Initial validation of the Mass. General Neuropathy Exam Tool (MAGNET) for evaluation of distal small-fiber neuropathy.

INTRODUCTION/AIMS: Diagnosis of small-fiber neuropathy (SFN) is hampered by its subjective symptoms and signs. Confirmatory testing is insufficiently available and expensive, so predictive examinations have value. However, few support the 2020 SFN consensus-case-definition requirements or were validated for non-diabetes neuropathies. Thus we developed the Massachusetts General Hospital Neuropathy Exam Tool (MAGNET) and measured diagnostic performance in 160 symptomatic patients evaluated for length-dependent SFN from any cause and 37 healthy volunteers. METHODS: We compared prevalences of abnormalities (vital signs, pupil responses, lower-limb appearance, pin, light touch, vibration and position sensitivity, great-toe strength, muscle stretch reflexes), and validated diagnostic performance against objective SFN tests: lower-leg skin-biopsy epidermal neurite densities and autonomic function testing (AFT). Sensitivity/specificity, feasibility, test-retest and inter-rater reliability, and convergence with the Utah Early Neuropathy Scale were calculated. RESULTS: Patients' ages averaged 48.5 ± 14.7 years and 70.6% were female. Causes of neuropathy varied, remaining unknown in 59.5%. Among the 46 with abnormal skin biopsies, the most prevalent abnormality was reduced pin sharpness at the toes (71.7%). Inter-rater reliability, test-retest reliability, and convergent validity excelled (range = 91.3-95.6%). Receiver operating characteristics comparing all symptomatic patients versus healthy controls indicated that a MAGNET threshold score of 14 maximized predictive accuracy for skin biopsies (0.74) and a 30 cut-off maximized accuracy for predicting AFT (0.60). Analyzing patients with any abnormal neuropathy-test results identified areas-under-the-curves of 0.87-0.89 for predicting a diagnostic result, accuracy = 0.80-0.89, and Youden's index = 0.62. Overall, MAGNET was 80%-85% accurate for stratifying patients with abnormal versus normal neuropathy test results. DISCUSSION: MAGNET quickly generates research-quality metrics during clinical examinations.

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.

Characterization of mononeuropathy of the lateral cutaneous nerve of the calf.

INTRODUCTION/AIMS: Isolated injuries to the lateral cutaneous nerve of the calf (LCNC) branch of the common peroneal nerve can cause obscure chronic posterolateral knee and upper calf pain and sensory symptoms. Routine examination and electrodiagnostic testing do not detect them because the LCNC has no motor distribution and it is not interrogated by the typical peroneal nerve conduction study. There are only about 10 prior cases, thus scant physician awareness, so most LCNC injuries remain misdiagnosed or undiagnosed, hindering care. METHODS: We extracted pertinent records from seven patients with unexplained posterolateral knee/calf pain, six labeled as complex regional pain syndrome, to investigate for mononeuropathies. Patients were asked to outline their skin area with abnormal responses to pin self-examination independently. Three underwent an LCNC-specific electrodiagnostic study, and two had skin-biopsy epidermal innervation measured. Cadaver dissection of the posterior knee nerves helped identify potential entrapment sites. RESULTS: Initiating events included knee surgery (three), bracing (one), extensive kneeling (one), and other knee trauma. All pin-outlines included the published LCNC neurotome. One oftwo LCNC-specific electrodiagnostic studies revealed unilaterally absent potentials. Longitudinal, controlled skin biopsies documented profound LCNC-neurotome denervation then re-innervation contemporaneous with symptom recovery. Cadaver dissection identified the LCNC traversing through the dense fascia of the proximolateral gastrocnemius muscle insertion. DISCUSSION: Isolated LCNC mononeuropathy can cause unexplained posterolateral knee/calf pain syndromes. This series characterizes presentations and supports patient pin-mappings as a sensitive, globally available, low-cost diagnostic aid. Improved recognition may facilitate more rapid, accurate diagnosis and, thus, optimize management and improve outcomes.

Sensory and autonomic deficits in a new humanized mouse model of familial dysautonomia.

Familial dysautonomia (FD) is an autosomal recessive neurodegenerative disease that affects the development and survival of sensory and autonomic neurons. FD is caused by an mRNA splicing mutation in intron 20 of the IKBKAP gene that results in a tissue-specific skipping of exon 20 and a corresponding reduction of the inhibitor of kappaB kinase complex-associated protein (IKAP), also known as Elongator complex protein 1. To date, several promising therapeutic candidates for FD have been identified that target the underlying mRNA splicing defect, and increase functional IKAP protein. Despite these remarkable advances in drug discovery for FD, we lacked a phenotypic mouse model in which we could manipulate IKBKAP mRNA splicing to evaluate potential efficacy. We have, therefore, engineered a new mouse model that, for the first time, will permit to evaluate the phenotypic effects of splicing modulators and provide a crucial platform for preclinical testing of new therapies. This new mouse model, TgFD9; Ikbkap(Δ20/flox) was created by introducing the complete human IKBKAP transgene with the major FD splice mutation (TgFD9) into a mouse that expresses extremely low levels of endogenous Ikbkap (Ikbkap(Δ20/flox)). The TgFD9; Ikbkap(Δ20/flox) mouse recapitulates many phenotypic features of the human disease, including reduced growth rate, reduced number of fungiform papillae, spinal abnormalities, and sensory and sympathetic impairments, and recreates the same tissue-specific mis-splicing defect seen in FD patients. This is the first mouse model that can be used to evaluate in vivo the therapeutic effect of increasing IKAP levels by correcting the underlying FD splicing defect.

Treatments for chronic inflammatory demyelinating polyradiculoneuropathy (CIDP): an overview of systematic reviews.

BACKGROUND: Chronic inflammatory demyelinating polyradiculoneuropathy (CIDP) is a chronic progressive or relapsing and remitting disease that usually causes weakness and sensory loss. The symptoms are due to autoimmune inflammation of peripheral nerves. CIPD affects about 2 to 3 per 100,000 of the population. More than half of affected people cannot walk unaided when symptoms are at their worst. CIDP usually responds to treatments that reduce inflammation, but there is disagreement about which treatment is most effective. OBJECTIVES: To summarise the evidence from Cochrane systematic reviews (CSRs) and non-Cochrane systematic reviews of any treatment for CIDP and to compare the effects of treatments. METHODS: We considered all systematic reviews of randomised controlled trials (RCTs) of any treatment for any form of CIDP. We reported their primary outcomes, giving priority to change in disability after 12 months.Two overview authors independently identified published systematic reviews for inclusion and collected data. We reported the quality of evidence using GRADE criteria. Two other review authors independently checked review selection, data extraction and quality assessments.On 31 October 2016, we searched the Cochrane Database of Systematic Reviews, the Database of Abstracts of Reviews of Effects (in theCochrane Library), MEDLINE, Embase, and CINAHL Plus for systematic reviews of CIDP. We supplemented the RCTs in the existing CSRs by searching on the same date for RCTs of any treatment of CIDP (including treatment of fatigue or pain in CIDP), in the Cochrane Neuromuscular Specialised Register, CENTRAL, MEDLINE, Embase, and CINAHL Plus. MAIN RESULTS: Five CSRs met our inclusion criteria. We identified 23 randomised trials, of which 15 had been included in these CSRs. We were unable to compare treatments as originally planned, because outcomes and outcome intervals differed. CorticosteroidsIt is uncertain whether daily oral prednisone improved impairment compared to no treatment because the quality of the evidence was very low (1 trial, 28 participants). According to moderate-quality evidence (1 trial, 41 participants), six months' treatment with high-dose monthly oral dexamethasone did not improve disability more than daily oral prednisolone. Observational studies tell us that prolonged use of corticosteroids sometimes causes serious side-effects. Plasma exchangeAccording to moderate-quality evidence (2 trials, 59 participants), twice-weekly plasma exchange produced more short-term improvement in disability than sham exchange. In the largest observational study, 3.9% of plasma exchange procedures had complications. Intravenous immunoglobulinAccording to high-quality evidence (5 trials, 269 participants), intravenous immunoglobulin (IVIg) produced more short-term improvement than placebo. Adverse events were more common with IVIg than placebo (high-quality evidence), but serious adverse events were not (moderate-quality evidence, 3 trials, 315 participants). One trial with 19 participants provided moderate-quality evidence of little or no difference in short-term improvement of impairment with plasma exchange in comparison to IVIg. There was little or no difference in short-term improvement of disability with IVIg in comparison to oral prednisolone (moderate-quality evidence; 1 trial, 29 participants) or intravenous methylprednisolone (high-quality evidence; 1 trial, 45 participants). One unpublished randomised open trial with 35 participants found little or no difference in disability after three months of IVIg compared to oral prednisone; this trial has not yet been included in a CSR. We know from observational studies that serious adverse events related to IVIg do occur. Other immunomodulatory treatmentsIt is uncertain whether the addition of azathioprine (2 mg/kg) to prednisone improved impairment in comparison to prednisone alone, as the quality of the evidence is very low (1 trial, 27 participants). Observational studies show that adverse effects truncate treatment in 10% of people.According to low-quality evidence (1 trial, 60 participants), compared to placebo, methotrexate 15 mg/kg did not allow more participants to reduce corticosteroid or IVIg doses by 20%. Serious adverse events were no more common with methotrexate than with placebo, but observational studies show that methotrexate can cause teratogenicity, abnormal liver function, and pulmonary fibrosis.According to moderate-quality evidence (2 trials, 77 participants), interferon beta-1a (IFN beta-1a) in comparison to placebo, did not allow more people to withdraw from IVIg. According to moderate-quality evidence, serious adverse events were no more common with IFN beta-1a than with placebo.We know of no other completed trials of immunosuppressant or immunomodulatory agents for CIDP. Other treatmentsWe identified no trials of treatments for fatigue or pain in CIDP. Adverse effectsNot all trials routinely collected adverse event data; when they did, the quality of evidence was variable. Adverse effects in the short, medium, and long term occur with all interventions. We are not able to make reliable comparisons of adverse events between the interventions included in CSRs. AUTHORS' CONCLUSIONS: We cannot be certain based on available evidence whether daily oral prednisone improves impairment compared to no treatment. However, corticosteroids are commonly used, based on widespread availability, low cost, very low-quality evidence from observational studies, and clinical experience. The weakness of the evidence does not necessarily mean that corticosteroids are ineffective. High-dose monthly oral dexamethasone for six months is probably no more or less effective than daily oral prednisolone. Plasma exchange produces short-term improvement in impairment as determined by neurological examination, and probably produces short-term improvement in disability. IVIg produces more short-term improvement in disability than placebo and more adverse events, although serious side effects are probably no more common than with placebo. There is no clear difference in short-term improvement in impairment with IVIg when compared with intravenous methylprednisolone and probably no improvement when compared with either oral prednisolone or plasma exchange. According to observational studies, adverse events related to difficult venous access, use of citrate, and haemodynamic changes occur in 3% to17% of plasma exchange procedures.It is uncertain whether azathioprine is of benefit as the quality of evidence is very low. Methotrexate may not be of benefit and IFN beta-1a is probably not of benefit.We need further research to identify predictors of response to different treatments and to compare their long-term benefits, safety and cost-effectiveness. There is a need for more randomised trials of immunosuppressive and immunomodulatory agents, routes of administration, and treatments for symptoms of CIDP.

Natural history and biomarkers in hereditary sensory neuropathy type 1.

INTRODUCTION: Hereditary sensory and autonomic neuropathy type 1 (HSAN1) is most commonly caused by missense mutations in SPTLC1. In this study we mapped symptom progression and compared the utility of outcomes. METHODS: We administered retrospective surveys of symptoms and analyzed results of nerve conduction, autonomic function testing (AFT), and PGP9.5-immunolabeled skin biopsies. RESULTS: The first symptoms were universally sensory and occurred at a median age of 20 years (range 14-54 years). The onset of weakness, ulcers, pain, and balance problems followed sequentially. Skin biopsies revealed universally absent epidermal innervation at the distal leg with relative preservation in the thigh. Neurite density was highly correlated with total Charcot-Marie-Tooth Examination Score (CMTES; r2 = -0.8) and median motor amplitude (r2 = -0.75). CONCLUSIONS: These results confirm sensory loss as the initial symptom of HSAN1 and suggest that skin biopsy may be the most promising biomarker for future clinical trials.

Relapsing-Remitting Immunotherapy Responsive Small-Fiber Neuropathy: Longitudinal Tracking Through 10 Years Including Pregnancies.

OBJECTIVES: To expand understanding of the pathogenesis, presentations, and treatment of initially idiopathic small fiber polyneuropathy (SFN). METHODS: We longitudinally readministered validated metrics to track disease course and treatment responses in a previously healthy woman with acute, postinfectious, skin biopsy-confirmed, idiopathic SFN. RESULTS: During 5 years, viral respiratory infections triggered 3 separated episodes of acute, disabling burning hand, foot, and face pain (erythromelalgia). The initial 2 resolved with high-dose prednisone, and the third responded to repeated immunoglobulin treatments. Pregnancy with miscarriage triggered a fourth exacerbation refractory to corticosteroids and cyclosporin. Immunoglobulins restored total remission for 2 months; then, 2 rituximab doses slightly improved later flaring. Subsequently, daratumumab initiated 100-day remission later maintained by belimumab, initiated to permit another pregnancy. Remission continued after gestational week 13 all-treatment withdrawal. A week 30 fifth flare responded to plasmapheresis, with healthy birth at week 40. At 11-week postpartum, as symptoms returned, restarting belimumab restored remission maintained during ≥19 months of breastfeeding. DISCUSSION: This decade of tracking characterizes a relapsing-remitting course of SFN with initially separated monophasic episodes becoming more confluent, as with multiple sclerosis. This tempo and responsiveness to 5 immunotherapies suggest dysimmune causality. Validated metrics helped define the course and track treatment efficacy, particularly during pregnancy and breastfeeding. CLASSIFICATION OF EVIDENCE: This is a single observational study without controls. This provides Class IV evidence.

Sacral Tarlov perineurial cysts: a systematic review of treatment options.

Tarlov perineurial spinal cysts (TCs) are an underrecognized cause of spinal neuropathic symptoms. TCs form within the sensory nerve root sleeves, where CSF extends distally and can accumulate pathologically. Typically, they develop at the sacral dermatomes where the nerve roots are under the highest hydrostatic pressure and lack enclosing vertebral foramina. In total, 90% of patients are women, and genetic disorders that weaken connective tissues, e.g., Ehlers-Danlos syndrome, convey considerable risk. Most small TCs are asymptomatic and do not require treatment, but even incidental visualizations should be documented in case symptoms develop later. Symptomatic TCs most commonly cause sacropelvic dermatomal neuropathic pain, as well as bladder, bowel, and sexual dysfunction. Large cysts routinely cause muscle atrophy and weakness by compressing the ventral motor roots, and multiple cysts or multiroot compression by one large cyst can cause even greater cauda equina syndromes. Rarely, giant cysts erode the sacrum or extend as intrapelvic masses. Disabling TCs require consideration for surgical intervention. The authors' systematic review of treatment analyzed 31 case series of interventional percutaneous procedures and open surgical procedures. The surgical series were smaller and reported somewhat better outcomes with longer term follow-up but slightly higher risks. When data were lacking, authorial expertise and case reports informed details of the specific interventional and surgical techniques, as well as medical, physical, and psychological management. Cyst-wrapping surgery appeared to offer the best long-term outcomes by permanently reducing cyst size and reconstructing the nerve root sleeves. This curtails ongoing injury to the axons and neuronal death, and may also promote axonal regeneration to improve somatic and autonomic sacral nerve function.

Corneal innervation as a window to peripheral neuropathies.

The cornea receives the densest sensory innervation of the body, which is exclusively from small-fiber nociceptive (pain-sensing) neurons. These are similar to those in the skin of the legs, the standard location for neurodiagnostic skin biopsies used to diagnose small-fiber peripheral polyneuropathies. Many cancer chemotherapy agents cause dose-related, therapy-limiting, sensory-predominant polyneuropathy. Because corneal innervation can be detected non-invasively, it is a potential surrogate biomarker for skin biopsy measurements. Therefore, we compared hindpaw-skin and cornea innervation in mice treated with neurotoxic chemotherapy. Paclitaxel (0, 5, 10, or 20 mg/kg) was administered to C57/Bl6 mice and peri-mortem cornea and skin biopsies were immunolabeled to reveal and permit quantitation of innervation. Both tissues demonstrated dose-dependent, highly correlated (r = 0.66) nerve fiber damage. These findings suggest that the quantification of corneal nerves may provide a useful surrogate marker for skin peripheral innervation.

Common neuropathic itch syndromes.

Patients with chronic itch are diagnosed and treated by dermatologists. However, itch is a neural sensation and some forms of chronic itch are the presenting symptoms of neurological diseases. Dermatologists need some familiarity with the most common neuropathic itch syndromes to initiate diagnostic testing and to know when to refer to a neurologist. This review summarizes current knowledge, admittedly incomplete, on neuropathic itch caused by diseases of the brain, spinal cord, cranial or spinal nerve-roots, and peripheral nerves.

Peripheral Neuropathy Evaluations of Patients With Prolonged Long COVID.

BACKGROUND AND OBJECTIVES: Recovery from severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) infection appears exponential, leaving a tail of patients reporting various long COVID symptoms including unexplained fatigue/exertional intolerance and dysautonomic and sensory concerns. Indirect evidence links long COVID to incident polyneuropathy affecting the small-fiber (sensory/autonomic) axons. METHODS: We analyzed cross-sectional and longitudinal data from patients with World Health Organization (WHO)-defined long COVID without prior neuropathy history or risks who were referred for peripheral neuropathy evaluations. We captured standardized symptoms, examinations, objective neurodiagnostic test results, and outcomes, tracking participants for 1.4 years on average. RESULTS: Among 17 patients (mean age 43.3 years, 69% female, 94% Caucasian, and 19% Latino), 59% had ≥1 test interpretation confirming neuropathy. These included 63% (10/16) of skin biopsies, 17% (2/12) of electrodiagnostic tests and 50% (4/8) of autonomic function tests. One patient was diagnosed with critical illness axonal neuropathy and another with multifocal demyelinating neuropathy 3 weeks after mild COVID, and ≥10 received small-fiber neuropathy diagnoses. Longitudinal improvement averaged 52%, although none reported complete resolution. For treatment, 65% (11/17) received immunotherapies (corticosteroids and/or IV immunoglobulins). DISCUSSION: Among evaluated patients with long COVID, prolonged, often disabling, small-fiber neuropathy after mild SARS-CoV-2 was most common, beginning within 1 month of COVID-19 onset. Various evidence suggested infection-triggered immune dysregulation as a common mechanism.

Neuropathic symptoms with SARS-CoV-2 vaccination.

Background and Objectives: Various peripheral neuropathies, particularly those with sensory and autonomic dysfunction may occur during or shortly after acute COVID-19 illnesses. These appear most likely to reflect immune dysregulation. If similar manifestations can occur with the vaccination remains unknown. Results: In an observational study, we studied 23 patients (92% female; median age 40years) reporting new neuropathic symptoms beginning within 1 month after SARS-CoV-2 vaccination. 100% reported sensory symptoms comprising severe face and/or limb paresthesias, and 61% had orthostasis, heat intolerance and palpitations. Autonomic testing in 12 identified seven with reduced distal sweat production and six with positional orthostatic tachycardia syndrome. Among 16 with lower-leg skin biopsies, 31% had diagnostic/subthreshold epidermal neurite densities (≤5%), 13% were borderline (5.01-10%) and 19% showed abnormal axonal swelling. Biopsies from randomly selected five patients that were evaluated for immune complexes showed deposition of complement C4d in endothelial cells. Electrodiagnostic test results were normal in 94% (16/17). Together, 52% (12/23) of patients had objective evidence of small-fiber peripheral neuropathy. 58% patients (7/12) treated with oral corticosteroids had complete or near-complete improvement after two weeks as compared to 9% (1/11) of patients who did not receive immunotherapy having full recovery at 12 weeks. At 5-9 months post-symptom onset, 3 non-recovering patients received intravenous immunoglobulin with symptom resolution within two weeks. Conclusions: This observational study suggests that a variety of neuropathic symptoms may manifest after SARS-CoV-2 vaccinations and in some patients might be an immune-mediated process.

Suprascapular neuropathy in a shoulder referral practice.

HYPOTHESIS: Suprascapular neuropathy (SSN) is considered a rare condition, and few studies have analyzed how commonly it is encountered in practice. Electrophysiologic studies are the gold standard for diagnosis; however, there is no consensus on diagnostic criteria. We hypothesized that SSN would be frequently diagnosed by electrophysiologic testing in a subset of patients with specific clinical and radiographic findings suggestive of the pathology. This study characterizes SSN in an academic shoulder referral practice and documents the electrodiagnostic findings that are currently being used to diagnose the condition. MATERIALS AND METHODS: A retrospective review of a 1-year period was used to identify all patients who completed electrodiagnostic studies to evaluate the suprascapular nerve. Clinical exam findings and associated shoulder pathology was documented. The specific electromyography (EMG) and nerve conduction studies (NCS) findings were analyzed. RESULTS: Electrodiagnostic results were available for 92 patients, and 40 (42%) had confirmed SSN. Patients with a massive rotator cuff tear were more likely to have an abnormal study than those without a tear (P = .006). The most common electrodiagnostic abnormalities were abnormal motor unit action potentials (88%), whereas only 33% had evidence of denervation. The average latency in studies reported as diagnostic of SSN was 2.90 ± 0.08 milliseconds for the supraspinatus and 3.78 ± 0.14 milliseconds for the infraspinatus. DISCUSSION: An electrodiagnostically confirmed diagnosis of SSN was seen in 4.3% of all new patients and in 43% of patients with clinical or radiographic suspicion of SSN. Clinical evaluation may be difficult because other shoulder pathology can have overlapping symptoms. CONCLUSION: Shoulder surgeons should consider electrophysiologic evaluation of patients with clinical or radiographic signs of SSN and be cognizant of the parameters that constitute an abnormal study.

Comparing partial and total tibial-nerve axotomy: long-term effects on prevalence and location of evoked pain behaviors.

Monophasic (one-time) nerve injuries heal without clinically significant residua in most cases, but rare individuals are left with neuropathic pain, even after seemingly minor lesions. The effects of lesion size on risk for chronic pain persistence are not well understood, particularly as concerns the complex regional pain syndrome, which is defined in part by pain "disproportionate" to the severity of the causative lesion, and extending outside the autonomous territory of a single nerve. To better clarify the expected prevalence of pain behaviors after nerve injury, we compared the effects in rats of different-sized axotomies on the prevalence and location of evoked pain behaviors. To highlight clinical relevance, we also describe a patient with iatrogenic tibial-nerve injury causing similar chronic neuralgia. Adult male Sprague-Dawley rats were anesthetized and had either 1/3, 2/3 or their entire left tibial nerves tightly ligated at two sites just below the sciatic trifurcation and the interposed nerve was cut. Unoperated rats provided controls. Sensory function in the tibial and sural-innervated territories of both plantar hindpaws was assessed for as long as 6 months postoperatively. Soon after surgery, evoked pain behavior developed in the ipsilesional sural-innervated site in a subset of axotomized rats and recovery was variable. The relationship between lesion size and prevalence and severity of hyperalgesia varied for different pain behaviors, with pinprick hyperalgesia clearly more likely after larger axotomies. In summary, partial tibial-nerve injury in rats models human disease and suggests that expectations of proportionality between lesion size and development of neuropathic pain may need revision.

Epidermal Neurite Density in Skin Biopsies From Patients With Juvenile Fibromyalgia.

OBJECTIVE: Fibromyalgia (FM) is defined by idiopathic, chronic, widespread musculoskeletal pain. In adults with FM, a metaanalysis of lower-leg skin biopsy demonstrated 45% pooled prevalence of abnormally low epidermal neurite density (END). END < 5th centile of the normal distribution is the consensus diagnostic threshold for small-fiber neuropathy. However, the clinical significance of END findings in FM is unknown. Here, we examine the prevalence of small-fiber pathology in juvenile FM, which has not been studied previously. METHODS: We screened 21 patients aged 13-20 years with FM diagnosed by pediatric rheumatologists. Fifteen meeting the American College of Rheumatology criteria (modified for juvenile FM) underwent lower-leg measurements of END and completed validated questionnaires assessing pain, functional disability, and dysautonomia symptoms. The primary outcome was proportion of FM patients with END < 5th centile of age/sex/race-based laboratory norms. Cases were systematically matched by ethnicity, race, sex, and age to a group of previously biopsied healthy adolescents with selection blinded to biopsy results. All 23 controls matching demographic criteria were included. RESULTS: Among biopsied juvenile FM patients, 53% (8/15) had END < 5th centile vs 4% (1/23) of healthy controls (P < 0.001). Mean patient END was 273/mm2 skin surface (95% CI 198-389) vs 413/mm2 (95% CI 359-467, P < 0.001). As expected, patients with FM reported more functional disability, dysautonomia, and pain than healthy controls. CONCLUSION: Abnormal END reduction is common in adolescents with FM, with similar prevalence in adults with FM. More studies are needed to fully characterize the significance of low END in FM and to elucidate the clinical implications of these findings.