Section

Complex lenses four

presbyopiaLenses for 24CHAPTER

24.1 Patient selection

24.1.1 New patients

Indications

•Patients who fulfil the normal criteria for successful contact lens wear.

•Fairly tolerant patients who can accept some compromise in their distance and near vision.

•Patients without exacting visual requirements.

Contraindications

•Patients almost emmetropic for distance.

•Where there are basic contraindications to contact lens wear.

•Very critical patients.

•Patients who need good sustained close vision for work require particular care.

•Where the reason for fitting is obviously a spectacle dispensing problem.

•Poor handling.

©2010 Elsevier Ltd, Inc, BV

DOI: 10.1016/B978-0-7506-7590-1.00011-X

Section four Complex lenses

PRACTICAL ADVICE

To give prior assessment of adaptation, fit single vision distance first or try disposables if:

•The patient is uncertain about contact lenses.

•There are practitioner doubts about suitability.

•Handling may be a problem.

24.1.2 Existing lens wearers

•Early presbyopes often cope with +0.50 D added to the distance correction, usually in one (non-dominant) eye but it may be accepted binocularly.

•Many patients are happy to wear reading spectacles over their contact lenses to avoid both complications and expense.

•Some patients inadvertently using monovision because of refractive change are less happy when refitted with bifocals.

•If the patient is a successful rigid lens wearer, soft bifocals should not be fitted as the first choice.

Indications

•Long-term lens wearers who do not wish to resume wearing spectacles, even for reading.

•Physical problems with spectacles.

•Nuisance value of spectacles for one specific task.

Contraindications

•Poor volume or quality of tears, common with presbyopes.

•Where tolerance is becoming marginal with single vision lenses.

•Patients taking systemic drugs (e.g. for arthritis or sometimes hormone replacement therapy).

24.2 Monovision

Monovision is a technique for correcting presbyopia in which reading power  is incorporated into a single-vision contact lens worn usually on the non-  dominant eye.

24.2.1 Advantages and disadvantages of monovision

Advantages

•The least complicated method of dealing with presbyopia.

•It is unnecessary to make any compromise in the fitting.

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•Patient acceptability is high, provided that the concept has been explained.

•Stability of vision.

•Patients usually decide rapidly that they can or cannot accept the technique.

•Less expensive.

•Maintains peripheral fusion.

•Refractive balancing not an issue.

Disadvantages

•Reduced stereopsis, but at optical infinity this is negligible.

•Intermediate range (e.g. VDUs) may be lost.

•Some loss of contrast sensitivity, although this is also true of most bifocal and multifocal contact lenses.

•Unacceptable blurring may reduce tolerance.

•Cannot be used with monocular patients.

•Requires fairly strong eye dominance.

•Care is required with driving, particularly at night.

PRACTICAL ADVICE

•There is a natural limit to how much reading power can be tolerated. This is the limit of disparity beyond which monovision ceases to provide effective or comfortable vision. For most patients, once the addition reaches +2.25 D, the disparity between the two eyes may be difficult to accept.

•A reduced addition can be supplemented with top-up reading spectacles.

24.2.2 Fitting for monovision

•Lenses, rigid or soft, are fitted according to normal criteria.

•The non-dominant eye is generally used for near vision.

•The left is more often the non-dominant eye; in the UK, this is more practicable for driving.

•The least minus or most plus is found for distance in the dominant eye.

•The minimum plus power for adequate near vision is included for the non-dominant eye.

Eye dominance

Correct determination of eye dominance is essential for successful monovision. Crossed dominance (i.e. right handed and left eyed, or vice versa) is not uncommon. Methods used include:

•Determine which eye gives better corrected acuity.

•Use a lens between +1.25 D and +2.00 D in front of each eye to see which gives less subjective blur or change in binocular acuity. Even +0.50 can sometimes be sufficient.

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Section four Complex lenses

•Pointing with the forefingers of both hands touching.

•Fixating through a cardboard tube or hole in a sheet of paper.

•View a spotlight in a dim room, the near add being moved from eye to eye while assessing the worse starburst effect.1

There is frequent ambiguity between these various methods and the practitioner must decide on which technique to place greatest reliance.

PRACTICAL ADVICE

•Generally the monocular subjective blur test gives the most reliable result and simulates what actually happens in practice.

•With disposables, patients can be given lenses to try distance and reading reversed.

•If right and left eyes are similar in prescription, the patient can experiment with which eye to use for near.

•If near vision is the main use, try changing the dominance and adapting the method to the patient’s needs – crossed monovision.

•For occasional optimum distance vision (e.g. prolonged driving), consider prescribing either a third, distance lens for the non-dominant eye or spectacles to wear over the lenses.

Partial monovision

A low add of +0.50 D or +0.75 D often gives sufficient convenience for intermittent near vision (e.g. price tags, menus, headlines) and patients are then happy to use reading spectacles for prolonged close work. This technique can sometimes be particularly effective with front surface aspherics.

24.3 Presbyopic lens designs

24.3.1 Simultaneous designs (non-translating bifocals or multifocals)

Simultaneous designs essentially provide distance and near vision together and do not rely on lens movement. They are commonly found with concentric or aspheric lenses where they are usually referred to as centre near (CN) or centre distance (CD).

Factors for successful fitting

Stability of fit and the ability to discern between distance and near are important factors. Pupil size is also very significant in the performance of all simultaneous bifocals or multifocals.

One of the main issues with simultaneous bifocals is a loss of contrast sensitivity of the superimposed retinal images. The problem is worse in low illumination and can give particular difficulties at near vision.

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Centre near lenses

•Low illumination with CN concentric types favours distance vision because of the increase in pupil size.

•High illumination with CN concentric types favours near vision because of the decrease in pupil size; thus drivers should wear sunglasses.

•With CN aspherics, the larger the pupil the better the distance vision. Older patients with small pupils may not achieve good distance acuity.

Centre distance lenses

•Low illumination with CD concentric types favours near vision.

•High illumination with CD concentric types favours distance vision; thus sunglasses should be worn to read on a beach.

•With CD aspherics, small pupils make available less reading addition; thus, the older the patient the less suitable for near.

24.3.2 Alternating designs (translating bifocals)

Alternating lenses contain two distinct sectors. These may be either fused or solid portions, or extend across the entire width of the lens (Figure 24.1). Segment lenses are more common, but the concept is equally valid with concentric designs. Distance and near portions can never be used in the same direction of gaze or at the same time. Lens stability and position are controlled by either prism or truncation.

Factors for successful fitting

•The lens must move upwards on downward gaze to bring the near portion in front of the pupil area.

•A relatively taut lower lid is necessary; if it is too slack, the lens edge slides across the lower limbus.

A B

Figure 24.1  Bifocal lens design: (A) fused or solid portion; (B) solid portion extending across lens width

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