The Incidence of Eosinophilia in Refractory Chronic Pain Requiring Neuromodulation

Objectives: Eosinophilia is a hallmark of a variety of disease processes, but its association with chronic pain has not been well-established. We evaluated the incidence and clinical relevance of eosinophilia in patients who underwent spinal cord stimulation (SCS) or intrathecal drug pump (ITP) implantation for high-impact chronic pain. Materials and Methods: We retrospectively reviewed 212 patients who underwent SCS or ITP implantation between 2017 and 2024 at the University of Arizona, Tucson. Of these, 114 had preoperative complete blood counts with differential and were included in the final analysis. Eosinophilia was defined as absolute eosinophil count (AEC) ≥350 cells/μL. Demographic, clinical, and laboratory data were compared in patients with and without eosinophilia using Fisher exact test and Welch's t-tests. Results: Overall, 14 patients (12.3%) met the criteria for eosinophilia, with a mean AEC of 547 cells/μL compared with 138 cells/μL in the noneosinophilic group. Patients with eosinophilia had higher eosinophil percentages (6.3% vs 1.9%), whereas total white blood cell counts and C-reactive protein levels did not differ. Age, sex, or implant type, and preoperative diagnoses—including chronic neuropathic pain, persistent spinal pain syndrome type 1, and complex regional pain syndrome—were comparable in the groups. The improvement in numerical rating scale scores after implantation did not differ significantly (−1.90 vs −1.60; p = 0.676). Comorbidities were similar in the groups, with the eosinophilic group showing migraines in 21.4%, asthma in 21.4%, fibromyalgia in 7.10%, and rheumatoid arthritis in 7.10%. No patients with eosinophilia experienced complications within 30 days or one year, nor did they require revision surgeries or device removal. Conclusions: Although eosinophilia is reported in <1% of the general population, it was present in 12.3% of patients who underwent neuromodulation for chronic pain. Although not associated with adverse short-term outcomes, eosinophilia may reflect a broader immunologic state characteristic of high-impact chronic pain.