First off, let me clarify that I am not an expert. But as we were dealing with Alex's head-banging when he was younger, we began to hear about a condition called CTE (Chronic Traumatic Encephalopathy appearing in former NFL players. They would become violent and delusional. It was linked to being hit in the head.
Since Alex was self-inflicting multiple head-hits per day and doing it hard enough that it echoed through the house, we were naturally concerned. We saw a neurologist who explained that we were right to be concerned but that CTE could only be diagnosed at autopsy and could only be suspected through a change in behaviour. Alex couldn't show a change in behaviour which would not be attributed to the fact that he was changing from a toddler to a preschooler and we certainly were not about to do an autopsy. She did reassure us that the preliminary evidence suggested that it took repeated trauma done over decades to create the conditions for CTE, so if we could work on Alex's head-banging, that would be helpful. If we could even reduce it to one bang per incident, that would be helpful, as the theory was that CTE happened when the brain received another trauma before the first one had a chance to heal. We asked if a helmet would help and the doctor told us that it wouldn't. The problem is when the brain inside slams against the skull and, short of surgery, there's no way to install cushioning in there.
We worked on it and I accepted that we'd never know if CTE was a factor until it was too late.
This weekend, I watched the movie Concussion which is a docu-drama about the discovery of CTE by Dr. Bennet Omalu, the coroner who insisted on doing an autopsy on Mike Webster, a former football star who ended up homeless, self-mutilating and delusional. The movie suggests that this was not the first former football player to go on a downward spiral and end up committing suicide, but it was the first time that someone insisted that there must be something physically wrong for it to happen. (Good film, very powerful)
It brought back my original concerns and how helpless I felt as my child would injure himself over and over. These days the head-banging is better. We usually only have one bang and those fairly widely-spaced, maybe 1 a week.
If he has injured himself sufficiently to cause brain damage, then I can only hope that we're prepared to deal with it. Maybe it will be less frightening to him, as the autism already means he lives in a world which doesn't make intuitive sense to him. Maybe he will be easier as he's used to being under constant care and likely will be for the rest of his life, regardless.
But we won't know. Not for a very long time.