Abstract
Awake craniotomy is a procedure in which the patient is awake for some period of time to allow for neurological testing. These awake procedures include epilepsy surgery, resection of tumors, or vascular malformations in proximity to eloquent regions of the brain and functional neurosurgery. The challenges for the anesthesiologist include providing adequate analgesia and anxiolysis for the patient especially during painful parts of the procedure, maintaining systemic and cerebral hemodynamic homeostasis, establishing airway patency with adequate ventilation, and ensuring patient cooperation during electrophysiological mapping. This chapter focuses on the anesthetic considerations in patients undergoing awake craniotomy for tumor excision and epilepsy surgery. The issues discussed include rationale for awake surgery, preoperative assessment, preparation of the operating room, patient positioning and monitoring, anesthetic techniques, cortical mapping and functional testing, and potential complications with their management strategies.
Keywords
Awake craniotomy, Cerebral tumor, Cortical mapping, Electrocorticography, Epilepsy surgery, Functional testing
Outline
Introduction 489
Patient Selection 490
Awake Craniotomy for Tumor Surgery 490
Rationale 490
Indications 490
Preoperative Assessment 490
Operating Room Preparation, Patient Positioning, and Monitoring 491
Local Anesthesia 492
Intraoperative Anesthetic Management 492
Premedication 492
Anesthetic Techniques 492
Intraoperative Cortical and Subcortical Mapping 494
Complications 494
Postoperative Care 496
Patient Perception and Satisfaction 496
Awake Craniotomy for Epilepsy 496
Introduction 496
Rationale 496
Preoperative Assessment 496
Intraoperative Anesthetic Management 497
Anesthetic Techniques 497
Intraoperative Electrocorticographic Monitoring 497
Effects of Anesthetic Drugs on Electrocorticography Monitoring 497
Inhalational Agents 498
Intravenous Agents 498
Opioids 498
Local Anesthetics 498
Neuromuscular Blocking Agents 498
Anticholinergics and Anticholinesterases 498
Complications 499
Conclusion 499
References 499
Introduction
Awake craniotomy is a procedure in which the patient is awake for some period of time to allow for neurological testing. This concept is not new and developed along with the concept of brain mapping, which was initially used to localize the epileptic focus in patients undergoing epilepsy surgery. With the advancement in neurosurgical techniques, neuroimaging, intraoperative cortical mapping, and anesthesia, the scope of awake craniotomy then expanded from epilepsy surgery to surgery for supratentorial tumors, vascular malformations, and other lesions located near eloquent areas of the brain function. The challenges for the anesthesiologist include providing adequate analgesia and anxiolysis for the patient especially during painful parts of the procedure, maintaining systemic and cerebral hemodynamic homeostasis, establishing airway patency with adequate ventilation, and ensuring patient cooperation during electrophysiological mapping. This chapter will focus on the anesthetic considerations in patients undergoing awake craniotomy for tumor excision and epilepsy surgery.
Patient Selection
The selection of appropriate patients is critical to the success of an awake procedure. The patient must be cooperative, motivated, and have understanding of the procedure and the reasons for it. The criteria for patient selection vary between institutions and even between surgeons in the same institution. The decision to proceed with awake craniotomy is often made on a case-by-case basis after weighing the benefits and risks for every individual patient. Patient cooperation is vital during the procedure, therefore, patients with significant dysphasia, severely impaired motor function, confusion, or somnolence may not be suitable candidates. The lack of understanding of the spoken language may be a problem, but it can be remedied with the presence of an appropriate translator. In patients with tumors, a trial of steroid therapy may be attempted to improve their preoperative symptoms by reducing mass effect from the tumor, and thus enable the possibility of an awake surgery. An absolute contraindication is patient refusal. Other relative contraindications include morbid obesity, obstructive sleep apnea, psychiatric disorders, poorly controlled seizures, increased intracranial pressure, uncontrolled coughing, and anticipated difficult airway.
Awake Craniotomy for Tumor Surgery
Rationale
For tumors located within proximity of eloquent areas of the brain function, awake craniotomy allows for neurological assessment and cortical mapping, which facilitates a greater extent of resection while minimizing damage to the eloquent areas. Further benefits include the ability to perform detailed language mapping, which cannot be performed during general anesthesia, and the ability to help predict neurological outcome. Patients undergoing awake craniotomies for tumors located near eloquent regions have a lower incidence of new and permanent neurological deficits postoperatively compared to those undergoing surgery under general anesthesia. The performance of cortical mapping in an awake patient also facilitates a greater extent of tumor resection potentially resulting in improved patient survival, increased time to recurrence, improved success of subsequent adjuvant therapy, greater functional and symptom improvement, and decreased risk of malignant transformation.
The avoidance of general anesthesia may also have other benefits such as the use of less invasive monitoring and anesthesia drugs and decrease in incidence of postoperative nausea and vomiting. Shorter hospital stays including day surgery, which are associated with less hospital-related complications such as infection, are possible, all of which decrease resource utilization. Awake craniotomy is also considered in some patients irrespective of the need for intraoperative mapping.
Indications
Awake craniotomy is generally indicated for resection of supratentorial intra-axial tumors located near or within eloquent areas of brain, which control somatosensory, motor, language, and executive functions. Surgical factors that preclude an awake craniotomy include highly vascular tumors (risk of massive bleeding), infratentorial tumors due to uncomfortable surgical positions (prone or park bench), or tumors with extensive dural involvement (resection can cause severe pain) ( Box 29.1 ).
Preoperative
- 1.
Establish rapport, psychological preparation, reassurance, reinforce indication for awake surgery.
- 2.
Optimize other medical problems.
- 3.
Continue antiepileptic medications on day of surgery for tumor surgery. For epilepsy surgery, discuss with neurologist and neurosurgeon.
- 4.
Continue steroid coverage for tumor surgery.
- 5.
Avoid benzodiazepines for preoperative sedation in epilepsy surgery.
Intraoperative
- 1.
Surgical position may be supine, lateral, or semisitting. Ensure comfortable positioning.
- 2.
Standard monitors are used with invasive monitoring when indicated. Supplemental oxygen with means of monitoring of end tidal CO 2 and respiratory rate.
- 3.
Ensure adequate local anesthesia for pin sites, scalp, and incision.
- 4.
Surgical drapes should form a tent around the patient’s face to allow access to the airway and communication with the patient.
- 5.
Anesthetic technique options include conscious sedation (monitored anesthesia care) and asleep-awake-asleep. Common sedative agents used include propofol, midazolam, fentanyl, remifentanil, dexmedetomidine.
- 6.
Discontinue propofol infusion 20 min before cortical mapping or electrocorticography monitoring.
- 7.
Functional tests done for cortical mapping include motor, sensory, and language.
- 8.
Continuous vigilance and rapid treatment of complications. Complications include seizures, respiratory adverse events, and lack of patient cooperation.
Postoperative
- 1.
Postoperative disposition depends on institutional practices.
- 2.
Monitor for postoperative neurologic deficits, seizures.
- 3.
Treatment of pain and nausea and vomiting.
- 4.
Overall patient satisfaction is good.
Preoperative Assessment
The selection of a patient for an awake craniotomy is initially done by the surgeon. However, each patient should also be seen and evaluated by an anesthesiologist, preferably in a preoperative anesthesia clinic. The preoperative encounter between the anesthesiologist and patient is important not only for optimization of the underlying medical comorbidities but also for providing opportunity for the anesthesiologist to establish rapport and provide counseling. The assessment is similar to that of any patient with a neurological disorder coming for a craniotomy. Medications for ongoing medical conditions should be continued, especially the continuation of dexamethasone for treatment of edema associated with a brain tumor. Patients with low-grade gliomas commonly present with seizures and are often on antiepileptic medications, which should be continued on the day of surgery. Patients are understandably anxious, but those who are adequately informed tend to tolerate the procedure better. The rationale for awake surgery should be discussed to reinforce patient cooperation. Patients should be informed that they may recall certain parts of the procedure, and reassured that adequate analgesia and sedation will be provided during painful parts such as local anesthesia infiltration, application of the head frame, and bone drilling. The requirements for intraoperative cortical mapping should be explained, and this involves performance of motor, memory, and/or language tests.
Operating Room Preparation, Patient Positioning, and Monitoring
The operating room should be prepared before the patient enters, and unnecessary traffic into and out of the room should be minimized to maintain a calm environment. Depending on the surgical requirements, the patient may be positioned supine, semisitting, or lateral. The head is usually immobilized with rigid pin fixation or occasionally placed on a donut-shaped gel pad. It is important to ensure that the patient is positioned comfortably prior to the procedure as they would remain in that position for several hours, with minimal adjustments possible subsequently. Discomfort from prolonged positioning is a common complaint among patients undergoing this procedure. Strategies to provide comfort include adequate soft padding especially on pressure areas, placing pillows under the knees, avoiding excessive flexion, extension or turning of the neck, and maintaining normothermia. Surgical drapes should be placed such that they form a tent around the patient’s face and allows the anesthesiologist access to the patient’s airway and to monitor for signs of distress. This “window” also facilitates communication between the medical team and patient during functional testing. A microphone can be placed near the patient’s face or a video camera used to record facial movements for the surgical team’s observation during functional testing.
Standard monitors such as electrocardiography, pulse oximetry, noninvasive blood pressure measurement, and end tidal CO 2 monitoring are mandatory in all patients. The use of other monitoring modalities such as intra-arterial blood pressure, central venous catheter, indwelling urinary catheter, temperature and bispectral index (BIS)/entropy depend on institutional practices and individual patient requirements.
Local Anesthesia
Providing adequate anesthesia to the scalp is vital to facilitate craniotomy in an awake patient. Different techniques for scalp blockade have been described. One technique involves bilateral selective blockade of sensory nerves (auriculotemporal, zygomaticotemporal, supraorbital, supratrochlear, lesser occipital, and greater occipital nerves), infiltration of the pin insertion sites, and along the surgical incision. Selective scalp blockade has been shown to be more effective than local infiltration in minimizing hemodynamic and stress responses to head pinning. An alternative technique is infiltration of the local anesthetic in a circular fashion or “a ring block” around the surgical incision and at the pin insertion sites. This technique is faster to perform than selective scalp blockade but has a potential higher risk of local anesthetic toxicity due to requirements for larger volumes of local anesthetic. Lidocaine is usually used for infiltration at the pin sites, but longer acting agents such as bupivacaine, levobupivacaine, and ropivacaine should be used for scalp blockade. Adrenaline (5 μg/mL, 1:200,000) should be added to the local anesthetic mixture to minimize systemic absorption and maximize block duration as the scalp is a highly vascularized area. A total of 30–40 mL of local anesthetic mixture is usually required to achieve adequate anesthesia of the scalp. With addition of adrenaline, the maximum allowable doses for lidocaine, bupivacaine, levobupivacaine, and ropivacaine are 7 mg/kg, 2.5 mg/kg, 2.5 mg/kg, and 4 mg/kg, respectively.
Dissection of the dura does not usually produce any pain except when it is done in close proximity to the meningeal vessels. This may happen during craniotomies over the middle cranial fossa, and local anesthesia with lidocaine alone can be supplemented in the potential space between the two layers of dura mater along the meningeal vessels.
Intraoperative Anesthetic Management
The anesthetic regimens used in awake craniotomies vary according to institutional practices, and there is no ideal formula. The aim is to provide adequate analgesia and anxiolysis during painful parts of the procedure and ensure that the patient is subsequently awake to cooperate with functional testing during intraoperative cortical mapping. Other objectives include ensuring a patent airway and adequate ventilation to prevent hypercarbia or hypoxia, and maintaining stable systemic and cerebral hemodynamics. The painful or stimulating parts of the procedure are during local anesthesia infiltration, application of the rigid head pin fixation, bone drilling, dissection of the temporalis muscle, and traction on the dura. Historically, neuroleptanesthesia with droperidol and fentanyl was the anesthetic technique of choice. Some variations include substitution of fentanyl with alfentanil or sufentanil. This technique was associated with higher incidences of intraoperative seizures, nausea and vomiting, excessive sedation, and dysphoria.
Newer anesthetic drugs with shorter duration of action and improved pharmacodynamic profiles such as propofol, midazolam, and dexmedetomidine have since replaced droperidol. Analgesic drugs that are commonly used include fentanyl and remifentanil. These drugs can be given as intermittent boluses (physician or patient controlled), continuous infusions, and target-controlled infusions.
Premedication
Premedication is optional but is used in some institutions with administration of benzodiazepines [e.g., zolpidem 10 mg orally (PO)] or clonidine (3–4 μg/kg PO). Some centers routinely administer anticonvulsants and mannitol to all patients intraoperatively. In addition, high-dose steroids [e.g., dexamethasone 10 mg intravenously (IV)] should be administered to most patients.
Anesthetic Techniques
The anesthetic techniques used for awake craniotomy can generally be classified as conscious sedation or monitored anesthesia care with no or minimal manipulation of the airway, and the asleep-awake-asleep usually with manipulation of the airway for the asleep parts. The technique of choice depends predominantly on the preferences of the neurosurgeon, neuroanesthesiologist, and institution.
Conscious Sedation
The aim of conscious sedation is to achieve a sedation score of around 3 on the Modified Observer’s Assessment of Alertness/Sedation scale or its equivalent, whereby patients respond only after their name is called loudly or repeatedly. Sedation is administered during the initial stimulating parts of the procedure, then stopped or reduced during cortical mapping, and resumed for resection and closure. Spontaneous ventilation is maintained throughout the procedure with oxygen supplementation from a face mask or nasal cannula. If needed, nasal trumpets or nasopharyngeal airways can be used to maintain airway patency during periods of sedation. The use of conscious sedation avoids repeated manipulation of the airway and potentially prevents wide fluctuations in systemic hemodynamics. However, with excessive sedation, there is an increased risk of airway obstruction, respiratory depression, and apnea leading to hypercarbia and hypoxia.
Drugs commonly used for conscious sedation are propofol, midazolam, remifentanil, fentanyl, and dexmedetomidine. Any combination of these drugs may be administered as continuous infusions, bolus injections, target-controlled infusions, or patient-controlled boluses. A well-accepted technique has been propofol infusion (50–150 μg/kg/min) and an opioid (e.g., fentanyl boluses 0.5–1 μg/kg or remifentanil 0.01–0.05 μg/kg/min). Remifentanil infusion can be continued at low doses (0.01–0.02 μg/kg/min) during mapping. Dexmedetomidine is an α 2 -adrenoceptor agonist with sedative and analgesic properties that is increasingly used for conscious sedation in awake craniotomy. When administered as a continuous infusion, the patient is sedated but easily arousable. Generally, it does not cause respiratory depression and has minimal interference on electrophysiology recordings and cortical mapping. A bolus dose of 0.5–1 μg/kg is administered over 10 min followed by a continuous infusion rate of 0.2–0.7 μg/kg/h. Other agents are usually added to provide more analgesia (e.g., fentanyl, remifentanil), and for more amnesia and less alertness (e.g., propofol, midazolam). During intraoperative cortical mapping, dexmedetomidine can be continued at a low dose (0.2 μg/kg/h) to provide anxiolysis and analgesia to the patient. Rapid administration of the bolus dose (<10 min) may lead to transient hypertension in some patients that is mediated by peripheral vasoconstriction from activation of α 2B -adrenoceptors. Dexmedetomidine can also produce dose-dependent hypotension and bradycardia secondary to agonist effects at the central α 2 -adrenoceptors.
Asleep-Awake-Asleep
Asleep-awake-asleep is perhaps the most common technique used. For the initial asleep phase, general anesthesia is induced with propofol and maintained with propofol infusion, a volatile agent, or dexmedetomidine infusion. Analgesia is usually provided with remifentanil infusion or fentanyl boluses. Airway management includes spontaneous ventilation via airway adjuncts such as face mask, nasal trumpet, nasopharyngeal airway and cuffed oropharyngeal airway, or controlled ventilation via laryngeal mask airway (LMA) or endotracheal intubation. After opening of the dura, the patient is awakened and the airway removed for cortical mapping. Propofol infusion should be terminated approximately 15–20 min before mapping. Once cortical mapping is completed, general anesthesia is induced again with or without airway instrumentation for ventilation. Techniques for endotracheal reintubation include blind nasal intubation, fiber-optic intubation, use of a tube exchanger, or video laryngoscopy. The advantages of LMA over endotracheal intubation are its easier removal and reinsertion for the later part of the procedure, and lower incidence of coughing. The asleep-awake method is a variation of the asleep-awake-asleep technique, wherein the patient remains awake until the end of the procedure after cortical mapping. More recently, the use of BIS monitor with target-controlled infusions of propofol and remifentanil has been suggested to improve the titration of anesthesia resulting in smoother and faster transition between the asleep and awake phases.
The asleep-awake-asleep method has the advantage of providing a more comfortable and stress-free environment for the patient during painful parts of the procedure, which facilitates subsequent patient cooperation during intraoperative cortical mapping. In addition, LMA insertion or endotracheal intubation for controlled ventilation reduces the risk of respiratory complications (e.g., airway obstruction, respiratory depression, apnea) and allows better control of PaCO 2 and PaO 2 levels. However, there is a risk of triggering laryngospasm or uncontrolled coughing during airway manipulation in between the asleep and awake phases, resulting in surgical complications such as intracranial bleeding and raised intracranial pressure.
Another variation of the aforementioned techniques that has been used and known as awake-awake-awake emphasizes therapeutic communication with the patient using rapport, nonverbal communication and hypnotic suggestions, with minimal use of sedatives and analgesics. Some of the side effects with anesthetic agents can be avoided such as airway obstruction, respiratory depression, apnea, and nausea and vomiting. However, this technique is not widely used and may only be suitable for a selected group of highly motivated patients.
Nonpharmacological Measures
Nonpharmacologic or social measures are very useful to help the patient through the procedure especially during periods of wakefulness. These include frequent reassurance and verbal comfort, warning the patient of loud noises (e.g., bone drilling) and painful moments, allowing the patient to intermittently move his or her extremities, providing ice chips, and nonverbal communication with sometimes just holding the patient’s hand.
Intraoperative Cortical and Subcortical Mapping
The correlation between cerebral anatomical landmarks and function is unreliable due to individual variation in cortical organization, disruption of cerebral topography by tumor infiltration, and functional remodeling of the brain (plasticity). Therefore, intraoperative cortical mapping in an awake patient is crucial in facilitating maximal resection of tumors near eloquent areas of the brain without compromising function. Depending on tumor location, cortical mapping is done to test motor, sensory, language, or cognitive functions. The surgeon maps the cortical area of interest using a bipolar or monopolar stimulator probe and decides on the extent of resection based on positive (identification of eloquent cortical tissue) and/or negative mapping (absence of eloquent cortical tissue). Cortical areas that produce an informative response during testing are labeled with numbered paper tickets.
During mapping a neuropsychologist/physiologist or speech language therapist may be involved on a routine basis in some institutions, or only for more complex language mapping in other centers. However, most often the anesthesiologist will be asked to assist with the mapping. For motor mapping, the patient should be monitored for abnormal involuntary movements, movement disturbances, or inhibition of movement in the face, arm, and/or leg. For testing of primary sensory cortex, the patient is instructed to report abnormal sensations such as paresthesia. Other sensory functions that may be tested include the vestibular system and visual field. Language mapping involves several tests such as naming objects, counting numbers, reading single words, repeating complex sentences, and writing words and sentences. Preoperative assessment of language function is usually done to establish the patient’s capability of performing the required language tests intraoperatively. During cortical stimulation, one observes for language deficits such as anomia (inability to name objects but has fluent speech), expressive aphasia (inability to express self through speech or writing), and receptive aphasia (patient has fluent but meaningless speech with inability to understand written or spoken language). The testing of other more complex cognitive functions such as counting, memory, visuospatial function, and emotions may also be performed.
Low- and high-grade gliomas often infiltrate into subcortical white matter tracts and may contain functional brain tissue, which can be damaged during tumor resection resulting in postoperative neurologic deficits. Subcortical mapping is an extension of cortical mapping, which helps to identify and preserve various subcortical pathways, such as the somatosensory and descending motor tracts, during tumor resection. The use of both cortical and subcortical mapping facilitate maximal tumor resection while minimizing the risk of permanent postoperative neurologic morbidity.
Complications
Intraoperative complications can occur, but with good vigilance most of these can be treated quickly. The most common and potentially harmful complications include intraoperative seizures, respiratory adverse events, and loss of patient cooperation. Other complications and their respective management strategies are outlined in Table 29.1 .
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