Preventing Chronification of Phantom Limb Pain Through Mirror Therapy in Conjunction With tDCS

Approximately 80% of amputees experience PLP, often severe, for years after amputation and most amputees will experience phantom limb sensations, including kinetic, proprioceptive (i.e. feeling of length or volume) and exteroceptive sensations (e.g. touch, pressure, itching). Treatment options for PLP have generally been limited, and there is no clear consensensus on the optimal treatment regimen. In PLP maladaptive plasticity associated with sensory defferentation following an amputation is one of the contributors for excessive pain.

This research project will focus on combined known neuromodulatory interventions of MT and tDCS, applied for the first time at the acute state of PLP in traumatic and non-traumatic amputees, with an aim to prevent maladaptive plasticity associated with PLP chronification and chronicity.

MT and tDCS related literature is based on data collected from patients who already developed PLP for years, before enrollment in the study. The investigators suggest that tDCS and MT applied at the acute stage of PLP will have synergistic effects on PLP intensity and thus avert its chronification. The researchers rationale is based on recent evidence from basic neuroscience on the phantom perception of other non-pain sensory modalities and the neuroscience of chronification of other pain conditions.

The fundamental mechanism for PLP suggests initial changes at the peripheral nervous system, including loss of afferent somatosensory input and exaggerated input from ectopic activity generated in the axotomized nociceptive neurons. Subsequent central changes include thalamic and cortical functional reorganization in the sensorimotor cortex, manifested as maladaptive cortical reorganization: an "invasion" of neighboring areas to the representation of the amputated limb.

Evidence in the literature suggests that the 2 noninvasive neuromodulation techniques- MT and tDCS applied to motor cortex (M1-tDCS)-may interfere with the maladaptive plasticity accompanied PLP and respectively will reduced PLP and negative affects. With MT, the moving, healthy limb reflected in the mirror makes the missing limb appear intact and functioning. The analgesic effect attributed to MT likely results from enhanced coherence between sensory feedback and motor command, counterbalancing the amputation-induced maladaptive neuroplasticity. In chronic PLP, MT conducted over several weeks reversed sensorimotor reorganization and reduced PLP intensity, suggesting a correlation between reorganization and PLP. MT also modulates the activity of the multisensory integration network to resolve the post-amputation perceptual incongruence, and modulates the activity of networks which help integrate the perceptual and motor areas to affect the sense of body ownership and agency. Indeed, an improved sense of agency has been reported after MT. Yet, these studies examined patients with chronic PLP and found mild analgesic effects. The investigators argue that MT applied at the acute PLP stage may interfere with the maladaptive plasticity of the multisensory integration network, and accordingly will improve the sense of agency and ownership over phantom limb, and reduce PLP intensity.

Motor areas (M1) stimulation through neuromodulation techniques alters the functional connectivity between brain areas, thus modulating brain networks rather than just affecting the local stimulation target. Specifically, it modulates ascending sensory input; brain areas of the fronto-striatal circuit, limbic brain areas, and midbrain nuclei involved in descending pain inhibition. Thus, tDCS applied to M1 may modulate brain areas composed of the subcircuits connected to mPFC and the salience network , i.e., areas presumably involved in pain chronification and chronicity. Although the electrical current only affects the cortex below the electrode, remote cortical and subcortical connected areas are affected as well. Previous work has described two putative analgesic effects of anodal M1 tDCS: 1) modulation of thalamic activity by descending corticothalamic pathways originating in the primary motor cortex and 2) inhibition of the primary somatosensory cortex via corticocortical pathways. Clinically, tDCS has immediate, sustained, and long-term analgesic effects. The investigators expect that M1-tDCS will restore the function of the mPFC and the salience network, and will thus improve endogenous pain inhibition, reduce PLP intensity and negative affect, yet will not eliminate pain chronification.

Because MT and tDCS, when given alone, produced only mild analgesic effects, their clinical adoption was limited. Soler et al. were the first to study the analgesic effects of a combination of visual illusion (similar to MT) and tDCS on neuropathic pain after spinal cord injury. The investigators found that the combination reduced pain intensity significantly more than did any single intervention; this effect lasted at least 12 weeks post-treatment. Gunduz et al. found no cumulative effect of MT and tDCS on patients with limb amputation. Whether studies examined the clinical effects of MT, tDCS, or a combination of both, the entire literature is based on data collected from patients with chronic PLP and did not consider time since amputation as an exclusion criterion. Therefore, time since amputation might be a key factor in the efficacy of these treatments. In patients who had only recently undergone amputation, the abnormal neuroplasticity might not yet be fixated; therefore, counterbalancing it should be easier. A preliminary study from Treister's lab is the first to show that a combination of MT and tDCS applied at the acute stage of PLP can prevent PLP chronification. Considering Treister's preliminary results and given that MT and M1-tDCS exert neuroplastic effects on different neural networks with similar outcome (i.e., PLP intensity), the investigators expect that a combination of MT and tDCS applied at the acute stage of PLP will have synergistic effects on PLP intensity and thus avert its chronification.