How does the InterStim (sacral nerve stimulation) implant work?

How InterStim Implant Works for Sacral Nerve Stimulation

The InterStim implant works by delivering continuous pulsed electrical stimulation to the sacral nerves through a battery-operated stimulator, modulating nerve activity to improve bowel control and reduce fecal incontinence episodes. 1, 2

Mechanism of Action

InterStim therapy operates through the following mechanisms:

• Sacral nerve modulation: The device targets the S3 sacral nerve root, which controls bowel, bladder, and pelvic floor function 1
• Electrical stimulation: Continuous low-level electrical pulses are delivered to normalize neural communication between the bowel and brain 2
• Two-phase implementation:
  • Initial test phase with temporary external stimulator (2-3 weeks)
  • Permanent implantation if ≥50% reduction in symptoms is achieved 1

Clinical Implementation Process

1. Patient selection: Candidates include those with moderate to severe fecal incontinence who have failed conservative management and biofeedback therapy 2

2. Test stimulation phase:

   • Temporary lead placement near sacral nerves
   • External stimulator provides test stimulation for 2-3 weeks
   • Success defined as ≥50% reduction in fecal incontinence episodes 1

3. Permanent implantation:

   • Only performed if test phase is successful (approximately 90% of patients qualify) 1
   • Battery-powered neurostimulator implanted beneath the skin
   • Connected to the lead that stimulates the sacral nerves 1, 3

Efficacy and Outcomes

InterStim therapy demonstrates significant clinical benefits:

• Long-term efficacy: 89% of patients show therapeutic success at 5-year follow-up 1
• Complete continence: Achieved in 36% of patients with long-term use 1
• Symptom reduction: Average reduction from 9.4 to 1.7 incontinent episodes per week 3
• Quality of life: Significant improvements in all domains of fecal incontinence quality of life measures 3

Physiological Effects

The exact mechanism remains somewhat unclear, but research suggests InterStim works by:

• Inducing colonic retrograde propagated sequences: May delay colonic transit 1
• Normalizing pelvic floor function: Improves coordination between sphincter muscles 2
• Modulating sensory pathways: May improve rectal sensation and awareness 1

Important Considerations

• Battery replacement: Required approximately every 7 years 1
• Common adverse events: Pain at implant site (28%), paresthesia (15%), changes in stimulation sensation (12%), and infection (10%) 3
• Contraindications: Not typically recommended for patients with large external sphincter defects (>120°) 1

Comparative Effectiveness

• Superior to medical treatment: Randomized controlled trials show better outcomes compared to medication alone 1
• Similar to PTNS: Not significantly better than percutaneous tibial nerve stimulation in direct comparisons 1
• Cost considerations: More expensive than conservative approaches at approximately $35,818 (2013 dollars) 1

InterStim therapy represents an effective minimally invasive surgical option for carefully selected patients with fecal incontinence who have failed conservative approaches.