Hello Cornelis

It sounds like your body of literature is not yet mature enough for any sort of quantitative systematic review.  Back in the old days they wrote 'narrative reviews' where the findings often took the form of summary tables with columns showing year, author(s), definition, main findings, relevant comments, etc.  I think these are now called qualitative reviews.  As with any review, transparency and replicability of your search are important.

When you say there is no gold standard available, this could mean that 'experts have not yet agreed on a gold standard' (a theoretical problem), or it could mean that a 'GS exists but is almost never done' (a practical problem).   Either way, a thoughtful systematic review of previous definitions would be a valuable contribution to any researchers planning to move this particular measurement literature forward.  Without a GS you can address aspects of the reliability of the different definitions used (i.e. similarities and differences among them), but you can't tackle the problem of validity.  

Thank you for your clear explanation. I confirmed my efforts in trying to make a systematic review but fail in the "quality assessment" or "risk of bias". I suppose a narrative review is a good option. 

I am not sure if this is the answer you are looking for - but there is INHAND - a collaboration in harmonization of pathology (toxicology) terminology and definitions - it is still on-going but you can google it to find the completed organ systems published and accessible through the Toxicologic Pathology Journal.