12 Risk Evaluation and Anesthesia for Intracranial and Spinal Meningiomas
The evaluation of a patient’s fitness to undergo the stress of an anesthetic and surgical procedure constitutes the anesthesiologist’s assessment of risk. This assessment is based on the knowledge of prevalence rates for unwanted consequences in population groups sharing the same characteristics. The anesthesiologist must identify the risks of the surgery and either avoid the conditions that would predispose the patient to that risk or develop a means to alter the consequences of the surgical intervention that would lead to the risk. The risks for meningioma resection consist of those related to the operative site and those related to the anesthetic. The operative site is influenced by the position of the tumor, its characteristics, and its size. Depending on the location, these tumors may engulf nerves and blood vessels, invade large vascular structures such as the cavernous sinus, and extend into multiple cranial fossa and foramina. As a result, these surgeries may be time consuming, with the potential for large blood loss and fluid shifts.
Risk factors such as age, poor preoperative clinical condition, and tumor characteristics have been associated with increased morbidity and mortality of meningioma surgery.1 Tumor characteristics that should be known to the anesthesiologist for assessment of risk include size of the vessels supplying and draining the meningioma and whether the tumor has been embolized. Blood dyscrasias secondary to medication or chronic alcohol use could increase the propensity for bleeding. The role of aspirin and other nonsteroidal antiinflammatory agents cannot be discounted as important risk factors for bleeding, especially in patients with chronic pain due to headache, rheumatoid arthritis, osteoarthritis, gout, and ankylosing spondylitis.
Hypertension, cardiomyopathies, diabetes, pneumopathies, and peripheral vascular disease may all affect the patient’s surgical risk. The preoperative risk assessment should be based on a thorough and efficient evaluation of the patient that addresses the following key points:
Does the physical status of the patient increase the risk of mortality and morbidity during the perioperative period?
What current disease processes and medications influence the intraoperative and postoperative course?
What immediate medical action would be most beneficial to the patient to increase the chances for a successful outcome? The assessment may also include decisions for noninvasive testing to better estimate risk.
The anesthesiologist’s risk assessment is uniquely suited to the patient and includes an adequate and specific history with physical exam that will lead to confirmatory laboratory tests. This history is crucial to the discovery of cardiac and other symptomatic diseases that could place the patient in a higher risk category.
The history has been demonstrated to give primary information concerning a patient’s physical state in ~60% of the cases.2 The history should identify serious cardiac conditions, signs of congestive heart failure, arrhythmias, or severe valvular disease. The first key aspect of the history is the assessment of exercise tolerance. Usually patients are asked if they can climb two sets of stairs, which is equivalent to 4 metabolic equivalent tasks (METs) of activity ( Table 12.1 ). The inability to perform this task should arouse suspicion of possible congestive heart failure or coronary artery disease. Other basic questions determining vitality, mobility, and fitness are asked, along with a review of systems to determine evidence of chronic disease, pulmonary disorders, and recent upper airway or genitourinary infection. Histories of hospitalization and surgeries, family history, and social history, including alcohol intake and use of drugs, are important considerations.
Age must always be considered in patients undergoing meningioma resection.3 This factor can contribute to increased morbidity and mortality associated with the meningioma resection. Although much of the increased morbidity related to age is appropriately attributed to comorbidity, a patient > 80 years of age will have reduced physiological reserve. Numerous studies have found a significant increase in mortality related to surgery beginning at the age of 70. The elderly patient has diminished cardiac reserve and a higher incidence of atrial fibrillation secondary to degenerative changes in the conduction system.4 Depression, dementia, and other neurological disorders may also be associated with advanced age, and neurological impairment, visual loss, or changes in vision should be assessed. Vital capacity and forced expiratory volume at 1 second decrease roughly 1% per year.5 Pre-operative questions concerning cough, sputum production, hemoptysis, pneumonias, shortness of breath, chest pain, and reduced physical activity may point to changes in either cardiac or respiratory reserve, which may need further evaluation before surgery.
Age also affects renal function and liver synthetic activity. Glomerular filtration rates decline linearly from 80 mL/min/m2 at age 30 to 58 mL/min/m2 by age 80.6 Blood flow to the kidneys will be half that of young adults by age 65. This may result in a decreased ability to concentrate urine and an inability to regulate electrolytes. Liver function and blood flow may be reduced in elderly patients, and the nutritional status may be questionable. Drug clearance and metabolism are reduced, and serum albumin may be low, producing an increase in the free fraction of highly protein bound drugs. The history obtained by the anesthesiologist should focus on eating habits, bowel and bladder function, and overall general nutrition.
The next major anesthetic assessment to determine risk is the ability to intubate and ventilate the patient for the surgical procedure. Closed claims analysis reveals that 85% of airway incidents involved brain damage or death due to the inability to ventilate and intubate the patient.7 This is only exacerbated in an individual with increased intracranial pressure secondary to a large brain mass. The identification of a patient with a difficult airway is vital in planning the anesthetic management and assessment of risk the patient will assume. Malformation of the face, acromegaly, cervical spondylosis, occipitoatlantoaxial disease, tumors of the airway, and long-term diabetes producing stiff joint syndrome carry added risk. Head movements and the ability to hyperextend the neck with a thyromental distance of greater than 6.5 cm and the ability of the patient to prognath the jaw and an interincisor gap of > 5 cm would provide evidence of a large mouth opening and easy laryngoscopy. Dental pathology, especially loose teeth, should be identified as they could become dislodged and be aspirated during laryngoscopy, which could add considerable morbidity to the procedure ( Table 12.2 ). The Mallampati Classification consists of the criteria most used for assessing difficulty with intubation and adjusting patient risk for obtaining a patent airway.8 Class I–IV designations of anatomical appearance predict ease of intubation, with class (IV) the most difficult to intubate. Dyspnea related to airway compression, dysphagia, and sleep apnea along with clinical signs (obesity, limited mouth opening, or large tongue) should be treated as portending a difficult airway until proven otherwise.
The next major assessment of perioperative risk involves the cardiovascular system and the patient’s fitness to undergo the procedure without major morbidity or mortality. The cardiovascular examination should include an assessment of vital signs, carotid pulse, evidence of jugular venous distention, auscultation of the lungs, precordial palpation and auscultation, abdominal palpation, and examination of the extremities for edema and vascular integrity. The patient who presents for major noncardiac procedures poses the biggest problem for assessment of fitness to undergo a surgical procedure. The general appearance of the patient provides evidence of overall physical health. Cyanosis, pallor, dyspnea during conversation or with minimal activity, Cheyne-Stokes respiration, obesity, skeletal deformities, tremor, and anxiety are a few clues of underlying disease that can be recognized by a skilled physician.
High-risk cardiac patients, as assessed from the history and physical exam, need further investigation to determine functional status and to help in developing the plan for intraoperative monitoring ( Table 12.3 ). An electrocardiogram (ECG) sometimes uncovers occult disease in older adults, but it rarely shows clinically important abnormalities in younger asymptomatic patients. Preoperative resting ECGs are recommended for patients undergoing large meningioma resection if they have evidence of coronary artery disease, peripheral vascular disease, high blood pressure, diabetes, history of congestive heart failure (CHF), shortness of breath, and cigarette smoking.9 ECG abnormalities that have the potential to alter management include atrial flutter or fibrillation; first-, second-, or third-degree atrioventrivular (AV) block; ST segment changes suggestive of ischemia, premature ventricular and atrial contractions; left ventricular (LV) or right ventricular (RV) hypertrophy; short PR interval; Wolff-Parkinson-White syndrome; prolonged QT interval; peaked T waves and small voltages indicative of cardiomyopathy.
Further cardiac studies to stratify risk may be beneficial for patients who by history or physical exam are considered to be at intermediate risk of cardiac complications. The ECG exercise treadmill test is useful in patients who can exercise but is rarely applicable to patients with ischemic lower extremities. A positive exercise test only slightly increases the likelihood of coronary artery disease, and a negative test correlates poorly with the absence of heart disease.9
Pharmacological stress testing should be considered for patients with an abnormal ECG (including left and possibly right bundle branch block) or a history of myocardial infarction. It should also be considered for those taking digoxin and in those who cannot exercise to acceptable levels. Studies of prospective cardiac risk using dipyridamole thallium scans (DTSs) suggest that patients with normal studies have a low risk for cardiac complications, but the prognostic implication of an abnormal scan is less well established. Studies have shown that reversible perfusion defects, which reflect jeopardized viable myocardium, carry the greatest risk of cardiac death or myocardial infarction (MI).10 In more recent publications, the positive predictive value of myocardial perfusion imaging has decreased significantly. This fact has reduced this procedure’s usefulness for evaluating assessment of surgical risk.
The predictive value of 24- or 48-hour ambulatory ECG monitoring for determining the perioperative risk of MI in patients undergoing high-risk noncardiac surgery is not widely used because differences in study protocols and ambulatory versus in-hospital monitoring may account for variability in the predictive value of the test.
Radionuclide ventriculography (RNVG) for assessment of LV function and ejection fraction (EF) can predict peri-operative cardiac morbidity in patients undergoing high-risk procedures. The scan shows ventricular wall motion abnormalities and systolic/diastolic dysfunction. Pasternak et al11 demonstrated that a calculated EF of < 35% was associated with a perioperative MI rate of 20%. The combined relative risk with the stipulated EF was 3.7, delineating a positive result. Measurement of EF using this technique is one of the strongest predictors of overall and late survival, especially after vascular surgery.
Dobutamine stress echo (DSE) was developed as a tool for assessing the presence of coronary artery disease. It has become the method of choice for pharmacological stress testing. The test assesses the effect of incremental infusions of supratherapeutic doses of dobutamine, which increases myocardial contractility and heart rate. Significant coronary disease can be identified by induction of LV ischemic regional wall motion abnormalities.12 DSE is recommended in patients with intermediate clinical predictors (prior MI, compensated CHF, diabetes, and mild angina). Integration of these clinical risk factors with ischemic wall motion abnormalities enhances the value of DSE in predicting perioperative nonfatal MI or death with high-risk procedures.
Coronary angiography is not recommended for risk assessment in patients having noncardiac surgery unless there is clinical evidence of coronary artery disease and the patient is undergoing a moderate- to high-risk procedure. If coronary artery disease or cardiac dysfunction is severe enough to indicate coronary angiography, the anesthesiologist presumes the risk to be so high that the patient will undergo a cardiac event. The anesthesiologist also presumes a corrective procedure will be performed if an appropriate lesion is found. The presence of significant coronary stenosis does not always indicate that an MI is unavoidable and that an invasive procedure is needed before surgery because the involved artery may supply scar tissue and not viable myocardium.
The potential complexity of perioperative cardiac risk evaluation makes the need for a simple algorithm apparent ( Table 12.4 ). Eagle et al used multivariate predictors of age > 70, angina, Q waves on ECG, ventricular ectopic activity, and diabetes to divide patients by these clinical variables.13 Patients with no risk factors had a 3.1% incidence of a myocardial event, whereas those that had three or more factors had a 50% incidence. Patients with one or two variables who were subjected to DTS with negative results had a 3.2% incidence of cardiac events, whereas those with a positive DTS test had a 29.2% incidence of cardiac complications. Thus the incorporation of invasive tests along with known cardiac risk factors improved the positive predictive value of the known risk factors.
The evaluation of pulmonary function is important in assessing patient risk for surgery. Pulmonary complications are common with abdominal and thoracic procedures, with less morbidity associated with intracranial procedures. In addition to pneumonia, postoperative pulmonary complications may include massive lobar collapse due to mucous plugging, pneumonitis, atelectasis, or a combination of one or more of these problems, which are exacerbated by prolonged intubation and intensive care unit (ICU) stays. The high incidence of these complications and the associated costs make it imperative that patients at risk be identified and pulmonary function optimized before the surgical procedure.
A variety of metabolic diseases could accompany the presentation of the meningioma. Diabetes predisposes the patient to specific risks, and its presence should heighten the suspicion of coronary artery disease because older patients with diabetes are more likely to develop heart failure after surgery than those without diabetes. Management of blood glucose levels may be difficult in the perioperative period, especially in patients receiving glucocorticoids to reduce cerebral edema and increased intracranial pressure associated with some large meningiomas. The risk assessment by the anesthesiologist must take into account the problems associated with both hyper- and hypoglycemia and the increased risk of postoperative infection and metabolic abnormalities. Azotemia is commonly associated with cardiac disease, and renal function should also be assessed in patients undergoing meningioma resection. Maintenance of adequate intravascular volume for renal perfusion during diuresis of a patient with an intracranial mass and associated renal insufficiency may be challenging.
Anemia in the surgical patient will produce additional stresses during and after surgery, which could place the patient at risk of perioperative morbidity.14 Cardiac output and tissue perfusion can increase fourfold with anemia in patients with normal heart function. If heart function is not compromised, normal blood flow can be maintained at hemoglobin levels as low as 5 gm/dL. In a patient with coronary artery disease, however, hemoglobin levels less than 10 gm/dL may be detrimental to ventricular function. Few studies have considered the effect of preoperative anemia on mortality. However, an inverse relationship exists between the perioperative hemoglobin level and the incidence of mortality in Jehovah’s Witness patients. The morbidity associated with transfusion, especially that of infection or human error associated with administration of the wrong blood, must be factored into the risk of undergoing surgery.
The numerous factors that are considered in the peri-operative assessment of risk are difficult to convey; thus a generalized scoring system that stratifies patients according to the severity of their illness is imperative to better estimate morbidity and mortality. The scoring system most often used by anesthesiologists to assess risk is the American Society of Anesthesiologists (ASA) Physical Status Classification.15 Patients are allocated to one of five categories (1 best, 5 worst) based on the medical history and physical exam ( Table 12.5 ). A variant of the ASA scoring system has been developed (by Klotz et al16) in which patients are assigned to one of three risk groups on the basis of a score obtained from a combination of four variables: (1) severity of the operation, (2) ASA grade, (3) presence of malignancy, (4) symptoms of respiratory disease. This scoring system provides a more accurate assessment of risk but is not widely used.
Once operative risk has been identified, the anesthesiologist should make specific recommendations for strategies to reduce risk in preparation for surgery. The preoperative “tune up” can be done on an outpatient basis if time and the nature of the surgery permit. Patients with pulmonary disease should have their bronchodilator therapy maximized and any evidence of infection cleared. Incentive spirometry done before surgery helps recruit lung units and reduces the incidence of perioperative hypoxia and atelectasis. Cardiac patients should be allowed to continue their medications in the preoperative period. Antiarrhythmics can be temporarily switched to intravenous forms, and sublingual medications like nifedipine can be switched to IV formulation for the surgery.
Airway concerns with spinal meningiomas are usually minimal unless the tumor is located in the cervical portion of the cord and could compress structures with neck movement, especially hyperextension, during laryngoscopy. An awake fiberoptic intubation may be the appropriate option to secure the airway with tumor involvement of the cervical cord. Intracranial and skull base meningiomas, depending on size and location, may have associated increased intracranial pressure (ICP). If the patient presents with evidence of increased ICP, the anesthetic induction must blunt any increase in blood pressure associated with laryngoscopy but not lower blood pressure, which could jeopardize cerebral perfusion. Selection of the anesthetic agent for induction is less important than careful titration of the drug to control hemodynamics. The ideal intravenous induction agent in patients with increased ICP should maintain cerebral perfusion pressure, prevent changes in mean arterial pressure, and preferably decrease ICP. Thiopental has a successful history of use during induction of anesthesia in these patients. ICP is lowered by cerebral vasoconstriction, but myocardial depression and peripheral vasodilatation may be profound. A reduced dose of this drug coupled with the administration of narcotics often reduces the myocardial depression observed. Etomidate tends to maintain cardiovascular stability on induction with a dose-dependent decrease in cerebral blood flow. Use of this drug is not widespread because associated myoclonus resembles seizure activity, and its administration is associated with adrenal suppression. Propofol can also be used to induce general anesthesia. Systemic vasodilation may reduce mean arterial pressure to such a degree that it could compromise cerebral perfusion. A common induction sequence for a patient with increased ICP would be 3 to 5 mg/kg of thiopental, 3 to 5 μg/kg fentanyl with 100 mg of lidocaine with small doses of esmolol, if needed, to blunt the cardiovascular response to laryngoscopy.
Use of muscle relaxants can be controversial. Pancuronium is a sympathomimetic and may increase mean arterial pressure. Vercuronium and rocuronium produce stable hemodynamics, with rocuronium used to replace succinylcholine if a rapid sequence induction is needed. Succinylcholine use in patients with high ICP is controversial because fasciculations and an increase in muscle spindle activity have been shown to increase ICP. Volatile anesthetics have been used for many years in patients undergoing both spinal and intracranial meningioma resection. Isoflurane produces moderate vasodilatation, which can be attenuated by hyperventilation. Newer volatile agents, such as desflurane and sevoflurane, behave in a similar fashion, producing mild cerebral vasodilatation coupled with a decrease in cerebral metabolic rate. Responsiveness to CO2 is maintained so the effect of increased cerebral blood flow is attenuated with hyperventilation. Narcotic use in these long and sometimes stimulating cases is the foundation for maintenance anesthesia. These drugs provide hemodynamic stability without deleterious effects on ICP and allow for rapid emergence at the conclusion of the surgery. Remifentanil, with its rapid clearance by esterase metabolism, may provide rapid emergence from general anesthesia, enabling a quick neurological assessment. Meningiomas have large vascular footprints, and care must be taken to control blood pressure with emergence from remifentanil anesthesia. Hypertension after meningioma resection could produce bleeding at the site of the tumor bed, which could increase the incidence of a subdural hematoma after surgery.
The anatomical position of the tumor will also dictate the operative positioning of the patient. Certain skull base meningiomas dictate that the patient be placed in a semisitting, lateral decubitus, or park bench orientation. Abnormal flexion or extension of the extremities is avoided, and care is taken to avoid abnormal neck and arm position, which could produce brachial plexus injury.
Positioning can also dictate the type of invasive lines that should be placed for the surgical procedure. The surgical field above the heart (sitting or park bench) could offer better anatomical exposure with less blood loss. However, these positions produce a high prevalence of venous air entrainment. Convexity meningiomas, especially if there is bony involvement, should also be considered for venous air embolus (VAE) monitoring.17 The bone has been noted to be a source of VAE in 43% of all sitting craniotomies. If the procedure warrants VAE monitoring, a central multiorifice catheter should be placed and positioned in the right atrium 1 to 2 cm above the tricuspid orifice. Monitoring for VAE has been extensively described. We prefer the use of the precordial Doppler (Versitone Model D8, Medasonics, Inc., Fremont, CA). Although many feel that the transesophageal echo is more sensitive in detecting venous air entrainment, the increased sensitivity produces a high false-positive rate, which reduces its specificity as a monitor to detect air emboli.
Blood loss during meningioma resection can be quite large. The position of the tumor, close to vascular structures, could produce a great deal of blood loss during resection. Many of these tumors are highly vascular and have been shown to produce a tissue type plasminogen activator that leads to significant fibrinolysis, producing increased blood loss during surgery.18 Some investigators have noted a disseminated intravascular coagulopathy during primary brain tumor resections. This could develop from tumor-specific antigens or destruction of the blood–brain barrier leading to liberation of factors activating hemolysis. Large-bore IV access should be initiated in these patients before surgery. If anticipated blood loss is large, central venous access should be obtained, with fluid and blood administration managed with the use of a Swan-Ganz catheter.
Intraoperative neurophysiological monitoring for re-section of spinal or cranial meningiomas is an important consideration when developing the anesthetic plan for the procedure. Cranial nerve monitoring may be employed if the tumor is located in the skull base and surrounds nerve structures. Electromyography of the facial, vagus, or trigeminal nerve may be used during surgical resection to identify the nerve and preserve its integrity. The use of muscle relaxants, in conjunction with electro-myographic (EMG) monitoring, is problematic, with the best conditions realized by complete avoidance of muscle relaxants.19 Maintenance of anesthesia with low-dose desflurane administration in a 50:50 air/O2 mixture with a baseline infusion of fentanyl 2 to 3 μg/kg/hr or remifentanil 0.25 to 0.35 μg/kg/min will produce the intraoperative conditions necessary to monitor cranial nerves and provide a stable surgical field without the use of muscle relaxants.
Other neurophysiological monitors may also be utilized for skull base or spinal cord meningioma resection. Brain stem auditory evoked responses or electrocochleography can be used for skull base meningiomas, especially if there is a possibility of vascular compromise or brain stem manipulation. Both techniques are not markedly affected by inhalational anesthesia, with concentrations as high as 1 minimum alveolar concentration (MAC) used with minimal effects on the response.20 Somatosensory evoked potentials (SSEPs) and motor evoked potentials (MEPs) could be used to assess the integrity of the brain stem and spinal cord during meningioma resection. Because SSEPs and MEPs have cortical components to their measurement, volatile anesthetics produce a dose-dependent increase in latency due to an increase in central conduction time and a decrease in amplitude.21 Satisfactory monitoring of both SSEPs and MEPs is possible with 0.5 to 0.75 MAC of isoflurane, desflurane, or sevoflurane. The effect of volatile agents on SSEPs and MEPs is compounded by N2O, which is usually not used as an anesthetic for these procedures. The newer volatile agents, desflurane and sevoflurane, affect these monitoring techniques in a fashion similar to isoflurane but can be used at slightly higher concentrations. A continuous infusion of low-dose propofol with opioids coupled with low concentrations of background inhalation anesthetics is ideal and recommended for SSEP and MEP monitoring during resection of spinal cord and infratentorial meningiomas.
After completion of the surgical procedure, the decision to extubate is predicated on the patient’s ability to follow commands, ventilate, and maintain oxygenation. Hemodynamic stability, blood loss, and fluid replacement also play a role in the decision to extubate. Recovery from anesthesia is expedited by the use of short-acting, low-solubility inhalational agents and infusions of short- or intermediate-acting opioids. Nausea, vomiting, and pain are important problems that must be treated in the immediate postoperative period. The incidence and magnitude of pain are not well characterized after craniotomy but are different depending on approach to the tumor and the anatomical structures disrupted. Pain after spinal procedures may be severe, depending on trauma to muscle and the number of laminectomies performed. There is evidence that neurosurgical patients receive inadequate analgesia from currently practiced regimens.22 Although no ideal analgesic exists, many practitioners have begun the use of patient-controlled analgesia with potent opioids. This produces a method to titrate analgesia, allows patients to control their pain, and may alleviate some of the physiological stress associated with pain.
Nausea and vomiting could be severe, depending on opioid administration or the area of surgery. Evidence suggests that a higher incidence of vomiting may be associated with infratentorial and skull base approaches rather than supratentorial or spinal cord resection of meningiomas.23 Different antiemetic strategies may be employed to provide relief from emesis after these procedures. Droperidol, a dopaminergic antagonist, has been used with success, but the possibility of extrapyramidal side effects and its synergism with opioids to increase sedation may limit its use. Ondansetron and other serotonin receptor antagonists, along with dexamethasone, have been used with some success to reduce nausea and vomiting.24 A multimodal approach to the treatment of nausea and vomiting after these surgeries may be more effective than single-therapy regimens.
In conclusion, the perioperative management of patients undergoing intracranial or spinal cord meningioma resection must account for the physical status of the patient and their ability to undergo the surgical procedure. The risks associated with anesthesia, blood loss, and surgical trauma are weighed against the patient’s estimated physical ability to successfully survive the surgery with minimal associated morbidity. Once the estimate of surgical risk is determined, the anesthesiologist must develop the anesthetic technique that will provide optimum conditions for intraoperative monitoring, a stable surgical field, and a hemodynamically stable patient. The anesthesiologist can improve postoperative outcomes and reduce morbidity by knowing information concerning tumor size, characteristics, surgical approach, and involved neurological structures. Successful perioperative evaluation and management of surgical patients undergoing meningioma resection require teamwork and communication between the surgeon, anesthesiologist, primary caregiver, and consultants to produce an optimal outcome with low morbidity. The preoperative assessment and estimate of operative risk and the patient’s acceptance of that risk play a vital role in the informed consent process and are key to a successful outcome.
REFERENCES
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