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Presbyopia is a condition where a person has problems of both
nearsightedness and farsightedness together. Presbyopia is corrected
with lenses that have both distance prescription as well as reading
prescription in each lens. People born between 1946 and 1964 are
people who are prone to presbyopia and though spectacles can rectify
this problem, many opt for more comfort and go for alternative
methods of correction to suit their ever active lifestyles. These
people thoroughly accept the advantages of this mode of vision
correction. Thus, bifocal and multifocal contact lenses are more
in demand than ever and there are a wide variety of options to
choose from. Bifocal contact lenses are now available for frequent
replacement as also daily disposables, soft as well as rigid gas
permeables and multi packaged disposables.
Bifocal contact lenses are a little difficult to fit but have
many benefits. The adjustment period for bifocal lenses is usually
long and it requires several changes before the appropriate lenses
can be fitted. So the patient has to be extremely cooperative
and practical to obtain the perfect fit.
The six decade old concept of alternating and simultaneous versions
still provide the basis of contact lens design for presbyopia
and are constituted of different materials and designs.
Simultaneous Designs and Alternating Designs
Simultaneous vision is when the patient can see both near and
far objects at the same time. Thus, the lens for correction should
have annular or concentric, concentric or diffractive designs.
The lenses used are non prismatic and the patient should have
extreme concentration. The distance and near refraction are superimposed.
Both the eye and brain focus on the distance or near light rays.
The near power is situated on the outer edge of the lens but can
also be at the centre of the lens. For good intermediate acuity,
aspheric design should be used.
A majority of translating and alternating bifocal lenses have
a prism to stop lens rotation and small optic zone diameters so
that the patients’ fixation alternates between zones as
required. They have separate distance, near and intermediate areas
of power. The eyes must move upward to allow sharp acuity in different
positions of gaze. To view distant objects the inferior part of
the lens is to be used. The lower eyelid is responsible for holding
the lens with downward gaze.
The Different Kinds of Bifocal Contact Lenses
Rigid Bifocal Contact Lenses have steep vision compared to soft
multifocal contacts. While fitting these lenses things to keep
in mind are the location of the corneal apex, pupil size and lower
lid position and performance of the lenses.
Rigid simultaneous gas permeable contact lenses have the ability
to facilitate viewing of objects, which are near in all gazes
but is up to the patient whether to view the distance or the nearby
image as both are focused on the retina. To obtain perfect vision
the patient must train his/her eyes to ignore the hazy near image
while focusing on the distant view and vice versa.
Bifocal contact lenses with an aspheric design are easier to
fit and offer better viewing than translating gas permeable lens
designs. They offer very good distance as well as intermediate
acuity and the near vision is rectified in straight ahead gaze.
The important things to keep in mind while fitting these types
of lenses are pupil size, lens movement and lens concentration.
Rigid annular gas permeable designs have distance, intermediate
or near powers arranged in a concentric design. In respect of
greater add powers they are better than aspheric gas permeable
lenses. The zone size needs to be adjusted according to the pupil
size of the patient. A large zone at centre provides better distance
vision but compromises on the quality of near vision. The converse
is also true.
Soft Bifocal Contact Lenses
These have corresponding designs as gas permeable ones and though
they are available in various parameters most soft bifocal lenses
use a simultaneous vision design. As with rigid simultaneous contacts
both distant and near images are focused on the retina simultaneously
and perfect vision can be obtained by selective viewing of near
or distant images by the patient. The steopsis for the patient
at distance as well as near is excellent but there is a decrease
of contrast acuity in stopic conditions.
Soft annular bifocal designs may be near centered or distant
and have both distance and near zone located in the pupil area.
For near centered concentric lenses, pupils are an important factor.
Aspheric soft contact lenses have an anterior or posterior aspheric
surface. In the former, the plus power is located towards the
centre of the lens and is ideal for patients requiring better
distance vision in bright light. In the latter the lenses have
increasing plus power peripherally and are ideal for people whose
jobs demand near vision in bright light.
Bifocal toric lenses are available in conventional vial and disposable
or planned replacement multi packs and are offered in a wide range
of powers.
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