Comparison of intraocular pressure and central corneal thickness in non-glaucomatous North- East Indian tribals versus general Indians

Purpose: To compare the intraocular pressure (IOP) and central corneal thickness (CCT) of non-glaucomatous North-East (NE) Indian tribals and general Indians. Materials and methods: In a prospective, cross-sectional study, the IOP and CCT of non-glaucomatous NE Indian tribals (n = 50) and non-glaucomatous general Indians (n = 50) were compared. Glaucoma was ruled out by history, detailed ocular examinations, and investigations. Results: There was very good correlation between the CCT and IOP values of right and left eyes (p = 0.940 and 0.847, respectively). The difference in the IOP values in the two groups was not statistically significant (p = 0.312 for Oculus Dexter [OD], p = 0.400 for Oculus Sinister [OS]). Similarly, the difference in the CCT values in the two groups was not statistically significant (p = 0.736 for OD and 0.613 for OS). The mean CCT and IOP OD for the whole population was 530.50 ± 35.42 μm and 13.80 ± 2.760 mmHg, respectively. By linear regression analysis, the IOP OD of the whole population had good correlation with CCT OD of the whole population (adjusted r2 = 0.084, p = 0.002), but not with age (adjusted r2=0.000, p=0.314) and sex (adjusted r2 = 0.010, p = 0.163). Similarly, CCT OD for the whole population did not have good correlation with age (adjusted r2 = -0.009, p = 0.762) and sex (adjusted r2 = -0.007, p = 0.603). Conclusions: In this study of individuals with normal corneas and without glaucoma, no racial variation was found in the CCT and IOP values of the two groups. The IOP OD of whole population had good correlation with CCT, but not with age and sex. There was good correlation between OD and OS values of IOP and CCT.


Introduction
Increased intraocular pressure (IOP) is a well-recognised risk factor for glaucoma and central corneal thickness (CCT) is known to have a definite influence on IOP. Studies have shown that ethnic variation exists in IOP and CCT values. [1][2][3][4][5] These variations have been studied in Caucasians, African Americans and Asians. Aghaian et al. found that the Japanese have thinner corneas than the Chinese and the Filipinos. 2 They concluded that differences in CCT may exist among different Asian subgroups. As per Foster et al., variation in CCT is a significant source of variation in IOP measurements between individuals. 4 It is important to find the variations in IOP and CCT in ethnic groups, otherwise we may falsely diagnose cases of normal tension glaucoma, ocular hypertension, and glaucoma or miss them depending on the CCT.
Some population-based studies have been carried out determining the IOP and CCT in East and South-East Asia. Chua et al. carried out a study to assess the ethnic variations in IOP and CCT using uniform study designs among three different Asian ethnic groups. They confirmed the presence of ethnic variation in IOP and CCT among Asian subgroups. They highlighted the need to study Asian subgroups individually and not in aggregate in future studies. 5 There have also been studies reporting the IOP and CCT in general Indians. 6-9 However, North-East (NE) Indians are ethnically distinct and different from the general Indian population. To the best of our knowledge, there has been to-date no study done to determine IOP and CCT in non-glaucomatous NE tribals and compare these parameters with those of non-glaucomatous general Indians. Therefore, we wanted to determine if there exists any ethnic variation in the CCT and IOP values between these two groups. If there is a difference, using the established normative values of IOP and CCTs will lead to inaccurate measurements in NE Indians, which in turn may lead to misdiagnosis of glaucoma in NE Indians.

Materials and methods
This was a prospective, cross-sectional study conducted in the department of Ophthalmology of a medical college of NE India. Permission was obtained from the Institute Ethics Committee for the study. Informed written consent was taken from every patient. We adhered to the tenets of the Declaration of Helsinki.
The medical institute shares its location with the headquarters of many central government agencies, such as Eastern Air Command, Border Security Forces, Geological Society of India, a central university, etc. Therefore, many of these central government employees living in and around the medical college come for regular medical check-ups here. Being a regional institute, it caters to people from all of the North-Eastern states. Two to three eligible patients from each state were enrolled in this study. Hence, the study population represents the entire Indian population.
Glaucoma was ruled out first by family history, detailed ocular examination, Humphrey visual field analyzer 30-2, IOP by Goldmann applanation tonometer (GAT), and gonioscopy. Only non-glaucomatous, consecutive, adult (> 40 years) patients attending the Outpatient Department of Ophthalmology and willing to take part were included in the study. Since these were consecutive patients taken each day of the week, there was minimal or negligible chance of selection bias. The study was conducted for a period of three months. These subjects were divided into two groups of 50 individuals each. Group 1 consisted of the non-glaucomatous general Indians and Group 2 consisted of the ethnic tribals of NE India visiting our tertiary care centre. Equal representation was ensured from all the states of NE India for the NE Indians and other states for the general Indians. Inclusion criteria were non-glaucomatous patients with both eyes normal. Exclusion criteria were any corneal patholoies, corneal or intraocular surgery, recent contact lens use and poor fixation of eyes, patients with glaucoma suspect or glaucoma, myopia > -2 dioptres, and one eyed. IOP and CCT were recorded by a single examiner. Measurements were done in the morning hours only, between 9 AM and 12 PM. IOP was measured first followed by CCT. Proparacaine 0.5% (Carecain,Sunways P Ltd., Mumbai, India) eye drops were instilled in both eyes twice in a 5-minute interval. IOP was measured by GAT (AT 900, CE 1250, Haag-Streit International) in both eyes. The right eye was measured first followed by the left eye. A drop of proparacaine 0.5% solution was instilled once again after 15 minutes. CCT was then measured with ultrasonic pachymeter (Sonomed, Micropach, 200P+, USA). Measurements were done with the patient looking straight and fixating at a distant target. Three consecutive error-free measurements were taken in each eye. The average of these three measurements was taken.

Statistical analysis
Clinical characteristics were found by descriptive statistics. Comparisons of parameters between the two groups were done using the t-test. Correlations of IOP and CCT with other parameters individually were done using Pearson correlation coefficients. Regression analysis was performed to see the correlation of IOP and CCT with age and sex. A 5% level of significance was adopted. Therefore, a P-value of < 0.05 was considered statistically significant. Statistical analysis was performed using the SPSS software package (SPSS for Windows, version 22.0; SPSS, Inc., Chicago, IL, USA).

Results
There were 50 participants in each group. The profile of participants and the distribution of IOP and CCT values by age are shown in Table 1. The two groups were well matched by age (p = 0.816) and sex (p = 0.692) ( Table 1). The IOP and CCT characteristics of the two groups are shown in Table 2. The differences in IOP values between the two groups were not statistically significant (p = 0.312 for OD, p = 0.400 for OS) (Table 2). Similarly, the difference in CCT values between the two groups was not statistically significant (p = 0.736 for OD and 0.613 for OS) ( Table 2). The difference in IOP and CCT values when compared by gender was also not statistically significant (Table 3).  There was very good correlation between the CCT values of right and left eyes (P = 0.940) (Fig. 1). Similarly, there was good correlation between the IOP values of right and left eyes of the whole population (P = 0.847). The Pearson correlation coefficients of IOP and CCT of right eyes of the two groups with other parameters are shown in Table 4.
The mean CCT OD of the whole group was 530.50 ± 35.42µm and the mean IOP OD of the whole population was 13.80 ± 2.760 mmHg. The CCT OD in the whole population had a normal distribution (Fig. 2). Similarly, the IOP OD in the whole population had a normal distribution. By linear regression analysis, the IOP OD of the whole population had good correlation with CCT OD of the whole the population (adjusted r 2 = 0.084, P = 0.002) (Fig. 3), but not with age (adjusted r2 = 0.000, p = 0.314) (Fig. 4) and sex (adjusted r 2 = 0.010, p = 0.163). Similarly, CCT OD of the whole population did not have good correlation with age (adjusted r2 = -0.009, p = 0.762) and sex (adjusted r 2 = -0.007, p = 0.603).

Discussion
CCT and IOP measurements are part and parcel of any glaucoma work up. The ethnic variation in CCT and IOP has been established in earlier studies. In our study, we have attempted to compare IOP and CCT in general Indians and ethnic   tribals of NE Indians, since no such study is available to date. We did not find any statistically significant difference in mean CCT and IOP between t general Indians and NE Indians.
To conclude, in this study in individuals with normal corneas and without glaucoma, no racial variation was found in CCT and IOP values between the two groups. The IOP OD of the whole population had good correlation with CCT, but not with age and sex. There was good correlation between OD and OS values of IOP and CCT.