According to this paper, high dose opioid use can trigger a seizure. The key may be high doses. The rate of seizure in Tramadol was apparently significantly increased when it was first introduced and dropped dramatically when doses were lowered. Seizure does not appear to be common at all in low to normal dose morphine use.
Some 'seizures' attributed to opiods may be high rates of myclonic jerks which do not show seizure activity using EEG's.
For more check out this page.
Be sure to talk to your doctor about this...