Mechanism of Incontinence in Normal Pressure Hydrocephalus
No, incontinence in NPH is NOT overflow incontinence from urinary retention—it is primarily urge incontinence caused by detrusor overactivity (detrusor hyperreflexia) due to disrupted cortical inhibition of the micturition reflex. 1
Pathophysiology of Urinary Dysfunction in NPH
The urinary symptoms in NPH result from a fundamentally different mechanism than overflow incontinence:
- Detrusor overactivity is the dominant urodynamic finding, occurring in 95.2% of NPH patients, which causes urgency, frequency, and urge incontinence rather than retention 1
- The mechanism involves disruption of cortical control pathways that normally inhibit the micturition reflex, similar to other suprapontine neurological lesions 1
- Bladder recovery after shunt surgery correlates with increased mid-cingulate perfusion, suggesting restoration of cortical inhibitory control over the micturition reflex 1
Clinical Presentation Pattern
The urinary symptoms manifest distinctly from overflow incontinence:
- Isolated overactive bladder symptoms occur in 64% of patients, while frank urinary incontinence occurs in 57% 1
- Urinary incontinence is part of the classic NPH triad (gait disturbance, cognitive impairment, urinary incontinence), with gait disturbance typically appearing first in approximately 70% of patients 2
- The incontinence pattern is characterized by urgency and urge incontinence, not the continuous dribbling or dampness typical of overflow incontinence 1
Key Distinction from Overflow Incontinence
This is a critical diagnostic point:
- Overflow incontinence results from urinary retention with bladder overdistension, causing continuous leakage when bladder pressure exceeds sphincter resistance—this produces dampness rather than soaking episodes 3
- NPH incontinence is urge incontinence from uninhibited detrusor contractions, producing episodic loss of urine control with preserved bladder sensation 1
- The urodynamic pattern in NPH shows detrusor hyperreflexia, not the detrusor underactivity and high post-void residuals seen in overflow incontinence 1
Treatment Implications
Understanding the correct mechanism guides appropriate management:
- Neurosurgical shunt placement is effective for urinary symptoms in 61.5% of NPH patients, as it addresses the underlying pathophysiology by reducing ventricular enlargement and restoring cortical function 1
- Medical management includes anticholinergic medications (those unable to cross the blood-brain barrier), transcutaneous electrical nerve stimulation, and sacral neuromodulation—all targeting overactive bladder, not retention 1
- Properly selected patients have an 80-90% chance of responding to shunt surgery, with potential improvement in all symptoms including urinary dysfunction 2, 4