Aetiology

The cause is unknown. IIH in adults is highly associated with female sex and obesity (often associated with recent weight gain).[6][10] Many aetiologies have been suggested but none proven. 

Pathophysiology

Cerebrospinal fluid (CSF) absorption occurs both via the arachnoid granulations and along nerve root sheaths, especially along the olfactory nerve through the cribriform plate.[11] It is thought that there is increased resistance to CSF outflow along one or both of these pathways, but abnormalities of these absorption pathways in patients with IIH remain unproven. With CSF absorption block, intracranial pressure must rise for CSF to be absorbed. Although interstitial and intracellular oedema have been reported in brain biopsy specimens, study with current methods of analysis has concluded that the histological features of the brain parenchyma are normal, and the findings of previous studies are artefacts of slide preparation.[12][13] In addition, changes in cerebral haemodynamics, including increased cerebral blood volume and decreased cerebral blood flow, have been reported, but their significance is unclear.

The relationship between papilloedema and loss of vision is complex and usually due to intraneuronal ischaemia related to increased CSF pressure transmitted along the optic nerve sheath.[14] With the loss of axons, papilloedema can regress while vision worsens. More often, however, the more severe the papilloedema, the worse the loss of vision. This is true for large case series, but, occasionally, patients have marked papilloedema and little loss of vision.[15]

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