Anesthesia for Functional Neurosurgery

Research Article

Austin J Anesthesia and Analgesia.2014;2(3): 1016.

Anesthesia for Functional Neurosurgery

Alex Bekker1,*

Jean Eloy2

Rutgers New Jersey Medical School, USA

*Corresponding author: Alex Bekker, Rutgers New Jersey Medical School, 185 South Orange Ave, MSB E538 Newark, NJ, 0710, USA.

Received: January 30, 2014; Accepted: March 18, 2014; Published: March 20, 2014

Introduction

Functional neurosurgery refers to the surgical management of neurological disorders that lack a gross structural or anatomical marker [1-2]. It produces therapeutic results by modifying or interrupting malfunctioning pathways and altering their underlying physiology. This is in contrast to non-functional neurosurgery also called anatomic or lesional neurosurgery, which refers to the dissection and excision of distinct lesions with the goals of removing these entities and preserving functionality in the remaining tissue[3]. Whereas non-functional neurosurgery is typically used to treat pathologic parenchymal or vascular lesions, the goal of functional neurosurgery is to improve quality of life by providing symptomatic relief to patients with movement disorders, psychiatric disorders, and chronic pain [4].

Awake neurosurgery is performed when patient participation is required to guide the surgical intervention, and may be used for either functional or non-functional procedures [5]. In the case of functional procedures, interaction with the patient allows the surgeon to maximize clinical improvement and minimize untoward side effects. Non-functional procedures are performed awake when the lesion to be excised is close to a vital area of cortex, like the speech or motor cortex commonly referred to as "eloquent" areas. In this case,communication with the patient allows the surgeon to remove the pathological lesion while minimizing damage to the adjacent areas.

This review will focus on the anesthetic considerations for the most commonly performed awake functional procedures; [6-7] specifically,Deep Brain Stimulator (DBS) implantation for the management of Parkinson's Disease (PD) and Spinal Cord Stimulator (SCS) insertion for the management of chronic pain syndromes will be discussed. It should be noted that DBS implantation is also performed for the management of other movement disorders (e.g. dystonia, familial tremor), certain psychiatric conditions, and chronic pain syndromes [8-10].

Deep Brain Stimulation

Background

The most common and well recognized of the movement disorders is Parkinson's disease (PD), but other entities include chorea, dystonia, and tremor [4,7,8]. The common feature of these conditions is a disruption in the patient's ability to control body movements. PD is due to the progressive degeneration of dopaminergic neurons in the basal ganglia and is characterized by tremor, rigidity, bradykinesia, and postural instability. Historically, the surgical management of PD involved the production of lesions in deep brain structures like the thalamus, globus pallidus, or cingular cortex. Although effective, the ablative procedures were associated with high complication rates and were irreversible [9,10]. These drawbacks, combined with the introduction of medical therapy for PD, led to the abandonment of ablative surgical therapy in the late 1960s.

The goal of medical therapy is to restore balance between dopaminergic (inhibitory), and cholinergic (excitatory) tone, and may be achieved by dopaminergic augmentation, cholinergic inhibition, or both.

As the adverse effects of long-term medical management of PD became more apparent, the advent of computer-guided stereotactic surgery and deep-brain stimulator (DBS) technology fostered a renewed interest in the surgical management of PD [11,12]. Stereotactic surgery refers to the use of a computer that correlates external reference points also defined by a headframe applied to the skull with internal structures seen on CT or MRI to guide the optimal approach and trajectory needed to reach a given target. Deep-brain stimulators are implantable devices that consist of electrodes precisely placed at target brain structures connected by wires to a remotely located module [13-15]. A conceptually analogous and more familiar device is the cardiac pacemaker. While the older ablative procedures could only coagulate and destroy targeted regions of the brain, the newer DBS can modulate the function of the same areas without destroying them. In addition, the DBS can be adjusted to deliver different types of stimulation to maximize benefit, or be turned off completely if it produced untoward effects. The ability of the surgeon to employ technological advances that allow precise localization and reversible modulation of deep brain structures represents an enormous advancement in the management of PD [16,-18].

Anesthetic Goals

The anesthesiologist faces many challenges in caring for patients who are undergoing placement of a DBS device. The anesthesiologist's role includes keeping the patient responsive and cooperative, often for a very long period of time. Many of these patients are elderly, presenting with complex medical problems. Moreover, often these patients are on multiple medications, raising the specter of possible anesthesia-drug interactions. All of these issues require constant vigilance to maintain a stable hemodynamic and minimize respiratory depression. In addition, the anesthesiologist should familiarize him/ herself with the potential effects of sedatives on micro recording (see below), because the altered signal may lead to the imprecise placement of the electrode.

When medical therapy fails or produces unacceptable side effects, surgical intervention may be considered [19-21]. The selection of a highly motivated patient is crucial, and the patient must be psychologically prepared for the experience of undergoing an invasive procedure while fully awake [22-24]. During the preoperativeinterview, the anesthesiologist must explain clearly that adequate sedation and analgesia will be provided for the painful portions of the procedure, and that there will be a period during the operation in the patient will be awake, alert, and required to follow instructions. It should be emphasized that the awake portion of the procedure is not painful and that the anesthesiologist will be at the bedside throughout the procedure to administer sedation and analgesia as necessary [25] Premedication, particularly benzodiazepines, is generally avoided as it may precipitate unpredictable responses (e.g. disinhibition) in the elderly.

All patients must follow the standard "nothing by mouth" orders.

Preoperative Considerations

The Preoperative Interview

When medical therapy fails or produces unacceptable side effects, surgical intervention may be considered [19-21]. The selection of a highly motivated patient is crucial, and the patient must be psychologically prepared for the experience of undergoing an invasive procedure while fully awake [22-24]. During the preoperative interview, the anesthesiologist must explain clearly that adequate sedation and analgesia will be provided for the painful portions of the procedure, and that there will be a period during the operation in the patient will be awake, alert, and required to follow instructions. It should be emphasized that the awake portion of the procedure is not painful and that the anesthesiologist will be at the bedside throughout the procedure to administer sedation and analgesia as necessary [25] Premedication, particularly benzodiazepines, is generally avoided as it may precipitate unpredictable responses (e.g. disinhibition) in the elderly.

All patients must follow the standard "nothing by mouth" orders.

Disease-Specific Considerations

In addition to the effects adverse effects caused by the medications used to treat it, PD itself produces systemic manifestations that may interfere with anesthetic management. Generally, these are the result of either autonomic dysfunction or muscle rigidity. Autonomic derangements result in orthostatic hypotension, delayed gastric emptying, excessive salivation, excessive sweating, and bladder disturbances. Muscle rigidity of the upper airway can result in pharyngeal and laryngeal dysfunction, increasing the risk of pulmonary aspiration and laryngospasm, and predisposing for anemia, hypovolemia, and malnutrition due to chronic dysphagia. Respiratory muscle rigidity can decrease inspiratory and/or expiratory flows, producing restrictive, obstructive, or mixed patterns of pulmonary dysfunction. Also seen is an increased perception of dyspnea, which leads to tachypnea and abnormal respiratory patterns [25-27].

Airway Considerations

Because of the inability to use advanced airway management devices during the awake portion of the procedure and the relative inaccessibility of the airway due to the presence for the head frame, a detailed airway evaluation must be performed with specific effort devoted to eliciting any history of obstructive sleep apnea. Options for securing the airway emergently at any point of the procedure must be considered and planned for preoperatively.

The Off-Drug State

To facilitate physiologic mapping and clinical testing, some patients are told to stop taking their usual anti-Parkinson's medication prior to surgery. This may significantly worsen the parkinsonian signs and symptoms described above. In severe cases a compromise may be necessary, with the administration of reduced doses of the usual regimen to partially control the symptoms while still allowing for adequate testing. This must be discussed with the neurosurgical teamand decided upon jointly.

Intraoperative Management

Overview

DBS insertion is a three-part procedure (Table 1). The first part involves attaching the stereotactic headframe and obtaining imaging studies; the second part involves placing and testing the electrodes; the third part entails tunneling the wires and implanting the pulse generator. The first and third portions of the procedure can be intensely stimulating, but the patient only needs to be awake andcooperative during the second portion. This had led to the use of the "asleep-awake-asleep" technique, in which general anesthesia is provided for the first and third parts, and light sedation is given for the second part. The details of the procedure and the "asleep-awakeasleep" anesthetic technique are discussed below [26-28].

Table 1: Phases of DBS insertion procedure and anesthetic goals.




  

    
Phase

    
Anesthetic    Goals

  

  

    
Headframe placement and imaging

    
Analgesia, deep sedation or GA

  

  

    
Electrode placement

    
Awake, cooperative, comfortable patient

  

  

    
Tunneling and generator implantation

    
Analgesia, deep sedation or GA

  









Table 1:  Phases of DBS insertion procedure and anesthetic goals.

Headframe Placement and Imaging

The implantation of a DBS begins with obtaining stereotactic data to guide the surgical approach. This involves the application of a stereotactic head frame followed by an MRI. While most patients easily tolerate non-invasive imaging modalities like MRI without issue, the placement of the headframe itself is far from trivial; in fact it is quite stimulating, involving the fixation of multiple sharp pins securely to the skull. Analgesia for the placement of the head frame is an absolute necessity. Options include infiltration of local anesthetic at the sites of pin insertion, nerve blocks (e.g. supraorbital, greater occipital,lesser occipital, auriculotemporal, and zygomaticotemporal), general anesthesia, or a combination of these techniques; local or regional anesthesia is frequently inadequate as a sole technique and must often be accompanied by some amount of sedation or general anesthesia [27,29]. This is not problematic because there is no particular need for the patient to be responsive during this portion of the procedure. However, of particular note is that once positioned, the headframe blocks access to the airway from the usual approach. It is therefore paramount that the anesthesiologist confirms adequate ventilation and oxygenation prior to its placement. Complicating matters further is that headframe application is often performed in a location remote from the operating room. The anesthesiologist must ensure that the usual monitoring requirements are met and that emergency equipment is readily available.

At the authors' institution, the most common practice is to induce general anesthesia, place a Laryngeal Mask Airway, and then allow the surgeon to administer local anesthetic at the anticipated pin insertion sites prior to placement of the headframe. The patients are monitored with an ECG, a non-invasive blood pressure measurement device, pulse oximetry, and caphnography. Anesthesia is maintained intravenously with a propofol infusion. Once the headframe is in place the imaging study is obtained. The patient may be awakened prior to the MRI, or may be left asleep for the duration of the study and then awakened prior to being transported to the operating room.

Electrode placement

In the operating room, the headframe is attached securely to the operating table and the patient is situated in a supine or semi-sitting position. After infiltration of local anesthetic, a small burr-hole is drilled in the skull and microelectrodes are inserted into the brain under stereotactic guidance to approach the targets identified on MRI.

As the electrode is advanced toward the target it detects the activity of individual neurons and transmits the data to a computer. The computer amplifies the signals and displays them graphically for interpretation by a neurophysiologist. These are referred to as microelectrode recordings (MERs). Recognition of the characteristic spontaneous activity of various types of neurons allows the neurophysiologist to identify the specific nucleus in which the tip of the electrode lies at any given time. Once the electrode is in the correct anatomic location, electrical pulses are sent through the electrode to the brain (macrostimulation) and the efficacy and adverse effects of the stimulation are assessed clinically.

The insertion of the electrodes must be performed with the patient awake for at least two reasons. The first reason is that the precise effects of sedatives and anesthetics on individual nuclei and their characteristic MERs are unknown, and altering MER patterns would confound interpretation and hinder proper electrode placement. The second reason is that macrostimulation relies on a clinical assessment of the patient for symptomatic improvement and the presence of untoward side effects. Sedation alone, especially with GABA-ergic agents, may suppress parkinsonian symptoms and result in false positives regarding surgical improvement; conversely, t may suppress the patient's ability to recognize and/or verbalize the presence of side effects, resulting in false negatives regarding adverse outcomes.

Sedation Strategies

Providing sedation for long periods of intraoperative wakefulness without compromising the patient's ability to cooperate fully represents a significant anesthetic challenge, the solution to which has changed with time [30]. Traditionally a combination of droperidol and opioids has been used to produce analgesia, immobility, and a state of indifference. This is referred to as neuroleptanalgesia. High complication rates, difficult titration, and the recent FDA "black box" warning pertaining to the association between droperidol and fatal dysrhythmias have led to an abandonment of this technique.

Several authors have described the use of propofol, with or without remifentanil, to provide sedation during awake neurosurgical procedures [31-33]. These techniques offer rapid kinetics and easytitration, but oversedation with ensuing respiratory depression, airway compromise, and/or loss of patient cooperation are of particular concern. Some authors have advocated the use of brain function monitoring to avoid over sedation [34].

Dexmedetomidine is a newer medication that has been advocated for providing sedation during awake neurosurgical procedures. It is a selective alpha-2 agonist, promoting sedation by producing central sympatholysis rather than augmentation of the GABA system, as is the case for most sedatives [35-38]. Dexmedetomidine produces a unique form of sedation, perhaps due to its subcortical mode of action, which allows patients to transition easily between quiet sleep and cooperative wakefulness [38,39]. It is not associated with disinhibition or respiratory suppression as is commonly seen with sedatives that act on the GABA system, but it may still cause upper airway obstruction by reducing pharyngeal muscle tone. Due to its sympatholytic mechanism hypotension and bradycardia are prominent adverse effects, though hypertension after the initial loading dose has been documented [35,36]. Over sedation, is defined as the inability to perform an adequate neurological exam is rare, but has also been reported [37,38].

(Table 2) summarizes sedation strategies used for DBS placement.

Regardless of the sedation strategy chosen, it may be difficult for many patients to remain motionless to the degree required for precise electrode placement. The anesthesiologist must be exceedingly diligent during the initial positioning to confirm patient comfort, and special care should be taken to ensure the stability of the initial position to prevent passive movement of the patient relative to the table during the procedure. A pillow should be placed beneath the knees to ease lower-back strain, and the involvement off a professional to massage any cramps that the patient may experience can be helpful. If possible, avoiding the placement of a urinary catheter can greatly minimize patient discomfort. Finally, maximizing the distance between the patient's face and the surgical drapes, and the use of clear plastic drapes rather than opaque ones,will help minimize anxiety. Attention to these seemingly mundane details can be the difference between a smooth operative experienceand one fraught with difficulty.

Table 2: Sedation strategies for awake neurosurgery.




  

    
Method

    
Advantages

    
Disadvantages

  

  

    
Neuroleptanalgesia

    
Analgesia, immobility, state of indifference

    
High complication rates, difficult titration, FDA    "black box" warning

  

  

    
Propofol +/- remifentanil

    
Rapid kinetics, easy titration

    
Potential for oversedation, respiratory    depression, airway compromise, and loss of cooperation

  

  

    
Dexmedetomidine

    
Subcortical, non-GABA mechanism, easy transition    between sleep and wakefulness, no disinhibition, no respiratory depression

    
Potiental for hypotension and bradycardia.  Oversedation and loss of upper airway tone    are possible.

  









Table 2:  Sedation strategies for awake neurosurgery.

Tunneling and Generator Implantation

Once the electrodes are successfully placed and tested, the pulse generator must be implanted. The electrodes are tunneled beneath the scalp and then subcutaneously on the lateral aspect of the neck. They are ultimately connected to the pulse generator, which is usually implanted in the subcutaneous tissue of the infraclavicular region. As with most tunneling procedures, this can be highly stimulating. Fortunately, there is no requirement that the patient remain awake during this portion of the procedure, nor is there a need for the headframe to remain in place. With the headframe removed and full airway access restored, general anesthesia may be induced intravenously and then maintained with an inhalational agent via Laryngeal Mask Airway for the duration of the procedure [27-29].

Complications

The potential for perioperative complications in awake neurosurgical procedures is not trivial. The diligent anesthesiologist must be acutely aware of these hazards and take measures to prevent them before they threaten the success of the procedure or the safety of the patient [40,41].

Airway Complications

As mentioned previously, the presence of the headframe and the position of the patient severely restrict access to the airway, making it extremely important to avoid any sort of airway compromise. Arranging the head and neck in a slight sniffing position will help prevent airway obstruction and facilitate emergent airway management, if necessary. Oversedation can easily lead to respiratory depression or airway obstruction. Restlessness and shifting of the body, with the position of the head fixed to the operating table, can result in a mechanical obstruction of the airway. Although airway complications are rare, one study indicated that respiratory arrest or severe airway obstruction can occur in up to 1.6% of the cases, and intraoperative cough, moaning, or sneezing in up to 1.2% [42].

In addition, intracranial events, such as seizure or hemorrhage, can result in an altered level of consciousness and consequent airway compromise [43,44]. Lastly, the dysfunction of the respiratory muscles and/or upper airway muscles often seen in PD increases the risk for perioperative respiratory complications.

Cardiovascular Complications

Intraoperative hypertension has been associated with an increased incidence of intracranial hemorrhage, and as such must be avoided. Anxiety, agitation, and pain may all result in hypertension in the awake patient. If hypertension cannot be controlled with sedation and analgesia, antihypertensive medications should be administered. Common choices include labetalol, esmolol, nicardipine or clevidipine. Nitrates should be avoided because they may increase cerebral blood volume leading to increased intracranial pressure. Thechoice of agent is less important than the effect: though precise blood pressures are poorly defined, recommendations include systolic pressures <140 or an increase of no more than 20% above the patient's baseline. Myocardial ischemia (ST changes and increased serum troponin levels) has been described in the absence of coronary artery disease, suggesting that patients are at risk for coronary vasospasm during these procedures. Invasive blood pressure monitoring may be useful, but is not an absolute requirement.

Neurologic Complications

Focal neurological deficits and seizures have been known to complicate awake procedures, especially during cortical stimulation. Seizures, when they occur, are most commonly partial seizures and require no acute management, though small doses of propofol or benzodiazepines may be used for immediate control. Focal deficits such as weakness or confusion require no acute treatment. More significant neurological sequelae, however, such as generalized seizures or sudden loss of consciousness, must be treated more ggressively.

Air Entrainment

Due to the combination of a sitting or semi-sitting position and spontaneous respiration, venous air embolism is a rare but everpresent threat [45,46]. The sudden onset of severe coughing during creation of the burr hole may signal the presence of an air embolism [47,48]. Acute hypotension, tachycardia, and hypoxia are more ominous signs. Precordial Doppler may be useful to detect small air emboli before they become clinically significant, but may interfere with neurologic recording [49]. Tension pneumocephalus has also been reported [50]. Despite these concerns, central venous catheter insertion is not routinely recommended.

Pain Management

Background

Neurosurgical approaches for the management of chronic pain are either ablative or augmentative [51,52]. Ablative procedures involve the irreversible destruction of nervous structures, and may be performed at the cranial, spinal, or peripheral levels. Examples include thalamotomy, cordotomy, dorsal rhizotomy, and neurolysis. These procedures are typically reserved for the management of pain associated with advanced malignancy. Augmentative procedures have the benefit of reversibility, and include the implantation of intrathecal pumps to infuse opiates, electrodes for spinal cord stimulation, or electrodes for deep brain stimulation [53,54].

Intrathecal pump implantation requires no patient interaction and no specific anesthetic consideration. The considerations for implanting electrodes for deep brain stimulation to manage chronic pain are nearly identical to those described above for PD, but without the specific concerns related to PD and its medical management [55,56]. SCS implantation is slightly different because the patient must be in the prone position.

Spinal Cord Stimulator Implantation

Spinal cord stimulation is a common method for the management of non-malignant pain [57,58]. The procedure entails placing electrodes into the epidural space, either through a Tuohy needle of via open laminotomy, and is performed with the patient in the prone position. The electrode is manipulated until stimulation produces paresthesias that overlap the distribution of the pain. Proper placement of the electrode requires significant patient interaction and is performed awake with minimal or no sedation. Just as with DBS insertion, once the electrode is positioned properly it must be tunneled beneath the skin and attached to a pulse generator, which is implanted subcutaneously in the flank. Again, the tunneling is often painful, but there is no longer a requirement for patient cooperation. However, the prone position all but precludes the placement of an LMA, as is commonly done in DBS procedures. The surgeon must provide adequate local anesthetic, and the anesthesiologist must provide adequate sedation for the patient to tolerate the procedure, but be extremely vigilant not to compromise the airway in the prone position.

Summary

Functional stereotactic neurosurgery is used in the treatment of patients with neurologic disorders that lack a gross structural lesion. This procedure poses many challenges to the anesthesiologist.

  • Patients must be well selected and highly motivated.
  • Rapid conversion from deep sedation or general anesthesia to a fully awake and cooperative state and back again is necessary.
  • Access to the airway is limited.
  • PD itself results in unique anesthetic considerations.
  • Anesthetic agents must minimally affect MERs.
  • Dexmedetomidine causes subcortical sympatholysis, producing comfortable sedation that allows an easy transition between quiet sleep and cooperative wakefulness while minimizing respiratory depression and cognitive disinhibition.

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Citation: Bekker A, Eloy J. Anesthesia for Functional Neurosurgery. Austin J Anesthesia and Analgesia. 2014;2(3): 1016. ISSN: 2381-893X