how does the abuse cause this? Is it the high stress?
Of course this is the key question. How?? What is the mechanism?
The answer I think is not completely certain at this point. I have talked it over with my T and he suggests that maybe it has to do with the connection between the right and left halves of the brain (right and left cerebral hemispheres). These two halves are connected by a large bundle of connecting nerve fibers. The connection is called the corpus callosum.
We know that abuse has the ability to cause the brain to shut down the transmission of impulses in certain nerve fibers (they don't die, there is just a kind of roadblock implemented). This is the neurological cause of dissociation. So, if the transmission of impulses between the left and right hemispheres is impaired, the coordination of the left and right hemispheres is also impaired.
This impaired communication between the left and right cerebral hemispheres would have all kinds of implications including coordination of the eye muscles and the visual fields. It would also impair learning because the right hemisphere is concerned with visual and coordination interpretations. The left hemisphere is concerned with verbal intelligence. If they are not coordinated, then it is easy to postulate all kinds of learning effects.
Notice also, and not last or least in importance is the impairment of fine motor coordination involved in a child's learning to write and draw and play marbles, among other activities. Therapy would involve decreasing the burden of anxiety and fear connected with the abuse with counseling and psychotherapy, while giving remedial training to coordination of the eyes and hands. Learning to calm the effects of anxiety might help to decrease the effects of abuse that mimic HD and ADHD.