Myopia
and Hyperopia, are common eye problems caused by
the inability of the eye to focus correctly. Commonly
known as nearsightedness farsightedness respectively,
these are refractive errors of the eye.
Refractive disorders of the eye
When the eyes fail
to refract light properly to focus iton the
fovea of the retina, refractive disorders occur.
Refraction means the bending of
light but in
medical terms, it
refers to the method of measuring the strength
of the lens needed to focus images on the eye
retina.
Refraction occurs when light slows down as
it travels from one medium (like air) to another
of greater density (like water). when light hits
a flat surface at right angles all the light
particles slow down concurrently. Hence, the
light continues to travel in a straight line.
However, if light hits a translucent surface
at an angle, the light particles that first enter
the new
medium
slow down first and then the rest follow
causing light to bend. When light passes
from
air into the relatively dense eye cornea, with
the help
of
the
lens of
the eye, it
refracts and then focuses onto the retina.
The eye needs to adjust to refract
light from various objects, angles
and distances. The eye lens
power adjusts accordingly for focusing.
The lens of the eye
Lens power is calculated in dioptres (D). There
are mainly two groups of lenses:
- A 'plus' lens
is convex and focuses light inwards
- A 'minus'
lens is concave and turns light outwards
The more the dioptre of a lens, the stronger
is its power in either plus or minus count.
Myopia or Shortsightedness
In myopia, the eye’s focusing power is
stronger than required for a comparatively large
eyeball. Hence, the eye power must be weakened
with minus power lenses. Severe myopia can lead
to serious complications like retinal injury
and detachment.
Clinical characteristics of myopia
The physiological condition that marks myopia
involves the bulging and swelling of the eyeball.
This phenomenon is clinically termed as the origin
of the myopic crescent. This is accompanied by
significant choroidal and retinal degenerative
transformation, retinal detachment, glaucoma
and staphyloma development.
Types of myopia
There are mainly three broad categories of Myopia:
- =2D (D for diopters) type Myopia or low
myopia (also called Physiological or School
Myopia).
- Moderate myopia or the 2–6D
type
- >6D type or high myopia (also called
pathological myopia)
Physiological or school myopia
Myopia caused by stress related factors is called
Physiological or School or =2D type Myopia. It
is the low myopic condition and is the most common
type. This kind of myopia is often caused by
stressful habits like excessive reading from
an early age. Statistics have shown that over
60% of the average medical school students suffer
from myopia of this kind. However, this kind
of myopia also shows a racial and ethnic variation.
Research based on racial and ethnic groups have
reported that it is occurs more frequently among
Asians and Jews and than among African-Americans.
Often it is a combination of a congenital cause
and stress related factors that result in myopia.
Treatment for myopia
The treatment of myopia involves refractive
correction. Wearing bifocals combined with the
continuing use of atropine eye drops is the most
common way to control myopia by means of refractive
correction. Contact lenses and laser technology
can also be used for treatment of myopia.
Pathological myopia
This is the other kind of myopia also termed
as the >6D type or high myopia. Certain studies
have revealed that 2–3% of the population
suffer from this type of myopia.
