WASHINGTON, DC—Patients with intractable epilepsy who undergo intracranial electrode implantation prior to epilepsy surgery have a significantly greater number of days until their first seizure during intracranial EEG monitoring, compared with during scalp monitoring, according to research presented at the 67th Annual Meeting of the American Academy of Neurology. This “implantation effect” is caused by surgery itself, the researchers concluded, because variables, including anesthesia, morphine, and method of surgery, did not affect seizure frequency. The average number of seizures per day was comparable during both monitoring periods.
“Anecdotally, it had been noticed that patients that have had intracranial electrode implantation have had a transient decrease in seizure frequency in comparison to scalp monitoring,” said Michael Lane, MD, a neurologist at Dartmouth-Hitchcock Medical Center in Lebanon, New Hampshire. The effect also has been noted in some studies evaluating deep brain stimulation, with decreased seizure frequency within the first month in the sham group as well as in the treatment group.
To investigate the effect, Dr. Lane and colleagues conducted a retrospective review of 193 patients between 1992 and 2009. The patients underwent scalp monitoring prior to intracranial monitoring, essentially acting as their own controls, Dr. Lane said. Researchers reviewed the number of seizures per day, days to first seizure, and variables including pain medications, types of intracranial electrodes, number of electrode contacts, and anesthetic agents. Researchers also corrected for antiseizure medications.
During scalp monitoring, patients experienced an average of 1.09 seizures per day, compared with 1.24 seizures per day during intracranial monitoring. While the difference in seizure frequency per day was not significant, there was a significantly longer time to first seizure. Patients experienced their first seizure after an average of 1.87 days during scalp monitoring, compared with 2.47 days during intracranial monitoring. The effect was smaller than researchers had anticipated, Dr. Lane said.
The average number of monitoring days was higher after implantation than during scalp monitoring. Morphine, and whether craniotomy or burr holes were the method of surgery, did not have statistical relevance to seizure frequency, the researchers said. The cause of the implantation effect is not known, Dr. Lane said. Theories include the possibility that the increase in time to first seizure is a protective effect that occurs following surgery, or that it is a result of brain cooling, or of electrolyte abnormalities caused by inflammation.
—Jake Remaly
