Anesthesia 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.