ADHD has a high rate of comorbidity. If untreated, it can lead to anxiety and depression. In my personal experience, ADHD often can leave me feeling completely overwhelmed with life - everything just seems like complete chaos most of the time. It's really easy to find some way (possibly unhealthy such as SI) for feeling an ounce of control again. SI is not directly a symptom of ADHD. I do think that it is very plausible that it results from symptoms of ADHD that are not being treated and thus causing depression and anxiety.