Taking your staff member down the performance management/ disciplinary route could leave your employer in an IT before you blink, if she has dyslexia and if she mentioned it to somebody in the past. She may not have dyslexia, she may not know she has dyslexia. Still, if her under-performance in this areas has not been mentioned to her, why wait until a formal appraisal to raise the issue? It instantly escalates a difficult situation. Well before the appraisal meeting, take her to one side and address the issue of poor written work directly. Show her examples, discuss the impact on you and her colleagues. Then ask, Why? Make sure she understands that she has a problem that needs to be addressed. Agree an action plan (the diagnostic and therapeutic approaches noted above sound good) with impact measures and timescales. The action plan can then be used to monitor her performance. If the under-performance continues then you have grounds for entering formal performance management/disciplinary processes. However, the appraisal is not a disciplinary mechanism. Your HR people should be giving you this advice by the way. That they are not is worrying and leaves your employer vulnerable.