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 Table of Contents  
ORIGINAL ARTICLE
Year : 2018  |  Volume : 5  |  Issue : 3  |  Page : 69-73

Central corneal thickness measurement of non-glaucomatous adults in Ahmadu Bello University Sick Bay, Samaru, Zaria


Department of Ophthalmology, Ahmadu Bello University Teaching Hospital, Shika-Zaria, Kaduna, Nigeria

Date of Web Publication29-Jul-2019

Correspondence Address:
Kehinde Oladigbolu
Department of Ophthalmology, Ahmadu Bello University Teaching Hospital, Shika-Zaria, Kaduna
Nigeria
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DOI: 10.4103/ssajm.ssajm_30_18

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  Abstract 


Background: Central corneal thickness (CCT) is the measurement of the center of the cornea and also a parameter for assessing the corneal health status. Normal mean values for CCT range between 521 μm and 554 μm due to racial and age differences; this is an important factor to consider when measuring intraocular pressure (IOP). The CCT is supposed to influence the IOP measurement in the cornea with overestimation of IOP in thicker corneas and an underestimation in thinner corneas.
Objectives: To determine the mean CCT of nonglaucomatous adults in the Eye Clinic of Ahmadu Bello University (ABU) Sick Bay Samaru, Zaria, and the correlation between CCT and IOP, age, gender, and refractive error.
Materials and Methods: Consenting adults who met the inclusion criteria were selected using simple random sampling. The participants had visual acuity assessment for both distance and near and detailed ocular examination. IOP and CCT were measured with Perkins handheld applanation tonometer and PacScan 300AP ultrasonic pachymeter, respectively. Data were entered in a pretested questionnaire and analyzed using Statistical Package for the Social Sciences (SPSS) version 23.0.
Results: More female participants were recorded in the study, 55 (67.9%). The mean age of all participants was 40.01 years (SD ± 13.86). The combined mean CCT for both eyes was 531.18 µm (95% CI, 522.65–539.71). A moderate inverse correlation shows the CCT decreases with increasing age (r = –0.42, P < 0.001) and a weak linear correlation with presbyopia was significant (r = 0.23, P = 0.03). No significant correlation was found between CCT and IOP, gender, and other types of refractive error.
Conclusion: The mean CCT of nonglaucomatous adults in this study is comparable to those of other Nigeria and African figures. There is a very significant inverse correlation between CCT and age.

Keywords: Central corneal thickness, intraocular pressure, nonglaucomatous adults


How to cite this article:
Oladigbolu K, Abdullahi H, Abdulsalam H, Gana O, Kadala N, Pam V. Central corneal thickness measurement of non-glaucomatous adults in Ahmadu Bello University Sick Bay, Samaru, Zaria. Sub-Saharan Afr J Med 2018;5:69-73

How to cite this URL:
Oladigbolu K, Abdullahi H, Abdulsalam H, Gana O, Kadala N, Pam V. Central corneal thickness measurement of non-glaucomatous adults in Ahmadu Bello University Sick Bay, Samaru, Zaria. Sub-Saharan Afr J Med [serial online] 2018 [cited 2019 Nov 19];5:69-73. Available from: http://www.ssajm.org/text.asp?2018/5/3/69/263566




  Introduction Top


The cornea is the outermost layer of the eyeball anteriorly, and it acts as a clear refractive surface and a protective barrier to infection and trauma. The cornea is about 1-mm thick at the periphery and 0.5-mm thick at the center.[1] Central corneal thickness (CCT) is the measurement of the center of the cornea, is a parameter for assessing the corneal health status, and provides valid information about corneal physiological condition and possible changes associated with diseases, traumas, and hypoxia.[2]

CCT has been identified as an important factor to consider when measuring intraocular pressure (IOP).[3] Although Goldmann applanation tonometer (GAT) is the accepted “gold standard” of tonometry, other methods of measurement are also in use, including, among others, Perkins handheld, various forms of noncontact tonometer (NCT), and the dynamic contour tonometer (DCT).[4]

Clinically, the measurement of IOP is useful not only in diagnosis of glaucoma but also to assess its response to treatment and the progression of the disease.[5] The measurement of IOP is known to be affected by corneal biomechanical factors such as corneal curvature, CCT, hydration, elasticity, hysteresis, and rigidity.[6]

The CCT is supposed to influence the IOP measurement in the cornea with overestimation of IOP in thicker corneas and an underestimation in thinner corneas. Correction of the IOP is based on using the linear correction formula[7]:



Based on this fact, CCT should be taken into consideration for all glaucoma patients. For every 30–100 μm difference in CCT from the mean in either way, there is an approximately 1.1–9.8 mmHg difference in the estimated IOP from the mean IOP (13.40 mmHg).[6],[8] Normal mean values for CCT range between 521 μm and 554 μm due to the racial difference.[9] African American and other Africans have thin cornea (521.0–530.2 μm)[4],[5],[7],[10] compared to the Caucasian population (537.8–551.9 μm).[5],[7],[11]

Reports from the previous studies varied on the effect of refractive status on CCT. Some reported thicker CCT in myopic, whereas others found thinner CCT, and yet others found no correlation between CCT and myopia.[12],[13] The introduction of laser refractive procedure (photorefractive keratectomy [PRK] and laser in situ keratomileusis [LASIK]) has also brought about increasing interest in determination and evaluation of differences in normal CCT.[14] The CCT is also used for contact lens fitting.

In line with the global trend, glaucoma is the second commonest cause of blindness in Nigeria with a prevalence of 0.7%.[3],[15] IOP is the most significant risk factor for glaucoma and remains at present the only modifiable parameter for which treatment has been demonstrated to reduce incidence and progression.[16]

Currently, there is no data on the mean CCT in nonglaucomatous adults available in ABU Zaria for management of patients seen in the eye clinic. There is also no data available in North Western zone of Nigeria for reference, obtaining a mean CCT will aid in the diagnosis and management of those patients.

This study aims to provide a mean CCT measurement and the correlation between CCT and IOP, age, gender, and refractive error of nonglaucomatous adults seen in the eye clinic, ABU Sick Bay, Samaru, Zaria.


  Materials and Methods Top


The ABU Sick Bay serves staff and students of the university and members of the host community from a diverse ethnic background. Participants for this cross-sectional hospital-based study, conducted in March 2016, were drawn from patients attending the eye clinic that can be conceivable as a fair representation of the area in general. Written informed consent was obtained from all participants who met the inclusion criteria and were selected by simple random sampling method. Ethical approval was also obtained from the Health Research Ethics Committee (HREC) and study conducted in accordance with the Helsinki declaration.

Inclusion criteria: Participants more than 18 years of age with no history of diabetes, contact lens wear, ocular trauma, previous eye surgery, glaucoma, corneal scar, and use of drugs that affect the cornea such as carbonic anhydrase inhibitor, prostaglandins, and steroids. An IOP of less than 21 mmHg and refractive error is also one of the criteria.

Sample size determination: The sample size was calculated using appropriate formula for descriptive study; N = 4σ2 (Zcrit)2/D2, where N = sample size of the single study group, σ = the assumed standard deviation of the group = +33.98 µm,[4] Zcrit = standard normal deviate, set at 1.960, which corresponds to the 95% confidence interval, and D = the total width of the expected confidence interval (CI). In this study, 5.8 µm above and 5.8 µm below was considered as the width of the CI = 11.6. One hundred and sixty eyes of 80 participants were the calculated sample size.

Procedures: Visual acuity was assessed for distance, unaided and with pinhole, and near using the Snellen’s and tumbling E charts. Urinalysis was carried out with dipstick. A detailed history was followed by eye examination with the slit lamp biomicroscope (BM 900, Haag Streit, UK). Refraction was performed using Nidek autorefractor (Nidek Co. Ltd., Japan), taking an average of two readings, whereas the CCT was measured taking an average of five readings using PacScan 300AP (Sonomed, Lake Success, New York, USA) ultrasonic pachymeter, in the multiple point, multiple reading mode.

The IOP was measured, between 09.00 am and 12.00 noon, using Perkins MK3 handheld applanation tonometer (Haag Streit, UK) taking average of three readings. Topical anesthesia was achieved with 0.5% amethocaine hydrochloride instilled into the inferior conjunctival fornix of the participant’s eye, and the pachymeter probe and tonometer prisms were sterilized with 0.05% hypochlorite solution and allowed to air-dry.

Data entry and analysis: Demographics, examination findings, and all measurements were entered on a pretested questionnaire and analyzed with Statistical Package for the Social Sciences, version 23.0 (IBM SPSS Statistics, Chicago, Illinois, USA).


  Results Top


More female participants were recorded in the study, 55 (67.9%). The mean age of all participants was 40.01 years (SD ± 13.86), majority (31.25%) were in the 40–49 years age group. The combined mean CCT for both eyes was 531.18 µm (95% CI, 522.65–539.71), as shown in [Table 1]. A moderate inverse correlation shows the CCT decreases with increasing age (r = –0.42, P < 0.001) and a weak linear correlation with presbyopia was significant (r = 0.23, P = 0.03) [[Table 2]]. The mean IOP was 13.35 mmHg (SD ± 3.69). No significant correlation was found between CCT and IOP, gender, and other types of refractive error.
Table 1 Descriptive statistics of CCT measured in all participants

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Table 2 Correlation between CCT and age, IOP, and refractive status

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There was no statistically significant difference in the mean CCT of males 528.67 µm (SD ± 43.57) and females 533.69 µm (SD ± 34.50). Also, a one-way analysis of variance (ANOVA) shows that gender has no significant effect on CCT (P = 0.40) [[Table 3]].
Table 3 Descriptive statistics of CCT according to age and gender

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  Discussion Top


The management of glaucoma is a complex procedure using IOP control as a modifiable risk factor for monitoring treatment and disease progression. The effect of CCT cannot be overemphasized in making an appropriate diagnosis, as variations in mean CCT have been observed in different types of glaucoma.[17] The distribution in different population varies widely and may be influenced by factors such as race, age, gender, and ethnicity, as reported by various researchers.[4],[8],[18],[19],[20] IOP measured may be overestimated or underestimated in thick or thin corneas, respectively.

The mean CCT found in this study was 531.18 ± 38.33 µm for patients without glaucoma from a diverse ethnic origin in Nigeria. This value is slightly lower but comparable to earlier studies in Nigerians by Mercieca et al.,[18] 535 ± 38 µm, and Babalola et al., 537.9 µm.[4] It is also more like findings in nonglaucomatous population in Sudan, Cameroon, and Egypt with mean CCT of 530.15 ± 58.10 µm, 528.74 ± 35.89 µm, and 530 ± 38.03 µm in both eyes, respectively.[5],[8],[10] African Americans tend to have lower CCT than all races, 521.0–528.5 µm.[8] Iyamu et al.[14] reported a much higher value of 547 ± 29.5. These variations in the reported value of mean CCT across the country could be due to the age range of the subjects and method of measurement, depicting differences in a supposedly homogenous race. Higher values of mean CCT were reported in Polish (563.0 µm), Caucasians (550.4 µm), Chinese (555.6 µm), Filipino (550.6 µm), and Hispanic (548.1 µm) in a review.[6] Studies have shown that, depending on the method used, statistically different CCTs may be obtained.[21] Optical low coherence reflectometry represents the most precise pachymetric method available, with its measurements reproducible to 1 µm.[22]

This study showed a very significant negative correlation between age and CCT. The CCT was progressively thinner with increasing age. Although this finding agrees with several investigators who reported a significant effect of age on CCT,[23],[24],[25],[26] others found that age did not affect CCT.[5],[27] Using a regression model, previous studies by Iyamu and Osuobenib[23] have demonstrated an average of 5.0–7.0 µm decrease in CCT in Nigerians for every 10 years increase in age, whereas Aghaian et al.[28] reported a decrease of 3.0 µm per decade. Scientists have observed that keratocytes are the major cellular components of the cornea stroma, and the keratocytes density decreases with age with the breakdown of collagen fibers as part of the normal aging process. These changes, coupled with the effect of long-term exposure to environmental factors, are the most likely reasons for the observed reduction in CCT with age.[29],[30]

Although females had thicker cornea (533.69 ± 34.50 µm) than males (528.67 ± 43.57 µm) in our study, this was not statistically significant (P = 0.40) and could be due to larger number of female (67.9%) in the sample. This is consistent with the report of Iyamu et al., Aghaian et al. and Eysteinsson et al.[23],[28],[30] Mercieca et al.[18] found significant (P = 0.035) gender-related difference in CCT of Nigerian adults (males: mean, 541.0 ± 47.0 µm; females: mean, 522.0 ± 22.0 µm), whereas others reported thicker CCT in males than females in Caucasians and Latinos.[19],[31],[32]

The mean IOP in this study was 13.35 ± 3.69 mmHg; it had no correlation with CCT (P = 0.63). Handheld Perkins tonometer was used, and the finding was like that of other researchers who used Pulsair noncontact and GAT.[4],[18],[20] This demonstrates that the method of measurement of IOP may not be a barrier in the management of glaucoma patients if the equipment is properly calibrated and used correctly. Studies by Andre et al. and Nazim et al. observed a positive correlation between CCT and IOP with an increase of 2.8 mmHg in IOP for 100 µm rise in CCT.[5],[8] Appropriate correction needs to be made because underestimation of the IOP in patients with primary open angle glaucoma (POAG) who have thin corneas may lead to a misdiagnosis of normal tension glaucoma (NTG), whereas overestimation of the IOP in normal subjects who have thick corneas may lead to a misdiagnosis of ocular hypertension (OHT). Similarly, patients of NTG with thin cornea may be cases of POAG when CCT-corrected IOP is taken.[17]There are conflicting reports on the effect of refractive status on CCT. Although a study found that myopic subjects had thicker CCT,[12] others report thinner CCT, and yet others found no correlation between CCT and myopia.[8],[18],[25],[33] We found a weak linear correlation with presbyopia that was significant (r = 0.23, P = 0.03). The reason for this cannot be explained and has not been reported before. However, there was no correlation with other types of refractive errors. In a study by Chinawa et al., there was significant thinning of the myopic cornea compared to emmetropes, but no correlation was noted between CCT and the myopia. It was explained that although myopes in our environment have statistically significant thinner cornea, this does not account for their myopic status.[13]

Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.



 
  References Top

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Babalola OE, Kehinde AV, Iloegbunam AC, Akinbinu T, Moghalu C, Onuoha I. A comparison of the Goldmann applanation and non-contact (Keeler Pulsair EasyEye) tonometers and the effect of central corneal thickness in indigenous African eyes. Ophthalmic Physiol Opt 2009;29:182-8.  Back to cited text no. 4
    
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American Academy of Ophthalmology. Practical ophthalmology: A manual for beginning residents. 6th ed. San Francisco, CA: AAO; 2009. p. 213-2.  Back to cited text no. 9
    
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Kunert KS, Bhartiya P, Tandon R, Dada T, Christian H, Vajpayee RB. Central corneal thickness in Indian patients undergoing LASIK for myopia. J Refract Surg 2003;19:378-9.  Back to cited text no. 12
    
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Chinawa NE, Pedro-Egbe CN, Ejimadu CS. Association between myopia, and central corneal thickness among patients in a tertiary hospital in south-south Nigeria. Adv Ophthalmol Vis Syst 2016;5:00147.  Back to cited text no. 13
    
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