16.3 Neuroanaesthesia: Craniotomy, Sitting Position, and Traumatic Brain Injury

16.3 Neuroanaesthesia: Craniotomy, Sitting Position, and Traumatic Brain Injury

Neuroanaesthesia centers on a primary mandate: providing an immotile, "slack" operative field while preserving cerebral perfusion and preventing secondary neurological injury. The adult cranium is a rigid, non-distensible vault, making intracranial dynamics exceptionally sensitive to physiological perturbations.

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1. Principles of Neuroanaesthesia and Secondary Brain Injury

Intracranial compliance and cerebral blood flow regulation govern intraoperative cerebral stability.

Cerebral Perfusion Pressure and Autoregulation

Cerebral Perfusion Pressure (CPP) is the effective pressure gradient driving cerebral blood flow (CBF) across the cerebral capillary bed: $CPP = MAP - ICP \quad (\text{or } MAP - CVP, \text{whichever is higher})$

• Normal Physiological Values: Normal ICP is 5 to 15 mmHg in supine adults. Intracranial hypertension is defined as sustained $\text{ICP} > 20\text{ to }22\text{ mmHg}$ requiring active clinical intervention.
• Target CPP: In both elective neurosurgery and traumatic brain injury, target CPP is maintained between 60 and 70 mmHg. CPP $<60\text{ mmHg}$ induces focal or global cerebral ischemia, whereas excessive CPP ($>70\text{--}80\text{ mmHg}$) risks vasogenic edema, disruption of the blood-brain barrier (BBB), and acute lung injury.
• Cerebral Autoregulation: In the intact brain, CBF remains constant (~$50\text{ mL/100g/min}$) across a Mean Arterial Pressure (MAP) range of 50 to 150 mmHg (or CPP of 60 to 140 mmHg). Beyond these limits, CBF becomes pressure-passive. In acute traumatic brain injury or surrounding intracranial tumours, autoregulation is frequently impaired or completely abolished, rendering the brain exceptionally vulnerable to ischemic injury from even modest drops in blood pressure.

Avoidance of Secondary Brain Injury (The "H-Bombs" of Neuroanaesthesia)

Primary mechanical damage (contusion, laceration, hematoma) occurs at the time of impact. Secondary brain injury develops over subsequent hours to days via cellular, metabolic, and inflammatory cascades. Anaesthetic management is aimed at preventing systemic physiological triggers that exacerbate secondary injury:

1. Hypoxia ($P_a\text{O}_2 < 60\text{ mmHg}$ or $Sp\text{O}_2 < 90\%$): Triggers immediate intracellular energy failure, ATP exhaustion, failure of $Na^+/K^+$ ATPase pumps, cytotoxic edema, and doubles perioperative mortality.
2. Hypotension ($SBP < 100-110\text{ mmHg}$ or $MAP < 80\text{ mmHg}$): In the injured or non-compliant brain, drops in perfusion pressure directly compromise cerebral blood flow in ischemic penumbra regions.
3. Hyperthermia (Core Temp $>37.5\text{°C}$): Every 1°C increase in core temperature elevates the Cerebral Metabolic Rate of Oxygen ($CMRO_2$) by ~7%, accelerating neurotransmitter toxicity (glutamate excitotoxicity) and lipid peroxidation. Active normothermia (36.0°C-37.0°C) must be maintained.
4. Hyperglycemia (Blood Glucose $>10\text{ mmol/L}$ / $>180\text{ mg/dL}$): Accelerates anaerobic metabolism in ischemic neurons, precipitating intracellular lactic acidosis, reactive oxygen species formation, and disruption of the blood-brain barrier.
5. Extreme Hypo/Hypercapnia:
   • Hypercapnia ($P_a\text{CO}_2 > 45\text{ mmHg}$): Induces marked cerebral arteriolar vasodilation, expanding Cerebral Blood Volume (CBV) and causing acute spikes in ICP.
   • Severe Hypocapnia ($P_a\text{CO}_2 < 28-30\text{ mmHg}$): Triggers extreme cerebral arteriolar vasoconstriction, shifts the oxyhemoglobin dissociation curve to the left (Bohr effect, impairing oxygen unloading), and induces severe focal cerebral ischemia.

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2. The Monro-Kellie Doctrine and ICP Reduction Strategies

The Monro-Kellie Hypothesis dictates that the rigid adult cranial cavity has a fixed internal volume ($V_{\text{total}} \approx 1400\text{--}1700\text{ mL}$), partitioned into three mutually incompressible compartments: $V_{\text{total}} = V_{\text{brain}} (80\%) + V_{\text{blood}} (10\%) + V_{\text{CSF}} (10\%) = \text{Constant}$ An increase in the volume of any one compartment (or the introduction of a pathological mass, e.g. tumour, hematoma, abscess) must be compensated by an equal reduction in the volume of the other compartments. Initial compensation occurs via displacement of CSF into the spinal subarachnoid space and translocation of venous blood into the internal jugular veins. Once these compensatory spatial reserves are exhausted, the intracranial compliance curve steepens exponentially: tiny increases in volume produce massive, life-threatening spikes in ICP, culminating in fatal transtentorial or tonsillar herniation.

   ICP
    ^
    |                                           / (Decompensated Phase)
    |                                          /
    |                                         /   Exponential rise in ICP
    |                                        /
    |                     (Compensated      /
    |                       Phase)         /
    |                  +------------------+
    |                  | Displacement of  |
    |                  | CSF and venous   |
    |                  | blood            |
    |   _______________+                  |
    +--------------------------------------------------------> Intracranial Mass Volume

Therapeutic Modalities to Modulate Intracranial Compartments

Osmotherapy: Mannitol vs Hypertonic Saline

• Mannitol 20% (0.5 to 1.0 g/kg IV over 15-20 minutes):
  • Biphasic Action: First, an immediate rheological effect: expands plasma volume, dilutes hematocrit, lowers blood viscosity, and improves microcirculatory capillary flow, reflexively triggering cerebral autoregulatory vasoconstriction that reduces CBV within minutes. Second, an osmotic effect (onset 15-30 min, duration 4-6 hours): the intact BBB (reflection coefficient $\sigma = 0.9$) prevents mannitol entry, establishing an osmotic gradient that dehydrates brain parenchyma. Excreted unchanged in urine via osmotic diuresis.
  • Hazards: Initial transient volume expansion can precipitate acute pulmonary edema in heart failure. Subsequent intense osmotic diuresis causes hypovolemia, hypotension, hypokalemia, and hypernatremia. If serum osmolarity exceeds 320 mOsm/kg, severe hyperosmolar acute tubular injury occurs.
• Hypertonic Saline (HTS 3%, 7.5%, or 23.4%; typically 3% NaCl at 2 to 3 mL/kg or 150-250 mL bolus):
  • Mechanism: Highest reflection coefficient ($\sigma = 1.0$) across the BBB, meaning the intact membrane is completely impermeable to sodium ions. Superior osmotic pull per milliliter compared to mannitol. Additionally expands intravascular volume, increases MAP, bolsters CPP, restores neuronal resting membrane potentials, and modulates neuroinflammation.
  • Key Advantage: Does not induce mandatory osmotic diuresis; ideal in polytrauma patients presenting with hypovolemia or hemodynamic instability. Target serum sodium maintained at $\le 155\text{ mmol/L}$; target serum osmolarity $<320\text{ mOsm/kg}$.

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3. Neuroanaesthesia for Craniotomy and Awake Brain Mapping

Anaesthesia for elective supratentorial craniotomy must facilitate smooth induction, total immobility, blunting of extreme sympathetic reflexes during skull-pin application (Mayfield head clamp), a relaxed brain, and rapid, cough-free emergence to permit immediate neurological examination.

Awake Craniotomy for Eloquent Cortex Mapping

Indicated for resecting intra-axial tumours (e.g. low-grade gliomas) or epileptogenic foci adjacent to eloquent cortex (Broca's speech area in inferior frontal gyrus, Wernicke's receptive language area in superior temporal gyrus, primary motor cortex in precentral gyrus, or sensory cortex in postcentral gyrus).

• Anaesthetic Techniques:
  • Asleep-Awake-Asleep (AAA): Patient is placed under general anaesthesia with a supraglottic airway (LMA) or endotracheal tube during skull pinning, incision, and bone flap removal. Anaesthesia is discontinued, the airway device removed, and the patient awakened fully for direct cortical electrical stimulation and continuous language/motor testing. Following mapping and resection, the patient is re-anesthetized and the airway re-secured for closure.
  • Monitored Anaesthesia Care (MAC) / Conscious Sedation: Patient remains spontaneously breathing throughout the procedure. Target-Controlled Infusion (TCI) of Dexmedetomidine (loading 0.5-1.0 $\mu\text{g/kg}$, maintenance 0.2-0.7 $\mu\text{g/kg/hr}$) combined with low-dose remifentanil is the modern technique of choice. Dexmedetomidine provides cooperative sedation, anxiolysis, and analgesia without respiratory depression or blunting intraoperative electrocorticography (ECoG).

The Scalp Block: Regional Innervation of the Cranium

A flawless bilateral scalp block is essential for awake craniotomy and highly advantageous for all craniotomies to ablate the hemodynamic surge of skull-pin fixation. The scalp is innervated by six distinct nerves bilaterally (12 total injections):

                 [ REGIONAL INNERVATION OF THE SCALP ]

                                  Vertex
                                    |
          +-------------------------+-------------------------+
          |                                                   |
   ANTERIOR SCALP                                      POSTERIOR SCALP
(Trigeminal Nerve, CN V)                            (Cervical Spinal Nerves)
          |                                                   |
   - CN V1 (Ophthalmic):                               - C2-C3 Ventral Rami:
     * Supraorbital Nerve                                * Lesser Occipital Nerve
     * Supratrochlear Nerve                              (retroauricular groove)
   - CN V2 (Maxillary):                                - C2 Dorsal Ramus:
     * Zygomaticotemporal Nerve                          * Greater Occipital Nerve
   - CN V3 (Mandibular):                                 (medial to occipital artery)
     * Auriculotemporal Nerve                          - C3 Dorsal Ramus:
       (anterior to tragus)                              * Third Occipital Nerve

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4. The Sitting Position for Posterior Fossa Surgery and Venous Air Embolism

Neurosurgical procedures in the sitting (park-bench or upright) position are utilized for lesions of the posterior cranial fossa, cerebellopontine angle (acoustic neuroma), pineal region, and upper cervical spine.

Clinical Trade-Offs of the Sitting Position

• Advantages: Unrivaled surgical access and visibility; gravity-assisted drainage of blood and CSF away from the microscopic field; reduced intraoperative blood loss; lower intracranial venous pressure; preservation of lower cranial nerves.
• Disadvantages and Physiological Hazards:
  • Profound Postural Hypotension: Severe venous pooling in lower limbs and splanchnic circulation drops venous return, stroke volume, and MAP. Transducers must be zeroed at the external auditory meatus (level of the Circle of Willis); hydrostatic difference means arterial pressure at the brain is 15 to 20 mmHg lower than pressure measured at heart level.
  • Mid-Cervical Quadriplegia & Cord Ischemia: Severe hyperflexion of the neck can occlude the anterior spinal artery or compress the upper cervical spinal cord against the vertebral column. Ensure at least two fingerbreadths between the chin and sternum.
  • Macroglossia and Upper Airway Obstruction: Extreme neck flexion obstructs lingual venous and lymphatic drainage, causing massive tongue swelling postoperatively.
  • Venous Air Embolism (VAE): The preeminent life-threatening hazard.

Venous Air Embolism (VAE) Pathophysiology

When the operative site is elevated above the level of the right atrium, the hydrostatic pressure within dural venous sinuses drops below atmospheric pressure ($P_{\text{venous}} < P_{\text{atmosphere}}$). The dural venous sinuses are tethered to the rigid cranial periosteum and cannot collapse. When incised or torn by the surgeon, air is continuously sucked into the venous circulation.

• Consequences: Entrained air travels via the internal jugular veins to the right atrium, right ventricle, and pulmonary arterial circulation. Microvascular bubbles trigger acute pulmonary vasoconstriction, bronchoconstriction, pulmonary hypertension, and acute right ventricular failure. Large air volumes form an "air lock" in the right ventricular outflow tract (RVOT), causing instantaneous circulatory arrest.
• Paradoxical Air Embolism (PAE): In patients with a Patent Foramen Ovale (PFO) (present in 25-30% of the general population), elevated right atrial pressure from pulmonary embolization forces air right-to-left across the septum into the systemic circulation, causing immediate catastrophic stroke or coronary air embolism. Preoperative echocardiography with an agitated saline contrast (bubble) study is recommended to screen for a PFO before elective sitting-position surgery.

Diagnostic Sensitivity Ranking for VAE

  [ SENSITIVITY SPECTRUM FOR DETECTING VENOUS AIR EMBOLISM ]

  HIGHEST SENSITIVITY (Detects 0.02 mL/kg air) = Transesophageal Echocardiography (TEE)
       |
       v
  Precordial Doppler Ultrasound (Detects 0.05 mL/kg; roaring/mill-wheel sound)
       |
       v
  End-Tidal Carbon Dioxide (Sharp drop in ETCO2 due to alveolar dead space)
       |
       v
  End-Tidal Nitrogen (ETN2 rise; reflects expired nitrogen gas)
       |
       v
  Pulmonary Artery Catheter (Acute spike in PAP)
       |
       v
  LOWEST SENSITIVITY (Late/Catastrophic Signs) = Mill-wheel murmur on stethoscope,
                                                hypotension, arrhythmias, cardiac arrest

1. Transesophageal Echocardiography (TEE): The undisputed gold standard and most sensitive diagnostic monitor. Detects microbubbles as small as 0.02 mL/kg in the right atrium or entering the left atrium via a PFO.
2. Precordial Doppler Ultrasound: Non-invasive, highly sensitive (detects 0.05 mL/kg). Positioned over the right sternal border between the 3rd and 6th intercostal spaces. Detects turbulent air bubbles as a characteristic harsh, churning sound (the "roaring" or "mill-wheel" Doppler signal) long before hemodynamic changes occur.
3. Capnography ($ET\text{CO}_2$): Rapid, sharp decline in end-tidal $CO_2$ with a concomitant rise in $P_a\text{CO}_2$. Pulmonary microvascular air embolization occludes capillary beds, creating a massive increase in alveolar dead space (ventilated but unperfused alveoli).
4. End-Tidal Nitrogen ($ET\text{N}_2$): Detects exhaled nitrogen gas as entrained atmospheric air is cleared across the alveolar membrane (specific, but requires absence of background nitrous oxide).
5. Precordial Stethoscope: Detects the classical "mill-wheel murmur" (a loud, churning mechanical sound over the precordium). This is a late sign of massive air-lock and hemodynamic collapse.

Emergency Management Protocol for VAE

If air entrainment is detected, the team must immediately execute the following multi-step algorithm:

1. Notify the surgical team immediately: Surgeon must instantly flood the operative field with sterile normal saline and pack bone edges with bone wax and dural margins with wet cottonoids to stop air suction.
2. Discontinue Nitrous Oxide ($N_2O$) and administer 100% $O_2$: $N_2O$ diffuses rapidly into closed air bubbles, expanding their volume up to three-fold and drastically accelerating vascular occlusion.
3. Compress bilateral internal jugular veins: Compressing the jugular veins transiently raises cranial venous pressure above atmospheric, converting air entrainment into visible retrograde venous bleeding, which allows the surgeon to identify and coagulate the open vessel.
4. Aspirate air from the right atrium: Using a pre-placed multi-orifice right atrial catheter (e.g. Bunegin-Albin catheter, with its tip positioned 1 to 2 cm below the cavoatrial junction in the high right atrium), apply continuous syringe suction to aspirate foam and air bubbles.
5. Position adjustment: If feasible, lower the operative table to level the head, or position the patient in the left lateral decubitus position with Trendelenburg (Durant maneuver) to trap air at the right ventricular apex, relieving RVOT obstruction.
6. Hemodynamic support: Administer rapid IV crystalloid boluses, inotropes (epinephrine), and vasopressors to sustain perfusion; initiate external chest compressions if cardiac arrest supervenes (compressions help crush large air locks into smaller microbubbles that pass through the pulmonary capillary bed).

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5. Traumatic Brain Injury (TBI) and Brain Trauma Foundation Guidelines

Traumatic brain injury is a major cause of death and disability globally. Contemporary management focuses on secondary insult prevention, adherence to Brain Trauma Foundation (BTF 4th Edition) protocols, and timely surgical evacuation of mass lesions (extradural hematoma, acute subdural hematoma, depressed skull fracture).

                  [ BRAIN TRAUMA FOUNDATION (BTF) PROTOCOLS ]
                                     |
   +------------------+------------------+------------------+------------------+
   |                  |                  |                  |                  |
[ Blood Pressure ]   [ ICP Threshold ]   [ Target CPP ]    [ Ventilation ]    [ Contraindicated ]
- SBP >= 100 mmHg    - Treat if          - Target:         - Normocapnia:     - NO Steroids
  (age 50-69)          ICP > 22 mmHg       60-70 mmHg        PaCO2 35-40        (CRASH Trial)
- SBP >= 110 mmHg    - Place EVD or      - Avoid CPP       - NO Prophylactic  - NO Prophylactic
  (age 15-49, >70)     intraparenchymal    < 60 or > 70      Hyperventilation   Hypothermia

Evidence-Based BTF Clinical Management Targets

• Blood Pressure Targets:
  • Maintain $SBP \ge 100\text{ mmHg}$ for patients aged 50 to 69 years.
  • Maintain $SBP \ge 110\text{ mmHg}$ for patients aged 15 to 49 years or $>70$ years.
  • Rationale: A single episode of intraoperative or pre-hospital hypotension ($SBP < 90\text{ mmHg}$) is associated with a doubling of mortality.
• Intracranial Pressure (ICP) Monitoring Indications:
  • The BTF 4th edition recommends ICP monitoring in severe TBI to reduce in-hospital and 2-week mortality (Level IIB). Its indications are carried over from the 3rd edition (not supported by evidence meeting current standards): all salvageable patients with severe TBI (GCS 3 to 8 after resuscitation) and an abnormal admission CT scan (showing hematomas, contusions, swelling, herniation, or compressed basal cisterns).
  • Also indicated in severe TBI with a normal CT scan if two or more of the following risk factors are present: age $>40$ years, unilateral or bilateral motor posturing (decerebrate/decorticate), or systolic blood pressure $<90\text{ mmHg}$.
  • ICP Intervention Threshold: Initiate medical and surgical ICP-lowering therapies when sustained $\text{ICP} > 22\text{ mmHg}$. The 4th edition (Level IIB) recommends treatment above 22 mmHg because higher values are associated with increased mortality.
• Target Cerebral Perfusion Pressure (CPP):
  • Maintain target CPP strictly between 60 and 70 mmHg.
  • Avoid deliberate aggressive driving of CPP $>70\text{ mmHg}$ using vasopressors and high fluids, which significantly increases the incidence of Acute Respiratory Distress Syndrome (ARDS) via capillary leak without improving neurological outcome.
• Ventilatory Management:
  • Maintain strict normocapnia ($P_a\text{CO}_2 35\text{ to }40\text{ mmHg}$).
  • Prophylactic hyperventilation ($P_a\text{CO}_2 \le 25\text{ mmHg}$) is strictly avoided during the first 24 hours post-injury. During the initial 24 hours, global cerebral blood flow is naturally halved; severe hypocapnia-induced vasoconstriction precipitates catastrophic secondary cerebral ischemia. Hyperventilation is reserved strictly as a brief, emergency bridge ($P_a\text{CO}_2 30-35\text{ mmHg}$) for acute neurological deterioration or impending herniation while preparing for definitive decompression.
• Corticosteroids: Strictly contraindicated. The landmark CRASH trial (Corticosteroid Randomization After Significant Head Injury; $>10{,}000$ patients) demonstrated that high-dose methylprednisolone administration in acute TBI significantly increased 14-day and 6-month mortality and is a Class I harm recommendation.
• Seizure Prophylaxis: Post-traumatic seizure prophylaxis with Levetiracetam (Keppra) or Phenytoin is recommended for the first 7 days following severe TBI to reduce the incidence of early post-traumatic seizures. Anticonvulsants do not prevent late post-traumatic epilepsy and should be discontinued after day 7 unless clinical seizures occur.
• Sedation and Neuromuscular Blockade: Propofol or midazolam infusions titrated to control ICP, coupled with opioid analgesia (fentanyl/remifentanil). Routine continuous neuromuscular blockade is not recommended unless refractory shivering, coughing, or ICP spikes occur despite deep sedation.