K5i: Describe the measurement of intracranial pressure

Measurement of ICP

Invasive

• Extraventricular drain → gold standard
• Parenchymal catheter
• Extradural ICP monitor

Non-Invasive

• Transcranial Doppler US
• Tympanic membrane displacement
• Optic nerve sheath diam
• Fundoscopy & papilledema

Oscillating Pressure Curve

• Progressively ↓P₁, P₂, P₃ notches indicating propagation of cardiac pulse pressure

• P₁ → PLEXUS PULSATIONS: arterial pulse transmitted through choroid plexus
• P₂ → TIDAL WAVE: cerebral compliance, reflection of arterial pulse bouncing off brain parenchyma
• P₃ → DICROTIC WAVE: closure of AV valve. If P₁ > P₂ = ↑ICP

Invasive

Extra Ventricular Drain

• Catheter tip passes through brain tissue
• Tip sits in Lateral Ventricle via Burr Hole
• Distal end connects to P transducer & drain system → allows drainage of CSF
• Zero level → external auditory meatus
• Can be recalibrated any time
• Monitors ICP cont & intermittent drainage

Codman Monitor

• Catheter tip lies in brain tissue
• Strain gauge tipped catheter of fibreoptic device
• Monitors ICP
• No recalibration possible after placement

Extradural ICP Monitor

• As per Codman but does not penetrate dura

Non-Invasive

Transcranial Doppler Ultrasonography

• US applied to MCA to measure BF
• Pulsatility index correlates with ICP
• Requires training & regular repetition

Tympanic Membrane Displacement

• CSF communicates with perilymph
• Based on:
  • Stapedial reflex
  • Patent cochlear duct
• Stimulation of Stapedial reflex
• Tympanic membrane movement correlates with ICP
• High Cochlear Fluid P = inward displacement TM
• Low Cochlear Fluid P = outward displacement TM

Optic Nerve Sheath Diameter

• Optic n = part of CNS
• Covered with Dural Sheath
• Small SUBARACHNOID SPACE b/w sheath & white matter
• ↑ICP = ↑sheath size
• Measure with trans-ocular US
• Correlates with ICP

Fundoscopy / Papilloedema

• Papilledema is due to ↑ICP
• Graded by FROSSEN Scale
• Usually done to screen patients with suspected ↑ICP