Integrating case reports into systematic reviews: methodological strategies and challenges

Systematic reviews are used in medical research to synthesize evidence by collecting evidence from available literature without overlooking important sources or pieces. Several guidelines are available for the methodological implementation of a systematic review; however, none of them include case reports or case series as a potential source of information that could be pooled and analyzed. Although case reports may be criticized as an evidence source, they are important for documenting certain types of evidence such as rare conditions, adverse events, or novel emergent diseases. In this article, we attempt to set the foundation for a guideline on the systematic review of cases, and we suggest a step-by-step guide for investigators to employ for systematically reviewing case reports. We also dissect the application of this methodology in the available literature and review a case study of a systematic review of case reports. Generally, case reports can be synthesized into a systematic review, providing evidence on matters that may be hard to investigate using other methodologies, but concerns regarding quality and level of evidence remain present.