What is IIH?  Read more about Diagnosis & Treatments

Idiopathic Intracranial Hypertension (IIH) is a neurological condition of unknown cause defined by increased intracranial pressure (ICP) around the brain without the presence of tumour or disease.

The space around the brain is filled with water like fluid (Cerebrospinal Fluid - CSF). If, due to a variety of factors, the pressure around the brain rises then the space containing the fluid cannot expand. It is this excessively high CSF pressure that produces the symptoms of IIH.

History and Terminology

IIH is also known by its earlier name: Benign Intracranial Hypertension (BIH) is little used because the condition can cause visual loss and therefore is not harmless or benign. You may also occasionally see it referred to as Pseudotumor Cerebri (PTC) because some sufferers present with signs and symptoms of a brain tumour despite no tumour being present, ('pseudo' meaning false).

The precise medical definition of IIH changes over time as more becomes known about it. The American Neurosurgeon Dandy defined the initial criteria for IIH before the Second World War. This criteria has been enhanced and is now called the Modified Dandy Criteria.

The pressure-volume relationship between ICP, volume of CSF, blood, and brain tissue, and cerebral perfusion pressure (CPP) is known as the Monro-Kellie doctrine or the Monro-Kellie hypothesis.

The Monro-Kellie hypothesis states that the cranial compartment is incompressible, and the volume inside the cranium is a fixed volume. Any unwanted increase in CSF pressure therefore presses upon the brain, eyes and tissues inside the skull.

Statistics for incidence and prevalence in UK

Adults:  What is the difference between incidence and prevalence? Incidence relates to new cases of IIH in the population in a year whereas prevalence tells us how widespread it is across the whole population.  Since there is no established national registry associated with IIH here in the UK the actual number of IIH patients in the community is unknown, the figures available for incidence in England in 2011 was 6.21 per 100,000. When analysed further, the figures show that IIH affects 2 in 100,000 people but the rate of incidence increases to 2-4 per 100,000 of women of child bearing age and increases to 20 per 100,000 with a BMI >30. The highest rates of incidence of IIH in the UK are in North East England and the London area. IIH is on the increase.

A research project 'The IIH Life' registry has been set up by the University of Birmingham. It is planned that the registry will include 40 neurological hospitals across the UK as well as health care institutions in Denmark. The registry plans to progressively expand to other European sites. However, it is to early to derive any information from this database.

Information concerning the prevalence of IIH is limited. Since no complete disease registry data are available the exact number of individuals affected by IIH is unknown. From the available resources the prevalence of IIH in the EU general population ranges from 0.51 - 1.43 per 10,000 population. Which equates to around 5000 people in the UK.  

Other known statistics are: 93% of people with IIH 2 are obese (BMI = 30), 25% of all men with IIH have permanent severe visual loss 1 – but this would probably be avoidable in almost every case if men reported problems to their opticians early - however some local ophthalmology identification procedures are inadequate due to low levels of awareness. Twice as many men with IIH go blind as women.

Children: It is thought that there are 57 new cases of children aged 1 to 16 with IIH each year3 and the total number of children with IIH is in excess of 800. They suffer broken educations and their siblings are adversely affected spending non-school time with parents visiting hospital. There is no difference in the rate of IIH by gender in children but 50% of female children seen in 2011 were obese.


1 from the Academic Unit of Ophthalmology at the University of Birmingham (Dr Alexandra Sinclair, PhD MRC Fellow – a specialist in IIH)

2Idiopathic Intracranial Hypertension: Recent concepts and develpoments. ACNR.2010; 3: 10-14h

3Department of Health British Paediatrics Surveillance Unit (BPSU) Annual Report for 2009/2010 conducted by Dr Yim-Yee Matthews.