Background: Evidence Gap Maps (EGMs) are a tool for promoting evidence and identifying gaps in research. EGMs summarise, appraise critically and present evidence - often systematic reviews - in a user-friendly format. This paper describes how this tool was applied to assess the availability of evidence on cataract in low- and middle-income countries with the aim of understanding better how to avoid gaps in research and set up the future research agenda (Fig 1).

Methods: The first stage of developing the cataract EGM involved identifying and displaying all relevant reviews in a matrix of 14 themes against three degrees of strength of evidence. The next stage involved searching and mapping primary studies on cataract into the same matrix as the systematic reviews.

Results: In the first stage, 52 reviews were included in the cataract EGM. Clear gaps were identified on cataract-related health systems and uptake of surgery. The second search yielded 169 primary studies, of which 11 met the inclusion criteria. Out of 11 studies, four provided evidence on barriers to surgery uptake, five focused on improving patients’ knowledge and subsequent demand for surgery, and two studies identified interventions to improve health workers’ knowledge, attitude and practice. Among the primary studies, the outcomes, designs and interventions were heterogeneous, thus precluding a systematic review.

Conclusions: The cataract EGM is a useful tool for identifying priorities in research in a number of ways:
1. EGMs help identify methodological weaknesses of existing reviews and encourage more systematic or rigorous approaches to synthesising evidence;
2. EGMs show thematic areas where few or no reviews are available and suggest questions for future systematic reviews;
3. EGMs identify evidence gaps with no reviews or primary studies and suggest areas for future investment in research.