Key Facts

IIH is a disorder of raised CSF pressure without structural cause. Visual loss is the major morbidity. Weight loss is the single most effective treatment. Acetazolamide provides symptomatic benefit.

Epidemiology

Overall incidence ~1-2 per 100,000. In obese women aged 20-44: up to 20 per 100,000. F:M 8:1. Prevalence increasing in parallel with obesity epidemic. Rare in men and children (if occurs, investigate more aggressively for secondary causes).

Aetiology

• Idiopathic by definition; strongly associated with obesity (>90% are obese)
• Weight gain often precedes symptom onset
• Drug associations: tetracyclines (doxycycline, minocycline), vitamin A/retinoids, combined OCP (debated), lithium, growth hormone
• Exclude: cerebral venous sinus thrombosis (MRV essential), anaemia, obstructive sleep apnoea

Pathophysiology

Exact mechanism unclear. Hypotheses include: impaired CSF absorption at arachnoid granulations, increased CSF production, increased venous pressure from obesity (raised intra-abdominal pressure → raised intrathoracic → raised venous pressure → impaired CSF drainage), hormonal factors (adipokines, glucocorticoid metabolism). Transverse sinus stenosis is commonly found (may be cause or consequence).

Symptoms

• Headache (>90%): daily, diffuse, worse on waking, aggravated by coughing/straining/Valsalva; may mimic migraine or TTH
• Transient visual obscurations (~70%): brief (seconds) episodes of bilateral visual blurring/greying, often positional
• Pulsatile tinnitus (~60%): 'whooshing' sound synchronous with pulse; unilateral or bilateral
• Diplopia: horizontal (CN VI palsy — false localising sign)
• Visual loss: progressive if untreated; may be insidious

Signs

• Papilloedema: bilateral (hallmark — must be present for diagnosis in most criteria); grade I-V (Frisén scale)
• Enlarged blind spots: earliest visual field defect
• CN VI palsy: unilateral or bilateral
• Visual field loss: inferonasal defect → progressive constriction

Red Flags

• Rapidly progressive visual loss → urgent intervention
• Severe papilloedema (grade IV-V)
• Male patient or child with IIH features → investigate more aggressively for secondary causes
• Atypical features → consider venous sinus thrombosis

Imaging (Before LP)

• MRI brain + MR venogram: MANDATORY to exclude mass lesion and venous sinus thrombosis
• MRI findings in IIH: empty sella, flattened posterior globes, distended perioptic subarachnoid space, transverse sinus stenosis (common but non-specific)

Lumbar Puncture

• Opening pressure: >25 cmH₂O in adults (lateral decubitus, legs extended); diagnostic
• CSF composition: NORMAL (protein, glucose, cells) — if abnormal, consider alternative diagnosis
• Therapeutic LP: drainage of 20-40 mL provides temporary symptom relief

Visual Assessment (Essential — Repeated)

• Visual acuity: Snellen chart
• Visual fields: Humphrey automated perimetry — most sensitive for monitoring; enlarged blind spots, inferonasal defect
• Fundoscopy: papilloedema grading (Frisén scale)
• OCT: retinal nerve fibre layer thickness — objective measure of papilloedema

Bloods

• FBC: exclude anaemia (severe anaemia can cause papilloedema)
• TFTs: exclude hypothyroidism
• Coagulation, thrombophilia screen: if CVT suspected

Weight Loss (Most Important)

• 5-10% body weight loss significantly reduces ICP and improves papilloedema
• Weight management programme referral
• Consider bariatric surgery for BMI >35 with refractory IIH (IIH:WT trial)

Pharmacological

• Acetazolamide 250mg-2g/day: carbonic anhydrase inhibitor → reduces CSF production; IIH Treatment Trial showed improvement in visual field + papilloedema; side effects: paraesthesiae, taste disturbance, metabolic acidosis, renal stones
• Topiramate 25-100mg BD: reduces ICP + promotes weight loss; alternative to acetazolamide or adjunct
• Furosemide: sometimes used as adjunct (limited evidence)

Surgical/Interventional (Refractory or Fulminant)

• CSF diversion: lumboperitoneal shunt or ventriculoperitoneal shunt; effective but high revision rate (~50% require revision within 2 years)
• Optic nerve sheath fenestration: decompresses perioptic subarachnoid space; protects vision; does not reliably treat headache
• Venous sinus stenting: for significant transverse sinus stenosis with pressure gradient; emerging evidence; NICE IPG
• Repeated therapeutic LPs: temporising measure (not practical long-term)

Monitoring

• Regular visual field testing (Humphrey perimetry) every 1-3 months
• OCT for RNFL thickness monitoring
• Weight monitoring

Referral Criteria

• Neurology/neuro-ophthalmology: all IIH patients
• Ophthalmology: visual field monitoring
• Neurosurgery: refractory IIH requiring CSF diversion
• Bariatric surgery: BMI >35 with refractory IIH
• Weight management: all patients

With weight loss and acetazolamide, most patients improve. Visual loss is the main morbidity: ~25% have some degree of permanent visual loss; severe visual loss in ~5-10%. Risk factors for poor visual outcome: male sex, severe papilloedema, high-grade visual field loss at presentation, delay in treatment. IIH recurrence: ~10-40% relapse (usually associated with weight regain). Fulminant IIH (rapid visual loss over days) requires emergency intervention.

Modified Dandy Criteria for IIH

Papilloedema Grading (Frisén Scale)