Anesthesia for Seizure Surgery.docDocument Transcript
Anesthesia for Seizure Surgery
Adapted from the ASA PBLD Curriculum L-16
Copyright protected, For Educational Uses Only
1. Discuss epilepsy and the classification of seizures.
2. Describe the effects anesthetic agents have on EEG monitoring.
3. Describe the diagnostic workup for seizure surgery.
4. Discuss the anesthetic goals and management for the placement of intracranial
5. Discuss the anesthetic goals and management for the patient requiring seizure
A 32 year-old female diagnosed with intractable epilepsy presents for implantation of
depth and grid electrodes. The surgeon cannot use a stereotactic frame. A craniotomy
with intraoperative electrocorticography is required. Her medication includes
carbamazepine and phenytoin. However, they have been discounted for twenty-four
hours. Her seizure classification is partial evolving to generalized.
Key Questions for Discussion:
1. What preoperative medications would you administer?
2. What invasive monitors would you include? When would you place them?
3. How would you induce and maintain this patient prior to the
4. What anesthesia would you administer during EcoG (electrocorticography)?
EPILEPSY AND THE CLASSIFICATION OF EPILEPTIC SEIZURES:
Definition: Epilepsy is a clinical paroxysmal disorder of recurring seizures, excluding
alcohol or drug withdrawal, febrile seizures or insulin induced
hypoglycemia. Recurring seizures from tumors or strokes is a form of
Incidence: Worldwide incidence is 0.5-2%. The incidence in the United States is
estimated at 0.625%, effecting 300,000 people. Approximately 30% of
patients with epilepsy are refractory to medical therapy; therefore, almost
100,000 patients are potential surgical candidates. It is estimated 5,000
new surgical cases present every year (2).
Classification of Epilepitic Seizures:
I. Partial Seizures – focal onset limited to one hemisphere
A. Simple – no impairment of consciousness “auras”
B. Complex – altered consciousness “psychomotor”, “temporal lobe”
C. Partial evolving to generalized seizures – starts focally then spreads to
involve bilateral hemispheres
II. Generalized Seizures – involvement of bilateral hemispheres, convulsive
A. Absence – nonconvulsive, “petit mal”
B. Atonic – loss of muscle tone
C. Myoclonic – single extensor movement
D. Clonic – repetitive successive contraction and relaxation
E. Tonic – sustained contraction
III. Pseudoseizures – may mimic any seizure but with atypical behavior
IV. Unclassified epileptic seizures
Seizure surgeries are most frequently performed for patients with partial seizures,
particularly complex partial, as they are most likely to be refractory to medical therapy
and often have a specific lesion. These lesions can be tumors, arteriovenous
malformations, tubers of tuberous sclerosis, post traumatic, infarction, and ischemia.
Concomitant medical problems associated with epilepsy are psychiatric disorders,
tuberous sclerosis, neurofibromatosis, multiple endocrine adenomatosis, Jervell and
Lange-Nielson Syndrome, sudden death syndrome and sleep deprivation.
Diagnostic Workup for Seizure Surgery:
Phase I: Goal-To determine if epilepsy is intractable and if there is an unifocal
onset. A non-invasive evaluation is performed. This includes a review of
prior medical management, a history and physical exam, a
neuropsychological evaluation, structural imaging (CT or MRI),
functional imaging and EEG monitoring.
Phase II: Goal- To accurately determine the site of the seizure onset and to localize
language and memory function if a temporal lobectomy is required. This
can be accomplished by intracranial electrode placement with monitoring,
and W ADA testing.
Phase III: Surgical resection.
Phase IV: Evaluation of outcome.
*W ADA testing – is performed to determine if the contralateral hemisphere can support
language and memory function for patients undergoing temporal lobectomies. Language
and memory are testes after an intracarotid injection of sodium amobarbital. This isolates
and anesthetizes one cerebral hemisphere based on flow distribution of the carotid artery
injected. However, the hippocampal region is supplied primarily by the posterior
cerebral arteries. Therefore, if required a “superselective W ADA test” is performed by
angiographically injecting the posterior cerebral arteries.
Noninvasive EEG recordings can supply erroneous information. If EEG recordings
cannot be validated, intracranial electrodes are placed intraoperatively.
There are different types of intracranial electrodes:
Depth electrodes – are placed within the brain tissues.
Subdural and Epidural electrodes- are arranged in strips or grids and placed over the
cortical surface of the brain.
The placement of grid electrodes usually requires a craniotomy.
The placement of depth electrodes may require a sterotactic frame.
The placement of strip electrodes can be done through burr holes.
Intracranial electrodes are able to record both interictal and ictal activity of the brain.
Electrocorticography (ECoG) refers to recordings from intracranial electrodes. Once
intracranial electrodes are placed, patients are admitted to a hospital floor where
electrocorticography can be monitored until the site of seizure onset is localized. This is
dependent on the frequency of a patient’s seizure activity. Commonly two weeks of
monitoring are required before the patient can return for definitive resection of their
Consideration for Preoperative Preparation:
Goals of the preoperative visit include obtaining informed consent, reviewing the medical
history, anticonvulsant therapy and the clinical presentations of seizures, creating rapport,
alleviating anxiety, informing and educating, and reviewing positioning concerns. When
electrocorticography is planned intraoperatively, it is important to avoid medications that
may suppress recordings. Patients will often have their anticonvuldant medications
discontinued prior to surgery.
Considerations for Monitoring:
Patients with epilepsy often have thicker skulls resulting in greater blood loss
(approximately 300-500 cc’s). When intracranial electrodes are placed stereotactically, a
craniotomy is not required. However, stereotactic frames may hinder airway
management, necessitating a craniotomy.
Considerations for Anesthetic Management:
The anesthetic goals for intracranial placement with interoperative electrocorticographic
monitoring are to provide optimal patient comfort, operating conditions and to cause
minimal effect on neurophysiological monitoring. Other considerations are: effects
anticonvulsants may have on anesthetic care, avoid delayed emergence, ensuring
immobitity and minimizing awareness.
Anticonvulsants are associated with a wide variety of side effects. Some side effects of
carbamazepine are leukopenia, diplopia, anorexia, and dizziness. Side effects of
phenytoin are gingival hypertrophy, hirsutism, and ataxia. In general, anticonvulsants
that induce hepatic enzymes will increase resistance to neuromuscular blockers and
increase anesthetic requirements. Fentanyl requirements may increase fourfold. Both,
carbamazepine and phenytoin induce enzyme activity. They also increase the oxidative
and reductive metabolism of halothane. Of note, erythromycin and cimetidine may
interfere with the metabolism of carbamazepine.
Stem Case Continued:
The surgeon has encountered a problem and would like to delineate the motor strip with
monitoring. He/She requests that the patient has no muscle relaxants, no nitrous and the
isoflurane decreased to <0.25%. Your response is?
ANESTHETIC EFFECTS ON EEG:
Proconvulsant and Anticonvulsant Properties of Anesthetics:
The table below is modified from reference (I). Please refer to it for a detailed listing of
the references for the anesthetics included in the table. Another excellent reference is (2)
which summates results from numerous references.
Proconvulsant and Anticonvulsant Properties of Anesthetics
ANESTHETIC Proconvulsant Anticonvulsant Proconvulsant Anticonvulsant
Nitrous Oxide + - ++ ----
Halothane + ++
Enflurane +++ + ++ +++
Isoflurane ++ +++ +++
Thiopental ++ +++ +++
Methohexital +++ +++
Etomidate +++ +++
Benzodiazepines --- +++ +++
Ketamine ++ + +++
Propofol ++ ++
Opioids +++ +++ +
Positive studies: +, isolated case : ++, several cases (1-5): +++, reproducible controlled
studies or many cases. Negative studies: -, isolated case: --, several cases (1-5): ---,
reproducible controlled studies or many studies.
In general, EEG activity is affected not only by various anesthetic agents but also by the
depth of anesthesia. Therefore, avoiding boluses and maintaining a steady state of
anesthesia is important for EEG monitoring.
To avoid interference during electrocorticography these recommendations have been
Halothane < 0.5%
Isoflurane < 0.25 - 0.5%
Enflurane – evidence suggests produces less focal activation but nonspecific activity,
therefore, not very useful.
Nitrous Oxide – reported to potentiate anesthetic effects on EEG.
Barbituates – have induced epileptiform activity in epileptic patients but not in normal
Thiopental – can be used for induction, however, controversy over ability to isolate
Methohexital – controversy over its specificity and clinical used to locate a seizure
Propofol – has induced epileptiform activity in epileptic patients but not in normal
patients. Gaining wider acceptance as an induction agent, but disparate results on EEG
Etomidate – similar to barbiturates, but like methohexital it can be used to elicit EcoG
Ketamine – controversial, no practical value.
Opioids – effects have been assumed to be nominal suppression, but evidence supports
further studies warranted.
Benzodiazepines – may falsely localize seizure focus, reduces spike activity after
Droperidol – no effect on EEG when given alone, potentiates fentanyl suppression.
Stem Case Continued:
The patient returns two weeks later for resection of her seizure focus, which is near the
motor strip. The surgeon would like to perform cortical mapping (evaluate language and
memory function and define the motor strip) during the procedure.
Key Questions for Discussion:
1. What anesthesia would you provide for the numerous injections to infiltrate
the scalp with local anesthetic?
2. What anesthetic regimen would you provide until assessment of the language
and motor function is complete?
3. What anesthetic regimen will you provide once assessment is complete and
4. Which patients might not be candidates for such procedures?
Considerations for Anesthetic Management:
Of 354 patients receiving conscious sedation analgesia for seizure resection there was no
perioperative morbidity or mortality (11): however,
• 7 patients lost their airway and required general anesthesia – can
attempt blind nasal first
• 16% had convulsions intraoperatively
• 8% experienced nausea and vomiting-metoclopromide or droperidol
• 3% has excessive sedation – loss of airway reflexes or inability to test
memory and language
• 1.4% has a tight brain
• 2% received toxic levels of local anesthetic
Prior to Discussion – ALSO consider:
• the lengthy procedure (>5 hours) with uncomfortable positioning
• minimizing coughing and movement
• possible recall
• pain-from application of alcohol to freshly shaven scalp, injection of
local anesthetic and separation of dura with removal of bone flap
• inducing seizure activity-may be accomplished by decreasing PaCO2
20-25 mmHg, decreasing Isoflurane to less than 0.25%, administering
methohexital boluses of 10-25 mg, or with boluses of etomidate
• interference on neurophysiologic monitoring
• noise of drill may disturb the patient