Abstract
Posterior fossa neurosurgical procedures pose significant risk of damage to the brain stem and cranial nerves contained within, resulting in severe life-threatening consequences. In addition, during posterior fossa surgeries, patients are susceptible to intraoperative hemodynamic instability, blood loss, cardiac arrhythmia, venous air embolism, and specific positioning related complications.
Proper patient positioning is thus of paramount importance to provide optimal surgical access and help prevent serious adverse events associated with positioning. Modified sitting position aiming to achieve a positive venous pressure at the operation site increases the safety of the procedure by decreasing the incidence and severity of venous air embolism. Intraoperative monitoring with precordial doppler or transesophageal echocardiography improves the detection of small venous air embolism enabling its early treatment and diminishing its consequences. Recent literature suggests that under strict team protocol, patients with known patent foramen ovale can be operated on in a sitting position, with few reported clinical venous air embolism and no paradoxical air embolism.
Keywords
Modified semisitting position, Paradoxical air embolism, Posterior fossa surgery, Sitting craniotomy, Venous air embolism
Outline
Introduction 255
Anatomy 255
Clinical Presentation 256
Perioperative Management of Patients for Posterior Fossa Surgery 256
Preoperative Evaluation 256
Surgical Approach 257
Anesthetic Considerations During Posterior Fossa Craniotomy 257
Premedication 258
Patient Positioning 258
Supine 258
Lateral 258
Park-Bench (Semiprone) 258
Prone 258
Sitting or Semisitting Position 259
Technical Considerations 259
Anesthetic Management 261
Respiratory Management 261
Hemodynamic Management 262
Intraoperative Monitoring 263
Venous Air Embolism 264
Incidence 264
Pathophysiology 264
Clinical Presentation of Venous Air Embolism 265
Monitoring 265
Grading of Venous Air Embolism 268
Paradoxical Air Embolism 268
Management of Venous Air Embolism 270
Postoperative Management 271
Complications 272
Abbreviations 272
References 273
Introduction
Posterior fossa surgery poses significant challenges to both the anesthesiologist and surgeons with a wider variety of complications than surgery in the supratentorial compartment. Apart from the general perioperative considerations involving any intracranial lesion, highlights of posterior fossa lesions include unusual surgical positioning and its complications, potential for brain stem injury, lengthy surgical procedures, perioperative cardiovascular and respiratory embarrassment, and acute obstructive hydrocephalus.
A thorough understanding of the patient’s history, neurological findings, imaging studies, operative anatomy, as well as all potential adverse events associated with the procedure is thus of paramount importance to minimize complications.
Anatomy
Posterior cranial fossa is the largest and deepest cranial fossa. It houses the brain stem (midbrain, pons, and upper medulla), the cerebellum, 3rd to 12th cranial nerve nuclei, the ascending and descending tracts, and the vertebrobasilar vascular system.
The presence of structures vital for control of airway, cardiovascular and respiratory systems within the narrow confines of this rigid and compact space makes the surgical anatomy unique and challenging. The presence of any space occupying pathology may lead to mass effect on vital brain stem structures. In addition, the cerebrospinal fluid (CSF) pathway is very narrow through the cerebral aqueduct, and a minor obstruction can cause acute hydrocephalus with significant increase in intracranial pressure (ICP). Presence of multiple large venous sinuses contained within the dural folds of the tentorium further adds to the risk of bleeding and air entrainment during surgery.
Clinical Presentation
Symptomatology of lesions in the posterior cranial fossa differs from supratentorial tumors in terms of presentation and rapid worsening, as seen with acute hydrocephalus. Most posterior fossa tumors present with signs and symptoms of increased ICP including headache, nausea, or vomiting and papilledema.
Signs and symptoms typical to the site of the lesion include movement disorders, altered tonicity, and ocular signs such as nystagmus, strabismus, diplopia, and pupillary abnormality. Additionally, there may be cranial nerve dysfunction, bulbar palsy, bradycardia, respiratory embarrassment, and sudden brain stem herniation leading to death. The classic triad of symptoms referable to a mass in the posterior fossa is said to be headache , vomiting , and ataxia . Unlike supratentorial tumors, seizures are rare. From pathological perspective, lesion in the posterior fossa may be neoplastic (most common), developmental, vascular, and traumatic ( Table 14.1 ).
| Intraaxial tumors | Cerebellum | Fourth ventricle and pons | Brain stem |
|---|---|---|---|
| Astrocytoma Hemangioblastoma Metastasis | Medulloblastoma Ependymoma Choroid plexus papilloma Hemangioblastoma | Glioma Hemangioblastoma | |
| Extraaxial tumors | Cerebellopontine angle | Skull base | |
| Vestibular schwanoma Meningioma Epidermoid tumor Glomus jugulare tumor | Metastasis Chordoma Chondrosarcoma | ||
| Vascular malformations | |||
| Posterior cerebellar artery aneurysm Vertebral/vertebrobasillar aneurysm Basillar tip aneurysm AV malformations Cerebellar hematoma Cerebellar infarction | |||
| Cysts | |||
| Epidermoid cyst Arachnoid cyst | |||
| Cranial nerve lesion | |||
| Trigeminal neuralgia (cranial nerve V) Hemifacial spasm (cranial nerve VII) Glossopharyngeal neuralgia (cranial nerve IX) | |||
| Craniocervical abnormalities | |||
Atlanto-occipital instability
| |||
Perioperative Management of Patients for Posterior Fossa Surgery
Preoperative Evaluation
As a routine, preoperative evaluation should include a thorough history and clinical evaluation of respiratory, cardiovascular, and neurologic systems; airway anatomy and necessary investigations based on patient’s requirements; and institutional protocols. Specific considerations for posterior fossa lesions include evaluation based on surgical approach and the intended patient positioning.
The focus of preoperative evaluation should be on the identification as well as optimization of any coexisting medical conditions. Quantification and risk stratification of patients with known coronary artery disease and carotid disease is essential as it poses excessive risks in certain surgical positions. Hypertension resets autoregulatory range and might result in significant perfusion deficits due to hypotension associated with positions such as sitting and prone.
A decrease in the level of consciousness and altered respiratory pattern may indicate the presence of elevated ICP. External ventricular drainage or other shunt procedures may be indicated to manage hydrocephalus before surgery or intraoperatively. Raised ICP may be associated with vomiting and inadequate intake resulting in hypovolemic status, which may give rise to significant hemodynamic perturbations on induction of anesthesia and positioning. In addition, presence of diabetes insipidus, administration of diuretics, and use of intravenous (IV) contrast agents to facilitate imaging may contribute to dehydration and electrolyte disturbances. Preoperative administration of IV fluid and optimization of electrolytes should be considered on an individual basis. Cerebellar hemangioblastomas often secrete erythropoietin, resulting in polycythemia, which should be taken into consideration during preoperative evaluation.
Preoperative evaluation and documentation of dysphagia, cough, gag, and other cranial nerve dysfunctions; evaluation of cerebellar functions; vision and auditory functions may be needed in specific types of tumor. In patients with bulbar dysfunction, loss of gag and cough reflex increases the risk of aspiration pneumonitis and extubation failure. Hence, possibility of need for postoperative ventilation or tracheostomy and extended intensive care unit (ICU) stay should be explained preoperatively.
Patients with atlantoaxial subluxation and lack of neck movement secondary to craniocervical fusion can present challenges during airway management and positioning, especially in sitting and prone position surgery. Hence, a preoperative assessment of cervical spine by dynamic flexion and extension views and Doppler study of neck vessels to look out for carotid insufficiency should be done in all patients where extreme neck flexion is anticipated, especially in the elderly.
For patients to be operated on in a sitting position, there is not a uniform approach on what special preoperative evaluation is necessary. However, detailed evaluation should be conducted to minimize complications that are preventable if known. If sitting position is planned, it is prudent to rule out a patent foramen ovale (PFO) using a transthoracic echocardiography and perform PFO closure if present, to prevent paradoxical air embolism (PAE) (discussed in detail later).
Surgical Approach
There are several surgical approaches to the posterior fossa, which include suboccipital (retrosigmoid) approach and midline posterior approach, which can be subtentorial or transtentorial. There are less common ones such as translabyrinthine, subtemporal/middle cranial fossa approaches or combinations of the above.
Anesthetic Considerations During Posterior Fossa Craniotomy
The surgical complexity of the posterior fossa and the hazards of different patient positioning make the intraoperative management of a patient posted for posterior fossa craniotomy quite challenging and unique. In addition to the basic neuroanesthetic considerations inherent to any neurosurgical procedure, the major intraoperative goals during posterior fossa craniotomy are:
- 1.
to provide optimal patient positioning and surgical access, with minimum possibility of positioning-related hazards to the patient;
- 2.
maintaining adequate depth of anesthesia while avoiding hemodynamic instability;
- 3.
to provide optimum conditions for intraoperative neurophysiological monitoring (IONM);
- 4.
prevention, early identification, and effective management of venous air embolism (VAE); and
- 5.
to allow smooth emergence with early awakening so as to facilitate neurological assessment.
Premedication
Premedication should include all regular medications, including steroids (dexamethasone). The role of sedative premedication is limited in patients with posterior fossa lesions with their inherent risk of hypoventilation and potential to increase ICP. However, short-acting benzodiazepine given under supervision may be reserved for anxious patients who are neurologically intact.
Patient Positioning
Proper patient positioning during posterior fossa surgery is one of the most important factors for success or failure of the procedure. All positions have advantages and disadvantages, assessed either from the surgical or anesthetic perspective. The greatest challenge for the anesthesiologist is to choose the most appropriate surgical position that provides the best surgical exposure as well as pose minimum positioning related risks to the patient. Great attention should thus be paid to the physical and physiologic consequences of different surgical positions to help prevent serious adverse events and associated complications.
Depending on the planned surgical approach and the lesion, the most common positions for posterior fossa surgery are supine, lateral, park bench (semiprone), prone, and semisitting. (Discussed in detail elsewhere in the book.) The surgical approach must be individualized for each patient because the risk of postoperative complications may vary greatly with patient’s age, neurological status, and lesion location.
Supine
Acoustic neuroma and cerebellopontine angle (CPA) tumors may be carried out in the supine position with the head turned to the opposite side and placement of a sandbag under the ipsilateral shoulder to minimize stretching of the brachial plexus.
Lateral
The lateral position facilitates gravity-assisted drainage of blood and CSF and provides good surgical access for unilateral procedures. Patient instability and brachial plexus injury are potential positioning hazards of this position.
Park-Bench (Semiprone)
The park-bench position is a modification of lateral position with back elevated and head turned about 30% facing down with maximum possible neck flexion. It provides better access of the posterior fossa, as compared with the lateral position and can be attained quickly. It may be used to gain rapid access to cerebellar hemispheres, for example, need for rapid evacuation of a posterior fossa bleed. It gives lesser hemodynamic perturbations but the surgical orientation of the neck may be lost after draping.
Prone
The prone position is the oldest and most suited for midline approach during infratentorial craniotomies. Over a period of time, it gained popularity because of much lower incidence of clinically significant VAE when compared with the sitting position (discussed in VAE section). However, a retrospective survey comparing the semisitting and prone positions for posterior fossa surgery in children found more intraoperative and postoperative complications in the prone position, with increased length of intensive care and hospital stay. In addition, the amount of blood loss is more than in sitting craniotomies. Logistically, it is the most difficult positioning due to challenges associated with providing adequate oxygenation, ensuring adequate ventilation, maintaining hemodynamics, and securing intravenous lines and the tracheal tube.
Sitting or Semisitting Position
In neurosurgical practice, sitting position remained popular during the 1960s and 1970s for procedures involving the cervicodorsal spine and posterior and lateral cranial fossae. Later, its use declined throughout the neurosurgical community because of its potential severe consequences and associated litigations, with an only absolute indication being the supracerebellar infratentorial surgical approach for pineal gland tumors. Indeed, there has been a great difference between countries in its use since its introduction into clinical practice, being popular mainly in Europe and India and is still a subject of controversy.
At present, however, there is no hard scientific evidence justifying the abandonment of this patient position with the recent literature proving its safety in experienced hands, with appropriate monitoring. Feigl et al. have demonstrated that under meticulous anesthesia and neurosurgical management, even patients with a PFO can be operated on in the semisitting position with only a very low risk for VAE. Henceforth, by keeping a high index of suspicion for possible VAE and managing them promptly to limit their progression, the sitting position is a safe alternative to the prone and lateral positions for posterior fossa and posterior cervical spine surgery.
Nonetheless, the potential advantages of the sitting position in emergency cases (reduction of cerebellar swelling) may be annihilated by the additional time spent for positioning. Therefore alternative techniques of positioning (e.g., concorde, semiprone, etc.) should be considered whenever applicable. The extra positioning time to put a patient into the sitting position takes approximately 15–20 min longer and consists of placing the transesophageal echocardiography (TEE) probe and bringing the patient in an upright position.
Technical Considerations
Sitting or semisitting position offers a number of technical advantages for the neurosurgeon and neuroanesthesiologist compared to other positions, especially for surgeries of large and vascularized tumors in the posterior cranial fossa and CPA ( Table 14.2) . Various studies have shown reduced operating time, less intraoperative venous bleeding with lower transfusion rates, and better preservation of cranial nerve function in the sitting position compared with horizontal positions, despite having a higher incidence of VAE.
For the neurosurgeon:
|
For the neuroanesthesiologist:
|
Sitting position, however, poses unique physiological challenges for the neuroanesthesiologist. Changing from the supine to the sitting position induces a significant decrease of cardiac index, stroke volume index, right atrial pressure, mean arterial pressure (MAP), mean pulmonary arterial pressure (PAP) and pulmonary wedge pressure, and an increase in pulmonary and systemic vascular resistance. Sitting may also affect brain arterial and venous pressure and alter the venous or arterial ratio with different blood distributions. Cerebral perfusion pressure (CPP) decreases in the sitting position in nonanesthetized patients and could further decrease under anesthesia because of vasodilation and impaired venous return. This can be further aggravated by jugular venous obstruction caused by unfavorable head and neck position.
Sitting position has the potential for serious complications ( Table 14.3 ). The increased risk of VAE, with its most feared sequela of PAE, is by and large the most feared complication (discussed in detail later). Paying meticulous attention during patient positioning is thus of paramount importance to prevent positioning related complications. Prerequisite for a safe and routine adaption of the sitting position is an interdisciplinary dialogue between neurosurgeons and neuroanesthesiologist considering the relative risk–benefit of sitting position surgery for the individual patient, based on physical status and specific intracranial pathology, absence of contraindications ( Table 14.4 ), anticipation of potential complications; level of comfort, both with the procedure, and with each other and careful perioperative monitoring.
|
Absolute
|
Since its introduction, this patient position has been modified for neurosurgery to a modified semisitting or lounging position as it is used today to reduce the risk of an air embolism. This position aims to achieve a positive venous pressure at the operation site by a combination of adjustments. The upper body and legs are elevated by bending the operating table to a position in which the hip is flexed to a maximum of 90 ° . A 30 ° flexion of the knees is maintained to avoid overstretching of the tendons and nerves of the leg. The patient’s head is flexed anteriorly and a two-finger space between the sternal notch and the chin is left to avoid venous outflow obstruction. The inclination of the whole operating table is then changed to a lower head and higher leg position, in which the legs of the patients are as high as the vertex. Arms are supported to avoid traction of the shoulders; and all pressure points including legs, arms, and heels are adequately padded.
The patient positioning should be done incrementally to avoid any hemodynamic instability. Positioning of the head should ideally be performed under electrophysiologic neuromonitoring to minimize cervical cord compression. Finally, it must be ensured that the anesthesiologist has adequate access to the patient with minimum possible disturbance to the surgical field.
Anesthetic Management
During anesthetic induction, care should be taken to avoid hypotension, hypoxia, and hypercapnia with attendant cerebral ischemia and brain stem herniation in view of low compliance of the posterior fossa.
Considering the choice of anesthetic drugs during posterior fossa craniotomies, total intravenous anesthesia has been reported as the most commonly employed anesthetic maintenance technique in recent studies. Anesthesia is usually maintained by using a continuous infusion of propofol (6–8 mg/kg/h) with supplemental administration of opioids (either repetitive boluses of fentanyl or continuous infusion of remifentanil or sufentanil titrated to effect). Few studies report on the use of sevoflurane with a maximum concentration of 1 MAC during anesthetic maintenance.
The use of nitrous oxide (N 2 O) is the most controversial in posterior fossa interventions, especially in sitting craniotomies. During supratentorial craniotomy, N 2 O has potential advantages in terms of stable hemodynamics, good surgical conditions, reduction of awareness with recall, and use in neurologically and cardiovascularly “at-risk” patients. However, the classic adverse characteristics, such as unfavorable effects on intracranial dynamics, expansion of gas-filled spaces, and postoperative nausea and vomiting (PONV) are often cited as reasons to avoid N 2 O-based anesthetic regimen during posterior fossa surgery.
Nitrous oxide has been hypothesized to convert a pneumocephalus into a tension pneumocephalus. However, there appears to be no difference in the volume of intracranial gas postcraniotomy in patients who have received N 2 O versus those who have had a nitrous-free anesthetic. In fact it may be advantageous to maintain anesthesia with high-inspired concentrations of N 2 O until dural closure so that the rapid washout of N 2 O may actually decrease the pneumocephalus when it is discontinued. It has been postulated that N 2 O equilibrates with the intracranial air-containing cavity while the dura is open, such that after dural closure, no further volume expansion and/or significant ICP increase will occur. Hence, it is not necessary to discontinue N 2 O prior to dural closure for reasons of avoiding expansion of intracranial air and increasing ICP.
Also of concern during posterior fossa surgery is the risk that N 2 O, being 34 times more soluble in blood than nitrogen, can dramatically increase the size of venous air emboli. However, Losasso and colleagues found no evidence that N 2 O increased the risk, volume, or clinical consequences of VAE, if its administration is discontinued immediately upon Doppler detection of VAE. Nonetheless, N 2 O administration in presence of VAE results in its volume augmentation and intensifies the hemodynamic alterations thus allowing for earlier detection and, consequently, prompter treatment of VAE.
Although it is not rational then to omit N 2 O solely on the fear of a VAE or tension pneumocephalus, it is still prudent to avoid it during a repeat craniotomy within 6–8 weeks after dural opening and stop its administration as soon as an air embolus is suspected intraoperatively. The emetogenic effect of N 2 O, although undesirable after a craniotomy, can be controlled with antiemetic prophylaxis.
Respiratory Management
The polyvinyl endotracheal tube (ETT) may kink during posterior fossa surgery from overbending of the softened tube (due to oral temperature) and neck flexion required to improve surgical access. Manual straightening of the tube may be helpful to relieve kinking of ETT. In a recent report the placement of Berman intubating airway was found helpful to relieve the kinking of the ETT in a prone patient. Nonetheless, most neuroanesthesiologist prefer reinforced ETT to prevent kinking in view of varied patient positioning during posterior fossa craniotomies. A gap of at least two-finger space should always be present between the chin and the chest, and head rotation should be minimized.
Patients are mechanically ventilated to maintain either normocapnia or mild hypercapnia [to allow for a change in end-tidal carbon dioxide (EtCO 2 ) to be more prominent]. A higher arterial partial pressure of carbon dioxide (PaCO 2 ) level of about 35 mmHg (slightly greater than during neurosurgery in other positions) may be acceptable in sitting position craniotomy because ICP is usually less in this position.
Fraction of inspired oxygen (FiO 2 ) is usually maintained between 0.4 and 1. Nonetheless, when administering anesthesia for operations involving risk of intrapulmonary right-to-left transmission, higher levels of FiO 2 should be maintained as hyperoxia may prevent or reduce blood flow through arteriovenous pathways bypassing the capillary system when they are exercise induced. It, however, remains unknown whether FiO 2 or oxygen tension specifically regulates these recruited anastomoses or opens them indirectly.
The use of positive end-expiratory pressure (PEEP) during sitting position craniotomies is controversial. PEEP has been proposed to lower the incidence of VAE essentially by increasing the central venous pressure (CVP) and has been found safe up to 10 cm H 2 O in terms of non-alteration of interatrial pressure difference. Nonetheless, VAE has been shown to occur during release of PEEP and repositioning after sitting position surgery. Earlier studies have documented that PEEP is potentially detrimental during sitting craniotomies as it does not decrease the incidence of VAE, impairs hemodynamic performance, and might predispose patients with a probe PFO to the risk of PAE. However, current literature gives mixed results in the usage of PEEP with many groups using PEEP from 6 to 10 cm H 2 O while some avoid it all together. It is generally contraindicated in patients with PFO. However, Ammirati et al. have established biphasic PEEP (7–10 cm H 2 O) in patients with proven PFO to increase the intrathoracic pressure.
Use of spontaneous ventilation for the time period of tumor excision has been advocated by some authors to monitor the structural and functional integrity of vital brain stem structures but is no longer in vogue. During spontaneous ventilation, changes in the respiratory pattern would provide the surgeon with a warning signal for potential damage of these structures; thus avoiding any iatrogenic injury. However, it is necessary to maintain adequate depth of anesthesia to avoid coughing and patient movement.
Hemodynamic Management
In addition to the effects of anesthetic agents, patient’s cardiovascular system is exposed to the effects of gravity with venous pooling of blood in lower extremities during sitting position craniotomy. Intraoperative VAE or cranial nerve manipulation further adds to hemodynamic instability, thereby jeopardizing cerebral blood flow (CBF), especially in patients with disturbed autoregulation. Hence, any hemodynamic instability during induction and positioning should be avoided and aggressively managed. Normovolemia must be maintained at all times as dehydration exacerbates the low venous pressure and increases the risk of VAE. To combat positioning related hypotension, prepositioning controlled fluid loading and use of antigravity devices have been advised. Fluid loading should be done meticulously in patients with reduced cardiovascular reserves (e.g., elderly patients) taking into account their existing intravascular volume status.
In the first randomized study focusing on fluid therapy in neurosurgical patients operated on in sitting position with a stroke volume–guided therapy, Lindroos et al. found that 6% hydroxyethyl starch (HES) boluses resulted in 34% smaller infusion volume and less positive fluid balance than crystalloid while significantly increasing cardiac and stroke volume indexes. Furthermore, no difference was observed in thromboelastometry coagulation analysis between Ringer’s acetate and HES groups. Authors thus suggested that use of stroke volume–guided HES therapy might be advantageous during sitting craniotomies, especially in patients with decreased brain compliance. Later, similar study was done by the same author group in patients undergoing neurosurgery in prone position utilizing the same protocol of stroke volume–directed administration of HES (130/0.4) and Ringer’s acetate. Although, the amount of HES needed for comparable hemodynamics was 24% less, a slight disturbance in coagulation parameters was observed, and hence authors suggested caution while using colloids in neurosurgical patients.
Use of intermittent sequential compression device on the lower extremities is another simple and effective method to decrease intraoperative hypotensive episodes and improve cerebral oxygen saturation.
With hypotension induced by the upright sitting position, both intracranial blood flow velocity and cerebral oxygen saturation are reported to decrease. Arterial pressure should thus be maintained close to preinduction values, to preserve cerebral perfusion and reduce any risk of cerebral injury or postoperative cognitive dysfunction. Lindroos et al. have suggested maintaining a target MAP of 60 mmHg or higher at the brain level during sitting craniotomies. Unfortunately, a single targeted value of MAP may not suffice in all the patients. Moreover, a reported CPP of 60 mmHg may vary from a true head-level value of 43–60 mmHg, depending on reference point, head-of-bed elevation and height of the patient. Emphasis should thus be given on use of individualized targeted MAP with the help of bedside autoregulation testing, to continuously adjust the “better MAP” to maintain CBF and brain oxygenation without increasing cerebral blood volume.
Ephedrine and phenylephrine are the most frequently used vasopressors to treat intraoperative hypotension during sitting craniotomies. However, when comparing both agents, cerebral oxygenation was found to decrease significantly after phenylephrine bolus treatment and remained unchanged after ephedrine bolus treatment, even though MAP was significantly increased by both agents. Phenylephrine infusion has also been associated with cerebral oxygen desaturation, possibly caused by cerebral vasoconstriction, despite preventing hypotension in the upright position. Norepinephrine also negatively affects cerebral oxygenation.
Intraoperatively, various frequent changes in cardiovascular responses including bradycardia, tachycardia, hypotension, or hypertension and arrhythmia occur during surgical manipulation of the lower pons, upper medulla, floor of the fourth ventricle, and the cranial nerve nuclei. Hence, drugs that would mask these sentinel cardiovascular responses, including anticholinergic medications and/or long acting beta-adrenergic blockers, should be avoided. The surgeon should be notified immediately, with most of these changes subsiding immediately after the surgical stimulus is withdrawn and pharmacological treatment is generally not required. Transcutaneous pacing should, however, be considered in high-risk patients in view of bradycardia and risk of asystole during the surgery.
In a rare cause of bradycardia during posterior fossa surgery, Prabhakar et al. have reported reproducible bradycardia following hydrogen peroxide irrigation at the end of surgery, which resolved following aspiration of the effervescent solution.
Intraoperative Monitoring
During posterior fossa surgeries, patients are susceptible to intraoperative hemodynamic instability, blood loss, cardiac arrhythmias, VAE, and specific positioning related complications. Hence, in addition to “routine” neuroanesthesia monitoring, such as electrocardiography (ECG), pulse oximetry, capnography, temperature, urine output, invasive arterial blood pressure (ABP), arterial blood gases and CVP, additional specific VAE (discussed in detail in VAE section) and neurophysiological monitoring, with minimal interruptions during positioning, is required to increase the safety of the procedure.
Invasive arterial monitoring allows continuous ABP monitoring and repeated blood gas analysis. During sitting craniotomies, the arterial line transducer should be located and zeroed at the level of tragus to estimate the CPP correctly. Central venous catheter is essential as it is helpful for aspirating air during VAE, in addition to monitoring CVP (discussed in detail in VAE section).
Electroencephalography-based monitors can be used to detect cerebral hypoperfusion as well as to determine the depth of anesthesia, especially when spontaneous ventilation is planned during tumor resection. IONM techniques including somatosensory evoked potentials (SSEPs); transcranial electrical motor evoked potentials; brain stem auditory evoked responses (BAERs); and spontaneous electromyography (EMG) offer a great tool for live monitoring of the integrity of central nervous structures. Thus, any dysfunction can be identified early and prompt modification of the surgical technique or operating conditions helps to avoid permanent structural damage. SSEPs help to monitor spinal cord ischemia (related to hypotension in the sitting position) and should be opted during neck positioning whenever feasible to reduce the risk of midcervical flexion myelopathy. However, the incidence of false positives during SSEP monitoring is very high, particularly for cases of brain stem monitoring.
The vestibulocochlear nerve (CN VIII) and, to a greater extent, the auditory pathways (as they pass through the brain stem) are especially at risk during CPA, posterior/middle fossa, or brain stem surgery. The CN VIII can be damaged by several mechanisms, from vascular compromise to mechanical injury by stretch, compression, dissection, and heat injury. Additionally, cochlea itself can be significantly damaged during temporal bone drilling, by noise, mechanical destruction, or infarction, and because of rupture, occlusion, or vasospasm of the internal auditory artery. Intraoperative monitoring of CN VIII can be successfully achieved by live recording of the function of one of its parts, using the BAERs, electrocochleography (ECochG), and compound nerve action potentials of the cochlear nerve.
The BAERs is the most widely used method for hearing preservation as it has high sensitivity and reliability to detect cochlear nerve damage. Latency increase of wave V of 1.0 s and amplitude decrease of 50% are the most widely used criteria to warn the surgeon about potential cranial nerve damage and thus encourage redirection of the operative plan of action. However, BAERs are prone to presenting false-positive results and there is a significant time delay of several seconds to minutes to deliver reliable response of wave changes. In this interim, a permanent damage to the cochlear nerve could happen, preventing the surgical team from detecting and avoiding it. In comparison, ECochG and direct stimulation of CN VIII are “near-field” techniques with shorter latency periods and provide immediate feedback on the state of the auditory system.
Facial nerve injury is a complication of major concern after posterior fossa surgery due to severe negative impact on patient’s quality of life. Continuous intraoperative facial nerve monitoring helps to minimize accidental damage to the nerve during CPA surgery and skull base tumor surgery. Current standard facial nerve monitoring modalities include direct electrical stimulation, free-running continuous EMG, and facial MEP. However, a lack of standardization in electrode montage and stimulation parameters precludes a definite conclusion regarding the best method of monitoring.
In addition to the seventh and the eighth nerve monitoring, lower cranial nerves (CN IX–XII) can be monitored similarly by EMG, using needle electrodes within their respective musculature. Use of these monitors requires modification of the anesthetic technique to minimize interference with the monitoring (discussed elsewhere in the book).
Related posts:
Stay updated, free articles. Join our Telegram channel
Full access? Get Clinical Tree
