Normal Pressure Hydrocephalus: Pathophysiology and Management
Pathophysiology
Normal pressure hydrocephalus (NPH) occurs when the brain ventricles progressively expand despite normal cerebrospinal fluid (CSF) pressure measurements, leading to the classic triad of gait disturbance, urinary incontinence, and dementia. 1, 2
The key pathophysiological features include:
Communicating hydrocephalus with ventricular dilatation (Evans index > 0.3) despite CSF pressures that remain within normal ranges on single or even multiple measurements 3, 2
Progressive CSF accumulation occurs even though individual pressure recordings are normal, suggesting intermittent pressure elevations or impaired CSF absorption that standard pressure monitoring fails to capture 3
The mechanism involves impaired CSF reabsorption rather than overproduction or obstruction, distinguishing it from obstructive hydrocephalus 4, 2
Diagnostic Approach
Diagnosis requires MRI with contrast to confirm ventriculomegaly and exclude other causes of the clinical syndrome. 5, 6
Critical diagnostic elements:
Clinical triad assessment: Gait disturbance (most responsive), cognitive impairment (intermediate response), and urinary incontinence (least responsive to treatment) 1, 7
Imaging confirmation: Evans index > 0.3 on MRI with communicating hydrocephalus 2
CSF diversion trial: Temporary CSF drainage via lumbar drain with standardized pre- and post-drainage testing is the most reliable predictor of shunt responsiveness 8
Patient Selection for Shunting
The most effective approach uses standardized gait, balance, and cognitive testing before and after temporary CSF diversion (lumbar drain trial) to identify surgical candidates. 8
The validated testing protocol includes:
Gait and balance assessments: Timed "Up & Go", Tinetti Gait and Balance Assessment, Berg Balance Scale 8
Cognitive testing: Mini Mental Status Exam, Trail Making Test Part B, Rey Auditory and Visual Learning Test 8
Response criteria: Patients showing clinically meaningful improvement (particularly in Tinetti scores, mean increase of 4.27 points) after lumbar drain trial have 89.6% likelihood of symptomatic improvement at 6 weeks post-shunt 8
A common pitfall is relying solely on CSF pressure measurements or ventricular size to guide treatment decisions—neither predicts surgical outcome. 5, 3
Surgical Management
Primary Treatment: Ventriculoperitoneal Shunt
VP shunt placement remains the definitive treatment for NPH, with sustained clinical improvement achievable for 5-7 years in appropriately selected patients. 5, 6, 7
Key management principles:
Laparoscopic technique is preferred over open placement, showing fewer distal shunt failures (6 of 155 vs. 7 of 77), shorter operating times, and reduced hospital stays 1
Expected outcomes by symptom:
Shunt failure rates: Overall 15-16% failure rate, with 53% of patients requiring at least one revision over long-term follow-up 1, 7
Alternative: Endoscopic Third Ventriculostomy
ETV has emerged as an alternative with lower long-term complication rates, though it shows higher early failure rates (first 3 months) compared to shunts. 5, 6, 4
Decision algorithm for ETV vs. VP shunt:
ETV is preferred when suitable anatomy exists and in cases of aqueductal stenosis, as failure rates become lower than shunts after the 3-month mark 6, 4
VP shunt is preferred for communicating hydrocephalus (as in NPH) or when anatomy is unsuitable for ETV 4
Both modalities demonstrate equivalent overall outcomes in many clinical scenarios when adjusted for patient age and etiology 5, 6, 4
Serial Lumbar Punctures
Serial lumbar punctures are NOT recommended as definitive treatment for NPH. This approach fails to provide sustained CSF diversion and does not prevent disease progression. 9, 6
Complications and Long-Term Management
Shunt obstruction is the primary cause of treatment failure and requires vigilant monitoring—87% of revisions are performed for malfunction, with 74% of revisions resulting in clinical improvement. 7
Critical management points:
Complication rates: Subdural fluid collections occur in 18% of patients, with other serious complications (seizure, hemorrhage, stroke) in 6% 8
Infection risk: Approximately 11% of initial shunt placements develop infection within 24 months 9, 4
Infection management: Complete shunt removal with systemic antibiotics is the standard approach 9, 4
Long-term monitoring: Regular assessment of shunt function and neurological status is essential, as approximately 45% of patients report symptom recurrence at 1 year despite initial improvement 8
A critical pitfall is assuming initial improvement guarantees long-term success—nearly half of patients experience symptom recurrence by one year, necessitating ongoing surveillance. 8
Expected Outcomes
Moderate-certainty evidence demonstrates that CSF-shunting produces large reductions in disability (risk ratio 2.08) and probable improvements in gait speed in the short term. 2
Realistic outcome expectations:
Short-term success: 89.6% report symptomatic improvement at 6 weeks 8
Long-term maintenance: Clinical improvement can be sustained for 5-7 years in appropriately selected patients, though multiple revisions may be necessary 7
Symptom hierarchy: Gait shows highest sustained improvement, followed by cognition, then urinary symptoms 7
Cognitive outcomes: Evidence remains very uncertain regarding the magnitude of cognitive improvement 2