Our 8 year old was 1year post surgery when he had to have an appendectomy.  We have been told by our neurosurgeon that 2 things are important for future surgeries:  hydration and blood pressure during anesthesia.  A third thing I would include in that is letting the surgery team know that your child has mm.  When our son had his appendectomy they made sure they understood our concerns and they understood his history.  We had no complications or problems.  They gave him one dose of tylenol with codeine overnight.  I didn't let them give him more the next day as he seemed to be managing the pain well.  We used basic tylenol if he was uncomfortable during the week afterward.  good luck!  Research and voicing your concerns and questions is the best!