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KMSG Journal |
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Jairo
E. Hoyos, MD – Melania Cigales, MD Hartstein1 was the first to note contact lens-induced changes in corneal shape and to refer to them as corneal warpage More recent publications2 define the term corneal warpage as denoting all contact lens-induced changes in corneal topography, reversible or permanent, that are not associated with corneal edema. Patients with contact lens-induced corneal warpage are commonly asymptomatic3. These patients frequently do not use glasses and depend on their contact lenses for their refractive error. Some may also notice intolerance to contact lenses or decreased visual acuity with glasses. Reported signs of contact lens-induced corneal warpage1,4-7 include changes in refraction and keratometric readings (relative steepening of mean corneal curvature in some patients, and flattening in others) and distortion of keratometer or keratoscope mires. But the keratometer evaluates corneal curvature from only four paracentral points, approximately 3 mm apart. The keratoscope provides information from a larger portion of the corneal surface, but the data are qualitative in nature. For these reasons, the best system to study contact lens-induced corneal warpage is computer-assisted topographic analysis of videokeratoscopic images.8 Contact lens-induced topographical abnormalities of the cornea include: 1-Central irregular astigmatism. 2-Loss of radial symmetry. 3-Reversal of the normal topographic pattern of progressive flattening of corneal contour from the center to the periphery. 4-Keratoconus-like images.2,3,9-12 Some studies13,14 show corneal thickness modifications induced by contact lens wear. There have been reports of increased corneal thickness measured by an optical pachymeter which have shown that this finding is mainly due to oxygen deprivation leading to corneal edema. Other researchers15 have found, in histopathological studies, that there is a reduced corneal thickness resulting from epithelial thinning. We have not found reports of studies done with the use of ultrasound pachymetry.
REFRACTIVE SURGERY IN CONTACT LENS WEARERS Laser Assisted "In situ" Keratomileusis (LASIK) is a refractive surgery technique in which an attempt is made to correct ametropia by modifying the anterior surface of the cornea. Contact lens wear may induce transient modifications on the corneal surface, with refractive changes that have a negative impact on the predictability of the procedure. In some cases, contact lenses may even produce corneal thinning, requiring the surgeon to program a smaller optic zone in order to correct the ametropia to avoid removing too much tissue. Hence the importance of knowing the length of time such patients should discontinue the use of contact lenses before undergoing surgery, as well as the parameters that might allow us to suspect corneal warpage derived from contact lens wear. Studies have demonstrated that topographic alterations are common in normal wearers of soft and rigid gas permeable (RGP) contact lenses. It is important to identify such topographic abnormalities before surgery, because they are likely to have an adverse effect on predictability an other determinants of the efficacy of refractive surgical procedures. Patients with contact lenses are required to discontinue their use before refractive surgery. Some authors16 recommend 1 week for soft lenses and 2 weeks for RGP lenses, while others17 recommend 2 weeks for all types of contact lenses. It is our routine practice to discontinue contact lens wear before refractive surgery, 1 or 2 weeks for soft lenses and 1 month for RGP lenses. Despite this practice, we have found topographic patterns of corneal warpage in 22 eyes of 12 patients, 13 eyes with RGP lenses and 9 eyes with soft lenses, followed every 2 to 4 weeks. At each visit, these patients were checked by topography, cycloplegic refraction, visual acuity and ultrasound pachymetry. Follow-up was continued until a topographic pattern, normal or abnormal, was found to persist without changes for at least 1 month.3 All eyes returned to their normal topographic pattern, except for one RGP lens wearer, who improved substantially and stabilized at 8 weeks, but never returned to a normal pattern. The mean time required for returning to a normal (or abnormal but stable), topographic pattern was 9 weeks (range from 4 to 10) for soft contact lens wearers, and 11 weeks (range from 8 to 16) for RGP contact lens wearers. These results are comparable to those obtained by Wilson and Klyce.3 Before refractive surgery, RGP contact lenses must definitely be discontinued for a longer period of time than soft contact lenses. A period of 1 or 2 weeks for soft lenses and 1 month for RGP lenses may be appropriate, but if there are any topographical signs of corneal warpage, patients must be delayed until their patterns normalize and/or stabilize.
PARAMETRIC DESCRIPTORS OF CORNEAL TOPOGRAPHY Topographic analysis is the most sensitive method to detect subclinical or occult corneal warping, and as such should be the indicator for the time during which contact lens wear must be discontinued before refractive surgery. In our study, we used a computerized topographic analysis with the TMS-1 topographer (Computed Anatomy Inc., software release 1.61, New York, NY). This instrument includes 25 videokeratoscopic rings covering almost the entire corneal surface, and digitizes 256 points along each mire. The international scale color code mapping was used to monitor corneal topography. Three topographic parameters were analyzed for follow-up: Simulated Keratoscope Reading (Sim K), Surface Asymmetry Index (SAI), and Surface Regularity Index (SRI).3,10,18 Simulated Keratoscope Reading (Sim K) The Sim K is calculated from the maximum meridian powers of rings, 6, 7 and 8. The display includes the average of those maximum powers, the axis at which the average value occurs, and the average power of the corneal surface for the same rings at the meridian located 90º away. Clinically insignificant cylindrical readings lower than 0.20 diopters (D) are not reported; instead, the spherical equivalent is reported in those cases. Sim K is used to quantify the dioptric power of the cornea in order to calculate the value of the cylinder and of the topographic axis, thus identifying differences between the initial and final examinations.18 Surface Asymmetry Index (SAI) The SAI is the centrally weighted sum of the differences in corneal power between corresponding points on the TMS-1 mires located 180º apart. The power distribution across a normal corneal surface is highly symmetrical, making the SAI a useful quantitative indicator for monitoring changes in corneal topography. Normal corneas generally have SAI values lower than 0.5. The SAI is correlated with potential visual acuity (PVA) as originally described by Klyce and Wilson, and is, to our knowledge, the first attempt at correlating the optical quality of corneal surfaces to visual acuity.2,3 Surface Regularity Index (SRI) The SRI is a quantitative descriptor which, like SAI, attempts to correlate the optical quality of the corneal surface with PVA. The SRI is calculated on the basis of the local regularity of the surface over the corneal area enclosed by an average virtual pupil of approximately 4.5 mm. Like SAI, the SRI of normal cornea surfaces is relatively low, and higher SRI values indicate surface of lesser optical quality.2,3
SOFT LENS-INDUCED CORNEAL CHANGES Our results were consistent with those of previous studies3,10,11 showing that soft contact lens-induced corneal warpage determines a topographic pattern of corneal steepening and increased myopia. Changes occurring between the first and the last examinations revealed a reduction of myopia in 88.9% of the eyes, with an average of –2.11 D (range from -1 to -6), associated with an average topographic flattening of 1.64 D (range from 0.05 to 4.15). These changes proved to be statistically significant and there was a positive correlation between them (p<0.05). The greater change were found in cases of corneal warpage with keratoconus-like images. Figures 1-A and 1-B show a clinical case of corneal warpage in a soft contact lens wear, with typical topographic pattern.
Figures 2-A and 2-B show a clinical case of soft contact lens-induced corneal warpage, with keratoconus-like image.
Our study did not reveal changes in average astigmatic values after discontinuing contact lens wear. However, these values were found to be reduced in 55.5% of cases, increased in 44.5%, and with axis modifications of more than 20º in 22.2% of cases. Visual acuity improved from one to five Snellen lines in 66.7% of cases, coinciding with improved regularity and radial symmetry in the topographic image (SAI and SRI decreased in 90% and 100% of cases, respectively, reaching a final value ≤ 0.5 in all cases). Improvements in visual acuity, and their relationship with symmetry (SAI) and regularity (SRI) indices have been described by other authors.3,8,10,16,18 Central corneal thickness was followed with ultrasound pachymetry (DGH-500 pachymeter, DGH Technology, Inc. USA), revealing an increase in corneal thickness after discontinuation of soft contact lens wear in 90% of cases. The average increase in thickness was 20 microns (range from 0 to 30) and was statistically significant (p<0.05). Wilson and Klyce3,9,11 have associated this corneal thinning with the presence of keratoconus, but our results show topographic and pachymetric reversibility once contact lens wear is discontinued, ruling out the presence of keratoconus in those cases. In only one case did we diagnose keratoconus on the basis of persistent topographic alterations and corneal thinning, and this case was excluded from the study. To summarize, soft contact lenses may induce corneal warpage with topographic steepening (with an occasional keratoconus-like image) and increased myopia, as well as central corneal thinning. If all factors are not taken into consideration before refractive surgery, the result will be overcorrection and the selection of a smaller optic zone.
RGP LENS-INDUCED CORNEAL CHANGES Our results show that RGP contact lens-induced corneal warpage is reflected in a topographic pattern of central corneal flattening and decreased myopia, and in cases of contact lens decentering, there is a keratoconus-like image. These results are consistent with the findings of other reported studies.2,3,10-12 Changes occurring between the first and the last examination revealed an increase in myopia in 77% of eyes, with an average of –0.53 D (range from +1 to –1.12), associated with an average topographic steepening of 0.43 D (range from 0.15 to 0.95). These changes proved to be statiscally significant and there was a positive correlation between them (p<0.05). In this group, there were a few cases of relative topographic flattening with increased myopia after discontinuing RGP lens use. This coincided with decentered lenses, causing a flattening at the site of contact with the cornea and steepening on the other side, giving rise to a keratoconus-like image.3 Figures 3-A and 3-B show a clinical case of corneal warpage in a RGP contact lens wear, with typical topographic pattern.
Figures 4-A and 4-B show a clinical case of RGP lens-induced corneal warpage, with keratoconus-like image.
Our study did not reveal changes in average astigmatic values after discontinuing contact lens wear. However, these values were found to be reduced in 18.2% of cases and increased in 18.2%, and there were axis modifications of more than 20º in 27.3% of cases. In our study, we have found greater cylinder changes when soft contact lenses are discontinued (reduction in 55.5% and increase in 18.2 % of cases) than the ones found in RGP lens wearers (reduction in 18.2% and increase in 18.2% of cases). Wilson and Klyce3 report that astigmatic modifications are greater in RGP lens wearers in terms of a cylinder increase when lens wear is discontinued, contrary to what happens with soft contact lenses. There was an improvement of one to four Snellen lines of visual acuity in 38.5% of cases, coinciding with improved regularity and radial symmetry in the topographic image (SAI and SRI decreased in 92% and 61.5% of cases, respectively, reaching a final value ≤ 0.5 in all cases). Improvements of visual acuity, and of symmetry and regularity indices, has been described by other authors.2,3,8,10,16,18 Follow-up on central corneal thickness with ultrasound pachymetry showed an increase in corneal thickness after RGP lenses were discontinued in 90% of cases, with an average increase of 15.5 microns (range from 0 to 25), which was statiscally significant (p<0.05). No differences were observed concerning pachymetric modifications in soft and RGP contact lens wearers. To summarize, RGP contact lenses may induce corneal warpage with topographic flattening (with keratoconus-like images when decentered) and decreased myopia, as well as central corneal thinning. If all factors are not taken into consideration before refractive surgery, the result will be undercorrection and the selection of a smaller optic zone.
CONCLUSION It is essential to perform a critical evaluation of the corneal topography of all candidates prior to refractive surgery. The tendency to proceed immediately to surgery if the topography appears normal should be avoided because the initial topography may depart significantly from the one obtained before contact lens fitting. Contact lens wear should be discontinued before refractive surgery for a period of 1 to 2 weeks for soft lenses, and 1 month for RGP lenses. However, in the event that any topographic signs of corneal warpage are observed, they must be followed until they normalize and/or stabilize. Therefore, topography will dictate the timing for refractive surgery. The presence of topographic signs of corneal warpage must lead us to suspect the possibility of corneal thinning as a result of contact lens wear, requiring close topographic as well as pachymetric follow-up in these patients. Careful operative evaluation and management are likely to improve the quality and predictability of corneal surgery of individual surgeons. Perhaps more importantly, the overall efficacy of procedures such as LASIK might be enhanced once unpredictable variables of contact lens-induced warpage are controlled.
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