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Table of Contents
REVIEW ARTICLE
Year : 2020  |  Volume : 10  |  Issue : 2  |  Page : 21-26

New frontiers of nerve regeneration in ear, nose, and throat surgery


Core Surgical Training, University Hospitals Birmingham, England, UK

Date of Submission08-Dec-2020
Date of Acceptance12-Dec-2020
Date of Web Publication18-Feb-2021

Correspondence Address:
Sofia Anastasiadou
CT2 ENT, University Hospitals Birmingham, Birmingham, England,
UK
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Source of Support: None, Conflict of Interest: None


DOI: 10.4103/jlv.jlv_20_20

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   Abstract 


Nerve injury is common in various specialties. Apart from the traditional end-to-end direct repair in limited nerve injuries, there are many new techniques that are being investigated worldwide. At the moment, synthetic nerve grafts, called scaffolds, made by tissue-friendly materials and covered by different kinds of cells such as Schwann cells or stem cells are being tested. In order to maximize the tissue integration and accelerate the nerve regeneration, various nerve growth factors are being used as well with promising results. Otolaryngology patients will certainly benefit from most of these techniques as facial nerve, recurrent laryngeal nerve, ear hair cells, and other kinds of nerves could be generated using innovative treatments in the future. The aim of this article is to review the main advancements in ear, nose, and throat (ENT) nerve regeneration. A literature review was performed in PubMed and Medline databases using the following keywords: nerve regeneration, neuroregeneration, otolaryngology, ENT, head, and neck. A significant number of articles and reviews were found as well as trials on nerve regeneration in otolaryngology. Many different techniques are being investigated currently in order to regenerate damaged nerves. It is a very complex procedure as it requires axonal regrowth in the correct direction as well as sensory and motor function restoration to full. The gold standard nowadays remains the end-to-end primary repair; nevertheless, there are various other methods with very promising results.

Keywords: Nerve regeneration, reinnervation, neuroregeneration


How to cite this article:
Raja H, Anastasiadou S, Sultana A. New frontiers of nerve regeneration in ear, nose, and throat surgery. J Laryngol Voice 2020;10:21-6

How to cite this URL:
Raja H, Anastasiadou S, Sultana A. New frontiers of nerve regeneration in ear, nose, and throat surgery. J Laryngol Voice [serial online] 2020 [cited 2021 Jun 13];10:21-6. Available from: https://www.laryngologyandvoice.org/text.asp?2020/10/2/21/309676




   Introduction Top


Nerve injuries can be life-long noncurable traumatic procedures that severely impact patient's physical ability and quality of life. Peripheral nerve injury is very common reaching 360,000 cases every year in the USA, leaving thousands of people with permanent sensory or motor deficits.[1] However, peripheral nerve injury is not the only one that plays a dominant role in the neurological damage worldwide, but cranial nerve injury has also significant prevalence. It is true that the majority of nerve injuries can result in denervation of important muscles and organs leading to severe permanent disabilities. This is the point where nerve regeneration appears in order to restore neural function and prevent complete atrophy with subsequent immobility.

Undoubtedly, it is crucial to understand and translate the regrowth pathway after a nerve injury in order to simulate the same conditions to reinforce the procedure. Scientific research has been developing new ways to treat nerve injuries and effective methods to improve nerve regeneration in order to offer a viable solution to patients suffering from them.[2] However, human nervous system is a complex, functional network of neurons that form local and distal connections. Having explored this system to a certain extent, it has been found that the neuronal growth is facilitated by a variety of extracellular and intracellular signaling molecules.[3] Nevertheless, nature is not always capable to repair the damage, and therefore, nerve regeneration science has evolved as an answer to that human imperfection. This article summarizes the current and future practices that are being followed in nerve regeneration sector and analyses the essential factors that are required for techniques to be successful.

There are four major pillars when it comes to nerve regeneration science which lead to various other branches. These are the microsurgical repair, the different kinds of cells' transplantation, the material engineering, and gene therapies.[4]


   Nerve Regeneration Techniques Top


Microsurgery

Regarding microsurgery, the gold standard for nerve regeneration at the moment is end-to-end repair of the injured nerve. Neurorrhaphy is feasible when the gap between the two ends permits approach without tension. However, this is not an option in severe nerve injuries where the distal and proximal ends cannot be brought in contact and there is segmental loss of the nerve trunk. Autologous grafts are used when the gap is more than 3 cm which are usually obtained from another less functional part of the body.[5] Unfortunately, there are significant disadvantages such as increased donor site morbidity, limited donor nerves, and mismatch between the two different kinds of nerves. In an attempt to override the above, allografts from cadavers have been used that are cellular or acellular, however, they often raised immune system responses with frequent rejections.[1] It is evident that all the existing solutions using either autologous grafts or allografts are linked to several complications and increased morbidity. Consequently, innovation and pioneer practices have emerged as a response to the insufficiency of traditional nerve injury treatments in order to improve nerve regeneration.

Material engineering

After exploring the microsurgical options to restore a nerve, it is interesting to introduce the three pillars left which are material engineering, cells, and gene therapies. Nowadays, research has been focusing in analyzing the neurobiology behind nerve regeneration which involves precisely observing nerve injury in various environments. Studies are being conducted mostly on animal models trying to investigate and evaluate normal nerve regeneration process. However, recently there is an outburst of different kinds of studies which focus on controlled cellular injury in vitro models.[6] These innovative research patterns enable the monitoring of a real-time injury response and permit simulation of the factors, chemical and nonchemical, that are being released in that very moment of the injury.[7] As a result, new horizons are emerging to improve surgical, biomaterial, or scaffold-based therapies, using fresh valuable information.

New frontiers of nerve regeneration involve cellular and acellular conduits to provide a skeleton to support nerve repair. These conduits are tube-like structures, called scaffolds, that provide an environment which enhances nerve repair.[5] Throughout literature, polyglycolic acid scaffolds are the most popular. They consist of woven fibers that form a porous nerve guide[8] which enables the injured nerve to grow toward the correct direction in order to cover the gap between the two ends. At the minute, there are countless synthetic nerve scaffolds that are being investigated focusing on the creation of a friendlier environment with less inflammation and less scar tissue formation.

Apart from the correct direction for nerve outgrowth, scaffolds need to comply with a number of other important criteria. For example, the need to be biocompatible and tissue-friendly must not be ignored as well as the fact that they must permit cell colonization and proliferation in the environment that they create.[9] More precisely, these biomaterial conduits need to offer the best circumstances for a nerve to regrow which means that they need to comply with blood components, cells, neurotrophins, synthetic matrixes, and various other factors to enhance the procedure. In addition, scaffold shape and elasticity are crucial as well as the porosity (size of pores in the material) which can be either cell development enhancing or destructive. To summarize, the challenges to develop a scaffold with all the above qualities are immense and various studies are being conducted on animal models to produce the best quality conduit.

Cell transplantation

In addition to the correct direction for nerve regeneration, neural conduits can be used as a matrix colonized by myriad cells or growth factors to establish a successful, faster, and more effective nerve outgrowth. In terms of cells that help mimicking the natural nerve regeneration procedures, these can be either Schwann cells (SCs), olfactory ensheathing cells (OECs), and stem cells or combinations of the above.[1]

Schwann cells

SCs are normally found in the myelin sheath that covers the peripheral nerve system. These cells are major facilitators of nerve regeneration, and they have been found to migrate in injured nerves' environment to enhance the recovery process.[10] However, formation of schwannomas in regions where SCs were transplanted previously and neuropathic pain have been significant disadvantages in patients who tried the technique.[11],[12]

Olfactory cells

OECs can only be found in the olfactory system, and they are also involved in natural nerve regeneration.[13] In fact, they can be very easily obtained from the nasal septum of the patient gaining the advantage to be autologous avoiding immune response and rejection. Furthermore, OECs provide a very good tissue integration which enables them to enhance the nerve function in many cases and to be extremely valuable in nerve regeneration process.[14] Despite the several OEC advantages, until recently, they have been always a complementary therapy and required adjuvant treatment before their application.[15]

Stem cells

Last major cell entity that is currently under research are stem cells (CS), either adult or embryonic. These kinds of cells are highly favored because of their regenerative and vastly expandable nature. To support that argument, several studies were conducted in which CS have been differentiated in neurons and were transplanted in injured nerve sights. Recently, CS that were differentiated in oligodendrocyte progenitors were used to treat nerve injury in animal models by Asterias Biotherapeutics, and clinical trial results are not published yet.[16] Despite the very promising capabilities of stem cells, ethical issues are prominent and are not allowing further experiments to investigate more. In addition to that, immunogenicity and tumorigenicity are also crucial issues that need addressing before trials progress to human participants.[1]

Gene therapies

Proteins

Apart from cells, other factors that facilitate the scaffold integration and therefore nerve regeneration are extracellular matrix proteins and growth factors. The first are peptides that can prolong transplanted cell life expectancy and accelerate cell differentiation. Many different kinds of proteins have been used to enhance nerve regeneration such as laminin, fibrin, and fibronectin that can be combined with collagen to reinforce new forming nerve axons.[17] Some of them are directly injected into the damaged nerve area, while others are expressed by cells that are being transplanted in the same region with significant positive impact. On the other hand, growth factors must be very wisely used as they require certain quantities and distribution into scaffolds. As shown by Wang et al., growth factors in specific concentrations enhance nerve outgrowth, however, overexpression of growth factors leads to reduction of nerve regeneration speed or even destruction of the process.[18],[19] In addition, as proven by a research field called surface topographic guidance, it is important that growth factors are distributed in a linear way with certain orientation into the scaffold to enable the nerve axon to grow in the correct direction.[18]

Thyroid and erythropoietin hormones

There are also several natural factors that have been investigated in terms of facilitating nerve regeneration. One of them is the thyroid hormone which has played a critical role in research fields lately. Barakat-Walter and Kraftsik have been trialing thyroxine to stimulate nerve outgrowth in various environments. In fact, it seems that it is not a direct factor of growth acceleration or a neurotrophin itself, however, it aids the nerve regeneration by increasing the function of SCs. It is also found that in hypothyroid animals, nerve regeneration was eliminated. The hormone has been tried using everyday injections, however, currently, it is expressed in cells that fill the various nerve conduits and scaffolds in an attempt to create a better environment for nerve regeneration.[20] Another choice, apart from thyroxin, is erythropoietin. It has been used for years to treat anemia, but the approval for that opened new horizons in treatment of nerve injury. Indeed, erythropoietin has been the main candidate for clinical trials about neuroprotection and nerve regeneration.[21] As it crosses the blood–brain barrier, it is a valuable factor for brain diseases, nerve degeneration pathologies, and traumatic injuries. In addition, hydrogen sulfide is another factor that facilitates nerve repair and has been trialed in animals already with successful results.[22] Matrix metalloproteinases are also another variety of factors which are developmentally regulated and participate in neuronal migration as well as myelination. These enzymes are able to accelerate nerve growth and have been used in various pathologies.[23]

Need for different approaches

Given the enormous heterogeneity of nerve injury, there are various other methods of repairing it that are being explored currently. Noninvasive brain stimulation using electrodes has been found to improve the damaged nerve function. Based on individual electromyograms, high- or low-frequency stimulant signals are training the nerve to reproduce the same signal as the stimulant. In this way, even without actual nerve regrowth, there is functional result which can be optimized with physiotherapy. This technique has been used in various contexts such as pain, spasticity, vision, and space acknowledgment even mood and psychological disorders.[22]


   Nerve regeneration in otolaryngology Top


Auditory system

In terms of nerve regeneration in ear, nose, and throat science, one of the main domains that research has been focusing is restoring the damaged structures of the auditory system. Cochlear implantation is one of the dominant strategies, however, hair cell regeneration and reconstruction have been investigated. This idea started back in the past when birds were found able to regenerate their auditory system, however, it is still in its infancy and has not been trialed in humans yet.[24] Since then, there has been significant progress in hair cell regeneration using growth factors, stem cells, and gene therapy. Furthermore, Kawamoto et al. have described injecting specific viruses that express nerve growth factors to facilitate nerve regeneration in guinea pigs.[25] Another study conducted by Lefebvre et al. showed that regeneration of hair cells is possible in vivo and in vitro in animal models and can work in favor of inner ear function.[26] To conclude, it is evident that tissue engineering and nerve regeneration are in their infancy regarding hair cells while cochlear implantation with hearing aids is the dominant treatment choice. However, as the current practice has certain disadvantages due to cases that are not compliant, the field is open for research to improve efficacy of the treatment options.[25]

Facial nerve

Facial nerve damage is very common in otolaryngology due to its anatomical characteristics. Because of its specific nature with sensory, motor, and parasympathetic fibers, its restoration is extremely challenging and various ways have been investigated. Until recently, there are limited approaches that are being used: direct repair and nerve grafts with end-to-end anastomosis. The option of vascularized nerve grafts has been explored in the literature regarding facial nerve regrowth. Villarreal et al. mention a significant positive impact on facial nerve regeneration when reconstructing with anterolateral thigh and vascularized nerve grafts after extended parotidectomy.[27] In addition to microvascular free tissue transplantation, regional muscle transfers have been studied by Magill CK et al. with preliminary results.[28] Furthermore, platysma transfer for eye closure has been proven to be the most efficacious technique in longstanding facial nerve paralysis.[29]

Nevertheless, there is ongoing research using stem cells, nerve growth factors, scaffolds, and SCs in order to support facial nerve regeneration with various results.[30] For example, there is a specific hormone, the pituitary adenylate cyclase-activating polypeptide that has been found to be a strong neurotrophic factor by reinforcing the action of other significant growth molecules such as glial cell line-derived neurotrophic factor. Their function is summarized in activation of mitotic procedures that lead to accelerated cell proliferation.[31] The pituitary adenylate cyclase-activating polypeptide has been used in various contexts, and it is proven to increase the myelination of the regenerated axons of the facial nerve. This enables the faster and more effective facial nerve recovery.[32]

However, nerve growth factors are not a monotherapy for facial nerve injury. There are several mechanical studies such as eye blink implantable devices developed by Ripple et al. that offer satisfactory eye blink and moisturization of the conjunctiva.[29] Furthermore, in unilateral facial nerve paralysis, neural signs from the healthy side can be extracted to treat the paralyzed side. Various kinds of electrodes have been also used in order to stimulate the paralyzed facial nerve to restore its function to a certain extent.[33] Last but not least, artificial muscles have been tried with limited results and exclusively in animal models by Langhals et al., while research is still ongoing.[33]

To summarize, all the above-named techniques must be carefully analyzed according to the specific needs of each patient. This involves a thorough analysis of the etiology and background of the nerve injury and a comprehensive comparison between the various different techniques, in order to conclude to the most appropriate one.

Accessory nerve

In terms of shoulder dysfunction due to accessory nerve injury after neck dissections, various techniques have been tried to accelerate nerve recovery and improve muscle function. This has been very challenging as nerve regeneration is not evolved in the field and there are limited donors for this area.[34] Until recently, traditional nerve autologous grafts have been tried in order to achieve satisfactory results such as nerve fascicles from C7 nerve[34] or the obturator nerve itself.[28] In case of nerve damage intraoperatively, Barber et al. have looked into brief intraoperative nerve stimulation, and they found that electrical stimuli aid in the nerve regeneration process and result in better postoperative recovery.[35] To conclude, it is evident that further effort is required in order to find innovative ways to treat accessory nerve injuries and improve the quality of life and motor capacity of patients that suffered them.

Recurrent laryngeal nerve

In terms of recurrent laryngeal nerve injuries, this is another sector with vast interest in otolaryngology nerve regeneration. Until recently, vocal cord injections and medialization techniques as well as speech and language therapies have been dominant in treating complications of laryngeal nerve injury. However, as our understanding of nerve regeneration evolves, it is becoming evident that gene therapies and tissue engineering can be combined to treat these patients.[36] In animal models only, Wang et al. have managed to prove that neurotrophic factors' expression in the thyroarytenoid and cricoarytenoid muscles increases reinnervation and recurrent laryngeal nerve recovery.[37] In addition to that, Kaneko et al. showed very recently that significantly more neuromuscular junctions are formed in mice who had recurrent laryngeal nerve transection and were injected with growth factors compared to the control group.[38] Finally, gene studies by Ren and Xu also proved that in adult rats, neural cell proliferation was enhanced with the expression of the polysialic acid neural cell adhesion molecule in the area of the injury.[39] The above research results imply that there is an abundance of growth factor injections that can be used to treat recurrent laryngeal nerve injuries in humans and ensure normal muscle function and phonation.


   Conclusion Top


Nerve tissue regeneration has been rigorously investigated. Extensive studies and research projects are ongoing to improve nerve grow and subsequently muscle function. It is extremely important to clarify that patients that suffered a nerve injury are complex systemically and psychologically and thorough care is required to aim for recovery. However, results are optimistic and medical research is targeting excellent and fully developed techniques in order to improve the life quality of these patients.

Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.



 
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