What is a PNI (Peripheral Nerve Implant) sacral nerve implant trial?

PNI Sacral Nerve Implant Trial: A Two-Stage Procedure for Treating Fecal Incontinence

A PNI (Peripheral Nerve Implant) sacral nerve implant trial is the initial testing phase of sacral nerve stimulation therapy, where temporary electrodes are placed at the sacral nerves to evaluate if a patient will respond to permanent implantation for treating fecal incontinence.

Overview of the Procedure

• A sacral nerve stimulation (SNS) trial involves a two-stage approach, with the first stage being the peripheral nerve evaluation (PNE) test to determine if a patient is a suitable candidate for permanent implantation 1, 2
• During the PNE test, temporary electrodes are placed percutaneously in the sacral foramina (typically S3) and connected to an external electrical stimulator for a period of 2-3 weeks 1
• If the frequency of fecal incontinence decreases by 50% or more during this trial period, the patient is considered a good candidate for permanent implantation 1

Patient Selection Criteria

• SNS is indicated for patients with fecal incontinence who have failed conservative therapies 1, 2
• Most clinical trials have focused on patients with a structurally intact anal sphincter or a defect smaller than 120° 1
• Some studies suggest SNS may be beneficial regardless of sphincter injury, but larger trials are needed to confirm this finding 1
• Patients with significant anatomical defects (e.g., rectovaginal fistula, full-thickness rectal prolapse) should have these corrected surgically before considering SNS 1

Procedure Techniques

• The PNE test can be performed using different approaches:

  • Conventional wire electrodes (traditional approach) 3
  • Permanent electrodes in a "two-stage implantation" approach, which has shown higher success rates (81.8% vs 47.6% with conventional PNE) 3
  • CT-guided placement, which can be helpful in patients with anatomical variations or sacral malformations 4

• The procedure typically involves:

  1. Identifying the sacral foramina (usually S3)
  2. Inserting a needle to locate the correct position
  3. Placing the temporary electrode
  4. Confirming proper placement through motor and sensory responses
  5. Connecting to an external stimulator for the trial period 5, 4

Efficacy and Outcomes

• In the pivotal US multicenter trial, 90% of 133 patients proceeded from temporary to permanent stimulation 1
• Long-term follow-up data (5 years) showed 36% of patients achieved complete continence, and 89% were deemed therapeutic successes 1
• Controlled studies have shown SNS to be significantly better than medical treatment for fecal incontinence 1
• A Cochrane review found that SNS can improve continence in a proportion of patients with fecal incontinence 6

Potential Complications

• The most common adverse events include:
  • Pain at the insertion site (up to 10% of patients) 1, 2
  • Infection at the insertion site (up to 10% of patients) 1, 2
  • Electrode migration during the trial period (reported in 18.6% of cases in one study) 5
  • Seroma formation 6
  • Excessive tingling sensations 6

Post-Trial Considerations

• If the trial is successful (≥50% reduction in fecal incontinence episodes), a permanent stimulator is implanted beneath the skin 1
• The battery for the permanent implant needs replacement approximately every 7 years 1, 2
• If SNS fails, alternative approaches include:
  • Ongoing medical management with emphasis on suppositories and enemas 2
  • Evaluation for pelvic floor structural abnormalities 2
  • Management of colonic motor dysfunction with prokinetic agents 2
  • Consideration of anterograde colonic enema procedures (though these have lower long-term success rates in adults) 2

Clinical Pearls and Pitfalls

• The discrepancy between symptom improvement and relatively minor effects on anorectal functions is puzzling; SNS may work by inducing colonic retrograde propagated sequences that delay colonic transit 1
• In crossover studies, some patients experienced equal symptoms with the stimulator on or off, suggesting a potential placebo effect 1
• Careful patient selection is crucial for optimizing outcomes 7
• The PNE test is generally well-tolerated and can be performed on an outpatient basis 5