# Normal Pressure Hydrocephalus

## Introduction

Normal pressure hydrocephalus (NPH) is a potentially reversible cause of dementia, gait disturbance, and urinary incontinence in older adults. The classic Hakim triad of gait apraxia, cognitive decline, and urinary incontinence, combined with ventriculomegaly out of proportion to cortical atrophy, defines this condition. NPH is surgically treatable with CSF shunting, making accurate diagnosis essential. However, distinguishing NPH from neurodegenerative diseases and age-related ventriculomegaly remains a significant clinical challenge.

## Classification

### Idiopathic NPH (iNPH)

Idiopathic NPH has no identifiable cause and is the most common form in adults over 60. Its estimated prevalence is 1 to 6 percent of individuals over 65 years. The course is chronic and progressive, and the condition may be underdiagnosed. The pathophysiology is incompletely understood, though reduced CSF absorption at the arachnoid granulations is the leading hypothesis.

### Secondary NPH

Secondary NPH results from a known antecedent event that impairs CSF dynamics. Causes include subarachnoid hemorrhage, meningitis, head trauma, and intracranial surgery. It may occur at any age and tends to have a more predictable response to shunting. It often develops weeks to months after the inciting event.

## Clinical Presentation: The Hakim Triad

### Gait Disturbance

Gait disturbance is typically the earliest and most prominent symptom. The characteristic gait is wide-based, short-stepped, and shuffling, often described as a "magnetic gait" because the feet appear stuck to the floor. Patients have difficulty initiating gait, and turns are executed with multiple small steps. Postural instability with frequent falls is common. Unlike Parkinson disease, there is no tremor, no significant rigidity, and no improvement with levodopa. Importantly, the gait response to CSF removal is the most reliable predictor of shunt response.

### Cognitive Impairment

Cognitive impairment in NPH follows a subcortical pattern characterized by psychomotor slowing, executive dysfunction, and impaired attention and concentration. Memory impairment is present but less prominent than in Alzheimer disease. Apathy, decreased motivation, and frontal lobe features are typical. Cognitive symptoms usually develop after gait disturbance and may be subtle initially.

### Urinary Incontinence

Urinary symptoms initially present as urgency and frequency before progressing to frank incontinence. The mechanism involves frontal lobe disinhibition of the micturition reflex. Urinary incontinence is typically the last symptom to appear and the least responsive to shunting. While the presence of all three triad components increases diagnostic confidence, it is not required for diagnosis.

## Diagnosis

### Imaging

MRI of the brain is the modality of choice. Ventriculomegaly is assessed using the Evans index, which is the ratio of maximum frontal horn width to maximum biparietal diameter, with a value greater than 0.3 indicating ventriculomegaly. The DESH pattern (disproportionately enlarged subarachnoid space hydrocephalus), characterized by tight high convexity sulci with dilated Sylvian fissures, is a characteristic finding supporting iNPH. A callosal angle less than 90 degrees on coronal images at the level of the posterior commissure is another supportive feature. A flow void sign at the cerebral aqueduct on T2-weighted images represents an aqueductal flow jet. Periventricular T2/FLAIR hyperintensity indicates transependymal CSF migration. CT of the head demonstrates ventriculomegaly but is less sensitive for the DESH pattern and flow dynamics.

### CSF Drainage Tests

The large-volume lumbar puncture, or tap test, involves removing 30 to 50 mL of CSF and assessing gait and cognition before and after at 1, 4, and 24 hours. A positive response is defined as improvement in gait speed, step length, or cognitive testing. The sensitivity of the tap test is approximately 50 to 60 percent, with specificity around 90 percent, meaning a negative tap test does not exclude NPH. The opening pressure is typically normal, below 18 cm H2O, by definition.

Extended lumbar drainage involves continuous drainage of 150 to 300 mL of CSF over 72 hours through a lumbar drain. It has higher sensitivity at 80 to 90 percent compared to the single tap test, and requires hospitalization. Risks include infection, overdrainage, and subdural hematoma. It is recommended when the tap test is negative but clinical suspicion remains high.

### Additional Studies

CSF infusion studies measure resistance to CSF outflow (Rout), and an elevated Rout greater than 12 mmHg/mL/min suggests impaired CSF absorption and predicts a favorable shunt response. Quantitative gait analysis measuring velocity, stride length, and cadence before and after CSF removal adds objectivity. Neuropsychological testing at baseline and post-drainage documents cognitive changes.

![Diagnostic algorithm for suspected normal pressure hydrocephalus](illustration-nph-diagnostic-algorithm.jpg)

## Treatment

### CSF Shunting

The ventriculoperitoneal (VP) shunt is the most common procedure, diverting CSF from the lateral ventricle to the peritoneal cavity. A ventriculoatrial (VA) shunt is an alternative when the peritoneal cavity is unsuitable due to prior surgery or adhesions. Lumboperitoneal (LP) shunts are less commonly used because of higher rates of overdrainage and revision. Programmable valves allow postoperative adjustment of shunt pressure settings without surgery and are widely used. Anti-siphon devices reduce overdrainage in the upright position.

### Outcomes

Improvement is seen in 60 to 80 percent of carefully selected patients after shunting. Gait is the most responsive symptom, with improvement often seen within days to weeks. Cognition may improve over weeks to months, though the response is more variable. Incontinence is the least responsive symptom, with better results when treated before incontinence becomes well established. Positive predictors of shunt response include gait improvement after the tap test, short symptom duration (less than six months), a known secondary cause, and the DESH pattern on MRI.

| Predictor | Favors Shunt Response | Unfavorable |
|-----------|----------------------|-------------|
| Tap test | Gait improvement after LP | No improvement |
| Symptom duration | <6 months | >2 years |
| Etiology | Known secondary cause (SAH, meningitis) | Idiopathic with atypical features |
| Imaging | DESH pattern, callosal angle <90° | Proportional atrophy |
| Dominant symptom | Gait predominant | Dementia predominant |
| Comorbidities | Few comorbidities | Significant vascular disease, Alzheimer overlap |

### Complications

Subdural hematoma or hygroma from overdrainage occurs in 2 to 17 percent of cases and is managed by adjusting the programmable valve to a higher pressure setting. Shunt infection has approximately a 5 percent incidence, is most commonly caused by coagulase-negative staphylococci, and requires shunt removal, external drainage, and IV antibiotics. Shunt malfunction from obstruction, disconnection, or migration causes symptom recurrence. Seizures carry a low risk with frontal catheter placement, and intracerebral hemorrhage is rare but related to catheter passage through brain parenchyma.

![VP shunt placement and programmable valve adjustment](illustration-vp-shunt-nph.jpg)

## Differential Diagnosis

Alzheimer disease presents with more prominent memory impairment and cortical atrophy proportional to ventricular enlargement, without early gait apraxia. Parkinson disease features tremor, rigidity, and bradykinesia that respond to levodopa. Vascular dementia shows step-wise cognitive decline with white matter ischemic changes. Progressive supranuclear palsy presents with vertical gaze palsy, axial rigidity, and falls. Chronic subdural hematoma may cause similar symptoms but shows an extra-axial collection on imaging rather than isolated ventriculomegaly.

![Comparison of imaging findings in NPH vs Alzheimer disease vs vascular dementia](illustration-nph-differential-imaging.jpg)

## Clinical Pearls

Gait disturbance is the earliest, most prominent, and most shunt-responsive symptom in NPH, and its presence should prompt evaluation. The Evans index greater than 0.3 indicates ventriculomegaly, but distinguishing NPH from ex vacuo ventriculomegaly requires assessment of the DESH pattern and clinical correlation. A negative tap test does not exclude NPH, and extended lumbar drainage has higher sensitivity and should be considered when clinical suspicion is high. Programmable valves allow non-invasive adjustment of shunt pressure and significantly reduce the need for revision surgery due to overdrainage. Early intervention with shorter symptom duration is associated with better shunt outcomes, and delays in diagnosis and treatment reduce the chance of meaningful recovery.

## References

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2. Hashimoto M, Ishikawa M, Mori E, et al. Diagnosis of idiopathic normal pressure hydrocephalus is supported by MRI-based scheme (SINPHONI). *Cerebrospinal Fluid Res*. 2010;7:18.
3. Halperin JJ, Kurlan R, Schwalb JM, et al. Practice guideline: Idiopathic normal pressure hydrocephalus (AAN). *Neurology*. 2015;85(15):1312-1318.
4. Williams MA, Malm J. Diagnosis and treatment of idiopathic normal pressure hydrocephalus. *Continuum (Minneap Minn)*. 2016;22(2):579-599.