PRK: Undercorrection vs. Regression

BY JAMES V. AQUAVELLA, M.D.
NOV. 1996

When considering a patient for PRK retreatment, it's important to understand the mechanism behind the residual myopia and to establish a unified comanagement approach. Taking a few simple precautions will help reduce the risk for overcorrection that can accompany a hasty recommendation.

UNDERCORRECTION VS. REGRESSION

Regression is a myopic shift that occurs only after a stable level of correction has been achieved. While refractive stability may occur earlier, a sixth-month postoperative evaluation is usually necessary to assess ultimate stability.

Undercorrection, on the other hand, is residual (unplanned) myopia that is apparent in the immediate or early postoperative period, and is often a result of variability in the healing response.

Many patients, but not all, who are undercorrected because of a florid healing response can be managed by increasing steroid dosage in the postoperative period and maintaining the dosage at a high level for several months. Undercorrection can also result from decentered ablations, excessive tearing during the procedure (which can reduce the laser energy delivered to the corneal surface) or technical problems in transferring the appropriate refractive information into the computer at the time of surgery.

ESTABLISH REFRACTIVE STABILITY

Delay any considerations for retreatment until stability of refraction has been documented by less than a half-diopter change in both sphere and cylinder over three refractions conducted at least one month apart. Residual myopia, as a result of undercorrection or regression, does not necessarily need to be treated if the patient is comfortable with the refractive status. In any event, myopia of less than one diopter is seldom an indication for retreatment.

YIELD TO HAZE

If haze is present, delay retreatment until the haze has regressed or stabilized. Haze will almost always disappear spontaneously within the first postoperative year. Haze that causes a reduction in the best corrected visual acuity will typically exaggerate myopia, so retreatment based on the subjective refraction can result in an overcorrection.

We recently examined a patient with haze that did not resolve several months after his original PRK was performed in Canada. Three months after he returned from a second trip to Canada, at which time the eye was retreated, slit lamp examination revealed 2+ subepithelial haze with best corrected visual acuity of 20/25 with a refraction of -1.00 SPH. Retreatment for haze poses a significant risk of overcorrection, and given his history, haze could recur as a result of remodeling following any additional retreatments. I advised the patient to continue to use his topical steroids three times a day and to avoid retreatment for several months.

SCAN FOR CENTRAL ISLANDS

Videokeratographic analysis is also extremely important to ascertain if the residual myopia is caused by a central island. To retreat a patient with a central island, the surgeon must tailor the ablation zone specifically to the size of the island to avoid overcorrection. The risk of overcorrection in retreating eyes with residual haze or central islands is significant.

COMANAGEMENT COMMUNICATION

To maintain patient confidence, particularly in cases of regression and undercorrection, the comanagement team must communicate regularly and agree upon a therapeutic plan based on a mutually compatible philosophical approach.

We recently treated a 44-year-old, pre-presbyopic woman with 20/20 uncorrected vision in the right eye following initial PRK, which was combined with a T-cut since she adamantly wanted to have very good distance vision. Following PRK of the left eye, she was initially unhappy with a 20/25 result and a residual refraction of -0.50 -0.50 x 180. Both the surgeon and her referring optometrist counseled her about the benefits of slight undercorrection in her case, and she soon realized that the rewards of near vision significantly outweighed the slight blurriness she experienced occasionally in the distance. Had there been less than unanimity, both in the philosophical approach and in the advice offered, this patient could have remained unhappy. CLS

Dr. Aquavella is chairman of the Genesee Valley Eye Institute and director of the corneal research lab at the University of Rochester.