251
LETTERS
€ Intraoperative corneal thickness 4. Kaya V, Utine CA, Yılmaz OF. measurements during corneal collagen cross-linking with hypoosmolar riboflavin solution in thin corneas. Cornea 2012; 31:486–490 5. Pantanelli S, MacRae S, Jeong TM, Yoon G. Characterizing the wave aberration in eyes with keratoconus or penetrating keratoplasty using a high-dynamic range wavefront sensor. Ophthalmology 2007; 114:2013–2021
Reply : The CXL standard protocol (Dresden) applies to eyes with a minimum stromal thickness of 400 mm. This would limit the ultraviolet-A irradiance at the corneal endothelium to well below the damage threshold for this tissue. However, during standard CXL, there is a significant decrease in central corneal thickness caused by the oncotic effect of dextran 20% concentration contained in isoosmolar riboflavin and the evaporative water loss from the deepithelialized cornea.1 A simple measure such as avoiding the eyelid speculum during riboflavin instillation results in a significantly smaller decrease in stromal thickness during standard CXL.2 Collagen crosslinking with isoosmolar riboflavin has been shown to be toxic to corneal endothelium, and there is overall agreement that it should be avoided in eyes with a stromal thickness less than 400 mm.3 However, patients with keratoconus often present with corneal thicknesses less than that, making modifications of the standard procedure necessary. Options include transepithelial CXL and the use of hypoosmolar riboflavin. In our study, we used hypoosmolar riboflavin 0.1% solution every 2 minutes for 60 minutes to swell the cornea before and during irradiation in eyes with a stromal thickness between 350 mm and 400 mm. This protocol has been used in eyes with thin corneas and has stabilized the ectasia and had no side effects for the corneal endothelium.4 Repeated application of hypoosmolar riboflavin during the irradiation process seems to prevent intraoperative corneal stromal thinning. We agree that HOAs are the main source of visual loss in keratoconic eyes, and this is the reason to use a topography-guided ablation algorithm to regularize the corneal shape. As tissue sparing is a prime consideration, stromal ablation is limited to 50 mm. However, lower-order aberrations (LOAs) and HOAs cannot be seen as separate entities, since variations in LOAs also affect HOAs. Reduction in defocus and astigmatism in an asymmetrically decentered system such as the keratoconic cornea reduces spherical aberration and coma, the latter being the most prominent aberration in the keratoconic eye.5 Finally, a significant number of patients in our study requested refractive surgery to reduce the need for optical aids and thus a global strategy was considered to improve the
visual function, both corrected and uncorrected.d Gonzalo Mu~ noz, MD, PhD, FEBO, Cesar Albarran-Diego, MSc, Hani Sakla, MD, PhD, Wassim Altroudi, MD REFERENCES 1. Kymionis GD, Kounis GA, Portaliou DM, Grentzelos MA, Karavitaki AE, Coskunseven E, Jankov MR, Pallikaris IG. Intraoperative pachymetric measurements during corneal collagen cross-linking with riboflavin and ultraviolet A irradiation. Ophthalmology 2009; 116:2336–2339 2. Soeters N, van Bussel E, van der Valk R, Van der Lelij A, Tahzib NG. Effect of the eyelid speculum on pachymetry during corneal collagen crosslinking in keratoconus patients. J Cataract Refract Surg 2014; 40:575–581 3. Kymionis GD, Portaliou DM, Diakonis VF, Kounis GA, Panagopoulou SI, Grentzelos MA. Corneal collagen crosslinking with riboflavin and ultraviolet-A irradiation in patients with thin corneas. Am J Ophthalmol 2012; 153:24–28 4. Raiskup F, Spoerl E. Corneal cross-linking with hypo-osmolar riboflavin solution in thin keratoconic corneas. Am J Ophthalmol 2011; 152:28–32 5. Maeda N, Fujikado T, Kuroda T, Mihashi T, Hirohara Y, Nishida K, Watanabe H, Tano Y. Wavefront aberrations measured with Hartmann-Shack sensor in patients with keratoconus. Ophthalmology 2002; 109:1996–2003
Preventing toric intraocular lens rotation Miyake et al.1 describe 6 cases with more than 20 degrees of postoperative rotation after implantation of a toric intraocular lens (IOL). This degree of rotation is clearly unacceptable as it leads to a loss of two-thirds of the astigmatic correction. It is notable that these cases were in highly myopic eyes with axial lengths of 25 mm or longer. It is logical that IOLs are more likely to rotate in the presence of such long eyes with presumably larger capsular bags. One important surgical step that we have adopted when using toric IOLs is to completely remove the bed of ophthalmic viscosurgical device (OVD) that lies beneath the implanted IOL along with removing the OVD anterior to the IOL. Although Miyake et al. recognize the importance of complete OVD removal in toric IOL implantation, they do not mention whether OVD was removed from behind the IOL in these 6 cases. If this was not done, it is not surprising that the IOLs rotated so much. If OVD was removed from behind the IOL, perhaps we should be giving more thought to implanting larger diameter customized toric IOLs in highly myopic eyes. Ronald Yeoh, FRCS (Glas), FRCS (Ed) FRCOphth (UK), FAM (Singapore) Singapore Dr. Yeoh is on the speaker panel for Alcon Laboratories, Inc. and Abbott Medical Optics, Inc.
J CATARACT REFRACT SURG - VOL 41, JANUARY 2015
252
LETTERS
REFERENCE 1. Miyake T, Kamiya K, Amano R, Iida Y, Tsunehiro S, Shimizu K. Long-term clinical outcomes of toric intraocular lens implantation in cataract cases with preexisting astigmatism. J Cataract Refract Surg 2014; 40:1654–1660
Reply : We agree with Dr. Yeoh that complete removal of the OVD is essential to prevent toric IOL rotation, as mentioned in our article. We intended to completely remove the OVD in all cases, not only the OVD anterior to the IOL but also the OVD that lies beneath the implanted IOL, to prevent postoperative IOL rotation. Nevertheless, in 6 of 378 eyes (1.6%), the IOL rotated more than 20 degrees. In these eyes, the axial length was 25 mm or longer, corneal astigmatism was with-the-rule (WTR), and the IOLs rotated relatively soon after surgery. In our experience, a large IOL rotation may occur in the early postoperative period in some eyes with relatively long axial lengths and WTR astigmatism even if the OVD that lies beneath the IOL has been removed.d Toshiyuki Miyake, MD, Kazutaka Kamiya, MD, PhD, Kimiya Shimizu, MD, PhD
Corneal stromal demarcation line after high-intensity (accelerated) collagen crosslinking In the discussion section of the article by Tomita et al.1 regarding accelerated corneal collagen crosslinking (CXL), the authors stated that the corneal stromal demarcation line was at the depth of approximately 350 mm in the accelerated CXL group (3 minutes of ultraviolet-A [UVA] irradiation at 30 mW/cm2 intensity) and of approximately 380 mm in the conventional CXL group (30 minutes of UVA irradiation at 3 mW/cm2 intensity).1 Yet in the results section, the authors stated that the mean corneal stromal demarcation line depth was 294.38mm G 60.57 (SD) for accelerated CXL and 380.78 G 54.99 mm for conventional CXL, while they reported no statistically significant difference in the corneal stromal demarcation line depth between the 2 groups. We think it is necessary to point out that the authors do not provide any information regarding the identification and depth measurement of the corneal stromal demarcation line and that the between-group difference in the mean corneal stromal demarcation line depth seems to be remarkable (294 mm versus 380 mm) even though the authors reported no statistically significant difference between the 2 treatment groups.1 A study by
Touboul et al.2 showed that with accelerated CXL (3-minute treatment with 30 mW/cm2 UVA irradiation), the corneal stromal demarcation line was generally located between depths of 100 mm and 150 mm. In addition, in our recent article,3 we showed that the corneal stromal demarcation line was significantly deeper after standard CXL (30-minute treatment with 3 mW/cm2 UVA irradiation) than after high-intensity CXL (10-minute treatment with 9 mW/cm2 UVA irradiation). Specifically, the corneal stromal demarcation line was 350.78 G 49.34 mm after standard CXL and 288.46 G 42.37 mm after high-intensity CXL. It has been suggested that the corneal stromal demarcation line is correlated to the effective depth of the CXL.4 The findings of our recent study indicate that high-intensity CXL could provide less effective CXL as the corneal stromal demarcation line depth was shallower after high-intensity CXL than after standard CXL.3 Moreover, the corneal stromal demarcation line depth may be different depending on the CXL protocol (time and UVA irradiation intensity) and unknown factors may also play a role. Since it has not been established which depth of the corneal stromal demarcation line is sufficient for effective and safe CXL, we believe that the corneal stromal demarcation line depth of the standard CXL should be the gold standard target after every high-intensity (accelerated) or modified CXL treatment protocol. Michael A. Grentzelos, MD George D. Kymionis, MD, PhD Heraklion, Crete, Greece Editor's Note: The lack of a statistical power analysis is a weakness that limits the interpretation of the nonsignificant results. REFERENCES 1. Tomita M, Mita M, Huseynova T. Accelerated versus conventional corneal collagen crosslinking. J Cataract Refract Surg 2014; 40:1013–1020 2. Touboul D, Efron N, Smadja D, Praud D, Malet F, Colin J. Corneal confocal microscopy following conventional, transepithelial, and accelerated corneal collagen cross-linking procedures for keratoconus. J Refract Surg 2012; 28:769–776 3. Kymionis GD, Tsoulnaras KI, Grentzelos MA, Plaka AD, Mikropoulos DG, Liakopoulos DA, Tsakalis NG, Pallikaris IG. Corneal stroma demarcation line after standard and high-intensity collagen crosslinking determined with anterior segment optical coherence tomography. J Cataract Refract Surg 2014; 40:736–740 4. Kymionis GD, Grentzelos MA, Plaka AD, Tsoulnaras KI, Diakonis VF, Liakopoulos DA, Kankariya VP, Pallikaris AI. Correlation of the corneal collagen cross-linking demarcation line using confocal microscopy and anterior segment optical coherence tomography in keratoconic patients. Am J Ophthalmol 2014; 157:110–115
J CATARACT REFRACT SURG - VOL 41, JANUARY 2015
LETTERS
€ Intraoperative corneal thickness 4. Kaya V, Utine CA, Yılmaz OF. measurements during corneal collagen cross-linking with hypoosmolar riboflavin solution in thin corneas. Cornea 2012; 31:486–490 5. Pantanelli S, MacRae S, Jeong TM, Yoon G. Characterizing the wave aberration in eyes with keratoconus or penetrating keratoplasty using a high-dynamic range wavefront sensor. Ophthalmology 2007; 114:2013–2021
Reply : The CXL standard protocol (Dresden) applies to eyes with a minimum stromal thickness of 400 mm. This would limit the ultraviolet-A irradiance at the corneal endothelium to well below the damage threshold for this tissue. However, during standard CXL, there is a significant decrease in central corneal thickness caused by the oncotic effect of dextran 20% concentration contained in isoosmolar riboflavin and the evaporative water loss from the deepithelialized cornea.1 A simple measure such as avoiding the eyelid speculum during riboflavin instillation results in a significantly smaller decrease in stromal thickness during standard CXL.2 Collagen crosslinking with isoosmolar riboflavin has been shown to be toxic to corneal endothelium, and there is overall agreement that it should be avoided in eyes with a stromal thickness less than 400 mm.3 However, patients with keratoconus often present with corneal thicknesses less than that, making modifications of the standard procedure necessary. Options include transepithelial CXL and the use of hypoosmolar riboflavin. In our study, we used hypoosmolar riboflavin 0.1% solution every 2 minutes for 60 minutes to swell the cornea before and during irradiation in eyes with a stromal thickness between 350 mm and 400 mm. This protocol has been used in eyes with thin corneas and has stabilized the ectasia and had no side effects for the corneal endothelium.4 Repeated application of hypoosmolar riboflavin during the irradiation process seems to prevent intraoperative corneal stromal thinning. We agree that HOAs are the main source of visual loss in keratoconic eyes, and this is the reason to use a topography-guided ablation algorithm to regularize the corneal shape. As tissue sparing is a prime consideration, stromal ablation is limited to 50 mm. However, lower-order aberrations (LOAs) and HOAs cannot be seen as separate entities, since variations in LOAs also affect HOAs. Reduction in defocus and astigmatism in an asymmetrically decentered system such as the keratoconic cornea reduces spherical aberration and coma, the latter being the most prominent aberration in the keratoconic eye.5 Finally, a significant number of patients in our study requested refractive surgery to reduce the need for optical aids and thus a global strategy was considered to improve the
visual function, both corrected and uncorrected.d Gonzalo Mu~ noz, MD, PhD, FEBO, Cesar Albarran-Diego, MSc, Hani Sakla, MD, PhD, Wassim Altroudi, MD REFERENCES 1. Kymionis GD, Kounis GA, Portaliou DM, Grentzelos MA, Karavitaki AE, Coskunseven E, Jankov MR, Pallikaris IG. Intraoperative pachymetric measurements during corneal collagen cross-linking with riboflavin and ultraviolet A irradiation. Ophthalmology 2009; 116:2336–2339 2. Soeters N, van Bussel E, van der Valk R, Van der Lelij A, Tahzib NG. Effect of the eyelid speculum on pachymetry during corneal collagen crosslinking in keratoconus patients. J Cataract Refract Surg 2014; 40:575–581 3. Kymionis GD, Portaliou DM, Diakonis VF, Kounis GA, Panagopoulou SI, Grentzelos MA. Corneal collagen crosslinking with riboflavin and ultraviolet-A irradiation in patients with thin corneas. Am J Ophthalmol 2012; 153:24–28 4. Raiskup F, Spoerl E. Corneal cross-linking with hypo-osmolar riboflavin solution in thin keratoconic corneas. Am J Ophthalmol 2011; 152:28–32 5. Maeda N, Fujikado T, Kuroda T, Mihashi T, Hirohara Y, Nishida K, Watanabe H, Tano Y. Wavefront aberrations measured with Hartmann-Shack sensor in patients with keratoconus. Ophthalmology 2002; 109:1996–2003
Preventing toric intraocular lens rotation Miyake et al.1 describe 6 cases with more than 20 degrees of postoperative rotation after implantation of a toric intraocular lens (IOL). This degree of rotation is clearly unacceptable as it leads to a loss of two-thirds of the astigmatic correction. It is notable that these cases were in highly myopic eyes with axial lengths of 25 mm or longer. It is logical that IOLs are more likely to rotate in the presence of such long eyes with presumably larger capsular bags. One important surgical step that we have adopted when using toric IOLs is to completely remove the bed of ophthalmic viscosurgical device (OVD) that lies beneath the implanted IOL along with removing the OVD anterior to the IOL. Although Miyake et al. recognize the importance of complete OVD removal in toric IOL implantation, they do not mention whether OVD was removed from behind the IOL in these 6 cases. If this was not done, it is not surprising that the IOLs rotated so much. If OVD was removed from behind the IOL, perhaps we should be giving more thought to implanting larger diameter customized toric IOLs in highly myopic eyes. Ronald Yeoh, FRCS (Glas), FRCS (Ed) FRCOphth (UK), FAM (Singapore) Singapore Dr. Yeoh is on the speaker panel for Alcon Laboratories, Inc. and Abbott Medical Optics, Inc.
J CATARACT REFRACT SURG - VOL 41, JANUARY 2015
252
LETTERS
REFERENCE 1. Miyake T, Kamiya K, Amano R, Iida Y, Tsunehiro S, Shimizu K. Long-term clinical outcomes of toric intraocular lens implantation in cataract cases with preexisting astigmatism. J Cataract Refract Surg 2014; 40:1654–1660
Reply : We agree with Dr. Yeoh that complete removal of the OVD is essential to prevent toric IOL rotation, as mentioned in our article. We intended to completely remove the OVD in all cases, not only the OVD anterior to the IOL but also the OVD that lies beneath the implanted IOL, to prevent postoperative IOL rotation. Nevertheless, in 6 of 378 eyes (1.6%), the IOL rotated more than 20 degrees. In these eyes, the axial length was 25 mm or longer, corneal astigmatism was with-the-rule (WTR), and the IOLs rotated relatively soon after surgery. In our experience, a large IOL rotation may occur in the early postoperative period in some eyes with relatively long axial lengths and WTR astigmatism even if the OVD that lies beneath the IOL has been removed.d Toshiyuki Miyake, MD, Kazutaka Kamiya, MD, PhD, Kimiya Shimizu, MD, PhD
Corneal stromal demarcation line after high-intensity (accelerated) collagen crosslinking In the discussion section of the article by Tomita et al.1 regarding accelerated corneal collagen crosslinking (CXL), the authors stated that the corneal stromal demarcation line was at the depth of approximately 350 mm in the accelerated CXL group (3 minutes of ultraviolet-A [UVA] irradiation at 30 mW/cm2 intensity) and of approximately 380 mm in the conventional CXL group (30 minutes of UVA irradiation at 3 mW/cm2 intensity).1 Yet in the results section, the authors stated that the mean corneal stromal demarcation line depth was 294.38mm G 60.57 (SD) for accelerated CXL and 380.78 G 54.99 mm for conventional CXL, while they reported no statistically significant difference in the corneal stromal demarcation line depth between the 2 groups. We think it is necessary to point out that the authors do not provide any information regarding the identification and depth measurement of the corneal stromal demarcation line and that the between-group difference in the mean corneal stromal demarcation line depth seems to be remarkable (294 mm versus 380 mm) even though the authors reported no statistically significant difference between the 2 treatment groups.1 A study by
Touboul et al.2 showed that with accelerated CXL (3-minute treatment with 30 mW/cm2 UVA irradiation), the corneal stromal demarcation line was generally located between depths of 100 mm and 150 mm. In addition, in our recent article,3 we showed that the corneal stromal demarcation line was significantly deeper after standard CXL (30-minute treatment with 3 mW/cm2 UVA irradiation) than after high-intensity CXL (10-minute treatment with 9 mW/cm2 UVA irradiation). Specifically, the corneal stromal demarcation line was 350.78 G 49.34 mm after standard CXL and 288.46 G 42.37 mm after high-intensity CXL. It has been suggested that the corneal stromal demarcation line is correlated to the effective depth of the CXL.4 The findings of our recent study indicate that high-intensity CXL could provide less effective CXL as the corneal stromal demarcation line depth was shallower after high-intensity CXL than after standard CXL.3 Moreover, the corneal stromal demarcation line depth may be different depending on the CXL protocol (time and UVA irradiation intensity) and unknown factors may also play a role. Since it has not been established which depth of the corneal stromal demarcation line is sufficient for effective and safe CXL, we believe that the corneal stromal demarcation line depth of the standard CXL should be the gold standard target after every high-intensity (accelerated) or modified CXL treatment protocol. Michael A. Grentzelos, MD George D. Kymionis, MD, PhD Heraklion, Crete, Greece Editor's Note: The lack of a statistical power analysis is a weakness that limits the interpretation of the nonsignificant results. REFERENCES 1. Tomita M, Mita M, Huseynova T. Accelerated versus conventional corneal collagen crosslinking. J Cataract Refract Surg 2014; 40:1013–1020 2. Touboul D, Efron N, Smadja D, Praud D, Malet F, Colin J. Corneal confocal microscopy following conventional, transepithelial, and accelerated corneal collagen cross-linking procedures for keratoconus. J Refract Surg 2012; 28:769–776 3. Kymionis GD, Tsoulnaras KI, Grentzelos MA, Plaka AD, Mikropoulos DG, Liakopoulos DA, Tsakalis NG, Pallikaris IG. Corneal stroma demarcation line after standard and high-intensity collagen crosslinking determined with anterior segment optical coherence tomography. J Cataract Refract Surg 2014; 40:736–740 4. Kymionis GD, Grentzelos MA, Plaka AD, Tsoulnaras KI, Diakonis VF, Liakopoulos DA, Kankariya VP, Pallikaris AI. Correlation of the corneal collagen cross-linking demarcation line using confocal microscopy and anterior segment optical coherence tomography in keratoconic patients. Am J Ophthalmol 2014; 157:110–115
J CATARACT REFRACT SURG - VOL 41, JANUARY 2015
