The Pathophysiology of Raised Intracranial Pressure in Idiopathic Intracranial Hypertension

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The Pathophysiology of Raised Intracranial
Pressure in Idiopathic Intracranial Hypertension

• Páraic Meaney

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Introduction

• Idiopathic Intracranial Hypertension (IIH)
  Pseudotumour Cerebri Benign
  Intracranial Hypertension
• IIH raised ICP with normal CSF, normal imaging
  and the absence of neurological signs.
• It is a diagnosis of exclusion.

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Aims and Methods

• Objective To evaluate the mechanisms of raised
  intracranial pressure in IIH.
• Methods Review of the relevant literature was
  performed using internet resources such as
  Science-Direct and Pub-Med.
• Manual search of the references of original and
  review articles completed the search strategy.

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Results

• The potential mechanisms proposed in the
  literature for the raised intracranial pressure
  in IIH include
• CSF Outflow Reduction.
• Intracranial Venous Hypertension.
• Increased Cerebral Volume.
• Increased CSF Production.
• Associated Medical Disorders.

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Normal Intracranial Pressure

• Normal ICP 5-15 mmHg.
• ICP is influenced by cerebral autoregulation,
  cerebral venous pressure, CSF formation and CSF
  absorption.
• IIH develops when these compensatory mechanisms
  are no longer functioning adequately.

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CSF Outflow Reduction

• Inflammation of the Arachnoid Villi.
• IIH like-syndrome develops following Cryptoccocal
  Meningitis.

• Protein Hypothesis.
• In normal circumstances protein is absorbed with
  CSF into the venous system.
• In IIH, there is a reduction in the absorption of
  CSF by the Arachnoid Villi.

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Intracranial Venous Hypertension

• ? Unrecognised non-occlusive thrombus lining the
  dural vessels could cause IIH.
• Anticardiolipin antibodies
• Important part in the pathogenesis of IIH in
  non-obese patients who lack other risk factors .
• Prothrombic abnormalities
• Incidence of antiphospholipid antibody in IIH
  group was 30 compared to 6 in the control
  group.

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Increased Cerebral Volume

• On CT imaging the scans of IIH patients showed
  smaller cranial CSF spaces, the reduction in the
  ventricular size indicated an increase in
  cerebral volume.
• Studies using non-invasive MR imaging techniques
  have suggested that cerebral oedema is not a
  feature of IIH.

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Increased CSF Production

• Case-Study of a 5 yo girl developed an IIH-like
  syndrome 10 days after surgical repair of a
  temporal lobe arteriovenous malformation.
• Pressure gradient across the choroid plexus is
  one of the main determinants of CSF production.
• Therefore, increases in the transcapillary
  pressure will result in increased transudation
  from the capillaries leading to increased CSF
  formation.

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Associated Medical Disorders

• Obesity
• Weight reduction is associated with a resolution
  of the symptoms of IIH.
• Oestrogen
• Shown to occur with greater frequency in those
  taking the OCP and in early pregnancy.
• Vitamin A
• Hypervitaminosis A may cause CSF hypersecretion.

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Conclusion

• There is a lack of convincing evidence in the
  literature that cerebral oedema or increased CSF
  production are the primary mechanisms behind IIH.
• Autopsies have failed to show evidence of
  interstitial cerebral oedema.
• Mathematical modelling of ventricular size in the
  presence of increased CSF production results in
  hydrocephalus not IIH.

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Conclusion contd.

• The studies which looked at decreased CSF
  absorption or elevated cerebral venous pressure
  as potential causes of IIH are far more
  persuasive.
• High pressures in venous sinuses have been
  measured in patients with IIH.
• Obesity may be the immediate cause in a suitably
  predisposed individual.

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