Q: Is it possible to publish a subsequent treatment of a previously published case report?

First of all, you have to be really sure that this case report warrants another publication or if the subsequent treatment can be added to the existing article as an addendum or correction. If the subsequent treatment is just an extension of the previous treatment, it is possible that it does not merit another publication. In this case, it might be considered salami slicing, which refers to the practice of breaking up a single research paper into their “least publishable units,” with each paper reporting different findings from the same study. A set of papers are referred to as salami publications when more than one paper covers the same population, methods, and research question. This is an unethical practice. 

However, having said that, not all related studies or follow-up studies are unethical. If you feel that this case report is important enough to be published, you can publish it as the second part to the previous case report. thus, you should cite the published report and make it very clear that this report describes the subsequent treatment. Additionally, you should clarify in the cover letter that you have another paper on a related topic and provide a copy of the previous paper with your manuscript at the time of submission.

Recommended reading:

The pitfalls of "salami slicing": Focus on quality and not quantity of publications