Re: presbyopia
- From: "Mike Tyner" <mtyner@xxxxxxxxxxxxxx>
- Date: Fri, 10 Feb 2006 06:46:35 GMT
"Eye Question" <junk@xxxxxxxxxx> wrote
Prior to lasik I could see clearly to a distance of 3". Now have blurred
vision if an object is closer than 6" to the eye. I understand that the
lasik correction for distance vision will reduce the focusing ability up
close. Is this much of a change normal?
Absolutely. To understand it better, look into the definition of "diopter."
A 3-inch "nearpoint" represents approximately 13 diopters.
A 6-inch nearpoint converts to approximately 6.5 diopters.
So the effect of LASIK was to increase your nearpoint by about 6.5 diopters.
Measured in diopters, you'd expect an identical change in your distance
refraction. *If* you got perfect results from surgery, we would suspect that
your spectacle prescription was about -6.50 before the procedure.
Since diopters are defined as 1/distance, the steps are much smaller at 3
inches than at 3 feet. That makes them much harder to measure accurately, so
if the -6.50 estimate is off, it could likely be refined by more careful
measurements, and by considering your post-LASIK refraction.
What is "normal" up close vision (distance before blurring) for someone
with
no correction needed for distance vision? At what age does presbyopia
normally begin? Is it gradual over several decades
It's gradual early on, but it accelerates predictably after 35 such that age
can often be calculated from measurements of accommodation. Age 40 is the
classic "age of onset" but that only means your nearpoint increases past a
nominal reading distance (16" or 18") at age 40.
Do exercises that force the eye to focus at close range then
distance delay its progression?
Probably negligible. Some people claim dramatic results, but after 20 years
none of them have ever made it into my office. When the measurements depart
from expected values, the variability is usually explained by astigmatism,
or small pupils. My own astigmatism at age 52 gives me "adequate" vision at
20", though only for vertical lines. Serendipitously, straining to
accommodate also shrinks the pupils, improving vision at near without true
accommodation.
-MT
.
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