Topical Lidocaine
Lidocaine, in the form of 5% patches, was efficacious and had an excellent tolerability profile in randomized controlled trials of patients with post-herpetic neuralgia and allodynia, as well as in patients with allodynia due to neuropathic pain [
17]. Lidocaine blocks voltage-gated sodium channels that are expressed by nerve fibers, which are responsible for the propagation of action potentials. The number, localization, subtype expression, and activity of these channels are altered in different forms of neuropathic pain [
18]. Because topical lidocaine can penetrate no deeper than 8–10 mm, it is therefore indicated in well-localized neuropathic pain. Its efficacy has been documented in different types of localized neuropathic pain, including post-herpetic neuralgia, painful diabetic neuropathy, post-surgical and post-traumatic pain related to incision of the skin [
19]. The most common adverse effects of lidocaine are mild local reactions due to its topical application. Lidocaine’s lack of systemic absorption and of drug interactions can be particularly beneficial in older patients [
19].
Opioids
Strong opioids, such as morphine, oxycodone, and hydromorphone, and weak opioids, such as tramadol, are efficacious when compared with other drugs used for neuropathic pain and are similar to antidepressants in terms of the numbers needed to treat [
5]. Nevertheless, they have always been considered second-line drugs [
1], and more recently third-line drugs [
5], due to adverse drug reactions and concerns about abuse, diversion, and addiction. Tapentadol represents a new class of dual opioid analgesics, combining a less potent agonistic activity at mu-opioid receptors with inhibition of noradrenaline uptake, and exploiting the synergy between the two mechanisms. The innovative pharmacodynamics and a favorable pharmacokinetic profile make tapentadol a unique opioid analgesic. However, the paucity of available studies prevented tapentadol from being included in the most recent systematic review and meta-analysis on neuropathic pain [
20‐
22]. For this reason it will not be further discussed in this review.
The analgesic effect of opioids is due to their action in the brain, brainstem, spinal cord, and, under certain circumstances, on peripheral terminals of primary afferent neurons. All endogenous opioid peptides, including β-endorphin, enkephalins, and dynorphins, bind to seven transmembrane G protein-coupled receptors, which are divided into three classes: mu, delta, and kappa receptors. Opioid receptors are coupled to inhibitor G proteins, with receptor activation inhibiting the adenylate cyclase as well as the intracellular production of cAMP. However, the coupling of opioid receptors to calcium and potassium channels is thought to be a central mechanism of analgesia production by both endogenous and exogenous opioids.
In the dorsal horn of the spinal cord, mu receptors make up the majority of opioid receptors with over 70% of the presynaptic location occurring at the central terminals of nociceptors (C and A delta fibers). Opioid receptors located postsynaptically on dendrites of second-order spinothalamic neurons and on interneurons make up the remaining 30%. Interneurons are predominately responsible for the release of endogenous opioids beta-enkephalin and endorphins, which act on mu receptors in the dorsal horn of the spinal cord. The activation of interneurons is dependent on the activity of descending pathways or, in a direct manner, by descending fibers. Inhibition of calcium ion channels is caused by the activation of presynaptic mu receptors, thus preventing neurotransmitter release. Activation of potassium ion channels is caused by the activation of postsynaptic mu receptors resulting in the efflux of potassium ions and hyperpolarization of the projecting cell. Therefore, stimulation of mu-opioid receptors in the spinal cord is an effective mechanism of blocking synaptic transmission, which restricts the number of nociceptive stimuli reaching the thalamus and cortex, in which a conscious perception of pain occurs [
23].
In spite of their efficacy, the role of opioids in the long-term treatment of nonmalignant pain is controversial for a number of reasons, including concerns over tolerability, possible development of tolerance to the analgesic effect, and the risk of addiction [
24]. A systematic review of randomized controlled trials of oral opioids for chronic nonmalignant pain indicated that approximately 50% of patients experienced an adverse event with opioids and more than 20% discontinued treatment because of adverse events [
25]. A more recent Cochrane review of long-term opioid management of chronic non-cancer pain reported a rate of discontinuation due to adverse events of 22.9% for oral opioids and 12.1% for transdermal opioids [
26]. There was a significant difference in discontinuation rates for orally administered weak (11.4%) compared with strong (34.1%) opioids. However, many studies in this Cochrane review specified that most adverse events were minor and some of them diminished over time [
26]. The most frequent adverse drug reactions to opioid therapy are nausea and vomiting (tend to diminish with increasing tolerance), constipation (remains a constant problem), pruritus, respiratory depression (very uncommon), dry mouth, urinary retention, drowsiness, and cognitive impairment. Drowsiness and cognitive impairment should be considered, along with constipation, the most serious adverse drug reactions to opioids, because they can seriously affect the patients’ quality of life. Over the last few years, addiction, diversion, and abuse have become the subject of a worldwide debate that started in the USA in response to the persistent increase of deaths due to unintentional overdose. A detailed discussion of this issue is beyond the scope of this review. Although the risk of addiction is inherent to opioids, its relevance in patients suffering from chronic pain, who take these drugs for pain relief, remains to be elucidated. It is likely that people with a previous experience of drug abuse or people with psychiatric diseases, such as depression or schizophrenia, are at higher risk of developing addiction after taking opioids. In other patients who take opioids for pain relief, the risk is probably very close to that in the general population [
27]. Nevertheless, the use of appropriate tools to identify at-risk patients prior to initiating treatment with opioids, constant vigilance on the behavior associated with opioid assumption, and frequent re-evaluation of the balance between risks and benefits of long-term opioid therapies should become a normal attitude among physicians [
27].
Other Drugs
A number of additional drugs have shown efficacy in the treatment of neuropathic pain; however, these were either single randomized controlled trials or their efficacy was inconsistent across multiple randomized controlled trials. These drugs represent the third or even fourth line of treatment options for neuropathic pain and include specific antidepressants (e.g., bupropion) and antiepileptic drugs (carbamazepine, lamotrigine), and topical low-concentration capsaicin. In general, these drugs should be reserved for patients who are unable to tolerate or who fail to respond to first- and second-line medications.