REFRACTIVE ERRORS AND TREATMENT
What is refractive error?
The function of the eye
see clearly the objects around us. The inability of the eye to
accurately focus the rays of light coming from distance on the retina is
called refractive error. This condition may be either because the eye is
too short or long in length, or because the cornea or lens does not have
the required refractive power. There are three types of refractive
this is the condition in which the eye is too long and
the light is focused in front of the retina. Distant objects are blurred
but the near objects are seen clearly. The eye has too much optical
power and to correct it the optical power is reduced by either minus
glasses or contact lenses, or by surgery.
this is the condition in
which the eye is too short and the light is focused behind the retina.
The eye has less optical power than is needed. When young the eye can
use the lens within the eye to compensate, but reading glasses are
needed at a relatively early age. Later, distance glasses (plus) are
needed as well, such that glasses for distance and near are required.
this is the condition where the eye
does not focus the light evenly, usually due to the cornea of the eye
being more curved in one direction than the other. It may occur on its
own or may be associated with myopia or hypermetropia.
What is Presbyopia?
Presbyopia is the normal aging process, where the lens progressively
loses its capacity to increase its power for near vision (loss of
accommodation). The distance vision may be normal, but the near vision
becomes blurred with age greater than about 45 years. This is corrected
by wearing reading glasses (plus) for the near work. This condition may
occur in itself or may be present alongwith pre-existing myopia,
hypermetropia or astigmatism.
What are the ways of treating
The refractive errors may be treated by either of the following ways:
- Glasses: this is the simplest and most popular way of
correcting refractive errors
- Contact lens: this option is cosmetically much better
though the maintenance and cleaning of contact lenses may be
cumbersome. There is also a slight risk of infection to the cornea,
if the contact lenses are not cleaned properly before use.
- Refractive surgeries:
these are increasingly becoming popular with the advent of more
predictable laser treatments.
What are the various
The refractive power of the eye can be changed by any of the three
- Changing the curvature of the cornea: this is the most
popular mode of refractive surgery.
- Removing the natural lens and replacing it with an
artificial lens of adequate power: it is similar to a routine
except that it is done in a clear lens and not a lens with cataract.
As it is an invasive procedure and it also increases the chances of
retinal detachment in eyes with high myopia, it is not recommended
- Putting an additional artificial lens within the eye on
top of the existing natural lens: this technique is still not very
popular as it is also invasive and may increase the chances of
What are the ways of changing the curvature of cornea?
The commonly used methods for refractive surgery are:
- PhotoRefractive Keratotomy (PRK) : It is being used less
- Laser-Assisted In-Situ Keratomileusis (LASIK) :This is
the most popular form of Laser treatment.
- LASEK (or Epi-LASIK) :This newer form of treatment may be
suitable in some selected patients with very high power.
What is Excimer laser?
Excimer laser is a far Ultra Violet (UV) light energy of wavelength 193
microns. It is invisible to human eye. This laser breaks the chemical
bonds within the molecules (photoablation) with minimal thermal damage
to the surrounding tissues. Thus it is very precise and can remodel the
cornea with an accuracy of more than a thousandth of a millimeter.
How does Excimer laser correct the refractive error?
The effect of excimer laser on the cornea is very similar to grinding
of a glass lens to change its refractive power. In eyes with refractive
errors, excimer laser because of its high level of precision, can change
the shape of the cornea to change its refractive power to the desired
state and thus correcting the refractive error.
central part of the cornea is made flatter, and thus decreasing the
refractive power of the cornea and of the eye, and thus correcting the
Similarly in hypermetropia
the laser removes a ring of tissue from the peripheral part of the
cornea and thus makes the central part steeper and corrects the
As we know, in astigmatism
the cornea is more curved in one direction. Excimer laser can correct
astigmatism also by selectively ablating the cornea in the required
What is LASIK?
LASIK involves putting the PRK treatment not on the surface of the
cornea, but under a protective corneal flap. A very thin (about 0.16 mm)
and precise flap is raised by a special instrument known as
microkeratome. The result is a corneal flap attached at one edge, the
hinge. The surgeon folds the flap to expose the inner stromal layer of
the cornea. The excimer laser treatment is applied on this stromal bed
to remodel it. After this the flap is repositioned to its original
position and it does not require any suture. Since the corneal
epithelium has only been minimally disturbed, there is only mild
discomfort after the procedure.
What is Customised LASIK?
This is a special form of LASIK in which the treatment parameters are
customised for the particular patient, based not only on the refractive
error, but also on the corneal map of the eye and other findings
detected by special tests. This procedure tries to correct aberrations,
maintains normal shape of the cornea and gives better night vision.
Who is a suitable candidate for LASIK laser surgery?
The person must be 18 years or older with a stable power. A contact
lens user must discontinue the use of contact lenses at least 2 weeks
before the procedure. A detailed eye checkup is done to look for
suitability for the procedure. Before the Laser is done the eyes are
checked with special machines to determine the exact power, the corneal
mapping is done and corneal thickness is measured. In patients with high
minus power, a special retinal checkup is done for detecting any
possible weak areas in the retina, which may need to be treated before
the LASIK is performed. The LASIK laser is performed only after ruling
out any contraindication and confirming the suitability of the
What happens during the Laser surgery?
The LASIK surgery is done as an outpatient procedure and does not
require any admission. It is painless and is done after putting the
anesthetic drops and does not require any injections. The laser
procedure takes approximately 15-20 minutes for both eyes. After the
procedure, the patient can go back home after 20-30 minutes.
What are the complications of LASIK?
LASIK is a very safe procedure with a majority of patients achieving
very good results. However, since it is a surgical procedure, it does
carry some chances of complications as well, which would be discussed
with you before the surgery. The overall rate of significant
complications in LASIK is only of the order of 1-2%. Some of these
complications may be:
What are the precautions to be followed after LASIK, and
when can one resume work?
- Undercorrection or Overcorrection
- Glare and difficulty in night driving
- Flap complications, perforation
- Scarring of the cornea
After LASIK surgery one needs to avoid using cosmetics in and around
the eye for a week or two. One should also avoid wetting or rubbing the
eyes for some period. Use the medications regularly and report
immediately in case of any discomfort, redness, injury or any other
problem. There is no restriction in reading, watching TV, going for
walks etc. In majority of cases, routine office or simple household work
may be resumed in a day or two.
How are the results of refractive surgery?
Before undergoing any refractive surgery procedure, one must realize
that none of these procedures can guarantee you perfect vision without
glasses in all the cases. These surgeries are based on the average
calculations from a large population, but as different individuals may
respond differently to surgery, there might be some variation in the
outcome results obtained. The aim is to decrease your dependence on
glasses/contact lenses. The reliability of the procedure is quite good
in mild to moderate levels of refractive errors, with most of people
being able to carry out their daily activities without glasses. But for
high degrees of refractive errors, the variation is more and some people
may still need glasses, though of much lesser power than before, to see
If my refractive error is not corrected fully, can I go
in for a repeat procedure?
If there is a significant residual refractive
error, then your surgeon might advise you to undergo a repeat surgery at
an appropriate time. In LASIK the same flap can be raised again and the
laser treatment applied as before.
Can I get my reading glasses (presbyopia) removed by
As we know,
the inability of natural lens to change its power according to the need
of the person. None of these procedures described can help in this.
There are a few techniques being developed to correct this problem, but
they are in the stage of investigation and as of today, are not
What is the minimum age before which refractive
surgeries are not done?
None of the refractive surgeries are performed till the power of the
eye becomes stable. This generally occurs after 18 years of age, when
the eyeball stops growing. Even after this age, the refractive error
should be stable over a period of few months before a surgery is
I am a contact lens wearer. What additional
precaution must I take before going for refractive surgery?
Since the contact lens can change the curvature of cornea, it is
advised to discontinue use of contact lenses at least 2 weeks before the
What are the situations in which refractive surgery is
Refractive surgery may not be considered appropriate in the following
conditions, as there may be additional risk or poor outcome may be
- Age below 18 yrs
- Unstable refraction in adulthood
- When the person has only one eye with good vision
- Keratoconus (conical cornea)
- Severe dry eye disease
- Significant ocular abnormality
- Those who are inappropriately motivated, who have
I have a high degree of myopia. I was told that because
of high myopia, my retina is 'weak' and I am at a risk of getting
retinal detachment. Will correction of myopia by LASIK lead to decrease
in risk of retinal detachment also?
No. You should remember that the treatment involves only reshaping of
cornea, so that the light gets focused properly on your retina. It does
not correct the 'weakness' of your retina and the risk of retinal
detachment remains. Even after refractive surgery you should continue
taking the special precautions and regular checkups with your
People who have minus number glasses
more than 6 diopter in power are said to have high or pathologic myopia.
The glasses number of such patients may even be as high as 15 to 20
diopters. The eyeball in such cases is enlarged leading to thinned out
coats of the eyeball so the central area may be very weak (chorioretinal
degeneration) leading to poor vision. The retina in these eyes is weak
in the periphery also and usually has some degeneration, atrophic holes,
or even retinal tears. These retinal holes or tears may sometimes lead
to a serious condition of retinal detachment, leading to sudden loss of
vision, and may require major surgery urgently to settle the retina.
So a regular retinal checkup is very essential in these patients to
look for the holes and to treat them with laser or else they might lead
to a retinal detachment
needs surgery. Also, whenever the patient with high number develops
symptoms like flashes or floaters
he/she must immediately get the eyes examined to look for retinal hole,
tear or retinal detachment.
High myopes are also predisposed to develop abnormal new vessels in the
central area (macula) called Choroidal Neovascular Membrane (CNVM),
which leads to sudden marked fall in vision due to hemorrhage or fluid
leakage. The treatment modalities being tried out for myopic CNVM are
anti-VEGF agents like Lucentis,
Avastin and Macugen
with verteporfin, Transpupillary
Thermo Therapy (TTT)
laser and surgical options like macular