Normal Pressure Hydrocephalus

• Impaired CSF absorption at the arachnoid granulations: The primary pathophysiologic mechanism involves reduced clearance of CSF at the arachnoid villi due to fibrosis, adhesions, or dysfunction of the arachnoid granulation complexes. Under normal conditions, CSF is absorbed through the pacchionian bodies (arachnoid granulations) into the dural sinuses; in NPH, this absorption is compromised, leading to gradual ventricular enlargement. The reduced absorptive capacity may result from age-related degeneration, prior inflammation from infection or subarachnoid hemorrhage, or idiopathic fibrosis of the subarachnoid space. Cerebrospinal fluid pressure remains relatively normal (typically <25 cm H₂O) because the slow accumulation allows compensatory expansion of the ventricular system. This distinguishes NPH from acute obstructive hydrocephalus, where CSF pressure rises acutely.

• Altered CSF pulsatile flow and wave propagation: In NPH, the normal pulsatile CSF flow dynamics are disrupted, with reduced compliance of the ventricular system and altered transmission of intracranial pressure waves. Normally, CSF pulsations with the cardiac cycle facilitate parenchymal fluid exchange and waste clearance through the glymphatic system. In NPH, enlarged ventricles transmit abnormal pressure waves to the surrounding parenchyma, particularly affecting periventricular white matter. Advanced imaging demonstrates altered CSF flow patterns with increased force of ventricular fluid jets impacting the frontal horns. These abnormal flow dynamics contribute to parenchymal damage independent of elevated pressure. The mechanics of abnormal flow are particularly important for understanding why some patients respond to shunt placement despite normal opening pressure.

• Periventricular white matter ischemia and glymphatic dysfunction: Chronic ventricular enlargement causes mechanical stretching and compression of periventricular white matter tracts, leading to demyelination and axonal dysfunction. Additionally, the abnormal CSF dynamics impair the glymphatic system—the brain's interstitial fluid clearance mechanism dependent on aquaporin-4 water channels in astrocytes and appropriate CSF-interstitial fluid exchange. Accumulation of neurotoxic proteins (including tau and amyloid-beta) results from impaired clearance, contributing to cognitive decline. White matter ischemia also occurs secondary to stretching of periventricular small penetrating arteries and arterioles, creating zones of hypoperfusion in the frontal and parietal white matter. Magnetic resonance imaging (MRI) reveals characteristic periventricular T2/FLAIR hyperintensities reflecting this white matter change. This mechanism explains why cognitive symptoms may improve following successful shunting, as restoration of normal CSF dynamics enhances glymphatic clearance.

• Disruption of frontal-subcortical neural circuits: The selective involvement of periventricular structures—particularly those crucial for gait control and cognition—explains the specific symptom profile. Damage to the superior longitudinal fasciculus and other frontal projection fibers disrupts circuits essential for coordinated gait initiation and cognitive processing. The gait disturbance specifically reflects involvement of the paramedian mesencephalic locomotor region connections and frontal supplementary motor area fibers. Cognitive decline predominantly affects executive function and processing speed (frontosubcortical dementia pattern) rather than memory early, reflecting the anatomical distribution of white matter injury. The urinary dysfunction results from disruption of frontal-pontine connections that inhibit micturition reflexes. These circuit-specific effects help explain why NPH presents with this particular combination of symptoms rather than global cognitive decline.

• Compensatory hydrocephalus and chronic changes: In idiopathic NPH, the process develops insidiously over years, allowing brain parenchyma to partially compensate through ventricular enlargement while initially maintaining relatively normal CSF pressure. Over time, the chronically enlarged ventricles create a state of "compensated" hydrocephalus where the brain has adapted but functions suboptimally. The Monro-Kellie doctrine explains this: the intracranial compartment contains brain, blood, and CSF; as CSF volume increases, brain volume decreases through gliosis and neuronal loss. This chronic adaptation means that acutely reducing CSF volume (through shunting) may not immediately restore function; instead, gradual improvement occurs as cerebral perfusion improves and some degree of neural reorganization occurs. The degree of chronicity and parenchymal damage are critical factors determining whether shunting will produce functional improvement.

• Idiopathic NPH (most common, ~50% of cases): The majority of NPH cases develop without a recognized precipitating event. The underlying etiology in idiopathic NPH likely involves age-related changes in arachnoid granulation function, degeneration of subarachnoid space compliance, and age-associated changes in CSF production rates. Some evidence suggests subclinical inflammation or viral activation may trigger arachnoid fibrosis. Genetics appears to play a role; family clustering has been reported in rare cases, and recent genome-wide association studies have identified genetic variants associated with NPH susceptibility. The idiopathic form typically develops in individuals over 60 years without prior CNS events, making it important to exclude secondary causes through careful history and imaging.

• Secondary NPH following subarachnoid hemorrhage (most common secondary cause): Traumatic or spontaneous subarachnoid hemorrhage initiates an inflammatory cascade within the subarachnoid space, leading to organization of blood clots and fibrosis of the arachnoid granulations. NPH can develop months to years after the acute hemorrhage as chronic scarring compromises CSF absorption. This represents the most common identifiable cause of NPH, occurring in approximately 5-10% of subarachnoid hemorrhage survivors. The timeline for symptom onset after hemorrhage is variable but typically occurs within 6-12 months, though later presentations are well-documented. Patients often report a lucid interval followed by progressive decline, distinguishing this from acute hydrocephalus from the hemorrhage itself.

• Secondary NPH following head trauma: Moderate to severe traumatic brain injury can precipitate NPH through development of subdural hematomas, subarachnoid blood, and subsequent arachnoid fibrosis. The closed head injury creates inflammation and tissue damage that extend into the subarachnoid space, triggering the same fibrotic cascade seen after spontaneous subarachnoid hemorrhage. NPH may develop months after the acute injury when immediate post-traumatic neurological recovery seems complete, representing a delayed consequence of CNS trauma.

• Secondary NPH following meningitis: Bacterial meningitis causes acute inflammation of the meninges and subarachnoid space, with subsequent fibrosis and adhesion formation compromising CSF circulation. NPH developing after meningitis is well-recognized, occurring in the chronic phase after apparent recovery from the acute infection. Tuberculous meningitis has particular propensity for causing chronic hydrocephalus due to extensive meningeal fibrosis. Fungal meningitis and neurosyphilis similarly carry risk for developing chronic hydrocephalus.

• Secondary NPH following arachnoiditis: Chronic adhesive arachnoiditis from any cause (prior surgery, spinal cord injury, chronic inflammation) can impair CSF circulation and predispose to NPH development. Spinal cord injury with subsequent tethering and arachnoiditis represents a recognized risk factor.

• Age as fundamental risk factor: Age remains the strongest demographic risk factor, with NPH predominantly affecting those older than 60 years. Age-related degenerative changes in arachnoid granulation structure and function, combined with reduced cerebral compliance, create the substrate for NPH development. Vascular risk factors including hypertension and atherosclerosis may predispose to both idiopathic NPH and secondary forms through effects on cerebrovascular function and glymphatic system integrity.

• Gait disturbance (earliest and most consistent symptom): The characteristic "magnetic gait" or "frontal gait" represents the hallmark symptom, though terminology varies. Patients describe difficulty initiating walking, sensation of feet "sticking to the floor," or short shuffling steps reminiscent of Parkinson's disease but typically with preserved arm swing (important distinguishing feature). The gait is broad-based, slow, and cautious, reflecting disruption of frontal supplementary motor area control of gait initiation. Patients often report the gait worsens in confined spaces (like aisles) and improves with external cues or walking up stairs—opposite to Parkinson's disease. The pathophysiology reflects damage to frontostriatal connections and the paramedian mesencephalic locomotor region projections. Falls become increasingly common as gait dysfunction progresses, and balance is often relatively preserved early compared to true Parkinsonian syndrome. Lower-extremity weakness is notably absent, which helps distinguish NPH from other causes of gait disturbance.

• Cognitive decline and dementia: Cognitive changes typically manifest as frontosubcortical dementia, with prominent impairment of executive function, processing speed, and attention rather than early memory loss. Patients exhibit apathy, psychomotor slowing, impaired complex problem-solving, and decreased initiative. In early stages, memory may be relatively preserved; when memory loss occurs, it reflects retrieval deficits rather than encoding failure (distinguishing this from Alzheimer's disease patterns). Cognitive changes may be subtle initially, with family noting personality changes, loss of motivation, or decreased work performance before frank dementia becomes apparent. Progression can be insidious over months to years, or more rapid depending on the underlying etiology. Some patients present with dementia as the primary symptom before gait changes become obvious. Reversibility of cognitive symptoms with shunting is variable; earlier intervention and less severe baseline impairment correlate with better cognitive recovery.

• Urinary incontinence (typically later symptom): Urinary urgency and incontinence develop later in the disease course, reflecting disruption of frontal inhibitory control of the micturition reflex. Characteristically, incontinence is urge-type with preserved sensation; patients report sudden urgent need to void with difficulty maintaining continence. Patients may deny incontinence initially, attributing symptoms to other causes or being unaware of episodes during sleep. The incontinence reflects loss of the normal frontal cortical inhibition of the pontine micturition center, resulting in hyperactive bladder function. Importantly, fecal incontinence is less common and its presence should prompt consideration of alternative diagnoses. Nocturia commonly precedes daytime incontinence.

• Additional early manifestations: Beyond the classic triad, patients often present with other features that direct attention toward NPH diagnosis. Bradykinesia and hypokinesia manifest as generalized slowing of movements, reduced spontaneous activity, and decreased initiation of purposeful movement—reflecting frontosubcortical dysfunction. Psychomotor retardation is prominent, distinct from depression (though depression frequently coexists). Some patients present with apathetic or akinetic mutism in advanced stages. Vertigo or dizziness occurs in some patients, though the mechanism is unclear—possibly reflecting cerebellar effects of pressure changes or white matter involvement. Headaches may occur but are not prominent features; their presence should prompt consideration of other diagnoses.

• Physical examination findings: Beyond the characteristic gait, formal neurological examination reveals important findings. Cognitive testing demonstrates impaired executive function (poor performance on Trail Making Test B, verbal fluency tests, clock drawing) with relatively preserved memory early in disease. Grasp reflex and other frontal release signs may be present. Hyperreflexia and increased tone may develop as an early sign, contrasting with typical hypokinetic disorders. Nystagmus or smooth pursuit abnormalities occasionally occur. Pupillary responses remain normal, which helps exclude other diagnoses. Importantly, motor strength is preserved, and there should be no focal neurological deficits—their presence suggests alternative pathology.

• Important clinical variants and presentations: Not all NPH patients present with the complete classic triad. Some present primarily with gait disturbance ("gait-predominant NPH"), others with cognitive symptoms ("dementia-predominant NPH"), and still others with urinary dysfunction. The proportion of each symptom varies, and one symptom may predominate early. Some patients present acutely with rapid symptom progression following a clear precipitating event (secondary NPH), while idiopathic cases progress insidiously. The severity spectrum ranges from minimal symptoms diagnosed incidentally on imaging to severely disabled patients. Patients with secondary NPH may show greater symptom variability related to the timing and nature of the precipitating event. Importantly, age of presentation varies; while NPH predominantly affects the elderly, occasional cases occur in younger patients, particularly following trauma or infection.

• Clinical diagnostic criteria: The most widely used diagnostic framework combines clinical presentation with imaging findings. Probable NPH requires: (1) the clinical triad of gait disturbance, cognitive impairment, and urinary incontinence; (2) characteristic imaging findings of ventriculomegaly with normal or only mildly elevated CSF opening pressure; and (3) absence of other explanation for symptoms. Possible NPH includes patients with only 1-2 elements of the triad plus appropriate imaging. Definite NPH (which can only be confirmed retrospectively) requires clinical improvement following shunt placement. These criteria, developed by consensus groups, help standardize diagnosis and improve patient selection for surgery. The presence of all three triad symptoms significantly increases the likelihood of NPH diagnosis.

• Imaging findings - MRI (gold standard for initial assessment): MRI should be the first neuroimaging study and demonstrates several characteristic findings. Ventricular enlargement is documented by measuring the Evans index (ratio of maximum frontal horn width to maximum biparietal skull width; >0.3 suggests ventriculomegaly) or absolute ventricular measurements. In NPH, ventricular enlargement is disproportionate to brain atrophy—the ventricles are large while brain sulci remain relatively normal or narrowed (important distinguishing feature from normal aging where both ventricles and sulci enlarge proportionally). Periventricular white matter changes appear as T2/FLAIR hyperintensities surrounding the lateral ventricles, particularly at the frontal and parietal horns, reflecting chronic interstitial edema from CSF transudation and white matter ischemia. Reduced flow voids within the aqueduct of Sylvius and at the foramen magnum may be appreciated on conventional MRI, with increased pulsatile flow visible on phase-contrast cine MRI. Enlarged subarachnoid spaces at the vertex and frontal convexities, paradoxically contrasting with compressed basilar cisterns, constitute the "ventricular disproportionality sign"—a helpful diagnostic feature. The combination of these findings is more sensitive than any single feature.

• Imaging findings - CT: When MRI is contraindicated, CT demonstrates ventricular enlargement and may show calcifications or other findings suggesting etiology. However, CT is less sensitive for white matter changes and cannot assess CSF flow dynamics, limiting its diagnostic utility. CT should not be used as primary imaging for NPH diagnosis when MRI is available.

• Cerebrospinal fluid opening pressure and composition: Normal to mildly elevated opening pressure (5-25 cm H₂O) is characteristic; markedly elevated pressure (>25 cm H₂O) suggests alternative diagnosis. Importantly, normal opening pressure does not exclude NPH—the name "normal pressure hydrocephalus" reflects this key feature that distinguishes it from obstructive hydrocephalus. CSF analysis should show normal protein and glucose with normal or near-normal cell counts—significant pleocytosis or biochemical abnormalities suggest infectious, inflammatory, or malignant processes.

No effective medical therapy exists — the disease is mechanical, so the treatment is mechanical

• Acetazolamide (carbonic anhydrase inhibitor): reduces choroid plexus CSF production and is occasionally used as a temporizing measure in patients who are poor surgical candidates, but evidence is weak and it is not a substitute for shunting. Expect metabolic acidosis, paresthesias, and hypokalemia.
• Serial large-volume lumbar punctures: therapeutic benefit is transient (days to weeks) and they are used diagnostically/prognostically, not as definitive treatment.

Step 1 — confirm shunt responsiveness before operating

• **High-volume lumbar puncture (tap test)**: removal of a large volume of CSF (on the order of 30–50 mL) with objective, timed gait testing before and after. Improvement in gait speed or step length predicts shunt response and is the single best next step in a patient with the triad plus ventriculomegaly.
• Extended lumbar drainage: a lumbar catheter draining CSF over several days is more sensitive than a single tap and is pursued when the tap test is negative but clinical suspicion remains high. Some centers add CSF infusion testing to measure outflow resistance.

Step 2 — definitive management

• Ventriculoperitoneal shunting: the treatment of choice. The American Academy of Neurology's 2015 practice guideline on idiopathic NPH concluded that shunting may be offered to appropriately selected patients, with the caveat that the strength of evidence is limited and response is not guaranteed. Ventriculoatrial and lumboperitoneal shunts are alternatives when the peritoneum is unusable.
• Programmable (adjustable) valves: allow non-invasive downward or upward titration of drainage, reducing reoperation for over- or underdrainage — the standard in contemporary US neurosurgical practice.

What not to do

• Endoscopic third ventriculostomy: designed for obstructive hydrocephalus; NPH is communicating, so ETV is not the standard operation.
• Anticholinergic bladder agents (e.g., oxybutynin): worsen cognition in this population and appear on the AGS Beers Criteria as potentially inappropriate in older adults.
• Levodopa: does not improve the magnetic gait; a robust levodopa response argues for Parkinson disease instead.

Complications of untreated disease

• Falls and hip fracture: the magnetic gait with impaired postural adjustment leads to recurrent falls; a fall is often the presenting event and drives loss of independence.
• Progressive irreversible dementia: prolonged periventricular white matter injury and axonal loss mean that late shunting improves gait far more reliably than cognition. Signal: a patient with years of symptoms who improves in gait but not memory after surgery.
• Immobility-related morbidity: aspiration pneumonia, deconditioning, pressure ulcers, and urinary tract infection from incontinence.

Complications of shunting — overdrainage

• Subdural hematoma or hygroma (emergency): excessive CSF egress collapses the ventricles and stretches bridging veins, which tear. Signal: a patient who improved after shunting and then declines, with new headache, somnolence, hemiparesis, or falls weeks to months later. Obtain non-contrast head CT; management is valve upward-adjustment and, if large or symptomatic, surgical evacuation. Concurrent antiplatelet/anticoagulant use markedly raises risk.
• Low-pressure (postural) headache: worse upright, relieved supine — the mechanism is siphoning through the distal catheter; treat by raising the valve setting or adding an antisiphon device.

Complications of shunting — underdrainage and hardware

• Shunt obstruction/malfunction (emergency if acute): choroid plexus or debris occludes the proximal catheter. Signal: return of the triad with re-enlarging ventricles; abrupt obstruction can produce headache, vomiting, and depressed consciousness from acute hydrocephalus.
• Shunt infection (emergency): usually coagulase-negative staphylococci (Staphylococcus epidermidis) or S. aureus seeded at implantation, presenting within the first months with fever, meningismus, shunt-tract erythema, or unexplained decline. Diagnosis requires CSF sampled from the shunt reservoir; IDSA healthcare-associated ventriculitis and meningitis guidance recommends hardware removal plus intravenous antimicrobials, with vancomycin dosed to a 24-hour AUC/MIC of 400–600 per the 2020 IDSA/ASHP consensus.
• Catheter-tract hemorrhage or seizure: from parenchymal passage of the ventricular catheter.
• Abdominal complications: CSF pseudocyst, peritonitis, or viscus perforation from the distal catheter — signal is abdominal pain with recurrent hydrocephalus.
• Ventriculoatrial-specific: bacteremia, pulmonary thromboembolism, and immune-complex shunt nephritis with hematuria and low complement.

• "Wet, wacky, and wobbly": urinary incontinence, dementia, and gait apraxia. Examiners expect you to know that wobbly comes first — gait disturbance is the earliest symptom, the most reliably reversible, and the domain used to judge shunt response.
• The single best next step in an elderly patient with the triad and ventriculomegaly is a high-volume lumbar puncture with pre- and post-tap gait testing, not immediate surgery. The LP also documents the normal-to-mildly-elevated opening pressure that defines the syndrome.
• Definitive therapy is a ventriculoperitoneal shunt, offered to selected responders per the American Academy of Neurology's iNPH practice guideline. Gait improves most, cognition least.
• Imaging buzzwords: Evans index >0.3, ventriculomegaly out of proportion to sulcal atrophy, and tight high-convexity/medial sulci with dilated Sylvian fissures (DESH pattern). A flow void in the cerebral aqueduct is a supportive sign.
• **The classic distractor is hydrocephalus *ex vacuo*: in Alzheimer disease and generalized atrophy, ventricles and** sulci enlarge together, and the dementia is amnestic/cortical with early encoding failure. NPH is a frontosubcortical dementia with executive dysfunction, apathy, and psychomotor slowing.
• The second distractor is Parkinson disease: NPH gait is broad-based, magnetic, and arm swing is preserved, with no resting tremor and no meaningful levodopa response. Do not start dopaminergic therapy.
• The association examiners test: prior subarachnoid hemorrhage, head trauma, or meningitis (especially tuberculous) causing arachnoid granulation fibrosis — secondary NPH presenting months to years after a lucid interval.
• Post-shunt deterioration after initial improvement = subdural hematoma from overdrainage until proven otherwise; get a non-contrast head CT. New fever or shunt-tract erythema means shunt infection, typically Staphylococcus epidermidis.
• Avoid anticholinergics for the incontinence — they worsen cognition and are flagged by the AGS Beers Criteria in older adults.