|
Myopia
(Short-sightedness): When the rays of
light are focused in front of the retina.
Need concave or – (minus) glasses for
correction.
Hypermetropia (Far-sightedness): When
the rays of light behind the retina. It is
corrected by convex or + (plus) lenses.
Astigmatism: Here the rays of light
are focused in an irregular fashion due to
uneven focusing power around the eye. This
requires cylindrical lens oriented at a
specific axis for its correction.
Presbyopia: It is not a true
refractive error but a weakness of ability
to focus for near objects. This is a normal
aging phenomenon and manifests at about
40-45 years of age. It is corrected by using
convex or + (plus) lenses / power which is
ADDED to the distance power of the person. |
|
What causes it? |
|
Most refractive
errors are inherent / inherited conditions
caused by following mechanisms:
Axial: The eyeball is longer (in
myopia) or shorter (in hypermetropia) than
normal.
Curvature: Abnormal curvature of
cornea or lens of the eye.
Index: Changes in the refractive
index of the optical parts of the eye
Presbyopia: is caused by reduction in
flexibility of the lens of the eye thereby
reducing its ability to focus for near
objects. This is part of normal aging
phenomenon.
Refractive errors are nor caused by
excessive reading, watching TV, working on
computers or any dietary deficiencies. |
|
What are the symptoms & signs? |
|
Often refractive
errors (especially low numbers) do not cause
any symptoms or signs and get picked up on
routine eye examination. However, following
are the common symptoms seen:
-
Blurring of
distance vision seen in myopia &
significant astigmatism
-
Blurring of near
vision is noticed in Presbyopia and high
Hypermetropia
-
Eye-strain and
discomfort seen commonly with
hypermetropia & astigmatism
-
Squint may be
seen with high hypermetropia in children
-
Redness,
watering, pain in eyes, headache, etc.
may occur with any of the refractive
errors.
|
|
How is it diagnosed? |
|
Refractive errors
are diagnosed with the help of following
tests:
-
Visual Acuity
Assessment: is measurement of vision
or ability of eye to seen distant small
objects with the help of special
illuminated charts (Snellen’s Chart) or
Projected Charts. The visual acuity is
denoted by a fraction numerator /
denominator. Where numerator denotes the
distance (in meters or feet) at which
the subject reads the chart and
denominator is the farthest distance (in
meters or feet) at which a normal person
would be able to read the same line.
Normal visual acuity is 6/5, 6/6 or 6/9.
-
Refraction
Assessment: This is the measurement
of refractive error or spectacle power
of the eye. This is done by the
conventional method of Retinoscopy
or by the automated instrument
called Autorefractometer (the so
called computerized vision testing). The
number so obtained is verified by
placing trial lenses in a frame worn on
patient’s eyes – trial frame or
phoropter.
In special
situations it is required to instill some
eye-drops in the patient’s eye to dilate the
pupil and to relax the inner eye muscles (ciliary
muscles) in order to measure the accurate
refractive error – Cycloplegic Refraction.
This is required especially in children and
young adults or an unexpected increase in
refractive error is observed or where the
pupil is too small for accurate measurement.
The patient is called once a gain after a
few days for the final prescription of
spectacles – Post-Cycloplegic Test. |
|
What is the treatment? |
|
The refractive
errors are corrected with the help of:
-
Spectacles
-
Contact Lenses
-
Surgery
The spectacles are
the oldest, the commonest, the safest but
not the cheapest way of correcting
refractive error. The cumulative cost of
spectacles to be used by a person in his
lifespan can be exorbitant ! |
|
What are the surgical options? |
|
Fortunately there
are some dependable and permanent surgical
treatment for refractive errors:
There is no
satisfactory solution for presbyopia till
date, however, lens replacement surgery with
implantation of multifocal IOLs is probably
the best available option. |
|
What are the outcomes? |
|
|
|
What are the complications? |
|
|
|
What is the time course? |
|
|
|
What is the expense? |
|
The cumulative cost
of spectacles over a person’s lifespan can
be mind-boggling. Even the contact lenses
turn out to be more expensive when compared
to the surgical options. |
|
What is Pseudo-myopia? |
|
Prolonged near work
in the form of reading or working on
computers can lead to a state of spasm of
inner eye muscles (ciliary muscles). This
causes blurring of distant vision and gives
a false appearance of myopia in the eye.
When eyes are tested after dilating the eye
this pseudo-myopia disappears. This (element
of) myopia is not to be treated with glasses
but rather by proper visual hygiene and
regular eye exercises. |
|
Author: Dr.
Sanjay Dhawan
Updated on: 25 November 2008 |
