Glaucoma is a multifactorial optic neuropathy, and it has been suggested that the choroid is associated with the development and progression of glaucomatous optic neuropathy and visual field loss.
29 Several studies have demonstrated that blood flow to the choroid is diminished in patients with POAG, and some previous histologic reports have suggested that the choroid might be thinner in eyes with glaucoma than in nonglaucoma pathology specimens.
3,30 In contrast, some researchers have detected significant thickening of the choroid in POAG cases. However, histologic studies have not reflected the choroid in vivo and have not controlled for the confounding effects of age, axial length, IOP, and so on. Enhanced depth imaging OCT is a reliable and repeatable technique that has evaluated both the macular and PCT in vivo.
18 Since the first report by Spaide et al.,
31 there has been growing interest in the relationship between choroidal thickness and POAG in recent years. Although great efforts have been made, the association between POAG and choroidal thickness is still controversial. With the increase of OCT studies, synthesizing the available data to resolve persistent difficulties in obtaining robust, replicable results is highly encouraged.
In this study, we observed that choroidal thickness was similar in the glaucomatous eyes and the controls. After a systematic search of several databases, we reviewed 16 studies, plus ours, that compared choroidal thickness in patients with POAG with that of controls. The data from these papers were combined, as they had similar study designs and methods. Our findings of nonsignificance were also reflected in the meta-analysis, even across different subgroups, which are less prone to chance results, indicating the robustness of our findings. As far as we known, this is the first report of MCT in Chinese patients with POAG, and the first meta-analysis on this topic.
Several strengths distinguishing the present investigation merit adequate consideration. First, to date, this is the first synthesis exploring the association of choroidal thickness with POAG. Second, the results of the present cross-sectional study were in line with that of the corresponding meta-analysis, and restricting the analysis to studies on POAG or NTG generated similar findings. Third, our results are less prone to selection bias, in view of the low probability of publication bias. Fourth, The Beijing Eye Study, which included 3468 participants, found that SFCT was associated with age and axial length, and not with BP, OPP, IOP, or arterial hypertension.
32 Our findings agree with this large study.
Some limitations should be considered when interpreting our findings. First, only two studies with a mode sample size provided data on NTG; therefore, further pragmatic studies are needed to evaluate the association between choroidal thickness and NTG. Second, choroidal thickness was measured manually; thus, the determination of the scleral border may be subjective in part, and poorly reproducible. However, several studies have reported that choroidal thickness measurement by EDI-OCT is highly reproducible and repeatable.
33,34 Third, the use of topical and systemic medications could affect choroidal thickness. Several studies have confirmed that some antiglaucomatous agents, such as topical α-2 agonists and carbonic anhydrase inhibitors, can increase choroidal blood flow.
35–37 Intravenous acetazolamide also increases choroidal thickness, and there are a number of other vasoactive substances that could potential influence the choroid.
38–40 However, in studies by EDI-OCT, other reseachers
18,41 found that antiglaucomatous agents weren't likely to influence choroidal thickness. Fourth, substantial heterogeneity among studies regarding the relationship between SFCT and POAG was observed. However, our subgroup analysis found that the heterogeneity predominantly derived from two studies that had not used Spectralis. After omitting those studies, there was no heterogeneity across our analyses. Fifth, residual confounding is of concern. Many factors can produce effects on choroidal thickness. Although most studies have controlled for various known confounding factors, the possibility of residual or unmeasured confounding cannot be ruled out. Finally, we focused on only several locations in macular and average PCT, and we did not cover the choroid in other areas. Given these limitations, we cannot jump to any conclusions until further verification of our findings in vitro, in vivo, and in large prospective studies.
In conclusion, this cross-sectional study of Chinese POAG patients, along with the comprehensive meta-analysis, failed to confirm the association of choroidal thickness with POAG, even across different subgroups. Nevertheless, the results should be interpreted cautiously, as the relevant evidence remains limited, and the findings must be confirmed through future larger research studies.