This is a link to a long extract from this publication:
Behrman: Nelson Textbook of Pediatrics, Sixteenth Edition, Copyright © 2000 W. B.
Chapter 22 - Anxiety Disorders
Basically around p32-35 (chapter written by Melvin D Levine) I found some references to working memory problems, which can often impede written output.
Will keep hunting for more specific documentation on this topic. Email any leads or links to firstname.lastname@example.org please.