Diagnosing small fiber neuropathy in clinical practice: a deep phenotyping study

Evaluation of small fibre neuropathies

Small fibre neuropathies (SFNs) are common and can significantly affect patients' lives due to debilitating pain and autonomic symptoms. We explain the tests that neurologists can use to diagnose SFNs and how neurophysiologists perform and interpret them. This review focuses on neurophysiological tests that can be used to investigate SFNs, their sensitivity, specificity and limitations. Some of these tests are available only in specialist centres. However, newer technologies are emerging from scientific research that may make it easier to diagnose these conditions in the future.

The Evolving Landscape of Small Fiber Neuropathy

Small fiber neuropathy (SFN) belongs to a heterogeneous group of disorders in which thinly myelinated Aδ and unmyelinated C-fibers are primarily affected, leading to neuropathic pain and autonomic symptoms. SFN can be associated with systemic conditions such as diabetes, autoimmune diseases, exposure to drugs and toxins, and infection, with the list of associated diseases continuing to expand. Variants in the SCN9A, SCN10A, and SCN11A genes encoding Nav 1.7, Nav 1.8, and Nav 1.9 sodium channel subunits, as well as in the TRPA1 gene, have been found in SFN patients, expanding the spectrum of underlying conditions and enhancing our understanding of pathophysiological mechanisms. There is also growing interest in immune-mediated forms that could help identify potentially treatable subgroups. According to international criteria, diagnosis is established through clinical examination, the assessment of intraepidermal nerve fiber density, and/or quantitative sensory testing. Autonomic functional tests allow for a better characterization of dysautonomia in SFN, which can be subclinical. Other tests can support the diagnosis. Currently, the management of SFN prioritizes treating the underlying condition, if identified, within a multidisciplinary approach that combines symptomatic pain therapy, lifestyle changes, and biopsychological interventions. Emerging insights from the molecular characterization of SFN channelopathies hold promise for improving diagnosis, potentially leading to the discovery of new drugs and refining trial designs in the future. This article reviews the clinical presentation, diagnostic workup, and advancing knowledge of associated conditions and interventional management of SFN.

Serum neurofilament light chain levels in patients with small-fiber neuropathy

BACKGROUND

Serum neurofilament light chain (sNfL) levels are an increasingly employed tool for the assessment of active axonal injury in a variety of neurological disorders including polyneuropathy. Injury to the small nerve fibers can lead to small fiber neuropathy (SFN), a neurological condition which is clinically manifested by combination of burning pain, various sensory disturbances and symptoms of autonomic dysfunction. SFN is evident by identifying reduced epidermal nerve fiber density (ENFD) in skin biopsy.

OBJECTIVE

To assess the utility of sNfL measurement as a marker for active axonal injury in a population of patients who were referred for skin biopsy as part of the evaluation of chronic sensory symptoms.

METHODS

sNfL levels were measured in 94 patients at the time of skin biopsy. Clinical, electrodiagnostic, and imaging data were collected and neurological comorbidities including central nervous system disorders, large-fiber polyneuropathy, and radiculopathy which may increase sNfL levels were reviewed.

RESULTS

Eighty-six patients had pathological skin biopsy result supporting the diagnosis of SFN. sNfL was increased in 9 (10%) SFN patients. Seven of them had neurological comorbidities which may explain the elevated sNfL. The other two patients had history of hypothyroidism and endometriosis.

CONCLUSIONS

sNfL levels are typically normal in patients with SFN. Furthermore, when sNfL are elevated, evaluation for another neurological disorder should be considered.

Current Perspectives in Pre- and Diabetic Peripheral Neuropathy Diagnosis and Management: An Expert Statement for the Gulf Region

Peripheral neuropathy (PN) significantly impacts the quality of life, causing substantial morbidity and increased mortality, as well as escalating healthcare costs. While PN can have various causes, the most common form, diabetic peripheral neuropathy, poses considerable risks for potential complications. Diabetic peripheral neuropathy (DPN) affects over 50% of people with prediabetes and diabetes. Despite its prevalence, a global gap in diagnosis and management exists, exacerbated by the COVID-19 pandemic. This expert consensus was formulated through a comprehensive evaluation by a panel of experts, informed by a focused literature review, aiming to establish a clinically robust approach to diagnosing and managing pre- and diabetic PN with the early utilization of neurotropic B vitamins. This document offers a consensus perspective on the existing challenges in diagnosing and managing PN, focusing on DPN. The expert panel proposes measures to address this underdiagnosed burden, highlighting the importance of early intervention through innovative screening methods, integrated care approaches, and therapeutic strategies. The document advocates for increased awareness, targeted campaigns, and proactive care strategies to bridge gaps in the patient care of individuals with diabetes, emphasizing the importance of early detection and timely management to improve overall health outcomes. Specific recommendations include incorporating simplified questionnaires and innovative screening methods into routine care, prioritizing neurotropic B vitamin supplementation, optimizing glucagon-like peptide 1 (GLP-1) receptor agonist treatments, and adopting a holistic approach to neuropathy management. The consensus underscores the urgent need to address the underdiagnosis and undertreatment of PN, offering practical measures to enhance early detection and improve health outcomes for individuals with DPN.

Idiopathic Distal Sensory Polyneuropathy and Fibromyalgia Syndrome: A Comparative Phenotyping Study

INTRODUCTION

Painful idiopathic distal sensory polyneuropathy (IDSP) and fibromyalgia syndrome (FMS) are cryptogenic chronic pain syndromes. The contribution of small fibre pathology (SFP) in FMS remains controversial. This study aims to quantify small nerve pathology in participants with IDSP and FMS and identify relationships of SFP with sensory phenotypes.

METHODS

In this study, 73 individuals (FMS: 25, IDSP: 23, healthy volunteers: 25) underwent comprehensive assessment, including neurological exams, questionnaires, sensory tests, and corneal confocal microscopy.

RESULTS

IDSP participants displayed lower wind-up ratio (WUR) relative to FMS (p < 0.001), loss of function to thermal and mechanical stimuli and elevated neuropathy disability scores compared to FMS and healthy volunteers (all p < 0.001). FMS participants demonstrated gain of function to heat and blunt pressure pain responses relative to IDSP, and healthy volunteers (heat: p = 0.002 and p = 0.003; pressure: both p < 0.001) and WUR (both p < 0.001). FMS participants exhibited reduced corneal nerve fibre density (p = 0.02), while IDSP participants had lower global corneal nerve measures (density, branch density, and length) relative to healthy volunteers (all p < 0.001). Utilising corneal nerve fibre length, SFP was demonstrated in 66.6% of participants (FMS: 13/25; IDSP: 22/23).

CONCLUSION

Participants with SFP, in both FMS and IDSP, reported symptoms indicative of small nerve fibre disease. Although distinctions in pain distributions are evident between individuals with FMS and IDSP, over 50% of participants between the two conditions displayed both a loss and gain of thermal and mechanical function suggestive of shared mechanisms. However, sensory phenotypes were associated with the presence of SFP in IDSP but not in FMS.

Pain and small fiber pathology in men with fibromyalgia syndrome

Introduction

Small fiber pathology may be involved in the pathophysiology of pain in women with fibromyalgia syndrome (FMS).

Objectives

This prospective single-center case-control study provides detailed pain phenotyping and small fiber pathology data in a cohort of men with FMS on a morphological and functional level.

Methods

Forty-two men with FMS underwent a comprehensive pain-related interview and neurological examination, a questionnaire and neurophysiological assessment, and specialized small fiber tests: skin punch biopsy, quantitative sensory testing including C-tactile afferents, and corneal confocal microscopy. Data were compared with those of healthy male controls.

Results

Men with FMS reported generalized and permanent pain with additional pain attacks and a mostly pressing pain character. Intraepidermal nerve fiber density was reduced at ≥1 biopsy site in 35 of 42 (83%) men with FMS (controls: 32/65, 49%). Compared with male controls, men with FMS had elevated cold (P < 0.05) and warm detection thresholds (P < 0.001) and an increased mechanical pain threshold (P < 0.05) as well as an impairment of C-tactile afferents (P < 0.05). Corneal nerve fiber density was lower in male patients with FMS vs healthy men (P < 0.01). Male FMS patients with pathological skin innervation at ≥1 biopsy site compared with those with normal skin innervation had a higher clinical Widespread Pain Index (P < 0.05) indicating an association between the severity of cutaneous denervation and symptom load.

Conclusion

We show a distinct pain phenotype and small nerve fiber dysfunction and pathology in male patients with FMS. These findings may have implications for the diagnosis and management of men with FMS.

Small fiber neuropathy

Small fiber neuropathy (SFN) is a condition involving the small nerve fibers of the peripheral nervous system, specifically the thinly myelinated Aδ and unmyelinated C fibers. It is an increasingly acknowledged condition within the spectrum of neuropathic pain disorders, leading to a rise in diagnosed patients. SFN is characterized by neuropathic pain, that is often described as burning, and typically presents in the hands and feet ascending proximally. Since small nerve fibers are involved in the autonomic nervous system, SFN can also lead to autonomic dysfunction. In the clinical setting, SFN diagnosis is frequently based on the Besta Criteria, which include skin biopsy and quantitative sensory testing. For clinical trials, the ACTTION criteria are also recommended. However, the diagnostic process is often complex, prompting research towards more accessible diagnostic methods. The pathophysiology of SFN remains unclear, thereby challenging therapeutic strategies. A large variety of underlying conditions has been associated with SFN, including metabolic, immune-mediated, infectious, toxic and hereditary conditions. The discovery of genetic sodium channelopathies in SFN provides insight into its underlying mechanisms. Newly discovered mutations within these genes reveal that SFN often shows overlapping clinical presentations with other sodium channelopathies. This chapter provides an in-depth look at SFN, including its clinical features, diagnostic methods, underlying conditions and possible therapeutic strategies.

Age-dependent small fiber neuropathy: Mechanistic insights from animal models

Small fiber neuropathy (SFN) is a common and debilitating disease in which the terminals of small diameter sensory axons degenerate, producing sensory loss, and in many patients neuropathic pain. While a substantial number of cases are attributable to diabetes, almost 50% are idiopathic. An underappreciated aspect of the disease is its late onset in most patients. Animal models of human genetic mutations that produce SFN also display age-dependent phenotypes suggesting that aging is an important contributor to the risk of development of the disease. In this review we define how particular sensory neurons are affected in SFN and discuss how aging may drive the disease. We also evaluate how animal models of SFN can define disease mechanisms that will provide insight into early risk detection and suggest novel therapeutic interventions.

[Sequence variants of unknown significance in small fiber neuropathy : Characterization of a heterogeneous patient population]

BACKGROUND

In almost half of patients suffering from small fiber neuropathies (SFN), the etiology remains elusive. For these patients with "idiopathic SFN", symptomatic analgesic therapy is the only option. Reports on a potential genetic background of neuropathic pain syndromes are increasing and particularly in SFN patients, several genetic variants were found mainly located in genes encoding voltage-gated sodium channels. Although up to 30% of SFN patients show genetic alterations, most of these remain of "unknown pathogenic significance" and little is known about "genetic SFN".

OBJECTIVES

The study aimed to determine clinical characteristics of SFN patients carrying a rare genetic variant of unknown significance in pain-associated genes.

MATERIALS AND METHODS

From 2015 to 2020, 66 patients with primarily idiopathic SFN were examined and rare gene variants of unknown significance detected in 13/66 (20%) of these. A detailed medical history with focus on pain was recorded and patients filled in standardized questionnaires to assess physical and emotional burden due to pain.

RESULTS

The authors found 13/66 (20%) patients with rare variants of unknown significance located in pain-associated genes who reported pain refractory to analgesic treatment, a higher number of external factors influencing clinical symptoms, and a higher level of physical impairment and emotional stress due to pain compared with patients without such genetic variants.

CONCLUSIONS

Early genetic assessment is recommended to optimize the management of patients with potentially hereditary SFN. Early access to rehabilitation and mental support as well as a consequent elimination of external triggering factors should be granted.

[The EAN-NeuPSIG guideline on the diagnosis of neuropathic pain-a summary]

In this joint guideline of the scientific societies and working groups mentioned in the title, evidence-based recommendations for the use of screening questionnaires and diagnostic tests in patients with neuropathic pain were developed. The systematic literature search and meta-analysis yielded the following results: Of the screening questionnaires, Douleur Neuropathique en 4 Questions (DN4), I‑DN4 (self-administered DN4), and Leeds Assessment of Neuropathic Symptoms and Signs (LANSS) received a strong recommendation, while S‑LANSS (self-administered LANSS) and PainDETECT received weak recommendations for their use in the diagnostic workup of patients with possible neuropathic pain. There was a strong recommendation for the use of skin biopsy and a weak recommendation for quantitative sensory testing and nociceptive evoked potentials. The role of confocal corneal microscopy is still unclear. Functional imaging and peripheral nerve blocks are helpful in elucidating the pathophysiology, but current literature does not support their use in diagnosing neuropathic pain. In selected cases, genetic testing in specialized centers may be considered.

Small fibre neuropathy in Fabry disease: a human-derived neuronal in vitro disease model and pilot data

Acral burning pain triggered by fever, thermal hyposensitivity and skin denervation are hallmarks of small fibre neuropathy in Fabry disease, a life-threatening X-linked lysosomal storage disorder. Variants in the gene encoding alpha-galactosidase A may lead to impaired enzyme activity with cellular accumulation of globotriaosylceramide. To study the underlying pathomechanism of Fabry-associated small fibre neuropathy, we generated a neuronal in vitro disease model using patient-derived induced pluripotent stem cells from three Fabry patients and one healthy control. We further generated an isogenic control line via gene editing. We subjected induced pluripotent stem cells to targeted peripheral neuronal differentiation and observed intra-lysosomal globotriaosylceramide accumulations in somas and neurites of Fabry sensory neurons using super-resolution microscopy. At functional level, patch-clamp analysis revealed a hyperpolarizing shift of voltage-gated sodium channel steady-state inactivation kinetics in isogenic control neurons compared with healthy control neurons (P < 0.001). Moreover, we demonstrate a drastic increase in Fabry sensory neuron calcium levels at 39°C mimicking clinical fever (P < 0.001). This pathophysiological phenotype was accompanied by thinning of neurite calibres in sensory neurons differentiated from induced pluripotent stem cells derived from Fabry patients compared with healthy control cells (P < 0.001). Linear-nonlinear cascade models fit to spiking responses revealed that Fabry cell lines exhibit altered single neuron encoding properties relative to control. We further observed mitochondrial aggregation at sphingolipid accumulations within Fabry sensory neurites utilizing a click chemistry approach together with mitochondrial dysmorphism compared with healthy control cells. We pioneer pilot insights into the cellular mechanisms contributing to pain, thermal hyposensitivity and denervation in Fabry small fibre neuropathy and pave the way for further mechanistic in vitro studies in Fabry disease and the development of novel treatment approaches.

Studying serum neurofilament light chain levels as a potential new biomarker for small fiber neuropathy

BACKGROUND AND PURPOSE

Diagnosing small fiber neuropathies can be challenging. To address this issue, whether serum neurofilament light chain (sNfL) could serve as a potential biomarker of damage to epidermal Aδ- and C-fibers was tested.

METHODS

Serum NfL levels were assessed in 30 patients diagnosed with small fiber neuropathy and were compared to a control group of 19 healthy individuals. Electrophysiological studies, quantitative sensory testing and quantification of intraepidermal nerve fiber density after skin biopsy were performed in both the proximal and distal leg.

RESULTS

Serum NfL levels were not increased in patients with small fiber neuropathy compared to healthy controls (9.1 ± 3.9 and 9.4 ± 3.8, p = 0.83) and did not correlate with intraepidermal nerve fiber density at the lateral calf or lateral thigh or with other parameters of small fiber impairment.

CONCLUSION

Serum NfL levels cannot serve as a biomarker for small fiber damage.

Earlier diagnosis of peripheral neuropathy in primary care: A call to action

Peripheral neuropathy (PN) often remains undiagnosed (~80%). Earlier diagnosis of PN may reduce morbidity and enable earlier risk factor reduction to limit disease progression. Diabetic peripheral neuropathy (DPN) is the most common PN and the 10 g monofilament is endorsed as an inexpensive and easily performed test for DPN. However, it only detects patients with advanced neuropathy at high risk of foot ulceration. There are many validated questionnaires to diagnose PN, but they can be time-consuming and have complex scoring systems. Primary care physicians (PCPs) have busy clinics and lack access to a readily available screening method to diagnose PN. They would prefer a short, simple, and accurate tool to screen for PN. Involving the patient in the screening process would not only reduce the time a physician requires to make a diagnosis but would also empower the patient. Following an expert meeting of diabetologists and neurologists from the Middle East, South East Asia and Latin America, a consensus was formulated to help improve the diagnosis of PN in primary care using a simple tool for patients to screen themselves for PN followed by a consultation with the physician to confirm the diagnosis.

In vitro characterization of cells derived from a patient with the GLA variant c.376A>G (p.S126G) highlights a non-pathogenic role in Fabry disease

Fabry disease (FD) is a life-limiting disorder characterized by intracellular globotriaosylceramide (Gb3) accumulations. The underlying α-galactosidase A (α-GAL A) deficiency is caused by variants in the gene GLA. Variants of unknown significance (VUS) are frequently found in GLA and challenge clinical management. Here, we investigated a 49-year old man with cryptogenic lacunar cerebral stroke and the chance finding of the VUS S126G, who was sent to our center for diagnosis and initiation of a costly and life-long FD-specific treatment. We combined clinical examination with in vitro investigations of dermal fibroblasts (HDF), induced pluripotent stem cells (iPSC), and iPSC-derived sensory neurons. We analyzed α-GAL A activity in iPSC, Gb3 accumulation in all three cell types, and action potential firing in sensory neurons. Neurological examination and small nerve fiber assessment was normal except for reduced distal skin innervation. S126G iPSC showed normal α-GAL A activity compared to controls and no Gb3 deposits were found in all three cell types. Baseline electrophysiological characteristics of S126G neurons showed no difference compared to healthy controls as investigated by patch-clamp recordings. We pioneer multi-level cellular characterization of the VUS S126G using three cell types derived from a patient and provide further evidence for the benign nature of S126G in GLA, which is of great importance in the management of such cases in clinical practice.

Interaction of human keratinocytes and nerve fiber terminals at the neuro-cutaneous unit

Traditionally, peripheral sensory neurons are assumed as the exclusive transducers of external stimuli. Current research moves epidermal keratinocytes into focus as sensors and transmitters of nociceptive and non-nociceptive sensations, tightly interacting with intraepidermal nerve fibers at the neuro-cutaneous unit. In animal models, epidermal cells establish close contacts and ensheath sensory neurites. However, ultrastructural morphological and mechanistic data examining the human keratinocyte-nerve fiber interface are sparse. We investigated this exact interface in human skin applying super-resolution array tomography, expansion microscopy, and structured illumination microscopy. We show keratinocyte ensheathment of afferents and adjacent connexin 43 contacts in native skin and have applied a pipeline based on expansion microscopy to quantify these parameter in skin sections of healthy participants versus patients with small fiber neuropathy. We further derived a fully human co-culture system, visualizing ensheathment and connexin 43 plaques in vitro. Unraveling human intraepidermal nerve fiber ensheathment and potential interaction sites advances research at the neuro-cutaneous unit. These findings are crucial on the way to decipher the mechanisms of cutaneous nociception.

Distinguishing fibromyalgia syndrome from small fiber neuropathy: a clinical guide

Introduction

Fibromyalgia syndrome (FMS) and small fiber neuropathy (SFN) are distinct pain conditions that share commonalities and may be challenging as for differential diagnosis.

Objective

To comprehensively investigate clinical characteristics of women with FMS and SFN to determine clinically applicable parameters for differentiation.

Methods

We retrospectively analyzed medical records of 158 women with FMS and 53 with SFN focusing on pain-specific medical and family history, accompanying symptoms, additional diseases, and treatment. We investigated data obtained using standardized pain, depression, and anxiety questionnaires. We further analyzed test results and findings obtained in standardized small fiber tests.

Results

FMS patients were on average ten years younger at symptom onset, described higher pain intensities requiring frequent change of pharmaceutics, and reported generalized pain compared to SFN. Pain in FMS was accompanied by irritable bowel or sleep disturbances, and in SFN by paresthesias, numbness, and impaired glucose metabolism (P < 0.01 each). Family history was informative for chronic pain and affective disorders in FMS (P < 0.001) and for neurological disorders in SFN patients (P < 0.001). Small fiber pathology in terms of skin denervation and/or thermal sensory threshold elevation was present in 110/158 (69.7 %) FMS patients and 39/53 (73.6 %) SFN patients. FMS patients mainly showed proximally reduced skin innervation and higher corneal nerve branch densities (p<0.001) whereas SFN patients were characterized by reduced cold detection and prolonged electrical A-delta conduction latencies (P < 0.05).

Conclusions

Our data show that FMS and SFN differ substantially. Detailed pain, drug and family history, investigating blood glucose metabolism, and applying differential small fiber tests may help to improve diagnostic differentiation and targeted therapy.

Cutaneous nerve biopsy in patients with symptoms of small fiber neuropathy: a retrospective study

OBJECTIVES

We aimed to investigate to what extent small fiber tests were abnormal in an unselected retrospective patient material with symptoms suggesting that small fiber neuropathy (SFN) could be present, and to evaluate possible gender differences.

METHODS

Nerve conduction studies (NCS), skin biopsy for determination of intraepidermal nerve fiber density (IENFD) and quantitative sensory testing (QST) were performed. Z-scores were calculated from reference materials to adjust for the effects of age and gender/height.

RESULTS

Two hundred and three patients, 148 females and 55 males had normal NCS and were considered to have possible SFN. 45.3 % had reduced IENFD, 43.2 % of the females and 50.9 % of the males. Mean IENFD was 7.3 ± 2.6 fibers/mm in females and 6.1 ± 2.3 in males (p<0.001), but the difference was not significant when adopting Z-scores. Comparison of gender differences between those with normal and abnormal IENFD were not significant when Z-scores were applied. QST was abnormal in 50 % of the patients (48.9 % in females and 52.9 % in males). In the low IENFD group 45 cases out of 90 (50 %) were recorded with abnormal QST. In those with normal IENFD 51 of 102 (50 %) showed abnormal QST.

CONCLUSIONS

Less than half of these patients had reduced IENFD, and 50 % had abnormal QST. There were no gender differences. A more strict selection of patients might have increased the sensitivity, but functional changes in unmyelinated nerve fibers are also known to occur with normal IENFD. Approval to collect data was given by the Norwegian data protection authority at University Hospital of North Norway (Project no. 02028).

Advances in diagnosis and management of distal sensory polyneuropathies

Distal sensory polyneuropathy (DSP) is characterised by length-dependent, sensory-predominant symptoms and signs, including potentially disabling symmetric chronic pain, tingling and poor balance. Some patients also have or develop dysautonomia or motor involvement depending on whether large myelinated or small fibres are predominantly affected. Although highly prevalent, diagnosis and management can be challenging. While classic diabetes and toxic causes are well-recognised, there are increasingly diverse associations, including with dysimmune, rheumatological and neurodegenerative conditions. Approximately half of cases are initially considered idiopathic despite thorough evaluation, but often, the causes emerge later as new symptoms develop or testing advances, for instance with genetic approaches. Improving and standardising DSP metrics, as already accomplished for motor neuropathies, would permit in-clinic longitudinal tracking of natural history and treatment responses. Standardising phenotyping could advance research and facilitate trials of potential therapies, which lag so far. This review updates on recent advances and summarises current evidence for specific treatments.

Corneal confocal microscopy in small and mixed fiber neuropathy-Comparison with skin biopsy and cold detection in a large prospective cohort

BACKGROUND AND AIMS

The diagnosis of small fiber neuropathy (SFN) is supported by reduced intraepidermal nerve fiber density (IENFD). The noninvasive method corneal confocal microscopy (CCM) has the potential to be a practical alternative. We aimed to estimate the diagnostic accuracy of CCM compared with IENFD and cold detection thresholds (CDT) in SFN and mixed fiber neuropathy (MFN).

METHODS

CCM was performed in an unselected prospective cohort of patients with a clinical suspicion of polyneuropathy. Predefined criteria were used to classify SFN and MFN. Neuropathy scores, including the Utah early neuropathy scale (UENS), were used to describe severity. Patients with established other diagnoses were used for diagnostic specificity calculations.

RESULTS

Data were taken from 680 patients, of which 244 had SFN or MFN. There was no significant difference in sensitivities [95%CI] of CCM (0.44 [0.38-0.51]), IEFND (0.43 [0.36-0.49]), and CDT (0.34 [0.29-0.41]). CCM specificity (0.75 [0.69-0.81]) was lower (p = .044) than for IENFD (0.99 [0.96-1.00]) but not than for CDT (0.81 [0.75-0.86]). The AUCs of the ROC curves of 0.63, 0.63 and 0.74 respectively, was lower for corneal nerve fiber density (p = .0012) and corneal nerve fiber length (p = .0015) compared with IENFD. While UENS correlated significantly with IENFD (p = .0016; R2  = .041) and CDT (p = .0002; R2  = .056), it did not correlate with CCM measures.

INTERPRETATION

The diagnostic utility of CCM in SNF and MFN is limited by the low specificity compared with skin biopsy. Further, CCM is less suitable than skin biopsy and CDT as a marker for neuropathy severity.

Small fiber neuropathy with normal intra-epidermal nerve fiber density but reduced sweat gland density after third BNT162b2 shot

Small fiber neuropathy (SFN) has not been reported after the third dose of BNT162b2 in a previously healthy vaccinee. A 44-year-old previously healthy female developed pain and sensory disturbances in varying locations after the third BNT162b2 dose. Additionally, she developed recurrent tinnitus, headaches, arthralgia, neck stiffness, and motor dysfunction. A skin biopsy five months after symptom onset revealed normal intra-epidermal nerve fiber density (IENFD) but reduced sweat gland nerve fiber density. She is intended for a first series of intravenous immunoglobulins. SARS-CoV-2 vaccinations may be complicated by SFN; the diagnosis SARS-CoV-2 vaccination SFN may be delayed; IENFD may be normal, but sweat gland nerve fiber density may document SFN; and full recovery after SFN cannot always be achieved quickly.

Peripheral mechanisms of peripheral neuropathic pain

Peripheral neuropathic pain (PNP), neuropathic pain that arises from a damage or disease affecting the peripheral nervous system, is associated with an extremely large disease burden, and there is an increasing and urgent need for new therapies for treating this disorder. In this review we have highlighted therapeutic targets that may be translated into disease modifying therapies for PNP associated with peripheral neuropathy. We have also discussed how genetic studies and novel technologies, such as optogenetics, chemogenetics and single-cell RNA-sequencing, have been increasingly successful in revealing novel mechanisms underlying PNP. Additionally, consideration of the role of non-neuronal cells and communication between the skin and sensory afferents is presented to highlight the potential use of drug treatment that could be applied topically, bypassing drug side effects. We conclude by discussing the current difficulties to the development of effective new therapies and, most importantly, how we might improve the translation of targets for peripheral neuropathic pain identified from studies in animal models to the clinic.

Thermal threshold testing: call for a balance between the number of measurements and abnormalities in the diagnosis of sarcoidosis-associated small fiber neuropathy

Introduction

Several recent studies of diagnosing small fiber neuropathy (SFN) have shown a lack of uniformity in thermal threshold testing (TTT) or quantitative sensory testing (QST) which makes it a challenge to compare the data. It is known that the chance of finding an abnormality increases with increasing number of measurements.

Objectives

With this study, we first wanted to investigate whether TTT could benefit from a new approach focusing on the balance between the number of measurements, depending on the selection of parameters and measuring sites, and on number of abnormalities (NOAs). Second, we wanted to address the role of the method of levels (MLe) in possible desensitization during TTT measurements.

Methods

One hundred seventeen participants were included (48 patients with sarcoidosis with probable SFN, 49 without SFN, and 20 healthy controls). Thermal threshold testing measurements and Small Fiber Neuropathy Screening List (SFNSL) questionnaire were used to assess SFN.

Results

A combination of measuring all thermal threshold parameters at both feet except for MLe showed the best diagnostic performance. Increasing TTT NOAs correlates with the severity of SFN. Adding the SFNSL questionnaire further improves diagnostic performance.

Discussion

Looking at TTT NOAs in all TTT parameters except for MLe at both feet should be considered as a new approach to improve the consistency and balance between the selection of TTT parameters, measuring sites, and definition of "abnormal QST." Moreover, the SFNSL questionnaire is a valuable tool to quantify SFN symptoms and could improve SFN diagnosis.

Pain associated with COVID-19 vaccination is unrelated to skin biopsy abnormalities

Introduction

Previous clinical observations raised the possibility that COVID-19 vaccination might trigger a small-fibre neuropathy.

Objectives

In this uncontrolled observational study, we aimed to identify small fibre damage in patients complaining of generalized sensory symptoms and pain after COVID-19 vaccination.

Methods

We collected clinical data, including a questionnaire for assessing autonomic symptoms (Composite Autonomic Symptom Score-31), and investigated quantitative sensory testing (QST) and skin biopsy in 15 prospectively enrolled patients with generalized sensory symptoms and pain after COVID-19 vaccination. Nine patients complaining of orthostatic intolerance also underwent cardiovascular autonomic tests.

Results

We found that all patients experienced widespread pain, and most of them (11 of 15) had a fibromyalgia syndrome. All patients had normal skin biopsy findings, and in the 9 patients with orthostatic intolerance, cardiovascular autonomic tests showed normal findings. Nevertheless, 5 patients had cold and warm detection abnormalities at the QST investigation.

Conclusions

In our study, most patients complaining of generalized sensory symptoms and pain after COVID-19 vaccination had clinical and diagnostic test findings compatible with a fibromyalgia syndrome. Although the abnormal QST findings we found in 5 patients might be compatible with a small-fibre neuropathy, they should be cautiously interpreted given the psychophysical characteristics of this diagnostic test. Further larger controlled studies are needed to define precisely the association between small fibre damage and COVID-19 vaccination.

Joint European Academy of Neurology-European Pain Federation-Neuropathic Pain Special Interest Group of the International Association for the Study of Pain guidelines on neuropathic pain assessment

BACKGROUND AND PURPOSE

In these guidelines, we aimed to develop evidence-based recommendations for the use of screening questionnaires and diagnostic tests in patients with neuropathic pain (NeP).

METHODS

We systematically reviewed studies providing information on the sensitivity and specificity of screening questionnaires, and quantitative sensory testing, neurophysiology, skin biopsy, and corneal confocal microscopy. We also analysed how functional neuroimaging, peripheral nerve blocks, and genetic testing might provide useful information in diagnosing NeP.

RESULTS

Of the screening questionnaires, Douleur Neuropathique en 4 Questions (DN4), I-DN4 (self-administered DN4), and Leeds Assessment of Neuropathic Symptoms and Signs (LANSS) received a strong recommendation, and S-LANSS (self-administered LANSS) and PainDETECT weak recommendations for their use in the diagnostic pathway for patients with possible NeP. We devised a strong recommendation for the use of skin biopsy and a weak recommendation for quantitative sensory testing and nociceptive evoked potentials in the NeP diagnosis. Trigeminal reflex testing received a strong recommendation in diagnosing secondary trigeminal neuralgia. Although many studies support the usefulness of corneal confocal microscopy in diagnosing peripheral neuropathy, no study specifically investigated the diagnostic accuracy of this technique in patients with NeP. Functional neuroimaging and peripheral nerve blocks are helpful in disclosing pathophysiology and/or predicting outcomes, but current literature does not support their use for diagnosing NeP. Genetic testing may be considered at specialist centres, in selected cases.

CONCLUSIONS

These recommendations provide evidence-based clinical practice guidelines for NeP diagnosis. Due to the poor-to-moderate quality of evidence identified by this review, future large-scale, well-designed, multicentre studies assessing the accuracy of diagnostic tests for NeP are needed.

Where Art Thou O treatment for diabetic neuropathy: the sequel

INTRODUCTION

Having lived through a pandemic and witnessed how regulatory approval processes can evolve rapidly; it is lamentable how we continue to rely on symptoms/signs and nerve conduction as primary endpoints for clinical trials in DPN.

AREAS COVERED

Small (Aδ and C) fibers are key to the genesis of pain, regulate skin blood flow, and play an integral role in the development of diabetic foot ulceration but continue to be ignored. This article challenges the rationale for the FDA insisting on symptoms/signs and nerve conduction as primary endpoints for clinical trials in DPN.

EXPERT OPINION

Quantitative sensory testing, intraepidermal nerve fiber density, and especially corneal confocal microscopy remain an after-thought, demoted at best to exploratory secondary endpoints in clinical trials of diabetic neuropathy. If pharma are to be given a fighting chance to secure approval for a new therapy for diabetic neuropathy, the FDA needs to reassess the evidence rather than rely on 'opinion' for the most suitable endpoint(s) in clinical trials of diabetic neuropathy.

Expanding the genetic causes of small-fiber neuropathy: SCN genes and beyond

Small-fiber neuropathy (SFN) is a disorder that exclusively affects the small nerve fibers, sparing the large nerve fibers. Thinly myelinated Aδ-fibers and unmyelinated C-fibers are damaged, leading to development of neuropathic pain, thermal dysfunction, sensory symptoms, and autonomic disturbances. Although many SFNs are secondary and due to immunological causes or metabolic disturbances, the etiology is unknown in up to half of the patients. Over the years, this proportion of "idiopathic SFN" has decreased, as familial and genetic causes have been discovered, thus shifting a proportion of once "idiopathic" cases to the genetic category. After the discovery of SCN9A-gene variants in 2012, SCN10A and SCN11A variants have been found to be pathogenic in SFN. With improved accessibility of SFN diagnostic tools and genetic tests, many non-SCN variants and genetically inherited systemic diseases involving the small nerve fibers have also been described, but only scattered throughout the literature. There are 80 SCN variants described as causing SFN, 8 genes causing hereditary sensory autonomic neuropathies (HSAN) described with pure SFN, and at least 7 genes involved in genetically inherited systemic diseases associated with SFN. This systematic review aims to consolidate and provide an updated overview on the genetic variants of SFN to date---SCN genes and beyond. Awareness of these genetic causes of SFN is imperative for providing treatment directions, prognostication, and management of expectations for patients and their health-care providers.

Functional and structural markers of peripheral microvascular autonomic neuropathy

INTRODUCTION/AIMS

Autonomic dysfunction is a common complication of small-fiber neuropathy (SFN). In this study we aimed to assess the applicability of autonomic microvascular indices as a potential marker for SFN assessment.

METHODS

Fifteen patients with confirmed SFN (idiopathic neuropathy [n = 10], chemotherapy-induced peripheral neuropathy [n = 2], impaired glucose tolerance [n = 1], hereditary transthyretin amyloidosis (hATTR) [n = 1], pulmonary sarcoidosis [n = 1]) and 15 matched control subjects underwent assessment of vascular skin responses assessed through laser Doppler flowmetry and evaluation of microvascular vessel and nerve density in skin biopsies. All participants underwent peripheral autonomic evaluation by quantitative sudomotor axon reflex testing (QSART).

RESULTS

We found no significant differences in vascular skin responses, or in any microvascular skin biopsy markers, when comparing SFN with control subjects. We found no correlation between vascular skin responses and skin biopsy indices. We saw no significant difference in any microvascular indices when comparing subjects with and without impaired sudomotor function.

DISCUSSION

Our findings suggest markers of peripheral microvascular innervation and function are not associated with the diagnosis of SFN. Furthermore, we saw no association between microvascular markers and sudomotor function, suggesting that these are independent and unrelated components of the autonomic nervous system.

Cytokine expression profiles in white blood cells of patients with small fiber neuropathy

BACKGROUND

The role of cytokines in the pathophysiology, diagnosis, and prognosis of small fiber neuropathy (SFN) is incompletely understood. We studied expression profiles of selected pro- and anti-inflammatory cytokines in RNA from white blood cells (WBC) of patients with a medical history and a clinical phenotype suggestive for SFN and compared data with healthy controls.

METHODS

We prospectively recruited 52 patients and 21 age- and sex-matched healthy controls. Study participants were characterized in detail and underwent complete neurological examination. Venous blood was drawn for routine and extended laboratory tests, and for WBC isolation. Systemic RNA expression profiles of the pro-inflammatory cytokines interleukin (IL)-1ß, IL-2, IL-8, tumor necrosis factor-alpha (TNF) and the anti-inflammatory cytokines IL-4, IL-10, transforming growth factor beta-1 (TGF) were analyzed. Protein levels of IL-2, IL-8, and TNF were measured in serum of patients and controls. Receiver operating characteristic (ROC)-curve analysis was used to determine the accuracy of IL-2, IL-8, and TNF in differentiating patients and controls. To compare the potential discriminatory efficacy of single versus combined cytokines, equality of different AUCs was tested.

RESULTS

WBC gene expression of IL-2, IL-8, and TNF was higher in patients compared to healthy controls (IL-2: p = 0.02; IL-8: p = 0.009; TNF: p = 0.03) and discriminated between the groups (area under the curve (AUC) ≥ 0.68 for each cytokine) with highest diagnostic accuracy reached by combining the three cytokines (AUC = 0.81, sensitivity = 70%, specificity = 86%). Subgroup analysis revealed the following differences: IL-8 and TNF gene expression levels were higher in female patients compared to female controls (IL-8: p = 0.01; TNF: p = 0.03). The combination of TNF with IL-2 and TNF with IL-2 and IL-8 discriminated best between the study groups. IL-2 was higher expressed in patients with moderate pain compared to those with severe pain (p = 0.02). Patients with acral pain showed higher IL-10 gene expression compared to patients with generalized pain (p = 0.004). We further found a negative correlation between the relative gene expression of IL-2 and current pain intensity (p = 0.02). Serum protein levels of IL-2, IL-8, and TNF did not differ between patients and controls.

CONCLUSIONS

We identified higher systemic gene expression of IL-2, IL-8, and TNF in SFN patients than in controls, which may be of potential relevance for diagnostics and patient stratification.

Small fiber neuropathy

Small fiber neuropathy (SFN) is a peripheral nervous system disease due to affection of A-delta or C-fibers in a proximal, distal, or diffuse distribution. Selective SFN (without large fiber affection) manifests with pain, sensory disturbances, or autonomic dysfunction. Though uniform diagnostic criteria are unavailable, most of them request typical clinical features and reduced intra-epidermal nerve fiber density on proximal or distal skin biopsy. Little consensus has been reached about the treatment of SFN, why this narrative review aims at summarizing and discussing treatment options for SFN. Treatment of SFN can be classified as symptomatic, pathophysiologic, or causal. Prerequisites for treating SFN are an established diagnosis, knowledge about the symptoms and signs, and the etiology. Pain usually responds to oral/intravenous pain killers, antidepressants, anti-seizure drugs, or topical, transdermal specifications. Some of the autonomic disturbances respond favorably to symptomatic treatment. SFN related to Fabry disease or hATTR are accessible to pathogenesis-related therapy. Immune-mediated SFN responds to immunosuppression or immune-modulation. Several of the secondary SFNs respond to causal treatment of the underlying disorder. In conclusion, treatment of SFN relies on a multimodal concept and includes causative, pathophysiologic, and symptomatic measures. It strongly depends on the clinical presentation, diagnosis, and etiology, why it is crucial before initiation of treatment to fix the diagnosis and etiology. Due to the heterogeneous clinical presentation and multi-causality, treatment of SFN should be individualized with the goal of controlling the underlying cause, alleviating pain, and optimizing functionality.

Small fiber involvement is independent from clinical pain in late-onset Pompe disease

BACKGROUND

Pain occurs in the majority of patients with late onset Pompe disease (LOPD) and is associated with a reduced quality of life. The aim of this study was to analyse the pain characteristics and its relation to a small nerve fiber involvement in LOPD patients.

METHODS

In 35 patients with LOPD under enzyme replacement therapy without clinical signs of polyneuropathy (19 females; 51 ± 15 years), pain characteristics as well as depressive and anxiety symptoms were assessed using the PainDetect questionnaire (PDQ) and the hospital anxiety and depression scale (HADS), respectively. Distal skin biopsies were analysed for intraepidermal nerve fiber density (IENFD) and compared to age- and gender-matched reference data. Skin biopsies from 20 healthy subjects served as controls to assure validity of the morphometric analysis.

RESULTS

Pain was reported in 69% of the patients with an average intensity of 4.1 ± 1.1 on the numeric rating scale (NRS; anchors: 0-10). According to PDQ, neuropathic pain was likely in one patient, possible in 29%, and unlikely in 67%. Relevant depression and anxiety symptoms occurred in 31% and 23%, respectively, and correlated with pain intensity. Distal IENFD (3.98 ± 1.95 fibers/mm) was reduced in 57% of the patients. The degree of IENFD reduction did not correlate with the durations of symptoms to ERT or duration of ERT to biopsy.

CONCLUSIONS

Pain is a frequent symptom in treated LOPD on ERT, though a screening questionnaire seldom indicated neuropathic pain. The high frequency of small nerve fiber pathology in a treated LOPD cohort was found regardless of the presence of pain or comorbid risk factors for SFN and needs further exploration in terms of clinical context, exact mechanisms and when developing novel therapeutic options for LOPD.

Small Fiber Neuropathy

PURPOSE OF REVIEW

This narrative review aims to summarize advances in the field of small fiber neuropathy made over the last decade, with emphasis on novel research highlighting the distinctive features of SFN.

RECENT FINDINGS

While the management of SFNs is ideally aimed at treating the underlying cause, most patients will require pain control via multiple, concurrent therapies. Herein, we highlight the most up-to-date information for diagnosis, medication management, interventional management, and novel therapies on the horizon. Despite the prevalence of small fiber neuropathies, there is no clear consensus on guidelines specific for the treatment of SFN. Despite the lack of specific guidelines for SFN treatment, the most recent general neuropathic pain guidelines are based on Cochrane studies and randomized controlled trials (RCTs) which have individually examined therapies used for the more commonly studied SFNs, such as painful diabetic neuropathy and HIV neuropathy. The recommendations from current guidelines are based on variables such as number needed to treat (NNT), safety, ease of use, and effect on quality of life.

Corneal Confocal Microscopy to Image Small Nerve Fiber Degeneration: Ophthalmology Meets Neurology

Neuropathic pain has multiple etiologies, but a major feature is small fiber dysfunction or damage. Corneal confocal microscopy (CCM) is a rapid non-invasive ophthalmic imaging technique that can image small nerve fibers in the cornea and has been utilized to show small nerve fiber loss in patients with diabetic and other neuropathies. CCM has comparable diagnostic utility to intraepidermal nerve fiber density for diabetic neuropathy, fibromyalgia and amyloid neuropathy and predicts the development of diabetic neuropathy. Moreover, in clinical intervention trials of patients with diabetic and sarcoid neuropathy, corneal nerve regeneration occurs early and precedes an improvement in symptoms and neurophysiology. Corneal nerve fiber loss also occurs and is associated with disease progression in multiple sclerosis, Parkinson's disease and dementia. We conclude that corneal confocal microscopy has good diagnostic and prognostic capability and fulfills the FDA criteria as a surrogate end point for clinical trials in peripheral and central neurodegenerative diseases.

Subepidermal Schwann cell counts correlate with skin innervation - an exploratory study

INTRODUCTION/AIMS

Schwann cell clusters have been described at the murine dermis-epidermis border. We quantified dermal Schwann cells in the skin of patients with small fiber neuropathy (SFN) compared to healthy controls to correlate with the clinical phenotype.

METHODS

Skin punch biopsies from the lower legs of 28 patients with SFN (eleven men, 17 women, median age 54 years [19-73]) and 9 healthy controls (five men, four women, median age 34 years [25-69]) were immunoreacted for S100 calcium-binding protein B as a Schwann cell marker, protein-gene product 9.5 as a pan-neuronal marker, and CD207 as a Langerhans cell marker. Intraepidermal nerve fiber density (IENFD) and subepidermal Schwann cell counts were determined.

RESULTS

Skin samples of patients with SFN showed lower IENFD (p<0.05), fewer Schwann cells/mm (p<0.01), and fewer Schwann cell clusters/mm (p<0.05) than controls. When comparing SFN patients with reduced (n=13, median age 53 years, 19-73 years) and normal distal (n=15, median age 54 years, 43-68 years) IENFD, the number of solitary Schwann cells/mm (p<0.01) and subepidermal nerve fibers associated with Schwann cell branches (p<0.05) were lower in patients with reduced IENFD. All three parameters positively correlated with distal IENFD (p<0.05 to p<0.01), while no correlation was found between Schwann cell counts and clinical pain characteristics.

DISCUSSION

Our data raise questions about the mechanisms underlying the interdependence of dermal Schwann cells and skin innervation in SFN. The temporal course and functional impact of Schwann cell presence and kinetics need further investigation.

Corneal Confocal Microscopy in the Diagnosis of Small Fiber Neuropathy: Faster, Easier, and More Efficient Than Skin Biopsy?

Chronic pain may affect 30–50% of the world’s population and an important cause is small fiber neuropathy (SFN). Recent research suggests that autoimmune diseases may be one of the most common causes of small nerve fiber damage. There is low awareness of SFN among patients and clinicians and it is difficult to diagnose as routine electrophysiological methods only detect large fiber abnormalities, and specialized small fiber tests, like skin biopsy and quantitative sensory testing, are not routinely available. Corneal confocal microscopy (CCM) is a rapid, non-invasive, reproducible method for quantifying small nerve fiber degeneration and regeneration, and could be an important tool for diagnosing SFN. This review considers the advantages and disadvantages of CCM and highlights the evolution of this technique from a research tool to a diagnostic test for small fiber damage, which can be a valuable contribution to the study and management of autoimmune disease.

Subclinical Corneal Nerve Fiber Damage and Immune Cell Activation in Systemic Lupus Erythematosus: A Corneal Confocal Microscopy Study

Purpose

The purpose of this study was to evaluate the utility of corneal confocal microscopy (CCM) in identifying small nerve fiber damage and immune cell activation in patients with systemic lupus erythematosus (SLE).

Methods

This cross-sectional comparative study included 39 consecutive patients with SLE and 30 healthy control participants. Central corneal sensitivity was assessed using a Cochet-Bonnet contact corneal esthesiometer and a laser scanning CCM (Heidelberg, Germany) was used to quantify corneal nerve fiber density (CNFD), nerve branch density (CNBD), nerve fiber length (CNFL), and Langerhans cell (LC) density.

Results

Age was comparable among patients with SLE (33.7 ± 12.7) and controls (35.0 ± 13.7 years, P = 0.670) and the median duration of disease was 3.0 years (2.0–10.0 years). CNBD (P = 0.003) and CNFL (P = 0.019) were lower and mature LC density (P = 0.002) was higher, but corneal sensitivity (P = 0.178) and CNFD (P = 0.198) were comparable in patients with SLE compared with controls. The SELENA-SLEDAI score correlated with CNFD (ρ = −0.319, P = 0.048) and CNFL (ρ = −0.373, P = 0.019), and the total and immature LC densities correlated with CNBD (ρ = −0.319. P = 0.048, and ρ = −0.328, P = 0.041, respectively). Immature LC density was higher (P = 0.025), but corneal sensitivity and nerve fiber parameters were comparable between patients with (33%) and without neuropsychiatric symptoms and SLE.

Conclusions

Corneal confocal microscopy identifies distal corneal nerve fiber loss and increased immune cell density in patients with SLE and corneal nerve loss was associated with disease activity.

Translational Relevance

Corneal confocal microscopy may enable the detection of subclinical corneal nerve loss and immune cell activation in SLE.

A translational study: Involvement of miR-21-5p in development and maintenance of neuropathic pain via immune-related targets CCL5 and YWHAE

Neuropathic pain occurs in more than half of the patients suffering from peripheral neuropathies. We investigated the role of microRNA (miR)-21 in neuropathic pain using a murine-human translational approach. We applied the spared nerve injury (SNI) model at the sciatic nerve of mice and assessed the potential analgesic effect of perineurial miR-21-5p inhibitor application. Immune-related targets of miR-21-5p were determined by a qRT-PCR based cytokine and chemokine array. Bioinformatical analysis identified potential miR-21-5p targets interacting with CC-chemokine ligand (CCL)5. We validated CCL5 and tyrosine 3-monooxygenase/tryptophan 5-monooxygenase activation protein (YWHAE), an interaction partner of miR-21-5p and CCL5, by qRT-PCR in murine common peroneal and tibial nerves. Validated candidates were then investigated in white blood cell and sural nerve biopsy samples of patients with focal to generalized pain syndromes, i.e. small fiber neuropathy (SFN), polyneuropathy (PNP), and nerve lesion (NL). We showed that perineurial miR-21-5p inhibition reverses SNI-induced mechanical and heat hypersensitivity in mice and found a reduction of the SNI-induced increase of the pro-inflammatory mediators CCL5 (p < 0.01), CCL17 (p < 0.05), and IL-12ß (p < 0.05) in miR-21-5p inhibitor-treated mice. In silico analysis revealed several predicted and validated targets for miR-21-5p with CCL5 interaction. Among these, we found lower YWHAE gene expression in mice after SNI and perineurial injections of a scrambled oligonucleotide compared to naïve mice (p < 0.05), but this was not changed by miR-21-5p inhibition. Furthermore, miR-21-5p inhibition led to a further increase of the SNI-induced increase in TGFß (p < 0.01). Patient biomaterial revealed different systemic expression patterns of miR-21-5p, with higher expression in SFN and lower expression in NL. Further, we showed higher systemic expression of pro-inflammatory mediators in white blood cells of SFN patients compared to healthy controls. We have conducted a translational study comparing results from animal models to human patients with three different neuropathic pain syndromes. We identified CCL5 as a miR-21 dependent common player in the mouse SNI model and the human painful disease SFN.