Incidence of arachnoid cysts after head trauma in an adult

  What is the incidence of arachnoid cysts after head trauma and what is the relationship of arachnoid to seisure disorder. How common is seisure caused by arachnoid cyst. How much head trauma is required to cause an arachnoid cysts?
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Precise figures are not available in the scientific literature for all of
your questions, specifically there is no data on incidence of arachnoid cysts
after head trauma, probably because this is such a varied group ranging from
closed injuries to gunshot wounds. In some cases arachnoid cysts have been
seen to decrease in size following head trauma, presumably because they
rupture due to the impact, so the relationship is complex and unpredictable.
It is really not possible to quantify the amount of head trauma required to
cause an arachnoid cyst, obviously it doesn't happen with very mild injuries and
would be nmore common with very severe head injuries but the is no clear cut
point on a scale above which the are more commom.
Arachnoid cysts are associated with epilepsy usually when they form part of a
congenital brain malformation, the incidence of epilepsy increases with the
size of the cyst, I have attached a reference to a paper which explores
this relationship.
Koch CA.  Moore JL.  Voth D.
Department of Medicine, Ohio State University Hospitals, Columbus, USA.
Arachnoid cysts: how do postsurgical cyst size and seizure outcome
correlate?.
Neurosurgical Review.  21(1):14-22, 1998.