Raised Intracranial Pressure (ICP) : Pathophysiology, diagnosis, treatment and complications

Raised Intracranial Pressure (ICP)

1. Introduction and Definitions

• Definition: Abnormally high pressure within the rigid, non-compliant skull cavity.

• Normal ICP Range: to $15\text{ mmHg}$ (Supine adult). Pressures $>20\text{ mmHg}$ are generally considered pathological and require intervention.

• The Monro-Kellie Doctrine: States that the total volume within the cranial vault is fixed and is comprised of three components:

  • Brain Parenchyma ($\sim 80\%$)

  • Cerebrospinal Fluid (CSF) ($\sim 10\%$)

  • Intracranial Blood ($\sim 10\%$)

• 

  Compensation: An increase in the volume of one component must be offset by a decrease in one or both of the other components to maintain a stable ICP.

  • Initial compensation primarily involves displacement of CSF into the spinal subarachnoid space and compression of cerebral veins.

2. Pathophysiology and Mechanisms

• Pressure-Volume Curve (Compliance):

  • High Compliance (Initial Phase): Small increases in volume cause minimal change in ICP (flat part of the curve) due to compensatory mechanisms.

  • Low Compliance (Decompensation): Once compensation is exhausted, the curve becomes steep; a small increase in volume causes an exponential increase in ICP.

• Cerebral Perfusion Pressure (CPP): The net pressure gradient causing blood flow to the brain. Maintenance is critical for preventing secondary brain injury.

  • Formula: $CPP=MAP-ICP$ (Mean Arterial Pressure minus Intracranial Pressure).

  • Goal: Maintain $CPP$ typically $>60\text{ mmHg}$. If $ICP$ increases, $CPP$ decreases, leading to ischemia.

• Causes of Raised ICP:

  • Mass Lesions: Tumors (primary/metastatic), hematomas (epidural, subdural, intracerebral), cerebral abscesses.

  • Cerebral Edema:

    • Vasogenic: Breakdown of the blood-brain barrier (e.g., tumors, infection, trauma).

    • Cytotoxic: Cellular swelling due to ischemia (e.g., stroke, hypoxia, DKA).

  • Hydrocephalus: Impaired CSF flow or absorption (e.g., subarachnoid hemorrhage, meningitis, obstruction of the aqueduct of Sylvius).

  • Increased CBV: Hypercapnia (${\text{CO}}_2$ retention causes vasodilation), venous outflow obstruction (e.g., jugular vein thrombosis, position).

  • Idiopathic: Pseudotumor Cerebri (Idiopathic Intracranial Hypertension).

3. Clinical Features

Stage	Signs and Symptoms	Key Findings
Early / Compensated	Mild headache, worse in the morning or with strain (Valsalva). Transient visual obscurations. Nausea/vomiting (may be projectile).	Diplopia (CN VI palsy). Papilledema (optic disc swelling – may be late sign).
Late / Decompensated	Cushing’s Triad: 1. Hypertension (often widening pulse pressure). 2. Bradycardia. 3. Irregular respirations.	Decline in Glasgow Coma Scale (GCS). Fixed, dilated pupils (Uncal herniation). Posturing (decorticate or decerebrate).

4. Diagnosis and Monitoring

• Neuroimaging (Initial):

  • CT/MRI: Essential to identify underlying cause (mass lesion, hemorrhage, edema, hydrocephalus) and signs of herniation.

  • Warning: Lumbar Puncture (LP) is ABSOLUTELY CONTRAINDICATED in the presence of a known or suspected mass lesion or signs of herniation, as it can precipitate transtentorial or tonsillar herniation.

• ICP Monitoring (Gold Standard):

  • External Ventricular Drain (EVD): Provides continuous ICP measurement, allows sampling of CSF, and permits therapeutic drainage of CSF to lower ICP.

  • Fiber Optic Transducers: Devices placed in the parenchyma or subdural space.

5. Management (Tiered Approach)

The primary goal is to maintain $CPP>60\text{ mmHg}$ and $ICP<20\text{ mmHg}$.

Tier 0: General and Supportive Measures

• Positioning: Head of bed elevation to $30$ degrees (promotes venous outflow).

• Vitals: Maintain Normothermia (fever increases metabolism and ICP). Treat pain and agitation (sedation/analgesia).

• Hemodynamics: Maintain normovolemia and $MAP$ (to support $CPP$).

Tier 1: First-Line Therapies

• CSF Drainage: If an EVD is in place, initiate continuous or intermittent CSF drainage.

• Hyperosmolar Therapy: Creates an osmotic gradient to draw water out of the brain parenchyma into the intravascular space.

  • Mannitol ($0.25$ to $1.0\text{ g/kg}$ IV bolus): Requires intact blood-brain barrier (BBB) and maintenance of serum osmolarity $<320\text{ mOsm/L}$.

  • Hypertonic Saline (e.g., $3\%$ or $23.4\%$): Draws fluid from brain, increases $MAP$, and avoids the rebound ICP effect sometimes seen with Mannitol.

Tier 2: Second-Line Therapies (If ICP remains

$>20\text{ mmHg}$)

• Ventilation: Controlled, transient hyperventilation (Target $pC{O}_{2}30-35\text{ mmHg}$). Causes cerebral vasoconstriction, rapidly reducing $CBV$ and $ICP$. Use cautiously as it can cause ischemia.

• Diuresis: Repeat hyperosmolar therapy.

Tier 3: Third-Line Therapies (Refractory ICP)

• Barbiturate Coma (e.g., Pentobarbital): Decreases cerebral metabolic rate and $CBV$ (powerful vasoconstrictor). Requires continuous $EEG$ monitoring and full $ICU$ support.

• Decompressive Craniectomy: Surgical removal of a portion of the skull to allow the edematous brain to swell outward, physically reducing pressure.

• Hypothermia: Induced mild hypothermia (Controversial; used in select, severe cases).

6. Complications

• Cerebral Herniation: Life-threatening displacement of brain tissue due to severe pressure gradient.

• Uncal Herniation: Medial temporal lobe (uncus) through the tentorial notch. Causes ipsilateral fixed and dilated pupil (CN III compression) and contralateral hemiparesis.

• Tonsillar Herniation: Cerebellar tonsils through the foramen magnum. Compresses the brainstem, leading to cardiorespiratory arrest.

• Secondary Brain Injury: Ischemia and infarction caused by critically low $CPP$.

• Central Diabetes Insipidus/SIADH/Cerebral Salt Wasting: Endocrine/electrolyte disturbances often associated with severe brain injury.