Anesthesia for posterior fossa craniotomy

INTRODUCTION — The posterior fossa contains the brainstem and cerebellum. Due to the location and the critical nature of the structures contained within the posterior fossa, surgery in this area introduces unique challenges to the anesthesia team. This topic reviews posterior fossa anatomy, indications for posterior fossa surgery, and the unique anesthetic considerations related to surgery involving the posterior fossa.

General principles and concerns regarding anesthesia for craniotomy are discussed separately. (See "Anesthesia for craniotomy in adults".)

ANATOMY OF THE POSTERIOR FOSSA — The posterior fossa is separated from the supratentorial intracranial compartment by the tentorium cerebelli (figure 1); posterior fossa craniotomy is also referred to as infratentorial craniotomy. The main neurologic structures within the posterior fossa include the cerebellum, midbrain, pons, and medulla oblongata (figure 2). These structures support many vital functions (eg, cardiovascular and respiratory control, awareness). The nuclei of cranial nerves III to XII are located in the midbrain, pons, and medulla. The posterior fossa also contains the ascending and descending tracts of the spinal cord.

Structures of the posterior fossa are supplied by the posterior cerebral artery, superior cerebellar artery, the pontine arteries, basilar artery, anterior inferior cerebellar arteries, posterior inferior cerebellar artery, vertebral arteries, and the anterior spinal artery (figure 3).

The pineal gland is not within the posterior fossa, but may be accessed through a posterior fossa craniotomy.

INDICATIONS FOR SURGERY — Indications for posterior fossa surgery include the following:

●Tumors – Posterior fossa tumors are more common in children than in adults; the most common pediatric tumors are medulloblastomas, astrocytomas, and ependymomas. In adults, metastatic lesions to the posterior fossa are the most common tumors, although primary brain tumors can rarely occur. Primary cerebellopontine tumors in adults include acoustic neuromas, meningiomas, schwannomas, and glomus jugulare tumors.  

●Vascular lesions – Surgery may be necessary for aneurysms (most commonly involving the basilar artery and posterior inferior cerebellar artery) or for arteriovenous malformations.

●Cranial nerve compression – Cranial nerve compression syndromes can also be treated with surgical decompression by access through a suboccipital craniotomy.

●Congenital malformations – Chiari malformations and Dandy-Walker malformations may be treated by suboccipital craniotomy.

●Hemorrhage – Surgical decompression may be required for hemorrhage in the posterior fossa, most commonly located in the cerebellum. Hemorrhage may be from vascular lesions, or may occur after intracranial surgery, either at the surgical site, or remotely. Remote cerebellar hemorrhage can rarely occur after supratentorial craniotomy, or after spine surgery [1-3].

SIGNS AND SYMPTOMS OF POSTERIOR FOSSA PATHOLOGY — The clinical presentation of patients with posterior fossa pathology varies based on location of the lesion, rate of lesion development, and the impact on cerebral spinal fluid flow and intracranial pressure. Lesions in the posterior fossa can cause focal deficits due to cranial nerve dysfunction (eg, ocular palsy, nystagmus, dysphagia, laryngeal dysfunction, arrhythmias), problems with respiratory regulation, sensory-motor deficits, or cerebellar signs and symptoms (ataxia, tremor, motor discoordination).

If flow of cerebral spinal fluid is compromised or a large lesion compresses the contents of the posterior fossa, the initial presentation can include the signs and symptoms of elevated intracranial pressure (eg, headache, nausea, vomiting, papilledema, or altered consciousness).

PREOPERATIVE ASSESSMENT AND MANAGEMENT — Patients undergoing posterior fossa craniotomies require the same preoperative history and physical examination as patients undergoing supratentorial craniotomies (see "Anesthesia for craniotomy in adults", section on 'Preoperative evaluation').

The neurologic examination in a patient with a posterior fossa lesion should focus on cranial nerve deficits. Patients with cranial nerve deficits resulting in dysphagia, loss of a gag reflex, and/or laryngeal nerve dysfunction may be at increased risk of aspiration, and may require delayed extubation at the end of surgery. (See 'Postoperative care' below.)

The risk of venous air embolism is high during craniotomy in the sitting position. If the sitting position is contemplated, we obtain a transthoracic echocardiogram preoperatively, or perform a transesophageal echocardiogram after induction of anesthesia, to rule out a patent foramen ovale (PFO) or other intracardiac shunt (eg, atrial septal defect, ventricular septal defect) and to assess cardiac function. We consider an intracardiac shunt a relative contraindication to the use of the sitting position. VAE in a patient with an intracardiac shunt can result in paradoxical air embolism (ie, entrance of air into the left heart chambers), which can result in myocardial ischemia or infarction or stroke. In addition, patients with pulmonary hypertension or right ventricular dysfunction may decompensate with even small amounts of intravenous (IV) air. Venous air embolism during craniotomy is discussed in detail separately. (See "Intraoperative venous air embolism during neurosurgery".)

ANESTHETIC MANAGEMENT

Premedication — Premedication for craniotomy should be individualized based on the patient's level of anxiety, baseline neurologic status, and comorbidities. (See "Anesthesia for craniotomy in adults", section on 'Premedication'.)

Patients with posterior fossa pathology may be especially sensitive to the respiratory depressant effects of benzodiazepines and opioids, and may be at increased risk of aspiration and excessive sedation. Sedatives should be titrated to effect using small doses of medication (eg, midazolam 1 to 2 mg IV, administered in 0.5 mg increments). For patients symptomatic with neurologic deficits who require premedication, we withhold sedation until the patient is fully monitored and in a setting that would allow immediate airway management.  

Monitoring — Physiologic monitoring during posterior fossa surgery is similar to monitoring during supratentorial craniotomy (see "Anesthesia for craniotomy in adults", section on 'Monitoring'). Monitoring considerations specific to posterior fossa surgery include the following:

●Electrocardiogram – Importantly, the electrocardiogram (ECG) can provide indications of traction on or manipulation of cranial nerves or the brainstem during surgery and allows adjustment of retractors or instruments.  

Brainstem manipulation or traction on cranial nerves V, IX, and X can produce abrupt profound bradycardia or transient asystole, via increased afferent activity and the resulting vagal response. The surgeon should be notified immediately if bradycardia occurs. The heart rate generally returns to normal or rebounds when surgical stimulation is adjusted, but administration of an anticholinergic (ie, atropine or glycopyrrolate) or epinephrine may be necessary. Rarely, chest compressions may be required to circulate the medications if asystole does not resolve after release of traction.

Administration of an anticholinergic may prevent or attenuate bradycardia and make the ECG an unreliable monitor for traction on cranial nerves or other sensitive neurologic structures. Thus some neurologic surgeons request that anticholinergics be avoided when possible prior to dissection near the brainstem.

Bradycardia may also signify an increase in intracranial pressure, particularly when associated with hypertension.

●Monitoring for venous air embolism – Monitoring for venous air embolism (eg, precordial Doppler, transesophageal echocardiography [TEE], end tidal carbon dioxide [ETCO₂]) is indicated for patients who are placed in the sitting position, and is discussed separately. (See "Intraoperative venous air embolism during neurosurgery", section on 'Monitoring for venous air embolism'.)

●Neuromonitoring – Electrophysiologic monitoring, or neuromonitoring, may be used for some posterior fossa surgeries to determine the integrity of cerebral function or to avoid injury of cranial nerves. Monitoring modalities and the anesthetic implications for patients who are monitored are discussed separately (table 1 and table 2). (See "Neuromonitoring in surgery and anesthesia".).

Induction of anesthesia — The goals for induction of anesthesia for posterior fossa craniotomy are the same as for supratentorial craniotomy: maintain hemodynamic stability and cerebral perfusion pressure, and avoid increases in intracranial pressure. Induction of anesthesia for craniotomy is discussed in detail separately. (See "Anesthesia for craniotomy in adults", section on 'Induction of anesthesia'.).

Positioning — The surgical approach to posterior fossa craniotomy determines the required patient positioning (figure 4).  

●The midline suboccipital approach is frequently used to decompress Chiari malformations or tumor resections, and often requires the patient to be positioned prone or sitting.

●The lateral suboccipital approach is often used for decompression of cranial nerves and provides access to lateral cerebellar lesions, lesions of the cerebellopontine angle, and vascular lesions of the posterior circulation. For this approach the patient may be placed in the lateral decubitus or park bench position, or supine with the head turned.

Patient positioning, including physiologic effects, nerve injuries, and particular concerns related to specific positions, are discussed separately. (See "Patient positioning for surgery and anesthesia in adults".)

Maintenance of anesthesia — The choice of anesthetic agents, hemodynamic management, fluid administration and management of ventilation are similar for posterior fossa and supratentorial craniotomies, and are discussed in detail separately. (See "Anesthesia for craniotomy in adults", section on 'Maintenance of anesthesia'.)

Overarching goals are to maintain hemodynamic stability and cerebral perfusion pressure, and to achieve a rapid emergence to facilitate an immediate postoperative neurologic examination. Anesthetic management may be affected by the need (if present) for brain relaxation, and by neuromonitoring, if used. (See "Anesthesia for craniotomy in adults", section on 'Brain relaxation' and "Neuromonitoring in surgery and anesthesia", section on 'Anesthetic effects on neuromonitoring'.)

Prophylaxis for postoperative nausea and vomiting — Patients who undergo posterior fossa surgery should receive prophylaxis for postoperative nausea and vomiting (PONV). Posterior fossa surgery is a risk factor for PONV, since the surgical site is near the vomiting center. Nonsedating antiemetics should be used to avoid delayed emergence or effects on the postoperative neurologic examination. Ondansetron or another serotonin receptor antagonist is a good choice for this reason. The dose and timing of dexamethasone administration for antiemesis or for brain relaxation should be discussed with the surgeon preoperatively.  

Prophylaxis for PONV is discussed in detail separately. (See "Postoperative nausea and vomiting".)

Plan for postoperative pain control — Similar to other craniotomies, we titrate short acting opioids as needed following extubation and a postoperative neurologic examination (see "Anesthesia for craniotomy in adults", section on 'Emergence from anesthesia'). However, infratentorial and occipital craniotomy may be more painful than frontal craniotomy, and higher doses of opioids may be required [4].

For patients having pain after surgery, we typically administer fentanyl 0.5 to 1 mcg/kg IV after the neurologic examination and titrate further doses of fentanyl as needed. Acetaminophen may be used as part of multimodal analgesia. Nonsteroidal anti-inflammatory drugs are usually avoided due to concerns about bleeding.

Emergence and extubation

●Emergence – The ideal emergence from any craniotomy should be smooth, with avoidance of coughing, straining, and/or hypertension (ie, systolic blood pressure >160 mmHg), and with the patient awake enough for an adequate neurologic examination (eg, responding to commands, moving all extremities on command, adequate vision and cranial nerve assessment). (See "Anesthesia for craniotomy in adults", section on 'Emergence from anesthesia'.)

●Extubation – Most patients are awoken and extubated in the OR after craniotomy, when awake, following commands, and demonstrating return of protective airway reflexes. Extubation may be delayed if there are concerns for cranial nerve dysfunction that might impact airway reflexes. For all patients who are intubated for anesthesia, a plan should be created for safe extubation, including risk stratification and backup strategies. This is discussed in detail separately. (See "Extubation following anesthesia", section on 'Extubation risk stratification'.)

The plan for extubation should be discussed with the surgeon. Factors to consider when deciding whether to delay extubation after posterior fossa surgery include the following:

•Preexisting neurologic deficits – Patients with preexisting laryngeal muscle dysfunction, dysphagia, or respiratory compromise may be unable to maintain or protect the airway after extubation, particularly with any residual anesthetic, or surgery related edema or trauma of the brainstem or cranial nerves.

•Extent and location of surgery – The possibility of injury of brain stem structures or cranial nerves that might compromise respiration or airway protection should be discussed with the surgeon. In particular, dissection in the floor of the fourth ventricle and around the cranial nerve nuclei can result in postoperative airway compromise.

•Airway edema related to positioning – Facial and/or airway edema may occur as a result of prone positioning, with the head turned, or with the neck flexed. If there are signs of facial edema (eg, scleral edema, swollen lips), the patient should be placed in the head up position to reduce edema, and extubation should be delayed. An endotracheal tube cuff leak test may be performed; if there is no leak, extubation should be delayed, but presence of a leak does not rule out the possibility of postextubation airway obstruction. (See "Extubation following anesthesia", section on 'Cuff-leak test'.)

Posterior fossa syndrome, which may develop immediately after surgery, can include dysarthria, cranial nerve deficits, hypotonia, and dysphagia, and can therefore affect the patient’s ability to maintain the airway. Posterior fossa syndrome is rare in adults, but can occur in up to 29 percent of children who have posterior fossa tumor surgery, particularly for medulloblastoma or tumors that invade the brainstem. Posterior fossa syndrome is discussed separately. (See "Treatment and prognosis of medulloblastoma", section on 'Posterior fossa syndrome'.)

High risk extubation and the use of airway exchange catheters for patients who may be unable to protect the airway or with potential airway edema is discussed in detail separately. (See "Extubation following anesthesia", section on 'High risk extubation procedure'.)  

POSTOPERATIVE CARE — Patients who undergo posterior fossa surgery recover from anesthesia and surgery in the intensive care unit. The primary indication for intensive care is to allow serial (usually hourly) neurologic examinations and rapid response to abnormalities. In addition, postoperative intensive care allows continuous blood pressure monitoring and control, intracranial pressure (ICP) monitoring when required, and treatment of pain and postoperative nausea and vomiting (PONV).

SUMMARY AND RECOMMENDATIONS

●Indications for surgery — Posterior fossa craniotomy may be performed for tumors, vascular lesions, cranial nerve compression syndromes, or congenital malformations that involve the brain stem, cerebellum, or bony structures in the posterior fossa. (See 'Indications for surgery' above.)

●Preoperative evaluation — Preoperative evaluation should focus on cranial nerve deficits. Patients with cranial nerve deficits may be at increased risk of aspiration and may require delayed extubation at the end of surgery. Increased intracranial pressure can result from lesions that compromise flow of cerebrospinal fluid. (See 'Signs and symptoms of posterior fossa pathology' above.)

For procedures that will be performed in the sitting position, we assess for patent foramen ovale or other right to left intracardiac shunt, and overall cardiac function with a preoperative echocardiogram or intraoperative transesophageal echocardiogram, due to the high risk of venous air embolism in the sitting position. (See 'Preoperative assessment and management' above.)

●Anesthetic management specific to posterior fossa surgery

•Patients with posterior fossa pathology may be especially sensitive to the respiratory depressant effects of sedatives and opioids. For patients who require premedication, we titrate to effect using small doses, and for patients with preoperative neurologic deficits, we withhold sedation until the patient is fully monitored and in a setting that allows immediate airway management. (See 'Premedication' above.)

•Monitoring the electrocardiogram can provide indications of manipulation of cranial nerves or the brainstem and allows surgical adjustment. Administration of anticholinergic medication may make the ECG an unreliable indicator, as it prevents or attenuates bradycardia. Monitoring for venous air embolism is indicated for patients who are placed in the sitting position for surgery. Neuromonitoring may be indicated for some posterior fossa procedures and may affect the choice of anesthetics (table 2). (See 'Monitoring' above.)

•Posterior fossa surgery may be performed in the prone, lateral decubitus, or sitting position. (See 'Positioning' above.)

•All patients who undergo posterior fossa surgery should receive prophylaxis for postoperative nausea and vomiting. (See 'Prophylaxis for postoperative nausea and vomiting' above.)

•The plan for extubation at the end of surgery should be discussed with the surgeon. Factors to consider when deciding whether to delay extubation after posterior fossa surgery include preexisting neurologic deficits, extent and location of surgery, and potential airway edema related to positioning, any of which can result in airway compromise after extubation. (See 'Emergence and extubation' above.)

●Postoperative care — Patients who undergo posterior fossa surgery recover in the intensive care unit, to allow frequent neurologic examination, blood pressure control, and intracranial pressure monitoring when indicated. (See 'Postoperative care' above.)