Measuring peripheral nerve involvement in Friedreich's ataxia

OBJECTIVE

Experimental therapies under development for Friedreich's Ataxia (FRDA) require validated biomarkers. In-vivo reflectance confocal microscopy (RCM) of skin is a noninvasive way to quantify Meissner's corpuscle (MC) density and has emerged as a sensitive measure of sensory polyneuropathies. We conducted a prospective, cross-sectional study evaluating RCM of MCs and conventional peripheral nerve measures as candidate peripheral nerve markers in FRDA.

METHODS

Sixteen individuals with FRDA and 16 age- and gender-matched controls underwent RCM of MC density and morphology, skin biopsies for epidermal nerve fiber density (ENFD), nerve conduction studies (NCS), and quantitative sensory testing (QST) including touch, vibration, and cooling thresholds.

RESULTS

MC densities were measurable in all participants with FRDA, and were lower at digit V (hand), thenar eminence, and arch (foot) compared to controls. By contrast, sensory NCS showed floor effects and were obtainable in only 13% of FRDA participants. QST thresholds for touch, vibration, and cooling were higher at the hand and foot in FRDA than controls. Reductions in ENFDs were present in more severely affected individuals with FRDA (Friedreich's Ataxia Rating Scale (FARS) >60) compared to matched controls, although skin biopsies were not well tolerated in children. MC densities, ENFDs, and touch and vibration thresholds were associated with clinical disease severity (FARS and modified FARS) and duration since symptom onset.

INTERPRETATION

MC density, ENFD, and QST thresholds provide structural and physiologic markers of sensory involvement in FRDA. Longitudinal evaluation is needed to determine whether these measures can identify changes associated with disease progression or treatment.

In-vivo reflectance confocal microscopy of Meissner's corpuscles in diabetic distal symmetric polyneuropathy

OBJECTIVE

To evaluate in-vivo reflectance confocal microscopy (RCM) of Meissner's corpuscles (MC) in diabetic distal symmetric polyneuropathy (DSP).

METHODS

Forty-three adults with diabetes and 21 control subjects underwent RCM of MC density at the fingertip of digit V, thenar eminence (TE), and arch of the foot, ankle skin biopsy for epidermal nerve fiber density (ENFD), electrophysiological studies, monofilament threshold testing, and timed vibration at the toe. Subjects with diabetes were subdivided into groups with and without clinical DSP using the American Academy of Neurology (AAN) case definition and neuropathy outcomes were compared across groups.

RESULTS

Both diabetic groups (with and without AAN clinical DSP criteria) had objective evidence of peripheral sensory involvement using conventional sensory measures, although those with clinical DSP criteria had greater abnormalities. MC densities were lower in the entire diabetic group at the TE and digit V relative to controls. MC densities at all imaging sites were associated with corresponding conventional sensory measures. MC densities were reduced in subjects without AAN clinical DSP criteria at the TE and digit V compared to controls whereas conventional upper limb sensory measures did not differ between these groups.

CONCLUSIONS

In-vivo RCM of MC density at digit V is a non-invasive, painless, objective marker in diabetes that offers a window into early large fiber sensory nerve terminal loss. Further studies are needed to determine whether RCM of MCs can identify quantitative changes in DSP associated with disease progression or treatment.

Tracking Epidermal Nerve Fiber Changes in Asian Macaques: Tools and Techniques for Quantitative Assessment

Quantitative assessment of epidermal nerve fibers (ENFs) has become a widely used clinical tool for the diagnosis of small fiber neuropathies such as diabetic neuropathy and human immunodeficiency virus-associated sensory neuropathy (HIV-SN). To model and investigate the pathogenesis of HIV-SN using simian immunodeficiency virus (SIV)-infected Asian macaques, we adapted the skin biopsy and immunostaining techniques currently employed in human patients and then developed two unbiased image analysis techniques for quantifying ENF in macaque footpad skin. This report provides detailed descriptions of these tools and techniques for ENF assessment in macaques and outlines important experimental considerations that we have identified in the course of our long-term studies. Although initially developed for studies of HIV-SN in the SIV-infected macaque model, these methods could be readily translated to a range of studies involving peripheral nerve degeneration and neurotoxicity in nonhuman primates as well as preclinical investigations of agents aimed at neuroprotection and regeneration.

Persistent Peripheral Nervous System Damage in Simian Immunodeficiency Virus-Infected Macaques Receiving Antiretroviral Therapy

Human immunodeficiency virus (HIV)-induced peripheral neuropathy is the most common neurologic complication associated with HIV infection. In addition to virus-mediated injury of the peripheral nervous system (PNS), treatment of HIV infection with combination antiretroviral therapy (cART) may induce toxic neuropathy as a side effect. Antiretroviral toxic neuropathy is clinically indistinguishable from the sensory neuropathy induced by HIV; in some patients, these 2 processes are likely superimposed. To study these intercurrent PNS disease processes, we first established a simian immunodeficiency virus (SIV)/pigtailed macaque model in which more than 90% of animals developed PNS changes closely resembling those seen in HIV-infected individuals with distal sensory neuropathy. To determine whether cART alters the progression of SIV-induced PNS damage, dorsal root ganglia and epidermal nerve fibers were evaluated in SIV-infected macaques after long-term suppressive cART. Although cART effectively suppressed SIV replication and reduced macrophage activation in the dorsal root ganglia, PGP 9.5 immunostaining and measurements of epidermal nerve fibers in the plantar surface of the feet of treated SIV-infected macaques clearly showed that cART did not normalize epidermal nerve fiber density. These findings illustrate that significant PNS damage persists in SIV-infected macaques on suppressive cART.

Clinical approach to peripheral neuropathy: anatomic localization and diagnostic testing

PURPOSE OF REVIEW

This article provides a clinical approach to peripheral neuropathy based on anatomic localization and diagnostic testing.

RECENT FINDINGS

Advances have been made in the evaluation of small fiber neuropathy and in the known genetic causes of neuropathy.

SUMMARY

History and physical examination remain the most useful tools for evaluating peripheral neuropathy. Characterization of a neuropathy aids in limiting the differential diagnosis and includes consideration of temporal profile (tempo of onset and duration), heredity, and anatomic classification. Anatomic classification involves (1) fiber type (motor versus sensory, large versus small, somatic versus autonomic), (2) portion of fiber affected (axon versus myelin), and (3) gross distribution of nerves affected (eg, length-dependent, length-independent, multifocal). Diagnostic testing may include serologic and CSF evaluation, electrodiagnosis, skin biopsy, quantitative sensory testing, autonomic testing, nerve biopsy, confocal corneal microscopy, and laser Doppler imager flare.

Sensory neuropathies, from symptoms to treatment

PURPOSE OF REVIEW

The present review focuses on recent developments in diagnosis and treatment of sensory neuropathies. It does not seek to establish a comprehensive classification of sensory neuropathies, nor treatment guidelines per se.

RECENT FINDINGS

Diagnostic criteria and guidelines have been developed for distal symmetric polyneuropathies, small fiber sensory neuropathies and sensory neuronopathies. Novel diagnostic tools such as skin biopsies now allow diagnosis of small fiber sensory neuropathies. Genetic testing has defined new subtypes of mitochondrial neuropathies and inherited neuropathies with sensory involvement. Intravenous immunoglobulin and tumor necrosis factor-alpha inhibitors show promise for some dysimmune sensory neuropathies or neuronopathies. Additional options for management of neuropathic pain are emerging.

SUMMARY

Diagnostic methods for both acquired and hereditary sensory neuropathies have progressed in recent years, leading to earlier and more specific diagnoses and a better understanding of disease mechanisms. Much progress remains to be made regarding symptomatic and disease-modifying therapy for a range of sensory neuropathies, including those due to diabetes, HIV infection and from dysimmune or hereditary causes.

Analysis of nerve and neuropeptide patterns in vacuum-assisted closure-treated diabetic murine wounds

BACKGROUND

Reestablishment of the peripheral nervous system occurs in parallel with wound healing. With accelerated wound healing seen with the vacuum-assisted closure device, the authors studied its effects on nerve fiber regeneration, nerve sprouting, and the stimulation of neuropeptides and neurotrophins.

METHODS

A vacuum-assisted closure device was applied to a full-thickness diabetic mouse wound using continuous or cyclical modes and compared with foam dressing or occlusive dressing controls, using 10 mice per group. Nerve fibers, substance P, calcitonin gene-related peptide, and nerve growth factor were analyzed using two-dimensional immunohistochemistry and real-time reverse-transcriptase polymerase chain reaction.

RESULTS

A significant increase in dermal and epidermal nerve fiber densities and in substance P, calcitonin gene-related peptide, and nerve growth factor expression was seen in vacuum-assisted closure-treated wounds. Cyclical treatment mode correlated with the largest increase in granulation tissue production, wound surface microdeformations, and a slightly faster wound closure rate.

CONCLUSIONS

This study suggests that vacuum-assisted closure therapy can modulate nerve fiber and neuropeptide production in the wound. Optimized kinetics of vacuum-assisted closure application may provide an opportunity for clinicians to further improve wound healing in denervated wounds such as pressure sores and diabetic foot ulcerations.

Utility of skin biopsy to evaluate peripheral neuropathy

Skin biopsy for epidermal nerve fiber analysis provides an important objective test for the diagnosis of peripheral neuropathy, particularly small fiber sensory neuropathy (SFSN). The determination of epidermal nerve fiber density (ENFD) is reliable, with high diagnostic specificity and good sensitivity. Because of false negatives, biopsy results must be interpreted in conjunction with neurologic findings and laboratory results, including objective tests of sensory and autonomic function. SFSN most commonly is length dependent and is idiopathic in about half the patients. Biopsy of a proximal site (thigh) and a distal site (calf) typically shows greater abnormality of ENFD distally than proximally. More severe abnormality of ENFD in the thigh than in the calf raises the possibility of a non-length-dependent SFSN. The causes of this type of neuropathy, such as Sjögren's syndrome, sarcoidosis, and celiac disease, may be treatable.

The lamellar cells in human Meissner corpuscles express TrkB

Cutaneous Meissner corpuscles depend for development and survival exclusively on the NT system TrkB/BDNF/NT-4 unlike other types of sensory corpuscles and nerve endings, which have very complex neuronal and growth factor dependence. However, the pattern of expression of TrkB in human Meissner corpuscles is not known. The experiments in these studies were designed to pursue further findings that suggest that BDNF and NT-4 have critical roles in the development and maintenance of Meissner corpuscles by analyzing the pattern of expression of TrkB, their high-affinity receptor, in human glabrous skin. These experiments showed that TrkB is expressed in different patterns by the lamellar cells of Meissner corpuscles and not by the axon. The studies also show that while the percentage of Meissner corpuscles that express TrkB remains constant from birth till 50-year old cases, it decreases approximately 3-fold in subjects older than 50 years. These results are important since the study of Meissner corpuscles from cutaneous biopsies to diagnose some neurological diseases has rapidly become of high interest and therefore the proteins expressed in these corpuscles are potential diagnostic tools.

Non-invasive imaging of nerve regeneration

The need for non-invasive imaging of peripheral nerves that can reliably assess extent of nerve fiber degeneration and regeneration is increasingly realized. Availability of such a technology has several immediate clinical and preclinical applications. Diffusion tensor imaging (DTI) is an emerging magnetic resonance based technology that is particularly suited for imaging nerve fiber tracts. This review highlights immediate clinical and preclinical uses of non-invasive imaging of peripheral nerve regeneration and DTI as a potential technology that can fulfill these clinical and research needs.