The Impact of Immunological Parameters on the Development of Phantom Pain after Major Amputation

Nerve-myeloid cell interactions in persistent human pain: a reappraisal using updated cell subset classifications

ABSTRACT

The past 20 years have seen a dramatic shift in our understanding of the role of the immune system in initiating and maintaining pain. Myeloid cells, including macrophages, dendritic cells, Langerhans cells, and mast cells, are increasingly implicated in bidirectional interactions with nerve fibres in rodent pain models. However, our understanding of the human setting is still poor. High-dimensional functional analyses have substantially changed myeloid cell classifications, with recently described subsets such as epidermal dendritic cells and DC3s unveiling new insight into how myeloid cells interact with nerve fibres. However, it is unclear whether this new understanding has informed the study of human chronic pain. In this article, we perform a scoping review investigating neuroimmune interactions between myeloid cells and peripheral nerve fibres in human chronic pain conditions. We found 37 papers from multiple pain states addressing this aim in skin, cornea, peripheral nerve, endometrium, and tumour, with macrophages, Langerhans cells, and mast cells the most investigated. The directionality of results between studies was inconsistent, although the clearest pattern was an increase in macrophage frequency across conditions, phases, and tissues. Myeloid cell definitions were often outdated and lacked correspondence with the stated cell types of interest; overreliance on morphology and traditional structural markers gave limited insight into the functional characteristics of investigated cells. We therefore critically reappraise the existing literature considering contemporary myeloid cell biology and advocate for the application of established and emerging high-dimensional proteomic and transcriptomic single-cell technologies to clarify the role of specific neuroimmune interactions in chronic pain.

Immune cell-mediated opioid analgesia

Pathological pain is regulated by a balance between pro-algesic and analgesic mechanisms. Interactions between opioid peptide-producing immune cells and peripheral sensory neurons expressing opioid receptors represent a powerful intrinsic pain control in animal models and in humans. Therefore, treatments based on general suppression of immune responses have been mostly unsuccessful. It is highly desirable to develop strategies that specifically promote neuro-immune communication mediated by opioids. Promising examples include vaccination-based recruitment of opioid-containing leukocytes to painful tissue and the local reprogramming of pro-algesic immune cells into analgesic cells producing and secreting high amounts of opioid peptides. Such approaches have the potential to inhibit pain at its origin and be devoid of central and systemic side effects of classical analgesics. In support of these concepts, in this article, we describe the functioning of peripheral opioid receptors, migration of opioid-producing immune cells to inflamed tissue, opioid peptide release, and the consequent pain relief. Conclusively, we provide clinical evidence and discuss therapeutic opportunities and challenges associated with immune cell-mediated peripheral opioid analgesia.

Leukocyte opioid receptors mediate analgesia via Ca(2+)-regulated release of opioid peptides

Opioids are the most powerful analgesics. As pain is driven by sensory transmission and opioid receptors couple to inhibitory G proteins, according to the classical concept, opioids alleviate pain by activating receptors on neurons and blocking the release of excitatory mediators (e.g., substance P). Here we show that analgesia can be mediated by opioid receptors in immune cells. We propose that activation of leukocyte opioid receptors leads to the secretion of opioid peptides Met-enkephalin, β-endorphin and dynorphin A (1-17), which subsequently act at local neuronal receptors, to relieve pain. In a mouse model of neuropathic pain induced by a chronic constriction injury of the sciatic nerve, exogenous agonists of δ-, μ- and κ-opioid receptors injected at the damaged nerve infiltrated by opioid peptide- and receptor-expressing leukocytes, produced analgesia, as assessed with von Frey filaments. The analgesia was attenuated by pharmacological or genetic inactivation of opioid peptides, and by leukocyte depletion. This decrease in analgesia was restored by the transfer of wild-type, but not opioid receptor-lacking leukocytes. Ex vivo, exogenous opioids triggered secretion of opioid peptides from wild-type immune cells isolated from damaged nerves, which was diminished by blockade of Gαi/o or Gβγ (but not Gαs) proteins, by chelator of intracellular (but not extracellular) Ca(2+), by blockers of phospholipase C (PLC) and inositol 1,4,5-trisphosphate (IP3) receptors, and was partially attenuated by protein kinase C inhibitor. Similarly, the leukocyte depletion-induced decrease in exogenous opioid analgesia was re-established by transfer of immune cells ex vivo pretreated with extracellular Ca(2+) chelator, but was unaltered by leukocytes pretreated with intracellular Ca(2+) chelator or blockers of Gαi/o and Gβγ proteins. Thus, both ex vivo opioid peptide release and in vivo analgesia were mediated by leukocyte opioid receptors coupled to the Gαi/o-Gβγ protein-PLC-IP3 receptors-intracellular Ca(2+) pathway. Our findings suggest that opioid receptors in immune cells are important targets for the control of pathological pain.

Chronic postsurgical pain: still a neglected topic?

Background

Surgical injury can frequently lead to chronic pain. Despite the obvious importance of this problem, the first publications on chronic pain after surgery as a general topic appeared only a decade ago. This study tests the hypothesis that chronic postsurgical pain was, and still is, represented insufficiently.

Methods

We analyzed the presentation of this topic in journal articles covered by PubMed and in surgical textbooks. The following signs of insufficient representation in journal articles were used: (1) the lack of journal editorials on chronic pain after surgery, (2) the lack of journal articles with titles clearly indicating that they are devoted to chronic postsurgical pain, and (3) the insufficient representation of chronic postsurgical pain in the top surgical journals.

Results

It was demonstrated that insufficient representation of this topic existed in 1981–2000, especially in surgical journals and textbooks. Interest in this topic began to increase, however, mostly regarding one specific surgery: herniorrhaphy. It is important that the change in the attitude toward chronic postsurgical pain spreads to other groups of surgeries.

Conclusion

Chronic postsurgical pain is still a neglected topic, except for pain after herniorrhaphy. The change in the attitude toward chronic postsurgical pain is the important first step in the approach to this problem.

Phantom limb phenomena in cancer amputees

PURPOSE

The objective of this study was to determine the prevalence of phantom pain and correlated conditions such as phantom sensations and stump pain in a population of cancer patients who had undergone limb amputation.

METHOD

A cross-sectional study was carried out in adult patients submitted to limb amputation, who were being followed up at the Physiotherapy Department between April 3 and November 30, 2006. The presence of phantom pain and associated conditions was quantified using a verbal numerical scale. The data obtained were analyzed for means, medians, and proportions with their respective confidence intervals, as appropriate.

RESULTS

Seventy-five patients participated in this study, 50 men (66.7%) and 25 women (33.3%). Mean age was 54.4 years (SD +/- 18.5); range 19 to 88 years. The prevalence of phantom pain was 46.7% (95%CI: 35.1 to 58.6), phantom sensation 90.7% (95%CI: 81.7 to 96.2), and surgical stump pain 32.0% (95%CI: 21.7 to 43.8).

CONCLUSION

Phantom pain and phantom sensations are highly prevalent among cancer patients. Further studies should be carried out to determine the main factors associated with their onset.

Immune cell-derived opioids protect against neuropathic pain in mice

The analgesic effects of leukocyte-derived opioids have been exclusively demonstrated for somatic inflammatory pain, for example, the pain associated with surgery and arthritis. Neuropathic pain results from injury to nerves, is often resistant to current treatments, and can seriously impair a patient's quality of life. Although it has been recognized that neuronal damage can involve inflammation, it is generally assumed that immune cells act predominately as generators of neuropathic pain. However, in this study we have demonstrated that leukocytes containing opioids are essential regulators of pain in a mouse model of neuropathy. About 30%-40% of immune cells that accumulated at injured nerves expressed opioid peptides such as beta-endorphin, Met-enkephalin, and dynorphin A. Selective stimulation of these cells by local application of corticotropin-releasing factor led to opioid peptide-mediated activation of opioid receptors in damaged nerves. This ultimately abolished tactile allodynia, a highly debilitating heightened response to normally innocuous mechanical stimuli, which is symptomatic of neuropathy. Our findings suggest that selective targeting of opioid-containing immune cells promotes endogenous pain control and offers novel opportunities for management of painful neuropathies.

Reflexology treatment for patients with lower limb amputations and phantom limb pain--an exploratory pilot study

OBJECTIVES

The objectives of the study were to evaluate the possibility of reflexology being used as a non-invasive form of phantom limb pain relief and of empowering patients to maintain any positive results with self-treatment.

SETTING

Prosthetic Services Centre, Herbert Street, Wolverhampton, West Midlands, England.

DESIGN

A same-subject, experimental pilot study, recording the intensity of phantom limb pain in weekly pain diaries over a 30-week period, which was divided into five phases: phase 1 gave a baseline of pain, whilst phase 3 was a resting phase. Phases 2, 4 and 5 provided the reflexology interventions.

SAMPLE

Ten participants with unilateral lower limb amputations and phantom limb pain were selected from the database at the Prosthetic Centre. REFLEXOLOGY INTERVENTIONS: In phase 2, six weekly reflexology treatments were given, which consisted of: full foot reflexology to the remaining foot and full hand reflexology to the hand of the amputated side of the body. In phase 4, six weekly hand reflexology teaching sessions were carried out; patients copied on their own hands what the therapist did on hers. A hand reflexology booklet gave the sequence of the treatment and was used as a reference. In phase 5, the patients self-treated for 6 weeks at home, using the reference material.

RESULTS

Over the 30-week period, there was an improvement in the perception of the presence and the intensity of the phantom limb pain, with a corresponding improvement in the duration of the pain and the affect on the person's lifestyle. The improvement was maintained when the clients self-treated. FOLLOW-UP QUESTIONNAIRE: A follow-up questionnaire was carried out in 2007--12 months after the project had ended--to elicit whether the patients had suffered from phantom pain over the previous 12 months, whether they still had relief from phantom limb pain and whether they still self-treated.

CONCLUSIONS

The project indicated that reflexology treatment, teaching and self-treatment were effective in eradicating or reducing the intensity and duration of phantom limb pain, in this group of clients. The follow-up questionnaire revealed that there was a maintained improvement in the intensity of phantom limb pain the patients experienced and that the majority still self-treated.

Surgical Treatment of Chronic Phantom Limb Sensation and Limb Pain after Lower Limb Amputation

BACKGROUND

Therapy for phantom sensation and phantom limb pain following amputation is still difficult, because pathophysiologic mechanisms have not been clarified. This report illustrates a simple and useful surgical intervention. The authors propose that changes at the peripheral nerve site can influence the central feeling of phantom sensation and pain.

METHODS

Fifteen patients (mean age, 56 years) with lower limb amputation were included in the study. In all patients, the sciatic nerve was split at a point approximately 3 cm proximal to the popliteal fossa, and the two parts were reconnected in a sling fashion using an epiperineurial technique under microscopic vision. The nerves were covered with a fibrin patch and anesthetics were applied by means of a local pain catheter. Frequency, duration, intensity, and quality of phantom pain were compared preoperatively and 1 week, 3 months, 6 months, and 1 year postoperatively.

RESULTS

Fourteen of 15 patients defined the procedure as very helpful. Average, maximum, and minimum pain intensity were significantly reduced 1 week, 3 months, 6 months, and 1 year postoperatively (p < 0.001). Pain intensity scores decreased significantly over the long term after surgical intervention (median visual analogue scale score: preoperatively, 7; 1 year postoperatively, 4) (p < 0.001). The duration of pain attack shortened from approximately 120 minutes to 5 to 10 minutes.

CONCLUSIONS

This study shows that accurate treatment of the peripheral nerve can help to successfully reduce phantom limb pain. The authors feel encouraged to perform future investigations to test their operative method in a prospective, randomized, matched control study including electrophysiologic tests for more objective pain assessment.