Skin biopsy in painful and immune-mediated neuropathies

Cutaneous and muscular afferents from the foot and sensory fusion processing: Physiology and pathology in neuropathies

The foot-sole cutaneous receptors (section 2), their function in stance control (sway minimisation, exploratory role) (2.1), and the modulation of their effects by gait pattern and intended behaviour (2.2) are reviewed. Experimental manipulations (anaesthesia, temperature) (2.3 and 2.4) have shown that information from foot sole has widespread influence on balance. Foot-sole stimulation (2.5) appears to be a promising approach for rehabilitation. Proprioceptive information (3) has a pre-eminent role in balance and gait. Reflex responses to balance perturbations are produced by both leg and foot muscle stretch (3.1) and show complex interactions with skin input at both spinal and supra-spinal levels (3.2), where sensory feedback is modulated by posture, locomotion and vision. Other muscles, notably of neck and trunk, contribute to kinaesthesia and sense of orientation in space (3.3). The effects of age-related decline of afferent input are variable under different foot-contact and visual conditions (3.4). Muscle force diminishes with age and sarcopenia, affecting intrinsic foot muscles relaying relevant feedback (3.5). In neuropathy (4), reduction in cutaneous sensation accompanies the diminished density of viable receptors (4.1). Loss of foot-sole input goes along with large-fibre dysfunction in intrinsic foot muscles. Diabetic patients have an elevated risk of falling, and vision and vestibular compensation strategies may be inadequate (4.2). From Charcot-Marie-Tooth 1A disease (4.3) we have become aware of the role of spindle group II fibres and of the anatomical feet conditions in balance control. Lastly (5) we touch on the effects of nerve stimulation onto cortical and spinal excitability, which may participate in plasticity processes, and on exercise interventions to reduce the impact of neuropathy.

Long non coding RNAs involved in MAPK pathway mechanism mediates diabetic neuropathic pain

Diabetes is the largest global epidemic of the 21st century, and the cost of diabetes and its complications comprise about 12% of global health expenditure. Diabetic neuropathy is the most common complication of diabetes, affecting up to 50% of patients over the course of their disease. Among them, 30%-50% develop neuropathic pain, which has typical symptoms that originate from the toes and progress to foot ulcers and seriously influence quality of life. The pathogenesis of diabetic neuropathic pain (DNP) is complicated and incompletely understood and there is no effective treatment except supportive treatment. Long noncoding RNAs (lncRNAs), a class of noncoding RNAs exceeding 200 nucleotides in length, have been shown to play key roles in fundamental cellular processes, and are considered to be potential targets for treatment. Recent research indicates that lncRNA is involved in the pathogenesis of DNP. Certain overexpressed lncRNAs can enhance the purinergic receptor-mediated neuropathic pain in peripheral ganglia and inflammatory cytokines are released due to receptors activated by adenosine triphosphate. In recent years, our laboratory also has been exploring the relationship and pathogenesis between lncRNAs and DNP. In this review, we focus on the recent progress in functional lncRNAs associated with DNP and investigate their roles related to respective receptors.

Small molecule drugs for the treatment of pruritus in patients with atopic dermatitis

Chronic pruritus is a cardinal symptom of the inflammatory skin disease atopic dermatitis (AD). Pathogenic mechanisms in the periphery, spinal cord and the brain have been implicated in AD-related pruritus. Therefore, both systemic and topical administration of drugs could potentially provide relief. Despite efforts to elucidate the mechanisms behind AD-related pruritus and the relative contribution of peripheral nervous system and central nervous system (CNS), specific and successful treatment options have not yet been developed. Several small molecule drugs are currently being investigated to treat AD and AD-related pruritus. These small molecule drugs can be applied systemically but also topically, as they are able to penetrate into the skin due to their small size. Small molecule drugs specifically targeting peripheral itch transmission, e.g. peripherally selective κ-opioid receptors agonists and neurokinin 1 receptors antagonists, have so far been unable to improve AD-related pruritus when applied systemically, possibly because of the lack of CNS activity. Current evidence from clinical and preclinical trials with centrally acting or peripherally selective oral κ-opioid receptors agonists implies that CNS activity is required for an antipruritic effect. CNS activity is, however, directly associated with CNS-mediated side-effects. On the other hand, topical application of small molecules with anti-inflammatory activity such as Janus kinase inhibitors and phosphodiesterase 4 inhibitors, and also of κ-opioid receptor agonists, has shown promising results regarding their ability to reduce AD-related pruritus. In conclusion, topical application of anti-inflammatory compounds appears to be a highly promising strategy for the treatment of AD-related pruritus.

A new look at painful diabetic neuropathy

The prevalence of diabetes mellitus and its chronic complications continue to increase alarmingly. Consequently, the massive expenditure on diabetic distal symmetrical polyneuropathy (DSPN) and its sequelae, will also likely rise. Up to 50% of patients with diabetes develop DSPN, and about 20% develop neuropathic pain (painful-DSPN). Painful-DSPN can cast a huge burden on sufferers' lives with increased rates of unemployment, mental health disorders and physical co-morbidities. Unfortunately, due to limited understanding of the mechanisms leading to painful-DSPN, current treatments remain inadequate. Recent studies examining the pathophysiology of painful-DSPN have identified maladaptive alterations at the level of both the peripheral and central nervous systems. Additionally, genetic studies have suggested that patients with variants of voltage gated sodium channels may be more at risk of developing neuropathic pain in the presence of a disease trigger such as diabetes. We review the recent advances in genetics, skin biopsy immunohistochemistry and neuro-imaging, which have the potential to further our understanding of the condition, and identify targets for new mechanism based therapies.

Susceptibility of different mouse strains to oxaliplatin peripheral neurotoxicity: Phenotypic and genotypic insights

Peripheral neurotoxicity is one of the most distressing side effects of oxaliplatin therapy for cancer. Indeed, most patients that received oxaliplatin experience acute and/or chronic severe sensory peripheral neuropathy. However, despite similar co-morbidities, cancer stage, demographics and treatment schedule, patients develop oxaliplatin-induced peripheral neurotoxicity with remarkably different severity. This suggests individual genetic variability, which might be used to glean the mechanistic insights into oxaliplatin neurotoxicity. We characterized the susceptibility of different mice strains to oxaliplatin neurotoxicity investigating the phenotypic features of neuropathy and gene expression profiles in dorsal root ganglia of six genetically different mice strains (Balb-c, C57BL6, DBA/2J, AJ, FVB and CD1) exposed to the same oxaliplatin schedule. Differential gene expression in dorsal root ganglia from each mice strain were assayed using a genome-wide expression analysis and selected genes were validated by RT-PCR analysis. The demonstration of consistent differences in the phenotypic response to oxaliplatin across different strains is interesting to allow the selection of the appropriate strain based on the pre-defined read-out parameters. Further investigation of the correlation between gene expression changes and oxaliplatin-induced neurotoxicity phenotype in each strain will be useful to deeper investigate the molecular mechanisms of oxaliplatin neurotoxicity.

Persistent Peripheral Nervous System Damage in Simian Immunodeficiency Virus-Infected Macaques Receiving Antiretroviral Therapy

Human immunodeficiency virus (HIV)-induced peripheral neuropathy is the most common neurologic complication associated with HIV infection. In addition to virus-mediated injury of the peripheral nervous system (PNS), treatment of HIV infection with combination antiretroviral therapy (cART) may induce toxic neuropathy as a side effect. Antiretroviral toxic neuropathy is clinically indistinguishable from the sensory neuropathy induced by HIV; in some patients, these 2 processes are likely superimposed. To study these intercurrent PNS disease processes, we first established a simian immunodeficiency virus (SIV)/pigtailed macaque model in which more than 90% of animals developed PNS changes closely resembling those seen in HIV-infected individuals with distal sensory neuropathy. To determine whether cART alters the progression of SIV-induced PNS damage, dorsal root ganglia and epidermal nerve fibers were evaluated in SIV-infected macaques after long-term suppressive cART. Although cART effectively suppressed SIV replication and reduced macrophage activation in the dorsal root ganglia, PGP 9.5 immunostaining and measurements of epidermal nerve fibers in the plantar surface of the feet of treated SIV-infected macaques clearly showed that cART did not normalize epidermal nerve fiber density. These findings illustrate that significant PNS damage persists in SIV-infected macaques on suppressive cART.

Acquired immune axonal neuropathies

PURPOSE OF REVIEW

This article discusses the clinical features, pathophysiology, and management of primary and secondary acquired immune axonal neuropathies.

RECENT FINDINGS

Although there are many collagen vascular disorders associated with vasculitic neuropathy, a quarter of cases have been described to be due to nonsystemic vasculitis of the peripheral nervous system. Enhanced surveillance and aggressive treatment of conditions such as cryoglobulin-related vasculitic neuropathy with cyclophosphamide, rituximab, and alfa interferons has led to improved morbidity and mortality, however, many cases of immune axonal acquired neuropathy are still associated with poor outcomes. Acute motor axonal neuropathy (AMAN) and acute motor sensory axonal neuropathy (AMSAN) are well-characterized variants of Guillain-Barré syndrome.

SUMMARY

Characterizing the clinical and electrophysiologic phenotype can help diagnose conditions such as nonsystemic vasculitic neuropathy, AMAN, AMSAN, and immune small fiber neuropathy, while careful evaluation of systemic features is key to identifying secondary immune axonal neuropathies such as vasculitic neuropathy related to collagen vascular disease. Additional research is needed to determine the exact immune pathogenesis and optimized treatment regimens for all acquired immune axonal neuropathies.

[Immunohistochemical examination of skin biopsy specimens with calculation of C fibers in the diagnosis of polyneuropathy]

The paper discusses the anatomy of innervation of the skin, the epidermis and dermis in particular, which are related to pain, the markers of skin nerves and cells. It gives data on the diagnosis of fine unmyelinated fibers, by immunohistochemically examining skin biopsy specimens. The paper also describes the morphometry of skin nerves: intraepidermal nerve fibers, dermal nerve fibers, and autonomic nerve fibers. It discusses whether a skin biopsy specimen may be used to diagnose polyneuropathies of different etiology: diabetic, immune, HIV-related, and hereditary ones.

Nerve demyelination increases metabotropic glutamate receptor subtype 5 expression in peripheral painful mononeuropathy

Wallerian degeneration or nerve demyelination, arising from spinal nerve compression, is thought to bring on chronic neuropathic pain. The widely distributed metabotropic glutamate receptor subtype 5 (mGluR5) is involved in modulating nociceptive transmission. The purpose of this study was to investigate the potential effects of mGluR5 on peripheral hypersensitivities after chronic constriction injury (CCI). Sprague-Dawley rats were operated on with four loose ligatures around the sciatic nerve to induce thermal hyperalgesia and mechanical allodynia. Primary afferents in dermis after CCI exhibited progressive decreases, defined as partial cutaneous denervation; importantly, mGluR5 expressions in primary afferents were statistically increased. CCI-induced neuropathic pain behaviors through the intraplantar injections of 2-methyl-6-(phenylethynyl)-pyridine (MPEP), a selective mGluR5 antagonist, were dose-dependently attenuated. Furthermore, the most increased mGluR5 expressions in primary afferents surrounded by reactive Schwann cells were observed at the distal CCI stumps of sciatic nerves. In conclusion, these results suggest that nerve demyelination results in the increases of mGluR5 expression in injured primary afferents after CCI; and further suggest that mGluR5 represents a main therapeutic target in developing pharmacological strategies to prevent peripheral hypersensitivities.

Unraveling the pathogenesis of HIV peripheral neuropathy: insights from a simian immunodeficiency virus macaque model

Peripheral neuropathy (PN) is the most frequent neurologic complication in individuals infected with human immunodeficiency virus (HIV). It affects over one third of infected patients, including those receiving effective combination antiretroviral therapy. The pathogenesis of HIV-associated peripheral neuropathy (HIV-PN) remains poorly understood. Clinical studies are complicated because both HIV and antiretroviral treatment cause damage to the peripheral nervous system. To study HIV-induced peripheral nervous system (PNS) damage, a unique simian immunodeficiency virus (SIV)/pigtailed macaque model of HIV-PN that enabled detailed morphologic and functional evaluation of the somatosensory pathway throughout disease progression was developed. Studies in this model have demonstrated that SIV induces key pathologic features that closely resemble HIV-induced alterations, including inflammation and damage to the neuronal cell bodies in somatosensory ganglia and decreased epidermal nerve fiber density. Insights generated in the model include: finding that SIV alters the conduction properties of small, unmyelinated peripheral nerves; and that SIV impairs peripheral nerve regeneration. This review will highlight the major findings in the SIV-infected pigtailed macaque model of HIV-PN, and will illustrate the great value of a reliable large animal model to show the pathogenesis of this complex, HIV-induced disorder of the PNS.

Intra-epidermal nerve fibres in human skin: back to the roots

Regarding the existence and the role of intra-epidermal nerve fibres, the literature is ambiguous. However, performing a literature search, a landmark paper turned up that even many dermatologists seem to have forgotten, or may not even know at all. This paper is entitled 'The innervation of human epidermis' written by Arthur and Shelley (J Invest Dermatol, 32, 1959, 397). The full text is available via http://www.nature.com/jid/journal/v32/n3/pdf/jid195969a.pdf. The authors present data on intra-epidermal nerves at 16 representative body areas. The existence of intra-epidermal nerve fibres is undisputable and does not only explain clinical symptoms but may even provide a promising target for drug development.

Unilateral peripheral neuropathic pain: The role of neurodiagnostic skin biopsy

According to the current definition of neuropathic pain (“pain arising as a direct consequence of a lesion or disease affecting the somatosensory system”), the demonstration of a lesion or disease involving the somatosensory system is mandatory for the diagnosis of definite neuropathic pain. Although several methods are currently available for this aim, none is suitable for every type of disease (or lesion). Neurodiagnostic skin biopsy (NSB) is a relatively new technique for the diagnosis of peripheral nerve lesions. It is an objective method, completely independent from the patient’s complaining, based on immunohistochemical staining techniques that allow measurement of the density of the epidermal nerve fibers, currently considered the free nerve endings of small diameter (A-delta and C) afferent fibers. NSB has the important property of being used to investigate the skin, allowing obtaining a diagnosis of small fiber axonal neuropathy of peripheral nerves supplying every body part covered by skin. This feature appears to be very important, particularly in cases of unilateral nerve lesions, because it allows going beyond the possibilities of neurophysiological tests which are available only for a limited number of peripheral nerves. All these characteristics make NSB a precious instrument for the diagnosis of peripheral unilateral neuropathic pain.

Animal models of diabetes-induced neuropathic pain

Neuropathy will afflict over half of the approximately 350 million people worldwide who currently suffer from diabetes and around one-third of diabetic patients with neuropathy will suffer from painful symptoms that may be spontaneous or stimulus evoked. Diabetes can be induced in rats or mice by genetic, dietary, or chemical means, and there are a variety of well-characterized models of diabetic neuropathy that replicate either type 1 or type 2 diabetes. Diabetic rodents display aspects of sensorimotor dysfunction such as stimulus-evoked allodynia and hyperalgesia that are widely used to model painful neuropathy. This allows investigation of pathogenic mechanisms and development of potential therapeutic interventions that may alleviate established pain or prevent onset of pain.

Skin matters: identifying pain mechanisms and predicting treatment outcomes

The skin acts as a complex sensory organ. The emerging new data on peripheral pain mechanisms from within the skin is presented. This data has led to new insights into the potential pain mechanisms for various pain conditions including neuropathic pain (from small fiber neuropathies) and Complex Regional Pain Syndrome. The somatosensory neurons that innervate our skin constantly update our brains on the objects and environmental factors that surround us. Cutaneous sensory neurons expressing nociceptive receptors such as transient receptor potential vanilloid 1 channels and voltage-gated sodium channels are critical for pain transmission. Epidermal cells (such as keratinocytes, Langerhans cells, and Merkel cells) express sensor proteins and neuropeptides; these regulate the neuroimmunocutaneous system and participate in nociception and neurogenic inflammation. In the past two decades, there has been widespread use of modalities such as punch skin biopsies, quantitative sensory testing, and laser-evoked potentials to evaluate small caliber nerve fibers. This paper explores these laboratory techniques as well as the phenomenon of small fiber neuropathy. Treatment using transdermal drug delivery is discussed. There is potential for these findings to predict treatment outcomes in clinical practice and to develop new therapies for different pain conditions. These findings should enhance the physician's ability to evaluate and treat diverse types of pain.