Limits of the sympathetic skin response in patients with diabetic polyneuropathy

Nociceptive flexion reflex in small fibers neuropathy and pain assessments†

BACKGROUND

The nociceptive flexion reflex (NFR) is a polysynaptic and multisegmental spinal reflex that develops in response to a noxious stimulus and is characterized by the withdrawal of the affected body part. The NFR possesses two excitatory components: early RII and late RIII. Late RIII is derived from high-threshold cutaneous afferent A-delta fibers, which are prone to injury early in the course of diabetes mellitus (DM) and may lead to neuropathic pain. We investigated NFR in patients with DM with different types of polyneuropathies to analyze the role of NFR in small fiber neuropathy (SFN).

METHODS

We included 37 patients with DM and 20 healthy participants of similar age and sex. We performed the Composite Autonomic Neuropathy Scale-31, modified Toronto Neuropathy Scale, and routine nerve conduction studies. We grouped the patients into large fiber neuropathy (LFN), SFN, and no overt neurological symptom/sign groups. In all participants, NFR was recorded on anterior tibial (AT) and biceps femoris (BF) muscles after train stimuli on the sole of the foot, and NFR-RIII findings were compared.

RESULTS

We identified 11 patients with LFN, 15 with SFN, and 11 with no overt neurological symptoms or signs. The RIII response on the AT was absent in 22 (60%) patients with DM and 8 (40%) healthy participants. The RIII response on the BF was absent in 31 (73.8%) patients and 7 (35%) healthy participants (P = .001). In DM, the latency of RIII was prolonged, and the magnitude was reduced. Abnormal findings were seen in all subgroups; however, they were more prominent in patients with LFN compared to other groups.

CONCLUSIONS

The NFR-RIII was abnormal in patients with DM even before the emergence of the neuropathic symptoms. The pattern of involvement before neuropathic symptoms was possibly related to an earlier loss of A-delta fibers.

Diagnosing Diabetic Neuropathy: Something Old, Something New

There are potentially many ways of assessing diabetic peripheral neuropathy (DPN). However, they do not fulfill U.S. Food and Drug Administration (FDA) requirements in relation to their capacity to assess therapeutic benefit in clinical trials of DPN. Over the past several decades symptoms and signs, quantitative sensory and electrodiagnostic testing have been strongly endorsed, but have consistently failed as surrogate end points in clinical trials. Therefore, there is an unmet need for reliable biomarkers to capture the onset and progression and to facilitate drug discovery in DPN. Corneal confocal microscopy (CCM) is a non-invasive ophthalmic imaging modality for in vivo evaluation of sensory C-fibers. An increasing body of evidence from multiple centers worldwide suggests that CCM fulfills the FDA criteria as a surrogate endpoint of DPN.

Electrophysiologic testing in diabetic neuropathy

Electrophysiologic studies provide objective data concerning nerve and muscle function. This information enables the diagnosis of disease states and monitoring of disease progression. This chapter describes the changes in electrophysiologic function in both prediabetes and diabetes and discusses the utility of this testing in patients with diabetes. Both the strengths and limitations of electrophysiology are discussed.

Neuromuscular complications of diabetes mellitus

PURPOSE OF REVIEW

Diabetes mellitus has become a modern global epidemic, with steadily increasing prevalence rates related to lifestyle such that 27% of individuals aged 65 years or older have diabetes mellitus, 95% of whom have type 2. This article reviews the effects of diabetes mellitus on the neuromuscular system.

RECENT FINDINGS

Diabetes mellitus leads to diverse forms of peripheral neuropathy as the major neuromuscular complication. Both focal and diffuse types of neuropathy can develop, with the most common form being diabetic sensorimotor polyneuropathy. Small fibers are damaged early in the development of diabetic sensorimotor polyneuropathy and are not assessed by nerve conduction studies. Small fiber damage occurs even in the prediabetes stage. No disease-modifying therapy for diabetic sensorimotor polyneuropathy is available at this time, but this complication can be limited in patients who have type 1 diabetes mellitus with strict glycemic control; the same outcome is not clearly observed in patients who have type 2 diabetes mellitus. Recently, the evidence base for symptomatic treatments of painful diabetic sensorimotor polyneuropathy underwent systematic review. Effective evidence-based treatments include some anticonvulsants (eg, pregabalin, gabapentin), antidepressants (eg, amitriptyline, duloxetine), opioids (eg, morphine sulfate, oxycodone), capsaicin cream, and transcutaneous electrical nerve stimulation.

SUMMARY

This article reviews the increasing prevalence of diabetes mellitus and diabetic sensorimotor polyneuropathy and discusses recent consensus opinion on the objective confirmation needed for the diagnosis in the clinical research setting. The evidence from clinical trials shows that intensive glycemic control reduces prevalence of diabetic sensorimotor polyneuropathy in patients with type 1 diabetes mellitus, but variable outcomes are observed in patients with type 2 diabetes mellitus. Finally, despite the lack of disease-modifying treatment, effective evidence-based therapy can control painful symptoms of diabetic sensorimotor polyneuropathy.

Small fibre neuropathy: role in the diagnosis of diabetic sensorimotor polyneuropathy

Small fibres constitute 70-90% of peripheral nerve fibres and regulate several key functions such as tissue blood flow, temperature and pain perception as well as sweating, all of which are highly relevant to the clinical presentation and adverse outcomes associated with foot ulcerations in patients with diabetes. Recent studies demonstrated significant abnormalities in the small fibres in subjects with impaired glucose tolerance and diabetes, despite normal electrophysiology, suggesting that the earliest nerve fibre damage is to the small fibres. Unfortunately, guidelines and consensus statements focus on large fibres and continue to advocate electrophysiology as a diagnostic modality and as a primary end point for the assessment of therapeutic benefit. (In part, this reflects the difficulties in quantifying small fibre dysfunction and damage.) We have therefore critically assessed currently available techniques that measure small fibre dysfunction in diabetic neuropathy, using quantitative sensory and sudomotor testing. We have assessed the role of identifying structural damage by quantifying intraepidermal nerve fibre density in skin biopsies and corneal nerve morphology using corneal confocal microscopy. Finally, we propose a definition for diabetic neuropathy that incorporates small fibre damage.

Reproducibility of in vivo corneal confocal microscopy as a novel screening test for early diabetic sensorimotor polyneuropathy

AIM

With the goal of identifying a valid biomarker of early diabetic sensorimotor polyneuropathy, we aimed to identify the most reliable in vivo corneal confocal microscopy (CCM) parameter for detection of abnormality of small nerve fibre morphology.

METHODS

Cross-sectional examination of 46 subjects (26 with Type 1 diabetes and 20 healthy volunteers) examined by corneal confocal microscopy for intra- and interobserver reproducibility by the intraclass correlation coefficient method. Corneal nerve fibre density, nerve branch density, nerve fibre length and tortuosity were measured on the same day that subjects underwent clinical and electrophysiological examination.

RESULTS

The 26 subjects with Type 1 diabetes had mean age and diabetes duration 42.8 ± 16.9 and 22.7 ± 16.4 years, respectively. Twelve of those subjects (46%) did not meet criteria for diabetic sensorimotor polyneuropathy, while five (19%) had mild, three (12%) had moderate and six (23%) had severe diabetic sensorimotor polyneuropathy. None of the healthy volunteers (mean age 41.4 ± 17.3 years) had polyneuropathy. Re-examination of selected corneal confocal microscopy images or sets of 40 images yielded very good to excellent intraclass correlation coefficients for all parameters. However, only one parameter (corneal nerve fibre length) emerged with consistently very good reproducibility using a clinically relevant 'study-level' protocol of subject re-examination (intra-observer intraclass correlation coefficient 0.72; interobserver intraclass correlation coefficient 0.73). Despite no differences in intraclass correlation coefficient between subgroups, corneal nerve fibre length was significantly lower (14.76 vs. 16.15 mm/mm(2), P = 0.04) in those with diabetes.

CONCLUSIONS

Development of corneal confocal microscopy may need to focus on the measurement of corneal nerve fibre length, as it appears to have superior reliability in comparison with other parameters, and as evidence exists for its potential as a clinical biomarker of early diabetic sensorimotor polyneuropathy.

Measurements of sweat response and skin vasomotor reflex for assessment of autonomic dysfunction in patients with diabetes

AIMS

Some physical or arousal stimuli induce a rise in sweat secretion (sympathetic sweat response or SSwR) and a reduction in skin blood flow (skin vasomotor reflex or SkVR) to the palm. We recorded SSwRs and SkVRs in diabetic patients and assessed the usefulness of these parameters for evaluating autonomic dysfunction in diabetes.

METHODS

We studied 42 diabetic patients (58+/-12 years) and 42 normal control subjects (59+/-11 years). Focal sweat secretion and skin blood flow were measured on the palm by a sudorometer and a Doppler flowmeter, respectively. SSwRs and SkVRs to deep inspiration, mental arithmetic, and isotonic exercise were recorded. SSwR amplitude was measured from baseline to peak, and SkVR amplitude (reduction rate) was calculated as: (blood flow reduction/basal blood flow)x100%. We also conducted head-up tilt tests and R-R interval variation tests (coefficient of variation of R-R intervals or CV(R-R)).

RESULTS

The SSwR or SkVR amplitudes in the diabetic group were significantly lower than those in the control group for any stimulus. CV(R-R) in the diabetic group was significantly less than that in the control group. The diabetic group showed a significantly greater reduction in systolic blood pressure during head-up tilt compared with the control group. In the diabetic group, there were significant correlations in SSwR or SkVR amplitudes versus blood pressure falls during the head-up tilt test, and CV(R-R) values.

CONCLUSION

We believe that SSwR and SkVR are useful indexes for the evaluation of autonomic involvement in diabetic patients.

Diagnostic validity of epidermal nerve fiber densities in painful sensory neuropathies

In this prospective study, intraepidermal nerve fiber densities (IENFD) and subepidermal nerve plexus densities (SENPD) were quantified by immunostaining in skin punch biopsies from the distal calf in 99 patients with clinical symptoms of painful sensory neuropathy and from 37 age-matched healthy volunteers. The clinical diagnosis was based on history and abnormal thermal thresholds on quantitative sensory testing (QST). In patients with neuropathy, IENFD and SENPD were reduced to about 50% of controls. Elevated warm detection thresholds on QST correlated with IENFD but not with SENPD. Using receiver-operating characteristic (ROC) curve analysis of IENFD values, the diagnostic sensitivity for detecting neuropathy was 0.80 and the specificity 0.82. For SENPD, sensitivity was 0.81 and specificity 0.88. With ROC analysis of both IENFD and SENPD together, the diagnostic sensitivity was further improved to 0.92. The combined examination of IENFD and SENPD is a highly sensitive and specific diagnostic tool in patients suspected to suffer from painful sensory neuropathies but with normal values on clinical neurophysiological studies.

Sympathetic skin response in idiopathic and diabetic carpal tunnel syndrome

BACKGROUND

In carpal tunnel syndrome (CTS), certain changes were expected in sympathetic skin response (SSR) because median nerve carries postganglionic unmyelinated fibres.

PURPOSE

To investigate the median and ulnar SSR in idiopathic and diabetic CTS without autonomic dysfunction in hands and to find possible relations with electrophysiological features of these diseases.

PATIENTS AND METHODS

SSRs were elicited by electrical stimulation on the supraorbital nerve and recorded from the median and ulnar territories in the hand from 20 diabetic patients with only CTS (29 hands), 24 idiopathic CTS patients (42 hands) and 13 normal subjects (26 hands). Hands with ulnar neuropathy at the wrist without symptoms and normal hands of unilateral CTS were excluded. In addition to classical parameters and comparative methods, SSR waveform changes and percentile method was used in finding abnormality.

RESULTS

Median SSRs had significant delayed latency compared to ulnar latency in both CTS patients but this was not important clinically (1.38/1.37 s for idiopatic CTS; 1.43/1.36 s for diabetic CTS). Median and ulnar SSR amplitude, area, median/ulnar latency difference, amplitude and area ratio were compared and only median/ulnar latency difference and median/ulnar latency ratio were found different between the three groups. Four idiopathic CTS hands were outside of the limits or absent (9.5%). SSR waveforms were significantly different from normal subjects in CTS patients. P type SSR replaced M type in idiopathic CTS and N type in diabetic CTS.

CONCLUSIONS

Findings regarding SSR parameters suggest that unmyelinated C fibers were affected in CTS. These values were not useful because they were too small. Data on SSR were not normally distributed and the percentile method seems to be more convenient for finding any abnormality in clinical practice. Also, SSR waveform analysis could give us valuable data and should add to the SSR examination parameters.

Relationship between the frequency of autonomic dysfunction and the severity of chronic obstructive pulmonary disease

OBJECTIVE

The relationship between the frequency of autonomic dysfunction (AD) and the severity of chronic obstructive pulmonary disease (COPD) has not been exactly known, despite its importance in the pathogenesis of COPD. Therefore, we aimed to evaluate the relationship between the clinical stage severity of the disease and the frequency of AD in COPD patients.

METHODS

The frequency and type of AD were determined according to the clinical severity of 35 stable COPD patients. The results were compared between the mild and moderate-severe COPD groups. Sympathetic system (SS) was evaluated with sympathetic skin response (SSR), QT and QTc intervals (ms) analyses. Parasympathetic system was evaluated with the heart rate interval variations (RRIV).

RESULTS

For the total group, an AD was detected in 20 patients (57%), a parasympathetic dysfunction (PD) in 14 (40%), a mixed-type dysfunction in five (14%) and a sympathetic dysfunction (SD) in only one patient (3%). For the 12 mildly affected patients, there were cases of isolated SD in only one patient (8.5%), isolated PD in five (42%) and AD in six patients (50%). For the 23 moderate-severe COPD patients, mixed AD was detected in five patients (22%), isolated PD in nine (39%) and AD in 14 patients (61%). There were no significant differences between the two patient groups, neither for isolated parasympathetic and sympathetic, mixed form nor for total AD (P>0.05).

CONCLUSIONS

The results suggested that PD was dominant in patients with COPD. However, the frequencies of autonomic parasympathetic and sympathetic dysfunction did not increase significantly with the severity of COPD. The severity of hypoxemia and/or airflow limitation may not be the only unique or definite reason for AD in COPD, as there was not an exact correlation between the presence of AD and the severity of airflow limitation and hypoxemia.

Distal site testing of sympathetic skin response (big toe) in diabetic polyneuropathy

The aim of this study was to determine whether the sympathetic skin response (SSR) recorded from the big toe is more sensitive than standard SSR recorded from the sole for the detection of sudomotor fiber dysfunction in diabetic neuropathy. We recorded big toe SSR (SSRBT) and plantar SSR (SSRP) in 17 diabetic patients with non-disabling neuropathy (group A), 13 patients with disabling neuropathy (group B) and 30 age-matched normal controls. With regard to controls, SSRP amplitude was reduced only in group B. In contrast, SSRBT amplitude was reduced in both groups of patients (p<0.0001). In 8 patients in group B, SSRBT was not recordable while the SSRP still persisted. Our results suggest that SS-RBT is a more sensitive test than SSRP in detecting distal sudomotor failure in patients with diabetic neuropathy.

Blunted circadian variation in autonomic regulation of sinus node function in veterans with Gulf War syndrome

PURPOSE

To test the hypothesis that subtle abnormalities of the autonomic nervous system underlie the chronic symptoms reported by many Gulf War veterans, such as chronic diarrhea, dizziness, fatigue, and sexual dysfunction.

METHODS

Twenty-two ill Gulf War veterans and 19 age-, sex-, and education-matched control veterans underwent measurement of circadian rhythm of heart rate variability by 24-hour electrocardiography, ambulatory blood pressure recording, Valsalva ratio testing, sympathetic skin response evaluation, sweat imprint testing, and polysomnography. Investigators were blinded to case- or control-group status.

RESULTS

High-frequency spectral power of heart rate variability increased normally 2.2-fold during sleep in controls but only 1.2-fold in ill veterans (P <0.0001). In ill veterans as compared with controls, it was lower at night (P = 0.0006), higher during the morning (P = 0.007), but no different during the rest of the day (P = 0.8). The mean heart rate of ill veterans also declined less at night (P = 0.0002), and their corrected QT intervals tended to be longer over the full 24 hours (P = 0.07), particularly at night (P = 0.03). Blunting of the nocturnal heart rate dip in ill veterans was confirmed by 24-hour automatic ambulatory blood pressure monitoring (P = 0.05) and polysomnography (P = 0.03). These differences remained significant after adjusting for potential confounders. Cases and controls were similar on measures of sympathetic adrenergic and sudomotor function, sleep architecture, respiratory function, and circadian variation in blood pressure and body temperature.

CONCLUSION

Some symptoms of Gulf War syndrome may be due to subtle autonomic nervous system dysfunction.

Diabetic neuropathy: a review emphasizing diagnostic methods

Diabetic sensorimotor polyneuropathy (DSP) is the most common complication of diabetes. In order to manage DSP effectively, it is necessary to formulate an accurate diagnosis and monitor subjects regularly. This review of important aspects of the diagnosis of DSP starts with a conceptual framework that includes elements of DSP epidemiology, pathophysiology, and therapy. The emphasis of the review is to present our current understanding of diagnostic methods for DSP including their utility and limitations. Screening for DSP in the diabetes clinic can be achieved successfully using simple clinical tests. Clinical neurophysiological methods are necessary to exclude other diagnoses, stage severity, and monitor the course of DSP. Novel investigative techniques are highly promising, but their usefulness in the clinic setting remains limited at this time. This article presents an overview of diagnostic methods for DSP.

Acquired peripheral neuropathy

The element of time is a crucial factor in the electrodiagnostic presentation of PN. The characteristic changes seen in various neuropathies evolve over time. If testing is performed very early in the course of the disease, abnormalities may not yet be present, in part, because the range of normal values for NCS parameters is broad. In addition, if the process is asymmetric, the affected nerves may not be sampled. Very late in the course of the disease, a multifocal process may appear diffuse and symmetric as the areas of focal involvement coalesce. A primarily, demyelinating process may begin to demonstrate secondary axonal involvement. In very severe neuropathies, it may become difficult to evoke any NCS responses to characterize the neuropathy. It is essential to place the electrodiagnostic findings within the context of the clinical progression to avoid drawing erroneous conclusions. Sometimes, the true nature of the neuropathy is clear only after sequential testing. The etiologic diagnoses of acquired polyneuropathy are vast. A well-crafted electrodiagnostic evaluation can categorize neuropathies into more specific diagnostic groups by identifying the descriptive diagnosis, significantly narrowing the list of possible etiologic diagnoses. Electrodiagnostic testing, which always starts with a pertinent history and physical examination, should always be viewed as a continuation and quantification of the physical examination. Only by knowing the extent and pattern of the clinical involvement is it possible to formulate a thorough electrodiagnostic evaluation. This knowledge is especially important in cases in which the presentation is multifocal, because the neuropathy can be missed entirely if the affected nerves are not evaluated. When evaluating a neuropathy, there are three important questions to answer: (1) Is the process diffuse or multifocal? (2) Is it demyelinating or axonal? (3) Does it predominantly involve the motor or sensory nerves? By assessing the neuropathy along these three axes, the large number of possible etiologic diagnoses becomes much more manageable.

Small-fiber neuropathy

Small-fiber neuropathy is a common disorder. It is often "idiopathic" and typically presents with painful feet in patients over the age of 60. Autoimmune mechanisms are often suspected, but rarely identified. Known causes of small-fiber neuropathy include diabetes mellitus, amyloidosis, toxins, and inherited sensory and autonomic neuropathies. Occasionally, small-fiber neuropathy is diffuse or multifocal. Depending on the type of small-fiber neuropathy, autonomic dysfunction can be significant or subclinical. Diagnosis is made on the basis of the clinical features, normal nerve conduction studies, and abnormal specialized tests of small-fiber function. These specialized studies include assessment of epidermal nerve fiber density as well as sudomotor, quantitative sensory, and cardiovagal testing. The sensitivities of these tests range from 59-88%. Each has certain advantages and disadvantages, and the tests may be complementary. Unless an underlying disease is identified, treatment is usually directed toward alleviation of neuropathic pain.