Axonal swellings are related to type 2 diabetes, but not to distal diabetic sensorimotor polyneuropathy.

AIMS/HYPOTHESIS: Distal diabetic sensorimotor polyneuropathy (DSP) is a common complication of diabetes with many patients showing a reduction of intraepidermal nerve fibre density (IENFD) from skin biopsy, a validated and sensitive diagnostic tool for the assessment of DSP. Axonal swelling ratio is a morphological quantification altered in DSP. It is, however, unclear if axonal swellings are related to diabetes or DSP. The aim of this study was to investigate how axonal swellings in cutaneous nerve fibres are related to type 2 diabetes mellitus, DSP and neuropathic pain in a well-defined cohort of patients diagnosed with type 2 diabetes. METHODS: A total of 249 participants, from the Pain in Neuropathy Study (UK) and the International Diabetic Neuropathy Consortium (Denmark), underwent a structured neurological examination, nerve conduction studies, quantitative sensory testing and skin biopsy. The study included four groups: healthy control study participants without diabetes (n = 45); participants with type 2 diabetes without DSP (DSP-; n = 31); and participants with evidence of DSP (DSP+; n = 173); the last were further separated into painless DSP+ (n = 74) and painful DSP+ (n = 99). Axonal swellings were defined as enlargements on epidermal-penetrating fibres exceeding 1.5 µm in diameter. Axonal swelling ratio is calculated by dividing the number of axonal swellings by the number of intraepidermal nerve fibres. RESULTS: Median (IQR) IENFD (fibres/mm) was: 6.7 (5.2-9.2) for healthy control participants; 6.2 (4.4-7.3) for DSP-; 1.3 (0.5-2.2) for painless DSP+; and 0.84 (0.4-1.6) for painful DSP+. Swelling ratios were calculated for all participants and those with IENFD > 1.0 fibre/mm. When only those participants with IENFD > 1.0 fibre/mm were included, the axonal swelling ratio was higher in participants with type 2 diabetes when compared with healthy control participants (p < 0.001); however, there was no difference between DSP- and painless DSP+ participants, or between painless DSP+ and painful DSP+ participants. The axonal swelling ratio correlated weakly with HbA1c (r = 0.16, p = 0.04), but did not correlate with the Toronto Clinical Scoring System (surrogate measure of DSP severity), BMI or type 2 diabetes duration. CONCLUSIONS/INTERPRETATION: In individuals with type 2 diabetes where IENFD is >1.0 fibre/mm, axonal swelling ratio is related to type 2 diabetes but is not related to DSP or painful DSP. Axonal swellings may be an early marker of sensory nerve injury in type 2 diabetes.

Painful and Painless Diabetic Neuropathies: What Is the Difference?

PURPOSE OF REVIEW: The prevalence of diabetes mellitus and its chronic complications are increasing to epidemic proportions. This will unfortunately result in massive increases in diabetic distal symmetrical polyneuropathy (DPN) and its troublesome sequelae, including disabling neuropathic pain (painful-DPN), which affects around 25% of patients with diabetes. Why these patients develop neuropathic pain, while others with a similar degree of neuropathy do not, is not clearly understood. This review will look at recent advances that may shed some light on the differences between painful and painless-DPN. RECENT FINDINGS: Gender, clinical pain phenotyping, serum biomarkers, brain imaging, genetics, and skin biopsy findings have been reported to differentiate painful- from painless-DPN. Painful-DPN seems to be associated with female gender and small fiber dysfunction. Moreover, recent brain imaging studies have found neuropathic pain signatures within the central nervous system; however, whether this is the cause or effect of the pain is yet to be determined. Further research is urgently required to develop our understanding of the pathogenesis of pain in DPN in order to develop new and effective mechanistic treatments for painful-DPN.

Reduced Thalamic γ-Aminobutyric Acid (GABA) in Painless but Not Painful Diabetic Peripheral Neuropathy.

Alterations in the structure, function, and microcirculation of the thalamus, a key brain region involved in pain pathways, have previously been demonstrated in patients with painless and painful diabetic peripheral neuropathy (DPN). However, thalamic neurotransmitter levels including γ-aminobutyric acid (GABA) (inhibitory neurotransmitter) and glutamate (excitatory neurotransmitter) in different DPN phenotypes are not known. We performed a magnetic resonance spectroscopy study and quantified GABA and glutamate levels within the thalamus, in a carefully characterized cohort of participants with painless and painful DPN. Participants with DPN (painful and painless combined) had a significantly lower GABA:H2O ratio compared with those without DPN (healthy volunteers [HV] and participants with diabetes without DPN [no DPN]). Participants with painless DPN had the lowest GABA:H2O ratio, which reached significance compared with HV and no DPN, but not painful DPN. There was no difference in GABA:H2O in painful DPN compared with all other groups. A significant correlation with GABA:H2O and neuropathy severity was also seen. This study demonstrates that lower levels of thalamic GABA in participants with painless DPN may reflect neuroplasticity due to reduced afferent pain impulses, whereas partially preserved levels of GABA in painful DPN may indicate that central GABAergic pathways are involved in the mechanisms of neuropathic pain in diabetes.

Magnetic resonance imaging of the central nervous system in diabetic neuropathy.

Diabetic 'peripheral' neuropathy (DPN) is one of the common sequelae to the development of both type-1 and type-2 diabetes mellitus. Neuropathy has a major negative impact on quality of life. Abnormalities in both peripheral vasculature and nerve function are well documented and, in addition, evidence is emerging regarding changes within the central nervous system (CNS) that are concomitant with the presence of DPN. The often-resistant nature of DPN to medical treatment highlights the need to understand the role of the CNS in neuropathic symptomatology and progression, as this may modulate therapeutic approaches. Advanced neuroimaging techniques, especially those that can provide quantitative measures of structure and function, can provide objective markers of CNS status. With that comes great potential for not only furthering our understanding of involvement of the CNS in neuropathic etiology but also most importantly aiding the development of new and more effective, targeted, analgesic interventions.

Nerve and Vascular Biomarkers in Skin Biopsies Differentiate Painful From Painless Peripheral Neuropathy in Type 2 Diabetes.

Painful diabetic peripheral neuropathy can be intractable with a major impact, yet the underlying pain mechanisms remain uncertain. A range of neuronal and vascular biomarkers was investigated in painful diabetic peripheral neuropathy (painful-DPN) and painless-DPN and used to differentiate painful-DPN from painless-DPN. Skin biopsies were collected from 61 patients with type 2 diabetes (T2D), and 19 healthy volunteers (HV). All subjects underwent detailed clinical and neurophysiological assessments. Based on the neuropathy composite score of the lower limbs [NIS(LL)] plus seven tests, the T2D subjects were subsequently divided into three groups: painful-DPN (n = 23), painless-DPN (n = 19), and No-DPN (n = 19). All subjects underwent punch skin biopsy, and immunohistochemistry used to quantify total intraepidermal nerve fibers (IENF) with protein gene product 9.5 (PGP9.5), regenerating nerve fibers with growth-associated protein 43 (GAP43), peptidergic nerve fibers with calcitonin gene-related peptide (CGRP), and blood vessels with von Willebrand Factor (vWF). The results showed that IENF density was severely decreased (p < 0.001) in both DPN groups, with no differences for PGP9.5, GAP43, CGRP, or GAP43/PGP9.5 ratios. There was a significant increase in blood vessel (vWF) density in painless-DPN and No-DPN groups compared to the HV group, but this was markedly greater in the painful-DPN group, and significantly higher than in the painless-DPN group (p < 0.0001). The ratio of sub-epidermal nerve fiber (SENF) density of CGRP:vWF showed a significant decrease in painful-DPN vs. painless-DPN (p = 0.014). In patients with T2D with advanced DPN, increased dermal vasculature and its ratio to nociceptors may differentiate painful-DPN from painless-DPN. We hypothesized that hypoxia-induced increase of blood vessels, which secrete algogenic substances including nerve growth factor (NGF), may expose their associated nociceptor fibers to a relative excess of algogens, thus leading to painful-DPN.

Determinants of Treatment Response in Painful Diabetic Peripheral Neuropathy: A Combined Deep Sensory Phenotyping and Multimodal Brain MRI Study.

Painful diabetic peripheral neuropathy (DPN) is difficult to manage, as treatment response is often varied. The primary aim of this study was to examine differences in pain phenotypes between responders and nonresponders to intravenous lidocaine treatment using quantitative sensory testing. The secondary aim was to explore differences in brain structure and functional connectivity with treatment response. Forty-five consecutive patients who received intravenous lidocaine treatment for painful DPN were screened. Twenty-nine patients who met the eligibility criteria (responders, n = 14, and nonresponders, n = 15) and 26 healthy control subjects underwent detailed sensory profiling. Subjects also underwent multimodal brain MRI. A greater proportion of patients with the irritable (IR) nociceptor phenotype were responders to intravenous lidocaine treatment compared with nonresponders. The odds ratio of responding to intravenous lidocaine was 8.67 times greater (95% CI 1.4-53.8) for the IR nociceptor phenotype. Responders to intravenous lidocaine also had significantly greater mean primary somatosensory cortex cortical volume and functional connectivity between the insula cortex and the corticolimbic circuitry. This study provides preliminary evidence for a mechanism-based approach for individualizing therapy in patients with painful DPN.

A new look at painful diabetic neuropathy.

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

Rare NaV1.7 variants associated with painful diabetic peripheral neuropathy.

Diabetic peripheral neuropathy (DPN) is a common disabling complication of diabetes. Almost half of the patients with DPN develop neuropathic pain (NeuP) for which current analgesic treatments are inadequate. Understanding the role of genetic variability in the development of painful DPN is needed for improved understanding of pain pathogenesis for better patient stratification in clinical trials and to target therapy more appropriately. Here, we examined the relationship between variants in the voltage-gated sodium channel NaV1.7 and NeuP in a deeply phenotyped cohort of patients with DPN. Although no rare variants were found in 78 participants with painless DPN, we identified 12 rare NaV1.7 variants in 10 (out of 111) study participants with painful DPN. Five of these variants had previously been described in the context of other NeuP disorders and 7 have not previously been linked to NeuP. Those patients with rare variants reported more severe pain and greater sensitivity to pressure stimuli on quantitative sensory testing. Electrophysiological characterization of 2 of the novel variants (M1852T and T1596I) demonstrated that gain of function changes as a consequence of markedly impaired channel fast inactivation. Using a structural model of NaV1.7, we were also able to provide further insight into the structural mechanisms underlying fast inactivation and the role of the C-terminal domain in this process. Our observations suggest that rare NaV1.7 variants contribute to the development NeuP in patients with DPN. Their identification should aid understanding of sensory phenotype, patient stratification, and help target treatments effectively.

Neuropathic pain drives anxiety behavior in mice, results consistent with anxiety levels in diabetic neuropathy patients.

BACKGROUND: Epidemiological studies in patients with neuropathic pain demonstrate a strong association with psychiatric conditions such as anxiety; however, the precipitating pathology between these symptoms remains unclear. To investigate this, we studied the effects of lifelong stress on levels of neuropathic pain-like behavior and conversely, the effects of chronic neuropathic injury on anxiety-like status in male and female mice. In addition, we assayed this link in painful and painless diabetic peripheral neuropathy patients. METHODS: Male and female mice were subject to ongoing life-stress or control living conditions. Baseline sensitivity and anxiety tests were measured followed by spared nerve injury (SNI) to the sciatic nerve. Subsequent sensory testing occurred until 3 weeks after SNI followed by anxiety tests between 4 and 6 weeks after SNI. RESULTS: Levels of tactile or cold allodynia did not differ between adult mice subject to lifelong chronic stress, relative to nonstressed controls, for at least 3 weeks after SNI. By contrast, longer-term neuropathic mice of both sexes displayed pronounced anxiety-like behavior, regardless of exposure to stress. If sex differences were present, females usually exhibited more pronounced anxiety-like behavior. These ongoing anxiety behaviors were corroborated with plasma corticosterone levels in distinct animal groups. In addition, data from patients with painful and nonpainful diabetic neuropathy showed a clear relationship between ongoing pain and anxiety, with females generally more affected than males. DISCUSSION: Taken together, these data demonstrate a strong link between chronic neuropathic pain and chronic anxiety, with the driver of this comorbidity being neuropathic pain as opposed to on-going stress.

Stratifying patients with peripheral neuropathic pain based on sensory profiles: algorithm and sample size recommendations.

In a recent cluster analysis, it has been shown that patients with peripheral neuropathic pain can be grouped into 3 sensory phenotypes based on quantitative sensory testing profiles, which are mainly characterized by either sensory loss, intact sensory function and mild thermal hyperalgesia and/or allodynia, or loss of thermal detection and mild mechanical hyperalgesia and/or allodynia. Here, we present an algorithm for allocation of individual patients to these subgroups. The algorithm is nondeterministic-ie, a patient can be sorted to more than one phenotype-and can separate patients with neuropathic pain from healthy subjects (sensitivity: 78%, specificity: 94%). We evaluated the frequency of each phenotype in a population of patients with painful diabetic polyneuropathy (n = 151), painful peripheral nerve injury (n = 335), and postherpetic neuralgia (n = 97) and propose sample sizes of study populations that need to be screened to reach a subpopulation large enough to conduct a phenotype-stratified study. The most common phenotype in diabetic polyneuropathy was sensory loss (83%), followed by mechanical hyperalgesia (75%) and thermal hyperalgesia (34%, note that percentages are overlapping and not additive). In peripheral nerve injury, frequencies were 37%, 59%, and 50%, and in postherpetic neuralgia, frequencies were 31%, 63%, and 46%. For parallel study design, either the estimated effect size of the treatment needs to be high (>0.7) or only phenotypes that are frequent in the clinical entity under study can realistically be performed. For crossover design, populations under 200 patients screened are sufficient for all phenotypes and clinical entities with a minimum estimated treatment effect size of 0.5.

Diabetic peripheral neuropathy may not be as its name suggests: evidence from magnetic resonance imaging.

Diabetic peripheral neuropathy (DPN) affects up to 50% of patients with diabetes and is a major cause of morbidity and increased mortality. Its clinical manifestations include distressing painful neuropathic symptoms and insensitivity to trauma that result in foot ulcerations and amputations. Several recent studies have implicated poor glycemic control, duration of diabetes, hyperlipidemia (particularly hypertryglyceridaemia), elevated albumin excretion rates, and obesity as risk factors for the development of DPN. However, similar data are not available for painful DPN. Moreover, although there is now strong evidence for the importance of peripheral nerve microvascular disease in the pathogenesis of DPN, peripheral structural biomarkers of painful DPN are lacking. However, there is now emerging evidence for the involvement of the central nervous system in both painful and painless DPN afforded by magnetic resonance imaging. This review will focus on this emerging evidence for central changes in DPN, hitherto considered a peripheral nerve disease only.

The Pain in Neuropathy Study (PiNS): a cross-sectional observational study determining the somatosensory phenotype of painful and painless diabetic neuropathy.

Disabling neuropathic pain (NeuP) is a common sequel of diabetic peripheral neuropathy (DPN). We aimed to characterise the sensory phenotype of patients with and without NeuP, assess screening tools for NeuP, and relate DPN severity to NeuP. The Pain in Neuropathy Study (PiNS) is an observational cross-sectional multicentre study. A total of 191 patients with DPN underwent neurological examination, quantitative sensory testing, nerve conduction studies, and skin biopsy for intraepidermal nerve fibre density assessment. A set of questionnaires assessed the presence of pain, pain intensity, pain distribution, and the psychological and functional impact of pain. Patients were divided according to the presence of DPN, and thereafter according to the presence and severity of NeuP. The DN4 questionnaire demonstrated excellent sensitivity (88%) and specificity (93%) in screening for NeuP. There was a positive correlation between greater neuropathy severity (r = 0.39, P < 0.01), higher HbA1c (r = 0.21, P < 0.01), and the presence (and severity) of NeuP. Diabetic peripheral neuropathy sensory phenotype is characterised by hyposensitivity to applied stimuli that was more marked in the moderate/severe NeuP group than in the mild NeuP or no NeuP groups. Brush-evoked allodynia was present in only those with NeuP (15%); the paradoxical heat sensation did not discriminate between those with (40%) and without (41.3%) NeuP. The "irritable nociceptor" subgroup could only be applied to a minority of patients (6.3%) with NeuP. This study provides a firm basis to rationalise further phenotyping of painful DPN, for instance, stratification of patients with DPN for analgesic drug trials.