Q: How do we determine the comparison groups in systematic review of case reports?
Thank you for your question. Systematic reviews include a meta-analysis component that involves using statistical techniques to synthesize data from several studies into a summary effect size. In contrast to traditional hypothesis testing that provides details about statistical significance (i.e., did the intervention group differ from the control group) but not necessarily clinical significance (i.e., was this difference clinically meaningful or large), effect sizes measure the strength of the relationship between two variables, thereby providing information about the magnitude of the intervention effect (i.e., small, medium, or large).
Systematic reviews of case reports have been reported previously, as you may see in this paper on PubMed and this one on PLOS ONE. As observed in these links, comparison will depend on the parameters that you wish to compare.
Hope this helps. All the best for your study!