It’s a Tuesday morning, and on your schedule is a 6 year old Maltese dog with a 3 year history of seizures that appear well controlled on phenobarbital monotherapy. She has a history of 1 seizure every 6-8 months and each seizure is less than 3 minutes in length. No interictal signs noted by the owners and she continues to have a normal neurologic exam. Today, they would like to discuss doing a dental for their dear little dog. What should you consider? How does general anesthesia for a patient with neurologic disease differ from those without?
The Risks of Anesthesia
General anesthesia is not necessarily bad. 😊 The biggest two risks of general anesthesia for dogs or cats with seizures are (1) hypotension and (2) negative effect on the seizure threshold. Let’s talk hypotension.
Your goals are to preserve cerebral blood flow, which is largely controlled by the cereal perfusion pressure, which is largely controlled by the mean arterial blood pressure (MABP). MABP is directly related to intracranial pressure (ICP). Thankfully there is a wide range at which MABP will have little negative effect on ICP. MABP between 50-150 mm HG should result in constant ICP, if other parameters are equal. Hypotension can be caused by some of the medications used (I’m looking at you acepromazine) or caused by CO2 levels. If the PaCO2 levels are above 50, a risk of vasodilation occurs which may decrease MABP. Monitoring CO2 can be quite useful to avoid this. If vasodilation occurs, and consequently decreased MABP, perfusion to the brain can be compromised. Hyperventilation will decrease the PaCO2, result in vasoconstriction and maybe lower the ICP. Some references suggest that PaCO2 should be between 30-35 for “appropriate” cerebral perfusion. Big disclaimer today – I am not an anesthesiologist so specific questions about anesthetic protocols should be directed to your local anesthesiologist! Long-standing, serious, hypotension can affect neuronal membranes and in rare situations could cause neuronal cell membrane damage, or a worsening seizure disorder. Thus, try to keep the PaCO2 in a normal range, and monitor blood pressure for pets with seizures undergoing anesthesia.
With respect to the seizure threshold, isoflurane, diazepam, midazolam, and possibly propofol are neuronal protective. If you have the luxury of choice, consider using one (or all!) of these medications in your anesthetic protocol. If you have a patient with poorly controlled seizures, administration of IV phenobarbital 20 minutes before starting anesthetic recovery may be useful to add seizure protection during the recovery process. ALWAYS go slow! Give the phenobarbital over 20-30 minutes as a slow infusion to avoid severe cardiovascular or respiratory suppression. This drug is closely related to pentobarbital, our common euthanasia solution. Most patients can receive benzodiazepine medication if active seizures are noted during the recovery process. Paradoxycal hyperactivity following benzodiazepine adminsitration occurs rarely but would be a reason to avoid using the drug in the future if a patient exibited these signs. Patients with significant hepatic disfunction such as those with portosystemic shunts should either avoid benzodiazepine drugs due to inadequate metabolism, or be adiminsered a 25% dose.
Any anesthetic event, even an uneventful one, can put a patient with a known seizure disorder at risk for seizure breakthrough. This risk persists for at least 24 hours following anesthesia but in rare patients it can be longer. Owners should avoid leaving the pet alone for extended periods of time such as traveling by airplane, boarding the pet or other unobserved time in the first 24 hours after anesthesia. Never withhold chronic anticonvulsant medications prior to anesthesia. Bromide can cause vomiting therefore it should be given with a small meal. Patients receiving bromide (liquid) can receive it rectally if they are anesthetized early in the morning and a small snack with their bromide is not possible. Phenobarbital, levetiracetam and zonisamide can be given on an empty stomach without high risk of GI upset.
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