What is the best course of action for a sacral neuromodulation (SNM) device with two of four electrodes showing abnormal impedances: reprogram the device or replace the lead?

Management of Sacral Neuromodulation Device with Abnormal Electrode Impedances

For a sacral neuromodulation device with two of four electrodes showing abnormal impedances, reprogramming the device should be attempted first before considering lead replacement, as this is often sufficient to restore functionality and avoid unnecessary surgery.

Understanding Electrode Impedance Issues

• Abnormal electrical impedance occurs in approximately 13% of sacral neuromodulation (SNM) implants and can manifest as either open circuits (impedance >4,000 Ω) or short circuits (impedance <50 Ω) 1
• Impedance measurements at 1 kHz are commonly used as a "lead integrity check" to detect connection failures in neuromodulation systems 2
• The 1 kHz impedance sampling feature in implantable pulse generators is specifically designed to detect open circuits due to failed connections 2

Diagnostic Approach

• Determine the type of impedance abnormality:
  • Open circuit: impedance >4,000 Ω 1
  • Short circuit: impedance <50 Ω and/or equivalence of impedance 1
• Assess clinical symptoms to determine if the impedance abnormalities are affecting device efficacy 1, 3
• Evaluate the timing of impedance changes, as short circuits typically present earlier (median 3.5 months) than open circuits (median 15 months) 1

First-Line Management: Reprogramming

• Almost one-third of patients with abnormal electrical impedance associated with clinical inefficacy can be treated conservatively, primarily through reprogramming 1
• Reprogramming should include:
  • Changes in electrode configuration to bypass the affected electrodes 3
  • Adjustments to stimulation amplitude, pulse frequency, and pulse width 3
  • Initial changes can often be performed remotely to avoid unnecessary hospital visits 3

When to Consider Lead Replacement

• Lead replacement should be considered when:
  • Reprogramming fails to restore adequate symptom control 3
  • Short circuits are present, as these more frequently require surgical intervention (75% vs 54.3% for open circuits) 1
  • Multiple electrodes show abnormal impedance that cannot be bypassed through reprogramming 1, 3
  • There is evidence of lead migration or hardware failure that cannot be addressed through reprogramming 4

Clinical Considerations

• Short circuits typically require more urgent attention as they present earlier and more frequently need surgical intervention 1
• Open circuits may be due to microfractures that accumulate over time at the sacral plate, resulting in later presentation 1
• Patient-specific factors such as trauma history and changes in body mass index have not been associated with abnormal electrical impedance 1

Practical Reprogramming Algorithm

1. Identify the specific electrodes with abnormal impedance 3, 5
2. Attempt to create new stimulation programs that bypass the affected electrodes 3
3. Adjust stimulation parameters (amplitude, frequency, pulse width) to optimize therapy with the remaining functional electrodes 3
4. Assess patient response to reprogramming using standardized sensory response tools 5
5. If reprogramming fails to restore adequate symptom control after multiple attempts, proceed to lead replacement 1, 3

Important Caveats

• Equalization of impedance measurements is the most commonly observed impedance abnormality in SNM devices requiring revision 4
• When impedance equalization is detected, check for fluid at the connection point before proceeding to lead replacement 4
• Standardized electrode placement techniques during replacement can improve outcomes by ensuring optimal electrode-nerve coupling 6
• The decision to reprogram versus replace should consider that sacral neuromodulation is primarily indicated for frequency/urgency symptoms and is much less effective for pain 2