There are only a handful of case studies of really bad cases I'm aware of. And they're not that exacting in follow-up.
Note research, not opinion pieces.
Good quality papers. I found just a handful. I'm having trouble fishing then out from the thousands of opinion pieces.
Edit: I found one credible meta-analysis so far, and the results are not good. Most of abuse is not reported. Impact is not known, how handling of it is done is unknown to affect severity.