A case report does not have to be about a new disease. However it "should contribute new knowledge to the understanding, diagnosis or management of a known disease and must give a clear message to lear, for the reader. Ideally, it should provide insight that can lead to further research on the topic."
Juyal D, Thaledi S, Thawani V. Writing patient case reports for publication. Educ Health (Abingdon) 2013; 26:126-9.
What are the CARE guidelines?
The CARE Guidelines are designed to increase the accuracy, transparency, and usefulness of case reports. They were developed by an international group of experts in 2013. Subsequently they have been endorsed by multiple medical journals and translated into nine languages.
The acronym “CARE” was created from CA from case and RE reports.
Gagnier JJ, Riley D, Altman DG, Moher D, Sox H, Kienle GS, for the CARE group: The CARE guidelines: Consensus-based clinical case reporting guideline development. Dtsch Arztebl Int 2013; 110(37): 603-8.