Microsoft Word - Mazharul Hassan 79 Original Article Relationship between Central Corneal Thickness and Intraocular Pressure in Selected Pakistani Population Mazhar ul Hassan, Aziz ur Rehman, Munawar Abbas, Umar Fawad, Nasir Bhatti, Ashraf Daud Pak J Ophthalmol 2010, Vol. 26 No. 2 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . See end of article for authors affiliations ………………………… Correspondence to: Mazhar ul Hassan Consultant Eye Surgeon Al-Ibrahim Eye Hospital/ Isra Postgraduate Institute of Ophthalmology, Old Thana, Malir, Karachi Received for publication July’ 2009 ………………………… Purpose: To explore the relationship between central corneal thickness (CCT) and intraocular pressure (IOP) in selected Pakistani population and to formulate recommendations for its application in clinical practice as well as future research. Material and Methods: The study was conducted at Al-Ibrahim Eye Hospital Karachi for six months, on five hundred eyes of 250 adults attending the outpatient. After informed consent, all patients underwent a comprehensive assessment including medical and ophthalmic history and examination. Intraocular pressure (IOP) was measured with Goldmann Applanation Tonometer and CCT with ultrasonic pachymeter. Results: Mean CCT in males was measured as 529.5 ± 33.6 (range 438-619 µm) and 524.1 ± 33.3 (range 443-623 µm) in females. Mean IOP in males was 12.75 ± 2.85 (range 8-20 mmHg) and in females 12.98 ± 2.39 (range 8-20 mmHg). There was a statistically significant association between CCT and IOP for normal subjects (Pearson correlation coefficient r=0.136, p=0.022). However, no statistically significant relationship was found between CCT and age (p=0.103). Conclusion: CCT is a significant predictor of IOP. Thin corneas lead to an underestimation and thick corneas, to an overestimation of intraocular pressure. Pakistani population in our study has comparatively thinner corneas as compared to Caucasian and African-American population. However further studies with sufficiently large sample size are needed to validate and to demonstrate its original value. he normal intraocular pressure varies from 10- 20 mmHg. Goldmann applanation tonometer is considered to be the gold standard in measuring intraocular pressure1. It works according to Imbert-Fick principle, which states that for an ideal, dry, thin walled sphere, the pressure inside equals to the force necessary to flatten its surface divided by the area of flattening2. A pachymeter is a device that uses ultrasound to determine the thickness of the cornea in any given location. Normal CCT is 490-560 µm2. The clinical use of central corneal thickness measurements has become so important that it directly affects glaucoma management strategy in 15% of patients3, 4. Various studies have been performed worldwide to study the significance of CCT in measuring IOP and glaucoma management. We want to perform a similar study to find out the relationship between central corneal thickness and intraocular pressure in Pakistani individuals. MATERIALS AND METHODS We conducted this study at Al-Ibrahim Eye Hospital (AIEH), Karachi. It was a prospective, non intervene- tional, comparative study which included 500 eyes of 250 adults who attended the outpatient department over a period of 6 months. We used non-purposive, convenience sampling. As this was a descriptive study, therefore no sample size calculation was made. The inclusion criterion was age between 40 and 60 yrs T 80 regardless of the gender. We excluded patients with preexisting ocular pathologies, history of contact lens wear, history of intraocular surgery, laser or trauma, corneal astigmatism greater than 3 diopters, patients with systemic illness or taking any topical or systemic medications. The patients were selected from outpatient department of AIEH. After informed consent, all patients underwent a comprehensive ophthalmic assessment which consisted of history regarding refractive errors, glaucoma, use of topical steroids, use of contact lenses, history of refractive surgery or laser. Best corrected visual acuity was obtained followed by slit lamp examination to rule out anterior segment pathologies corneal pathologies and infections. After anesthetizing the eye with topical proparacaine 0.5% and using the fluorescein strips 2%, we measured IOP in both eyes using Goldmann applanation tonometer. We examined all the patients between 9:00am-12:00noon. We took three consecutive readings and the mean was noted. CCT was measured with ultrasonic pachymeter (Pac Scan 300p digital biometric ruler). The ultrasound pachymeter was calibrated at the beginning of each day according to the manufacturer’s instructions. After anesthetising the cornea with topical proparacaine 0.5% and the patient looking in primary position of gaze, the pachymeter probe was placed on the centre of the cornea. Five measurements were taken from each eye and the average was used for analysis. The data was entered in MS Excel and was cleaned and analyzed using SPSS v. 10.0. Mean ± SD was calculated for all quantitative variables. Frequencies and percentage was computed for sex. Pearson correlation test was applied to find the relationship between corneal thickness and intraocular pressure at p ≤ 0.05 level of significance. RESULTS Out of 250 patients, 130 (52%) were males and 120 (48%) were females. Most of the patients 83(33.2%) belonged to the age group of 40-44 years. The mean CCT in males was 529.5 ± 33.6 (range 438-619µm) and 524.1 ± 33.3 (range 443-623µm) in females. The mean intraocular pressure in males was 12.75 ± 2.85 (range 8-20 mmHg) and 12.98 ± 2.39 (range 8-20 mmHg) in females. A significant association was found between central corneal thickness and intraocular pressure for normal subjects (Pearson correlation coefficient r=0.136, p=0.022). There was no statistically significant relationship between the central corneal thickness and age. (p =0.103). Table 1. Mean intraocular pressure for both genders according to central corneal thickness n=250 CCT (µm) Male Female n IOP (mm Hg) n IOP(mm Hg) < 525 61 12.67± 2.8 67 12.81± 2.25 525 – 575 58 12.75± 2.9 47 13.05± 2.35 > 575 11 13.5± 3.51 6 11± 4.2 CCT = Central Corneal Thickness Mid Values CCT (µm) 62 5 60 5 58 5 56 5 54 5 52 5 50 5 48 5 46 5 44 5 N um be r o f P at ie nt s 70 60 50 40 30 20 10 0 16 35 49 59 46 27 9 5 Fig. 1. Distribution of Central Corneal Thickness Mean± SD = 527± 3.5 (Range = 438 - 623 (µm) Fig. 2. Relationship between central corneal thickness and intraocular pressure Pearson Correlation Coefficient (r) = 0.136, P= 0.022 DISCUSSION Intraocular pressure is an important factor that has a significant influence in the diagnosis and follow-up of ocular hypertension and glaucoma patients. Know- 81 ledge of the central corneal thickness therefore, is important to know the validity of the intraocular pressure readings. To the best of our knowledge, this is the second hospital based study from Pakistan to determine the effect of CCT on IOP5. Dueker et al concluded that CCT measured by ultrasound pachymetry is a reliable indicator of risk for progression of ocular hypertension to glaucoma. Mixed evidence was found in terms of the association of CCT with the presence of glaucoma, therefore the value of CCT measurement as a screening tool for glaucoma appears to be negligible6. The Ocular Hypertension Treatment Study (OHTS) established corneal thickness as a risk factor for glaucoma. In the opinion of Kass, the OHTS demonstrated that moderate IOP reductions could be achieved and maintained during a median follow-up period of 72 months7. In a cross-sectional study arm of the OHTS, Brandt et al set out to determine if CCT is related to race. CCT was measured in 1301 patients with ocular hyper- tension8. Ultrasonic pachymeters of the same make and models were used in all sites. The mean CCT in Caucasians was 573 µm, while the mean CCT for African-American subjects was 555.7µm. The study demonstrated that African-American subjects have thinner corneas than white subjects. The effect of CCT may influence the accuracy of applanation tonometry in the diagnosis, screening and management of patients with glaucoma and ocular hypertension. La Ros reported a comparative study of CCT of Caucasians and African-Americans in glaucomatous and non glaucomatous populations. A statistically significant difference was found between the central corneal thickness of African-Americans (n=56) and Caucasians (n=32) who had suspected or confirmed glaucoma from control populations of African- Americans (n=56) and Caucasian (n=51) subjects who had no evidence of glaucoma, elevated intraocular pressure (IOP) or optic nerve damage. It is proposed that the finding that African-Americans have thinner corneas than Caucasians may lead to lower applanation, intraocular pressure readings, and potentially result in an underestimation of the actual level of intraocular pressure9. The Ocular Hypertension Treatment Study is the first to establish corneal thickness as a risk factor for glaucoma. Based on the results of this study, the American Academy of Ophthalmology Preferred Practice Pattern on Evaluation of the Glaucoma Suspect recommends measurement of corneal thickness with electronic pachymetry in evaluating the glaucoma suspect7,10. Our study had 500 eyes of 250 subjects. The mean CCT in males was 529.5 ± 33.6 and 524.1 ± 33.3 in females. In contrast the earlier studies had mentioned that the mean CCT in Caucasians was 573 µm, and 555.7µm in African-American subjects. Figure 1 illus- trates the normal Gaussian curve for distribution of CCT in our patients. It demonstrates that majority of patients had mean CCT in the range of 525µm, which is smaller when compared to the other ethnicities. Our study has the following limitation. Firstly, the sample size is relatively small and may not be able to detect the exact relationship between IOP and CCT. Secondly, we used convenience sampling and all the samples were taken from the hospital. Therefore, our findings cannot be extrapolated to the general Pakistani population. Thirdly, our hospital is a charity hospital and majority of our patients belong to under- privileged group of society therefore our findings may be biased with factors such as socioeconomic status, occupation, exposure to sunlight etc. In conclusion, thin corneas lead to an under- estimation and thick corneas, to an overestimation of intraocular pressure. CCT is a significant predictor of IOP. Pakistani population has comparatively thinner corneas when compared to Caucasian and African- American population. However further studies with sufficiently large sample size are needed to validate this finding and to demonstrate its value in the management of glaucoma. Author’s affiliation Dr. Mazhar ul Hassan Al-Ibrahim Eye Hospital / Isra Postgraduate Institute of Ophthalmology Old Thana, Malir, Karachi Dr. Aziz ur Rehman Al-Ibrahim Eye Hospital / Isra Postgraduate Institute of Ophthalmology Old Thana, Malir, Karachi Dr. Munawar Abbas Al-Ibrahim Eye Hospital / Isra Postgraduate Institute of Ophthalmology Old Thana, Malir, Karachi Dr. Umar Fawad Al-Ibrahim Eye Hospital / Isra Postgraduate Institute of Ophthalmology Old Thana, Malir, Karachi 82 Dr. Nasir Bhatti Al-Ibrahim Eye Hospital / Isra Postgraduate Institute of Ophthalmology Old Thana, Malir, Karachi. Dr. Ashraf Daud Al-Ibrahim Eye Hospital / Isra Postgraduate Institute of Ophthalmology Old Thana, Malir, Karachi REFERENCE 1. 0Kanski JJ. Clinical Ophthalmology, Fifth edition, Edinburgh, Elsevier Science. 2003; 100: 196. 2. Feltgen N, Leifert D, Funk J. Correlation between central corneal thickness, applanation tonometry and direct intracameral intraocular pressure readings. Br. J. Ophthalmol. 2001; 85: 85-7. 3. Weizer J S, Stinnett S S, Herndon L W. Longitudinal changes in central corneal thickness and their relation to glaucoma status: and 8 year follow up study. Br J Ophthalmol. 2006; 90: 732-6. 4. Doughty MJ, Zaman ML. Human corneal thickness and its impact on intraocular pressure measures: A review and meta- analysis approach. Surv Ophthalmol. 2000; 44: 367-408. 5. Mukhtar S. Central corneal thickness and its relation with measured intraocular pressure. Pak J Ophthalmol. 2004; 20: 23- 5. 6. Dueker DK, Singh K, Lin SC, et al. Corneal thickness measurement in the management of primary open angle glaucoma: a report by the American Academy of Ophthalmology. Ophthalmology. 2007; 114: 1779-87. 7. Kass MA, Heuer DK, Higginbotham EJ, et al. The Ocular Hypertension Treatment Study: A randomized trial determines that topical ocular hypertensive medication delays or prevents the onset of primary open-angle glaucoma. Arch Ophthalmol. 2002; 120: 701-13. 8. Brandt JD, Beiser JA, Kass MA, et al. Central corneal thickness in the Ocular Hypertension Treatment Study (OHTS). Ophthalmology. 2001; 108: 1779-88. 9. La Rosa FA, Gross RL, Orengo-Nania S. Cenral corneal thickness of Caucasians and African Americans in glaucomatous and nonglaucomatous populations; Arch Ophthalmol. 2001; 119: 23-7. 10. Gordon MO, Beiser JA, Brandt JD, et al. The Ocular Hypertension Treatment Study: Baseline factors that predict the onset of primary open angle glaucoma. Arch Opthalmol. 2002; 120: 714-20.