Though meta analysis is ranked in the top of the

Though meta-analysis is ranked in the top of the evidence based medicine pyramid, some aspects of meta-analysis seem mysterious in nature. By definition, meta-analysis is a statistical method for combining the results of several independent studies addressing similar hypotheses in order to gain a better estimate of the effect size of an intervention.Meta coming from the Greek prefix “meta” means “after” or “beyond”. Thus, the idea is to pool the results of two studies or more in order to look beyond the primary results of the individual studies. The combined result can be evaluated as a secondary outcome using one of the size effect measures such as odds ratio, relative risk, and mean difference. In order to produce useful and meaningful results out of meta-analysis, a good systematic review should take place first. Systematic review is an integral part of any meta-analysis which started with an appropriate research question followed by a systematic retrieval of relevant studies through pre-planned inclusion and exclusion criteria. Summarizing the characteristics of the included studies concludes a systematic review. Subsequently, statistical analysis of the pooled results provides us with meta-analysis.
The number of published systematic reviews and meta-analyses in ophthalmic field has been increasing progressively over the past years. The number of publications increased from 3 per year in 1994 to almost 100 per year in 2010. This increased publication reflects the need for such design, especially with presence of several studies on the effect of an intervention or risk factor with varying directions or varying significance of outcome. Meta-analysis helps to interpret and clarify the direction and magnitude of the size effect. The major ophthalmic subspecialties that publish systematic reviews and meta-analyses are GPCR Compound Library which represents 35% of the publication of this type followed by glaucoma with publication rate of 21%.
In this journal issue, another retinal meta-analysis discusses the risk and contributing impact of a specific vascular endothelial growth factor (VEGF) polymorphism on retinopathy of prematurity (ROP). It has been known that the retinopathy of prematurity is a major cause of childhood blindness. In Saudi Arabia, retinopathy of prematurity was found to be comparable to reports from other parts of the world. In a cohort of premature infants with a mean gestational age of 28.4weeks, the incidence of ROP with birth weight of <1500 grams and <1250 grams was 41% and 50.7%, respectively. Nineteen of the 73 children with ROP (26%) reached threshold ROP, and needed laser treatment or cryotherapy. Development of ROP passes through two phases. In the first, the relative high oxygen inhibits the physiologically driven retinal vascularization by inhibiting the normal vascular endothelial growth factor (VEGF) secretion which is a hypoxia-inducible cytokine. In the second, there is relative hypoxia as a result of increased metabolic demand in the process of retinal maturation. Hypoxia promotes high-expression of VEGF and stimulates abnormal retinal vasculogenesis leading to the clinical manifestations of ROP such as neovascularization and proliferative retinopathy. Thus, VEGF is a key in normal and pathological retinal vascularization. Many researchers believe that single nucleotide polymorphisms (SNPs) of VEGF are related to the changes of protein expression during ROP and hence the susceptibility to develop ROP. However in case of the single nucleotide polymorphisms at positions rs 2010963, there are conflicting results concerning the direction of impact of VEGF polymorphism on ROP risk. Malik et al., in this issue, employed meta-analysis to show that VEGF-634G/C (rs 2010963) polymorphism may not be associated with ROP risk based on the pool results of six case-control studies including 355 cases and 471 controls. The pooled results of these studies in meta-analysis help to increase the sample size and improve the statistical power compared to the individual studies. The current study analyzed the association between VEGF-634G/C polymorphism and ROP risk using different models (dominant versus recessive) at different levels (allele versus ethnicity level) to provide more convincing result. The lack of GPCR Compound Library association between VEGF-634G/C polymorphism and ROP risk is in agreement with previous meta-analysis by Liu et al. who demonstrates that advanced ROP is not associated with VEGF gene –634G/C polymorphism.