3% in 1 study
One study* found arnold-chiari type 1 malformation in 2 out of the 67 children
w arachnoid cysts.
Study was small; actual number could be different, also could be differences amongst different populations. *pascual-castroviejo i, et al "primary intracranial arachnoidal cysts. A study of 67 childhood cases." childs nerv syst. 1991;7(5):257.