Neurosurgery: Intracranial Tumor Debulking

Cranial Vault Composition

• Composed of three components: brain tissue (80%), blood (12%), and cerebrospinal fluid (CSF) (8%)
• These components together determine intracranial pressure (ICP)

Brain and Blood

• Brain tissue: 1300 grams (3 lb or 2% of total body weight)
• Blood: 15% to 20% of cardiac output, 750 mL/min

Cerebrospinal Fluid (CSF)

• Total volume: 150 mL in cranium and spinal cord
• Volume in cranium at any given time: 75 mL
• Production rate: approximately 400 mL/day by choroid plexuses in lateral ventricles
• Absorption: in the subarachnoid space by arachnoid villi of the venous system

Cerebral Circulation

• Circle of Willis: allows bilateral communication of internal carotid and vertebral artery blood flow

Tumor Locations

• Described as supratentorial or infratentorial

CSF Flow Pathway

• Produced by choroid plexus in lateral ventricles
• Flows through interventricular foramen of Monro and into the third ventricle
• Proceeds from the third ventricle through the aqueduct of Sylvius and into the fourth ventricle
• From the fourth ventricle, flows through the foramen of Magendie and two foramina of Luschka into the cerebellomedullary cistern (cistern magna) and then into the subarachnoid space and spinal column

Cerebral Blood Flow (CBF)

• Average CBF: 50 mL/100 g/min (total 750 ml/min), with a range of 10 to 300 mL/100 g/min
• CBF in gray matter (neuronal bodies): 80 mL/100 g/min
• CBF in white matter (axons): 20 mL/100 g/min
• Alterations in CBF can be either detrimental (decreasing blood flow to ischemic areas) or beneficial (providing more blood and oxygen to ischemic areas)

Factors Affecting CBF

• PaCO2: has a linear relationship with CBF, with a 1 mm Hg increase in PaCO2 resulting in a 1 to 2 mL/100 g/min increase in CBF
• PaO2: has a profound effect on CBF, but only at very low PaO2 levels

Intracranial Tumor Debulking

• Normal ICP is ≤10 mm Hg, but temporary elevation occurs during coughing, Valsalva maneuver, and hypertensive episodes.

ICP Management

• Sustained elevation of ICP >15 mm Hg decreases CPP and increases the risk of cerebral ischemia.
• Severely increased ICP may lead to brainstem herniation through the foramen magnum.
• Intracranial tumors are space-occupying lesions that may increase ICP, depending on their size and location.

Anesthetic Concerns

• Preventing increases in ICP is a primary anesthetic concern.

Decreasing ICP

• Hyperventilation decreases CBF by causing cerebral vascular vasoconstriction, which decreases CBF.
• Decreasing cerebral blood volume (CBV) decreases ICP.
• Diuretics decrease brain tissue water content, thereby decreasing ICP.
• Ventriculostomy and intrathecal catheters allow CSF to drain, which decreases ICP.

Glioblastomas

• Glioblastomas are tumors that rapidly expand and arise from white or gray matter usually in the frontal or temporal regions of the brain.
• These tumors are often surrounded by inflammatory and necrotic tissue.

Symptoms and Diagnosis

• Glioblastomas can become large rapidly before the patient develops significant symptoms.
• Tumors that infiltrate or displace the ventricles may cause obstructive hydrocephalus.

Treatment

• Treatment includes tumor debulking, CSF diversion, chemotherapy, and radiation to the affected site.
• Despite these treatments, survival remains low.

Pathophysiology of ICP

• Intracranial tumors may not cause serious symptoms initially because of compensatory physiologic mechanisms that help maintain normal ICP.
• Glioblastomas often develop rapidly and cause increases in ICP.

Cushing Reflex

• When CPP requirements exceed the arterial pressure, the hypothalamic sympathetic reflex increases blood pressure to restore CBF.
• The increased blood pressure stimulates carotid bodies, which lowers the heart rate by initiating the Cushing reflex.
• The Cushing reflex is one of the body's most potent physiologic responses that, when stimulated, dramatically increases sympathetic nervous system predominance.

Cardiovascular Response

• The cardiovascular response includes hypertension and bradycardia.
• Bradycardia is the result of baroreceptor stimulation in response to increased systemic vascular resistance.

Cushing Triad

• The Cushing triad includes hypertension, bradycardia, and irregular respirations, reflecting severe increases in ICP and severe cerebral ischemia.
• Up to 33% of patients with elevated ICP display all three components of the Cushing triad.
• Impending herniation of the brainstem down through the foramen magnum can occur if ICP continues to increase.

Intracranial Tumor Debulking

• The surgical procedure for intracranial tumor debulking involves an open craniotomy using a parietal approach with the patient in the lateral position.

Surgical Objectives

• The primary objective is to completely remove the tumor to prevent regrowth of tumor remnants.
• Intraoperative biopsy is performed to confirm and identify the tumor.
• Tumor boundaries (margins) are biopsied to assess the absence of tumor tissues before surgical closure.

Supratentorial and Infratentorial Tumors

• Supratentorial tumors occupy the area of the midbrain and cerebral cortex.
• Infratentorial tumors occupy the area of the vital centers of the cerebellum and brainstem.
• The location of an intracranial tumor necessitates specific positioning for neurosurgical access.

Positioning Risks

• Sitting position: Increased risk of venous air emboli (VAE).
• Lateral oblique position: Risk of brachial plexus injury.
• Prone position: Risk of postoperative vision loss (POVL), especially in diabetic patients, associated with lengthy prone position cases and intraoperative hypotension.
• Supine position: May have head and neck rotation or extension; may have cranial pinning/fixation in head tongs.

Anesthetic Management and Considerations for Intracranial Tumor Debulking

• Preoperative goals include maintaining or achieving normal ICP and CPP (minimum 70 mmHg) to optimize cerebral circulation and oxygenation.
• Maintenance of adequate CPP limits ischemia around the tumor and during intraoperative brain retraction.
• Midazolam 0.025 to 0.05 mg/kg IV may be given to attenuate increases in blood pressure and ICP related to anxiety.
• Preoperative interventions include placing at least one large-bore IV access preoperatively and another after induction.
• Avoid narcotics preoperatively as they depress respiratory function and raise PCO, causing cerebral vessel dilation and corresponding increases in ICP.
• Administer antibiotics and corticosteroids (dexamethasone 4 to 10 mg IV) per surgeon preference before induction.

Signs and Symptoms of Elevated ICP

• Symptoms progress on a continuum from mild to severe and include headache, difficulty concentrating, memory disturbances, vision disturbances, vertigo, syncope, nausea, vomiting, severe headache, the Cushing reflex, Cushing's triad, seizures, or coma.
• Not all intracranial tumors cause increases in ICP; small tumors and larger, slow-growing tumors associated with cerebral physiologic compensation may present with delayed increases in ICP.

Location of Intracranial Tumors

• The most prevalent location of intracranial tumors is supratentorial, involving the cerebral hemispheres.
• The cerebral hemispheres are divided by the medial longitudinal fissure and composed of the frontal, parietal, temporal, and occipital lobes.
• Infratentorial location refers to the brainstem, consisting of the midbrain, medulla, cerebellum, and pons, containing major motor and sensory pathways and cranial nerve nuclei.

Influence of Tumor Location on Patient Positioning

• Infratentorial tumors usually require a prone or lateral position for surgical access.
• Supratentorial tumors are often resected while the patient is positioned supine or lateral.
• Lateral or semi-lateral positions may use foam supports or a bean bag vacuum mattresses to support the patient.
• The sitting position is associated with an increased risk of VAE and excessive neck flexion, impeding venous drainage from the head.
• Mild head elevation (reverse Trendelenburg) may be done with any surgical position.
• The patient's head may be held by a horseshoe-shaped support, a foam support, or pinned in tongs, fixing the skull to a support frame.

Anesthetic Concerns during Induction for a Patient with Elevated ICP

• Primary concerns during induction: hypoxia, hypercarbia, hypertension, and hypotension
• Smooth induction with hyperventilation and proper mask ventilation is recommended
• Rapid sequence induction may be indicated for patients with severe GERD
• Continuous blood pressure monitoring is important, and placement of an arterial line may be beneficial
• Induction agents may depress cardiac function and cause hypotension, while laryngoscopy can cause sympathetic stimulation and hypertension

Primary Intraoperative Goals of Anesthesia Care for the Patient with Elevated ICP

• Decreasing intracranial volume to prevent increases in ICP
• Maintaining adequate cerebral perfusion pressure (CPP) by manipulating blood pressure
• Decreasing cerebral metabolic rate of oxygen (CMRO2)

Methods to Decrease CMRO2

• IV agents: propofol, etomidate, and midazolam lower CMRO2 and cerebral blood flow (CBF)
• Vasodilatory anesthetics (VAAs): decrease CMRO2, increase CBF, and uncouple the relationship between CMRO2 and CBF
• Temperature: mild decrease in body temperature (1°C decrease corresponds to 7% decrease in CMRO2)

Coupling and Uncoupling of CMRO2 and CBF

• Coupling: direct relationship between CMRO2 and CBF, where an increase in one corresponds to an increase in the other
• Uncoupling: disruption of the direct relationship between CMRO2 and CBF, caused by VAAs, which decrease CMRO2 while increasing CBF

Methods to Lower ICP

• Decreasing brain volume: diuretics (furosemide, mannitol) and hypertonic saline administration
• Decreasing blood volume: hyperventilation to decrease PCO2 and cause cerebral vasoconstriction
• Decreasing CSF volume: not mentioned in the passage

Rationale for Each Method to Decrease Intracranial Volume

• Brain: diuretics (furosemide, mannitol) and hypertonic saline administration to decrease brain volume
• Blood: hyperventilation to decrease intracranial blood volume

Postoperative Period for Intracranial Tumor Debulking

• Goals of the recovery phase of anesthesia include pain control and maintenance of adequate blood pressure.
• Postoperative monitoring involves observing for changes in mental status or signs and symptoms that may indicate an intracranial event, such as: • Increased ICP (Intracranial Pressure) • Vasospasm • Hemorrhage
• If the patient is extubated, close monitoring is essential to detect any potential complications.
• If the patient remains intubated, sedation using central nervous system depressant medications is necessary.