Corneal thickness measurement in the management of primary open angle glaucoma (POAG): a report by the American Academy of Ophthalmology

Methodological quality of the review: Low confidence

Author: Dueker DK, Singh K, Lin SC, Fechtner RD, Minckler DS, Samples JR, Schuman JS

Geographical coverage: Not reported

Sector: Glaucoma

Sub-sector: Open angle glaucoma

Equity focus: None specified

Review type: Evidence synthesis

Quantitative synthesis method: Narrative synthesis

Qualitative synthesis method: Not applicable

Background: This report evaluates whether central corneal thickness (CCT) is a valid risk factor to determine the presence, development or progression of glaucomatous optic nerve damage in relation to primary open angle glaucoma (POAG). The authors reviewed and graded several selected publications based on statistical association of CCT with the risk of having or developing glaucomatous optic nerve damage. This can potentially impact the screening of POAG by adding CCT measurement as an additional screening step.

Objectives: The authors aimed to evaluate the published literature to assess whether CCT can present as a risk factor for the presence, development or progression of glaucomatous optic nerve damage in relation to POAG.

Main findings: A total of 24 studies were included in the review. There was mixed evidence regarding the association of CCT with the presence of glaucoma. This conclusion was drawn from the fact that the CCT measurement values as a screening parameter in association with glaucoma were negligible in two large population studies, and marginally associated in a third study. Overall, this report found that there was strong evidence to suggest that the CCT measurement acts as an additional and effective component of ocular hypertension management.

The majority of the evidence indicated that CCT measurement was not useful in predicting the progression risk of existing glaucoma. Some clinical studies have suggested that CCT measurement is associated with the progression of glaucoma, while others have not found it to be so. Therefore, the available evidence does not present a definite conclusion.

Authors note that CCT measurement is still an influencing factor on the IOP measurement. Since IOP is the most important risk factor for the progression of glaucoma, CCT measurement can still be considered important in managing patients with suspected or proven open angle glaucoma.

Methodology: The authors conducted a literature search on PUBMED on November 15 2004 using the following keywords: corneal thickness, CCT or pachymetry and glaucoma. This search was restricted to English language articles and retrieved 195 articles in total. 57 articles were chosen after a panel reviewed the abstracts, acquired their full text and evaluated their relevance to the assessment questions. From the periodic updates of the literature search and the reference lists from the articles reviews, 24 additional studies were identified. 37 out of the 81 published reports identified for methodological review were based on the relevance to the assessment questions and study design quality.

The articles were rated based on the strength of evidence. A well designed, properly conducted randomised clinical trial, or similar quality cohort studies that have been validated with suitable reference standards, were assigned a Level I rating. A Level II rating was given to well-designed case control studies, exploratory cohort studies and other non-randomised clinical studies that lacked consistent reference standards. Finally, a Level III rating was assigned to poorly designed case control studies, case series and other papers that consisted solely of evidence-lacking opinions from experts. If the study found a statistically significant association of CCT measurement with the risk of developing or having glaucomatous optic nerve damage, it was graded as a positive, and vice versa if no association found.

Depending on how each article related to the three risk categories, the methodological review of literature was conducted. The results for each risk category were presented in three different tables. In some cases, papers presented results in two categories and as such were presented in both tables and indicated in the comments section of the table.

Applicability/external validity: Although the review did not find strong evidence to support the theory that CCT measurement could work as a risk assessment for glaucoma progression or established glaucoma, it does indicate the importance of it providing useful information regarding the IOP measurement by Goldmann applanation tonometry. It also can provide a deeper understanding of individual patients’ clinical course by separating CCT into broad categories.

Geographic focus: The authors did not specify the focus of the review geographically.

Summary of quality assessment: The inclusion criteria were not stated, although a fairly comprehensive literature search was carried out using standard databases. However, there were several search restrictions which could have potentially contributed to certain biases in the data screening. In addition to this, there were no independent reviewers further adding to the questionable reliability of the review. There was no effective risk assessment strategy mentioned and this invalidates the strength of evidence. Therefore, the quality assessment of this review has been assigned a low confidence in conclusion about effects.