Key Facts

NPH is an important and potentially treatable cause of dementia. The key diagnostic challenge is distinguishing NPH from cerebral atrophy with ex-vacuo ventriculomegaly. A positive response to therapeutic LP strongly predicts shunt responsiveness.

Epidemiology

Prevalence ~0.5-2.9% of over-65s. Incidence increases with age. More common in men. Often underdiagnosed. Accounts for ~5% of dementia cases.

Aetiology

• Idiopathic (iNPH): most common (~50%); age-related impaired CSF absorption at arachnoid granulations
• Secondary: post-SAH, post-meningitis, post-traumatic, post-neurosurgery (altered CSF dynamics)

Pathophysiology

Impaired CSF absorption (primarily at arachnoid granulations) → gradual ventricular enlargement → compression of periventricular white matter tracts (corona radiata — motor fibres to legs; frontal white matter — executive function; parasagittal fibres — bladder control). CSF pressure is 'normal' on single measurement but likely has intermittent elevations (B-waves on continuous monitoring). Ventriculomegaly compresses subcortical structures while cortical function is relatively preserved initially.

Gait Disturbance (Earliest, Most Treatable)

• Magnetic/apraxic gait: broad-based, shuffling, short steps, difficulty initiating (feet 'glued to floor')
• Falls common; difficulty turning
• Distinct from PD gait (PD: narrow-based, festinating) and cerebellar ataxia (wide-based, irregular)

Urinary Incontinence

• Initially urinary urgency and frequency (frontal lobe disinhibition of micturition reflex)
• Progresses to frank incontinence
• Faecal incontinence is late

Dementia

• Subcortical pattern: executive dysfunction, psychomotor slowing, apathy, reduced attention
• Memory retrieval impaired (improves with cues — unlike AD where encoding is impaired)
• Late feature and least responsive to shunting

Red Flags

• Gait + incontinence + dementia in elderly → NPH until proven otherwise
• Rapid onset after SAH/meningitis/neurosurgery → secondary NPH
• Full triad present → more likely to respond to shunting

Imaging

• MRI brain: ventriculomegaly disproportionate to cortical atrophy (key finding); Evans index >0.3 (ratio of maximal width of frontal horns to maximal internal diameter of skull); callosal angle <40° (on coronal view); periventricular signal change (transependymal CSF flow); tight high convexity sulci (DESH — disproportionately enlarged subarachnoid-space hydrocephalus)
• CT head: may show ventriculomegaly but MRI is preferred

CSF Dynamics (Diagnostic)

• Therapeutic lumbar puncture (tap test): drain 30-50mL CSF; assess gait improvement (timed 10-metre walk, TUG test) at 1 hour, 24 hours, and 72 hours; positive response predicts shunt success (sensitivity ~50-60%; but poor sensitivity means negative test does NOT exclude NPH)
• Extended lumbar drainage (ELD): 3-5 days continuous drainage via temporary lumbar drain; higher sensitivity (~80-90%) than single tap test; used if tap test equivocal but clinical suspicion high
• CSF opening pressure: typically normal or mildly elevated (<18 cmH₂O) — by definition

Neuropsychological Testing

• Before and after tap test: document improvement in processing speed, attention, executive function

Other

• Standard dementia screen bloods: exclude reversible causes

Surgical (Definitive)

• Ventriculoperitoneal (VP) shunt: diverts CSF from ventricle to peritoneal cavity; programmable valve (allows pressure adjustment without re-operation)
• Gait improves in ~60-80% of properly selected patients
• Incontinence improves in ~50-70%
• Dementia improves in ~30-50% (least responsive — often irreversible if long-standing)
• Patient selection is key: best outcomes with short symptom duration, gait-predominant presentation, positive tap test, identifiable secondary cause

Shunt Complications

• Subdural haematoma/hygroma (~10%): over-drainage
• Infection (~5%): shunt infection
• Shunt malfunction/obstruction
• Under/over-drainage (adjustable valves help manage)

Non-Surgical

• Serial therapeutic lumbar punctures: temporising if surgery declined/contraindicated
• Physiotherapy: gait training, falls prevention
• OT: home safety assessment
• Continence management

Referral Criteria

• Neurosurgery: all suspected NPH for shunt assessment
• Neurology: for diagnostic workup
• Consider in any patient with 'reversible' dementia screen

With VP shunt: ~60-80% improve overall; gait most responsive, dementia least. Earlier treatment → better outcomes. Without treatment: progressive decline in all three domains. Post-shunt improvement may be sustained for years. Shunt complication rate ~20-30% (most manageable). Long-term outcomes better in secondary NPH (identifiable cause) than idiopathic NPH. Patients with typical triad, short duration, and positive tap test have best shunt outcomes.

NPH Diagnostic Criteria (Key Imaging Features)

Response to VP Shunt by Symptom