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Reply: Bilateral corneal ectasia after laser in situ keratomileusis
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As a final point, we would like to mention that our group is processing new data from theses cases to extend our understanding of the causes of ICRS explantation. The value of ICRS explantation and its correlation to the surgical technique, type of segment, and year of implantation will be the focus of the next study.dConsuelo Ferrer, PhD, Jorge L. Alio, MD, PhD
Bilateral corneal ectasia after laser in situ keratomileusis
The recent case report by Saad and Gatinel1 highlights a patient who developed bilateral corneal ectasia 2 years after laser in situ keratomileusis despite normal preoperative parameters. In retrospect, the authors note that there was a large degree of pachymetric asymmetry in the patient prior to surgery. Despite otherwise normal parameters, the pachymetric asymmetry of 20 mm at the thinnest point is proposed as a potential risk factor. In 2008, we published an article documenting the standard variance of pachymetric asymmetry in otherwise normal corneas.2 Our study of 724 normal patients found that at the thinnest corneal point, the mean asymmetry was 9.0 mm G 8.3 (SD). The mean asymmetry values at the apex and pupil center were similar. In only 5% of the population was the thinnest point of pachymetric difference 23.2 mm. We proposed that pachymetric asymmetry should be evaluated when screening patients for refractive surgery. In the Saad and Gatinel case, the midperipheral pachymetric readings showed a variance up to 40.0 mm. It would be interesting to see the pachymetric progression graphs for this patient.3 Our paper established only the normal interocular pachymetric variance. Since the frequency of iatrogenic ectasia is very low, case reports such as the one by Saad and Gatinel will help determine the clinical significance of pachymetric asymmetry as a screening tool. Stephen S. Khachikian, MD Rapid City, South Dakota, USA
2015
REPLY: We agree with Khachikian and Belin about the importance of evaluating intrasubject pachymetry asymmetry when screening patients for refractive sur1 gery. In 2008, Khachikian et al. found an intrasubject pachymetry difference of 9.0 G 8.3 mm at the thinnest corneal point. More recently, Falavarjani et al.2 showed a similar result, with a difference of 8.42 mm measured with a Scheimpflug system. As Khachikian and Belin note, the interocular central and thinnest pachymetry difference between our patient's eyes3 was much higher than the mean found in normal eyes. After adjusting the nasal–temporal inversion of the pachymetry maps (Orbscan II, Bausch & Lomb), this difference reached a maximum of 28 mm in the mid superotemporal periphery.3 The patient in our case was referred to our institution, and at the time the article was published, the preoperative raw data of the topography maps were inaccessible. However, we were able to obtain them recently and calculate the following corneal spatial profiles: percentage of thickness increase, normalized percentage of anterior curvature variation, and normalized percentage of posterior curvature variation. These measurements were obtained from the raw elevation data after recentration of the
Michael W. Belin, MD Tucson, Arizona, USA REFERENCES 1. Saad A, Gatinel D. Bilateral corneal ectasia after laser in situ keratomileusis in patient with isolated difference in central corneal thickness between eyes. J Cataract Refract Surg 2010; 36:1033–1035 2. Khachikian SS, Belin MW, Ciolino JB. Intrasubject pachymetric asymmetry analysis. J Refract Surg 2008; 24:606–609 sio R Jr, Alonso RS, Luz A, Coca Velarde LG. Corneal3. Ambro thickness spatial profile and corneal-volume distribution: tomographic indices to detect keratoconus. J Cataract Refract Surg 2006; 32:1851–1859
Figure 1. A: Right eye percentage of thickness increase compared with the mean and 95% CI in a normal population. B: Left eye percentage of thickness increase compared with the mean and 95% CI in a normal population (PTI Z percentage of thickness increase; SD Z standard deviation).
J CATARACT REFRACT SURG - VOL 36, NOVEMBER 2010
2016
LETTERS
Figure 2. Left eye normalized percentage of anterior curvature variation compared with the mean and 95% CI in a normal population (PVAK Z percentage of variation of anterior curvature; SD Z standard deviation).
Figure 3. Left eye normalized percentage of posterior curvature variation compared with the mean and 95% CI in a normal population (PVPK Z percentage of variation of posterior curvature; SD Z standard deviation).
corneal wall on the thinnest point, as described in a recent publication.4 The profiles in Figures 1 to 3 compare the patient's percentage of thickness increase, normalized percentage of anterior curvature variation, and normalized percentage of posterior curvature variation with the mean and 95% confidence interval (CI) values in a normal population. The right and left eye percentage of thickness increase curves (Figure 1) did not show an evident abnormal shape; however, the left eye percentage of thickness increase was borderline with the lowest standard deviation curve. In the right eye, the normalized percentage of anterior curvature variation and the normalized percentage of posterior curvature variation were within the normal range. In the left eye, the normalized percentage of anterior curvature variation and the normalized percentage of posterior curvature variation were clearly abnormal (Figures 2 and 3). These findings suggest that caution may be required when pachymetric and curvature progression from the thinnest point to the periphery are altered in one or both eyes of LASIK candidates.4 We believe that in the future, these parameters, possibly associated with other data such as biomechanical indices,5,6 should be taken into consideration for proper screening of refractive surgery candidates.dAlain Saad, MD, Damien Gatinel, MD, PhD
thickness between eyes. J Cataract Refract Surg 2010; 36:1033–1035 4. Saad A, Gatinel D. Topography and tomography properties of forme fruste keratoconus corneas. Invest Ophthalmol Vis Sci 2010 June 16; [Epub ahead of print] 5. Saad A, Lteif Y, Azan E, Gatinel D. Biomechanical properties of keratoconus suspect eyes. Invest Ophthalmol Vis Sci 2010; 51:2912–2916 sio R Jr, Dawson DG, Saloma ~ o M, Guerra FP, Caiado AL, 6. Ambro Belin MW. Corneal ectasia after LASIK despite low preoperative risk: tomographic and biomechanical findings in the unoperated, stable, fellow eye. J Refract Surg 2010 May 19; [Epub ahead of print]
REFERENCES 1. Khachikian SS, Belin MW, Ciolino JB. Intrasubject corneal thickness asymmetry. J Refract Surg 2008; 24:606–609 2. Falavarjani KG, Modarres M, Joshaghani M, Azadi P, Afshar AE, Hodjat P. Interocular differences of the Pentacam measurements in normal subjects. Clin Exp Optom 2010; 93:26–30. Available at: http://onlinelibrary.wiley.com/doi/10.1111/j.1444-0938.2009.004 46.x/pdf. Accessed August 10, 2010 3. Saad A, Gatinel D. Bilateral corneal ectasia after laser in situ keratomileusis in patient with isolated difference in central corneal
Is central pachymetry asymmetry between eyes an independent risk factor for ectasia after LASIK?
Saad and Gatinel1 report bilateral ectasia after laser in situ keratomileusis (LASIK) in a patient with no risk factors identified preoperatively. They suggest the preoperative 20 mm difference in central pachymetry between eyes was a new risk factor. We have reviewed the records of 1609 consecutive patients treated with bilateral myopic LASIK between September 2003 and December 2008 using a femtosecond laser. Preoperatively, 89 patients (5.5%) had a betweeneye difference of more than 20 mm in central ultrasonic pachymetry readings. This was similar to the 5.0% rate of asymmetry exceeding 23 mm that Khachikian et al.2 detected by corneal tomography (Pentacam, Oculus, Inc.) in a separate cohort of 724 consecutive LASIK candidates. None of our 89 patients has developed ectasia at a mean of 4.5 years (range 1.6 to 6.9 years) after LASIK. This suggests that a pachymetry difference of more than 20 mm is not predictive of ectasia in the absence of other recognized risk factors, such as abnormal topography or expected residual bed thickness of
J CATARACT REFRACT SURG - VOL 36, NOVEMBER 2010
As a final point, we would like to mention that our group is processing new data from theses cases to extend our understanding of the causes of ICRS explantation. The value of ICRS explantation and its correlation to the surgical technique, type of segment, and year of implantation will be the focus of the next study.dConsuelo Ferrer, PhD, Jorge L. Alio, MD, PhD
Bilateral corneal ectasia after laser in situ keratomileusis
The recent case report by Saad and Gatinel1 highlights a patient who developed bilateral corneal ectasia 2 years after laser in situ keratomileusis despite normal preoperative parameters. In retrospect, the authors note that there was a large degree of pachymetric asymmetry in the patient prior to surgery. Despite otherwise normal parameters, the pachymetric asymmetry of 20 mm at the thinnest point is proposed as a potential risk factor. In 2008, we published an article documenting the standard variance of pachymetric asymmetry in otherwise normal corneas.2 Our study of 724 normal patients found that at the thinnest corneal point, the mean asymmetry was 9.0 mm G 8.3 (SD). The mean asymmetry values at the apex and pupil center were similar. In only 5% of the population was the thinnest point of pachymetric difference 23.2 mm. We proposed that pachymetric asymmetry should be evaluated when screening patients for refractive surgery. In the Saad and Gatinel case, the midperipheral pachymetric readings showed a variance up to 40.0 mm. It would be interesting to see the pachymetric progression graphs for this patient.3 Our paper established only the normal interocular pachymetric variance. Since the frequency of iatrogenic ectasia is very low, case reports such as the one by Saad and Gatinel will help determine the clinical significance of pachymetric asymmetry as a screening tool. Stephen S. Khachikian, MD Rapid City, South Dakota, USA
2015
REPLY: We agree with Khachikian and Belin about the importance of evaluating intrasubject pachymetry asymmetry when screening patients for refractive sur1 gery. In 2008, Khachikian et al. found an intrasubject pachymetry difference of 9.0 G 8.3 mm at the thinnest corneal point. More recently, Falavarjani et al.2 showed a similar result, with a difference of 8.42 mm measured with a Scheimpflug system. As Khachikian and Belin note, the interocular central and thinnest pachymetry difference between our patient's eyes3 was much higher than the mean found in normal eyes. After adjusting the nasal–temporal inversion of the pachymetry maps (Orbscan II, Bausch & Lomb), this difference reached a maximum of 28 mm in the mid superotemporal periphery.3 The patient in our case was referred to our institution, and at the time the article was published, the preoperative raw data of the topography maps were inaccessible. However, we were able to obtain them recently and calculate the following corneal spatial profiles: percentage of thickness increase, normalized percentage of anterior curvature variation, and normalized percentage of posterior curvature variation. These measurements were obtained from the raw elevation data after recentration of the
Michael W. Belin, MD Tucson, Arizona, USA REFERENCES 1. Saad A, Gatinel D. Bilateral corneal ectasia after laser in situ keratomileusis in patient with isolated difference in central corneal thickness between eyes. J Cataract Refract Surg 2010; 36:1033–1035 2. Khachikian SS, Belin MW, Ciolino JB. Intrasubject pachymetric asymmetry analysis. J Refract Surg 2008; 24:606–609 sio R Jr, Alonso RS, Luz A, Coca Velarde LG. Corneal3. Ambro thickness spatial profile and corneal-volume distribution: tomographic indices to detect keratoconus. J Cataract Refract Surg 2006; 32:1851–1859
Figure 1. A: Right eye percentage of thickness increase compared with the mean and 95% CI in a normal population. B: Left eye percentage of thickness increase compared with the mean and 95% CI in a normal population (PTI Z percentage of thickness increase; SD Z standard deviation).
J CATARACT REFRACT SURG - VOL 36, NOVEMBER 2010
2016
LETTERS
Figure 2. Left eye normalized percentage of anterior curvature variation compared with the mean and 95% CI in a normal population (PVAK Z percentage of variation of anterior curvature; SD Z standard deviation).
Figure 3. Left eye normalized percentage of posterior curvature variation compared with the mean and 95% CI in a normal population (PVPK Z percentage of variation of posterior curvature; SD Z standard deviation).
corneal wall on the thinnest point, as described in a recent publication.4 The profiles in Figures 1 to 3 compare the patient's percentage of thickness increase, normalized percentage of anterior curvature variation, and normalized percentage of posterior curvature variation with the mean and 95% confidence interval (CI) values in a normal population. The right and left eye percentage of thickness increase curves (Figure 1) did not show an evident abnormal shape; however, the left eye percentage of thickness increase was borderline with the lowest standard deviation curve. In the right eye, the normalized percentage of anterior curvature variation and the normalized percentage of posterior curvature variation were within the normal range. In the left eye, the normalized percentage of anterior curvature variation and the normalized percentage of posterior curvature variation were clearly abnormal (Figures 2 and 3). These findings suggest that caution may be required when pachymetric and curvature progression from the thinnest point to the periphery are altered in one or both eyes of LASIK candidates.4 We believe that in the future, these parameters, possibly associated with other data such as biomechanical indices,5,6 should be taken into consideration for proper screening of refractive surgery candidates.dAlain Saad, MD, Damien Gatinel, MD, PhD
thickness between eyes. J Cataract Refract Surg 2010; 36:1033–1035 4. Saad A, Gatinel D. Topography and tomography properties of forme fruste keratoconus corneas. Invest Ophthalmol Vis Sci 2010 June 16; [Epub ahead of print] 5. Saad A, Lteif Y, Azan E, Gatinel D. Biomechanical properties of keratoconus suspect eyes. Invest Ophthalmol Vis Sci 2010; 51:2912–2916 sio R Jr, Dawson DG, Saloma ~ o M, Guerra FP, Caiado AL, 6. Ambro Belin MW. Corneal ectasia after LASIK despite low preoperative risk: tomographic and biomechanical findings in the unoperated, stable, fellow eye. J Refract Surg 2010 May 19; [Epub ahead of print]
REFERENCES 1. Khachikian SS, Belin MW, Ciolino JB. Intrasubject corneal thickness asymmetry. J Refract Surg 2008; 24:606–609 2. Falavarjani KG, Modarres M, Joshaghani M, Azadi P, Afshar AE, Hodjat P. Interocular differences of the Pentacam measurements in normal subjects. Clin Exp Optom 2010; 93:26–30. Available at: http://onlinelibrary.wiley.com/doi/10.1111/j.1444-0938.2009.004 46.x/pdf. Accessed August 10, 2010 3. Saad A, Gatinel D. Bilateral corneal ectasia after laser in situ keratomileusis in patient with isolated difference in central corneal
Is central pachymetry asymmetry between eyes an independent risk factor for ectasia after LASIK?
Saad and Gatinel1 report bilateral ectasia after laser in situ keratomileusis (LASIK) in a patient with no risk factors identified preoperatively. They suggest the preoperative 20 mm difference in central pachymetry between eyes was a new risk factor. We have reviewed the records of 1609 consecutive patients treated with bilateral myopic LASIK between September 2003 and December 2008 using a femtosecond laser. Preoperatively, 89 patients (5.5%) had a betweeneye difference of more than 20 mm in central ultrasonic pachymetry readings. This was similar to the 5.0% rate of asymmetry exceeding 23 mm that Khachikian et al.2 detected by corneal tomography (Pentacam, Oculus, Inc.) in a separate cohort of 724 consecutive LASIK candidates. None of our 89 patients has developed ectasia at a mean of 4.5 years (range 1.6 to 6.9 years) after LASIK. This suggests that a pachymetry difference of more than 20 mm is not predictive of ectasia in the absence of other recognized risk factors, such as abnormal topography or expected residual bed thickness of
J CATARACT REFRACT SURG - VOL 36, NOVEMBER 2010