Functional Neurosurgery: Anesthetic Considerations

Irene P. Osborn, MD Department of Anesthesiology and Neurosurgery, Division of Neuroanesthesia, Icahn School of Medicine at Mount Sinai, New York, New York

Samuel D. Kurtis, BA Icahn School of Medicine at Mount Sinai, New York, New York

Ron L. Alterman, MD Division of Neurosurgery, Beth Israel Deaconess Medical Center, Boston, Massachusetts

Functional neurosurgical procedures are one of the fastest-growing areas of neurosurgery treatment.1 Most commonly performed for treatment of Parkinson disease (PD), these procedures are now utilized for treatment of essential tremor, dystonia, and some neuropsychiatric disorders.2 Procedures such as pallidotomy and thalamotomy were initially performed but are now less frequently utilized. These lesions are permanent, and there is risk for side effects such as stroke or neurological injury. Currently, most patients undergo deep brain stimulation (DBS), which involves the implantation of stimulatory electrodes and a generator that allows control of the electrodes (Fig. 1). DBS devices are implanted in 2 stages: the first stage, lead placement, is usually performed with the patient awake or with minimal sedation. The second stage involves generator placement and is performed under general anesthesia (GA). The anesthetic management of surgical PD patients is based on an understanding of the numerous risk factors, drug therapies, and potential perioperative complications. DBS has emerged as the treatment of choice for movement disorders that are refractory to medical therapy.3 The anesthetic challenges during DBS include providing sedation and analgesia with minimal respiratory depression or interference with intraoperative electrophysiological monitoring or clinical testing. The procedure involves multiple anesthetizing locations, careful titration of sedative agents, and control of blood pressure. DBS surgery is best performed by REPRINTS: IRENE P. OSBORN, MD, DEPARTMENT OF ANESTHESIOLOGY AND NEUROSURGERY, DIVISION OF NEUROANESTHESIA, ICAHN SCHOOL OF MEDICINE AT MOUNT SINAI, ONE GUSTAVE L. LEVY PLACE, BOX 1010, NEW YORK, NY 10029. E-MAIL: [email protected] INTERNATIONAL ANESTHESIOLOGY CLINICS Volume 53, Number 1, 39–52 r 2015, Lippincott Williams & Wilkins

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Figure 1. Implantable pulse generator for deep brain stimulation.

an experienced surgeon with specific expertise in stereotactic and functional neurosurgery with the help of a professional team consisting of a neurologist, a neurophysiologist, and an anesthesiologist. This chapter will discuss techniques for providing safe anesthesia for functional neurosurgery.



Preoperative Considerations

Patients presenting for functional neurosurgery are likely to have PD, dystonia, or other types of movement disorder. More recent indications have included obsessive-compulsive disorder and severe depression.4 Because patients are awake during a significant part of the procedure, extensive communication in the preoperative period is necessary. Preoperative assessment should provide information on disease severity, other medical problems, drug therapy, and complications of drug therapy. Major concerns for the anesthesiologist are the respiratory and cardiovascular system. Patients with longstanding PD may have autonomic dysfunction, impaired respiratory reserve, sleep www.anesthesiaclinics.com

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apnea, and increased risk for aspiration.5,6 These patients are often taking multiple prescription medications for disease treatment, creating a risk for perioperative drug interactions. Autonomic dysfunction and levodopa therapy may lead to orthostatic hypotension. If vasopressors are needed, direct acting agents, such as phenylephrine, should be considered. Obstructive sleep apnea, severe obesity, and uncontrolled hypertension all pose potential problems and increase the risk of surgery.4 The patient who will undergo a DBS insertion must understand what is likely to happen throughout this prolonged procedure (possibly 6 to 7 h). Preoperative cessation of anti-Parkinson therapy begins the evening before surgery to allow the symptoms to be observed during the procedure and demonstrate accurate placement of stimulating electrodes. Unfortunately, this eventually results in skeletal muscle rigidity, cramping, and other side effects.7 Another group of patients who undergo DBS includes younger individuals with dystonia who may be physically healthy but debilitated by spasticity or tremor.8 These patients often require additional sedation and for greater periods of time. The youngest patients may require a light general anesthetic for head frame placement with transition to monitored sedation and maintenance of the lowest anesthetic infusion that prevents movement.9 Anesthetic goals are the same, to have a responsive, cooperative patient during intracranial electrode placement and microelectrode recording (MER) (Table 1).10 ’

Procedural Sequence

The procedure of DBS involves several locales, as the patient is transported to different areas, which include the radiology suite and the

Table 1.

Characteristics of Common Diseases Requiring DBS Parkinson Disease

Age Prevalence

Most commonly 60+ 1%-2% over age 65

Medications

Dopamine precursors, dopamine agonists, anticholinergics, COMT inhibitors, MAO-B inhibitor, antiviral —

Pain sensitivity DBS targets

STN, GPi

Essential Tremor

Dystonia

Any age 4%-5.6% over age 40, 9% over age 60 b-blockers, anticonvulsants

Any age, often pediatric

— VIM

Dopamine precursors, anticholinergics, VMAT inhibitor, botulinum toxin Higher GPi

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operating room. The procedure may begin in the patient’s room or in the holding area where the patient is assessed, the intravenous line placed, and the hair may be shaved. At this time the patient is further assessed to determine the degree of movement (tremor) or anxiety, as this will govern the need for sedation during the first stage of the procedure—placement of the stereotactic head frame. Patients who are generally cooperative and able to control their movements can undergo this portion with little or no sedation. The frame is attached to the patient’s head after local anesthetic infiltration or “scalp block” for the pin sites.11 This portion of the procedure is extremely important, as the frame must be placed correctly to allow for the anatomic targeting required. In addition, the anesthesiologist should observe the features of the patient’s airway and the risk of airway compromise during positioning, as the head frame could interfere with airway access in an emergent scenario. After head frame placement, the patient is taken to computed tomography or magnetic resonance imaging (MRI) for imaging, which is used to determine the target and trajectory for the placement of the DBS leads.1 The patient might require sedation during this time for cooperation during the diagnostic scan. Patients who undergo MRI may be claustrophobic and request sedation, whereas others may have a severe tremor and movement, which produces artifacts. The optimal technique can be a low-dose infusion of propofol, which will diminish the tremor and provide a degree of sedation and anxiolysis.4,12 Supplemental oxygen with end-tidal CO2 monitoring and pulse oximetry is essential to perform this procedure safely, as the study may take 20 to 30 minutes. Although midazolam is also an effective agent for sedation, many neurosurgeons prefer to avoid its use as it may later affect neurophysiological recording.13,14 A computed tomography scan is shorter than MRI; however, the patient must be able to lie flat on the table, and, occasionally, sedation or analgesia is required for this purpose. Patient observation during this time is important to determine response to sedatives and to anticipate any problems regarding cooperation during the procedure. After imaging, the patient is transported to the operating room for lead placement. Positioning of the patient is critical. Most neurosurgeons prefer the patient in a sitting or recumbent position.15 Once positioned, the airway must be evaluated and a plan formed for securing the airway in case of emergency. Spontaneous respiration with oxygen supplementation is the normal plan for DBS lead placement. However, in the event of an emergency, the head frame, patient positioning, and surgical drapes combine to make endotracheal intubation very difficult. A laryngeal mask airway is relatively easy to insert and is well tolerated under light anesthesia, and is therefore a popular choice in such a scenario.4 www.anesthesiaclinics.com

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Intraoperative Monitoring and Management

Standard anesthesia monitors are applied, including electrocardiogram, noninvasive arterial blood pressure, oxygen saturation, and capnography. Technical difficulties with some monitors may be encountered in patients with severe tremor. Invasive arterial blood pressure monitoring may be considered for patients with severe cardiovascular or pulmonary disease.4 It is essential to provide proper patient positioning to ensure comfort while allowing access to the hands and lower extremities for testing of movement. Insertion of a urinary catheter is often necessary and may be performed while the patient is in the sedated or semiconscious state. The patient may be allowed to have considerable sedation during the preparation stages of the procedure, yet it is important to ensure proper head and neck positioning to avoid airway obstruction. Supplemental oxygen should be delivered by nasal cannula or mask with a sampling line for capnography. Capnography confirmation and an optimal pattern of respiration must be established before the head is fixed into place. ’

Scalp Block

The technique of blocking the nerves of the scalp has been used for decades to provide analgesia for awake craniotomy or as a means of supplementing GA.16,17 Application of bupivacaine 0.25% to 0.5% to the supraorbital and greater occipital nerves will provide for prolonged analgesia (6 to 8 h) and is useful for maintaining patient comfort. This modified block of only 2 nerves is easy to perform and provides analgesia for the surgical incision and burr holes as well as improved tolerance of the posterior pin sites11 (Fig. 2). A small amount of sedation is often required for easy acceptance and for younger patients. Some surgeons prefer to perform the block initially for head frame placement, whereas others use local infiltration for the pin sites. Analgesia is supplemented later before the procedure begins, often while the patient is comfortably sedated. Table 2 describes the scalp block procedure in detail. ’

Anesthetic Agents for Sedation

Monitored anesthesia care (MAC) with local anesthesia and sedation is our standard anesthetic modality for DBS lead placement. Most neurosurgeons desire a patient who is awake and cooperative throughout the testing portion of the surgery, to allow for macrostimulation: direct feedback on symptom reduction and adverse effects when placing the DBS leads. Therefore, GA and longer-acting sedatives should be avoided.18 www.anesthesiaclinics.com

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Figure 2. Placement of supraorbital nerve block in a patient wearing stereotactic frame.

Table 2. Scalp Blocks: Surgical Landmarks and Method Preferred local anesthetic: bupivacaine 0.25%-0.5% 1. Supraorbital—Supraorbital (S/O) notch is palpated by finger along the superior orbital rim, and needle is inserted approx. 1 cm medial to the foramen, with a medial direction to avoid the penetration of the foramen at all costs (causes nervous trauma). Classically, the puncture point is in line with the centered pupil, just above the orbital ledge. Adult 2 mL ( 2); children 1-1.5 mL (2) 2. Supratrochlear—Medially extending the supraorbital block, approx. 1 finger-breath medial to the S/O block, above the orbital ledge. Adults 1 mL (x2); children 0.5 mL ( 2) Alternatively, around the glabellar region, in the midline. Caution: intravascular injection of the angular vein. Adults 2 mL ( 1); children 1 mL ( 1) 3. Zygomaticotemporal—1 cm lateral and superior to lateral canthus of eye. Approx. lateral to the orbital margin, where the eyebrow ends. Adults 2 mL ( 2); children 1 mL ( 2) 4. Auriculotemporal—Innervating the front of the ear and temples. The puncture pt. is in front of the tragus, directed laterally. When needle hits bone (deep) 2 mL, withdraw needle superficially and inject 2 mL again. Total 4 mL ( 2) in adults 5. Posterior temporal (P/T)+great auricular—Both these nerves can be blocked together by injecting along the posterior border of upper 1/3rd of sternocleidomastoid muscle The specific posterior auricular block is along the line of tragus, posterior to the ear. It blocks the region mainly behind the ear, as well as below it. Adults 2 mL ( 2). The great auricular nerve supplies below and behind the ear as well, but the region is more inferior relative to the P/T nerve These 2 nerve blocks are especially important during placement of the DBS battery and extension wires 6. Greater occipital nerve (GON) and lesser occipital nerves (LON)—During head pinning before craniotomy, position the needle as close to the pins as possible, with care not to compromise the occipital artery. Can be injected as a fan block along occipital ridge to cover both GON and LON areas. Largest vol. of anesthetics is used—5-6 mL in adults ( 2), 2-4 mL in children ( 2). The best landmark is to palpate the occipital artery (if possible) and inject medially after careful aspiration

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Nonopioid, nonbarbiturate sedative hypnotic

Opioids

Benzodiazepine

Propofol

Remifentanil, fentanyl

Midazolam

Minimal effect on MER

Minimal effect on MER, provides sedation while retaining patient cooperation, minimal respiratory depression, attenuates physiological response to surgical incision, does not suppress PD tremor Rapid onset, short duration of action, easy titration allows for asleep-awake-asleep technique

Advantages

DBS indicates deep brain stimulation; MER, microelectrode recording; PD, Parkinson disease.

a2 agonist

Drug Class

Dexmedetomidine

Drug

Table 3. Common Anesthetic Drugs Used in DBS Surgery

Respiratory depression, may worsen rigidity in PD, suppresses PD tremor Reduces quality of MER, respiratory depression, suppresses PD tremor, impairs consciousness, long duration of action

Unclear effect on MER, unclear effect on PD tremor

No antiepileptic activity, bradycardia at high doses, relatively expensive

Disadvantages

Contraindicated

MER returns to baseline with discontinuation of infusion

Comment

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In functional neurosurgery, a major concern with anesthetic agents is potential interference with MER, an electrophysiological technique used by the surgeon to ensure proper lead placement. Common drugs for DBS sedation, along with their advantages and disadvantages, are listed in Table 3. At our institution, dexmedetomidine (Dex) is the preferred agent for sedation for DBS lead placement. As an a-2 agonist, Dex has minimal effect on MER and causes little respiratory depression.19 Dex provides anxiolysis and sedation but patients can be awakened easily with verbal stimulation. In addition, Dex in sedative doses does not suppress PD tremor, so it is ideal for macrostimulation testing in patients who are extremely anxious.20 Dex can be administered by bolus infusion of 1 mg/kg for 10 minutes, followed by a continuous infusion of 0.2 to 0.9 mg/kg/h. Many clinicians simply begin with an infusion of 0.7 mg/kg/h for an onset of effect in about 9 to 10 minutes.21 Propofol is frequently used for sedation in DBS procedures. Propofol has a rapid onset, short duration of action, and is easily titratable, making it ideal for surgeons who prefer an asleep-awakeasleep technique. Patients appreciate the degree of unconsciousness and, thus, amnesia, for parts of the procedure that are distressing: the burr hole, placement of urinary catheter, and wound closure. There is certainly potential for airway obstruction, apnea, and hypotension if propofol is given rapidly in patients with PD.22 A continuous infusion of 25 to 75 mg/kg/min provides a titratable degree of unconsciousness and may be administered alone or in combination with Dex or small doses of opioid. It is important to discontinue the agent at least 15 minutes before expected patient response, bearing in mind that prolonged infusions will require a longer time for elimination.23 There are concerns, however, that propofol may diminish the quality of MER, and this is likely to occur when propofol drug concentrations are relatively high.12 Benzodiazepines such as midazolam are generally avoided during DBS lead placement. As a GABA potentiator, midazolam can profoundly reduce the quality of MER, affecting lead placement. In addition, benzodiazepines suppress PD tremor and impair consciousness, which both adversely affect macrostimulation testing.14 Midazolam can be administered as soon as all testing is completed and provides anxiolysis and amnesia for the patient during wound closure. It is important to ascertain whether the patient is experiencing pain at this point and address this with a small dose of opioid, more scalp infiltration, or intravenous acetaminophen. Recent reports have suggested that remifentanil may be advantageous in certain patients undergoing DBS.5 When administered at very low infusion rates (0.03 to 0.05 mg/kg/min) it provides analgesia and moderate sedation with little effect on MERs.24 www.anesthesiaclinics.com

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Intraoperative Complications

The rate of intraoperative complications during DBS surgery varies widely with different studies. A summary of complication rates can be found in Table 4.25–29 Most common problems are related to sedation causing transient or disruptive hypoxemia, hypercarbia, or apnea. These are hopefully alleviated by stopping or decreasing the sedation, repositioning the head, or providing needed airway control. It is important to have a supraglottic airway readily available that can be used for airway access and ventilation in this emergency setting.4 Hypotension is also a potential problem, which may cause decreased cerebral perfusion and exacerbate the sedated state. Blood pressure control is one of the important goals of the anesthesiologist, and neurosurgeons require strict adherence to keeping the blood pressure no greater than systolic of 130 to 140 mm Hg. Hypertension has been associated with intracranial bleeding and is to be treated aggressively with b-blockers, vasodilators, or newer calcium-channel agents.28,30 Venous air embolism (VAE) is a dangerous complication that is relatively frequent in DBS surgery, with a reported incidence of 1.3% to 4.5%.31 It is postulated that the increased frequency of VAE is a consequence of the sitting or recumbent position of the patient, which leads to an increased negative pressure gradient between the venous system of the head and the right atrium.32 The classic sign of VAE is coughing during high-risk portions of the procedure, often accompanied by nonspecific changes in vital signs and end-tidal CO2. The mechanism of cough in VAE is unknown. Rarely, the presence of a patent foramen ovale can lead to a paradoxical air embolus and subsequent transient ischemic attack.33 The gold standard for diagnosing VAE is the precordial Doppler, which is rather impractical due to the noise created and difficult placement on an awake patient. Transesophageal echocardiography is the most sensitive diagnostic tool but is not applicable in this setting.34

Table 4.

Potential Complications of DBS

Complication Venous air embolism Intracerebral hemorrhage Seizures Respiratory complications Cardiac Coughing/sneezing Pulmonary embolism

Incidence (%) 1.3-4.5 in the recumbent position 0.9-5 0.8-4.5 1.6 0.4 1.2 0.3

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Treatment of VAE is supportive. There must be a focus on prevention of further introduction of air into the venous system, with copious irrigation of the surgical field with saline, lowering of the head of the bed below the level of the heart, and waxing of any exposed bone. The patient should be reassured and blood pressure supported with pressor agents. The symptoms usually subside quickly once air is no longer being entrained. The decision is made then to continue or abandon the case for another time. Intracerebral hemorrhage (ICH) is a potentially catastrophic complication in DBS lead placement, with a reported incidence of 0.9% to 5.0%.35,36 ICH severity can range from asymptomatic bleeds that are discovered on postoperative imaging, to large hemorrhages requiring premature termination of surgery, sometimes leading to postoperative hemiparesis and altered mental status. If not obvious from the surgeon’s perspective, ICH can be suspected if there is a change in mental status of an awake, sedated patient. Hypertension and advanced age are known patient risk factors for ICH in functional neurosurgery.14 Surgically, risk factors for ICH include increased number of MER passes and trajectories that involve a sulcus or ventricle.15



GA for DBS: What is the Future?

MAC with sedation has long been the preferred anesthesia modality for DBS lead placement. The major concern among surgeons is that drugs used in GA can diminish intraoperative MER used to ensure that the leads are in the proper location. GA also inhibits macrostimulation testing; by eliminating tremor and preventing patient cooperation and feedback, the surgeon loses another tool to ensure proper lead placement and lack of adverse effects. GA has normally been reserved for patients who cannot tolerate MAC with sedation, including those with a strong fear of awake surgery, pediatric patients, and dystonia patients with extreme movements. Recent studies suggest that the concerns surrounding GA for lead placement are overstated. There are sizeable studies demonstrating successful lead placement under GA.14,18,37–39 These studies show that, contrary to popular belief, MER can be successfully used under GA to identify the proper DBS targets, as long as the anesthetic agents are titrated to lower levels during MER. Additional studies compare GA with MAC and show similar outcomes in terms of success of the DBS procedures, measured by reduction in parkinsonian symptoms.40–42 It should be noted that, although the evidence supporting GA for DBS lead placement is growing, most studies have been retrospective, uncontrolled studies. Only 1 randomized, controlled, prospective study has yet to be performed.41 www.anesthesiaclinics.com

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Postoperative Recommendations

After the DBS procedure patients are likely to be fatigued and will often report exhaustion. Pneumocephalus is frequently seen on postoperative studies and is also a cause for postoperative discomfort or somnolence. Headache is a frequent complaint, and this is often relieved with acetaminophen and other analgesics. Postoperative pain is generally not severe, as the brain parenchyma does not have sensory nerve endings. Normally, the patient is admitted overnight for observation after DBS lead placement, although same-day discharge is conceivable in young, healthy patients who had short procedures with no complications and minimal sedation.43 It is important to administer anti-Parkinson medication as soon as allowable, frequently on admission to the PACU. This will allow the patient to recover more quickly and relieve the distress caused by withholding their meds.



Anesthesia for DBS Stage 2: Stimulator Placement

The second stage of DBS placement typically takes place a few weeks after lead placement. In this stage, a pacemaker-like stimulator is placed in the upper torso, and connected to the leads inserted in the first surgery. As the surgery does not involve craniotomy, neuromonitoring is not required. Therefore, this procedure can be performed effectively under standard GA, with no limitations as to the drugs that are used. After recovering from this procedure, the patient will typically return to the neurology clinic several weeks later, wherein the DBS system is turned on and its settings adjusted to meet the patient’s needs.



Conclusions

Since it was first described in 1987, DBS has revolutionized the treatment of advanced PD and other movement disorders, and it is showing great promise in the treatment of some refractory psychiatric conditions.2,44 With expanding indications and an aging population, the number of DBS procedures performed annually will continue to increase, as will the demand for anesthesiologists with expertise in functional neurosurgery. There is no single best anesthetic regimen for DBS lead placement, and research continues to emerge on the effects of anesthetic drugs on intraoperative MER. For DBS lead placement, we have seen excellent results using MAC with a bupivacaine scalp block, combined with Dex for cooperative sedation. There is promising research on the use of GA for lead placement, although for now, GA remains a second-line choice. www.anesthesiaclinics.com

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The authors have no conflicts of interest to disclose.



References

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41. Lefaucheur JP, Gurruchaga JM, Pollin B, et al. Outcome of bilateral subthalamic nucleus stimulation in the treatment of Parkinson’s disease: correlation with intraoperative multi-unit recordings but not with the type of anaesthesia. Eur Neurol. 2008;60:186–199. 42. Harries AM, Kausar J, Roberts SA, et al. Deep brain stimulation of the subthalamic nucleus for advanced Parkinson disease using general anesthesia: long-term results. J Neurosurg. 2012;116:107–113. 43. Blanshard HJ, Chung F, Manninen PH, et al. Awake craniotomy for removal of intracranial tumor: considerations for early discharge. Anesth Analg. 2001;92:89–94. 44. Benabid AL, Pollak P, Louveau A, et al. Combined (thalamotomy and stimulation) stereotactic surgery of the VIM thalamic nucleus for bilateral Parkinson disease. Appl Neurophysiol. 1987;50:344–346.

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