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Combined transepithelial phototherapeutic keratectomy and corneal collagen crosslinking for ectatic disorders: Cretan protocol
LETTERS
Combined transepithelial phototherapeutic keratectomy and corneal collagen crosslinking for ectatic disorders: Cretan protocol In the introduction of their recent study of transepithelial phototherapeutic keratectomy (PTK) combined with implantation of a single inferior intrastromal corneal ring segment (ICRS) and corneal collagen crosslinking (CXL) in keratoconus, Yeung et al.1 reported that Kanellopoulos2 used transepithelial PTK to remove the corneal epithelium before topographyguided photorefractive keratectomy (PRK) and CXL in patients with keratoconus. However, in the article by Kanellopoulos,2 the corneal epithelium was mechanically removed with the use of a 20% alcohol solution. We think it is necessary to point out that in 2010, we first described corneal epithelial removal using transepithelial PTK during CXL.3 Moreover, our fellow article regarding combined transepithelial PTK and CXL for progressive keratoconus is, to our knowledge, the first and the only prospective study that deals with comparative outcomes of corneal epithelial removal techniques during CXL and indicates the benefits of using transepithelial PTK (Cretan protocol).4 Our study, which was a prospective comparative analysis of well-matched groups, showed that transepithelial PTK during CXL results in better visual and refractive outcomes than mechanical epithelial debridement. Removal of the corneal epithelium using transepithelial PTK during CXL is a relatively new combined technique, and we believe it should be performed in any case of CXL for better visual and refractive outcomes, especially in cases in which PRK with CXL cannot be performed due to low corneal thickness. George D. Kymionis, MD, PhD Michael A. Grentzelos, MD Vardhaman P. Kankariya, MD Ioannis G. Pallikaris, MD, PhD Crete, Greece REFERENCES 1. Yeung SN, Low SAW, Ku JYF, Lichtinger A, Kim P, Teichman J, Iovieno A, Rootman DS. Transepithelial phototherapeutic keratectomy combined with implantation of a single inferior intrastromal corneal ring segment and collagen crosslinking in keratoconus. J Cataract Refract Surg 2013; 39:1152–1156 2. Kanellopoulos AJ. Comparison of sequential vs same-day simultaneous collagen cross-linking and topography-guided PRK for treatment of keratoconus. J Refract Surg 2009; 25:S812–S818 3. Kymionis GD, Grentzelos MA, Karavitaki AE, Kounis GA, Kontadakis GA, Yoo S, Pallikaris IG. Transepithelial phototherapeutic keratectomy using a 213-nm solid-state laser system Q 2013 ASCRS and ESCRS Published by Elsevier Inc.
followed by corneal collagen cross-linking with riboflavin and UVA irradiation. J Ophthalmol 2010 Article ID:146543. Available at: http://downloads.hindawi.com/journals/jop/2010/ 146543.pdf. Accessed August 28, 2013 4. Kymionis GD, Grentzelos MA, Kounis GA, Diakonis VF, Limnopoulou AN, Panagopoulou SI. Combined transepithelial phototherapeutic keratectomy and corneal collagen cross-linking for progressive keratoconus. Ophthalmology 2012; 119: 1777–1784
Additional basics for mesopic contrast sensitivity testing and need for use of artificial pupil This letter is responding to a recent review of contrast sensitivity.1 Its primary goal is to provide additional basics for mesopic contrast sensitivity testing and need for use of an artificial pupil. The contrast sensitivity curve given by Richman et al.1 in Figure 2 is for a well-focused test object for photopic conditions. The detection of any contrast is highly dependent on the mean retinal illuminance in trolands (td), which is determined by the mean luminance for the test object in cd/m2 area of pupil in mm2. My 1966 publication2 gave a set of 4 curves for spatial resolution of contrast based on sinusoidal grating target for 4 values of mean retinal illuminance of 1000, 100, 10, and 3 td, which corresponded to a mean test object luminance of photopic 318 and 32 and mesopic 3 and 0.3 cd/m2, respectively. The initial rise and fall of the contrast sensitivity curve between 0.1 and 10 cycles per degree as shown in Figure 2 of the review article1 correspond to 2 photopic light levels, but this rise and fall reduces2 significantly for mesopic 3.1 cd/m2 and even disappears at 0.31 cd/m2. In the mesopic range, the contrast threshold increased2 and thus its inverse contrast sensitivity significantly decreased and the maximum resolution of spatial frequency was also significantly lower2 at the maximum possible contrast of 100%. This dependency of contrast sensitivity on mean retinal illuminance leads to another important basic fact that unless the pupil size is controlled, the contrast sensitivity curve becomes pupil size–dependent for the same mean test object luminance. Pupil size varies with factors such as age and medications, even with the same mean object luminance. This results in a wide band of normal photopic contrast sensitivity when pupil size is not controlled. But for the mesopic light level, in addition to lower retinal illuminance, the larger natural pupil size brings in the role of refractive defocusing effect of higher-order aberrations and consequently on contrast detection. Thus, for any mesopic contrast sensitivity testing for comparative clinical studies to be meaningful, careful control of pupil size by an artificial pupil is required. Unfortunately, it is done in relatively 0886-3350/$ - see front matter http://dx.doi.org/10.1016/j.jcrs.2013.10.003
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Combined transepithelial phototherapeutic keratectomy and corneal collagen crosslinking for ectatic disorders: Cretan protocol In the introduction of their recent study of transepithelial phototherapeutic keratectomy (PTK) combined with implantation of a single inferior intrastromal corneal ring segment (ICRS) and corneal collagen crosslinking (CXL) in keratoconus, Yeung et al.1 reported that Kanellopoulos2 used transepithelial PTK to remove the corneal epithelium before topographyguided photorefractive keratectomy (PRK) and CXL in patients with keratoconus. However, in the article by Kanellopoulos,2 the corneal epithelium was mechanically removed with the use of a 20% alcohol solution. We think it is necessary to point out that in 2010, we first described corneal epithelial removal using transepithelial PTK during CXL.3 Moreover, our fellow article regarding combined transepithelial PTK and CXL for progressive keratoconus is, to our knowledge, the first and the only prospective study that deals with comparative outcomes of corneal epithelial removal techniques during CXL and indicates the benefits of using transepithelial PTK (Cretan protocol).4 Our study, which was a prospective comparative analysis of well-matched groups, showed that transepithelial PTK during CXL results in better visual and refractive outcomes than mechanical epithelial debridement. Removal of the corneal epithelium using transepithelial PTK during CXL is a relatively new combined technique, and we believe it should be performed in any case of CXL for better visual and refractive outcomes, especially in cases in which PRK with CXL cannot be performed due to low corneal thickness. George D. Kymionis, MD, PhD Michael A. Grentzelos, MD Vardhaman P. Kankariya, MD Ioannis G. Pallikaris, MD, PhD Crete, Greece REFERENCES 1. Yeung SN, Low SAW, Ku JYF, Lichtinger A, Kim P, Teichman J, Iovieno A, Rootman DS. Transepithelial phototherapeutic keratectomy combined with implantation of a single inferior intrastromal corneal ring segment and collagen crosslinking in keratoconus. J Cataract Refract Surg 2013; 39:1152–1156 2. Kanellopoulos AJ. Comparison of sequential vs same-day simultaneous collagen cross-linking and topography-guided PRK for treatment of keratoconus. J Refract Surg 2009; 25:S812–S818 3. Kymionis GD, Grentzelos MA, Karavitaki AE, Kounis GA, Kontadakis GA, Yoo S, Pallikaris IG. Transepithelial phototherapeutic keratectomy using a 213-nm solid-state laser system Q 2013 ASCRS and ESCRS Published by Elsevier Inc.
followed by corneal collagen cross-linking with riboflavin and UVA irradiation. J Ophthalmol 2010 Article ID:146543. Available at: http://downloads.hindawi.com/journals/jop/2010/ 146543.pdf. Accessed August 28, 2013 4. Kymionis GD, Grentzelos MA, Kounis GA, Diakonis VF, Limnopoulou AN, Panagopoulou SI. Combined transepithelial phototherapeutic keratectomy and corneal collagen cross-linking for progressive keratoconus. Ophthalmology 2012; 119: 1777–1784
Additional basics for mesopic contrast sensitivity testing and need for use of artificial pupil This letter is responding to a recent review of contrast sensitivity.1 Its primary goal is to provide additional basics for mesopic contrast sensitivity testing and need for use of an artificial pupil. The contrast sensitivity curve given by Richman et al.1 in Figure 2 is for a well-focused test object for photopic conditions. The detection of any contrast is highly dependent on the mean retinal illuminance in trolands (td), which is determined by the mean luminance for the test object in cd/m2 area of pupil in mm2. My 1966 publication2 gave a set of 4 curves for spatial resolution of contrast based on sinusoidal grating target for 4 values of mean retinal illuminance of 1000, 100, 10, and 3 td, which corresponded to a mean test object luminance of photopic 318 and 32 and mesopic 3 and 0.3 cd/m2, respectively. The initial rise and fall of the contrast sensitivity curve between 0.1 and 10 cycles per degree as shown in Figure 2 of the review article1 correspond to 2 photopic light levels, but this rise and fall reduces2 significantly for mesopic 3.1 cd/m2 and even disappears at 0.31 cd/m2. In the mesopic range, the contrast threshold increased2 and thus its inverse contrast sensitivity significantly decreased and the maximum resolution of spatial frequency was also significantly lower2 at the maximum possible contrast of 100%. This dependency of contrast sensitivity on mean retinal illuminance leads to another important basic fact that unless the pupil size is controlled, the contrast sensitivity curve becomes pupil size–dependent for the same mean test object luminance. Pupil size varies with factors such as age and medications, even with the same mean object luminance. This results in a wide band of normal photopic contrast sensitivity when pupil size is not controlled. But for the mesopic light level, in addition to lower retinal illuminance, the larger natural pupil size brings in the role of refractive defocusing effect of higher-order aberrations and consequently on contrast detection. Thus, for any mesopic contrast sensitivity testing for comparative clinical studies to be meaningful, careful control of pupil size by an artificial pupil is required. Unfortunately, it is done in relatively 0886-3350/$ - see front matter http://dx.doi.org/10.1016/j.jcrs.2013.10.003
1939