Definition

The posterior fossa is a small volume compartment with low compliance containing a number of vitally important neural structures, which are uniquely different in function and pathology. Traditionally in neurosurgical and neuroanesthesia literature, discussions on procedures related to the pathology found in the posterior fossa are combined into one chapter, although one may argue this approach as arbitrary at best but potentially also misleading. Each procedure performed on the various anatomic structures found in the posterior fossa carries its own dramatically unique challenges and the risk for potential complications. Intimate knowledge of these challenges and close cooperation and communication with the neurosurgical and neurophysiologic teams is required in order to design and execute the perioperative anesthesia care plan for each particular surgical intervention. Considering how extensive the list of pathologies found in the posterior fossa is ( Box 71.1 ), it is beyond the scope of this chapter to provide a full comprehensive review necessary to address every aspect of anesthesia care for these procedures. However, some common principles in regard to anticipating, recognizing, and managing anesthesia perioperative complications in the posterior fossa surgery are covered in this chapter.

Surgery in the posterior fossa, with the presence of vital neuronal and vascular structures within a limited space, presents challenges for both the surgeon and the anesthesiologist. It contains critical respiratory, vasomotor, and cardiac control centers, specifically within the brainstem. In addition to the midbrain, medulla, and pons, which form the brainstem, the cerebellum, lower cranial nerves, and many critical vascular structures are found in the posterior fossa. The vascular anatomy includes arteries of the vertebrobasilar circulation and venous sinuses traversing the fossa, including the sigmoid, transverse, and occipital. Manipulation of these regions, as seen with surgical retraction, can significantly influence respiratory drive, resting vascular tone, blood pressure, and heart rate. Small increases in volume within this space, mainly as a result of postoperative hematoma or edema, can result in rapid elevation of compartmental pressure, and subsequent life-threatening brainstem compression. Intraoperative goals are aimed at facilitating surgical access, accommodating requirements for reliable neurophysiologic monitoring of nervous tissue and cranial nerve integrity, and maintaining respiratory and cardiovascular stability. The cerebrospinal fluid (CSF) conduit is exceptionally narrow through the cerebral aqueduct. Minimal obstruction can result in significant increases in intracranial pressure. Patients with lesions in the posterior fossa have a varied presentation depending on the exact location and structures involved. As an example, a small lesion impinging on the cerebral aqueduct may result in obstructive hydrocephalus, with presenting symptoms of headache and altered mental status. Similarly, a small lesion located in the lateral pons may result in isolated cranial nerve dysfunction. Therefore it is critical in the preoperative setting to identify the exact location of the lesion, as well as structural involvement, and any associated neurologic or systemic compromise.

Recognition

When discussing complications related to posterior fossa surgery, it is important to point out the predominantly surgical and non–life-threatening nature of these complications. In a recent review of 500 cases, the overall complication rate was under 32% with the vast majority of those being CSF leaks, meningitis, wound infection, and cranial nerve palsies. Hydrocephalus and cerebellar hematoma represent a small percentage of all surgical complications. Overall mortality rate was less than 3%. Most of these complications are unlikely to be related to the anesthetic management. Serious perioperative complications that are not directly related to surgical technique or approach (such as venous air embolism) are traditionally and arguably considered to be in the domain of anesthesia management and are quite rare. Transitory hemodynamic instability, although more frequently observed in this type of surgery, can usually be treated promptly without consequences and should not be considered a complication. Most of the anesthesia complications can be divided into a number of broad, and sometimes overlapping, categories: (1) complications related to positioning for posterior fossa surgery; (2) failure to provide adequate anesthetic for complex intraoperative neurophysiologic monitoring needed to monitor the integrity of neuronal pathways at risk; (3) severe hemodynamic disturbances related to compression or irritation of vasomotor centers in the brainstem; and (4) postoperative complications related to an expanding hematoma and parenchymal edema, especially in patients with an unsecured airway.

Surgical exposure in the posterior cranial fossa is a well-recognized challenge. Both sitting and variations of horizontal positions are being used. In the past, the sitting position was viewed as providing superior surgical exposure and anatomic orientation with the CSF and blood being gravitationally drained from the operative field, improved venous drainage, decreased blood loss, free diaphragmatic movement, and easier access to the endotracheal tube and the airway. However, this has fallen out of favor in many centers due to potentially severe complications associated with this position and the risk of malpractice claims. Sitting craniotomy complications are well known and include venous air embolism (VAE), paradoxic air embolism (PAE), tension pneumocephalus, lingual and laryngeal edema, quadriparesis, peripheral nerve injuries, severe hemodynamic instability, and decreased cerebral perfusion. Use of horizontal positioning such as lateral, prone, or bench positions decreases the rate of VAE and PAE but presents its own challenges. Frequent use of extreme neck flexion to improve exposure may result in higher risk of cervical spinal cord ischemia, macroglossia, and airway edema. Prone position leads to increased intraabdominal and intrathoracic pressure, impaired venous drainage, and risk of postoperative vision loss. Bench and lateral positions can be associated with brachial plexus injury if appropriate measures are not undertaken to minimize the stretch and the compression in the dependent arm. All of these positions demand special equipment and significant expertise and experience, which is uncommon considering how increasingly rare the use of these positions has become. The most common approach in acoustic neuroma excision is retromastoid, where the patient remains supine with an elevated ipsilateral shoulder using a sandbag. This position is not associated with the increased risk of the aforementioned complications. A comprehensive discussion on the management of each individual position and complications related to it is beyond the scope of this text. Just a description of monitoring modalities and management of VAE and PAE would deserve a whole chapter. The practitioner faced with the planning for surgery involving any of these positions would be wise to allocate the right amount of time to make adequate preparations for this challenge, including close communication with the members of the team involved in the procedure.

The use of neurophysiologic monitoring to improve neural function preservation during posterior fossa surgery may involve somatosensory evoked potentials (SSEPs), brainstem auditory evoked potentials (BAEPs), and electromyography (EMG). Various anesthetic agents affect neurophysiologic monitoring to a different degree, and appropriate anesthetic management is needed to provide the best monitoring conditions. Failure to appreciate the specific monitoring needs for the proposed surgery may result in inadequate patient monitoring and suboptimal outcomes.

A profound hemodynamic instability is often expected in posterior fossa surgery. Surgical irritation or damage to brainstem cardiac and vasomotor centers can lead to rapid and unpredictable hemodynamic changes. Extreme heart rate and blood pressure alterations are common with surgical manipulation, and rapid recognition and treatment are required. The important caveat to remember is that the surgeons might rely on changes in blood pressure and heart rate for early detection of surgical damage to the important brainstem structure and cranial nerves. Therefore prophylactic treatment of hemodynamic changes must be undertaken only in agreement with surgical colleagues.

Finally, there are no adequate intraoperative monitors for a large number of important brainstem functions, such as airway maintenance and protection, swallowing, and respiratory control. Patients emerging after posterior fossa surgery are at increased risk of an endangered airway, respiratory compromise, and sudden neurologic deterioration due to rapid compression of the brainstem in the presence of even fairly small postoperative edema or hematoma. Thus anesthetic management must be planned to ensure rapid and clear emergence to allow reliable evaluations of the airway reflexes and neurologic status before extubation can be undertaken. The patient care team should discuss the range of acceptable hemodynamic parameters, expected neuronal or bulbar dysfunction, and measures needed to address anticipated alterations in airway protection and respiratory function should the need arise. Postoperative ventilatory support, intubation, or diagnostic studies (e.g., angiography, computed tomography, magnetic resonance imaging) should be discussed before emergence, and appropriate plans must be developed in light of those discussions and relayed to the intensive care unit team.

Risk Assessment

Knowledge of the anatomic location of the lesion of interest, the planned surgical procedure, and the actual structures involved in the surgery are all critical elements of posterior fossa surgery. Risk assessment is possible only after a review of the individual patient’s history and physical examination, an evaluation of radiologic studies, and a discussion with the neurosurgeon. The greatest risks are associated with tumors directly involving the brainstem (e.g., pons and medulla), lesions with direct involvement of the cranial nerves required for airway maintenance and protection, lesions involving the facial nerve, and surgeries conducted with the patient in the sitting position. The actual events encountered during surgery are impossible to predict, which contributes to the challenge of providing anesthesia for neurosurgery in general and for posterior fossa surgery in particular. At a minimum, plans for the diagnosis and management of hemodynamic instability, respiratory dysfunction, alterations in cranial nerve function, and venous air embolism should be made before starting any posterior fossa surgery.

Implications

As previously discussed, the risks to patients undergoing posterior fossa surgery have direct implications for the preoperative preparation, type of monitoring, and anesthetic technique necessary to secure optimal perioperative outcomes. Before surgery, patients must be carefully monitored and sedative-hypnotic and analgesic drugs must be titrated with extreme care. Intraoperative risks are predominantly hemodynamic instability and cardiovascular collapse. Especially with surgery involving the pons and medulla, extreme hemodynamic variability in heart rate and blood pressure may result in patient instability. This instability is usually limited to periods of direct surgical retraction and manipulation, but it can be clinically important. Hemodynamic collapse and cardiac arrest have resulted from venous air entrainment, and both are a constant risk during all posterior fossa (and skull base) surgeries, even in patients who are horizontally positioned. Use of an arterial line and right atrial catheter is warranted when needed. Anesthetic regimen based on modern short-acting intravenous agents (total intravenous anesthesia) such as propofol, remifentanil, and dexmedetomidine, with low stable levels of desflurane, would allow superior conditions for SSEPs and cranial nerve EMG monitoring. The BAEPs are resistant to the effects of most anesthetic agents in the doses used today.