Anaesthesia for neurosurgery without craniotomy (Neuroanaesthesia) Part 4

Anaesthetic considerations

The considerations for anaesthesia for third ventricu-lostomy and other neuroendoscopic procedures are similar to those for any patient with raised ICP. Te procedure itself is undertaken in the supine, head-up position. Standard monitoring and anaesthesia are used. Invasive monitoring is not normally indicated. Occasionally, the head may be fixed in a frame. Intraoperative arrhythmias and bradycardia in particular are common. Tis may respond to repositioning of the endoscope or removal of irrigation fluid but does also occasionally require pharmacological treatment. Post-operative analgesic requirements are mild to moderate, and opiates are rarely required beyond the recovery room.

Stereotactic neurosurgery and functional neurosurgery

Tie word ‘stereotactic’ is a mongrel word from the Greek stereo meaning ‘three-dimensional’ and the Latin tactus meaning ‘to touch’. Neurosurgical stereotactic techniques comprise methods for locating surgical targets within the brain relative to an external frame of reference, therefore assisting accurate navigation to a region without direct visualization. Its history can be traced back to animal studies by Sir Victor Horsley and Robert Clarke in the early 1900s. Modern stereotac-tic neurosurgery was pioneered by Lars Leksell, who invented a navigation apparatus suitable for clinical use, which is still in use today.


Frame-based versus frameless stereotactic navigation

Navigation to any surgical area of interest requires the establishment of an external reference system from which target location can be deduced by subsequent imaging. Te technique can be broadly classified into frame-based and frameless methods. Te frame-based technique utilizes a rigid stereotactic frame firmly attached to the patient’s skull to establish such an external reference. Te frame is held in place by pins that penetrate the outer layer of the skull. With the frame in situ, the patient’s head is imaged with either MRI or CT. Based on the radiological data, the coordinates of the area of interest and the most appropriate surgical approach can be planned.

Frameless stereotaxy is a recent technique aiming to reduce trauma and to improve surgical access to the patient. Special markers (fiducials) are pasted onto the patient’s scalp before imaging. In the subsequent surgical procedure, the patient’s head (together with the fi ducials) is secured to the operating table with a neuronavigation reference attachment. Using a special probe or pointer, the fiducials on the patient’s head are cross-referenced with those on the MR images, which will be used to direct the procedure and provide realtime interaction with MRI.

Anaesthetic implications for stereotactic surgery

Application of the rigid head frame for surgery imaging is often well tolerated by patients using local anaesthesia, with or without sedation. Te choice of whether or not this is done under general anaesthesia is often dictated by the invasiveness of the subsequent surgical intervention, the relationship of the operative site to critical structures or brain regions, and the degree and type of intraoperative neuromonitoring, specifically in deep brain stimulation procedures. In addition, there are a number of specific anaesthetic issues warranting special attention when deciding on the technique to be used.

A significant portion of the head frame is positioned over the nose and mouth and close to the patient’s shoulders. Tis can hinder access to the patient’s airway. For this reason, instruments that are required to release the frame should be readily available for as long as the head frame is in situ. Continuous observation of airway adequacy is crucial. If general anaesthesia is necessary for the subsequent surgical intervention, endotracheal intubation is best performed before the frame is applied.

Great care is given to patient positioning. The back portion of the frame can be close to the patient’s shoulder and decrease mobility of the head and neck. Poor head and back position can result in neuropraxia and local trauma, especially if the patient is under sedation or general anaesthesia and unable to notify any discomfort.

Deep brain stimulation and anaesthesia

The evolution of deep brain stimulation (DBS) can be traced back to the initial use of intraoperative electrical stimulation for target exploration prior to lesioning in patients with movement disorders, particularly but not exclusively in Parkinson’s disease. Deep brain stimulation provides advantages over the traditional ablative procedures such as thalamotomy and pallidotomy as it is non-destructive, reversible and adjustable.

Hialamic stimulation was first developed for tremor control. Subsequently, ventralis interme-dius nucleus (Vim), subthalamic nucleus (STN) and internal globus palludus (GPi) stimulation were also investigated. he effect of stimulation on various nuclei differs. Ventralis intermedius nucleus stimulation provides good tremor relief in Parkinson’s disease but little improvement of other symptoms such as akinesia, rigidity, bradykinesia and drug-induced dyskinesia. Stimulation of the GPi and in particular STN, on the other hand, can relieve most cardinal motor features of Parkinson’s disease, including rigidity, tremor, brady-kinesia, gait disturbances and motor fluctuations, and levodopa-induced dyskinesia in advanced Parkinson’s disease. Currently, the STN is the preferred target in this disease. Following its success in Parkinson’s disease, the indications and applications for DBS have extended into other disorders such as essential tremor, dystonia, epilepsy, chronic pain and psychiatric disorders. he exact mechanism of action of the success of neurostimulation, however, remains unclear.

Surgical technique

The DBS hardware commonly employed has four main components:

1. Multicontact intracranial quadripolar electrodes designed to be surgically inserted into the deep brain unilaterally or bilaterally via a burr hole.

2. A plastic ring and cap seated onto the burr hole to fix the electrodes.

3. A single- or dual-channel internal pulse generator (IPG) with battery unit.

4. An extension cable tunnelled subcutaneously from the cranial area to the chest or abdomen, connecting the DBS electrode(s) to the IPG.

The insertion of electrode(s) into the target area(s) of the brain, followed by internalization of extension cable and implantation of the IPG. Successful outcome relies on accurate insertion of electrodes. Frequently, this is achieved using a combination of methods including:

1. Stereotactic, frame-based imaging to identify the target nuclei and establish stereotactic coordinates. A frame-based stereotactic technique is used. he frame is usually applied under local anaesthesia, except for uncooperative patients or those with severe dystonia. With the stereotactic frame in place, MRI is performed to identify target nuclei and allow surgical planning to establish external coordinates for electrode insertion. In subjects with contraindication to MRI assessment, CT can also be used. he use of frameless stereotaxis in place of the frame-based technique has also been reported with the potential to provide better patient tolerability and ease of surgical planning. However, experience with a frameless stereotactic approach remains limited, mostly due to concerns regarding the accuracy of the method.

2. Electrophysiological guidance with the use of microelectrode recording. Due to brain shift during patient positioning and loss of CSF through the burr hole, and because of the intrinsic inaccuracies of current frame-guided navigation techniques, stereotactic navigation is not flawless. To further fine tune the location of the electrode, many centres utilize an electrophysiological mapping technique known as microelectrode recording (MER). A microelectrode is passed along the calculated trajectory to 10-15 mm above the target, then slowly advanced in 50 to 100 |xm increments while its tip records and amplifies neuronal discharges along its path. Specific brain structures can be identified based on their unique patterns of firing. his allows feedback of the actual trajectory and fine adjustments of position before inserting the final electrode.

3. Macrostimulation testing on an awake patient to observe symptoms improvement and side effects of neurostimulation. Finally, if the patient is awake, intraoperative macrostimulation through the deep brain electrode helps to confirm clinical improvement and assess side effects during neurostimulation. When the team is satisfied with the electrode location, it is secured and the burr hole can be closed off. A second electrode may be inserted in the contralateral side if bilateral deep brain stimulation is planned. Te electrode insertion is routinely followed by radiological confirmation.

Internalization of electrodes and IPG implantation can be performed under general anaesthesia either immediately or as second-stage surgery. Te electrode is connected to the external cable, which is tunnelled subcutaneously in the scalp and at the side of the neck to a pulse generator implanted in the chest or abdomen. Currently, there is no evidence favouring the best timing of this stage. Te decision is dependent on the patient’s condition, team preference and local hospital logistics. Oedema around freshly implanted electrodes leading to the so-called ‘microlesion effect’ may interfere with assessment of clinical symptoms post-operatively. For this reason, most centres do not initiate stimulation until 2-4 weeks following lead implantation.

Anaesthetic management

Pre-operative evaluation

The pre-operative preparation starts with careful patient selection, as this is a major determinant of successful outcome. Te decision to operate should be based on an individualized risk-to-benefit assessment, balancing the risk of the procedure against the perceived improvement in quality of life. Te level of patient disability, likelihood of improvement following DBS, risk factors for complications, general life expectancy and the patient’s motivation should all be taken into account. Tis is best accomplished using a multi-disciplinary approach addressing medical, neurological, anaesthetic, psychiatric and social issues by a team consisting of neurologists, neurosurgeons, anaesthetists, neuropsychologists and nurses.

Deep brain stimulation can be considered for Parkinson’s disease when the patient develops moderate to severe motor fluctuation, medication-fnduced dyskinesia, refractory tremor or intolerance to medications. Te disabling symptoms and their response to medications should be identified. Patients who do not improve significantly with levodopa are unlikely to improve with surgery. For patients with dystonia, DBS may benefit those with primary or secondary dystonia suffering from significant disability despite optimal medical management.

Patients are evaluated based on their general physical condition (in particular, cardiopulmonary comorbidities) and psychiatric and cognitive function. Tere is no specific age limit for DBS, but it is a factor affecting how the patient will cope with the surgical procedure and behave post-operatively. Older patients may have only modest motor improvement and have increased incidence of intraoperative delirium and cognitive dysfunction after STN stimulation.

The severity of the underlying conditions poses significant concerns to anaesthetists. Besides motor problems, patients with long-standing Parkinson’s disease can have autonomic dysfunction, increased aspiration risk, sleep apnoea, impaired respiratory reserve and impaired cough due to respiratory muscle dysfunction. Many have significant physiological or psychological comorbidities that can be age- or disease-related. Patients with dementia may be unable to tolerate and cooperate with the awake procedures typically employed in DBS insertion and may have trouble accurately observing and communicating their symptoms after the procedure, complicating the post-operative titration of stimulation parameters and medication.

Medical conditions that can substantially increase the surgical risk, such as poorly controlled hypertension and coagulopathy, should be identified and optimized before surgery. Polypharmacy is frequent in patients with Parkinson’s disease and there is a risk of perioperative drug interactions. Children with debilitating dystonia are often malnourished and hypovol-aemic. Contractures can cause skeletal deformity. Developmental delay and communication difficulties are common.

Pre-operative assessment helps to determine the optimal mode of anaesthesia. If an awake technique is contemplated, the patient should be motivated and able, physically and cognitively, to remain attentive and cooperative while undergoing a stressful procedure and testing requiring several hours of immobilization. A history of claustrophobia, psychiatric disorder or previous sedation failure warrants special attention. Regardless of the original anaesthetic plan, meticulous airway assessment is imperative to assess the risk of airway compromise and to assist in formulating a plan should intervention be required at any stage.

Patients are usually admitted the evening before surgery. A standard pre-operative fasting regimen is implemented. Anti-parkinsonism medication is often withheld to render the patients in an ‘off-drug’ state for intraoperative neurological testing, but such abrupt withdrawal of medication may result in significant patient discomfort and side effects. Pre-medication should be used judiciously, as many agents can interfere with patients’ cooperation and tremor interpretation.

Anaesthetic techniques

The anaesthetic priorities during deep brain electrode insertion are (i) to provide optimal surgical conditions and patient comfort during the procedure, (ii) to facilitate intraoperative monitoring and target localization and (iii) to rapidly diagnose and treat any complications. Various anaesthetic techniques have been described. hese include local anaesthesia with or without conscious sedation on an awake patient and methods that involve general anaesthesia, either throughout the entire procedure or temporarily as in an asleep-awake-asleep technique. Currently, there is no consensus regarding which technique is superior, and most centres have developed their own practice to address their team preference, local hospital setting and each patient’s individual needs. Certainly, general anaesthesia provides the highest degree of patient comfort and physiological control but may render many of the available intraoperative tests difficult or impossible. Teams who wish to perform MER and macrostimula-tion have to balance the conflicting interests of improving patient comfort through sedation and minimizing pharmacological interferences.

Awake technique

This technique is frequently employed. Clinical improvement and side effects of stimulation can readily be observed. his also avoids the emergence excitation and its associated haemodynamic fluctuations. Patients with less post-operative nausea and vomiting can also resume oral anti-parkinsonism medication earlier.

Intravenous access and monitoring is established first. Standard monitoring such as ECG, non-invasive arterial blood pressure and pulse and oxygen saturation can be challenging in the presence of a severe movement disorder. he degree of any additional monitoring is dictated by the patient’s comorbidities. Capnography and respiratory rate monitoring are particularly helpful. Patients with labile blood pressure may benefit from invasive monitoring for enhanced titration of antihypertensive infusions. Positioning must be done patiently with frequent patient feedback to ensure comfort. Attention to thermal control improves tolerability. Patients should be encouraged to void before surgery and urinary catheterization is undesirable particularly in males, where a sheath catheter is a good alternative. Excessive fluid administration is discouraged to prevent bladder distension. Draping should allow access to the patient’s face, arms and legs while maintaining a sterile surgical site. Attention to detail, good patient communication, patient reassurance and motivation are all necessary. Antibiotics are typically administered before incision. he scalp may be anaesthetized at sites of incision and pin attachments. A combined supraorbital and greater occipital nerve block is a good alternative.

To increase patient tolerability, some centres use intravenous sedation for the incisions and bony opening until electrophysiological mapping begins. Cerebral subcortical areas are extremely sensitive to Y-aminobutyric acid (GABA) receptor-mediated medications. he use of gabaminergic sedative medication, even in small doses, has been shown to affect the quality of MER. An ideal sedative agent should have no or at least a readily reversible effect on subcortical activity to allow MER and clinical testing. Benzodiazepines should be avoided as they can abolish MER and induce dyskinesia. Propofol has been used with success, although it is not yet clear to what extent propofol interferes with MER, and it is known to cause dyski-netic effects and abolish tremor. Although it is titrat-able with a rapid onset and offset, its pharmacokinetic behaviour in patients with Parkinson’s disease may differ from that of the population from which the target-controlled infusion models were developed. When it is used as part of the asleep-awake-asleep technique, delayed awakening can be a problem after cessation and use of the bispectral index (BIS) to titrate the anaesthetic depth does not seem to offer any advantage regarding times to arousal, total propofol consumption and cardiopulmonary stability.

In a number of centres, dexmedetomidine is the sedative agent of choice. Dexmedetomidine reliably produces conscious sedation mediated through activation of a2-adrenoreceptors in the locus coeruleus, a key modulator for arousal, sleep and anxiety. his, together with minimal respiratory depression, makes it an attractive agent to use in ‘awake’ functional neuro-surgery. Low-dose infusion of this drug (0.3-0.6 |xg kg-1 h-1) provides sedation from which patients are easily arousable and cooperative with verbal stimulation, allowing sophisticated cognitive tests to be successfully carried out. It has also been shown to attenuate the haemodynamic and neuroendocrine responses to headpin insertion in patients undergoing craniot-omy and to significantly reduce the concomitant use of antihypertensive medication. Even in the setting of a compromised cerebral circulation, there is, so far, no evidence of adverse effects on cerebral haemodynam-ics, and several animal studies have even suggested a neuroprotective ef ect. Dexmedetomidine does not ameliorate clinical signs of Parkinson’s disease and it seems that, at least in low-dose infusion, anxiolysis can be achieved with no effect on MER.

In a retrospective questionnaire interview of patients who underwent DBS electrode insertion, almost all recalled physical pain and psychological suffering during the procedure. Besides sedation, various measures have been implemented to improve patient tolerance, including intrathecal hydromorphone to alleviate lower back pain, intraoperative physiotherapy, local massage and respiratory exercises.

General anaesthesia

Although conscious sedation has been used successfully in some children, general anaesthesia may be necessary in others and in adults who cannot tolerate the awake technique, either due to concurrent psychiatric problems, dystonia or severe anxiety with associated hypertension. Te decision for general anaesthesia is best made before surgery after careful pre-operative assessment, as the presence of a stereotactic head frame can complicate airway management, and any unplanned conversion to general anaesthesia in the midst of the procedure carries significant risk. Te airway should be secured before head frame insertion, as access to the airway is restricted afterwards.

Concerns developed over whether the procedure performed under general anaesthesia would render MER impossible, and whether the lack of intraopera-tive assessment would result in a higher risk of subopti-mal electrode placement. Current evidence suggests that MER is possible under a light level of general anaesthesia with careful titration of desflurane and propofol. A study by Maltete on post-operative outcome in patients who underwent bilateral STN electrode placement found that residual motor disability and the intensity of stimulation appeared to be slightly higher in patients operated on under general anaesthesia with propofol, implying that STN stimulation was less precise in the absence of intraoperative clinical assessment. However, this result was not reproduced in other investigations. A cohort study also failed to demonstrate any definite influence of the type of anaesthesia on surgical outcome. Furthermore, continued advancement in neuroimaging is likely to bring about improvements in target localization under general anaesthesia. Recently, the placement of DBS electrodes under general anaesthesia using a skull-mounted aiming device under interventional MRI has been reported. Te technique eliminates the use of the traditional stereotactic frame and possibly the need for any intraoperative recording or testing and may become a future alternative method in patients who are unable to tolerate awake surgery.

Regardless of the anaesthetic technique employed, vigilance is necessary, as complications do occasionally occur. Monitoring in the recovery area should include frequent assessment of neurological status, good blood pressure control, attention to respiratory status and prompt treatment of any pain or nausea. If anti-parkinsonism medications have been withheld, they should be resumed as soon as possible to avoid motor fluctuations and deterioration in neurological and respiratory function.

Intraoperative anaesthetic-related complications

There is only limited data on the incidence of intraoperative anaesthetic complications during DBS. Overall, intraoperative complications are reported to occur in 5-16% of patients.

Hypertension is a common intraoperative problem, usually related to poor pre-operative control, patient distress or anxiety during the procedure. If necessary, a- or ^-blockers or calcium-channel antagonists may be used. A venous air embolism can occur at any time during the burr hole procedure both in the supine and in the semi-sitting position.

Potential loss of airway is an important consideration with the awake technique, especially if sedation is used. Te stereotactic head frame makes airway access difficult. A gradual shift of the body with neck flexion often occurs during surgery and may slowly compromise the airway. Oversedation can lead to both obstructive and central apnoea and further aggravate this situation. It is important to note that, while dexme-detomidine is known to cause minimal respiratory depression in healthy volunteers and patients without respiratory disease, it can still produce upper airway obstruction. Prompt laryngeal mask airway insertion and even head frame disengagement can be life-saving during an airway crisis.

Table 17.2 Summary of devices that may potentially interfere with a neurostimulator

Device

Potential interactions

Precaution(s)

ECG

Deep brain stimulation (DBS) may directly produce ECG artefacts

Bipolar stimulation of neurostimulator may minimize ECG artefacts

Severe tremor after DBS deactivation can lead to ECG artefacts

Short-wave diathermy

Induces heating of DBS electrodes leading to brain damage

Use of short-wave diathermy is contraindicated

Phacoemulsification

No interference reported

Electrocautery

Potential thermal injury to brain

Switching off pulse generator may decrease

Reprogramming and damage of DBS

damage to neurostimulator

Use of battery-operated heat-generating pulse generator

Use the lowest diathermy energy in short irregular pulses

Re-interrogate DBS system after surgery

Pacemakers

Cross-interference between the two devices

Bipolar DBS and bipolar pacemaker stimulation can decrease interference

Interrogation of the two devices before and after surgery

External defibrillator and internal cardioverter-defibrillator (ICD)

Tissue heating around the brain target Reprogramming and damaging of DBS

Position external defibrillator paddle as far away from neurostimulator as possible, perpendicular to the lead system

Bipolar DBS + ICD electrodes can minimize interference

Interrogation of DBS + ICD device after defibrillation

Peripheral nerve stimulator

No interference reported

Electroconvulsive therapy (ECT)

No interference reported

Place ECT electrodes away from DBS hardware

MRI

Electrode heating leading to brain damage

Follow safety MRI guidelines

Limit MRI exposure

DBS reprogramming and damage

MRI image artefacts

Seizure is the most common neurological complication and patients with multiple sclerosis undergoing DBS may be at particularly high risk. Most peripro-cedural seizures occur during test stimulation and are often self-limiting and focal in nature. However, generalized tonic clonic seizures do occasionally occur, and hence all indwelling catheters should be secured to prevent inadvertent dislodgement, and anticonvul-sants should always be readily available. Post-ictal airway patency must be ensured.

Haemorrhage, although rare, can be devastating. Te only consistent factor associated with the occurrence of haematoma is hypertension. Te number of MER penetrations is, at most, weakly correlated with the occurrence of haematoma in the absence of hypertension.

Other changes in neurological status such as confusion, speech deficit or limb weakness can occur both during and after the procedure. Te aetiology ranges from patient fatigue, medication withdrawal, seizures and intracranial bleeding to pneumoencephalus and can be difikult to determine in the midst of the procedure. Akinetic crisis due to acute drug withdrawal has been reported.

There is little information on the management of patients with existing deep brain stimulators who present for unrelated surgeries. Te potential exists that the neurostimulator may interfere with other monitoring and therapeutic equipment with possible severe consequences. Table 17.2 summarizes the current knowledge on possible interference.

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