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    Anesthesia for Seizure Surgery.doc Anesthesia for Seizure Surgery.doc Document Transcript

    • Anesthesia for Seizure Surgery Adapted from the ASA PBLD Curriculum L-16 Copyright protected, For Educational Uses Only Objectives: 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 electrodes. 5. Discuss the anesthetic goals and management for the patient requiring seizure surgery. Stem Case: 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 electrocorticography recordings? 4. What anesthesia would you administer during EcoG (electrocorticography)? Model Discussion: 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 epilepsy. 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 or non-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. INTRACRANIAL ELECTRODES: 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 seizure focus. 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 HUMAN ANIMAL ANESTHETIC Proconvulsant Anticonvulsant Proconvulsant Anticonvulsant Nitrous Oxide + - ++ ---- Halothane + ++ Enflurane +++ + ++ +++ Isoflurane ++ +++ +++ Sevoflurane ++ Desflurane -- 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 made (2). 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 patients.
    • Thiopental – can be used for induction, however, controversy over ability to isolate lesions. Methohexital – controversy over its specificity and clinical used to locate a seizure focus. 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 activity. Etomidate – similar to barbiturates, but like methohexital it can be used to elicit EcoG activity. 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 boluses. 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 resection begins? 4. Which patients might not be candidates for such procedures? Model Discussion: 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 bolus • 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