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Editor,—Pepose et al discuss the problem of an inaccurate measurement of intraocular pressure (IOP) after photorefractive keratectomy (PRK).1 To my knowledge, we were the first to demonstrate the apparent reduction of IOP after PRK for myopia in a group of 64 eyes (47 patients).2
We measured the IOP with the Goldmann tonometer in the central and temporal parts of the cornea before and after PRK for myopia during a period of 1 year. Whereas pressure values in the temporal part remained unchanged mean values in the central part were 2–3 mm Hg lower.2
Pepose et al say that “the small change in IOP measurement following PRK is probably not enough to alter a therapeutic decision in an individual patient known to have glaucoma”. However, in our study, the differences between central and temporal measurements were higher, the higher the degree of intended correction, and reached 10 and 12 mm Hg.
Thus, these patients with high myopia might really be in danger of losing visual function, while IOP might seem normal and evaluation of disc cupping might be very difficult. In these patients, we recommend also measuring the pressure in the temporal part of the cornea.
Pepose et al cite Holladay and say that “in patients with previous PRK or LASIK, either manual or automated keratometry will both overestimate the change in central refractive power following these procedures”. However, we showed that the reduction in dioptric power as measured with a manual keratometer or with videokeratography is less than the reduction in subjective refraction.3 This phenomenon was later described by others. So, underestimation must be presumed in these cases.
Editor,—I appreciate Dr Schipper’s comments regarding our commentary, as well as the important contributions of his group on intraocular pressure measurement following excimer laser photorefractive keratectomy. No attempt was made to present a literature review of this subject in the forum or our commentary. However, the suggestion and demonstration that excimer PRK would lead to underestimations of Goldmann applanation tonometry predates Schipper’s work by several years. Readers may also find the reports of Chatterjee et al, Faucheret al, and Kohlhaas et al on this subject to be of interest. The findings of our own studies are in total agreement with Schipper, as is our stated conclusion that the reduced central tonometry reading following PRK or LASIK might, in selected cases, delay the recognition that glaucoma is present following these procedures.
With regard to keratometry, the relation between the change in manifest refraction and change in corneal topography following excimer laser photorefractive keratectomy is complex. While studies by Schipper and others have shown smaller changes measured with videokeratography than by refraction, other investigators have shown that topography tended to overestimate refractive change for corrections of 5 dioptres or less and underestimate the change for corrections greater than 5 dioptres. Such inconsistences may reflect the idiosyncrasies and confounding inaccuracies inherent to corneal topography units. These include the use of algorithms for power calculation based on spherical rather than aspherical optical systems; inaccurate power calculations at points distant from tile corneal apex; data averaging across meridians; alignment of videokeratoscopes perpendicular to the comeal apex; post-surgical changes in corneal and visual reference axes; interpolation rather than measurement of central corneal power; interexamination variation; and employment of a keratometric index of refraction of 1.3375 rather than the actual comeal refractive index of 1.376.