Brainstem Manipulation During Surgery

Physiologic Considerations and Brain Stem Reflexes

• Autonomic reflexes are a primary concern during brainstem manipulation, particularly the trigeminocardiac reflex and the glossopharyngeal-vagal reflex. Activation of these reflexes can trigger a parasympathetic surge via the vagus nerve, leading to bradycardia, atrioventricular block, hypotension, or even asystole.
• Airway-related brainstem reflexes may also be triggered, particularly when manipulating regions near cranial nerves IX and X. Activation of the laryngeal adductor reflex and cough reflex can lead to laryngospasm, coughing, or apnea.
• Respiratory pattern changes may occur if medullary centers are transiently stimulated or suppressed. These are rare under general anesthesia due to controlled ventilation, but may manifest as apnea during awake or lightly sedated testing.
• Cushing’s reflex (hypertension with reflex bradycardia) may indicate rising intracranial pressure or brainstem compression. Afferent = baroreceptor stretch via CN IX → nucleus solitarius. Efferent = increased vagal output via CN X → bradycardia.
• Factors influencing the severity of autonomic reflex stimulation include anesthetic depth, local anesthetic infiltration, anticholinergic use, and timely communication between anesthesia and surgical teams.^1,3

Preoperative Assessment

• Preoperative evaluation of patients should focus on any preexisting neurologic deficits that could affect anesthetic management, such as patients presenting with dysphagia, hoarseness, or loss of gag reflex, and respiratory impairment or cardiovascular instability.
• Brainstem mapping is typically performed preoperatively using magnetic resonance imaging (MRI), functional MRI, and advanced methods such as diffusion tensor imaging (Figure 1, source link), which can guide the surgical course and help predict intraoperative complications.

• In addition to standard American Society of Anesthesiologists monitors, invasive arterial blood pressure monitoring is recommended. The transducer should be zeroed at the level of the circle of Willis to ensure accurate cerebral perfusion assessment.
• Central venous access (in the right atrium/superior vena cava) allows for monitoring of central venous pressure (CVP), administration of vasoactive medications, and aspiration of air during possible VAE.
• Precordial Doppler ultrasound and transesophageal echocardiography are employed in high-risk cases to monitor for VAE.²
• Positioning considerations and padding are integral to the anesthetic plan.^2,4
• The sitting position may be used for midline posterior fossa surgeries to enhance exposure via gravity-assisted brain relaxation and venous drainage, but it increases VAE risk and may cause hypotension due to reduced venous return. Pad sacrum, heels, and elbows, support arms securely, use compression devices or a G-suit to reduce venous pooling and VAE risk.
• The lateral oblique position is also used for cerebellopontine angle tumors. There is a lower risk of VAE in this position than in the sitting position. Pad the dependent shoulder, arm, and leg, use an axillary roll to protect the brachial plexus, and support nondependent limbs to avoid nerve compression.
• The prone position has the lowest VAE incidence, but can be challenging as airway access is limited, and there is a higher risk of airway edema and ocular pressure injuries. Protect eyes and face with a head cradle or Mayfield clamp; use chest rolls to offload the abdomen; pad arms, knees, and feet to prevent nerve injury.
• Positioning is ultimately chosen by the surgical team based on mapping and surgeon preference and must be adapted by the anesthetic team.
• Mayfield clamp application may cause an acute sympathetic surge and hypertension. Use short-acting agents to blunt responses and avoid large hemodynamic swings.
• Intraoperative neuromonitoring is highly recommended to help prevent neural injury.¹
• Brainstem auditory evoked potentials are used to monitor the integrity of the auditory pathway and lower brainstem (Figure 2, source link).

• Somatosensory evoked potentials and motor evoked potentials are used to monitor the dorsal column and corticospinal tracts.
• Electromyography (EMG) of cranial muscles to monitor cranial nerve nuclei and nerve roots.
• Please see the two OA summaries on evoked potentials for more details (Link) and (Link)
• Total intravenous anesthesia with a propofol infusion, often combined with a short-acting opioid, is frequently used to maintain anesthesia.¹
• The primary aim is for hemodynamic stability and potent enough anesthesia to prevent movement or awareness, while still permitting reliable neuromonitoring. For this reason, usage of muscle relaxants and volatile inhaled anesthetics are minimized.

Intraoperative Management

• Primary goals intraoperatively are to provide optimal surgical conditions, protect the brainstem and other neural structures, maintain adequate cerebral perfusion, and promptly manage any reflex disturbances.
• Hemodynamic goals: Ensure the MAP is sufficient to perfuse the brainstem, often targeting a MAP in the high normal range (80-90 mmHg) unless contraindicated by patient factors.⁵
  • Communication with the surgical team is critical, as sudden hypertension or tachycardia can indicate a noxious stimulus. Sudden bradycardia often indicates a brainstem stretch or traction.
  • Treat episodes of bradycardia or hypotension by first having the surgical team reduce any traction or pressure on brainstem structures, and then by administering short-acting vasopressors, such as ephedrine and phenylephrine, if hemodynamic changes persist.
  • Prevent extreme hypertension by deepening anesthetic levels or giving short-acting vasoactive drugs (e.g., esmolol or nicardipine).
• Ventilation and oxygenation: A slightly lower PaCO₂ (30-35 mmHg) should be targeted to cause mild vasoconstriction and modulate intracranial pressure.⁵
  • One hundred percent oxygen is administered any time a VAE is suspected to maximize oxygen delivery and help shrink intravascular air bubbles.
  • Use positive end-expiratory pressure (PEEP) carefully, as PEEP can increase intrathoracic pressure and impede venous return from the brain.²
• Fluid management: Aim for a euvolemic state. Adequate intravascular volume helps maintain blood pressure after positioning and reduces the likelihood of VAE.
  • Monitor urine output, arterial lactate, stroke volume variations, trends, and CVP trends to guide fluid therapy.
  • Balanced crystalloids are often used to maintain plasma sodium and acid/base balance within normal ranges.²
• Temperature management: The brainstem is involved in central thermoregulation. Extremes of body temperature can adversely affect both the patient and the neuromonitoring signals.
  • Use active warming devices to keep the patient normothermic (unless hypothermia is intentionally used for neuroprotection in select cases)
• VAE management: If a VAE is detected by Doppler (“mill-wheel” murmur), sudden decrease in EtCO₂, or unexplained hypotension or hypoxia, the surgeon must be notified to flood the surgical field with saline.
  • Immediately halt any dural opening or venous dissection. Administer 100% O₂, discontinue nitrous oxide if it was in use, and perform aspiration via the central venous catheter to withdraw air.
  • Other supportive measures include jugular vein compression and positioning the patient in left lateral decubitus and Trendelenburg if feasible.
  • Most VAEs are detected early and managed before catastrophic outcomes (literature reports a mortality rate around 1% from VAE in the sitting position) when proper precautions are in place.²

Emergence and Postoperative Considerations

• Coughing or bucking on the endotracheal tube during wake-up is a primary concern, as it can increase intracranial pressure, increase MAP, and increase the risk of cerebral hemorrhage.
• Edema of the airway can be an issue due to the lengthy prone or sitting positioning and should be assessed. Consider an endotracheal tube leak test prior to extubation.
• Any intraoperative indication of lower cranial nerve pathology (e.g., IX, X, or XII) is high risk for postoperative ventilatory failure or aspiration.
• Postoperative intubation and mechanical ventilation are commonly indicated until a neurologic exam can be conducted.
• Criteria favoring extubation: intact gag reflex, stable respiratory pattern, able to follow commands appropriately, and no significant surgical area swelling.³
• Pain Control: Posterior fossa and brainstem surgeries typically have minimal pain at the incision and can be managed with non-opioid analgesics.
  • Excessive sedation from opioids and benzodiazepines should be avoided, as it could mask the neurologic exam.
  • Short-acting analgesics (e.g., fentanyl) titrated to effect are preferred, and regional anesthesia (e.g., scalp blocks with local anesthetic) can be used for craniotomy incision pain without systemic side effects.³