Indistinct vision most commonly is caused by errors of
refraction.
Doctors do not often have to deal with this problem
because patients usually are prescribed glasses by an
optometrist.
However, if a patient presents complaining of
visual problems, it is extremely important to ask the question:
“Is this patient’s poor vision caused by a refractive error?”
The use of a simple “pinhole” made in a piece of card will
help to determine whether or not there is a refractive error.
In the absence of disease the vision will improve when the pinhole
is used unless the refractive error is extremely large.
Eye with no refractive error
In an eye with no refractive error (emmetropia) light rays from
infinity are brought to a focus on the retina by the cornea and
lens when the eye is in a “relaxed” state.
The cornea
contributes about two thirds and the lens about one third to
the eye’s refractive power. Disease affecting the cornea (for
example, keratoconus) may cause severe refractive problems.
The rays of light from closer objects, such as the printed
page, are divergent and have to be brought to a focus on the
retina by the process of accommodation.
The circular ciliary
muscle contracts, allowing the naturally elastic lens to assume a
more globular shape that has a greater converging power. In young people the lens is very elastic, but with age the
lens gradually hardens and even when the ciliary muscle
contracts the lens no longer becomes globular.
Thus from the
age of 40 onwards close work becomes gradually more difficult
(presbyopia).
Objects may have to be held further away to
reduce the need for accommodation, which leads to the
complaint “my arms don’t seem to be long enough.”
Fine detail
cannot be discerned.
Convex lenses in the form of reading glasses therefore are
needed to converge the light rays from close objects on to the
retina.
People who wear glasses to see clearly in the distance may
find it convenient to change to bifocal lenses in their glasses
when they become presbyopic.
In bifocal lenses the reading
lens simply is incorporated into the lower part of the lens.
Therefore, the person does not have to change his or her
glasses to read.
However, details at an intermediate distance
such as the prices of items on supermarket shelves are not
clear.
A third lens segment can be incorporated between that
for distance above and that for reading below, creating a
trifocal lens. However, many people cannot cope with the
“jump” in magnification inherent in the use of these lenses.
This has led to the introduction of multifocal lenses in which
the lens power increases progressively from top to bottom.
People may also have problems adapting to this type of lens, as
peripheral vision may be distorted.
Refractive errors do not get worse if a person reads in bad
light or does not wear their glasses.
The exceptions are young
children, however, who may need a refractive error corrected to
prevent amblyopia.
Myopic or shortsighted eye
In the myopic eye, light rays from infinity are brought to a
focus in front of the retina because either the eye is too long or
the converging power of the cornea and lens is too great.
To achieve clear vision the rays of light must be diverged by a
concave lens so that light rays are focused on the retina.
For near vision, light rays are focused on the retina with
little or no accommodation depending on the degree of
myopia and the distance at which the object is held.
This is the
reason why shortsighted people can often read without glasses
even late in life, when those without refractive errors need
reading glasses.
A certain type of cataract (nuclear sclerosis) increases the
refractive power of the lens, making the eye more myopic.
Patients with these cataracts may say their reading vision has
improved.
Patients with an extreme degree of shortsightedness
are more susceptible to retinal detachment, macular
degeneration, and primary open angle glaucoma.
Hypermetropic or longsighted eye
In the hypermetropic eye, light rays from infinity are brought
to a focus behind the retina, either because the eye is too short
or because the converging power of the cornea and lens is too
weak.
Unlike the young shortsighted person, the young
longsighted person can achieve a clear retinal image by
accommodating.
Extremely good distance vision can often be
achieved by this “fine tuning” for example, 6/4 on the
Snellen chart and this has given rise to the term
“longsighted.”
For near vision the longsighted person has to
accommodate even more.
This may be possible during the first
two to three decades of life, but the need for reading glasses
arises earlier than in the normal person.
As the ability to accommodate (and thus compensate for
the hypermetropia) fails with advancing years, the longsighted
person may require glasses for both distant and near vision
when none were needed before.
This may result in the
complaint of a deterioration in eyesight because the patient has
gone from not needing glasses to needing them for both
distance and near vision.
Longsighted people are more susceptible to closed angle
glaucoma because their smaller eyes are more likely to have
shallow anterior chambers and narrow angles.
Astigmatic eye
Astigmatism occurs when the cornea does not have an even
curvature.
A good analogy is that of a soccer ball (no
astigmatism) and a rugby ball (astigmatism).
The curvature of a
normal cornea may be likened to that of the back of a ladle
and that of the astigmatic eye to the back of a spoon.
This uneven curvature results in an uneven focus in different
meridians, and the eye cannot compensate by accommodating.
Astigmatism can be corrected by a lens that has power in
only one meridian (a cylinder).
Alternatively, an evenly curved
surface may be achieved by fitting a hard contact lens.
Astigmatism can be caused by any disease that affects the shape
of the cornea; for example, a meibomian cyst may press hard
enough on the cornea to cause distortion.
Contact lenses
Contact lenses have become increasingly popular in recent
years.
There are several types, which can be grouped into three
categories.
● Hard lenses are made of polymethylmethacrylate (plastic
material) and are not permeable to gases or liquids.
They
cannot be worn continuously because the cornea becomes
hypoxic and they are the most difficult lenses to get used to.
Because of their rigidity, however, they correct astigmatism
well and are durable.
Infection and allergy are less likely with
this type of lens.
They are now less commonly prescribed, but
there are still many people who have been using this type of
lens for a long time with no problems.
● Gas permeable lenses have properties between those of hard
and soft lenses.
They allow the passage of oxygen through to
the tear film and cornea, and they are better tolerated than
hard lenses.
Being semi-rigid they correct astigmatism better
than soft lenses.
They are, however, more prone to the
accumulation of deposits and are also less durable than hard
lenses. Gas permeable lenses usually are used as daily wear
lenses.
● Soft lenses have a high water content and are permeable to
both gases and liquids.
They are tolerated much better than
hard or gas permeable lenses and they can be worn for much
longer periods. Both infection and allergy, however, are more
common.
The lenses are less durable, are more prone to the
accumulation of deposits, and do not correct astigmatism as
well as the harder lenses. Nevertheless, because they are so
well tolerated, they are the most commonly prescribed
lenses.
Certain types of gas permeable and soft lenses can be worn
continuously for up to several months because of their high
oxygen permeability, but the risk of sight threatening
complications is higher than with daily wear lenses.
Disposable lenses are soft lenses that are designed to be
thrown away after a short period of continuous use.
They are
popular because no cleaning is required during this period.
However, it is important that the lenses are used as
recommended, or the risk of complications, such as corneal
infection, rise substantially.
Indications for prescribing contact lenses
Personal appearance and the inconvenience of spectacles are
common reasons for prescribing contact lenses.
They also may
considerably reduce the optical aberrations that are associated
with the wearing of glasses, particularly those with high power
that are sometimes prescribed for patients who have had
cataracts removed.
The brain cannot resolve the large
difference in the size of the retinal images that occurs when
the refractive power of the two eyes differs considerably.
For example, this occurs when a cataract has been
removed from one eye and a spectacle lens has been prescribed
but the other eye is normal.
A contact lens brings the image
size closer to “normal,” permitting the brain to fuse the two
images.
If a person is very myopic, the use of contact lenses
rather than spectacles may increase the image size on the retina
and improve the visual acuity.
A contact lens can also neutralise irregularities in the
cornea and correct the effects of an irregularly shaped
cornea (for example, keratoconus or that which occurs after
corneal graft surgery).
Relative contraindications to contact lens wear
Contraindications include a history of atopy, “dry eyes,”
previous glaucoma filtration surgery, and an inability to handle
or cope with the management of lenses.
These are, however,
relative contraindications; a trial of lenses may be the only way
to determine whether it is feasible for a particular patient to
wear contact lenses.
Complications of wearing contact lenses
The most serious complication of contact lens wear is a corneal
abscess.
This is most common in elderly patients who have
worn soft contact lenses for an extended period.
Certain bacterial pathogens such as pneumococci or Pseudomonas
species can cause severe corneal damage and even perforation.
Other pathogens such as acanthamoebae can contaminate
contact lenses or contact lens cases and can produce a chronic
corneal infection with severe pain.
Acanthamoebae live in
tap water and it is important to instruct all contact lens wearers
to avoid rinsing their lens cases with tap water. Corneal
abrasions are also fairly common.
Chronic lens overuse can
lead to ingrowth of blood vessels into the normally avascular
cornea.
Any contact lenses wearer with a red eye should have
the contact lens removed and the eye stained with fluorescein
to show up any corneal abrasion or abscess.
As fluorescein
stains soft contact lenses, the eye should be washed out with
saline before the lens is replaced.
If there is an abrasion or
infection the appropriate treatment should be given, and the
contact lens should not be worn again until the condition has
resolved.
The wearing time may have to be built up again,
particularly if hard or gas permeable lenses are worn.
Good hygiene is essential for contact lens wearers, to
minimise the risks of infection.
Lenses should never be licked
and replaced in the eye.
Non-sterile solutions may contain
contaminants such as amoebae, which can lead to intractable
ocular infection.
Refractive surgery
There has been much interest in operations to alter the
refractive state of the eye, particularly operations to treat
myopia.
The technique called radial keratotomy entails making
deep radial incisions in the peripheral cornea, which results in
flattening of the central cornea and refocusing of light rays
nearer the retina.
It is only of use in short sight, and possible
disadvantages include weakening of the cornea (particularly if
the eye subsequently sustains trauma), infection, glare, and
fluctuation of the refractive state of the eye.
If contact lenses
are still required after radial keratotomy has been performed,
they are much more difficult to fit.
Surface-photorefractive keratectomy (S-PRK)
A special (excimer) laser has been used to reprofile the surface
of the cornea.
This laser works by vaporising a very thin layer of
the corneal stroma after the corneal epithelium has been
debrided (photoablation), which reshapes the front surface of
the cornea, changing its focusing power.
This technique, known as surface-photorefractive keratectomy (S-PRK), is safer
than radial keratotomy, as it does not involve deep cuts into the
eye. Side effects include:
● pain for a few days after the laser treatment
● haze-regression reactions (a period when the vision becomes
hazy, along with a tendency for the refraction to regress back
towards myopia again)
● overcorrection with a hypermetropic shift (often poorly
tolerated)
● corneal opacification caused by scarring of the treated zone,
which may result in a reduction of best corrected visual acuity
(usually transient) and glare.
Predictability of the final refractive result is poorer if the
patient is very shortsighted. (This is particularly the case if the
patient has more than 6 dioptres of myopia.)
Laser assisted in situ keratomileusis (LASIK)
More recently, a technique called laser assisted in situ
keratomileusis (LASIK) has been introduced.
This entails
cutting a superficial hinged flap in the cornea (about 160 to
200 µm thick) with an automated microkeratome, carrying out
excimer laser reshaping of the underlying corneal stroma, and
then replacing the flap.
Advantages of the technique over
surface laser treatment include more rapid stabilisation of
vision, reduced corneal scarring (with a definite reduction in
haze-regression reactions), and much better correction of
higher degrees of myopia.
Accuracy of LASIK is optimal up to
-6.00 dioptre sphere (DS), good up to -8.00 DS, and starts to
become increasingly less accurate over -10.00 DS.
Disadvantages include complications associated with the
technical difficulties of cutting and replacing the thin surface
flap, which occur in 1–5% of patients.
A recent modification of LASIK is LASEK, in which an
epithelial flap is raised prior to stromal ablation and then
replaced.
Other methods of altering the refractive status of the
eye include corneal intrastromal rings, phakic intraocular
lenses (intraocular lenses when the natural lens remains), and
small incision clear lensectomy.
Laser techniques can also be
used to correct astigmatism and hypermetropia, although these
are used much less commonly.
Possible complications of surgery
Patients who are contemplating any type of refractive surgery
should be fully informed of the risks by the operating surgeon
and given time to evaluate the advantages and disadvantages
before undergoing a procedure that may cause irreversible
change.
This is especially important as many patients will have
pre-operative best corrected visual acuities of 6/6 or better
(although they will need glasses or contact lenses to achieve
this vision).
It should be emphasised that the risk of
complications is low, but complications are potentially
devastating to vision.
Complications that may occur include:
● infection corneal infection is a rare problem associated with
all refractive procedures and can substantially reduce vision.
● corneal perforation this may very rarely occur in association
with technical problems with the microkeratome in LASIK.
● corneal flap problems there may occasionally be problems in
cutting or replacing the corneal flap in LASIK.
Flap
irregularites, subflap foreign bodies, unstable flaps, and flap
melts all have been reported.
Epithelial ingrowth under the
corneal flap is a rare complication.
● corneal ectasia photoablative procedures all reduce the
corneal thickness.
If too much corneal stroma is removed
then the cornea can progressively thin and become ectatic.
● regression of refractive outcome in some patients the cornea
undergoes a period of remodelling after refractive surgery,
with a tendency to drift back towards the original refractive
status.
● refractive under- or overcorrection this occurs where the
anticipated refractive correction does not occur.
Overcorrection of myopia to produce hypermetropia often is
tolerated poorly by the patient.
● corneal stromal scarring postoperative corneal stromal scarring
produces corneal haze, which produces optical aberrations
(reduced best acuity, glare, reduced contrast, and problems
with night vision).
● optic neuropathy very rarely, patients have been reported to
lose vision as a result of optic nerve damage after refractive
procedures involving cutting a corneal flap.
Optic nerve
damage may be related to the transient but very high rise in
intraocular pressure that occurs when the microkeratome is
applied to the eye.
● retinal detachment this serious complication may possibly be
caused by tractional forces exerted on the eye when the
microkeratome is used during refractive surgery.
