Functional Threshold Changes in Patients with Inspire Hypoglossal Nerve Stimulation
The functional threshold for Inspire hypoglossal nerve stimulation typically increases over time in patients, requiring periodic adjustments to maintain therapeutic efficacy. 1
Understanding Functional Threshold in Inspire Therapy
Hypoglossal nerve stimulation (HGNS) with the Inspire device is a treatment for obstructive sleep apnea (OSA) that works by stimulating the hypoglossal nerve to maintain airway patency during sleep. The functional threshold refers to the minimum stimulation intensity needed to achieve therapeutic effect.
Mechanism of Threshold Changes
Several factors contribute to the increasing threshold phenomenon:
- Nerve fiber adaptation: Similar to what's observed in vagus nerve stimulation, repeated electrical stimulation leads to adaptation of nerve fibers, requiring higher stimulation intensities over time 1
- Tissue changes: Fibrosis around the stimulation lead can develop over time, increasing electrical impedance
- Electrode-tissue interface changes: Micro-movements and healing processes alter the contact between the electrode and nerve tissue
Timeline and Pattern of Threshold Changes
The threshold changes follow a predictable pattern:
- Initial period (0-3 months): Relatively stable thresholds during the healing and adaptation phase
- Mid-term (3-12 months): Most significant increases in threshold typically occur during this period
- Long-term (>12 months): Continued but more gradual increases in threshold 2
This pattern is similar to what's observed in other nerve stimulation therapies, where threshold changes tend to stabilize after the first year but continue to gradually increase over time.
Clinical Implications
The increasing threshold has several important clinical implications:
- Need for regular follow-up: Patients require periodic device adjustments to maintain efficacy
- Battery life considerations: Higher stimulation parameters accelerate battery depletion
- Therapeutic window narrowing: As threshold increases, the gap between effective stimulation and stimulation that causes arousal may narrow 1
Management Strategies
To address increasing thresholds:
- Regular titration sessions: Schedule follow-up visits at 1,3,6, and 12 months post-activation, then annually
- Parameter adjustments: Increase amplitude, pulse width, or frequency as needed to maintain efficacy
- Monitoring for side effects: Watch for symptoms like tongue soreness, discomfort during stimulation, or arousal from sleep
- Consideration of lead repositioning: In rare cases of significant threshold increases that limit therapy effectiveness
Comparison to Other Neurostimulation Therapies
The threshold increase phenomenon in Inspire therapy parallels observations in other neurostimulation treatments:
- Vagus nerve stimulation: Similar increases in stimulation thresholds have been documented over time 1
- Respiratory muscle training: Shows adaptation patterns where initial improvements plateau and require progressive increases in training load 1
Factors Affecting Threshold Changes
Several patient-specific factors may influence the rate and magnitude of threshold changes:
- Body habitus: Patients with higher BMI may experience more significant threshold increases 3
- Anatomical variations: Individual differences in nerve anatomy and tissue composition
- Device placement technique: Surgical technique and precise electrode positioning may affect long-term threshold stability 4
Common Pitfalls and Caveats
- Mistaking tolerance for device failure: Increasing thresholds are often misinterpreted as device malfunction
- Inadequate follow-up: Insufficient monitoring can lead to suboptimal therapy as thresholds increase
- Overlooking lead migration: Sometimes threshold increases are due to physical displacement of the stimulation lead rather than physiological adaptation 3
- Ignoring patient feedback: Subjective reports of decreased efficacy should prompt threshold reassessment
By understanding and anticipating threshold changes, clinicians can optimize the long-term management of patients with Inspire therapy, ensuring continued therapeutic benefit while minimizing side effects and maximizing device longevity.