Refractive disorders involve focusing problems caused
when the eye fails to bend light for focusing on
the fovea of the retina. Technically refraction means
the bending of light but in medical terms it can
also refers to the method of measuring the power
of the lens needed.
Now, what does bending of light mean? Light slows
down when it travels from one medium to another
of greater density. All the light particles slow
down simultaneously when light hits a flat surface
at right angles. Hence, it continues to travel
in a straight line.
However, if light hits a surface at an angle, the
particles that first enter the new medium slow
down first, followed by those that follow. This
phenomenon results in the bending or refracting
of light. So, when light passes from the air into
the cornea of the eye (the two mediums have a huge
difference in density) it refracts and then focuses
onto the retina with the help of the lens of the
eye.
The eye needs to do a lot of accommodation in
order to refract light from various objects, angles
and distances. Hence, the lens of the eye must
accordingly increase its power to ensure focusing
adaptability.
The lens of the eye and its types
The power of the lens is measured in dioptres
(D). There are two types of lenses:
- A 'plus' lens
is convex and focuses light inwards
- A 'minus'
lens is concave and turns light outwards
The more the dioptre count of a lens, the stronger
its power for either plus or minus type.
Astigmatism
Astigmatism occurs when the cornea of the eye
is incorrectly shaped and fails to refract light
properly. Hence, the retina fails to form a clear
image of objects.
A certain amount of astigmatism is present in
most people, caused by the pressure of the top
eyelid on the cornea. In mild cases, it is only
a negligible difference of half a dioptre.
Astigmatism is also often caused if a cataract
surgery distorts the cornea. The modern surgical
method of phacoemulsification has reduced this
risk, since it involves minimum invasion. Another
cause of astigmatism is corneal scars and an
even rarer cause is keratoconus or more commonly
called the conical cornea.
Prevalence of presbyopia
Astigmatism develops very early in life. Statistics
have shown that an estimated 50% of full-term
infants suffer from astigmatism of over 1D in
their initial phase of life. The frequency of
astigmatism reduces with adulthood and may occur
only due to some kind of intraocular surgery
like corneal transplants, cataract surgery or
treatment of corneal damages.
Correction of astigmatism
The conventional treatment is the use of spectacles
that correct the refractive power of the lens
of the eye. It has been advised that soft contact
lenses are not suitable for correcting astigmatism
as they fit into the contours of the cornea.
Lenses for spectacles should also be properly
aligned with the eye.
The spectacle prescribed for astigmatism have
a straight line through the lens and rotates
if the lens is moved both in clockwise and anti-clockwise
directions.
