Bedside Neuromodulation of Persistent Pain and Allodynia with Caloric Vestibular Stimulation

Abstract

Background: Caloric vestibular stimulation (CVS) is a well-established neurological diagnostic technique that also induces many phenomenological modulations, including reductions in phantom limb pain (PLP), spinal cord injury pain (SCIP), and central post-stroke pain.

Objective: We aimed to assess in a variety of persistent pain (PP) conditions (i) short-term pain modulation by CVS relative to a forehead ice pack cold-arousal control procedure and (ii) the duration and repeatability of CVS modulations. The tolerability of CVS was also assessed and has been reported separately.

Methods: We conducted a convenience-based non-randomised single-blinded placebo-controlled study. Thirty-eight PP patients were assessed (PLP, n = 8; SCIP, n = 12; complex regional pain syndrome, CRPS, n = 14; non-specific PP, n = 4). Patients underwent 1-3 separate-day sessions of iced-water right-ear CVS. All but four also underwent the ice pack procedure. Analyses used patient-reported numerical rating scale pain intensity (NRS-PI) scores for pain and allodynia.

Results: Across all groups, NRS-PI for pain was significantly lower within 30 min post-CVS than post-ice pack (p < 0.01). Average reductions were 24.8% (CVS) and 6.4% (ice pack). CRPS appeared most responsive to CVS, while PLP and SCIP responses were less than expected from previous reports. The strongest CVS pain reductions lasted hours to over three weeks. CVS also induced substantial reductions in allodynia in three of nine allodynic CRPS patients, lasting 24 h to 1 month. As reported elsewhere, only one patient experienced emesis and CVS was widely rated by patients as a tolerable PP management intervention.

Conclusions: Although these results require interpretative caution, CVS was found to modulate pain relative to an ice pack control. CVS also modulated allodynia in some cases. CVS should be examined for pain management efficacy using randomised controlled trials.

Keywords: allodynia; caloric vestibular stimulation; complex regional pain syndrome; non-invasive brain stimulation; persistent pain; phantom limb pain; spinal cord injury pain; vestibular neuromodulation.

Figure 1

Brain activity associated with caloric vestibular stimulation (CVS). Stronger and better-replicated evidence of CVS-induced brain activation and deactivation is indicated by the darker red and blue tones, respectively. CVS activates—via vestibular pathways (indicated by black arrows)—contralateral structures including the anterior cingulate cortex, insular cortex, putamen in basal ganglia and various temporoparietal areas. Several of these brain regions have been implicated in pain processing [33,34,35,36,37].

Figure 2

Mean pain intensity at baseline and up to 30 min post-CVS#1 and post-ice pack intervention for all patients who underwent both conditions (N = 34). Results revealed significantly lower pain ratings compared to baseline within 30 min post-CVS than post-ice pack intervention. Error bars designate standard deviations. * Results revealed significantly lower pain ratings compared to baseline within 30 min post-CVS than post-ice pack intervention.

Figure 3

Case illustrations of pain and allodynia modulation by CVS: (A) CRPS-12—a case of ≥50% short-term pain reduction following CVS that had returned to baseline by 24 h. (B) CRPS-13—a case of ≥30–49% short-term pain reduction following CVS that became a ≥50% reduction by 24 h, returning to baseline by 1 week. (C) NPP-1—a case of ≥30–49% short-term pain reduction following CVS that became a ≥50% reduction by 24 h, still evident after 1 week. (D) CRPS-2—a striking case of ≥50% short-term allodynia reduction following CVS, completely ameliorated allodynia by 24 h, two further CVS sessions on consecutive days keeping allodynia at low levels, repeat reduction evident after CVS#3 and allodynia reported to be still low (2/10) a month later. (E) CRPS-6—a case of a short-lived increase in allodynia following CVS with amelioration of allodynia by 24 h and allodynia still absent after 1 week. (F) CRPS-11—a case of <30% allodynia reduction following CVS that became a ≥50% allodynia reduction by 24 h, returning to baseline by 1 week and a repeatable reduction after a second CVS session.

Figure 4

Tolerability and side effects of CVS and patient willingness to repeat the intervention if it reduced their pain by 50% or more for one week or one month (figure reprinted from [47]). Solid bars indicate the percentage of patients (n = 25 with formal tolerability data available), while dotted lines indicate the reported intensity of CVS-induced discomfort/pain and side effects. The vast majority of patients reported that despite finding the procedure uncomfortable or painful or experiencing side effects, they were willing to repeat the intervention if it helped their pain.