Systemic local-anaesthetic-type drugs in chronic pain: a systematic review

Intravenous Lidocaine in Chronic Neuropathic Pain: A Systematic Review

OBJECTIVES

A systematic review of original research articles was conducted to evaluate the safety and efficacy of lidocaine infusion in the treatment of adult patients with chronic neuropathic pain.

MATERIALS AND METHODS

Original research from 1970 to September 2021 describing adult patients with chronic neuropathic pain receiving at least 1 dose of intravenous lidocaine was included. Extracted data included study design, sample size, patient demographics and comorbidities, etiology and duration of pain, pain intensity scores, time to pain resolution, lidocaine dose and administration frequency, lidocaine serum concentration, and adverse events. Each study was evaluated for level of evidence using the 2017 American Association of Neurology classification system.

RESULTS

Twenty-seven studies evaluating lidocaine infusion treatment in chronic neuropathic pain met inclusion criteria. One class I study was identified for patients with neuropathic pain due to spinal cord injury . Two Class II studies were identified, one describing neuropathic pain due to peripheral nerve injury and another due to diabetic neuropathy. Across all studies, study design, participants, and experimental interventions were heterogenous with wide variation.

DISCUSSION

This qualitative review found insufficient, heterogenous evidence and therefore no recommendation can be made for lidocaine infusion treatment in patients with chronic neuropathic pain due to spinal cord injury, peripheral nerve injury, diabetic neuropathy, postherpetic neuralgia, or complex regional pain syndrome type II. Larger randomized, double-blind, placebo-controlled studies are required to further establish the efficacy of lidocaine infusion in patients with these etiologies of chronic neuropathic pain.

Calixmexitil: Calixarene-based Cluster of Mexiletine with Amplified Anti-myotonic Activity as A Novel Use-dependent Sodium Channel Blocker

Mexiletine as the first choice drug in myotonia treatment is a chiral sodium channel blocker clinically used in its racemic form. The phenolic structure of this drug, prompted us to design its novel calix[4]arene-based cluster in a chalice-shaped structure. Therefore, the present study reports the synthesis and in-vitro anti-myotonic activity of the chalice-shaped cluster of mexiletine (namely calixmexitil) in comparison to its simple drug unit (mexitil) as the reference medication. The synthetic route included chemical modification of the calix[4]arene structure by grafting four 2-aminopropoxy moieties at the lower rim of the scaffold. Electrophysiological tests were performed for the determination of test compounds abilities to act as sodium channel blockers in inhibiting sodium currents (in use-dependent manner) in single skeletal muscle fibers. The experimental results showed an amplified (10-fold) potency in producing phasic block as an indication of the anti-myotonic activity and improved (3-fold) potency in producing use-dependent block for the cluster (calixmexitil) in relation to its monomer (mexiletine). The potency in producing phasic block and use-dependent block are two main factors to describe dose range, drug affinity, and side effects of an anti-myotonic agent. Therefore, compared to mexiletine, calixmexitil with these improved factors can be considered as a “selective” anti-myotonic agent with low dose range. These improved pharmaceutical effects are maybe attributed to clustering effect and improved interaction of four impacted mexiletine units of the cluster with the sodium channels’ structure in skeletal muscle fibers.

Inhibition of Fast Nerve Conduction Produced by Analgesics and Analgesic Adjuvants-Possible Involvement in Pain Alleviation

Nociceptive information is transmitted from the periphery to the cerebral cortex mainly by action potential (AP) conduction in nerve fibers and chemical transmission at synapses. Although this nociceptive transmission is largely inhibited at synapses by analgesics and their adjuvants, it is possible that the antinociceptive drugs inhibit nerve AP conduction, contributing to their antinociceptive effects. Many of the drugs are reported to inhibit the nerve conduction of AP and voltage-gated Na⁺ and K⁺ channels involved in its production. Compound action potential (CAP) is a useful measure to know whether drugs act on nerve AP conduction. Clinically-used analgesics and analgesic adjuvants (opioids, non-steroidal anti-inflammatory drugs, ₂-adrenoceptor agonists, antiepileptics, antidepressants and local anesthetics) were found to inhibit fast-conducting CAPs recorded from the frog sciatic nerve by using the air-gap method. Similar actions were produced by antinociceptive plant-derived chemicals. Their inhibitory actions depended on the concentrations and chemical structures of the drugs. This review article will mention the inhibitory actions of the antinociceptive compounds on CAPs in frog and mammalian peripheral (particularly, sciatic) nerves and on voltage-gated Na⁺ and K⁺ channels involved in AP production. Nerve AP conduction inhibition produced by analgesics and analgesic adjuvants is suggested to contribute to at least a part of their antinociceptive effects.

Conotoxins That Could Provide Analgesia through Voltage Gated Sodium Channel Inhibition

Chronic pain creates a large socio-economic burden around the world. It is physically and mentally debilitating, and many sufferers are unresponsive to current therapeutics. Many drugs that provide pain relief have adverse side effects and addiction liabilities. Therefore, a great need has risen for alternative treatment strategies. One rich source of potential analgesic compounds that has emerged over the past few decades are conotoxins. These toxins are extremely diverse and display selective activity at ion channels. Voltage gated sodium (Na_V) channels are one such group of ion channels that play a significant role in multiple pain pathways. This review will explore the literature around conotoxins that bind Na_V channels and determine their analgesic potential.

p.L1612P, a Novel Voltage-gated Sodium Channel Nav1.7 Mutation Inducing a Cold Sensitive Paroxysmal Extreme Pain Disorder

Background:

Mutations in the SCN9A gene cause chronic pain and pain insensitivity syndromes. We aimed to study clinical, genetic, and electrophysiological features of paroxysmal extreme pain disorder (PEPD) caused by a novel SCN9A mutation.

Methods:

Description of a 4-generation family suffering from PEPD with clinical, genetic and electrophysiological studies including patch clamp experiments assessing response to drug and temperature.

Results:

The family was clinically comparable to those reported previously with the exception of a favorable effect of cold exposure and a lack of drug efficacy including with carbamazepine, a proposed treatment for PEPD. A novel p.L1612P mutation in the Nav1.7 voltage-gated sodium channel was found in the four affected family members tested. Electrophysiologically the mutation substantially depolarized the steady–state inactivation curve (V1/2 from −61.8 ± 4.5 mV to −30.9 ± 2.2 mV, n = 4 and 7, P < 0.001), significantly increased ramp current (from 1.8% to 3.4%, n = 10 and 12) and shortened recovery from inactivation (from 7.2 ± 5.6 ms to 2.2 ± 1.5 ms, n = 11 and 10). However, there was no persistent current. Cold exposure reduced peak current and prolonged recovery from inactivation in wild-type and mutated channels. Amitriptyline only slightly corrected the steady–state inactivation shift of the mutated channel, which is consistent with the lack of clinical benefit.

Conclusions:

The novel p.L1612P Nav1.7 mutation expands the PEPD spectrum with a unique combination of clinical symptoms and electrophysiological properties. Symptoms are partially responsive to temperature but not to drug therapy. In vitro trials of sodium channel blockers or temperature dependence might help predict treatment efficacy in PEPD.

Effects of lidocaine administration via continuous rate infusion on the minimum alveolar concentration of isoflurane in New Zealand White rabbits (Oryctolagus cuniculus)

OBJECTIVE

To evaluate the effect of a continuous rate infusion (CRI) of lidocaine on the minimum alveolar concentration (MAC) of isoflurane in rabbits.

ANIMALS

Five 12-month-old female New Zealand White rabbits (Oryctolagus cuniculus).

PROCEDURES

Rabbits were anesthetized with isoflurane. Baseline isoflurane MAC was determined by use of the tail clamp technique. A loading dose of lidocaine (2.0 mg/kg, IV) was administered followed by a CRI of lidocaine at 50 μg/kg/min. After 30 minutes, isoflurane MAC was determined. Another loading dose was administered, and the lidocaine CRI then was increased to 100 μg/kg/min. After 30 minutes, isoflurane MAC was determined again. Plasma samples were obtained for lidocaine analysis after each MAC determination.

RESULTS

Baseline isoflurane MAC was 2.09%, which was similar to previously reported values in this species. Lidocaine CRI at 50 and 100 μg/kg/min induced significant reductions in MAC. The 50 μg/kg/min CRI resulted in a mean plasma lidocaine concentration of 0.654 μg/mL and reduction of MAC by 10.5%. The 100 μg/kg/min CRI of lidocaine resulted in a mean plasma concentration of 1.578 μg/mL and reduction of MAC by 21.7%. Lidocaine also induced significant decreases in arterial blood pressure and heart rate. All cardiopulmonary variables were within reference ranges for rabbits anesthetized with inhalation anesthetics. No adverse effects were detected; all rabbits had an uncomplicated recovery from anesthesia.

CONCLUSIONS AND CLINICAL RELEVANCE

Lidocaine administered as a CRI at 50 and 100 μg/kg/min decreased isoflurane MAC in rabbits. The IV administration of lidocaine may be a useful adjunct in anesthesia of rabbits.

Effects of various antiepileptics used to alleviate neuropathic pain on compound action potential in frog sciatic nerves: comparison with those of local anesthetics

Antiepileptics used for treating neuropathic pain have various actions including voltage-gated Na(+) and Ca(2+) channels, glutamate-receptor inhibition, and GABA(A)-receptor activation, while local anesthetics are also used to alleviate the pain. It has not been fully examined yet how nerve conduction inhibitions by local anesthetics differ in extent from those by antiepileptics. Fast-conducting compound action potentials (CAPs) were recorded from frog sciatic nerve fibers by using the air-gap method. Antiepileptics (lamotrigine and carbamazepine) concentration dependently reduced the peak amplitude of the CAP (IC50 = 0.44 and 0.50 mM, resp.). Carbamazepine analog oxcarbazepine exhibited an inhibition smaller than that of carbamazepine. Antiepileptic phenytoin (0.1 mM) reduced CAP amplitude by 15%. On the other hand, other antiepileptics (gabapentin, sodium valproate, and topiramate) at 10 mM had no effect on CAPs. The CAPs were inhibited by local anesthetic levobupivacaine (IC50 = 0.23 mM). These results indicate that there is a difference in the extent of nerve conduction inhibition among antiepileptics and that some antiepileptics inhibit nerve conduction with an efficacy similar to that of levobupivacaine or to those of other local anesthetics (lidocaine, ropivacaine, and cocaine) as reported previously. This may serve to know a contribution of nerve conduction inhibition in the antinociception by antiepileptics.

[Chronic pain and regional anesthesia in children]

Chronic pain is usually underestimated in children, due to lack of knowledge and its specific signs. In addition to suffering, chronic pain causes a physical, psychological, emotional, social, and financial burden for the child and his family. Practitioners may find themselves in a situation of failure with depletion of medical resources. Some types of chronic pain are refractory to conventional systemic treatment and may require the use of regional anesthesia. Cancer pain is common in children and its medical management is sometimes insufficient. It is accessible to neuroaxial or peripheral techniques of regional anesthesia if it is limited to an area accessible to one of these techniques and no contraindications (e.g., thrombopenia) are present. Complex regional pain syndrome 1 is not rare in children and adolescents, but it often goes undiagnosed. Regional anesthesia may contribute to the treatment of complex regional pain syndrome 1, mainly in case of recurrence, because it provides rapid effective analgesia and allows rapid implementation of intensive physiotherapy. These techniques have also shown interest in phantom limb pain after limb amputation, but they remain controversial for erythromelalgia pain or chronic abdominopelvic pain. Finally, the treatment of postdural puncture headache due to cerebrospinal fluid leak can be treated by performing an epidural injection of the patient's blood, called a blood-patch. Finally, the management of children with chronic pain should be multidisciplinary (pediatrician, physiotherapist, psychologist, surgeon, anesthesiologist) to support the child and her problem in its entirety.

Pain management in burn patients

BACKGROUND AND OBJECTIVES

Despite advances, inappropriate analgesic treatment for burn patients is still seen. The objective of this review was to collect data on pain management in burn patients.

CONTENT

We reviewed the mechanisms of pain, burn patient assessment, as well as pharmacological and non-pharmacological treatment.

CONCLUSION

Pain management in burn patients is still a challenge for the multidisciplinary team. Frequent and continuous evaluation of the patient's response is very important due to the various stages that the hospitalized burn patient goes through, as well as a combination therapy with analgesic and non-pharmacological measures. Understanding the complexity of the pathophysiological, psychological, and biochemical changes a burn patient presents is the first step to achieve success in analgesic management.

Evidence-based guideline for neuropathic pain interventional treatments: spinal cord stimulation, intravenous infusions, epidural injections and nerve blocks

BACKGROUND

The Special Interest Group of the Canadian Pain Society has produced consensus-based guidelines for the pharmacological management of neuropathic pain. The society aimed to generate an additional guideline for other forms of neuropathic pain treatments.

OBJECTIVE

To develop evidence-based recommendations for neuropathic pain interventional treatments.

METHODS

A task force was created and engaged the Institute of Health Economics in Edmonton, Alberta, to survey the literature pertaining to multiple treatments. Sufficient literature existed on four interventions only: spinal cord stimulation; epidural injections; intravenous infusions; and nerve blocks. A comprehensive search was conducted for systematic reviews, randomized controlled trials and evidence-based clinical practice guidelines; a critical review was generated on each topic. A modified United States Preventive Services Task Force tool was used for quality rating and grading of recommendations.

RESULTS

Investigators reviewed four studies of spinal cord stimulation, 19 studies of intravenous infusions, 14 studies of epidural injections and 16 studies of nerve blocks that met the inclusion criteria. The task force chairs rated the quality of evidence and graded the recommendations. Feedback was solicited from the members of the task force.

CONCLUSION

There is sufficient evidence to support recommendations for some of these interventions for selected neuropathic pain conditions. This evidence is, at best, moderate and is often limited or conflicting. Pain practitioners are encouraged to explore evidence-based treatment options before considering unproven treatments. Full disclosure of risks and benefits of the available options is necessary for shared decision making and informed consent.

Asymmetric hydrolytic kinetic resolution with recyclable macrocyclic Co(III)-salen complexes: a practical strategy in the preparation of (R)-mexiletine and (S)-propranolol

A chiral cobalt(III) complex (1e) was synthesized by the interaction of cobalt(II) acetate and ferrocenium hexafluorophosphate with a chiral dinuclear macrocyclic salen ligand that was derived from 1R,2R-(-)-1,2-diaminocyclohexane with trigol bis-aldehyde. A variety of epoxides and glycidyl ethers were suitable substrates for the reaction with water in the presence of chiral macrocyclic salen complex 1e at room temperature to afford chiral epoxides and diols by hydrolytic kinetic resolution (HKR). Excellent yields (47% with respect to the epoxides, 53% with respect to the diols) and high enantioselectivity (ee>99% for the epoxides, up to 96% for the diols) were achieved in 2.5-16 h. The Co(III) macrocyclic salen complex (1e) maintained its performance on a multigram scale and was expediently recycled a number of times. We further extended our study of chiral epoxides that were synthesized by using HKR to the synthesis of chiral drug molecules (R)-mexiletine and (S)-propranolol.

Neuropathic pain as a process: reversal of chronification in an animal model

Peripheral neuropathic pain arises from trauma to sensory nerves. Other types of acute neurotrauma such as stroke and spinal cord injury are treated immediately, largely to prevent secondary damage. To pursue the possibility that neuropathic pain may also be amenable to early treatment, a rat model of neuropathic pain was induced using a 2-mm polyethylene cuff implanted around one sciatic nerve. Within 24 hours, hypersensitivity to von Frey hair stimulation appeared, as indicated by decreased paw withdrawal thresholds. When the cuff was removed 24 hours after implantation, readings returned to pre-implantation levels starting as early as day 18. When the cuff was removed after 4 days, there was a period of initial hypersensitivity, and then an increase toward baseline at two time points near the end of the study; therefore, only a partial recovery toward pre-implantation values occurred. Having established that a temporal reversal can occur, the next step examined possible pharmacological reversal. The tachykinin NK₁ receptor antagonist, CP-96,345, produced a minor increase in withdrawal thresholds in animals with the cuff left permanently implanted. To determine the effect of early and repeated administration of CP-96,345, it was given daily on days 1–4. The cuff was removed on day 4. Six days later, readings showed reversal of tactile hypersensitivity. We suggest that persistent neuropathic pain occurs from processes that develop over several hours and days, and that some of these processes may be prevented by early medical intervention. Thus, nerve injury in the context of chronic neuropathic pain should be treated in a similar manner to nerve injury resulting from stroke, spinal cord injury, and other types of neurotrauma. We suggest that effective medical intervention within the first few hours after nerve injury may spare a patient from a chronic debilitating pain that may be refractory to later therapies.

Management of pain in children with burns

Burn injuries are common in children under 10 years of age. Thermal injury is the most common mechanism of injury and scalds account for >60% of such injuries. All children with burns will experience pain, regardless of the cause, size, or burn depth. Undertreated pain can result in noncompliance with treatment and, consequently, prolonged healing. It is acknowledged that the monitoring and reporting of pain in children with burns has generally been poor. Due to the adverse physiological and emotional effects secondary to pain, adequate pain control is an integral and requisite component in the management of children with burns. A multidisciplinary approach is frequently necessary to achieve a robust pain relief. Key to successful treatment is the continuous and accurate assessment of pain and the response to therapy. This clinical review article discusses the essential aspects of the pathophysiology of burns in children provides an overview of pain assessment, the salient principles in managing pain, and the essential pharmacodynamics of commonly used drugs in children with burn injuries. Both pharmacological and nonpharmacological treatment options are discussed, although a detailed review of the latter is beyond the scope and remit of this article.

Effect of systemic lidocaine on visceral and somatic nociception in conscious horses

REASONS FOR PERFORMING STUDY

Commonly used analgesics (nonsteroidal anti-inflammatory agents, opioids and alpha2-agonists) have unwanted side effects. An effective alternative with minimal adverse effects would benefit clinical equine pain management.

OBJECTIVES

To compare the effect of lidocaine or saline on duodenal and rectal distension threshold pressure and somatic thermal threshold in conscious mature horses.

HYPOTHESIS

Systemically administered lidocaine would increase somatic and visceral nociceptive thresholds.

METHODS

Lidocaine (2 mg/kg bwt bolus followed by 50 microg/kg bwt/min for 2 h) or saline was administered to 6 horses each carrying a permanently implanted gastric cannula, in a randomised, blinded cross-over design. Thermal threshold was measured using a probe containing a heater element placed over the withers which supplied heat until the horse responded. A barostatically controlled intraduodenal balloon was distended until a discomfort response was obtained. A rectal balloon was inflated until extruded or signs of discomfort noted.

RESULTS

Thermal threshold was increased significantly 30 and 90 mins after the start of lidocaine infusion. There was no change in duodenal distension pressure and a small but clinically insignificant change in colorectal distension pressure in the lidocaine group.

CONCLUSIONS

At the dose used, systemically administered lidocaine produced thermal antinociception but minimal changes in visceral nociception.

POTENTIAL RELEVANCE

At these doses, lidocaine may play a role in somatic analgesia in horses.

Deracemization of mexiletine biocatalyzed by omega-transaminases

(S)- as well as (R)-mexiletine [1-(2,6-dimethylphenoxy)-2-propanamine], a chiral orally effective antiarrhythmic agent, was prepared by deracemization starting from the commercially available racemic amine using omega-transaminases in up to >99% ee and conversion with 97% isolated yield by a one-pot two-step procedure. The absolute configuration could be easily switched to the other enantiomer, just by switching the order of the applied transaminases. The cosubstrate pyruvate needed in the first oxidative step was recycled by using an amino acid oxidase.

Governing role of primary afferent drive in increased excitation of spinal nociceptive neurons in a model of sciatic neuropathy

Previously we reported that the cuff model of peripheral neuropathy, in which a 2 mm polyethylene tube is implanted around the sciatic nerve, exhibits aspects of neuropathic pain behavior in rats similar to those in humans and causes robust hyperexcitation of spinal nociceptive dorsal horn neurons. The mechanisms mediating this increased excitation are not known and remain a key unresolved question in models of peripheral neuropathy. In anesthetized adult male Sprague-Dawley rats 2-6 weeks after cuff implantation we found that elevated discharge rate of single lumbar (L(3-4)) wide dynamic range (WDR) neurons persists despite acute spinal transection (T9) but is reversed by local conduction block of the cuff-implanted sciatic nerve; lidocaine applied distal to the cuff (i.e. between the cuff and the cutaneous receptive field) decreased spontaneous baseline discharge of WDR dorsal horn neurons approximately 40% (n=18) and when applied subsequently proximal to the cuff, i.e. between the cuff and the spinal cord, it further reduced spontaneous discharge by approximately 60% (n=19; P<0.05 proximal vs. distal) to a level that was not significantly different from that of naive rats. Furthermore, in cuff-implanted rats WDR neurons (n=5) responded to mechanical cutaneous stimulation with an exaggerated afterdischarge which was reversed entirely by proximal nerve conduction block. These results demonstrate that the hyperexcited state of spinal dorsal horn neurons observed in this model of peripheral neuropathy is not maintained by tonic descending facilitatory mechanisms. Rather, on-going afferent discharges originating from the sciatic nerve distal to, at, and proximal to the cuff maintain the synaptically-mediated gain in discharge of spinal dorsal horn WDR neurons and hyperresponsiveness of these neurons to cutaneous stimulation. Our findings reveal that ectopic afferent activity from multiple regions along peripheral nerves may drive CNS changes and the symptoms of pain associated with peripheral neuropathy.

Article Commentary: Pain after Mastectomy and Breast Reconstruction

Breast cancer is a potentially deadly disease affecting one in eight women. With the trend toward minimally invasive therapies for breast cancer, such as breast conserving therapies, sentinel node biopsies, and early treatments of radiation and chemotherapy, life expectancy after breast cancer has increased. However, pain after breast cancer surgery is a major problem and women undergoing mastectomy and breast reconstruction experience postoperative pain syndromes in approximately one-half of all cases. Patients post mastectomy and breast reconstruction can suffer from acute nociceptive pain and chronic neuropathic pain syndromes. Several preventative measures to control acute post operative pain and chronic pain states such as post mastectomy pain and phantom pain have been tried. This review focuses on the recent research done to control acute and chronic pain in patients receiving minimally invasive therapies for breast cancer, such as breast conserving therapies of mastectomies and breast reconstruction, sentinel node biopsies, and early treatments of radiation and chemotherapy.

Benzazepinone Nav1.7 blockers: potential treatments for neuropathic pain

A series of benzazepinones were synthesized and evaluated as hNa(v)1.7 sodium channel blockers. Several compounds from this series displayed good oral bioavailability and exposure and were efficacious in a rat model of neuropathic pain.

Intravenous Lidocaine: An Outdated or Underutilized Treatment for Pain?

BACKGROUND

Conventional analgesic treatment involves the use of oral and transdermal formulations of drugs that require repetitive administration for sustained pain relief to be achieved. Along with the potential of analgesia, the risk of ongoing side effects consequent on the use of these analgesics also exists and this may have a detrimental effect on the patient's quality of life. In contrast, an intriguing body of evidence suggests that short-term administration of intravenous lidocaine may produce pain relief that far exceeds both the duration of infusion and the half-life of the drug. When pain relief is produced, concomitant analgesic medication can be reduced, side effects from pain relieving medication minimized with a potential for very real improvement in the quality of life of the patient.

OBJECTIVE

To ascertain whether literature evidence supports the use of intravenous lidocaine in clinical practice.

DESIGN

A review of the currently available published evidence.

RESULTS

A reasonable body of evidence, along with extensive clinical experience, suggests that intravenous lidocaine can have a useful pain-relieving effect and is worth consideration in palliative care patients.

CONCLUSION

While this form of therapy is not commonplace in the terminally ill patient, it could be argued that its use has much merit in that field and should be considered.

Relationship Between the Firing Frequency of Injured Peripheral Neurons and Inhibition of Firing by Sodium Channel Blockers

Animal models of neuropathic pain in which a peripheral nerve is damaged result in spontaneous activity in primary afferents that can be inhibited by intravenous administration of sodium channel blockers. Many of these compounds exhibit use-dependent block of sodium current, leading to the prediction that they should more readily inhibit neurons that fire at higher frequencies. This prediction was tested in 2 rat models of nerve injury, L5 spinal nerve section and sciatic nerve section. Sciatic nerve section produced average firing frequencies that were higher than spinal nerve section and often manifested as high-frequency bursting. Inhibition of firing by intravenous sodium channel blockers was longer lasting in this model. Within each model, higher frequency of firing did not translate into more effective block. In the spinal nerve section model, there was a robust inverse correlation between frequency and inhibition. Within the sciatic section model, only neurons that fired in rhythmic bursts were inhibited, and again, those firing at lower mean frequencies were more effectively inhibited. These results indicate that the efficacy of sodium channel blockers depends on the nature of the injury and the pattern of the resulting activity rather than simply the frequency of action potentials generated.

PERSPECTIVE

This study examines the ability of frequency-dependent sodium channel blockers to inhibit spontaneous firing of injured peripheral nerves in vivo. It outlines the conditions under which inhibition is more and less effective and will provide insight into conditions under which sodium channel blockers are likely to be therapeutically useful.

Blocking sodium channels to treat neuropathic pain

Neuropathic pain remains a large unmet medical need. A number of therapeutic options exist, but efficacy and tolerability are less than satisfactory. Based on animal models and limited data from human patients, the pain and hypersensitivity that characterize neuropathic pain are associated with spontaneous discharges of normally quiescent nociceptors. Sodium channel blockers inhibit this spontaneous activity, reverse nerve injury-induced pain behavior in animals and alleviate neuropathic pain in humans. Several sodium channel subtypes are expressed primarily in sensory neurons and may contribute to the efficacy of sodium channel blockers. In this report, the authors review the current understanding of the role of sodium channels and of specific sodium channel subtypes in neuropathic pain signaling.

Local anesthetics

Local anesthetics are used broadly to prevent or reverse acute pain and treat symptoms of chronic pain. This chapter, on the analgesic aspects of local anesthetics, reviews their broad actions that affect many different molecular targets and disrupt their functions in pain processing. Application of local anesthetics to peripheral nerve primarily results in the blockade of propagating action potentials, through their inhibition of voltage-gated sodium channels. Such inhibition results from drug binding at a site in the channel's inner pore, accessible from the cytoplasmic opening. Binding of drug molecules to these channels depends on their conformation, with the drugs generally having a higher affinity for the open and inactivated channel states that are induced by membrane depolarization. As a result, the effective potency of these drugs for blocking impulses increases during high-frequency repetitive firing and also under slow depolarization, such as occurs at a region of nerve injury, which is often the locus for generation of abnormal, pain-related ectopic impulses. At distal and central terminals the inhibition of voltage-gated calcium channels by local anesthetics will suppress neurogenic inflammation and the release of neurotransmitters. Actions on receptors that contribute to nociceptive transduction, such as TRPV1 and the bradykinin B2 receptor, provide an independent mode of analgesia. In the spinal cord, where local anesthetics are present during epidural or intrathecal anesthesia, inhibition of inotropic receptors, such as those for glutamate, by local anesthetics further interferes with neuronal transmission. Activation of spinal cord mitogen-activated protein (MAP) kinases, which are essential for the hyperalgesia following injury or incision and occur in both neurons and glia, is inhibited by spinal local anesthetics. Many G protein-coupled receptors are susceptible to local anesthetics, with particular sensitivity of those coupled via the Gq alpha-subunit. Local anesthetics are also infused intravenously to yield plasma concentrations far below those that block normal action potentials, yet that are frequently effective at reversing neuropathic pain. Thus, local anesthetics modify a variety of neuronal membrane channels and receptors, leading to what is probably a synergistic mixture of analgesic mechanisms to achieve effective clinical analgesia.

Neuropathic pain: are we out of the woods yet?

Patients suffering from neuropathic pain continue to pose challenges in clinical practice. This descriptive review discusses the continuing debate on the definition and concerns about increasing incidence of neuropathic pain. The clinical features of neuropathic pain are outlined, and the current understanding of the possible mechanisms of neuropathic pain is highlighted. Current management strategies are reviewed, and future advances in our understanding of the mechanisms, accurate clinical diagnosis and more effective treatment strategies are eagerly awaited.

The Role of Sodium Channels in Chronic Inflammatory and Neuropathic Pain

Clinical and experimental data indicate that changes in the expression of voltage-gated sodium channels play a key role in the pathogenesis of neuropathic pain and that drugs that block these channels are potentially therapeutic. Clinical and experimental data also suggest that changes in voltage-gated sodium channels may play a role in inflammatory pain, and here too sodium-channel blockers may have therapeutic potential. The sodium-channel blockers of interest include local anesthetics, used at doses far below those that block nerve impulse propagation, and tricyclic antidepressants, whose analgesic effects may at least partly be due to blockade of sodium channels. Recent data show that local anesthetics may have pain-relieving actions via targets other than sodium channels, including neuronal G protein-coupled receptors and binding sites on immune cells. Some of these actions occur with nanomolar drug concentrations, and some are detected only with relatively long-term drug exposure. There are 9 isoforms of the voltage-gated sodium channel alpha-subunit, and several of the isoforms that are implicated in neuropathic and inflammatory pain states are expressed by somatosensory primary afferent neurons but not by skeletal or cardiovascular muscle. This restricted expression raises the possibility that isoform-specific drugs might be analgesic and lacking the cardiotoxicity and neurotoxicity that limit the use of current sodium-channel blockers.

PERSPECTIVE

Changes in the expression of neuronal voltage-gated sodium channels may play a key role in the pathogenesis of both chronic neuropathic and chronic inflammatory pain conditions. Drugs that block these channels may have therapeutic efficacy with doses that are far below those that impair nerve impulse propagation or cardiovascular function.

Tolerability of treatments for postherpetic neuralgia

Herpes zoster occurs in up to 20% of people infected with varicella-zoster virus, due to reactivation of the virus from latently infected sensory ganglia. Although pain is a typical feature of acute zoster, pain persisting for more than a month after resolution of the rash is less common and is termed postherpetic neuralgia (PHN). The pain associated with PHN is neuropathic in origin and is notoriously difficult to treat. The incidence of herpes zoster and its associated complications both increase with age, so PHN should be seen more commonly in an aging population. Vaccination with live, attenuated varicella vaccine is safe and efficacious, particularly in children. It decreases the incidence of acute varicella and subsequent herpes zoster. Aciclovir is well tolerated, with renal toxicity only at high intravenous doses. Treatment of acute varicella with aciclovir attenuates acute illness but does not prevent herpes zoster. Treatment of herpes zoster with aciclovir or its derivatives minimises symptoms and may reduce the rate of PHN. Foscarnet is an alternative for an aciclovir-resistant virus but its use is limited by renal and CNS toxicity. Corticosteroids reduce acute pain in herpes zoster but do not affect the incidence of PHN. Their use in some patients may be limited by adverse effects such as gastritis and impaired glucose tolerance. Treatment of established PHN is difficult and may require a holistic approach. Tricyclic antidepressants and gabapentin are the systemic agents with the most proven benefit, although opioids such as oxycodone and NMDA receptor antagonists such as ketamine may be useful in some people. Adverse effects from tricyclic antidepressants are common but usually mild, while gabapentin is generally well tolerated. Although effective, the relatively common adverse effects of opioids and ketamine limit their usefulness in treating PHN. Topical treatment with 5% lidocaine patch or capsaicin is of benefit in some patients and is generally well tolerated. Intrathecal methyl prednisolone may be considered for intractable pain but efficacy and safety have not been confirmed.

Neural Modulation by Blocks and Infusions

Neural blockade is widely used in clinical practice to alleviate acute or chronic pain, including neuropathic pain. However, to date there is little controlled evidence to confirm the efficacy of nerve blocks in neuropathic pain. The most common indication for nerve blocks, especially sympathetic blockade, is complex regional pain syndrome, in which success rates of up to 38% have been achieved, depending on the type of the block used. Greater efficacy has been achieved by combining a nerve block with patient-controlled analgesia. Sympathectomy is recommended for the treatment of neuropathic pain only after careful consideration of its usefulness, effectiveness, and risk of adverse effects. Current evidence and clinical experience suggest that neural blockade could be a useful adjunct in the management of refractory neuropathic pain, but further well-controlled studies would be of great benefit to support this type of therapy.

Sodium Channels and Mechanisms of Neuropathic Pain

Na+ channels are large transmembrane proteins with a voltage-gated central pore capable of selectively passing Na+ ions. They are critical determinants of the electrical excitability of sensory neurons and play a key role in pain sensation by controlling afferent impulse discharge. Injury and disease affecting peripheral nerves induces axonopathy and demyelination. These neuropathic changes, in turn, trigger membrane remodeling in injured afferents and perhaps also in uninjured neighbors. A major consequence of the remodeling is increased cellular excitability. This is due in large part to subtype-selective abnormalities in the expression and trafficking of Na+ channels and perhaps also to altered kinetic properties of unitary channels. Hyperexcitable neurons show enhanced membrane resonance, rhythmogenesis, and ectopic spiking. The resulting excess discharge constitutes a primary neuropathic pain signal. In addition, it triggers and maintains central sensitization. This amplifies residual afferent input, yielding tactile allodynia, and it also amplifies ongoing ectopia that exaggerates spontaneous pain. Membrane-stabilizing Na+ channel ligands suppress neuropathic pain by selectively reducing membrane resonance in injured afferents and hence ectopic hyperexcitability. The clinical usefulness of these peripherally acting drugs might be enhanced by reducing their central side effects.

PERSPECTIVE

Neuropathic pain is a complex outcome of multiple pathophysiological changes that develop in the peripheral nervous system (PNS) and the central nervous system (CNS) following nerve injury or disease. All or most of the CNS changes are thought to be due to abnormal signaling from the PNS, notably electrical hyperexcitability of peripheral sensory neurons. Because hyperexcitability is associated with abnormal sodium channel regulation, this process is a prime target for therapeutic intervention.