Definition of Intracranial Hypertension
Intracranial hypertension is defined as an intracranial pressure (ICP) greater than 20-25 mmHg, measured invasively via intraparenchymal monitors or ventricular catheters. 1, 2
Measurement Standards
- Normal ICP is below 10 mmHg in adults, and pathological elevation requiring treatment begins at or above 20-25 mmHg 3
- The threshold of 20 mmHg is the most commonly cited cutoff for defining intracranial hypertension across multiple guidelines 1, 2
- ICP measurement requires invasive monitoring using either intraparenchymal electronic/fiberoptic devices or ventricular catheters (external ventricular drains), which are the most reliable and accurate methods 1, 3
Context-Specific Definitions
Idiopathic Intracranial Hypertension (IIH)
- For IIH diagnosis, CSF opening pressure must be at least 25 cm H₂O (approximately 18 mmHg) measured via lumbar puncture in the lateral decubitus position, according to the modified Dandy criteria 1
- This represents a slightly different threshold specific to the IIH population
Pediatric Populations
- In children, elevated ICP is defined as opening pressure >280 mm H₂O (approximately 21 mmHg) when measured via lumbar puncture 1
- If the child is not sedated and not obese, the threshold is >250 mm H₂O (approximately 18 mmHg) 1
Cryptococcal Meningitis
- In both HIV-positive and HIV-negative patients with cryptococcal disease, elevated ICP is defined as opening pressure ≥200 mm H₂O (approximately 15 mmHg) measured in the lateral decubitus position 1
- This lower threshold reflects the specific pathophysiology of this infectious condition
Clinical Significance by Severity
- ICP 20-40 mmHg is associated with a 3.95-fold increased risk of mortality and poor neurological outcome, with consciousness typically impaired at these levels 2
- ICP >40 mmHg increases mortality risk 6.9-fold and is almost universally associated with severe consciousness impairment or coma, demanding immediate aggressive intervention 2
- Refractory intracranial hypertension is defined as ICP increases to more than 30 mmHg and/or cerebral perfusion pressure reductions to less than 60 mmHg for longer than 15 minutes, with failure to respond to maximum medical treatment 4
Important Caveats
- The exact threshold defining intracranial hypertension remains somewhat uncertain in the literature, with both lower and higher thresholds described depending on clinical context 1
- ICP values should never be interpreted in isolation but rather in the context of monitoring trends, cerebral perfusion pressure (CPP = MAP - ICP), and clinical evaluation 1, 2
- Continuous waveform assessment is essential to ensure accuracy and reliability of measurements, as instantaneous values alone can be misleading 1