LETTERS
Central Corneal Thickness and Intraocular Pressure in a Nepalese Population: The Bhaktapur Glaucoma Study To the Editor: We read with great interest the study by Thapa et al1 describing the characteristics of central corneal thickness (CCT) and intraocular pressure (IOP) in Nepalese population. Nepalese represent a Himalayan Asian population.1 The authors proved that the changes in CCT values cause a significant variation in IOP in this population. We agree with the authors about this subject; however, there are also some issues that should be mentioned. As this study was performed at a high altitude region (Bhaktapur, Nepal), the consistency of the results could have been affected. The measurement methods of the study by Thapa et al1 could have been affected by the weather conditions, atmosphere pressure, and hydroregulation of the body caused by high altitudes.2,3 Although there are not enough wideranged studies about the ophthalmologic changes at high altitudes, changes in corneal endothelial cells with the result of increase in CCT have been reported at the chronic hypoxic environment of high altitudes.3–5 Correspondingly, in this study, it was found that the Nepalese have thicker corneas. Besides, changes in IOP at high altitudes have been reported in some studies, but no consistent results were received.2,6,7 The changes in IOP and CCT may occur at acute altitude exposure secondary to pulmonary pressure changes, respiratory alkalosis, and even intracranial pressure changes. Although little decrease in those changes is usually seen during the acclimatization process, the duration of those changes is still not known exactly for chronic altitude exposure. Although the measurements taken by Disclosure: The authors declare no conflict of interest. DOI: 10.1097/IJG.0000000000000180
J Glaucoma
TO THE
EDITOR
Goldmann tonometry are corrected according to CCT, individual factors, especially corneal biomechanics, which are not included in this study, might have affected the results. This possibility was previously indicated in the study of Karadag˘ et al.8 In their study, although the IOP measurements taken by tonopen after short-term exposure to hypobaric hypoxic environment were corrected according to CCT, the results were still higher than the previous measurements as a result of the changes in corneal biomechanics secondary to hypoxic conditions. In conclusion, not only racial variations and different instrumentation but also hypoxic hypobaric conditions at high altitude may result in differences in CCT and IOP results in Nepalese population. The Bhaktapur Glaucoma Study, which was performed at a high altitude, is considered to be more realistic in reflecting the individuals’ results who live in highaltitude environments. Gokcen Gokce, MD* Cem Ozgonul, MDw *Department of Ophthalmology Sarıkamis Military Hospital, Kars wDepartment of Ophthalmology Anittepe Dispensary, Ankara, Turkey
REFERENCES 1. Thapa SS, Paudyal I, Khanal S, et al. Central corneal thickness and intraocular pressure in a Nepalese population: the Bhaktapur Glaucoma Study. J Glaucoma. 2012;21:481–485. 2. Karakucuk S, Mujdeci M, Baskol G, et al. Changes in central corneal thickness, intraocular pressure, and oxidation/ antioxidation parameters at high altitude. Aviat Space Environ Med. 2012;83: 1044–1048. 3. Karaku¨c¸u¨k S, Mirza GE. Ophthalmological effects of high altitude. Ophthalmic Res. 2000;32:30–40. 4. Morris DS, Somner JEA, Scott KM, et al. Corneal thickness at high altitude. Cornea. 2007;26:308–311. 5. Somner JE, Morris DS, Scott KM, et al. What happens to intraocular pressure at high altitude? Invest Ophthalmol Vis Sci. 2007;48:1622–1626. 6. Bayer A, Yumusak E, Sahin OF, et al. Intraocular pressure measured at ground level and 10,000 feet. Aviat Space Environ Med. 2004;75:543–545. 7. Ersanli D, Yildiz S, Sonmez M, et al. Intraocular pressure at a simulated altitude of 9000 m with and without 100% oxygen. Aviat Space Environ Med. 2006; 77:704–706.
Volume 24, Number 3, March 2015
8. Karadag˘ R, Sen A, Golemez H, et al. The effect of short-term hypobaric hypoxic exposure on intraocular pressure. Curr Eye Res. 2008;33:864–867.
Intraocular Pressure: Is it a Risk Factor in Normal Tension Glaucoma? To the Editor: I read with interest the article by Lee et al1 evaluating risk factors for progression in low (group A) and high (group B) intraocular pressure (IOP) groups of normal tension glaucoma (NTG). The authors found mean IOP as significant risk factor for progression in group B, whereas disc hemorrhage was a risk factor for group A. Although this suggests IOPdependent damage in eyes with higher IOP and vascular or systemic risk factors as main mechanism in eyes with low IOP, the interpretation becomes difficult when the results are viewed with attention. The authors found no difference in the incidence of progression and progression rate between the 2 groups, even though the mean IOP and mean decrement of IOP were different. The role of IOP reduction in halting or slowing progression in NTG has been demonstrated by randomized trials.2,3 This statement contradicts the theory that risk factors are different in the 2 groups as it seems that the negative effect of central corneal thickness in group A and mean IOP in group B got negated by other factors thereby making the rate of progression similar in the 2 groups. A close look at the methodology would explain the above observations. The mean IOP was calculated by all IOP obtained during follow-ups in the 2 groups. A relook into the clinical variables shows that the 2 groups were not significantly different in terms of other IOP parameters including fluctuation which meant group B would have been more aggressively treated Disclosure: The author declares no conflict of interest. DOI: 10.1097/IJG.0000000000000181
www.glaucomajournal.com |
251
Central Corneal Thickness and Intraocular Pressure in a Nepalese Population: The Bhaktapur Glaucoma Study To the Editor: We read with great interest the study by Thapa et al1 describing the characteristics of central corneal thickness (CCT) and intraocular pressure (IOP) in Nepalese population. Nepalese represent a Himalayan Asian population.1 The authors proved that the changes in CCT values cause a significant variation in IOP in this population. We agree with the authors about this subject; however, there are also some issues that should be mentioned. As this study was performed at a high altitude region (Bhaktapur, Nepal), the consistency of the results could have been affected. The measurement methods of the study by Thapa et al1 could have been affected by the weather conditions, atmosphere pressure, and hydroregulation of the body caused by high altitudes.2,3 Although there are not enough wideranged studies about the ophthalmologic changes at high altitudes, changes in corneal endothelial cells with the result of increase in CCT have been reported at the chronic hypoxic environment of high altitudes.3–5 Correspondingly, in this study, it was found that the Nepalese have thicker corneas. Besides, changes in IOP at high altitudes have been reported in some studies, but no consistent results were received.2,6,7 The changes in IOP and CCT may occur at acute altitude exposure secondary to pulmonary pressure changes, respiratory alkalosis, and even intracranial pressure changes. Although little decrease in those changes is usually seen during the acclimatization process, the duration of those changes is still not known exactly for chronic altitude exposure. Although the measurements taken by Disclosure: The authors declare no conflict of interest. DOI: 10.1097/IJG.0000000000000180
J Glaucoma
TO THE
EDITOR
Goldmann tonometry are corrected according to CCT, individual factors, especially corneal biomechanics, which are not included in this study, might have affected the results. This possibility was previously indicated in the study of Karadag˘ et al.8 In their study, although the IOP measurements taken by tonopen after short-term exposure to hypobaric hypoxic environment were corrected according to CCT, the results were still higher than the previous measurements as a result of the changes in corneal biomechanics secondary to hypoxic conditions. In conclusion, not only racial variations and different instrumentation but also hypoxic hypobaric conditions at high altitude may result in differences in CCT and IOP results in Nepalese population. The Bhaktapur Glaucoma Study, which was performed at a high altitude, is considered to be more realistic in reflecting the individuals’ results who live in highaltitude environments. Gokcen Gokce, MD* Cem Ozgonul, MDw *Department of Ophthalmology Sarıkamis Military Hospital, Kars wDepartment of Ophthalmology Anittepe Dispensary, Ankara, Turkey
REFERENCES 1. Thapa SS, Paudyal I, Khanal S, et al. Central corneal thickness and intraocular pressure in a Nepalese population: the Bhaktapur Glaucoma Study. J Glaucoma. 2012;21:481–485. 2. Karakucuk S, Mujdeci M, Baskol G, et al. Changes in central corneal thickness, intraocular pressure, and oxidation/ antioxidation parameters at high altitude. Aviat Space Environ Med. 2012;83: 1044–1048. 3. Karaku¨c¸u¨k S, Mirza GE. Ophthalmological effects of high altitude. Ophthalmic Res. 2000;32:30–40. 4. Morris DS, Somner JEA, Scott KM, et al. Corneal thickness at high altitude. Cornea. 2007;26:308–311. 5. Somner JE, Morris DS, Scott KM, et al. What happens to intraocular pressure at high altitude? Invest Ophthalmol Vis Sci. 2007;48:1622–1626. 6. Bayer A, Yumusak E, Sahin OF, et al. Intraocular pressure measured at ground level and 10,000 feet. Aviat Space Environ Med. 2004;75:543–545. 7. Ersanli D, Yildiz S, Sonmez M, et al. Intraocular pressure at a simulated altitude of 9000 m with and without 100% oxygen. Aviat Space Environ Med. 2006; 77:704–706.
Volume 24, Number 3, March 2015
8. Karadag˘ R, Sen A, Golemez H, et al. The effect of short-term hypobaric hypoxic exposure on intraocular pressure. Curr Eye Res. 2008;33:864–867.
Intraocular Pressure: Is it a Risk Factor in Normal Tension Glaucoma? To the Editor: I read with interest the article by Lee et al1 evaluating risk factors for progression in low (group A) and high (group B) intraocular pressure (IOP) groups of normal tension glaucoma (NTG). The authors found mean IOP as significant risk factor for progression in group B, whereas disc hemorrhage was a risk factor for group A. Although this suggests IOPdependent damage in eyes with higher IOP and vascular or systemic risk factors as main mechanism in eyes with low IOP, the interpretation becomes difficult when the results are viewed with attention. The authors found no difference in the incidence of progression and progression rate between the 2 groups, even though the mean IOP and mean decrement of IOP were different. The role of IOP reduction in halting or slowing progression in NTG has been demonstrated by randomized trials.2,3 This statement contradicts the theory that risk factors are different in the 2 groups as it seems that the negative effect of central corneal thickness in group A and mean IOP in group B got negated by other factors thereby making the rate of progression similar in the 2 groups. A close look at the methodology would explain the above observations. The mean IOP was calculated by all IOP obtained during follow-ups in the 2 groups. A relook into the clinical variables shows that the 2 groups were not significantly different in terms of other IOP parameters including fluctuation which meant group B would have been more aggressively treated Disclosure: The author declares no conflict of interest. DOI: 10.1097/IJG.0000000000000181
www.glaucomajournal.com |
251
