Why does tonsillar herniation not occur in idiopathic intracranial hypertension?

Idiopathic intracranial hypertension (IHH or pseudotumor cerebri) is an uncommon but important cause of headache, characterized by raised intracranial pressure (ICP) in the absence of infection and intracranial pathology on neuroimaging. Lumbar puncture (LP) is usually contra-indicated in situations where the ICP is suspected to be high. However, tonsillar herniation is hardly ever reported after an LP in patients suspected of having IHH. The author postulates that increasing brain stiffness reported in IHH may be the reason for this observation. On the other hand, tonsillar herniation may occur as a late complication following lumbo-peritoneal shunt insertion (occasionally used in the management of patients with IHH). This may be due to the decrease in brain stiffness, i.e. an increase in brain compliance as the condition improves following shunt insertion. Recent advances in magnetic resonance imaging techniques are likely to help us further in understanding the pathogenesis of this curious condition.