Overcorrection and Undercorrection

The most common complication (if it even deserves that name) is overcorrection - “over” or undercorrection - "under", meaning there is residual refractive error impairing vision.  Overcorrection and undercorrections are actually misnomers because is exceedingly rare that the surgeon attempts to correct the wrong prescription.  While the laser does what it’s told to do, the uncertainty is how the eye heals.  Every eye heals a little differently.  Eyes can heal more or less than expected, thereby determining whether the final prescription is more or less than what the surgeon targeted.  Consequently, it is often more correct to use the terms over and under “response”.
Over and under response is more common with high prescriptions.  As an example, take a high myope with a –10.00 diopter prescription.  If this patient under heals the intended correction by 10%, that leaves a residual nearsightedness of –1.00 diopter.  This usually causes a noticeable blur.  In contrast, if a patients starts with a –1.00 diopters of nearsightedness, a 10% under healing results in –0.10 diopter residual refraction error which even the most discriminating patient can not tell.  A doctor should tell you if you are likely to have a significant over and under correction at the preoperative exam.

Patients with a bothersome residual prescription can wear glasses or contact lenses.  Patients often ask, “What is the benefit of refractive surgery if they still need glasses and contacts afterwards?”  As described under Realistic Expectations, many patients with the “throw away glasses and contact lenses” attitude will be disappointed.  Nevertheless, it is possible to perform an enhancement refractive surgery procedure.  In LASIK, this typically involves lifting up the flap with a special instrument and reshaping the cornea with the laser.  Sometimes, astigmatic keratotomy (AK) is used as the enhancement procedure.  PRK enhancements should never be done on patients that have had previous LASIK because there is a high risk for developing severe corneal haze.  Enhancements are not performed until after 1-3 months of the original procedure, depending on when the doctor feels the prescription has stabilized.

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