Longitudinal follow‐up of biopsy‐proven small fiber neuropathy

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.

Dorsal Root Ganglion Stimulation in Chronic Painful Polyneuropathy: A Potential Modulator for Small Nerve Fiber Regeneration

OBJECTIVES

Neuromodulatory treatments like spinal cord stimulation and dorsal root ganglion stimulation (DRGS) have emerged as effective treatments to relieve pain in painful polyneuropathy. Animal studies have demonstrated that neurostimulation can enhance nerve regeneration. This study aimed to investigate if DRGS may impact intraepidermal nerve fiber regeneration and sensory nerve function.

MATERIALS AND METHODS

Nine patients with chronic, intractable painful polyneuropathy were recruited. Intraepidermal nerve fiber density (IENFD) quantification in 3 mm punch skin biopsy was performed 1 month before DRGS (placed at the level of the L5 and S1 dorsal root ganglion) and after 12- and 24-month follow-up. Quantitative sensory testing, nerve conduction studies, and a clinical scale score were also performed at the same time points.

RESULTS

In 7 of 9 patients, DRGS was successful (defined as a reduction of ≥ 50% in daytime and/or night-time pain intensity), allowing a definitive implantable pulse generator implantation. The median baseline IENFD among these 7 patients was 1.6 fibers/mm (first and third quartile: 1.2; 4.3) and increased to 2.6 fibers/mm (2.5; 2.9) and 1.9 fibers/mm (1.6; 2.4) at 1- and 2-years follow-up, respectively. These changes were not statistically significant (p = 1.000 and 0.375). Sensory nerve tests did not show substantial changes.

CONCLUSIONS

Although not significant, the results of this study showed that in most of the patients with implants, there was a slight increase of the IENFD at the 1- and 2-year follow-up. Larger-scale clinical trials are warranted to explore the possible role of DRGS in reversing the progressive neurodegeneration over time.

CLINICAL TRIAL REGISTRATION

The Clinicaltrials.gov registration number for the study is NCT02435004; Swiss National Clinical Trials Portal: SNCTP000001376.

Autoantibodies Against Trisulfated Heparin Disaccharide and Fibroblast Growth Factor Receptor-3 May Play a Role in the Pathogenesis of Neuropathic Corneal Pain

PURPOSE

The aim of this study was to describe cases of patients with presumable dysimmune small-fiber neuropathy (SFN)-related neuropathic corneal pain (NCP), presenting with autoantibodies against trisulfated heparin disaccharide (TS-HDS) or fibroblast growth factor receptor-3 (FGFR-3).

METHODS

This study was a case series of 3 patients with NCP with positive anti-TS-HDS and/or anti-FGFR-3 autoantibodies and systemic SFN as confirmed by positive skin biopsy results.

RESULTS

All 3 patients were women with a mean age of 34.3± 6.1 years. They suffered from moderate to severe persistent chronic ocular discomfort (10/10, 10/10, and 9/10 on a visual analogue scale, respectively). Although 1 patient suffered from ocular pain and photophobia alone, the other 2 patients experienced additional non-ocular pain. One of the patients had pain on her face and head, and 1 patient reported neck and lower back pain. Two patients had high anti-TS-HDS IgM titers, whereas 1 patient had both high anti-TS-HDS IgM and anti-FGFR-3 IgG titers. Skin biopsy confirmed the presence of SFN in all patients by demonstrating decreased intraepidermal nerve fiber density.

CONCLUSIONS

The presence of anti-TS-HDS and anti-FGFR-3 autoantibodies in patients with NCP with positive skin biopsy findings for SFN highlights the potential role of dysimmune SFN in the pathogenesis of this disease.

Limbic Connectivity Underlies Pain Treatment Response in Small-Fiber Neuropathy

OBJECTIVE

Small-fiber neuropathy (SFN) is characterized by neuropathic pain due to degeneration of small-diameter nerves in the skin. Given that brain reorganization occurs following chronic neuropathic pain, this study investigated the structural and functional basis of pain-related brain changes after skin nerve degeneration.

METHODS

Diffusion-weighted and resting-state functional MRI data were acquired from 53 pathologically confirmed SFN patients, and the structural and functional connectivity of the pain-related network was assessed using network-based statistic (NBS) analysis.

RESULTS

Compared with age- and sex-matched controls, the SFN patients exhibited a robust and global reduction of functional connectivity, mainly across the limbic and somatosensory systems. Furthermore, lower functional connectivity was associated with skin nerve degeneration measured by reduced intraepidermal nerve fiber density and better therapeutic response to anti-neuralgia medications, particularly for the connectivity between the insula and the limbic areas including the anterior and middle cingulate cortices. Similar to the patterns of functional connectivity changes, the structural connectivity was robustly reduced among the limbic and somatosensory areas, and the cognition-integration areas including the inferior parietal lobule. There was shared reduction of structural and functional connectivity among the limbic, somatosensory, striatal, and cognition-integration systems: (1) between the middle cingulate cortex and inferior parietal lobule and (2) between the thalamus and putamen. These observations indicate the structural basis underlying altered functional connectivity in SFN.

INTERPRETATION

Our findings provide imaging evidence linking structural and functional brain dysconnectivity to sensory deafferentation caused by peripheral nerve degeneration and therapeutic responses for neuropathic pain in SFN. ANN NEUROL 2022.

Implications of Nerve Fiber Density on the Diagnosis and Treatment of Juvenile Fibromyalgia

Juvenile fibromyalgia (JFM) is a condition that presents as chronic widespread musculoskeletal pain and affects children and adolescents. JFM remains a challenging diagnosis, as it is both based on subjective criteria and the pathogenesis is poorly understood. Small fiber neuropathy (SFN) is a distinct condition, which is characterized by pathology of small A-delta and C fibers, and can present similarly to JFM. Small fiber pathology is characterized by reduced intraepidermal nerve fiber density (IENFD) on skin biopsy. Recent studies have found that as many as half of patients with JFM can demonstrate decreased IENFD, in pattern similar to SFN. This phenomenon has been referred to as small fiber pathology. The meaning of these findings was disputed; however, the current consensus remains that fibromyalgia and SFN are distinct conditions. Additionally, among patients with fibromyalgia, there are two phenotypes: those with small fiber pathology and those without. The purpose of this review was to characterize the role assessment of IENFD plays in the clinical context. We conducted a narrative review of pertinent articles pertaining to JFM, SFN and small fiber pathology in fibromyalgia. We concluded that assessment of IENFD should be completed if SFN is suspected either when a patient first presents or in patients who were previously diagnosed with fibromyalgia and SFN is later suspected. Distinguishing between JFM and SFN is important because recommended therapies differ between the two conditions. However, there is no evidence to support the use of skin biopsy to distinguish between the two discussed fibromyalgia phenotypes. More studies are needed to elucidate whether IENFD varies with morbidity and if both fibromyalgia phenotypes vary in their response to different therapeutic regimens.

Small Fiber Neuropathy Incidence, Prevalence, Longitudinal Impairments, and Disability

BACKGROUND AND OBJECTIVES

There are limited population-based data on small fiber neuropathy (SFN). We wished to determine SFN incidence, prevalence, comorbid conditions, longitudinal impairments, and disabilities.

METHODS

Test-confirmed patients with SFN in Olmsted, Minnesota, and adjacent counties were compared 3:1 to matched controls (January 1, 1998-December 31, 2017).

RESULTS

Ninety-four patients with SFN were identified, with an incidence of 1.3/100,000/y that increased over the study period and a prevalence of 13.3 per 100,000. Average follow-up was 6.1 years (0.7-43 years), and mean onset age was 54 years (range 14-83 years). Female sex (67%), obesity (body mass index mean 30.4 vs 28.5 kg/m²), insomnia (86% vs 54%), analgesic-opioid prescriptions (72% vs 46%), hypertriglyceridemia (180 mg/dL mean vs 147 mg/dL), and diabetes (51% vs 22%, p < 0.001) were more common (odds ratio 3.8-9.0, all p < 0.03). Patients with SFN did not self-identify as disabled with a median modified Rankin Scale score of 1.0 (range 0-6) vs 0.0 (0-6) for controls (p = 0.04). Higher Charlson comorbid conditions (median 6, range 3-9) occurred vs controls (median 3, range 1-9, p < 0.001). Myocardial infarctions occurred in 46% vs 27% of controls (p < 0.0001). Classifications included idiopathic (70%); diabetes (15%); Sjögren disease (2%); AL-amyloid (1%); transthyretin-amyloid (1%); Fabry disease (1%); lupus (1%); postviral (1%); Lewy body (1%), and multifactorial (5%). Foot ulcers occurred in 17, with 71% having diabetes. Large fiber neuropathy developed in 36%, on average 5.3 years (range 0.2-14.3 years) from SFN onset. Median onset Composite Autonomic Severity Score (CASS) was 3 (change per year 0.08, range 0-2.0). Median Neuropathy Impairment Scale (NIS) score was 2 at onset (range 0-8, change per year 1.0, range -7.9 to +23.3). NIS score and CASS change >1 point per year occurred in only AL-amyloid, hereditary transthyretin-amyloid, Fabry, uncontrolled diabetes, and Lewy body. Death after symptom onset was higher in patients with SFN (19%) vs controls (12%, p < 0.001), 50% secondary to diabetes complications.

DISCUSSION

Isolated SFN is uncommon but increasing in incidence. Most patients do not develop major neurologic impairments and disability but have multiple comorbid conditions, including cardiovascular ischemic events, and increased mortality from SFN onsets. Development of large fiber involvements and diabetes are common over time. Targeted testing facilitates interventional therapies for diabetes but also rheumatologic and rare genetic forms.

Better Islet Function and Cardiovascular Autonomic Function in Chinese Type 2 Diabetic Patients with Pure Small Fiber Neuropathy than with Mixed Neuropathy

INTRODUCTION

The clinical characteristics and outcomes of small fiber neuropathy (SFN) in Chinese patients with type 2 diabetes mellitus (T2DM) have not been thoroughly described. In this study, we investigated the metabolic and neurological indexes and the prognosis of patients with T2DM based on skin biopsy.

METHODS

A total of 34 healthy Chinese volunteers were recruited for skin biopsy to establish the reference range of intra-epidermal nerve fiber density (IENFD), and 89 patients with T2DM attending the Nanjing Drum Tower Hospital were evaluated at baseline. Of these 89 patients, 17 with pure SFN and nine with mixed diabetic polyneuropathy (DPN) were reassessed at the end of the follow-up.

RESULTS

Glycated hemoglobin and postprandial blood glucose levels were lower (P = 0.005 and P = 0.041, respectively) and postprandial C-peptide and insulin levels were higher (P = 0.001 and P = 0.019, respectively) in the pure SFN group than in the mixed DPN group. A partial correlation study showed that there was a negative correlation between IENFD of the distal leg and cardiovascular autonomic reflex test (CART) scores (r = - 0.513, P = 0.001) after adjusting for age and duration of diabetes. Only vitamin B12 level (P = 0.028) and motor nerve conduction velocity (MCV) of the common peroneal nerve (P = 0.045) were increased in the patients with pure SFN at the final visit while MCVs of the common peroneal nerve (P = 0.025) and tibial nerve (P = 0.047) were decreased in the mixed DPN group at the final visit.

CONCLUSION

Better islet function and cardiovascular autonomic function were observed in patients with pure SFN compared with mixed DPN. The metabolic and neurological indexes remained relatively stable in the patients with pure SFN during the follow-up.

Non-length-dependent small fiber neuropathy: Not a matter of stockings and gloves

The clinical spectrum of small fiber neuropathy (SFN) encompasses manifestations related to the involvement of thinly myelinated A-delta and unmyelinated C fibers, including not only the classical distal phenotype, but also a non-length-dependent (NLD) presentation that can be patchy, asymmetrical, upper limb-predominant, or diffuse. This narrative review is focused on NLD-SFN. The diagnosis of NLD-SFN can be problematic, due to its varied and often atypical presentation, and diagnostic criteria developed for distal SFN are not suitable for NLD-SFN. The topographic pattern of NLD-SFN is likely related to ganglionopathy restricted to the small neurons of dorsal root ganglia. It is often associated with systemic diseases, but about half the time is idiopathic. In comparison with distal SFN, immune-mediated diseases are more common than dysmetabolic conditions. Treatment is usually based on the management of neuropathic pain. Disease-modifying therapy, including immunotherapy, may be effective in patients with identified causes. Future research on NLD-SFN is expected to further clarify the interconnected aspects of phenotypic characterization, diagnostic criteria, and pathophysiology.

Small fiber neuropathy: Swiss cohort characterization

Introduction/Aim

There is currently insufficient clinical and epidemiological data concerning small fiber neuropathy (SFN). This research analyzes data from medical records to determine epidemiology, demographics, clinical characteristics and etiology of SFN.

Methods

This is a retrospective, observational study of sequential patients diagnosed with definite SFN (typical clinical features, normal nerve conduction studies, abnormal epidermal nerve fiber density) from the end of November 2016 to the middle of July 2019 at the Cantonal Hospital Lucerne, central Switzerland.

Results

A total of 84 patients (64.3% female) with a mean age of 54.7 y were analyzed. Symptoms had been present in patients for an average of 4.8 y when entering the study. A length dependent clinical pattern was seen in 79.8%. All patients had sensory discomfort. Etiology could not be determined in 35.7% of patients, who were diagnosed with idiopathic SFN; 34.5% of patients had an apparently autoimmune SFN, followed by14.3% of patients with metabolic causes. The estimated incidence was at least 4.4 cases/100.000 inhabitants/y. The minimum prevalence was 131.5 cases/100.000 inhabitants.

Discussion

This study indicates significant incidence and prevalence rates of SFN in Switzerland. SFN can vary greatly in its symptoms and severity. Extensive work‐up resulted in two thirds of the patients being assigned an etiological association. The largest group of patients could not be etiologically defined, underlining the importance of further research on etiologic identification. We expect increased awareness of the developing field of SFN.

The challenge of differentiating fibromyalgia from small-fiber neuropathy in clinical practice

Fibromyalgia and small fibre neuropathy are two diseases leading to chronic widespread pain, and it is difficult to differentiate them in order to provide appropriate care. In this review, we will describe the pathophysiological and clinical differences between fibromyalgia and small fibre neuropathy. In fibromyalgia, pain is increased by dysregulation of central pain processing while small fibre neuropathy pain is related to loss or dysfunction of intraepidermal small nerve fibres. Higher pain intensity; stabbing pain and paraesthesia; allodynia; dry eyes/mouth; changed pattern or sweating on body; skin colour alterations/modifications; reduced hair/nail growth on lower extremities; warm or cold hypoesthesia could be more common in small fibre neuropathy whereas headache or temporo-mandibular disorder point toward fibromyalgia. Length-dependent distribution of pain is common in small fibre neuropathy but can also affect the whole body. Anxiety or depression are common in these two diseases, but post-traumatic stress disorder and physical or sexual abuse in childhood or adulthood suggest fibromyalgia. Inflammatory disease or musculoskeletal disease is frequently reported with fibromyalgia whereas metabolic disorders (especially diabetes mellitus), neurotoxic exposure, Sjogren's syndrome, sarcoidosis, HIV are the main diseases associated with small fibre neuropathy. Skin biopsy, quantitative sensory testing, laser evoked potentials, confocal corneal microscopy or electrochemical skin conductance can help to discriminate between fibromyalgia and small fibre neuropathy.

Small Fiber Neuropathy in the Elderly

Small fiber neuropathy is common and prevalent in the elderly. The disease can be associated with many medical conditions. It often has a negative impact on quality of life due to painful paresthesia, dizziness, and sedative side effects of pain medications. Skin biopsy is the gold standard diagnostic test. Screening for associated conditions is important, because etiology-specific treatment can slow down disease progression and ameliorate symptoms. Adequate pain control can be challenging due to safety and tolerability of pain medications in the elderly. Treatment should be individualized with the goals of controlling underlying causes, alleviating pain, and optimizing daily function.

The complex association between pain and neuropathy

BACKGROUND

Several studies of patients with polyneuropathy failed to show differences between patients with and without pain. In the current study, we aimed to explore the association between neuropathic symptoms, mainly pain, and polyneuropathy characteristics.

METHODS

A prospective cross-sectional study recruiting 151 patients with non-diabetic polyneuropathy was performed between November 2016 and May 2017. A total of 38 patients with chronic inflammatory demyelinating neuropathy were excluded. Patients underwent clinical, electrophysiological and functional assessments of their polyneuropathy. Polyneuropathy characteristics were compared depending on the presence and severity of neuropathic symptoms.

RESULTS

The presence and the severity of weakness were associated with a more severe neuropathy as measured by clinical, electrophysiological and functional assessments, while the presence of pain was associated with higher sural amplitudes, and the severity of pain showed a curvilinear association with neuropathy severity.

CONCLUSIONS

Our study shows a novel finding of a curvilinear association between pain and polyneuropathy severity.

Current View of Diagnosing Small Fiber Neuropathy

Small fiber neuropathy (SFN) is a disorder of the small myelinated Aδ-fibers and unmyelinated C-fibers [5, 6]. SFN might affect small sensory fibers, autonomic fibers or both, resulting in sensory changes, autonomic dysfunction or combined symptoms [7]. As a consequence, the symptoms are potentially numerous and have a large impact on quality of life [8]. Since diagnostic methods for SFN are numerous and its pathophysiology complex, this extensive review focusses on categorizing all aspects of SFN as disease and its diagnosis. In this review, sensitivity in combination with specificity of different diagnostic methods are described using the areas under the curve. In the end, a diagnostic work-flow is suggested based on different phenotypes of SFN.

A Systematic Review of Pharmacologic and Rehabilitative Treatment of Small Fiber Neuropathies

The aim of this systematic review is to guide the physician in defining the pharmacologic and rehabilitative therapeutic approaches for adopting the best strategies described in the current literature. The search was conducted in PubMed, EMBASE, Cochrane Library and Web of Science to identify the treatment of small fiber neuropathies. Two reviewers independently reviewed and came to a consensus on which articles met inclusion/exclusion criteria. The authors excluded the duplicates, animal studies and included the English articles in which the treatment of patients with small fiber neuropathies was described. The search identified a total of 975 articles with the keywords “small fiber neuropathy” AND “rehabilitation” OR “therapy” OR “treatment”. Seventy-eight selected full-text were analyzed by the reviewers. Forty-two publications met the inclusion criteria and were included in the systematic review to describe the rehabilitative and pharmacologic treatment of small fiber neuropathies. Despite the range of different protocols of treatment for small fiber neuropathy, other robust trials are needed. In addition, always different therapeutic approaches are used; a unique protocol could be important for the clinicians. More research is needed to build evidence for the best strategy and to delineate a definitive therapeutic protocol.

Small Fiber Neuropathy

Small fiber neuropathy has a broad array of presentations. Length-dependent symptoms and findings present little diagnostic difficulty, but non-length-dependent or multifocal symptoms can be challenging. Intraepidermal nerve fiber density (IENFD) testing in apparent fibromyalgia warrants further study, but skin biopsy testing of this patient population is reasonable. Avoidance of IENFD testing in situations where diagnosis of neuropathy is already clear or where neuropathy is not the cause of symptoms helps to prevent incorrect conclusions. Careful history and physical examination plus pretest probability are important factors to consider when assessing the results of an IENFD test report.

A Systematic Review of the Diagnostic Methods of Small Fiber Neuropathies in Rehabilitation

This systematic review describes the several methods to diagnose and measure the severity of small fiber neuropathies and aims to guide the physician to define all the diagnostic approaches for adopting the best strategies described in the current literature. The search was conducted in PubMed, EMBASE, Cochrane Library and Web of Science. Two reviewers independently reviewed and came to consensus on which articles met inclusion/exclusion criteria. The authors excluded all the duplicates, animals' studies, and included the English articles in which the diagnostic measures were finalized to assess the effectiveness of rehabilitation and pharmacologic treatment of patients with small fiber neuropathies. The search identified a total of 975 articles with the keywords "small fiber neuropathy" AND "rehabilitation" OR "therapy" OR "treatment". Seventy-eight selected full-text were analyzed by the reviewers. Forty-one publications met the inclusion criteria and were included in the systematic review. Despite the range of diagnostic tools for the assessment of small fiber neuropathy, other robust trials are needed. In addition, always different diagnostic approaches are used, a unique protocol could be important for the clinicians. More research is needed to build evidence for the best diagnostic methodologies and to delineate a definitive diagnostic protocol.