens has
replaced the cloudy natural lens.
Monovision Summary
If reading glasses are a hassle, mild
halos are acceptable, and you are able to tolerate the
imbalance between eyes, then monovision with monofocal IOLs may be the
most appropriate choice.
An accommodating IOL is designed to
change focus. When an accommodating IOL works well, a
patient is able to see distant objects and is also able to
change focus to see near objects. The only accommodating IOL
approved for the United States is the Crystalens. Click for
large pop-up
animation of the Crystalens accommodating IOL.
Accommodating IOL Advantages
An accommodating IOL mimics the
accommodation originally provide by the young crystalline
lens.
Like the
monofocal IOL, an accommodating IOL, allows 100% of the
light entering the eye to have the same focus. When
accommodating, that focus is able to change.
Accommodating IOL Limitations
Accommodating IOLs have a rather limited
range of maximum possible accommodation. The greatest
possible range of accommodation of the Crystalens is about
2.00 diopters. This would be about the same as using +2.00
power reading glasses.
Accommodating IOLs do not always work
and do not always work well. Sometimes they are unable to
accommodate at all. Sometimes they only provide a very small
amount of accommodation, such as 0.50 diopters. This would
be like +0.50 reading glasses.
It is difficult to predict if an
accommodating IOL will work with the patient’s eye structure
and will be able to accommodate until it is implanted.
If the accommodating IOL is implanted
and is not able to accommodate, you effectively have an
expensive monofocal IOL.
Accommodating IOL Summary
If reading glasses are a hassle, halos
are unacceptable, and the possibility that you may end up
with little or no actual accommodation is acceptable, then
accommodating IOLs may be the most appropriate choice.
Multifocal IOLs are lenses that have
rings of focus power variations. Think of a bullseye target
with different rings. Some rings provide focus for near
objects, some for mid-range objects, and some rings provide
the focus power for distant objects. Click to see
large image
of multifocal IOL.
Multifocal IOL Advantages
Because light is focused for near,
mid-distance, and distant objects, multifocal IOLs can
provide a full range of vision with a reduced need for
corrective lenses.
The transition from one ring to the next
can create blur that presents as halos around objects,
especially light emitting objects and especially at night.
Not all persons are bothered by this halo effect, but it is
a consideration.
Only a portion of the light entering the
eye through a multifocal IOL is focused for a particular
distance, and the remaining light is out of focus. As an
example, let’s assume a particular multifocal IOL has 33.3%
of the light passing through it focused for near
objects, 33.3% of the light for mid-range objects, and 33.3%
for distant objects. If you are looking at a near object and
your pupils are naturally dilated, such as in a low light
environment,
only 1/3rd of the light reaching your retina is fully
focused, with the remainder being out of focus. No matter
what you are looking at, when/if your pupil is large for low
light environments, 2/3rds of the light entering your
eye is out of focus.
The mix of focused light and
out-of-focus light all entering your eye at the same time
can create a blur effect and that can present as halos and
provide less than sharp vision. Contrast sensitivity
(ability to differentiate shades) may also be reduced.
ReZoom is very much like the example of
a lens in Multifocal
IOL Limitations with about equal areas for near, mid,
and distant vision. A balance of all three distances is
provided to give the most focused and greatest depth of
field. Click to see large
image
of ReZoom multifocal IOL.
The ReSTOR lens uses more light focused
on near and distant objects at the expense of mid-distance
vision quality. As an example the lens may have 40% near
vision, 15% mid vision, and 45% distance vision. These
numbers are not accurate (exact amounts are proprietary)
however a common complaint of the ReSTOR lens is loss of
mid-distance quality. You may be in the grocery store and
you can read the can of beans in your hand, see your car in
the parking lot, but you can’t see the items on the shelves
very well.
Like the example used for the ReZoom
multifocal IOL, no matter what you are looking at, when/if
your pupil is large for low light environments, the majority of the light entering your
eye is out of focus. Click to see large
image
of ReSTOR multifocal IOL
Some cataract doctors recommend the
ReZoom in one eye and the ReSTOR in the other. This is a
modified version of monovision and
an attempt to resolve the limitations of each lens with a
combination of the two. Results of this method have been
mixed.
The importance of pupil size with
multifocal IOLs cannot be overstressed.
The iris tends to constrict the size of
the pupil when looking at a near object. Pupils become small
in brightly lit environments and large in low light
environments. All multifocal IOLs rely on the size of the
pupil to help determine how much light from the different
powered focus rings enters the eye to be seen.
Our example multifocal IOL has the
majority of its near vision power in the center with
distance vision power in rings at the outer edge. If the
pupil constricts enough to block the distance vision portion
of the multifocal IOL thereby allowing only light from the
near vision portion of the IOL to enter the eye, then 100%
of the light entering the eye is focused on the near object.
This would provide the best possible near vision with a
multifocal IOL.
If the pupil does not constrict enough
when looking at a near object, then out-of-focus light from
the distance vision portion of the multifocal IOL would
enter the eye and combine with the light from the near
vision portion. This mix of near focus and distance focus
may provide blur and less clear vision quality.
Using the same example of a multifocal
IOL with near vision in the center and distance vision at
the outer edge, if the pupil does not naturally dilate
enough to allow light from the distance vision correction
outer edge of the IOL to enter the eye, then only light from
the near distance vision portion of the IOL will enter the
eye. In bright environments and in dark environments, a
person with a small pupil would only see through the near
vision portion of the multifocal IOL.
The size of the naturally changing
pupils under different light and vision conditions needs to
be matched to the type of multifocal IOL being proposed or
the patient may gain little or no benefit from the
multifocal ability of the IOL.
Multifocal IOL Summary
If reading glasses are a hassle, halos
would not be bothersome, pupil dilation and constriction are
consistent with the multifocal IOL, and being able to see adequately at
many distances is more important than seeing very well at
only one distance, then multifocal IOLs may be the most
appropriate choice.
|
