What is overcorrection? (Posted: jan 2005)

Overcorrection occurs when too much tissue is removed from the cornea, with the result that the target refractive error is overshot and the opposite type of refractive error is induced.

How does it happen? (posted: jan 2005)

Overcorrection happens to a certain number of patients without an obvious explanation. Sometimes the laser's correction profile (nomogram) may need adjustment. There are environmental factors such as humidity that affect the ablation process and have to be compensated for but this compensation is not always precise. In rare cases, the wrong correction may have been programmed into the laser. For people with high prescriptions, laser correction is not precise enough to always achieve the target and the laser program may assume a certain amount of regression; if the regression does not occur, the patient may remain overcorrected.

What does it mean for the patient? (posted: jan 2005)

Overcorrection is potentially much more problematic for the patient than undercorrection, for a number of reasons. First of all, an overcorrection removes more tissue than it was intended be removed and this may carry safety implications, if the safety threshold is exceeded. (See Patient FAQ on ectasia.) Secondly, for similar reasons, it may make the patient ineligible for further laser treatment. Thirdly, in an overcorrection the patient ends up with the opposite type of refractive error they originally had. That is, a short(near)sighted patient will be long(far)sighted, and vice versa. This can be very disorienting to someone who has never experienced it before. For myopic patients who are also presbyopic, an overcorrection will be particularly troublesome.

How is it diagnosed? (posted: jan 2005)

With a standard refraction (the "better 1, better 2?" type examination).

What are the treatment options? (posted: jan 2005)

Glasses or contact lenses are of course the safest option. If there is a gap between the two eyes, contact lenses may be the better (or, in extreme cases, the only) choice.

Further laser treatment ("enhancement") is an option, however, the risks of further surgery need to be weighed very carefully. Foremost amongst the considerations is the importance of determining the true residual stromal thickness, which cannot be measured accurately by standard equipment in most eye doctors' offices, only by confocal microscopy or Artemis VHF digital ultrasound scans. The doctor can calculate how much tissue has been removed, but what he cannot be sure of is the thickness of the flap that was cut, and in an overcorrected patient this number may be pivotal, especially in higher corrections.

Where can I learn more about overcorrections? (posted: jan 2005)

Click here for Encyclopedia entry, which may have additional articles and links.