Table of Contents
Overview – Intracranial Pressure
Intracranial pressure (ICP) refers to the pressure exerted within the skull by brain tissue, cerebrospinal fluid (CSF), and cerebral blood volume. It is a tightly regulated physiological parameter, essential to maintaining adequate cerebral perfusion and preventing brain herniation. Increases in ICP—due to trauma, mass lesions, or CSF imbalance—can rapidly compromise cerebral blood flow and lead to neurological deterioration. Understanding ICP regulation is critical for assessing conditions like head injury, hydrocephalus, and cerebral oedema.
What Is Intracranial Pressure?
- Pressure within the cranial vault, exerted on:
- Brain tissue
- Cerebral blood vessels
- Cerebrospinal fluid (CSF)
- Normal ICP: ~7–15 mmHg in adults
Determinants of ICP
- CSF Volume
- ↑ Production (e.g. choroid plexus tumours)
- ↓ Resorption (e.g. arachnoid granulation dysfunction)
- Brain Tissue Volume
- Tumour, inflammation, or trauma
- Blood Volume
- Intracranial haemorrhage or venous congestion
A rise in any one component must be offset by a decrease in another to maintain pressure homeostasis (see: Monroe-Kellie Doctrine)
Monroe-Kellie Doctrine
The cranial vault is a closed, fixed-volume space containing three elements:
- Brain (~80%)
- Blood (~10%)
- CSF (~10%)
Any increase in one component must be offset by a compensatory decrease in another:
- Initial compensation via displacement of CSF and venous blood
- Buffering capacity: ~100–120 mL
- Decompensation leads to raised ICP and reduced cerebral perfusion
High ICP – Clinical Consequences
Physiological Effects
- Compresses cerebral arteries → ↓ perfusion → ischaemia
- Can displace brain structures (herniation syndromes)
Signs & Symptoms
- Altered consciousness (e.g. drowsiness → coma)
- Bradycardia & hypertension (Cushing’s triad)
- Papilloedema, impaired pupillary reflexes
- Headache, nausea, vomiting
- Abnormal posturing or motor responses
Cerebral Blood Flow & ICP
- Brain receives ~750 mL/min (≈15% of cardiac output)
- CBF is tightly regulated through autoregulation
- Cerebral Perfusion Pressure (CPP) = MAP − ICP
- Normal CPP: 60–100 mmHg
- CPP < 50 mmHg = risk of ischaemia
Graph Insight
- Within a CPP range of ~60–150 mmHg, cerebral blood flow remains constant
- Beyond this range, autoregulation fails, and CBF becomes pressure-dependent
CSF Flow, Production & Resorption
Production
- Made by choroid plexuses in ventricles
- Total volume: ~150 mL
- Turnover: ~500 mL/day
Resorption
- Occurs via arachnoid villi (granulations) into the superior sagittal sinus
- CSF diffuses into venous circulation by pressure gradient
Disruption in drainage → hydrocephalus or raised ICP
Cerebral Oedema
Definition
Excess accumulation of water in intracellular or extracellular brain compartments
Types
1. Vasogenic Oedema
- Extracellular
- Due to disruption of the blood–brain barrier
- E.g. hydrostatic pressure pushing plasma out of capillaries
2. Cytotoxic Oedema
- Intracellular
- Cell membrane pump failure (Na⁺/K⁺-ATPase) → water retained inside cells
- Often seen in ischaemia
3. Osmotic Oedema
- Water shifts into brain tissue due to low plasma osmolality
- E.g. rapid rehydration after dehydration or SIADH
Summary – Intracranial Pressure
Intracranial pressure reflects the dynamic balance between brain tissue, CSF, and cerebral blood volume. When this equilibrium is disturbed, compensatory mechanisms (per the Monroe-Kellie Doctrine) may delay, but not prevent, pressure elevation. Left unchecked, raised ICP compromises cerebral perfusion and can result in fatal brain herniation. Clinical understanding of CSF flow, autoregulation of cerebral blood flow, and types of cerebral oedema is critical in managing neurological emergencies. For a broader context, see our Nervous System page.