Classification of corneal topographic patterns after PRK.

To the Editor: We read with interest the article by P.S. Hersh attempting to standardize the classification of topographic patterns after laser refractive surgery (Hersh PS. A standardized classification of corneal topography after laser refractive surgery. J Refract Surg 1997;13:571-575). To standardize a technique is always a very important and difficult task, but it is the best way to facilitate improvement in technology as people can compare their experiences, and with the “same language,” faster improvement can occur. To standardize a technique has already been realized in other areas of ophthalmology, such as ophthalmic echography, by Karl Ossoinig in the early 1970s (Ossoinig KC. Standardized Ophthalmic Echography of the Eye, Orbit and Periorbital Region, 3rd edition. Iowa City, IA: Goodfellow Company; 1985). For this reason, we suggest an improvement to Hersh’s classification. Hersh suggested using a differential map (derived from subtraction of the preoperative from postoperative values at corresponding points on the actual power maps) to follow-up patients, utilizing a 0.5 step. Since then, we have used his classification, but had some discrepancies (see figures). In Figure 1, the differential map automatically generated by the computer (EyeSys Vista) has a 0.5 step and according to Hersh’s classification, would be classified as a regular homogeneous pattern. In Figure 2, the same patient shows a toric with axis pattern. The difference between the two images are the scales. Figure 1 has 0 difference as a mean point, and goes from a difference of +3.50 to -3.50 D. In Figure 2, it goes from 0 to -7.00 D. Figure 2 was generated using a different scale. This is important because if we perform a treatment below 3.50 D, we will fit in the correct classification, but if we perform a high dioptric treatment, utilizing the first scale, it is quite obvious that we will get a uniform image, as most of the area will have a dioptric difference superior to the highest level of the scale. Moreover, this will allow better detection of small irregular areas. We suggest the following criterion: to assign the last step to the highest difference value obtained from the map. Hersh’s classification referred to topographic findings after treatment for myopia and myopic astigmatism. We suggest that this classification be extended to findings after hyperopic PRK; the differential map should derive from subtraction of the postoperative from preoperative values at corresponding points on the actual power maps. The result would be patterns similar to those found for myopia and myopic astigmatism, the only difference being that a toric with axis treatment will increase the patient’s corneal toricity (Fig 3).