EYE ADVICE: Professional Papers

D. Pro-active Prescribing for Presbyopes

(i) Phakic Presbyopes

Knowing that most reversible and more permanent ATR astigmatic changes that are observed in uncorrected presbyopes are due to spasm or overaction of PMA, I generally try to prescribe as little ATR cylinder as possible for the near correction, and direct my efforts on prescribing adequate spherical plus lenses. Prescribing the full subjective ATR cylinder will serve to consolidate the ATR refractive astigmatism, initially causing some discomfort and later permanent corneal toricity changes. On the other hand, giving too little ATR cylinder correction will initially give inadequate clarity of vision that may persist for several weeks and can especially be a problem if the prescribed glasses are to be used for distance vision as well eg. in the case of bifocals.

Some individuals prefer to delay presbyopia by using low-powered reading glasses or by not wearing glasses at all. Their hyperopia tends to increase at a slower rate, however, they also tend to develop greater amounts of ATR astigmatism. Normally, I try to give the minimum plus correction that will avoid ATR changes. As a rule, depending on the demands for distance vision, I try to omit any ATR correction if the corneal toricity is >0.5D WTR. In these cases it would not be unusual to see a 1D ATR refractive astigmatism disappearing by the next two-yearly visit, especially if the axis is close to 90°.

I have found that oblique ATR cylinders mostly originate from prior oblique WRT corneal toricity which becomes transformed to ATR via oblique axes. This cannot usually be fully eliminated by appropriate plus correction, however this would help to slow the progression towards ATR.

Mr C.F.  Age 66

Old glasses (5yrs)   R) +2.25/-0.50*57.5 (6/6=)   L) +2.25/-0.50*115 (6/6-).

Retinoscopy:   R) +2.50/-0.75*7.5   L) +2.50/-0.75*170.

Subjective:   R) +2.25/-0.50*15   L) +2.25/-0.75*160.

This is a typical case of increasing ATR astigmatism with age. ATR astigmatism is typically allowed to increase in presbyopes due to reactive prescribing, i.e. the lens offering the clearest vision at the time of the examination is prescribed. If the reversible effects of 2° PMA on the eye's astigmatism are removed by eliminating the cause of 2° PMA (i.e. by sufficiently correcting presbyopia), the ATR astigmatic correction will be needlessly higher than that which is required for both clear and comfortable vision.

Clear vision may still be attainable, however, the patient may initially complain of a tiring sensation when wearing the glasses, quite apart from the distortion in the vision. This fatigue would be due to the patient's efforts to re-create the necessary ATR astigmatism in order to maintain clear vision with the new glasses. As the cause of the 2° PMA is usually removed by the stronger plus lenses, 1° PMA must be used which requires purposeful effort and may be associated with asthenopia. These symptoms and any increase in the prescribed ATR cylinder may be avoided by pro-active prescribing as demonstrated in Mr C.F. who accepted a more WRT cylinder correction after repeated relaxation of the eyes. (See below for discussion on refractive techniques.)

In a case such as this, the practitioner may spend considerable time trying to refine the cylinder power and axis as the preferred cylinder keeps varying, due to neuromuscular unrest and consequent fluctuations in MA. One should aim to prescribe the least ATR cylinder finding.

Next