Methodological quality of the review: Low confidence
Author: Meng N, Zhang P, Huang H, Ma J, Zhang Y, Li H, Qu Y
Geographical coverage: Not reported
Sub-sector: POAG, colour doppler imaging
Equity focus: None specified
Review type: Other review
Quantitative synthesis method: Meta-analysis
Qualitative synthesis method: Not applicable
Background: Primary open-angle glaucoma (POAG) has been described as a leading cause of legal blindness and has been extensively studied for decades. These studies have attempted to identify the exact pathogenesis of POAG and diagnostic methods of detection.
Colour doppler imaging (CDI) measures the velocity of blood flow as opposed to actual blood flow, as it is impossible to precisely determine the diameter of orbital vessels in vivo using this technique. Despite this limitation, blood flow velocity is still a good indicator of blood flow within a given vessel. This meta-analysis was carried out to evaluate the difference in several blood flow parameters in retrobulbar vessels using CDI in patients with POAG. This can potentially be of substantial diagnostic value.
Objectives: To analyse the diagnostic value of CDI of blood flow in the retrobulbar vessels of eyes with POAG.
Main findings: The data from this meta-analysis suggests that the velocity of the blood flow was decreased, whereas the refractive index (RI) increased in all the retrobulbar vessels in POAG eyes.
The authors selected 23 prospective observational studies (1286 eyes with POAG and 1052 controls) for this meta-analysis. However, no geographical location was of any particular focus. The age range varied and the gender distribution was fairly similar between POAG patients and control subjects. Although clinical heterogeneity was observed in several areas, it was not analysed since these factors were not consistently reported in the publications.
The peak systolic velocity (PSV), end diastolic velocity (EDV) in the ophthalmic artery (OA), central retinal artery (CRA) and short posterior ciliary artery (SPCA) were assessed by the authors. The RI was also measured. In twenty studies, there was a significant reduction in the PSV in the CRA and OA in patients with POAG. Similarly, in eighteen studies PSV was decreased in the SPCA of POAG eyes. Significant heterogeneity was observed in treated and untreated groups when a sub-group analysis of PSV in OA, CRA and SPCA was carried out. EDV outcome also showed a reduction in OA, CRA and SPCA. Sub-group analysis of EDV in OA, CRA and SPCA showed significant heterogeneities in treated and untreated groups. With regards to RI, there was an increase in the OA, CRA and SPCA. Sub-group analysis revealed significant heterogeneities in OA and CRA due to drug treatment responsible for lowering IOP.
Overall, meta-analysis showed that the velocity of blood flow was reduced and RI increased in all retrobulbar vessels in individuals with POAG. Since the change in ocular blood flow is necessary in POAG development, CDI may be a promising diagnostic tool.
The authors of this meta-analysis acknowledge the limitations, which included the fact that observational studies – rather than randomised clinical controlled trials – were included. They also mention the possibility of the results being distorted due to the publication bias. They further recognise the heterogeneity between studies. Another important factor to consider is also the possibility of a diagnostic bias. All the above suggests the need to conduct a more thorough meta-analysis, even though this shows a significant outcome.
Methodology: The search for the studies was carried out independently by two researchers in three databases: Cochrane Central Register of Controlled Trials, PUBMED and ISI Web of Knowledge. The language was limited to English. Reference lists of case reports, review articles and studies were also reviewed. Studies that were either randomised clinical control trials or observational studies were included, along with those that compared blood flow velocities (PSV, EDV and RI in OA, CRA and SCPA) in normal and POAG eyes.
Using the PRISMA checklist, the authors ensured inclusion of recent studies. All the included studies had PSV and EDV measured by CDI in the OA, CRA and SPCA as the outcomes. For this meta-analysis, the RevMan software was used along with the chi-square based Cochrane’s statistics to test for between-study heterogeneity. A sub-group analysis was carried out depending whether POAG eyes were treated with an IOP-lowering drug and defined as treated or untreated groups.
Applicability/external validity: The authors do not discuss the applicability or external validity of the results of this meta-analysis. However, they detail the various limitations of the included studies. Despite this, the results show that CDI is a potential diagnostic tool for POAG, thereby making it safe to assume it is generally applicable.
Geographic focus: There is no mention of geographical focus in the study, however this can potentially aid in the diagnosis of POAG in low/middle-income countries.
Summary of quality assessment: The authors of this meta-analysis did not mention the timeframe of the data search, which leads one to question whether a comprehensive data search was carried out. In addition to this, the authors set up a language restriction, once again limiting the quality of the included studies. The authors acknowledge the limitations of the study that possibly affect the quality and reliability of the results. However, the outcome of this meta-analysis shows the effect that CDI has as a diagnostic tool for patients with POAG. Bearing in mind the limitations presented by this meta-analysis, low confidence was attributed in the conclusions about the effects of this review.