Pudendal nerve stimulation recruits the urethra during awake human cystometry

This study demonstrates that pudendal nerve stimulation in awake humans effectively recruits the urethra and alters lower urinary tract pressures without significantly impacting voiding efficiency, suggesting a mechanism of action similar to sacral neuromodulation.

The Gist

The Big Picture: Tapping the "Mystery Button"

Imagine your body has a complex plumbing system (your bladder and urethra) controlled by a sophisticated electrical network (your nerves). For years, doctors have been using a device called a Pudendal Nerve Stimulator to help people with bladder problems (like leaking, holding too much, or pain). It's like having a remote control for your pelvic floor.

However, while we knew the remote worked, we didn't really know how it worked. Did it squeeze the pipe? Did it tell the tank to empty? Or was it just a magic trick?

This study was the first time scientists turned on the "lights" to watch exactly what happens inside the plumbing of awake, conscious humans when they press that remote button.

The Experiment: The "Plumbing Inspection"

The researchers recruited 15 people who already had these nerve stimulators implanted to treat their bladder issues. They asked these volunteers to come in for a special check-up that involved:

1. Filling the Tank: They slowly filled the bladder with saline (salt water) until the person felt a strong urge to pee.
2. The Pressure Test: They inserted special pressure sensors (like tiny microphones) into the urethra (the tube that carries urine out) and the bladder.
3. The Remote Control: Once the person was ready to pee, the researchers turned on the stimulator. They tested different "frequencies" (like tuning a radio to different stations) to see what happened to the pressure in the pipes.

What They Found: The "Squeeze" and the "Surprise"

1. The Urethra Squeezes (The "Gatekeeper" Effect)

When they turned on the stimulator, the urethra didn't just sit there; it squeezed.

• The Analogy: Think of the urethra as a garden hose with a clamp on it. When the nerve was stimulated, the clamp tightened.
• The Details: In about 39% of the tests, the bottom part of the tube squeezed. In 35% of the tests, the top part squeezed.
• The Frequency Twist: They found that "High Frequency" settings (fast pulses) caused much stronger, longer-lasting squeezes than "Low Frequency" settings. However, the high settings were also more uncomfortable for the patients, so they couldn't turn the volume up as high.

Why this matters: If the urethra can be made to squeeze on command, this could be a game-changer for stress incontinence (leaking when you cough or sneeze). It's like having a remote-controlled clamp that you can tighten instantly to stop a leak.

2. The Bladder Surprise (The "Tank" Reacts)

In one very special participant, the stimulation didn't just squeeze the tube; it actually made the bladder itself contract (squeeze the tank).

• The Analogy: Usually, the bladder is like a passive balloon waiting to be filled. In this one case, the remote control made the balloon squeeze itself, pushing water out.
• The Catch: This only happened in one person, and it required a specific setting. It suggests that for some people, this nerve stimulation can wake up a "sleepy" bladder, which could help people who have trouble emptying their bladders (urinary retention).

3. The "Magic" of the Long Term (The "Software Update")

Here is the most interesting part: When the researchers asked people to actually pee while the stimulator was on, nothing changed. The flow rate was the same, and the efficiency was the same.

• The Analogy: Imagine you have a thermostat that fixes your house's temperature. When you walk into the room, the heater isn't blasting right now (the immediate effect is zero), but the room is warm because the thermostat changed the settings of the whole house over time.
• The Conclusion: The stimulator doesn't seem to work like a "remote control" that instantly fixes the plumbing while you are using it. Instead, it works like a software update for your nervous system. Over time, it rewires the reflex circuits in your spine and brain, teaching your body to manage the bladder better on its own.

The Takeaway

This study is a bit like finally opening the hood of a car to see the engine while it's running.

• Good News: We confirmed that this nerve stimulation can physically squeeze the urethra. This is great news for treating leaks.
• The Mystery Solved: We also learned that when people feel better after weeks of treatment, it's not because the machine is holding the door shut for them in the moment. It's because the machine has been "retraining" the nervous system's reflexes, much like physical therapy for the nerves.

In short: The remote control can tighten the gate to stop leaks, and over time, it teaches your body's brain to manage the plumbing better on its own.

Technical Summary

1. Problem Statement

Pudendal nerve stimulation (PNS) is an established off-label clinical treatment for lower urinary tract (LUT) disorders, including overactive bladder (OAB), urinary incontinence, retention, and chronic pelvic pain. However, the direct physiological mechanisms by which PNS affects the human LUT remain poorly understood.

• Knowledge Gap: While animal models and studies on anesthetized or spinal cord injury (SCI) patients suggest PNS can induce bladder and urethral contractions, there is a lack of data regarding these effects in awake, non-SCI human subjects.
• Clinical Question: Can PNS generate urethral contractions without causing intolerable pain in awake humans? Does PNS directly improve voiding efficiency, or does it act through indirect, long-term neural modulation (similar to sacral neuromodulation)?

2. Methodology

The study employed a single-center, observational design involving 15 participants (13 women, 2 men) who already had implanted pudendal neurostimulators for symptom management.

• Experimental Setup:

  • Participants: Seated in a reclined position.
  • Instrumentation:
    • Manometry: A 2.75 mm solid-state multichannel catheter (Medtronic) placed in the LUT to measure pressures at multiple points (proximal and distal urethra, bladder).
    • Urodynamics: A dual-sensor air-charged catheter (Laborie) for bladder filling and pressure recording.
    • Abdominal Pressure: Recorded via a rectal or vaginal catheter.
    • Stimulation: Participants' implanted pulse generators were used. Stimulation was applied in monopolar or bipolar configurations at two frequency ranges: Low Frequency (2–3.1 Hz) and High Frequency (30–33 Hz).
  • Protocol:
    1. Bladder was filled with saline until a "strong desire to void" was reported.
    2. Stimulation amplitude was increased to the Maximum Tolerable Stimulation Amplitude (MTSA).
    3. Trials were conducted with and without stimulation to measure urethral/bladder pressure changes.
    4. Participants attempted to void with catheters in place to assess voiding efficiency and flow rates.

• Data Analysis:

  • Statistical tests included Shapiro-Wilk (normality), Welch's t-test, paired t-tests, and Wilcoxon rank-sum tests ($\alpha = 0.05$).
  • Key metrics included pressure change amplitude, location of maximum pressure, voiding efficiency, and maximum flow rate.

3. Key Contributions

• First-in-Human Awake Data: This is the first study to characterize direct LUT activation (urethral and bladder) via PNS in awake, non-SCI human subjects.
• Frequency-Dependent Effects: The study differentiates the physiological effects of low-frequency vs. high-frequency stimulation on urethral dynamics.
• Mechanism Elucidation: It provides evidence suggesting that while PNS can acutely recruit urethral fibers, its therapeutic benefit for voiding disorders likely stems from long-term neural circuit modulation rather than immediate mechanical changes during voiding.

4. Key Results

• Urethral Recruitment:
  • Urethral contractions were observed in 11 out of 15 cystometry sessions.
  • Low Frequency (2–3.1 Hz): Induced contractions in 31/62 trials (50%). Contractions were distributed between distal (19) and proximal (12) urethra with no significant difference in location or amplitude.
  • High Frequency (30–33 Hz): Induced contractions in 24/71 trials. Crucially, the maximum pressure change occurred significantly more often in the proximal urethra (19 times) compared to the distal (5 times; $p=0.007$).
  • Amplitude: High-frequency contractions were significantly larger (Median Proximal: 33.8 cmH2O) than low-frequency contractions (Median Proximal: 2.6 cmH2O), though high-frequency stimulation had a significantly lower MTSA ($p=0.041$).
• Bladder Contractions:
  • In one highly responsive participant, PNS triggered four distinct bladder contractions with an average pressure change of 24.3 cmH2O. These occurred at volumes >70% capacity and required stimulation amplitudes $\le$ 2V.
  • No other participants exhibited stimulation-driven bladder contractions, suggesting this effect is rare or requires specific lead placement/nerve recruitment in non-SCI humans.
• Voiding Dynamics:
  • No Significant Change: There was no significant difference in voiding efficiency ($p=0.76$) or maximum flow rate ($p=0.45$) between trials with and without stimulation.
  • Observation: Pressure profiles showed a localized decrease in urethral pressure just prior to voiding, followed by a gradual pressure drop during flow, regardless of stimulation.

5. Significance and Conclusion

• Therapeutic Potential: The ability of PNS to recruit the urethra (particularly the proximal urethra via high-frequency stimulation) suggests a potential mechanism for treating stress urinary incontinence (SUI) by increasing urethral closure pressure on demand.
• Mechanism of Action: The lack of immediate improvement in voiding efficiency or flow rate implies that PNS does not act as a direct "pump" or sphincter relaxant during the voiding phase. Instead, its efficacy in treating OAB and retention likely relies on long-term plasticity and the modulation of spinal reflex circuits (similar to sacral neuromodulation), rather than acute mechanical changes.
• Clinical Implications:
  • High-frequency stimulation may be more effective for generating strong, tetanic urethral contractions to prevent leakage, provided tolerable amplitudes can be managed.
  • Future therapies could leverage on-demand urethral contraction for SUI, while relying on chronic stimulation for reflex modulation in OAB/retention.
• Limitations: The presence of dual catheters may have restricted natural voiding dynamics, and the small sample size for bladder contraction events limits generalizability. Future studies should consider removing infusion catheters during voiding attempts to reduce artifacts.