Patient is admitted to the Neurosurgical ICU in preparation for a posterior fossa craniotomy for tumor resection.

Posterior fossa is a small rigid space at the base of the skull that contains the brain stem, cranial nerves, cerebellum, and the 4th ventricle. All cranial nerves originate in the posterior fossa and all basic physiologic activities (e.g., heart rate, respirations, temperature, emetic) are regulated here. CSF circulates from the choroid plexus through the 4th ventricle into the spinal cord. Posterior fossa craniotomy can be challenging for both the neurosurgeon and the anesthesiologist. The surgeon needs good exposure to a small area, approximately 185 cm³, dense in vital structures that are also in close proximity to noncompressible venous sinuses increasing the risk of blood loss. Tumor or vascular surgery can disrupt the physiologic functions of the posterior fossa.

The anesthesiologist is responsible for managing usual physiologic functions (e.g., optimal BP, ventilation, cerebral blood flow, level of anesthesia) and responding to physiologic changes that occur due to surgery in the posterior fossa (e.g., venous air embolism, paradoxical arterial air embolism (AAR), arrhythmias from cardiovascular center or cranial nerve stimulation). Additional serious neurologic complications can arise from the intersection of positioning and preexisting disease like cervical cord stretch leading to quadriplegia, optical nerve compression causing blindness, brachial, or lumbosacral nerve stretch causing permanent palsy. While not an acute complication, recognizing, usually through specialized monitoring, injury to cranial nerves IX to XII is important to safe recovery from the surgical experience. Vigilance and anticipation is critical in the prevention of unsafe situations and complications.

Standard anesthesia evaluation should always be performed. Special attention should be placed on signs and symptoms associated with posterior fossa tumors like hearing loss, difficulty swallowing, or other cranial nerve preexisting injury.

Chronic hypertension with or without treatment, vascular disease, especially carotid vascular disease, may increase the risk of hypotension if the surgeon chooses the sitting position. Presence of carotid or vertebral vascular disease increases concerns about the ability to tolerate extreme head rotation or flexion. Some centers no longer use sitting position due to the increased anesthetic management complexities and patient hypoperfusion risks. Patients with coronary disease may not tolerate the sitting position. Dysrhythmias, tachycardia, bradycardia, conduction abnormalities from brain stem traction occur regardless of position, and in individuals with preexisting arrhythmia the difficulty managing them may be increased.

Posterior fossa tumors can present clinically with evidence of cranial nerve or brain stem deficits (e.g., respiratory irregularities, chronic aspiration, altered consciousness, dysphagia). Knowledge about the presence of respiratory and upper airway dysfunction based on the involvement of cranial nerves IX and X, XI, XII is important when deciding anesthetic management.

The patient scheduled for posterior fossa surgery could be hypovolemic from hypertonic treatment to decrease brain edema or ICP, or from use of diuretics. Hypovolemia could be poorly tolerated in sitting position. Vascular volume may need to be replenished with isotonic crystalloids. The use of albumin remains controversial but it is often administered. The controversy involves its use in traumatic brain injury where it is associated with higher mortality. The literature is silent in its use in other areas. In general, support of BP may require pressors and volume administration may be determined by acid base status.

Central venous access is not routinely necessary except in patients who have limited or very difficult vascular access or the need for a right atrial catheter. Central venous catheter via the internal jugular or subclavian veins are not without risks. Internal jugular vein catheters can partially obstruct venous outflow especially with some head positions or patient attributes (e.g., short thick neck, obesity). Use of ultrasound devices has reduced placement risk. The basilic vein is primarily used for placement of the right atrial catheter. Need for right atrial catheters have decreased with the decreased use of the seated position. When considering an approach or need for a central line, the ability of the patient to tolerate Trendelenburg/head turned position should be considered. Trendelenburg position increases the venous return, and may not be well tolerated by a patient with increased ICP.

Major damage can occur to CN III-XII; neurophysiologic monitoring should be dictated by the location of the tumor. Monitoring often includes brainstem auditory evoked potentials (BAEP), somatosensory evoked potentials (SEP), electromyography (EMG) in the soft palate, tongue, vocal cords, and rarely ocular muscles.

Hemodynamic monitoring should include arterial blood pressure. If VAE or Arterial Air Embolism (AAE) is probable, precordial doppler needs to be added to end-tidal CO₂ measurement to detect right atrial air.