Clinical application of sensory protection of denervated muscle

Novel Technologies to Address the Lower Motor Neuron Injury and Augment Reconstruction in Spinal Cord Injury

Lower motor neuron (LMN) damage results in denervation of the associated muscle targets and is a significant yet under-appreciated component of spinal cord injury (SCI). Denervated muscle undergoes a progressive degeneration and fibro-fatty infiltration that eventually renders the muscle non-viable unless reinnervated within a limited time window. The distal nerve deprived of axons also undergoes degeneration and fibrosis making it less receptive to axons. In this review, we describe the LMN injury associated with SCI and its clinical consequences. The process of degeneration of the muscle and nerve is broken down into the primary components of the neuromuscular circuit and reviewed, including the nerve and Schwann cells, the neuromuscular junction, and the muscle. Finally, we discuss three promising strategies to reverse denervation atrophy. These include providing surrogate axons from local sources; introducing stem cell-derived spinal motor neurons into the nerve to provide the missing axons; and finally, instituting a training program of high-energy electrical stimulation to directly rehabilitate these muscles. Successful interventions for denervation atrophy would significantly expand reconstructive options for cervical SCI and could be transformative for the predominantly LMN injuries of the conus medullaris and cauda equina.

Utilization of the Rat Tibial Nerve Transection Model to Evaluate Cellular and Molecular Mechanisms Underpinning Denervation-Mediated Muscle Injury

Peripheral nerve injury denervates muscle, resulting in muscle paralysis and atrophy. This is reversible if timely muscle reinnervation occurs. With delayed reinnervation, the muscle's reparative ability declines, and muscle-resident fibro-adipogenic progenitor cells (FAPs) proliferate and differentiate, inducing fibro-fatty muscle degradation and thereby physical disability. The mechanisms by which the peripheral nerve regulates FAPs expansion and differentiation are incompletely understood. Using the rat tibial neve transection model, we demonstrated an increased FAPs content and a changing FAPs phenotype, with an increased capacity for adipocyte and fibroblast differentiation, in gastrocnemius muscle post-denervation. The FAPs response was inhibited by immediate tibial nerve repair with muscle reinnervation via neuromuscular junctions (NMJs) and sensory organs (e.g., muscle spindles) or the sensory protection of muscle (where a pure sensory nerve is sutured to the distal tibial nerve stump) with reinnervation by muscle spindles alone. We found that both procedures reduced denervation-mediated increases in glial-cell-line-derived neurotrophic factor (GDNF) in muscle and that GDNF promoted FAPs adipogenic and fibrogenic differentiation in vitro. These results suggest that the peripheral nerve controls FAPs recruitment and differentiation via the modulation of muscle GDNF expression through NMJs and muscle spindles. GDNF can serve as a therapeutic target in the management of denervation-induced muscle injury.

Regenerative Peripheral Nerve Interface Surgery: Anatomic and Technical Guide

Regenerative peripheral nerve interface (RPNI) surgery has been demonstrated to be an effective tool as an interface for neuroprosthetics. Additionally, it has been shown to be a reproducible and reliable strategy for the active treatment and for prevention of neuromas. The purpose of this article is to provide a comprehensive review of RPNI surgery to demonstrate its simplicity and empower reconstructive surgeons to add this to their armamentarium. This article discusses the basic science of neuroma formation and prevention, as well as the theory of RPNI. An anatomic review and discussion of surgical technique for each level of amputation and considerations for other etiologies of traumatic neuromas are included. Lastly, the authors discuss the future of RPNI surgery and compare this with other active techniques for the treatment of neuromas.

Tissue-engineered grafts exploit axon-facilitated axon regeneration and pathway protection to enable recovery after 5-cm nerve defects in pigs

Functional restoration following major peripheral nerve injury (PNI) is challenging, given slow axon growth rates and eventual regenerative pathway degradation in the absence of axons. We are developing tissue-engineered nerve grafts (TENGs) to simultaneously "bridge" missing nerve segments and "babysit" regenerative capacity by providing living axons to guide host axons and maintain the distal pathway. TENGs were biofabricated using porcine neurons and "stretch-grown" axon tracts. TENG neurons survived and elicited axon-facilitated axon regeneration to accelerate regrowth across both short (1 cm) and long (5 cm) segmental nerve defects in pigs. TENG axons also closely interacted with host Schwann cells to maintain proregenerative capacity. TENGs drove regeneration across 5-cm defects in both motor and mixed motor-sensory nerves, resulting in dense axon regeneration and electrophysiological recovery at levels similar to autograft repairs. This approach of accelerating axon regeneration while maintaining the pathway for long-distance regeneration may achieve recovery after currently unrepairable PNIs.

Autonomic Nerve Fibers Aberrantly Reinnervate Denervated Facial Muscles and Alter Muscle Fiber Population

The surgical redirection of efferent neural input to a denervated muscle via a nerve transfer can reestablish neuromuscular control after nerve injuries. The role of autonomic nerve fibers during the process of muscular reinnervation remains largely unknown. Here, we investigated the neurobiological mechanisms behind the spontaneous functional recovery of denervated facial muscles in male rodents. Recovered facial muscles demonstrated an abundance of cholinergic axonal endings establishing functional neuromuscular junctions. The parasympathetic source of the neuronal input was confirmed to be in the pterygopalatine ganglion. Furthermore, the autonomically reinnervated facial muscles underwent a muscle fiber change to a purely intermediate muscle fiber population myosin heavy chain type IIa. Finally, electrophysiological tests revealed that the postganglionic parasympathetic fibers travel to the facial muscles via the sensory infraorbital nerve. Our findings demonstrated expanded neuromuscular plasticity of denervated striated muscles enabling functional recovery via alien autonomic fibers. These findings may further explain the underlying mechanisms of sensory protection implemented to prevent atrophy of a denervated muscle.SIGNIFICANCE STATEMENT Nerve injuries represent significant morbidity and disability for patients. Rewiring motor nerve fibers to other target muscles has shown to be a successful approach in the restoration of motor function. This demonstrates the remarkable capacity of the CNS to adapt to the needs of the neuromuscular system. Yet, the capability of skeletal muscles being reinnervated by nonmotor axons remains largely unknown. Here, we show that under deprivation of original efferent input, the neuromuscular system can undergo functional and morphologic remodeling via autonomic nerve fibers. This may explain neurobiological mechanisms of the sensory protection phenomenon, which is because of parasympathetic reinnervation.

Sensory nerve regeneration and reinnervation in muscle following peripheral nerve injury

Sensory afferent fibers are an important component of motor nerves and compose the majority of axons in many nerves traditionally thought of as "pure" motor nerves. These sensory afferent fibers innervate special sensory end organs in muscle, including muscle spindles that respond to changes in muscle length and Golgi tendons that detect muscle tension. Both play a major role in proprioception, sensorimotor extremity control feedback, and force regulation. After peripheral nerve injury, there is histological and electrophysiological evidence that sensory afferents can reinnervate muscle, including muscle that was not the nerve's original target. Reinnervation can occur after different nerve injury and muscle models, including muscle graft, crush, and transection injuries, and occurs in a nonspecific manner, allowing for cross-innervation to occur. Evidence of cross-innervation includes the following: muscle spindle and Golgi tendon afferent-receptor mismatch, vagal sensory fiber reinnervation of muscle, and cutaneous afferent reinnervation of muscle spindle or Golgi tendons. There are several notable clinical applications of sensory reinnervation and cross-reinnervation of muscle, including restoration of optimal motor control after peripheral nerve repair, flap sensation, sensory protection of denervated muscle, neuroma treatment and prevention, and facilitation of prosthetic sensorimotor control. This review focuses on sensory nerve regeneration and reinnervation in muscle, and the clinical applications of this phenomena. Understanding the physiology and limitations of sensory nerve regeneration and reinnervation in muscle may ultimately facilitate improvement of its clinical applications.

Direct Neurotization: Past, Present, and Future Considerations

ABSTRACT

Direct neurotization is a method that involves direct implantation of nerve fascicles into a target tissue, that is, muscle fibers, skin, cornea, and so on, with the goal of restoring aesthetic, sensation and or functional capacity. This technique has been implemented since the early 1900s, with numerous experimental and clinical reports of success. Applications have included both sensory and motor neurotization of muscle, as well as protective sensory provision for other organs. These techniques have been used to restore corneal sensation, repair brachial plexus injuries, reestablish tongue movement and function through direct tongue neurotization, and reinnervate multiple facial muscles in patients with facial paralysis. Most recently, these methods have even been used in conjunction with acellular cadaveric nerve grafts to directly neurotize skin. Indications for direct neurotization remain limited, including those in which neural coaptation is not feasible (ie, surgical or traumatic damage to neuromuscular junction, severe avulsion injuries of the distal nerve); however, the success and wide-range application of direct neurotization shows its potential to be implemented as an adjunct treatment in contrast to views that it should solely be used as a salvage therapy. The purpose of the following review is to detail the historic and current applications of direct neurotization and describe the future areas of investigation and development of this technique.

Reactivation of Denervated Schwann Cells by Embryonic Spinal Cord Neurons to Promote Axon Regeneration and Remyelination

In peripheral nerve injuries (PNIs) in which proximal axons do not regenerate quickly enough, significant chronic degeneration of Schwann cells (SCs) can occur at the distal stump of the injured nerve and obstruct regeneration. Cell transplantation can delay the degeneration of SCs, but transplanted cells fail to generate voluntary electrical impulses without downstream signal stimulation from the central nervous system. In this study, we combined cell transplantation and nerve transfer strategies to investigate whether the transplantation of embryonic spinal cord cells could benefit the microenvironment of the distal stump of the injured nerve. The experiment consisted of two stages. In the first-stage surgery, common peroneal nerves were transected, and embryonic day 14 (E14) cells or cell culture medium was transplanted into the distal stump of the CPs. Six months after the first-stage surgery, the transplanted cells were removed, and the nerve segment distal to the transplanted site was used to bridge freshly cut tibial nerves to detect whether the cell-treated graft promoted axon growth. The phenotypic changes and the neurotrophic factor expression pattern of SCs distal to the transplanted site were detected at several time points after cell transplantation and excision. The results showed that at different times after transplantation, the cells could survive and generate neurons. Thus, the neurons play the role of proximal axons to prevent chronic degeneration and fibrosis of SCs. After excision of the transplanted cells, the SCs returned to their dedifferentiated phenotype and upregulated growth-associated gene expression. The ability of SCs to be activated again allowed a favorable microenvironment to be created and enhanced the regeneration and remyelination of proximal axons. Muscle reinnervation was also elevated. This transplantation strategy could provide a treatment option for complex neurological injuries in the clinic.

Sensory neurotization of muscle: past, present and future considerations

Research has shown that temporary innervation by a sensory neuron can provide trophic support to a denervated muscle and stave off muscular atrophy until motor neuron transfer is viable. This so called 'sensory protection' allows for improved outcomes when motor reinnervation able to occur. The theoretical benefit of sensory neurotization is hypothesized to maintain tissue architecture of the end organ due to tropic effects of stimulation. While the literature supports direct motor neurotization from 2 to 4 months post-injury, patient factors including the location of the injury and loss of nerve can preclude this therapeutic window. When direct neurotization is not possible, or there is a long distance to traverse for reinnervation, sensory neurotization may be beneficial. The theorized trophic stimulation enabling end organ architectural maintenance provided by sensory neurotization has been shown to allow for delayed direct motor neurotization without the irreversible sequelae of prolonged denervation. This is a review of the pathogenesis of nerve injury and a literature review of sensory neurotization. An analytical search of the literature in PubMed was performed in order to find articles pertinent to the topic of sensory neurotization, including experimental data from both animal models and case reports in humans.

Use of Vascularized Sural Nerve Grafts for Sciatic Nerve Reconstruction After Malignant Bone and Soft Tissue Tumor Resection in the Lower Legs

BACKGROUND

Vascularized nerve grafting is normally associated with a good outcome, but can be difficult to use for nerve reconstruction in patients with long defects of the sciatic nerve given the graft thickness. We report 3 cases of large defect sciatic nerve reconstruction using the bilateral sural nerves of the lower legs harvested together with the fascia and lesser saphenous vein to form a vascularized flap.

METHODS

The subjects were 3 patients who required the reconstruction of a 10-cm or longer segment of the sciatic nerve. Priority was given to restoring sensation in the plantar region such that reconstruction of the sensory nerves corresponding to the tibial region.

RESULTS

Two patients were followed up for long term. There was some persistent perceptual deficit in the foot, minimal protective sensation had been achieved.

CONCLUSIONS

We were able to selectively reconstruct the sensory nerves to achieve sensation in the soles of the feet by using sural nerve grafts from both legs. As the prognosis for the underlying condition in cases necessitating this procedure is often poor, the costs and benefits of reconstruction should always be weighed carefully for each individual patient.

Cross-Face Nerve Grafting with Infraorbital Nerve Pathway Protection: Anatomic and Histomorphometric Feasibility Study

Smiling is an important aspect of emotional expression and social interaction, leaving facial palsy patients with impaired social functioning and decreased overall quality of life. Although there are several techniques available for facial reanimation, staged facial reanimation using donor nerve branches from the contralateral, functioning facial nerve connected to a cross-face nerve graft (CFNG) is the only technique that can reliably reproduce an emotionally spontaneous smile. Although CFNGs provide spontaneity, they typically produce less smile excursion than when the subsequent free functioning muscle flap is innervated with the motor nerve to the masseter muscle. This may be explained in part by the larger number of donor motor axons when using the masseter nerve, as studies have shown that only 20% to 50% of facial nerve donor axons successfully cross the nerve graft to innervate their targets. As demonstrated in our animal studies, increasing the number of donor axons that grow into and traverse the CFNG to innervate the free muscle transfer increases muscle movement, and this phenomenon may provide patients with the benefit of improved smile excursion. We have previously shown in animal studies that sensory nerves, when coapted to a nerve graft, improve axonal growth through the nerve graft and improve muscle excursion. Here, we describe the feasibility of and our experience in translating these results clinically by coapting the distal portion of the CFNG to branches of the infraorbital nerve.

The Dilator Naris Muscle as a Reporter of Facial Nerve Regeneration in a Rat Model

OBJECTIVE

Many investigators study facial nerve regeneration using the rat whisker pad model, although widely standardized outcomes measures of facial nerve regeneration in the rodent have not yet been developed. The intrinsic whisker pad "sling" muscles producing whisker protraction, situated at the base of each individual whisker, are extremely small and difficult to study en bloc. Here, we compare the functional innervation of 2 potential reporter muscles for whisker pad innervation: the dilator naris (DN) and the levator labii superioris (LLS), to characterize facial nerve regeneration.

METHODS

Motor supply of the DN and LLS was elucidated by measuring contraction force and compound muscle action potentials during stimulation of individual facial nerve branches, and by measuring whisking amplitude before and after DN distal tendon release.

RESULTS

The pattern of DN innervation matched that of the intrinsic whisker pad musculature (ie, via the buccal and marginal mandibular branches of the facial nerve), whereas the LLS seemed to be innervated almost entirely by the zygomatic branch, whose primary target is the orbicularis oculi muscle.

CONCLUSIONS

Although the LLS has been commonly used as a reporter muscle of whisker pad innervation, the present data show that its innervation pattern does not overlap substantially with the muscles producing whisker protraction. The DN muscle may serve as a more appropriate reporter for whisker pad innervation because it is innervated by the same facial nerve branches as the intrinsic whisker pad musculature, making structure/function correlations more accurate, and more relevant to investigators studying facial nerve regeneration.

Enhancement of facial nerve motoneuron regeneration through cross-face nerve grafts by adding end-to-side sensory axons

BACKGROUND

In unilateral facial palsy, cross-face nerve grafts are used for emotional facial reanimation. Facial nerve regeneration through the grafts takes several months, and the functional results are sometimes inadequate. Chronic denervation of the cross-face nerve graft results in incomplete nerve regeneration. The authors hypothesize that donor axons from regional sensory nerves will enhance facial motoneuron regeneration, improve axon regeneration, and improve the amplitude of facial muscle movement.

METHODS

In the rat model, a 30-mm nerve graft (right common peroneal nerve) was used as a cross-face nerve graft. The graft was coapted to the proximal stump of the transected right buccal branch of the facial nerve and the distal stumps of the transected left buccal and marginal mandibular branches. In one group, sensory occipital nerves were coapted end-to-side to the cross-face nerve graft. Regeneration of green fluorescent protein-positive axons was imaged in vivo in transgenic Thy1-green fluorescent protein rats, in which all neurons express green fluorescence. After 16 weeks, retrograde labeling of regenerated neurons and histomorphometric analysis of myelinated axons was performed. Functional outcomes were assessed with video analysis of whisker motion.

RESULTS

"Pathway protection" with sensory axons significantly enhanced motoneuron regeneration, as assessed by retrograde labeling, in vivo fluorescence imaging, and histomorphometry, and significantly improved whisker motion during video analysis.

CONCLUSION

Sensory pathway protection of cross-face nerve grafts counteracts chronic denervation in nerve grafts and improves regeneration and functional outcomes.

Sensory nerve cross-anastomosis and electrical muscle stimulation synergistically enhance functional recovery of chronically denervated muscle

BACKGROUND

Long-term muscle denervation leads to severe and irreversible atrophy coupled with loss of force and motor function. These factors contribute to poor functional recovery following delayed reinnervation. The authors' previous work demonstrated that temporarily suturing a sensory nerve to the distal motor stump (called sensory protection) significantly reduces muscle atrophy and improves function following reinnervation. The authors have also shown that 1 month of electrical stimulation of denervated muscle significantly improves function and reduces atrophy. In this study, the authors tested whether a combination of sensory protection and electrical stimulation would enhance functional recovery more than either treatment alone.

METHODS

Rat gastrocnemius muscles were denervated by cutting the tibial nerve. The peroneal nerve was then sutured to the distal tibial stump following 3 months of treatment (i.e., electrical stimulation, sensory protection, or both). Three months after peroneal repair, functional and histologic measurements were taken.

RESULTS

All treatment groups had significantly higher muscle weight (p<0.05) and twitch force (p<0.001) compared with the untreated group (denervated), but fiber type composition did not differ between groups. Importantly, muscle weight and force were significantly greater in the combined treatment group (p<0.05) compared with stimulation or sensory protection alone. The combined treatment also produced motor unit counts significantly greater than sensory protection alone (p<0.05).

CONCLUSIONS

The combination treatment synergistically reduces atrophy and improves reinnervation and functional measures following delayed nerve repair, suggesting that these approaches work through different mechanisms. The authors' research supports the clinical use of both modalities together following peripheral nerve injury.

Early sensory protection in reverse end-to-side neurorrhaphy to improve the functional recovery of chronically denervated muscle in rat: a pilot study

OBJECT

Early innervation by sensory nerves has been proposed to prevent atrophy of chronically denervated muscle, but conventional end-to-end (ETE) neurorrhaphy has been demonstrated to have adverse effects on muscle contractile function. The aim of the present study was to investigate the potential for modified sensory nerve protection in reverse end-to-side (ETS) neurorrhaphy as a way of improving the functional recovery of denervated muscle.

METHODS

Four groups of rats underwent surgical denervation of the tibial nerve projecting to the right hindlimbs (Group 1, unprotected controls; Group 2, positive control [immediate repair without delayed denervation]; Group 3, ETS protected; and Group 4, ETE protected). The proximal and distal stumps of the tibial nerve were ligated in all animals except for those in the immediate-repair group. Other animals underwent denervation without sural nerve protection, or with ETE or ETS neurorrhaphy. The ETE- and ETS-protected and unprotected groups underwent an additional surgery in which the trimmed proximal and distal tibial nerve stumps were sutured together. After 3 months of recovery, the tibial function index was determined, and electrophysiological, histological, and morphometric parameters were assessed.

RESULTS

Significant muscle atrophy was observed in the unprotected group, while a well-preserved ultrastructure was observed for the gastrocnemius muscle in the ETE- and ETS-protected groups. Enhanced recovery in the ETS-protected group was indicated by the tibial function index, motor nerve conduction velocity, muscle contractile force tests, and the histological results. In contrast, early sensory nerve protection in ETE neurorrhaphy impaired the recovery of the regenerated axons and diminished the contractile force of the denervated muscle.

CONCLUSIONS

Early sensory protection in reverse ETS neurorrhaphy is an effective method for improving the functional recovery of chronically denervated muscle following peripheral nerve injury in rats.

Rat whisker movement after facial nerve lesion: evidence for autonomic contraction of skeletal muscle

Vibrissal whisking is often employed to track facial nerve regeneration in rats; however, we have observed similar degrees of whisking recovery after facial nerve transection with or without repair. We hypothesized that the source of non-facial nerve-mediated whisker movement after chronic denervation was from autonomic, cholinergic axons traveling within the infraorbital branch of the trigeminal nerve (ION). Rats underwent unilateral facial nerve transection with repair (N=7) or resection without repair (N=11). Post-operative whisking amplitude was measured weekly across 10weeks, and during intraoperative stimulation of the ION and facial nerves at ⩾18weeks. Whisking was also measured after subsequent ION transection (N=6) or pharmacologic blocking of the autonomic ganglia using hexamethonium (N=3), and after snout cooling intended to elicit a vasodilation reflex (N=3). Whisking recovered more quickly and with greater amplitude in rats that underwent facial nerve repair compared to resection (P<0.05), but individual rats overlapped in whisking amplitude across both groups. In the resected rats, non-facial-nerve-mediated whisking was elicited by electrical stimulation of the ION, temporarily diminished following hexamethonium injection, abolished by transection of the ION, and rapidly and significantly (P<0.05) increased by snout cooling. Moreover, fibrillation-related whisker movements decreased in all rats during the initial recovery period (indicative of reinnervation), but re-appeared in the resected rats after undergoing ION transection (indicative of motor denervation). Cholinergic, parasympathetic axons traveling within the ION innervate whisker pad vasculature, and immunohistochemistry for vasoactive intestinal peptide revealed these axons branching extensively over whisker pad muscles and contacting neuromuscular junctions after facial nerve resection. This study provides the first behavioral and anatomical evidence of spontaneous autonomic innervation of skeletal muscle after motor nerve lesion, which not only has implications for interpreting facial nerve reinnervation results, but also calls into question whether autonomic-mediated innervation of striated muscle occurs naturally in other forms of neuropathy.

Functional recovery of denervated skeletal muscle with sensory or mixed nerve protection: a pilot study

Functional recovery is usually poor following peripheral nerve injury when reinnervation is delayed. Early innervation by sensory nerve has been indicated to prevent atrophy of the denervated muscle. It is hypothesized that early protection with sensory axons is adequate to improve functional recovery of skeletal muscle following prolonged denervation of mixed nerve injury. In this study, four groups of rats received surgical denervation of the tibial nerve. The proximal and distal stumps of the tibial nerve were ligated in all animals except for those in the immediate repair group. The experimental groups underwent denervation with nerve protection of peroneal nerve (mixed protection) or sural nerve (sensory protection). The experimental and unprotected groups had a stage II surgery in which the trimmed proximal and distal tibial nerve stumps were sutured together. After 3 months of recovery, electrophysiological, histological and morphometric parameters were assessed. It was detected that the significant muscle atrophy and a good preserved structure of the muscle were observed in the unprotected and protective experimental groups, respectively. Significantly fewer numbers of regenerated myelinated axons were observed in the sensory-protected group. Enhanced recovery in the mixed protection group was indicated by the results of the muscle contraction force tests, regenerated myelinated fiber, and the results of the histological analysis. Our results suggest that early axons protection by mixed nerve may complement sensory axons which are required for promoting functional recovery of the denervated muscle natively innervated by mixed nerve.

Electrical muscle stimulation after immediate nerve repair reduces muscle atrophy without affecting reinnervation

INTRODUCTION

Electrical stimulation of denervated muscle has been shown to minimize atrophy and fibrosis and increase force in animal and human models. However, electrical stimulation after nerve repair is controversial due to questions of efficacy.

METHODS

Using a rat model, we investigated the efficacy of short-term electrical muscle stimulation for increasing reinnervation and preventing muscle atrophy. After tibial nerve transection and immediate repair with the fibular nerve, 1 month of electrical stimulation was applied 5 days/week for 1 hour to the gastrocnemius muscle via implanted electrodes.

RESULTS

After 2 months of further recovery without stimulation, muscle weights, twitch forces, and type I fiber areas were significantly greater in stimulated animals than in repaired controls without stimulation. Motor unit size and numbers were not different between the 2 groups.

CONCLUSIONS

Short-term electrical muscle stimulation after nerve repair significantly reduces muscle atrophy and does not affect motor reinnervation.

Intensive care unit-acquired weakness: clinical phenotypes and molecular mechanisms

Intensive care unit-acquired weakness (ICUAW) begins within hours of mechanical ventilation and may not be completely reversible over time. It represents a major functional morbidity of critical illness and is an important patient-centered outcome with clear implications for quality of life and resumption of prior work and lifestyle. There is heterogeneity in functional outcome related to ICUAW across various patient populations after an episode of critical illness. This state-of-the art review argues that this observed heterogeneity may represent a clinical spectrum of disability in which there are recognizable clinical phenotypes for outcome according to age, burden of comorbid illness, and ICU length of stay. It further argues that these functional outcomes are modified by mood, cognition, and caregiver physical and mental health. This proposed construct of clinical phenotypes will be used as a framework for a review of the current literature on the molecular biology of muscle and nerve injury. This translational approach for the development of models pairing clinical phenotypes for different functional outcomes after critical illness with molecular mechanism of injury may offer unique insights into the diagnosis and treatment of muscle and nerve lesions.

Determining the effects of electrical stimulation on functional recovery of denervated rat gastrocnemius muscle using motor unit number estimation

The use of electrical muscle stimulation to treat denervated muscle prior to delayed reinnervation has been widely debated. There is evidence showing both positive and negative results following different protocols of electrical stimulation. In this study we investigated the role electrical stimulation has on muscle reinnervation following immediate and delayed nerve repair using motor unit estimation techniques. Rat gastrocnemius muscle was denervated and repaired using the peroneal nerve either immediately or following three-months with and without electrical stimulation. Motor unit counts, average motor unit sizes, and maximum compound action potentials were measured three-months following peroneal nerve repair. Motor unit counts in animals that were denervated and stimulated were significantly higher than those that were denervated and not stimulated. Both average motor unit sizes and maximum compound action potentials showed no significant differences between denervated and denervated-stimulated animals. These results provide evidence that electrical stimulation prior to delayed nerve repair increases muscle receptivity to regenerating axons and may be a worthwhile treatment for peripheral nerve injuries.

Nerve reconstruction in the hand and upper extremity

In the management of traumatic peripheral nerve injuries, the severity or degree of injury dictates the decision making between surgical management versus conservative management and serial examination. This review explores some of the recent literature, specifically addressing recent basic science advances in end-to-side and reverse end-to-side recovery, Schwann cell migration, and neuropathic pain. The management of nerve gaps, including the use of nerve conduits and acellularized nerve allografts, is examined. Current commonly performed nerve transfers are detailed with focus on both motor and sensory nerve transfers, their indications, and a basic overview of selected surgical techniques.

Reverse end-to-side nerve transfer: from animal model to clinical use

PURPOSE

Functional recovery after peripheral nerve injury is predominantly influenced by time to reinnervation and number of regenerated motor axons. For nerve injuries in which incomplete regeneration is anticipated, a reverse end-to-side (RETS) nerve transfer might be useful to augment the regenerating nerve with additional axons and to more quickly reinnervate target muscle. This study evaluates the ability of peripheral nerve axons to regenerate across an RETS nerve transfer. We present a case report demonstrating its potential clinical applicability.

METHODS

Thirty-six Lewis rats were randomized into 3 groups. In group 1 (negative control), the tibial nerve was transected and prevented from regenerating. In group 2 (positive control), the tibial and peroneal nerves were transected, and an end-to-end (ETE) nerve transfer was performed. In group 3 (experimental model), the tibial nerve and peroneal nerves were transected, and an RETS nerve transfer was performed between the proximal end of the peroneal nerve and the side of the denervated distal tibial stump. Nerve histomorphometry and perfused muscle mass were evaluated. Six Thy1-GFP transgenic Sprague Dawley rats, expressing green fluorescent protein in their neural tissues, also had the RETS procedure for evaluation with confocal microscopy.

RESULTS

Nerve histomorphometry showed little to no regeneration in chronic denervation animals but statistically similar regeneration in ETE and RETS animals at 5 and 10 weeks. Muscle mass preservation was similar between ETE and RETS groups by 10 weeks and significantly better than negative controls at both time points. Nerve regeneration was robust across the RETS coaptation of Thy1-GFP rats by 5 weeks.

CONCLUSIONS

Axonal regeneration occurs across an RETS coaptation. An RETS nerve transfer might augment motor recovery when less-than-optimal recovery is otherwise anticipated.

TYPE OF STUDY/LEVEL OF EVIDENCE

Therapeutic I.

The basis for diminished functional recovery after delayed peripheral nerve repair

The postsurgical period during which neurons remain without target connections (chronic axotomy) and distal nerve stumps and target muscles are denervated (chronic denervation) deleteriously affects functional recovery. An autologous nerve graft and cross-suture paradigm in Sprague Dawley rats was used to systematically and independently control time of motoneuron axotomy, denervation of distal nerve sheaths, and muscle denervation to determine relative contributions of each factor to recovery failure. Tibial (TIB) nerve was cross-sutured to common peroneal (CP) nerve via a contralateral 15 mm nerve autograft to reinnervate the tibialis anterior (TA) muscle immediately or after prolonging TIB axotomy, CP autograft denervation, or TA muscle denervation. Numbers of motoneurons that reinnervated TA muscle declined exponentially from 99 ± 15 to asymptotic mean (± SE) values of 35 ± 1, 41 ± 10, and 13 ± 5, respectively. Enlarged reinnervated motor units fully compensated for reduced motoneuron numbers after prolonged axotomy and autograft denervation, but the maximal threefold enlargement did not compensate for the severe loss of regenerating nerves through chronically denervated nerve stumps and for failure of reinnervated muscle fibers to recover from denervation atrophy. Muscle force, weight, and cross-sectional area declined. Our results demonstrate that chronic denervation of the distal stump plays a key role in reduced nerve regeneration, but the denervated muscle is also a contributing factor. That chronic Schwann cell denervation within the nerve autograft reduced regeneration less than after the denervation of both CP nerve stump and TA muscle, argues that chronic muscle denervation negatively impacts nerve regeneration.

Side-to-Side Nerve Grafts Sustain Chronically Denervated Peripheral Nerve Pathways During Axon Regeneration and Result in Improved Functional Reinnervation

BACKGROUND:

Progressive atrophy of Schwann cells in denervated nerve stumps is a major reason for progressive failure of functional recovery after peripheral nerve injury and surgical repair.

OBJECTIVE:

To examine whether side-to-side nerve bridges between an intact donor nerve and a recipient denervated distal nerve stump promote nerve growth and in turn, protect distal nerve stumps to improve axon regeneration after delayed surgical repair.

METHODS:

In Sprague-Dawley rats, 1 or 3 side-to-side common peroneal (CP) nerve bridges were used to bridge between the donor intact tibial (TIB) nerve and a recipient denervated CP distal nerve stump in the contralateral hind limb. No bridges were placed in control animals. After 4 months, either a fluorescent retrograde dye was applied to back-label TIB motoneurons with axons that had grown into the CP nerve stump or the proximal and distal CP nerve stumps were resutured in experimental and control animals to encourage CP nerve regeneration for 5 months. Retrograde dyes were again applied to count CP motoneurons that regenerated their axons through protected and unprotected nerve stumps.

RESULTS:

Significantly more donor TIB motoneurons regenerated axons into the recipient denervated CP nerve stump through 3 side-to-side CP nerve bridges compared with 1 bridge. This TIB nerve protection significantly increased the number of CP motoneurons regenerating axons through the denervated CP nerve stumps, the number of regenerated axons, and the weight of the reinnervated muscles.

CONCLUSION:

Multiple side-to-side nerve bridges protect chronically denervated nerve stumps to improve axon regeneration and target reinnervation after delayed nerve repair.

A New System and Paradigm for Chronic Stimulation of Denervated Rat Muscle

Traditionally, animal studies employing electrical stimulation for conditioning denervated muscle rely on 24-hour-based stimulation paradigms, most employing implantable stimulators. While these stimulators provide the necessary current to cause muscular contraction, they have problems with battery life, programmability, and long-term robustness. Continuous 24-hour stimulation, while shown to be effective in animals, is not easily translatable to a clinical setting. It is also difficult to evaluate animal comfort and muscular contraction throughout a 24-hour period. We have developed a system and stimulation paradigm that can stimulate up to five animals at one time for one hour per day. The constant current stimulator is a USB-powered device that can, under computer control, output trains of pulses with selectable shapes, widths, durations and repetition rates. It is an external device with no implantable parts in the animal except for the stimulating electrodes. We tested the system on two groups of rats with denervated gastrocnemius muscles. One group was stimulated using a one-hour-per-day, 5-days-per-week stimulation paradigm for one month, while the other group had electrodes implanted but received no stimulation. Muscle weight and twitch force were significantly larger in the stimulated group than the non-stimulated group. Presently, we are using the stimulator to investigate electrical stimulation coupled with other therapeutic interventions that can minimize functional deficits after peripheral nerve injuries.

Sensory protection of rat muscle spindles following peripheral nerve injury and reinnervation

BACKGROUND

Skeletal muscle structure and function are dependent on intact innervation. Prolonged muscle denervation results in irreversible muscle fiber atrophy, connective tissue hyperplasia, and deterioration of muscle spindles, specialized sensory receptors necessary for proper skeletal muscle function. The protective effect of temporary sensory innervation on denervated muscle, before motor nerve repair, has been shown in the rat. Sensory-protected muscles exhibit less fiber atrophy and connective tissue hyperplasia and maintain greater functional capacity than denervated muscles. The purpose of this study was to determine whether temporary sensory innervation also protects muscle spindles from degeneration.

METHODS

Rat tibial nerve was transected and repaired with either the saphenous or the original transected nerve. Negative controls remained denervated. After 3 to 6 months, the electrophysiologic response of the nerve to stretch in the rat gastrocnemius muscle was measured (n = 3 per group). After the animals were euthanized, the gastrocnemius muscle was removed, sectioned, stained, and examined for spindle number (n = 3 per group) and morphology (one rat per group). Immunohistochemical assessment of muscle spindle innervation was examined in four additional animals.

RESULTS

Significant deterioration of muscle spindles was seen in denervated muscle, whereas in muscle reinnervated with the tibial or the saphenous nerve, spindle number and morphology were improved. Histologic and functional evidence of spindle reinnervation by the sensory nerve was obtained.

CONCLUSION

These findings add to the known means by which motor or sensory nerves exert protective effects on denervated muscle, and further promote the use of sensory protection for improving the outcome after peripheral nerve injury.