Chronic pain affects an estimated 20% of people worldwide and remains difficult to treat. Although previous studies suggest that altering the central representation of the body through multisensory bodily inputs—such as visuotactile with the rubber hand illusion or visuomotor during mirror therapy—may be of relevance for the understanding and treatment of pain, typical multisensory stimulation procedures are not possible in many patients with chronic pain because movement or even gentle touches of the affected limb may increase pain.
To help address this current state, Marco Solca, MD, and colleagues used the latest research on multisensory body processing from their laboratory and recent advances in virtual reality (VR) technology to develop a new digital therapy for chronic upper limb pain that combines neuroscience principles from mirror therapy, immersive virtual reality VR, and multisensory stimulation, including cardiac signals. “Merging expertise from neuroscience and engineering, we have developed a new immersive digital therapy for chronic pain that we refer to as heartbeat-enhanced VR (HEVR),” Dr. Solca says. HEVR allows the application of prolonged and repeated doses of stimulation, enables the automatized integration with existing pain treatments, and minimizes the active involvement of patient and therapist.
What’s HEVR?
For the study, published in Neurology, patients with complex regional pain syndrome (CRPS) wore a VR headset to immerse them in an in-house developed virtual environment in which they were shown a realistic, 3-dimensional, virtual depiction of their affected limb, with different hand models used to account for laterality of the disease, gender, and skin color. During HEVR, participants were asked to view the virtual, which was flashing (illumination applied to the texture of the hand) in synchrony (treatment) or in asynchrony (control) with respect to their own, online-detected heartbeats.
Participants in the synchronous group saw the virtual hand flash after the detection of the R peak, with a delay of 33 milliseconds. During the asynchronous control condition, the virtual hand was illuminated at a frequency varying every 5 seconds randomly among 70%, 80%, 90%, 110%, 120%, or 130% of the ongoing heart rate. The flash lasted for 100 milliseconds (sinusoidal transparency of the illumination from 0%–100% visible). During stimulation, participants listened to white noise in order to mask surrounding auditory input. Healthy age- and gender-matched participants, matched to the side of each patient’s affected limb, served as controls. The experiment included synchronous and asynchronous conditions for each participant that were repeated three times consecutively. Outcomes were measured at baseline and after each trial (pain) or condition (force).
Encouraging Outcomes
“When compared with asynchronous stimulation, a single-session of synchronous HEVR decreased pain, improved limb function, and modulated a physiological pain marker,” explains Dr. Solca. “Without applying any touch or movement to the CRPS patient’s painful limb, we used a readily available signal (the heartbeat), avoiding the risk of allodynia. Thus, HEVR procedures can be used in most patients with chronic pain, including those with allodynia or somatosensory deficit.”
High-frequency heartrate variability (HRV) increased during synchronous trials but not during asynchronous trials, notes Dr. Solca (Figure). A negative correlation was observed between subjective pain rating and HRV. “It is important to stress that this HRV change was highly selective, as it occurred only during synchronous, and not asynchronous, HEVR and only for the high frequency HRV component, which has been shown to be reduced in chronic pain,” he adds.
According to Dr. Solca, the changes observed in the study were reliably induced and highly selective, when compared with previous VR or mirror therapies, and fully controlled and automatized by the HEVR system used in the study, with the potential for further personalization and combination with other therapy. “HEVR analgesic effects were stronger in participants receiving pharmacologic pain treatments, underscoring the potential benefit of integrating our digital therapy with existing pain treatments,” he says.
Potential Future Impacts
The ability of the HEVR procedure to facilitate application of higher doses of therapy is difficult to achieve with procedures requiring tactile stimulation, notes Dr. Solca. “HEVR is likely also efficient in other chronic pain conditions, such as fibromyalgia, and may also be advantageous in inducing artificial bodily sensations in stroke patients who suffer from somatosensory deficits, such as anesthesia or hypoesthesia, or in amputees in whom applied touches to the missing limb are not possible. We are in the beginning stages of the use of VR therapies for pain. VR and related media may soon be commonplace, further underscoring the potential future impact of our results.”
Rebecca Shover