Nigerian Journal of Ophthalmology

ORIGINAL ARTICLE
Year
: 2021  |  Volume : 29  |  Issue : 1  |  Page : 17--21

Exploring the Relationship Between Central Corneal Thickness and Intraocular Pressure Among Nonglaucoma Patients in a General Ophthalmology Clinic, South East Nigeria


Adaku I Mbatuegwu1, Ebere O Achigbu1, Chidi U Mbatuegwu2, Florence U Nkwogu3, Afekhide E Omoti4,  
1 Department of Ophthalmology, Federal Medical Centre, Owerri, Imo State, Nigeria
2 Department of Family Medicine, Federal Medical Centre, Owerri, Imo State, Nigeria
3 Department of Ophthalmology, Imo State University Teaching Hospital Orlu, Imo State, Nigeria
4 Department of Ophthalmology, University of Benin Teaching Hospital, Edo State, Nigeria

Correspondence Address:
Adaku I Mbatuegwu
Department of Ophthalmology, Federal Medical Centre, Owerri, Imo State
Nigeria

Abstract

Context: Glaucoma is an important cause of irreversible blindness and visual impairment in Nigeria. Studies have shown that intraocular pressure (IOP) is the only modifiable risk factor for glaucoma. The true IOP of an individual, in turn, is determined by the central corneal thickness (CCT). Aim: The aim of the study is to describe the relationship between IOP and CCT among nonglaucoma patients in Owerri, Imo State, Nigeria. Methods: This was a hospital-based, cross-sectional, analytical study done on nonglaucomatous patients aged 18 years and above. A comprehensive ocular examination was carried out for all participants and IOP was measured between 8 am and 12 noon to reduce the effect of diurnal variations. Results: Four hundred and twenty-two subjects with a mean age of 40.34 ± 14.468 years and a female to male ratio of 1.7:1 participated. The mean measured IOP was 16.77 ± 4.37 mmHg, and the mean CCT was 533.05 ± 33.92 μm. The mean corrected IOP was 17.61 ± 4.57 mmHg indicating a pattern of falsely low IOP. There was a positive, statistically significant correlation between IOP and CCT (P = 0.045). Conclusion: There was a positive correlation between IOP and CCT with the resultant effect that thin corneas presented with falsely low IOP and thick corneas with falsely high IOP. Routine CCT measurement in addition to IOP measurement should be done on every patient.



How to cite this article:
Mbatuegwu AI, Achigbu EO, Mbatuegwu CU, Nkwogu FU, Omoti AE. Exploring the Relationship Between Central Corneal Thickness and Intraocular Pressure Among Nonglaucoma Patients in a General Ophthalmology Clinic, South East Nigeria.Niger J Ophthalmol 2021;29:17-21


How to cite this URL:
Mbatuegwu AI, Achigbu EO, Mbatuegwu CU, Nkwogu FU, Omoti AE. Exploring the Relationship Between Central Corneal Thickness and Intraocular Pressure Among Nonglaucoma Patients in a General Ophthalmology Clinic, South East Nigeria. Niger J Ophthalmol [serial online] 2021 [cited 2021 Nov 30 ];29:17-21
Available from: http://www.nigerianjournalofophthalmology.com/text.asp?2021/29/1/17/321642


Full Text



 Introduction



Glaucoma is a cause of irreversible blindness and the second commonest cause of visual impairment and blindness in Nigeria in individuals who are 40 years and above.[1],[2] About 147,064 persons were estimated to have vision <6/60 in 2008 as a result of glaucoma and this figure is predicted to increase to 205,266 by the year 2020.[1] Intraocular pressure (IOP) is the most important modifiable risk factor associated with glaucoma.[3],[4]

Normal IOP ranges from 11 to 21 mmHg within the general population.[5] Although there is no absolute pathological cutoff point, 21 mmHg is considered the upper limit of normal and levels above this are viewed with suspicion.[5]

The Goldmann applanation tonometer (GAT) was designed to give accurate IOP readings with a central corneal thickness (CCT) of 520 μm.[6] Studies have shown that with thick corneae, the tonometer readings were falsely higher than the actual IOP and with thin corneae the readings were falsely lower than the actual IOP, thus producing apparent ocular hypertension (OHT) and normal tension glaucoma, respectively.[6],[7] Since population-based studies have shown that IOP is a major and the only realistic modifiable risk factor for glaucoma, it is imperative to study the factors that influence IOP.[3],[4],[8],[9] These factors include age, gender, family history, blood pressure, race, and central corneal thickness among others.[8],[10] However, CCT has been reported as the only significant predictor of IOP.[11] Variations in CCT have been reported in different races. The Africans, African Americans, and Asians have been reported to have thin corneae compared to the Caucasians.[12],[13],[14] Thin central cornea was a strong predictive factor for the development of glaucoma in subjects with OHT.[15] Subjects with a corneal thickness of 555 μm or less had a threefold increased risk of developing primary open angle glaucoma compared with participants with CCT greater than 588 μm.[15]

The Bhaktapur glaucoma study[12] and the Meiktila Eye study[11] showed that changes in CCT values caused a significant variation in IOP. The former reported that a 100 μm increase in CCT was associated with a 1.03 mmHg (95% CI: 0.79–1.26) increase in IOP after adjusting for age and sex.[12]

Average CCT ranging from 535.0 to 551.6 μm have been reported in Nigerian subjects with a positive correlation reported between IOP and CCT.[13],[14] In contrast, some other studies found no significant relationship between CCT and IOP.[13],[15] Thus, CCT may influence the accuracy of applanation tonometry in the diagnosis and management of patients with glaucoma and OHT.

The global target of the WHO in Vision 2020 is to reduce blindness prevalence to less than 0.5% in all countries or less than 1% in any community.[16] The interest in IOP, therefore, is mainly because of its association with glaucoma. In addition, the practice of limiting CCT measurements to only patients with a diagnosis of glaucoma can erroneously miss patients with thin corneae and falsely low IOP who may be at risk of developing glaucoma. Findings supporting a linear relationship between IOP and CCT would provide a basis for recommending the routine use of pachymeters for IOP estimation in our Eye Clinics.

 Materials and Methods



This was a hospital-based analytical cross-sectional study carried out at the Eye Clinic of the Federal Medical Centre, Owerri, Imo State, Nigeria (FMCOW), a tertiary health care hospital on adult consecutive nonglaucoma patients 18 years and above who were not on any IOP reducing medication or antihypertensives and who had no previously diagnosed ocular abnormality or surgery and willingly gave consent to participate in this study.

Using the Leslie-Kish formula,[17] a sample size of 384, which was corrected to 422 (using an attrition rate of 10%), was determined and data were collected using a questionnaire that was tested for reliability and validity in a pilot study.

A convenience, nonprobability sampling method was used and participants were recruited consecutively after obtaining an informed written consent. The principal researcher had the sole responsibility of administering the questionnaire, measuring the systemic blood pressure, IOP, CCT, carrying out the general ocular examination, treatment, and referral. All the instruments for measurements were adequately calibrated prior to use. Participants had visual acuity measurement, IOP measurement, CCT measurement, blood pressure measurement, and ocular examination. IOP was measured using a slit lamp mounted GAT. The CCT was measured with a Sonomed PacScan 300AP ultrasound pachymeter. Five consecutive readings of the CCT taken solely by the principal researcher were recorded to the nearest thousandth of a millimeter and an average value was displayed in addition to the corrected IOP value. All measurements were taken by the researcher between 8 am and 12 noon to avoid diurnal variations and interobserver error. Prior to measurements on each participant, prevention of cross infection was done by cleaning the measuring probe using cotton wool soaked in isopropyl alcohol and air-drying the probe.

 Results



Two hundred and sixty-four females and 158 males participated in the study. Majority (107: 25.4%) were in the 41 to 50 years age group. The minimum and maximum ages of participants were 18 and 85 years, respectively, with a mean age of 40.34 ± 14.47 years. Two hundred and ninety-eight (70.6%) had tertiary education and 124 (29.4%) were skilled workers.[Table 1]{Table 1}

Measured IOP: Pearson Chichi-Square square = 2.560, df = 2, pP-value = 0.278

Corrected IOP: Pearson Chichi-Square square = 1.0618, df = 2, Pp-value =0.588

Correlation analysis for both eyes: (r = 0.797, Pp < 0.001)

The mean corrected IOP was statistically significantly higher than the mean measured IOP in both eyes: t = –7.120, df = 421, P < 0.001 for right eye and t = 2.616, df = 421, P = 0.009 for left eye.[Table 2]{Table 2}

Pearson chi-square = 5.864, df = 5, P = 0.320 (r = 0.150, P = 0.002).

Pearson chi-square = 22.115, df = 10, P = 0.015 (r = 0.166, P = 0.001).

R= right eye

L= left eye

The 61 to 70 years age group had the highest percentage of ocular hypertensives compared to the other age groups.[Table 3]{Table 3}

RE: Pearson chi-square = 25.631, df = 10, P = 0.004 (r = −0.166, P = 0.001).

LE: Pearson chi-square = 30.272, df = 10, P = 0.001 (r = –0.196, P < 0.001).

Correlation analysis for both eyes (r = 0.821, P < 0.001)

The central cornea was thinnest in the older age groups (>41 years) than in the younger age groups.

A unit micrometre increase in CCT in the right eye was associated with an increase in IOP of 0.022 mmHg while controlling for SBP and DBP. This finding was statistically significant (P < 0.001). A unit micrometre increase in CCT in the left eye was associated with an increase in IOP of 0.021mmHg while controlling for SBP and DBP. This finding was statistically significant (P = 0.001).[Table 4]{Table 4}

 Discussion



The mean age recorded in this study was similar to that reported in other hospital-based studies in Nigeria[13],[14],[18] but contrasted with that of population-based studies with larger sample sizes in other settings.[11],[12],[19] Similar studies also noted a female preponderance.[14]

A clear correlation was noted between IOP in the right eye and IOP in the left eye. This correlation was statistically significant (r = 0.797, P < 0.001), and thus the right eye findings will be adopted for comparison with other studies. The mean measured IOP was 16.77± 4.37 mmHg. This was similar to the findings in other hospital-based studies in Nigeria.[14],[20] In contrast, another hospital-based study in Nigeria reported a much lower mean IOP of 13.47 ± 3.23 mmHg.[21] However, this was a multicenter study on glaucomatous patients and controls with the lower mean pressure recorded among the controls. Regional differences and thick corneae may have been contributory factors to this disparity.

While there was no gender difference in IOP, there was a statistically significant positive correlation between IOP and age similar to other studies.[4] In contrast, other studies reported either no relationship[18] or an inverse relationship with age.[22] The disparity in these results may arise from the methodology used and differences in race. This study measured IOP with GAT, and some others[18],[22] used noncontact tonometer. However, it has been suggested that IOP estimation using noncontact tonometer is more affected by CCT than that by GAT.[13]

A clear correlation between CCT in the right eye and left eye was also noted (r = 0.821, P < 0.001). The mean CCT noted in the index study is comparable to other studies.[12],[14],[19] This may be due to similarities in age ranges of participants and race. Black participants have been noted to have lower CCT values compared to Caucasians.[19],[23] In addition, lower CCT values were more prevalent in the older age groups than in the younger age groups with a significant inverse correlation between CCT and age (r = −0.166, P = 0.001) for right eye and left eye (r = −0.196, P < 0.001), respectively. Similar findings were described by other researchers.[12],[18],[22],[23] The Rotterdam study,[24] however, found no association between CCT and age. While they studied participants aged 55 years and above, the index study had a wider range of participants (18 years and above). The decrease in CCT values with increasing age may be due to a possible decrease in keratocytes and collagen fibres in the cornea that occurs with advancing age.[25]Changes in measured IOP were noted following pachymetry indicating that CCT affects IOP [Table 1]. The mean corrected IOP was higher than the mean measured IOP in either eye showing that the participants had falsely low IOP. This finding was statistically significant (t = –7.120, df = 421, P < 0.001) for right eye and left eye (t = 2.616, df = 421, P = 0.009).

In corroboration, a positive, linear correlation between IOP and CCT, which were statistically significant (P = 0.001, r = 0.03), was noted. The linear correlation between IOP and CCT was also noted in other studies[12],[14] contrary to that reported in some studies among Blacks.[18],[19] This may be attributed to the difference in age groups studied and methodology employed in terms of timing of measurements and instruments used. These findings have implications for the management of patients with glaucoma or OHT and generally for walk-in patients with thin cornea who may develop glaucoma with advanced optic neuropathy if CCT measurement is not practiced routinely.

Considering that there are other factors that affect IOP including blood pressure, which has been reported to be high in patients with raised IOP,[3],[4],[8],[20] a multivariate linear regression analysis showed a stronger correlation between IOP and CCT (from P = 0.001 to P < 0.001) with blood pressure held constant in the right eye and a slightly weaker though still very significant correlation between IOP and CCT in the left eye (from P < 0.001 to P = 0.001), suggesting a possible effect of blood pressure on IOP.

 Conclusion



The mean corrected IOP was higher than the mean measured IOP in both eyes and more participants had thin corneas than thick or normal cornea. There was a positive correlation between IOP and CCT with the resultant effect that thin corneas presented with falsely low IOP and thick corneas with falsely high IOP.

Recommendation

It is recommended that routine CCT measurement in addition to IOP measurement be done on every patient who presents to the eye clinic. This practice may help to identify and follow up patients with thin CCT who may be at risk of developing glaucoma later.

Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.

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