Abstract

Summary: We determined the value of central corneal thickness (CCT) in Congolese patients with primary open-angle glaucoma (POAG) and assessed its impact on intraocular pressure (IOP). Patients and Methods: We conducted a cross-sectional study at the University Hospital of Brazzaville. IOP was measured using a Goldmann applanation tonometer. Pachymetry was performed with an optical pachymeter (Topcon TRK-2P). Adjusted IOP was determined using the Ehlers correction table. Data were collected on standardized forms, entered using CSPro 7.4, and analyzed with the public health software SPSS 25. Results: Mean IOP in the right eye was 23.16 ± 8.66, with values ranging from 10 to 55. Mean IOP in the left eye was 23.21 ± 9.24, with values ranging from 10 to 57. Mean CCT in the right eye was 515.13 ± 38.23 µm, ranging from 434 to 590. Mean CCT in the left eye was 512.22 ± 34.56 µm, ranging from 435 to 582. Mean adjusted IOP in the right eye was 23.87 ± 8.05, ranging from 10 to 50. Mean adjusted IOP in the left eye was 23.88 ± 8.29, ranging from 10 to 57. The difference between mean adjusted IOP and non-adjusted IOP was statistically significant (p = 0.03). Conclusion: Patients with POAG have corneas that are thinner than the Caucasian average, which results in overestimated ocular pressures that must be taken into account for diagnosis, treatment, and follow-up.

Keywords

Primary Open-Angle Glaucoma, Central Corneal Thickness, Pachymetry, Intraocular Pressure

Share and Cite:

1. Introduction

Ocular hypertension (OHT) is not part of the definition of primary open-angle glaucoma (POAG), but it is an alert sign that guides evaluation and referral [1]. The reference instrument for measuring intraocular pressure remains the Goldmann applanation tonometer [2]. This instrument was calibrated for a central corneal thickness (CCT) of 520 µm [3] [4]. However, corneal thickness is not a constant; it varies between patients. It is thinner in African Americans and Africans than in White Caucasian subjects [5]. Thus, when CCT is higher than normal, IOP will be overestimated, and when CCT is lower than normal, IOP will be underestimated.

Worldwide studies place normal CCT between 527 and 560 µm [6] [7]. A few studies in sub-Saharan Africa report corneas that are thinner than the Caucasian average [8]-[11]. To date, no study on CCT has been conducted in Congo-Brazzaville.

The objective of this study is to assess the importance of measuring central corneal thickness in Congolese patients with glaucoma.

2. Patients and Methods

2.1. Study Type and Period

This was an analytical cross-sectional study conducted from January 1, 2025 to June 30, 2025 (6 months).

2.2. Study Setting

The study took place in the Ophthalmology Department of the University Hospital Center of Brazzaville (CHU-B).

2.3. Study Population

2.3.1. Target Population

The target population consisted of all patients with POAG who attended during the study period.

2.3.2. Inclusion Criteria

• Patients followed for POAG.

• Age 20 years or older.

• Patients who provided informed consent.

2.3.3. Non-Inclusion Criteria

• Patients younger than 20 years.

• Non-transparent ocular media.

• Corneal abnormalities.

2.3.4. Sampling

We used a systematic, non-probability sampling of patients meeting inclusion criteria during the study period.

2.4. Study Procedures

All selected patients underwent a complete ophthalmologic examination.

2.4.1. Interview

• Collection of socio-occupational data.

• Assessment of a history of glaucoma.

2.4.2. Refraction

Distance visual acuity was assessed using the Monoyer chart. Based on visual acuity, patients were grouped according to the WHO classification. Autorefractometry was performed with the Topcon TRK-2P.

2.4.3. Slit-Lamp Examination

To assess the transparency of ocular media.

2.4.4. IOP Measurement and Pachymetry

• IOP measured with a regularly calibrated Goldmann applanation tonometer in patients with or without glaucoma follow-up, regardless of treatment adherence; the IOP value retained for analysis was the mean of three consecutive measurements, taken at the same time each morning over three consecutive days.

• Pachymetry using the optical pachymeter (Topcon TRK-2P).

• Adjusted IOP determined using the Ehlers table [4].

2.4.5. Fundus Examination

Using a 90-diopter lens.

2.4.6. Visual Field Assessment and OCT

To confirm glaucoma.

2.4.7. Confirmation of the Diagnosis of Glaucoma

Based on the presence of at least 3 suggestive findings:

• Ocular hypertension (OHT)/elevated intraocular pressure (IOP).

• Increased optic disc cupping (increased papillary excavation).

• Visual field abnormalities.

• OCT abnormalities (optical coherence tomography).

2.5. Statistical Analysis

Data were collected on forms, entered using CSPro 7.4, and analyzed using SPSS 25. Tables and graphs were generated with Microsoft Excel 2016. Frequencies of qualitative variables were compared using the chi-square test. Statistical significance was set at p ≤ 0.05.

3. Results

The mean age of patients was 60.98 ± 13.40 years, with a range from 21 to 93 years. The most represented age group was 60 and older (54.63%) (see Table 1). By sex, mean age was 62.67 ± 10.57 years in men versus 59.57 ± 9.16 years in women (see Table 2). The sex ratio was 0.83.

58.33% of patients had visual acuity ≥ 3/10 (see Table 3). Only 11.11% of right eyes had IOP ≥ 30, compared with 15.74% of left eyes (see Table 4).

Table 1. Distribution of patients by age group.

Table 2. Mean age of patients by sex.

Table 3. Distribution of patients according to visual acuity.

RE = right eye; LE = left eye.

Table 4. Distribution of patients according to IOP.

Mean IOP in the right eye was 23.16 ± 8.66, with values ranging from 10 to 55.

Mean IOP in the left eye was 23.21 ± 9.24, with values ranging from 10 to 57.

Mean CCT in the right eye was 515.13 ± 38.23 µm, with values ranging from 434 to 590.

Mean CCT in the left eye was 512.22 ± 34.56 µm, with values ranging from 435 to 582.

The difference in mean CCT between the two eyes was not statistically significant (p = 0.084).

Mean CCT of the right eye in men was 512.26 ± 23.69 µm.

Mean CCT of the right eye in women was 517.83 ± 25.99 µm.

The difference in mean right-eye CCT between sexes was not statistically significant (p = 0.273).

Mean CCT of the left eye in men was 513.11 ± 23.20 µm.

Mean CCT of the left eye in women was 511.51 ± 25.60 µm.

The difference in mean left-eye CCT between sexes was not statistically significant (p = 0.357).

CCT decreased with age (see Table 5).

After adjusting IOP for pachymetry, 19.96% had IOP ≥ 30 in the right eye and 18.58% in the left eye (see Table 6).

Mean adjusted IOP in the right eye was 23.87 ± 8.05, with values ranging from 10 to 50.

Mean adjusted IOP in the left eye was 23.88 ± 8.29, with values ranging from 10 to 57.

The difference between mean adjusted IOP and non-adjusted IOP was statistically significant (p = 0.03) (see Table 7).

Table 5. Mean CCT by age group.

CCT = central corneal thickness; RE = right eye; LE = left eye.

Table 6. Distribution of patients according to adjusted IOP.

Table 7. IOP vs adjusted IOP.

4. Discussion

4.1. Sociodemographic Characteristics

4.1.1. Age

The mean age of patients was 60.98 ± 13.40 years. This varies across studies: Ndiaye-Sow in Senegal [12], Lee in Korea [13], Singh in Australia [14], and Baudoin in France [15] reported mean ages of 56.53 ± 11.29 years, 57.2 ± 12.6 years, 59 ± 16 years, and 63 ± 13 years, respectively. The most represented age group in our study was over 60 years (54.63%).

4.1.2. Sex

There was a female predominance (54.63%), with a sex ratio of 0.83. Denis et al. [16] and Baudoin [15] in France also found a female predominance (56.15% and 56%, respectively). Conversely, Watson [17] in England reported a male predominance (65.7%). Male sex is sometimes considered a risk factor for POAG [18].

4.2. Clinical Characteristics

4.2.1. Visual Acuity

Most patients had visual acuity greater than 3/10: 62.96% in the right eye and 58.33% in the left eye. Preservation of good visual acuity despite glaucoma progression is often described [19].

4.2.2 Non-Adjusted Intraocular Pressure

Mean IOP was 23.16 ± 8.66 mmHg in the right eye and 23.21 ± 9.24 mmHg in the left eye. Pretreatment IOP reported in the literature among glaucoma patients varies across studies and sampling. Ndiaye-Sow [12] found mean IOP of 27.54 ± 7.51 mmHg (right) and 26.76 ± 6.67 mmHg (left). Singh [14] in Australia reported 27.5 ± 5.1 mmHg. These were slightly higher than ours. Ouattara in Côte d’Ivoire [20], however, reported a mean IOP of 18.43 ± 4.18 mmHg.

4.2.3. Central Corneal Thickness

We found mean CCT of 515.13 ± 38.23 µm in the right eye and 512.22 ± 34.56 µm in the left eye. These results are close to those from African studies evaluating corneal thickness in POAG patients: 506.69 ± 35.08 µm reported by Tolesa [21] in Ethiopia; 519.6 ± 32.6 µm by Fanny [8] in Côte d’Ivoire; 525.40 ± 39.63 µm by Ndiaye-Sow [12] in Senegal; and 529.29 ± 35.9 µm by Eballé [9] in Cameroon.

This thickness appears higher in Caucasians; Khoramnia [22] in Germany found a CCT of 539.62 ± 31.87 µm. In the Nemesure study [23] in Barbados, CCT increased clearly from Black to White subjects: 529.8 µm in Blacks, 537.8 µm in Black–White mixed-race individuals, and 545.2 µm in Whites.

We did not find a sex-related difference in CCT, similar to Nemesure [23] in Barbados and Bron [24] in France. In contrast, Mercieca [25] in Nigeria found thinner corneas in men than in women.

CCT decreased with age in our study, as also observed by Nemesure [23]. However, we used optical pachymetry, which may yield lower values. In a meta-analysis, Doughty [7] found that optical measurements produced lower CCT values than ultrasound measurements.

4.2.4. Adjusted Intraocular Pressure

Many techniques are available for measuring intraocular pressure (IOP), but the reference technique in clinical practice remains Goldmann applanation tonometry, which is influenced by multiple parameters, particularly central corneal thickness (CCT). IOP should be systematically adjusted for CCT, since a thin cornea leads to underestimation of IOP values, whereas a thick cornea leads to overestimation [26].

Mean adjusted IOP was 23.87 mmHg in the right eye and 23.88 ± 8.29 mmHg in the left eye. Adjusted IOP was higher than non-adjusted IOP, and the difference was statistically significant (p = 0.03). This higher IOP reflects the thinner CCT. The thinner cornea in Black African subjects explains the generally underestimated IOP measurements [7] [27]; these are in fact “falsely” low measurements because they are underestimated.

The difference between non-adjusted and adjusted IOP was statistically significant (p = 0.03). The relationship between CCT and IOP was established by Ehlers et al. as early as 1975 [4]. This relationship is further supported by IOP variations that follow corneal thickness after LASIK [7] [28]. Because our patients had CCT values below the norm, they are exposed to underestimation of their IOP.

5. Conclusions

Central corneal thickness is an important parameter that should not be overlooked in the follow-up of chronic open-angle glaucoma.

This study showed that CCT in glaucoma patients followed at the University Hospital of Brazzaville is thinner than average. Therefore, IOP values obtained in clinic in glaucoma patients are usually underestimated to a non-negligible degree. This underestimation may lead the clinician to diagnostic errors or insufficient therapeutic adjustment of IOP, allowing glaucoma to continue progressing.

It is therefore necessary to take this underestimation into account in glaucoma management, especially in centers that do not yet have a pachymeter. Centers equipped with a pachymeter should systematically follow patients using adjusted IOP.

Conflicts of Interest

The authors declare no conflicts of interest regarding the publication of this paper.

References