The Monro–Kellie rule states that the intracranial pressure is determined by the amount of brain tissue, cerebrospinal fluid (CSF) and blood inside the bony cranial vault. Three theories therefore exist as to why the pressure might be raised in IIH: an excess of CSF production, increased volume of blood or brain tissue, or obstruction of the veins that drain blood from the brain.The first theory, that of increased production of cerebrospinal fluid, was proposed in early descriptions of the disease. However, there is no experimental data that supports a role for this process in IIH.The second theory posits that either increased blood flow to the brain or increase in the brain tissue itself may result in the raised pressure. Little evidence has accumulated to support the suggestion that increased blood flow plays a role, but recently Bateman et al. in phase contrast MRA studies have quantified cerebral blood flow (CBF) in vivo and suggests that CBF is abnormally elevated in many people with IIH. Both biopsy samples and various types of brain scans have shown an increased water content of the brain tissue. It remains unclear why this might be the case.The third theory suggests that restricted venous drainage from the brain may be impaired resulting in congestion. Many people with IIH have narrowing of the transverse sinuses. It is not clear whether this narrowing is the pathogenesis of the disease or a secondary phenomenon.
In children, there is no difference in incidence between males and females.From national hospital admission databases it appears that the need for neurosurgical intervention for IIH has increased markedly over the period between 1988 and 2002. This has been attributed at least in part to the rising prevalence of obesity, although some of this increase may be explained by the increased popularity of shunting over optic nerve sheath fenestration. History
The first report of IIH was by the German physician Heinrich Quincke, who described it in 1893 under the name serous meningitis The term "pseudotumor cerebri" was introduced in 1904 by his compatriot Max Nonne. Numerous other cases appeared in the literature subsequently; in many cases, the raised intracranial pressure may actually have resulted from underlying conditions. For instance, the otitic hydrocephalus reported by London neurologist Sir Charles Symonds may have resulted from venous sinus thrombosis caused by middle ear infection. Diagnostic criteria for IIH were developed in 1937 by the Baltimore neurosurgeon Walter Dandy; Dandy also introduced subtemporal decompressive surgery in the treatment of the condition.The terms "benign" and "pseudotumor" derive from the fact that increased intracranial pressure may be associated with brain tumors. Those people in whom no tumour was found were therefore diagnosed with "pseudotumor cerebri" (a disease mimicking a brain tumor).