Effects of Chronic Non-Invasive Vagus Nerve Stimulation in an Asplenic Individual with Rheumatoid Arthritis: A Single Subject Trial

Technical Summary: Effects of Chronic Non-Invasive Vagus Nerve Stimulation in an Asplenic Individual with Rheumatoid Arthritis

Problem Statement
Rheumatoid arthritis (RA) is a systemic autoimmune disorder characterized by chronic inflammation and autonomic imbalance, often requiring "treat-to-target" strategies to achieve remission. Non-invasive vagus nerve stimulation (nVNS) has emerged as a potential immunomodulatory therapy, leveraging the cholinergic anti-inflammatory pathway (CAP) to dampen pro-inflammatory cytokine release. However, the CAP relies heavily on the spleen as a primary target for vagal efferent stimulation. A critical gap exists in understanding the efficacy of nVNS in patients who have undergone splenectomy (asplenic individuals), as the anatomical substrate for the spleen-mediated anti-inflammatory reflex is absent. This study addresses whether chronic nVNS can improve disease activity, sympathovagal balance, and neuroimmunological indices in an asplenic patient with RA.

Methodology
This research was conducted as a 24-month, open-label, single-subject (n-of-1) interventional trial.

• Participant: A male in his 60s with a history of RA (diagnosed 2004) and post-traumatic splenectomy (1976). At baseline, the patient was drug-free, presenting with moderate disease activity (SDAI = 10.9) and elevated CRP (16.52 mg/L).
• Intervention Protocol: The study utilized a home-based nVNS protocol using a portable TENS device stimulating the left auricular branch of the vagus nerve. The 24-month period was divided into three 8-month blocks (Periods A, B, and C) with varying stimulation parameters:
  • Block A: 20 Hz, 300 µs, 45 minutes/day, 5 days/week.
  • Block B: 12 Hz, 300 µs, 120 minutes/day, 5 days/week.
  • Block C: 10 Hz, 300 µs, 120 minutes/day, 7 days/week.
• Outcome Measures:
  • Primary: Simplified Disease Activity Index (SDAI).
  • Secondary: Sympathovagal Balance (SVB), derived from Heart Rate Variability (HRV) analysis using the SD2/SD1 ratio (Poincaré plot).
  • Other: Neuroimmunological (NIM) index, calculated as RMSSD/CRP.
• Data Collection: Measurements were taken at baseline and every two months. HRV was recorded via 10-minute resting ECGs, and CRP levels were monitored to minimize short-term variability. Statistical analysis included paired t-tests and repeated-measures ANOVA.

Key Results
The study found that while chronic nVNS was safe and well-tolerated with no reported adverse events, it did not yield a globally significant clinical or physiological improvement in the asplenic participant.

• Disease Activity (SDAI): Contrary to therapeutic goals, disease activity worsened. The SDAI score significantly increased from a baseline of 10.9 (moderate activity) to 21.51 during Block C (10 Hz–300 µs), indicating a shift to high disease activity (p < 0.05).
• Sympathovagal Balance (SVB): A statistically significant improvement in SVB (reduction in SD2/SD1 ratio) was observed only during Block A (20 Hz–300 µs) compared to baseline (p < 0.001). No significant improvements were found in Blocks B or C.
• Neuroimmunological Index (NIM): The NIM index showed only moderate, non-significant improvement during Block A.
• Inflammatory Markers: CRP levels did not show significant reduction across any treatment block.
• Overall Efficacy: Repeated-measures ANOVA revealed no significant differences among the three treatment periods for SDAI, NIM index, or SVB, except for the specific baseline-to-Block A comparison for SVB.

Key Contributions and Claims

• First-in-Human Demonstration: This study provides the first documented assessment of chronic nVNS effects specifically in an asplenic patient with RA.
• Safety and Feasibility: The authors demonstrate that long-term, self-administered nVNS at home is feasible, safe, and well-tolerated, even in a patient managing RA without pharmacological intervention.
• Negative Efficacy Findings: The primary contribution is the empirical evidence suggesting that nVNS may not be an effective alternative to pharmacological treatment for asplenic RA patients. The null hypothesis—that chronic nVNS does not improve disease activity, SVB, or the NIM index in this population—was accepted.

Significance and Limitations
The paper posits that the lack of therapeutic benefit supports the hypothesis that an intact spleen is necessary for the vagus nerve to exert its full anti-inflammatory effect via the CAP. The authors suggest that the temporary improvement in SVB during the high-frequency (20 Hz) block may be mediated by central autonomic network (CAN) modulation rather than the peripheral CAP.

The authors maintain a modest tone regarding the generalizability of their findings. They explicitly acknowledge that as a single-subject case study, the results cannot be generalized to the broader population. Furthermore, they note limitations regarding the self-administered nature of the treatment, which introduces potential bias regarding adherence to the protocol. The paper concludes by calling for future randomized controlled trials with larger sample sizes to corroborate these findings in asplenic populations.