It was suggested to me that written output disorder sometimes might be a focal example of such traditional anxiety disorder categories as specific phobia and that there have been carefully designed programs of ERP (exposure response prevention therapy) have been shown in controlled trials to help greatly.
I have searched for references to read more about this, but I have found nothing yet that specifically links the two. I did find information about graphaphobia (sp? name?) but by all accounts that seems to be some sort of fear of writing in public. (I suppose writing in a school setting could qualify as such an example?) If anyone comes across or knows where to further research this please email suggestions to firstname.lastname@example.org Possibly I am not looking in the right places.
I post this suggestion here because it may have relevance to people reading this blog and may be something they wish to investigate further. For my own circumstances I can see no evidence that it has much resonance for my child, for whom I've seen the physical act of writing causes difficulties, but I see a desire to write, that is thwarted somewhat by the physical slowness, which frustrates him. I have seen marked improvements with modifications to the expectations and I have also seen that the introduction of the keyboarding/typing facilitates a much more natural and rapid output.
I think anxiety is also a natural by product of being in a classroom setting where you sense a disparity between yourself and other children. I think there's a lot to excavate in this question of anxiety for these children.