A statistical approach to fiber diameter distribution in human sural nerve

NRV: An open framework for in silico evaluation of peripheral nerve electrical stimulation strategies

Electrical stimulation of peripheral nerves has been used in various pathological contexts for rehabilitation purposes or to alleviate the symptoms of neuropathologies, thus improving the overall quality of life of patients. However, the development of novel therapeutic strategies is still a challenging issue requiring extensive in vivo experimental campaigns and technical development. To facilitate the design of new stimulation strategies, we provide a fully open source and self-contained software framework for the in silico evaluation of peripheral nerve electrical stimulation. Our modeling approach, developed in the popular and well-established Python language, uses an object-oriented paradigm to map the physiological and electrical context. The framework is designed to facilitate multi-scale analysis, from single fiber stimulation to whole multifascicular nerves. It also allows the simulation of complex strategies such as multiple electrode combinations and waveforms ranging from conventional biphasic pulses to more complex modulated kHz stimuli. In addition, we provide automated support for stimulation strategy optimization and handle the computational backend transparently to the user. Our framework has been extensively tested and validated with several existing results in the literature.

NRV: An open framework for in silico evaluation of peripheral nerve electrical stimulation strategies

Electrical stimulation of peripheral nerves has been used in various pathological contexts for rehabilitation purposes or to alleviate the symptoms of neuropathologies, thus improving the overall quality of life of patients. However, the development of novel therapeutic strategies is still a challenging issue requiring extensive in vivo experimental campaigns and technical development. To facilitate the design of new stimulation strategies, we provide a fully open source and self-contained software framework for the in silico evaluation of peripheral nerve electrical stimulation. Our modeling approach, developed in the popular and well-established Python language, uses an object-oriented paradigm to map the physiological and electrical context. The framework is designed to facilitate multi-scale analysis, from single fiber stimulation to whole multifascicular nerves. It also allows the simulation of complex strategies such as multiple electrode combinations and waveforms ranging from conventional biphasic pulses to more complex modulated kHz stimuli. In addition, we provide automated support for stimulation strategy optimization and handle the computational backend transparently to the user. Our framework has been extensively tested and validated with several existing results in the literature.

Morphology and morphometry of the ulnar nerve in the forelimb of pigs

The knowledge of the morphology and morphometry of peripheral nerves is essential for developing neural interfaces and understanding nerve regeneration in basic and applied research. Currently, the most adopted animal model is the rat, even though recent studies have suggested that the neuroanatomy of large animal models is more comparable to humans. The present knowledge of the morphological structure of large animal models is limited; therefore, the present study aims to describe the morphological characteristics of the Ulnar Nerve (UN) in pigs. UN cross-sections were taken from seven Danish landrace pigs at three distinct locations: distal UN, proximal UN and at the dorsal cutaneous branch of the UN (DCBUN). The nerve diameter, fascicle diameter and number, number of fibres and fibre size were quantified. The UN diameter was larger in the proximal section compared to the distal segment and the DCBUN. The proximal branch also had a more significant number of fascicles (median: 15) than the distal (median: 10) and the DCBUN (median: 11) segments. Additionally, the mean fascicle diameter was smaller at the DCBUN (mean: 165 μm) than at the distal (mean: 197 μm) and proximal (mean: 199 μm) segments of the UN. Detailed knowledge of the microscopical structure of the UN in pigs is critical for further studies investigating neural interface designs and computational models of the peripheral nervous system.

Median amplitude and frequency analysis of sensory nerve responses to intraepidermal stimulation

BACKGROUND

In clinical practice, small myelinated sensory fibers conveying pain and other sensations, Aδ-fibers, cannot be examined with available nerve conduction study techniques.

NEW METHOD

Equipment available in clinical neurophysiology laboratories is used to record from human sensory nerves multiple averaged responses to non-painful stimulation of intraepidermal nerves. Ten averaged responses are analyzed in all possible pair combinations with an algorithm applied to a 0.45 ms period of amplitude and frequency (power spectrum). The median of the algorithms is compared to control data to identify potentials generated as response to intraepidermal stimulation.

RESULTS

Median analysis of the algorithm applied to amplitude and frequency of multiple record pairs identifies potentials with conduction velocities of Aδ-fibers. The analysis of frequency (power spectrum) adds data to the analysis of amplitude. Median analysis of multiple record pairs yields more data than analysis of one pair of alternate averages with the same algorithms.

COMPARISON WITH EXISTING METHOD(S)

At present, analysis of one pair of alternate average records with an algorithm is the only method to identify Aδ-fiber generated potentials. Median analysis of the same algorithm applied to the amplitude of multiple record pairs increases the number of Aδ-fiber generated potentials identified. Neither median analysis of amplitude nor frequency of multiple records pairs has ever been used for conduction studies of nerve fibers, including Aδ-fibers.

CONCLUSIONS

Stimulation, recording and data analysis methods used in this study can be applied in the clinical EMG laboratory to identify the conduction velocities of Aδ-fibers in human sensory nerves.

RFC1 AAGGG repeat expansion masquerading as Chronic Idiopathic Axonal Polyneuropathy

BACKGROUND

A biallelic intronic AAGGG repeat expansion in the Replication Factor C subunit 1 (RFC1) gene has been recently associated with Cerebellar Ataxia, Neuropathy, Vestibular Areflexia Syndrome, a disorder often presenting as a slowly evolving sensory neuropathy at the onset. "Chronic Idiopathic Axonal Polyneuropathy" (CIAP) is a common indolent axonal neuropathy of adulthood which remains without an identifiable cause despite thorough investigations.

METHODS

We screened 234 probands diagnosed with CIAP for a pathogenic biallelic RFC1 AAGGG repeat expansion. Patients were selected from 594 consecutive patients with neuropathy referred to our tertiary-care center for a sural nerve biopsy over 10 years.

RESULTS

The RFC1 AAGGG repeat expansion was common in patients with pure sensory neuropathy (21/40, 53%) and less frequent in cases with predominantly sensory (10/56, 18%, P < 0.001) or sensorimotor (3/138, 2%, P < 0.001) neuropathy. The mutation was associated with sensory ataxia (τ_b = 0.254, P < 0.001), autonomic disturbances (35% vs 8%, Prevalence Odds Ratio-POR 6.73 CI 95% 2.79-16.2, P < 0.001), retained deep tendon reflexes (score 18.0/24 vs 11.5/24, R = 0.275, P < 0.001). On pathology, we observed absent/scant regenerative changes (τ_b = - 0.362, P < 0.001), concomitant involvement of large (100% and 99%, n.s.), small myelinated (97% vs 81%, POR 7.74 CI 95% 1.03-58.4, P = 0.02) and unmyelinated nerve fibers (85% vs 41%, POR 8.52 CI 95% 3.17-22.9, P < 0.001). Cerebellar or vestibular involvement was similarly rare in the two groups.

CONCLUSIONS

This study highlights the frequent occurrence of the RFC1 AAGGG repeat expansion in patients diagnosed with CIAP and characterizes the clinical and pathological features of the related neuro(no)pathy.

Effects of anthropometric factors on nerve conduction: an electrophysiologic study of feet

BACKGROUND

Previous studies have shown that age, sex, and body mass index (BMI) affect the amplitude of sensory nerve action potentials (SNAPs), but the total effects of multiple factors and the most prominently affected nerves have not been elucidated. We systematically investigated the effects of these factors on motor and sensory nerves of the feet.

METHODS

The amplitude, latency, and conduction velocity of compound muscle action potential (CMAP), SNAP, and mixed nerve action potential (MNAP) of the posterior tibial, calcaneal, distal posterior tibial, medial and lateral plantar, and sural nerves were measured in 30 healthy individuals (60 feet). The effects of age, sex, height, and BMI on each nerve were estimated by correlation and linear regression analyses.

RESULTS

The amplitude of posterior tibial CMAP and distal posterior tibial MNAP decreased with BMI. The amplitude of medial plantar MNAP and sural SNAP decreased with height. The conduction velocity of calcaneal SNAP and distal posterior tibial and lateral plantar MNAP decreased with height and BMI. The conduction velocity of medial plantar MNAP decreased only with height. The latency of posterior tibial CMAP increased with age and height. The latency of lateral plantar CMAP and calcaneal SNAP increased with height. The latency of lateral plantar MNAP increased with BMI.

CONCLUSIONS

The effects of age, sex, height, and BMI in foot nerve conduction studies are not identical. Height and BMI were shown to strongly affect motor, sensory, and mixed nerve conduction. Further investigations are needed.

Reproducibility in nerve morphometry: comparison between methods and among observers

We investigated the reproducibility of a semiautomated method (computerized with manual intervention) for nerve morphometry (counting and measuring myelinated fibers) between three observers with different levels of expertise and experience with the method. Comparisons between automatic (fully computerized) and semiautomated morphometric methods performed by the same computer software using the same nerve images were also performed. Sural nerves of normal adult rats were used. Automatic and semiautomated morphometry of the myelinated fibers were made through the computer software KS-400. Semiautomated morphometry was conducted by three independent observers on the same images, using the semiautomated method. Automatic morphometry overestimated the myelin sheath area, thus overestimating the myelinated fiber size and underestimating the axon size. Fiber distributions overestimation was of 0.5 μm. For the semiautomated morphometry, no differences were found between observers for myelinated fiber and axon size distributions. Overestimation of the myelin sheath size of normal fibers by the fully automatic method might have an impact when morphometry is used for diagnostic purposes. We suggest that not only semiautomated morphometry results can be compared between different centers in clinical trials but it can also be performed by more than one investigator in one single experiment, being a reliable and reproducible method.

Sural nerve biopsy may predict future nerve dysfunction

OBJECTIVE

Sural nerve pathology in peripheral neuropathy shows correlation with clinical findings and neurophysiological tests. The aim was to investigate progression of nerve dysfunction over time in relation to a baseline nerve biopsy.

METHODS

Baseline myelinated nerve fiber density (MNFD) was assessed in sural nerve biopsies from 10 men with type 2 diabetes, 10 with impaired and 10 with normal glucose tolerance. Nerve conduction and quantitative perception thresholds were estimated at baseline and follow-up (7-10 years later).

RESULTS

Subjects with low MNFD (< or = 4700 fibers/mm(2)) showed decline of peroneal amplitude (P < 0.02) and conduction velocity (P < 0.04), as well as median nerve sensory amplitude (P < 0.05) and motor conduction velocity (P < 0.04) from baseline to follow-up. In linear regression analyses, diabetes influenced decline of nerve conduction. MNFD correlated negatively with body mass index (r = -0.469; P < 0.02).

CONCLUSION

Low MNFD may predict progression of neurophysiological dysfunction and links obesity to myelinated nerve fiber loss.

Morphology and morphometry of the vagus nerve in male and female spontaneously hypertensive rats

The vagus nerve is an important component of the efferent arm of the baroreflex. Blood pressure levels as well as baroreflex control of circulation are significantly different in male and female spontaneously hypertensive rats (SHR). We proposed to investigate the morphometric differences between genders using the vagus nerve of SHR. Adult animals (20 weeks old) were anesthetized and had their arterial pressure (AP) and heart rate (HR) recorded by a computerized system. The rats were then systemically perfused with a fixative solution and had their cervical vagi nerves prepared for light microscopy. Proximal and distal segments of the left and right vagi nerves were evaluated for morphometric parameters including fascicle area and diameter, myelinated fiber number, density, area and diameter. Comparisons were made between sides and segments on the same gender as well as between genders. Differences were considered significant when p<0.05. Male SHR had significantly higher AP and HR. Morphometric data showed no differences between the same levels of both sides and between segments on the same side for male and female rats. In addition, no significant morphometric differences were observed when genders were compared. This is the first description of vagus nerve morphometry in SHR indicating that gender differences in AP and HR cannot be attributed to dissimilarities in vagal innervation of the heart. These data provide a morphological basis for further studies involving functional investigations of the efferent arm of the baroreflex in hypertension.

Morphometric aspects of peripheral nerves in adults and the elderly

There have been inconsistencies among reports of age-related differences in human peripheral nerves (PNs). For such studies, normal control values are necessary. Moreover, the diversity of methods employed makes it difficult to compare results. We used the same histological procedures and methods to measure 12 PNs: 8 in the cranial nerves, 2 motor nerves in the lower limb, and 2 nerves in the autonomic system. We performed a morphometric analysis of nerve fibers and estimated the change in the total number (TN) and average transverse area (ATA) of myelinated axons from adulthood to old age. The spinal nerves demonstrated notable age-related changes in TN and ATA. Most of the cranial nerves also demonstrated notable age-related changes in TN and ATA. However, some nerves demonstrated no such age-related changes and were affected more by other factors. With regards to the autonomic nerves, the lesser splanchnic nerve indicated age-related changes in TN, but the greater splanchnic nerve indicated no age-related changes in either TN or ATA. The autonomic nerves were affected not only by the aging process but also by the pathological changes to the peripheral tissues that they innervate.

Microscopic anatomy of brachial plexus branches in Wistar rats

In the present study, we analyze the morphology and morphometry of the lateral proper digital nerve of the third finger, and of the proximal and distal segments of the ulnar, median, and radial nerves, in Wistar rats 4 or 7 weeks old. The fascicular area and diameter were generally significantly greater in the proximal compared to distal segments and tended to be larger in 7-week-old compared to 4-week-old rats (e.g., median nerve area of 0.13 mm(2) for the proximal and 0.07 mm(2) for distal segments in 4-week-old rats, and 0.17 and 0.10 mm(2), respectively, for the proximal and distal segments of 7-week-old rats). The number of fascicles was significantly greater while the number of myelinated fibers was significantly less in the distal segments (e.g., 1,359 and 509 myelinated fibers, respectively, in the proximal and distal segments of the radial nerve 4-week-old rats). There was no significant difference in these parameters between the two age groups. The diameter of the myelinated fibers and their respective axons increased from 4 to 7 weeks of age (e.g., myelinated fiber diameter of 4.10 microm in 4-week-old animals and 4.7 microm in the ulnar nerve proximal segment of 7-week-old rats). The g-ratio regression line (axon diameter vs. fiber diameter quotient) was outlined for all the nerves studied here. Differences in myelinated fiber density were detected between the segments of the radial nerve, accompanying the number of myelinated fibers. Detailed knowledge of the microscopic anatomy of rat forelimb nerves provides control data for comparison with studies of experimentally induced neuropathies, which can shed more light on human neuropathies.

Compound sensory action potential in normal and pathological human nerves

The compound sensory nerve action potential (SNAP) is the result of phase summation and cancellation of single fiber potentials (SFAPs) with amplitudes that depend on fiber diameter, and the amplitude and shape of the SNAP is determined by the distribution of fiber diameters. Conduction velocities at different conduction distances are determined by summation of SFAPs of varying fiber diameters, and differ in this respect, also, from the compound muscle action potential (CMAP) for which conduction velocities are determined by the very fastest fibers in the nerve. The effect and extent of temporal dispersion over increasing conduction distance is greater for the SNAP than CMAP, and demonstration of conduction block is therefore difficult. In addition, the effect of temporal dispersion on amplitude and shape is strongly dependent on the number of conducting fibers and their distribution, and, with fiber loss or increased conduction velocity variability changes of the SNAP may be smaller than expected from normal nerve. The biophysical characteristics of sensory and motor fibers differ, and this may to some extent determine divergent pathophysiological changes in sensory and motor fibers in different polyneuropathies. In this review, different factors that characterize sensory fibers and set the SNAP apart from the CMAP are discussed to emphasize the supplementary and complementary information that can be obtained from sensory conduction studies. Sensory conduction studies require particular effort and attention to theory and practical detail that may be time consuming.

Accuracy of sampling methods in morphometric studies of human sural nerves

The aim of this study was to ascertain the minimum sample required to accurately measure the total number of myelinated fibres, mean myelinated fibre density (MFD), myelinated fibre diameter (Ds) and axonal diameter (Da) in morphometric studies of sural nerve biopsies. Measurements were obtained by sampling a single fascicle or systematic sampling of up to 50% of the total transverse fascicular area of two control and eighteen pathological sural nerves showing varying degrees of demyelination and axonal degeneration. MFD and fibre size were heterogeneous between fascicles in both control and pathological sural nerves, and morphometric results from one fascicle and systematic sampling of up to 50% of the total transverse fascicular area did not accurately represent the whole myelinated fibre population in the sural nerve. For accurate morphometric data it is necessary to quantitate all the myelinated fibres in the sural nerve.

Initial report on the limited value of hypoglossal nerve transfer to treat brachial plexus root avulsions

OBJECT

Hypoglossal nerve (12th cranial nerve) transfer was performed to treat the sequelae of brachial plexus root avulsion in 12 adults and two infants, and the patients were followed to assess the effectiveness of the surgery.

METHODS

The 12th cranial nerve was transected at the base of the tongue, and a sural nerve graft was used to bridge the gap between the donor (12th) and recipient nerves: C-5 spinal, axillary, suprascapular, or musculocutaneous nerve. The mean graft length in adult patients was 15.75 +/- 5.5 cm (+/- standard deviation, median 14.5 cm) and in the two infants the graft lengths were 7 and 8 cm, respectively. After a mean postoperative interval of 1138 +/- 254 days, electromyographic examination of the target muscles showed tongue movement-related activity in all patients. Muscle force strength measured according to the Medical Research Council's guidelines, was Grade 3 or higher in 21% of patients. Contraction, however, could only be attained by tongue movements, and volitional control was not achieved.

CONCLUSIONS

Although recovery of muscle strength was obtained by 12th cranial nerve transfer, the functional gain remained virtually nonexistent because central control was missing.

Conduction velocity distributions compared to fiber size distributions in normal human sural nerve

Compound action potentials (CAPs) were recorded from the sural nerve of healthy volunteers. A mathematical technique (inverse modeling) was used to compute conduction velocity (CV) histograms from the data. Results were compared to the morphology of age-matched normal sural nerve biopsies. Coefficients of variation (CoVs) revealed the statistical relationship between morphological data (diameter histograms) and electrophysiological data (CV histograms and conventional CAP parameters). No differences were found for the thick fiber group when comparing the CoVs of the diameter histogram parameters with the corresponding CV histogram parameters. Apparently, the same inherent biological interindividual variability is encountered. The CoVs of the CVs of the CAP's main phases are in good agreement with the CoVs of the estimated mean velocity of the thick fiber group. Inverse modeling increases the reliability of the estimation of the number of active fibers as compared to direct CAP amplitude interpretation.