mega cisterna magna

My grandson who is 17 has been having tics on his right eye for the past few months.  He also has been complaining of headaches.  He had these tics 3 years ago but it went away on its own.  He has been having problems with concentration and focusing at school, so he was diagnosed as ADD.  He has been taking adderol for the past 5 years.  When he was small he had speech problems and a speech therapist worked with him for 4 to 5 years. We took him to the neurologist who requested an MRI and this what the impression:
Mega cisterna magna, no abnormal enhancement.  Diffusion weighted sequences are normal.
What does this mean and what is the treatment?