Bifocal contact lenses and usage

 
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Bifocal Contact Lenses (for Presbyopia)

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.