Idiopathic intracranial hypertension (IIH) is a condition of unknown etiology, primarily affecting overweight females of childbearing age. Typically, patients experience headache and visual symptoms due to increased intracranial pressure (ICP) and papilledema. The diagnosis is difficult, and outcomes vary from no sequelae to blindness or chronic headaches. No clear prognostic indicators exist. Treatment consists of medication, weight loss, and possibly surgical intervention.There is an unmet need of defining biomarkers with prognostic or diagnostic value and defining predictors of a poor outcome.

This project is a prospective, population-based cohort study including clinical data and a biobank (blood samples and cerebrospinal fluid).

The investigator's primary aim is to identify biomarkers of diagnostic or prognostic value and to create a clinical IIH database. The clinical database will answer questions about patient characteristics at baseline and during follow-up, identify predictors of outcome, and help create a standardized programme for follow-up and

This study is a multicenter, prospective, population-based cohort study with consecutive inclusion of patients in which the diagnosis of IIH is suspected. This study is carried out in collaboration between the Danish Headache Center, Rigshospitalet-Glostrup, and the Neurological Department at Odense University Hospital.

Patients are eligible for inclusion into the study if:

1. IIH is suspected
2. > 18 years old and able to provide written informed consent.

At baseline included patients will have:

A.) Medical history B.) Neurological, ophthalmological and general medical examination C.) Relevant neuro-imaging D.) Blood samples and lumbar puncture F.) Evaluation by other specialist, including neuro-psychologists, if appropriate.

Subsequently patients are divided into three sub-groups according to revised Friedmann criteria:

1. Certain IIH or IIH-WOP
2. Suspected, but unconfirmed, IIH
3. IIH ruled out

Patients are followed at a headache center and by neuro-ophthalmologist according to standard clinical practice.

Inclusion Criteria:

1. Able to and willing to provide informed consent
2. More than 18 years of age
3. Suspicion of IIH (based on clinical evaluation by neurologist or opthalmologist)

Exclusion Criteria:

1.) Unable to consent (e.g. language, mental retardation).

All admitted or out-patient referrals where IIH is suspected. Patients will be identified and included consecutively once there is a clinical suspicion of IIH, if they wish to participate. Most often IIH will be suspected in a typical patient (female, child-bearing age, overweight) with either bilateral papiledema and normal neuro-imaging, high ICP by lumbar puncture or a new headache with associated visual disturbances or pulsatile tinnitus.

All hospitals in the Capital Region of Denmark and the Southern Region of Denmark can participate and refer patients.