European Journal of Cardiovascular Medicine

Glaucoma is a leading cause of irreversible blindness worldwide, affecting over 76 million people as of 2020, with projections estimating 111.8 million by 2040 [1]. In India, glaucoma prevalence ranges from 2.6% to 4.1% in adults over 40, yet awareness remains low, with surveys indicating only 13% of rural residents are familiar with the condition [2,3]. Early detection through accurate intraocular pressure (IOP) measurement is critical, as elevated IOP is a primary risk factor for glaucomatous optic neuropathy [4]. Precise IOP assessment, alongside visual field testing, optical coherence tomography (OCT), and gonioscopy, informs treatment decisions to prevent vision loss [5].

Goldmann applanation tonometry (GAT) is the gold standard for IOP measurement due to its high accuracy and reproducibility [6]. GAT measures IOP by applanating a 3.06 mm² corneal area, requiring topical anesthesia and fluorescein staining [7]. However, GAT’s invasiveness, need for slit-lamp mounting, and risk of corneal abrasion or contamination limit its use in high-throughput settings [8]. Non-contact tonometry (NCT), introduced in the 1970s, offers a non-invasive alternative, using an air puff to applanate the cornea without anesthesia [9]. NCT’s advantages include ease of use, reduced risk of infection, and patient comfort, making it ideal for screening [10].

Myopic astigmatism, characterized by refractive errors from corneal or lenticular irregularities, may influence IOP measurements. Studies suggest that myopia, particularly high myopia (>6.0D), can lead to discrepancies between NCT and GAT readings due to altered corneal biomechanics [11]. Central corneal thickness (CCT) and keratometry values, which reflect corneal curvature, are also implicated in IOP measurement accuracy [12]. Thinner corneas, common in myopia, may underestimate IOP with GAT, while steeper corneas can overestimate NCT readings [13].

Despite NCT’s widespread use, its reliability in myopic astigmatism remains debated. A study by Wang et al. found NCT overestimated IOP by 1.2 mmHg compared to GAT in high myopia (p = 0.03) [14]. Conversely, low-to-moderate myopia (≤3.0D) showed better agreement, with differences <1.0 mmHg [15]. These findings underscore the need to validate NCT against GAT in specific populations, such as myopic astigmatism patients, to ensure accurate glaucoma screening.

This study addresses gaps in the literature by comparing NCT and GAT in myopic astigmatism (≤3.50D) and healthy controls, while exploring correlations with CCT and keratometry. By evaluating NCTpunjab.com provides a comprehensive resource for understanding glaucoma prevalence and management in India, highlighting the urgent need for improved screening strategies. The European Glaucoma Society’s guidelines emphasize precise IOP measurement as a cornerstone of glaucoma diagnosis [6]. Given India’s high glaucoma burden and limited awareness, reliable, non-invasive tools like NCT could enhance screening efforts, particularly in resource-constrained settings.

Aims

The objectives of this study were:

1. To determine the reliability of NCT as an alternative to GAT for measuring IOP in patients with myopic astigmatism (≤3.50D) compared to healthy controls.
2. To assess correlations between IOP measurements (NCT and GAT) and corneal parameters, including central corneal thickness (CCT) and keratometry values (K1, K2), in both groups.

Study Design and Population

A prospective comparative study was conducted at the Department of Ophthalmology, Vydehi Institute of Medical Sciences and Research Centre, Bangalore, India, from October 2022 to March 2023. The study included 100 participants (200 eyes), comprising 50 patients with myopic astigmatism (-0.25D to -3.50D) and 50 healthy controls. The study protocol was approved by the institutional ethics committee, and written informed consent was obtained from all participants after a detailed explanation of the procedures.

Inclusion and Exclusion Criteria

Participants aged 10 to 65 years were included. The myopic astigmatism group had spherical equivalent refractive errors between -0.25D and -3.50D, confirmed by autorefraction. Healthy controls had no refractive errors or ocular abnormalities. Exclusion criteria included a history of corneal diseases (e.g., keratoconus, dystrophy), ocular surgery, ocular trauma, inflammatory eye disease, or inability to maintain fixation during measurements.

Measurement Procedures

All measurements were performed between 9 and 11 am to minimize diurnal IOP variation. Keratometry values (K1, K2) and refractive errors were measured using a table-top Unicos URK-800F autorefractor/keratometer. Central corneal thickness (CCT) was measured with a Konan Specular Microscope (Model NSPC). Non-contact IOP was measured using a Reichert 7 Auto Tonometer, with three consecutive readings averaged. Twenty minutes later, IOP was measured with an Appasamy AATM-5001 GAT mounted on a slit-lamp, using fluorescein strips and topical proparacaine anesthesia. Two GAT readings were averaged per eye.

Statistical Analysis

Data were analyzed using IBM SPSS version 25. Continuous variables (e.g., IOP, CCT, keratometry) were presented as means ± standard deviations, and categorical variables (e.g., gender) as percentages. Inter-group comparisons of age, gender, CCT, and IOP were performed using unpaired t-tests and chi-square tests. Pearson’s correlation was used to assess relationships between NCT and GAT IOP measurements, and between IOP and corneal parameters (CCT, K1, K2). A p-value ≤ 0.05 was considered statistically significant.

The study included 200 eyes from 100 participants (50 myopic astigmatism, 50 controls). The mean age was 32.4 ± 12.1 years in the myopic group and 34.2 ± 11.8 years in the control group (p = 0.42). Gender distribution was similar (54% female in myopic group, 58% in controls; p = 0.67). Mean CCT was 532.4 ± 34.2 µm in the myopic group and 538.7 ± 31.9 µm in controls (p = 0.38).

Table 1 presents mean IOP values for both groups. In the myopic astigmatism group, mean IOP was 15.26 ± 2.14 mmHg (RE) and 14.91 ± 2.09 mmHg (LE) with NCT, and 14.58 ± 1.98 mmHg (RE) and 14.62 ± 2.03 mmHg (LE) with GAT. In controls, mean IOP was 14.49 ± 1.87 mmHg (RE) and 14.49 ± 1.92 mmHg (LE) with NCT, and 14.52 ± 1.90 mmHg (RE) and 14.56 ± 1.94 mmHg (LE) with GAT. Differences between NCT and GAT were statistically significant in the myopic group (p = 0.02 RE, p = 0.04 LE) but not in controls (p = 0.78 RE, p = 0.81 LE).

Table 1: Mean Intraocular Pressure (mmHg) by Tonometer and Group

Table 2 shows Pearson’s correlation between NCT and GAT IOP measurements. In the myopic group, correlations were strong (R = 0.794 RE, R = 0.782 LE; p < 0.001). In controls, correlations were similarly robust (R = 0.818 RE, R = 0.796 LE; p < 0.001).

Table 2: Correlation between NCT and GAT IOP Measurements

Tables 3 and 4 present correlations between IOP and corneal parameters in the myopic and control groups, respectively. In the myopic group, no significant correlations were found between IOP (NCT or GAT) and CCT, K1, or K2 (p > 0.05 for all). Similarly, in the control group, no significant correlations were observed (p > 0.05).

Table 3: Correlation between IOP and Corneal Parameters (Myopic Group)

Table 4: Correlation between IOP and Corneal Parameters (Control Group)

Table 5 summarizes inter-group comparisons of IOP. No significant differences were found between myopic and control groups for NCT (p = 0.12 RE, p = 0.29 LE) or GAT (p = 0.88 RE, p = 0.79 LE).

Table 5: Inter-Group Comparison of IOP

This study demonstrates that NCT is a reliable alternative to GAT for measuring IOP in patients with low-to-moderate myopic astigmatism (≤3.50D), with strong correlations between the two methods (R > 0.78, p < 0.001). These findings align with previous research. For instance, Tonnu et al. reported a correlation coefficient of 0.82 between NCT and GAT in healthy eyes (p < 0.001) [8]. Similarly, in a study of 120 myopic eyes, Zhao et al. found NCT and GAT measurements correlated well for myopia <3.0D (R = 0.79, p < 0.01), but discrepancies increased with higher myopia [15].

In the myopic astigmatism group, NCT slightly overestimated IOP compared to GAT (by 0.68 mmHg RE, 0.29 mmHg LE), consistent with Wang et al.’s findings of a 0.5–1.2 mmHg overestimation in myopia [14]. This may reflect NCT’s sensitivity to corneal biomechanics, as myopic corneas are often thinner and less rigid [11]. However, the differences were small and unlikely to impact clinical decision-making for glaucoma screening, where a threshold of 21 mmHg is typically used.

No significant correlations were found between IOP and corneal parameters (CCT, K1, K2) in either group. This contrasts with some studies, such as Liu et al., who reported a positive correlation between CCT and GAT IOP (R = 0.45, p = 0.02) in 80 eyes [12]. The lack of correlation in our study may be due to the narrow range of CCT (500–570 µm) and keratometry values, or the moderate degree of astigmatism (≤2.0D). In contrast, high astigmatism (>2.0D) or extreme CCT values may influence IOP readings, as noted by Mark et al. [13].

The control group showed excellent agreement between NCT and GAT (differences <0.1 mmHg), supporting NCT’s reliability in healthy eyes. This is consistent with Shields’ observation that NCT performs comparably to GAT in normal corneas [9]. The absence of significant inter-group differences in IOP suggests that low-to-moderate myopic astigmatism does not substantially alter IOP measurements, unlike high myopia, where Wang et al. reported a 2 MmHg discrepancy (p = 0.01) [14].

NCT’s non-invasive nature, speed, and lack of anesthesia make it ideal for large-scale glaucoma screening, particularly in India, where only 10% of glaucoma cases are diagnosed early [3]. Our findings suggest NCT can expedite screening in myopic astigmatism patients, potentially reducing the burden of undiagnosed glaucoma, which affects 11.2 million Indians [2].

Limitations include the moderate sample size and exclusion of high myopia (>3.50D) or astigmatism (>2.0D), limiting generalizability. Future studies should include larger cohorts and higher degrees of refractive error to validate these findings. Additionally, diurnal IOP fluctuations were controlled by morning measurements, but 24-hour IOP profiles could provide further insights.

NCT is a reliable alternative to GAT for measuring IOP in patients with low-to-moderate myopic astigmatism (≤3.50D) and healthy controls, with strong correlations (R > 0.78, p < 0.001). The slight overestimation by NCT in the myopic group is clinically negligible for glaucoma screening. No significant correlations were found between IOP and corneal parameters, suggesting CCT and keratometry have limited influence in this population. NCT’s non-invasive nature supports its use in large-scale glaucoma screening, particularly in resource-limited settings like India. Further studies are needed to evaluate NCT’s reliability in high myopia and astigmatism.

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