Normal Pressure Hydrocephalus (NPH)

Normal Pressure Hydrocephalus (NPH)

Overview

Normal pressure hydrocephalus (NPH) is a neurological disorder characterized by the triad of gait disturbance, cognitive decline, and urinary incontinence, occurring in the presence of enlarged cerebral ventricles but normal cerebrospinal fluid (CSF) pressure on lumbar puncture[@adams1965][@hakim1965]. NPH represents one of the few potentially reversible causes of dementia, making its accurate diagnosis critically important. The condition was first described by Hakim and Adams in 1965, who reported a distinctive syndrome of ventricular enlargement, walking difficulty, and dementia that improved following CSF drainage[@hakim2009].

Despite being recognized for over 60 years, NPH remains underdiagnosed, with an estimated 80% of cases going unrecognized. Common misdiagnoses include [Alzheimer's disease](/diseases/alzheimers-disease) and [Parkinson's disease](/diseases/parkinsons-disease), underscoring the need for improved awareness among clinicians. When appropriately diagnosed and treated, 50-80% of patients experience significant clinical improvement following shunt surgery.

Epidemiology

Prevalence and Incidence

Normal Pressure Hydrocephalus (NPH)

Overview

Normal pressure hydrocephalus (NPH) is a neurological disorder characterized by the triad of gait disturbance, cognitive decline, and urinary incontinence, occurring in the presence of enlarged cerebral ventricles but normal cerebrospinal fluid (CSF) pressure on lumbar puncture[@adams1965][@hakim1965]. NPH represents one of the few potentially reversible causes of dementia, making its accurate diagnosis critically important. The condition was first described by Hakim and Adams in 1965, who reported a distinctive syndrome of ventricular enlargement, walking difficulty, and dementia that improved following CSF drainage[@hakim2009].

Despite being recognized for over 60 years, NPH remains underdiagnosed, with an estimated 80% of cases going unrecognized. Common misdiagnoses include [Alzheimer's disease](/diseases/alzheimers-disease) and [Parkinson's disease](/diseases/parkinsons-disease), underscoring the need for improved awareness among clinicians. When appropriately diagnosed and treated, 50-80% of patients experience significant clinical improvement following shunt surgery.

Epidemiology

Prevalence and Incidence

Idiopathic NPH (iNPH) primarily affects individuals over 60 years, with peak prevalence in the 70-79 age group. Population-based studies estimate the prevalence of iNPH at approximately 0.5-2.9% in individuals over 65 years, making it a significant contributor to reversible dementia in the elderly[@relkin2005]. Some studies suggest that up to 5-10% of individuals with dementia may have NPH as a contributing or sole factor.

The incidence of iNPH increases with age, estimated at 5.5 per 100,000 person-years overall, rising to 18 per 100,000 in those over 70. Men may be slightly more affected than women (ratio ~1.3:1).

Underdiagnosis

NPH is widely considered the most underdiagnosed reversible cause of dementia. Several factors contribute[@marmarou2005]:

• Symptom overlap with more common neurodegenerative diseases
• Gradual symptom onset that may be attributed to normal aging
• Incomplete clinical triad presentation (only 50-60% present with all three symptoms)
• Limited awareness of NPH among primary care physicians
• Reluctance to pursue invasive shunt surgery in elderly patients

Classification

Idiopathic Normal Pressure Hydrocephalus (iNPH)

The most common form, occurring in older adults without an identifiable cause. iNPH accounts for approximately 50-80% of all NPH cases and is considered a disease of aging, with emerging evidence linking it to cerebrovascular risk factors, glymphatic system dysfunction, and impaired CSF absorption at the arachnoid granulations[@relkin2005].

Secondary Normal Pressure Hydrocephalus

Results from known underlying conditions including[@ringstad2020]:

• Subarachnoid hemorrhage: The most common cause, disrupting arachnoid granulation function
• Traumatic brain injury: Post-traumatic CSF dynamics impairment
• Meningitis: Inflammatory damage to CSF absorption pathways
• Brain tumors: Mechanical obstruction or altered CSF dynamics
• Previous neurosurgery: Post-operative adhesions affecting CSF flow

Pathophysiology

CSF Dynamics

NPH involves a complex disturbance of CSF production, circulation, and absorption. The traditional model emphasizes impaired CSF absorption through the arachnoid granulations, but contemporary understanding recognizes a more nuanced pathophysiology[@kitagaki1998]:

• Impaired CSF absorption: Reduced clearance through arachnoid villi leads to gradual ventricular enlargement
• Altered CSF pulsatility: Increased ventricular pulse pressure transmits mechanical stress to periventricular white matter
• Retrograde transependymal CSF flow: CSF migration into periventricular tissue causes edema and white-matter damage
• Cerebral blood flow reduction: Compressed periventricular capillaries lead to chronic ischemia
• Reduced intracranial compliance: Loss of the normal buffering capacity of the craniospinal system

Glymphatic System Dysfunction

Emerging research has implicated glymphatic system dysfunction as a key mechanism in iNPH pathogenesis. The glymphatic system relies on perivascular spaces and AQP4 water channels on [astrocyte](/cell-types/astrocytes) endfeet to facilitate CSF-interstitial fluid exchange and clearance of metabolic waste products from the brain[@ringstad2020][@wang2025].

In iNPH[@mori2021]:

• Reduced AQP4 expression on astrocyte endfeet diminishes CSF periarterial inflow and CSF-ISF exchange, leading to accumulation of metabolic waste including [amyloid-beta](/proteins/amyloid-beta) and [tau](/proteins/tau)[@ding2022]
• Impaired perivascular drainage causes neurotoxic metabolite accumulation and contributes to cognitive impairment
• Sleep-dependent glymphatic clearance is disrupted, as glymphatic function is primarily active during sleep
• AQP4 autoantibodies have been detected in some iNPH patients, suggesting an autoimmune component

Affected Brain Regions

Periventricular White Matter: The frontal and parietal white matter is most vulnerable to damage from ventricular expansion, transependymal CSF flow, and chronic ischemia. Disruption of frontal-subcortical circuits that control gait, executive function, and bladder regulation produces the classic triad.

Corpus Callosum: Progressive thinning and callosal angle narrowing due to chronic ventricular enlargement disrupt interhemispheric communication.

Corona Radiata: Compression of descending corticospinal tract fibers contributes to the characteristic gait disorder.

Basal Ganglia: Distortion of basal ganglia circuits due to ventricular expansion may contribute to parkinsonian features.

Hippocampal and Cortical Structures: Secondary cortical atrophy may develop in advanced or longstanding cases, potentially indicating comorbid neurodegeneration.

Vascular Contributions

Cerebrovascular disease is frequently comorbid with iNPH and may contribute to its pathogenesis:

• Hypertension and arteriosclerosis impair CSF absorption and perivascular clearance
• White matter hyperintensities on [neuroimaging](/diagnostics/neuroimaging) are more prevalent in iNPH patients
• Reduced cerebral blood flow in periventricular regions correlates with symptom severity
• Endothelial dysfunction may impair the [blood-brain barrier](/mechanisms/blood-brain-barrier-breakdown) and CSF dynamics

Clinical Features

Classic Triad

The classic Hakim-Adams triad of symptoms typically develops insidiously over months to years:

Gait Disturbance: Often the first and most prominent symptom, present in over 90% of cases[@adams1965]. Characteristic features include:

• Magnetic gait: Feet appear stuck to the floor with difficulty initiating steps
• Wide-based, shuffling walk: Short stride length with reduced foot clearance
• Gait ignition failure: Difficulty starting to walk, especially from sitting
• Gait apraxia: Impaired motor planning for walking despite preserved leg strength
• Frequent falls: Due to postural instability and impaired balance reflexes
• En bloc turning: Turning requires multiple steps rather than smooth pivoting
• Preserved arm swing: Distinguishes from [Parkinson's disease](/diseases/parkinsons-disease) where arm swing is reduced

• Psychomotor slowing: Delayed processing speed is the earliest cognitive sign
• Executive dysfunction: Impaired planning, sequencing, set-shifting, and multitasking
• Attention deficits: Reduced sustained and divided attention
• Memory retrieval deficits: Impaired recall but relatively preserved recognition (distinguishing from [Alzheimer's disease](/diseases/alzheimers-disease) where encoding is impaired)
• Apathy and reduced motivation: Prominent behavioral change
• Preserved language and visuospatial function: Early in the disease

• Urgency: Detrusor overactivity causing sudden urges
• Frequency: Increased urination frequency, especially nocturia
• Urge incontinence: Inability to reach the bathroom in time
• Frank incontinence: In advanced disease, continuous incontinence with reduced awareness

Presentation Patterns

Only 50-60% of patients present with the complete triad. The most common presentation is gait disturbance alone or gait combined with cognitive decline. Isolated dementia or urinary symptoms without gait impairment is atypical and should prompt consideration of alternative diagnoses.

Diagnosis

Clinical Criteria

Probable iNPH:

Possible iNPH: Less stringent criteria allowing for incomplete triad or atypical features.

Neuroimaging

[Neuroimaging](/diagnostics/neuroimaging) is the preferred imaging modality:

• Evans Index >0.3: Ratio of maximum frontal horn width to maximum biparietal diameter on axial imaging; the primary screening measure for ventriculomegaly
• Callosal angle <90 degrees: Measured on coronal [neuroimaging](/diagnostics/neuroimaging) at the level of the posterior commissure; a specific finding for iNPH
• DESH pattern: Disproportionately Enlarged Subarachnoid-space Hydrocephalus, characterized by tight high-convexity and medial subarachnoid spaces combined with enlarged Sylvian fissures. DESH has high positive predictive value (77%) for shunt responsiveness
• Periventricular hyperintensities: T2/FLAIR signal changes reflecting transependymal CSF flow or chronic ischemia
• Flow void sign: Prominent CSF flow void in the aqueduct on T2 [neuroimaging](/diagnostics/neuroimaging), indicating hyperdynamic CSF flow
• Absent or minimal hippocampal atrophy: Distinguishes from [Alzheimer's disease](/diseases/alzheimers-disease) where hippocampal atrophy is prominent

Supplementary Diagnostic Tests

CSF Tap Test: Removal of 30-50 mL CSF via lumbar puncture with assessment of gait improvement over 30 minutes to 72 hours[@marmarou2005]. A positive response (>10% improvement in gait speed or step length) has:

• Positive predictive value: 73-100%
• Negative predictive value: 25-42% (a negative tap test does NOT exclude NPH)

• Sensitivity: 80-90%
• Specificity: 80-90%
• Better negative predictive value than the tap test

Intracranial Pressure Monitoring: Overnight ICP monitoring may reveal B-waves (intermittent pressure elevations at 0.5-2/minute) suggestive of reduced intracranial compliance.

Relationship to Alzheimer's Disease

The overlap between NPH and [Alzheimer's disease](/diseases/alzheimers-disease) is clinically significant and represents a major diagnostic challenge[@mullerschmitz2020]:

Comorbidity

Approximately 30-50% of iNPH patients show concomitant Alzheimer's pathology on biopsy or at autopsy. This overlap likely reflects shared risk factors (age, vascular disease) and a potential mechanistic link through impaired [amyloid-beta](/proteins/amyloid-beta) clearance due to glymphatic dysfunction.

Differential Diagnosis

CSF biomarkers can help differentiate iNPH from AD[@mullerschmitz2020]:

• iNPH typically shows normal or only mildly reduced A-beta42/40 ratio
• AD shows more pronounced A-beta42 reduction and elevated total/phosphorylated tau
• Neurofilament light chain (NfL) is elevated in both but tends to be higher in iNPH with white-matter damage

Mechanistic Overlap

Bidirectional Mendelian randomization studies have confirmed shared genetic architecture between iNPH and Alzheimer's disease[@bidirectional2024], suggesting convergent pathogenic pathways involving:

• Vascular dysfunction
• Impaired glymphatic/lymphatic clearance
• [Neuroinflammation](/mechanisms/neuroinflammation)
• Age-related brain changes

Treatment

Ventriculoperitoneal (VP) Shunt

VP shunt placement remains the standard of care for iNPH[@marmarou2005]:

Procedure: A catheter is placed in the lateral ventricle and tunneled subcutaneously to the peritoneal cavity, allowing CSF diversion. Modern systems include:

• Programmable valves: Allow noninvasive adjustment of shunt opening pressure via external magnetic device, enabling optimization of CSF drainage without reoperation
• Anti-siphon devices: Prevent excessive drainage in the upright position
• Gravitational valves: Compensate for postural changes in hydrostatic pressure

• Gait improvement: 70-90% of well-selected patients
• Cognitive improvement: 50-80% (primarily in psychomotor speed and attention)
• Urinary symptom improvement: 40-70%
• Overall clinical improvement: 50-80%

• Predominant gait symptoms (strongest predictor)
• Positive tap test or ELD response
• Shorter symptom duration (<2 years)
• Secondary NPH (vs. idiopathic)
• Absent or minimal cortical atrophy
• DESH pattern on [neuroimaging](/diagnostics/neuroimaging)
• Elevated Rout on infusion testing

• Subdural hematoma or hygroma: 2-17%
• Shunt malfunction or obstruction: 5-15%
• Infection: 3-6%
• Over-drainage headaches: managed by valve pressure adjustment
• Need for revision: 20-30% over 5 years

Endoscopic Third Ventriculostomy (ETV)

ETV creates an alternative CSF pathway by fenestrating the floor of the third ventricle, allowing CSF to bypass obstructed absorption pathways. Its role in iNPH is evolving:

• ETV may be effective in carefully selected iNPH patients, with improvement rates of 60-70% in some series
• Advantages over VP shunt: no implanted hardware, lower infection risk, no valve-related complications
• Limitations: lower success rate than VP shunt in most studies; more effective when CSF outflow resistance is high or when partial obstruction exists

Conservative Management

For patients who decline surgery or are not surgical candidates:

• Serial lumbar punctures for temporary symptom relief
• Physical therapy for gait training and fall prevention
• Cognitive rehabilitation
• Management of comorbidities (hypertension, diabetes)
• Monitoring for disease progression

Prognosis

Without treatment, iNPH progresses over months to years, leading to severe disability with wheelchair dependence, profound dementia, and total incontinence. Untreated NPH significantly increases morbidity and mortality from falls, aspiration, and immobility-related complications.

With appropriate shunt surgery:

• Gait is the symptom most likely to improve and responds most robustly
• Cognition shows meaningful improvement in 50-80%, particularly when intervention is early
• Urinary symptoms improve less consistently
• Sustained benefit: Long-term follow-up shows persistent improvement in most responders at 3-5 years
• Prognostic factors: Shorter symptom duration, predominant gait disturbance, positive supplementary tests, and absence of comorbid neurodegenerative disease predict better outcomes

Research Directions

Current research focuses on:

See Also

• [Alzheimer's Disease](/diseases/alzheimers-disease)
• [Parkinson's Disease](/diseases/parkinsons-disease)
• [Glymphatic System](/entities/glymphatic-system)
• [Cerebrospinal Fluid Dynamics](/mechanisms/csf-dynamics)
• [Vascular Cognitive Impairment](/mechanisms/vascular-cognitive-impairment-pathway)
• [Dementia: Differential Diagnosis](/mechanisms/dementia-differential-diagnosis)
• [White Matter Hyperintensities](/mechanisms/white-matter-hyperintensities)

External Links

• [NINDS: Normal Pressure Hydrocephalus](https://www.ninds.nih.gov/health-information/disorders/normal-pressure-hydrocephalus)
• [Hydrocephalus Association](https://www.hydroassoc.org/)
• [ClinicalTrials.gov: Normal Pressure Hydrocephalus](https://clinicaltrials.gov/search?cond=Normal+Pressure+Hydrocephalus)