(ii) Low Vision
Both cases described below have impaired
visual acuities. The commentary under the case of
Mr P.K. stresses the importance
of an adequate near addition to prevent
excessive demand on PSA leading to unnecessary 2° PMA and adverse ATR changes. The case of
Mr S.H, is of congenital onset.
This case emphasises the need to measure the
corneal toricity and to prescribe the maximum WTR
cylinder that is accepted to allow for a
constantly clear image, without relying on 1° PMA
for optical clarity. This would stabilise the
vision and allow for improved visual acuity in
cases of amblyopia. Keratometry is
a very important clinical measurement in both
cases. Experience will allow the practitioner to
vary the prescribed cylinder appropriately.
Mr P.K. Born 1930
Jan 1989, old gasses: R) -1.75/-1.50*105
May 1989 Sub: R) -1.00/-1.75*105
(6/12=) L) -0.75/-2.50*90
Jul 1990 Sub:
R) pl/-2.25*95 (6/12=)
L) -0.75/-2.75*90 (6/18=).
Decided to give: +7.00 Near Additon in S.V.
Apr 1992 Sub: R)
L) +0.50/-1.25*87.5 (6/18).
This persisted until last seen in May 1995.
2° PMA can be activated when low
vision patients who have insufficient PSA attempt
to accommodate at a much reduced working distance
without a correction. Mr P.K. suffered
moderately reduced vision due to lens opacities
and diabetic maculopathy. Before July 1990, he
wore near additions of less than +3.50. He
was reading extensively and at an extremely short
working distance due to the reduced vision. This
led to the ATR lenticular changes due to 2° PMA.
When the demand on PSA was reduced by the higher
near addition, the lenticular astigmatism was
also significantly reduced.
Master S.H. Born 1970
Condition: Leber's optic atrophy. No
glasses were previously worn as the patient was
told that they would not improve vision.
Jun 1984 Unaided vision: R)
6/30 L) 6/12-2.
Jun 1984 (other Optom.) Sub
R) +1.75/-2.25*15 (6/18=) L) +0.25/-0.50*170
Jul 1984 - Retinoscopy: R) +3.00/-3.00*4
Jul 1984 - Keratom. R) 43.95*12==47.50*102
(3.55) L) 44.70/169==46.75*79
Jul 1984 - Max subjective accepted R) +2.00/-2.75*10
(6/18) L) +0.75/-1.25*170
Jan 1987 - Subjective R) +2.00/-2.50*12.5
(6/18) L) +0.75/-1.25*165 (6.9-).
From the keratometry readings, and the subjective result as obtained
by another optometrist in June 1994, it is apparent that Mr S.H.
must have been compensating for a significant amount of corneal
toricity. By correcting the latent astigmatism up to the maximum
amount possible, the patient's demand on PMA was reduced. This not
only helped to prevent asthenopia but offered better vision, as
less astigmatic focussing was required for clear vision. As can
be seen, the patient's vision improved slightly by January 1997
- enough for him to pass the vision standard for driving a car.
The patient remarked : "the glasses have made a difference".