Expert: JM Starkman, MD - 12/5/2009

Question
I'm a night nurse on a med-surg floor (I'm a new grad).  Recently, I had an epileptic patient with a serum phenytoin level of only 5.7 who was scheduled to have a hip replacement surgery the next morning at 0730.  MD was aware of the patient's phenytoin level, but had ordered no phenytoin beyond the dose the patient had already been taking (the dose that had resulted in this low serum level).
As I passed the patient and his information on to the day shift nurse, we asked ourselves the question, "What if the patient were to have a seizure while under general anesthesia?"  If a neuromuscular blocking agent were used, and the patient ventilated, what, if any, would be the effects of a seizure while under general anesthesia?  Also, do inhalation anesthetics and their adjuncts raise the seizure threshold?  Enough so that low serum levels of anticonvulsants are not a deterrent to surgery?

Thank you so much for being available to answer these questions.
Sincerely,
Rebecca

Answer
Anesthetics, by definition, depress CNS activity and so, raise the seizure threshold. However, that overly-simplistic explanation does not account for the fact that some anesthetic agents, even though they "depress" do, in fact, LOWER the sz threshold: Some examples are Sevoflurane, a lousy, but widely marketed, promoted and therefore utilized inhalational anesthetic. Sufentanil, which is a potent narcotic. Ethrane, a mostly discontinued  inhalational (cheap, generic--no one was making any money on this one).  Of course, these agents are not usually used as the 'sole' agent during anesthesia, so the sum-total of many factors--including the manner in which they are dosed--will determine the overall effect on the sz threshold.

A seizure under GA can result in the same detrimental effects to the CNS as when the patient is 'awake'. The use of neuromuscular blocking agents would tend to obscure to an observer the fact that a motor sz was occurring.

With so many issues that have to come together today to 'pull-off' a scheduled surgery, most anesthesiologists would tend to take into account the lower blood level of DPH and either administer an iv dose pre or intra operatively or utilize any of a number of techniques that would also eliminate the likelihood of a perioperative seizure......rather than inconvenience all involved and postpone a surgery for an easily-remedied problem.