Monro-Kellie hypothesis in intracranial hypotension

This hypothesis states that

  • The volume of 3 components in brain (tissue, ICF and blood) is nearly constant in a state of equilibrium.
  • The average intracranial volume in an adult is 1700 mL = 1400 mL (brain tissue) + 150 mL (ICF) + 150 mL (blood, mainly in veins)

A decrease in the volume of one compartment is compensated by the increase in the volume of another compartment.

  • Eg in idiopathic intracranial hypertension, there is slit like ventricles (decreased ICF) and smaller size of pituitary gland i.e empty fossa (decreased blood flow)

How does this apply in intracranial hypotension?

  • Characteristic features in intracranial hypotension: venous distension sign, pachymeningeal enhancement, subdural effusion
  • A decrease in ICF is compensaterd by increased blood volume (venous hyperemia and engorgement)
    – Venous distension sign is due to venous engorgement (sinus becomes round instead of triangular)

    – Pachymeningeal enhancement is due to pressure gradient induced leak in gd into the pachymeningeal layer (which
    lacks blood-brain barrier unlike leptomeningeal layer)

    – Subdural effusion is due to leakage from venous engorgement (like edema in lower extremities)


  4. Bahram Mokri, MD. The Monro-Kellie hypothesis. Applications in CSF volume depletion. Neurology 2001;56:1746-48

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