The abstracts can help with this, of course. The main trick is coming up with, or spotting, the sub themes and that is simply a matter of brain work. But if it is done well, and you have taken the trouble to find good sources, then you will find, quite magically, that you have constructed the skeleton and a good bit of the flesh and blood of your essay or research project.
In fact, a good literature review can result in an essay that virtually writes itself.