6.2 Neurological Assessment in Critical Illness

Pathophysiology of Intracranial Hypertension

In neurocritical care, managing the patient with a severe traumatic brain injury (TBI), intracranial hemorrhage, or ischemic stroke requires a precise understanding of cerebral hemodynamics.

The Monro-Kellie Doctrine

The Monro-Kellie doctrine states that the cranial vault is a rigid, non-yielding sphere with a fixed volume. This volume is occupied by three components:

1. Brain parenchyma (~80%)
2. Intravascular blood (~10%)
3. Cerebrospinal fluid (CSF) (~10%) Under normal conditions, a state of dynamic equilibrium exists. If the volume of any one component increases (e.g., cerebral edema, tumor, or hematoma), the volume of the others must decrease to maintain normal pressure. Once these compensatory mechanisms are exhausted (e.g., CSF displacement into the spinal canal), any further volume increase causes a rapid, exponential rise in Intracranial Pressure (ICP).

Cerebral Perfusion Pressure (CPP)

Cerebral Perfusion Pressure (CPP) represents the net pressure gradient driving blood flow to the brain, ensuring adequate oxygen and nutrient delivery.

• Formula: $CPP = MAP - ICP$ where MAP is Mean Arterial Pressure and ICP is Intracranial Pressure.
• Clinical Targets:
  • Normal ICP: 5 to 15 mmHg.
  • ICP Treatment Threshold: Sustained ICP > 20 mmHg requires immediate intervention.
  • Target CPP: Maintain between 60 and 70 mmHg.
• Clinical Trap (Perfusion vs. Pressure): If a patient's ICP rises to 25 mmHg and their MAP is 75 mmHg, the CPP is 50 mmHg, which is insufficient and leads to cerebral ischemia. The clinician must not only work to lower the ICP but must also raise the MAP (typically using a vasopressor like norepinephrine) to at least 85 to 95 mmHg to restore the CPP to the target range of 60-70 mmHg.

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Clinical Neurological Assessment

The Glasgow Coma Scale (GCS)

The GCS is a standardized tool used to assess a patient's level of consciousness and gauge the severity of acute brain injury. It evaluates three behavioral responses:

1. Eye Opening (1 to 4 points): Spontaneous (4), To sound (3), To pressure/pain (2), None (1).
2. Verbal Response (1 to 5 points): Oriented (5), Confused (4), Inappropriate words (3), Incomprehensible sounds (2), None (1).
3. Motor Response (1 to 6 points): Obeys commands (6), Localizing pain (5), Normal flexion/withdrawal (4), Abnormal flexion/Decorticate posturing (3), Extension/Decerebrate posturing (2), None (1).

• Posturing Pathology:
  • Decorticate Posturing (Abnormal Flexion): Characterized by adduction and internal rotation of the arms and flexion of the wrists/fingers. Indicates lesions above the red nucleus (cortical/subcortical tracts).
  • Decerebrate Posturing (Extension): Characterized by rigid extension and pronation of the arms, and plantar flexion. Indicates severe damage to the brainstem (midbrain or pons) and carries a poorer prognosis.
• Intubation Threshold: A GCS score of 8 or less indicates a coma and a loss of protective airway reflexes (cough, gag, swallow), representing a mandatory indication for endotracheal intubation.

Pupillary Light Reflexes

• A unilateral fixed and dilated pupil (mydriasis) indicates compression of the ipsilateral Cranial Nerve III (oculomotor nerve). This is a medical emergency signifying uncal herniation, where the temporal lobe is forced downward through the tentorial notch.

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Interventions for Managing Elevated ICP

When ICP exceeds 20 mmHg, a tiered protocol is implemented to prevent brain herniation.

1. Positioning and Venous Drainage

• The head of the bed (HOB) must be elevated to 30 to 45 degrees to facilitate gravitational venous drainage from the cerebral circulation.
• The Neck Position Trap: The patient's head and neck must be maintained in a strict neutral, midline position. Any rotation, flexion, or lateral bending of the neck can compress the internal jugular veins, obstructing venous outflow and causing an immediate, severe spike in ICP.

2. Osmotic Therapy

Osmotic agents increase serum osmolality, drawing water out of the swollen brain tissue and into the vascular space to be excreted by the kidneys.

• Mannitol (20%): Administered as an IV bolus of 0.5 to 1.0 g/kg over 20-30 minutes.
  • Monitoring: The clinician must monitor serum osmolality (keep < 320 mOsm/kg to prevent acute kidney injury) and calculate the osmolar gap.
• Hypertonic Saline (3% or 23.4%): Pulls fluid from the intracellular to extracellular space.
  • Monitoring: Serum sodium must be monitored closely, targeting a range of 145 to 155 mEq/L. High-concentration saline (e.g., 23.4%) must be administered through a central venous catheter to prevent severe peripheral vein phlebitis and necrosis.

3. Hyperventilation and Cerebral Vasoconstriction

Carbon dioxide is a potent regulator of cerebral vascular tone.

• Mechanism: Hyperventilation lowers the $PaCO_2$, which increases the pH of the cerebrospinal fluid, causing cerebral arteriolar vasoconstriction. This constriction reduces cerebral blood volume and rapidly lowers ICP.
• Target $PaCO_2$: 30 to 35 mmHg.
• Critical Safety Rule: Prophylactic hyperventilation is strictly contraindicated. The resulting vasoconstriction can cause severe local cerebral ischemia. Hyperventilation must be used ONLY as a short-term, temporary bridge (minutes to hours) during acute herniation crises while other therapies (osmotic agents or surgery) are initiated.