RGP contact lens prescription question

I am very near-sighted - -10.75 R, -8.25 L. Before I went in for my
annual exam last week, my L lens prescription was -9.25. In the exam, I
could see only 20/40 with my L lens. So the doctor changed the
prescription to -8.25. Now I am confused. If my eyes got worse,
shouldn't the -9.25 have been increased to -10 or so? It seems
backwards to me. Can anyone explain?

Dear Tenplay,

You should ask you doctor about this.

If you want the stronger minus -- I am
certain he will provide it.

There are doctors who believe that
a very strong minus results in
your vision going "down" at a
steady rate -- and will attempt
to "under-prescribe" for that
reason.

You should ask him to explain, and
-- if that is the case -- then he
should have explained before
he gave you a "reduced" prescription.

Best,

Otis
(Engineer)

tenplay wrote:
> I am very near-sighted - -10.75 R, -8.25 L. Before I went in for my
> annual exam last week, my L lens prescription was -9.25. In the exam, I
> could see only 20/40 with my L lens. So the doctor changed the
> prescription to -8.25. Now I am confused. If my eyes got worse,
> shouldn't the -9.25 have been increased to -10 or so? It seems
> backwards to me. Can anyone explain?

You neglected to say whether the reduction in prescription made your
vision better or worse.

The goal of prescribing for myopia is to give only the power that is
required to achieve clear, comfortable vision at far...no more and no
less. There are various techniques for determining this. The eye
doctor must have determined that your prescription was too strong, not
too weak, and made the necessary adjustment. By itself, a Snellen
acuity of 20/40 says nothing about the cause. You could be farsighted,
nearsighted, astigmatic, a combination of the above, or have a retinal
lesion.

DrG

if you are a hammer, everything looks like a nail

tenplay wrote:
> I am very near-sighted - -10.75 R, -8.25 L. Before I went in for my
> annual exam last week, my L lens prescription was -9.25. In the exam, I
> could see only 20/40 with my L lens. So the doctor changed the
> prescription to -8.25. Now I am confused. If my eyes got worse,
> shouldn't the -9.25 have been increased to -10 or so? It seems
> backwards to me. Can anyone explain?

Since I glossed over the fact that this patient is wearing RGP lenses,
I will add to what others have already written by saying that I have
long observed a trend in RGP lens wearers of having to adjust the
prescription downward over time. Presumably this is due partly to the
effects of changing corneal shape as a result of wearing the lenses.

DrG