The use of oral mexiletine for the treatment of pain after peripheral nerve injury

Downregulation of tetrodotoxin-resistant sodium currents and upregulation of a rapidly repriming tetrodotoxin-sensitive sodium current in small spinal sensory neurons after nerve injury

Clinical and experimental studies have shown that spinal sensory neurons become hyperexcitable after axonal injury, and electrophysiological changes have suggested that this may be attributable to changes in sodium current expression. We have demonstrated previously that sodium channel alpha-III mRNA levels are elevated and sodium channel alpha-SNS mRNA levels are reduced in rat spinal sensory neurons after axotomy. In this study we show that small (C-type) rat spinal sensory neurons express sodium currents with dramatically different kinetics after axotomy produced by sciatic nerve ligation. Uninjured C-type neurons express both slowly inactivating tetrodotoxin-resistant (TTX-R) sodium current and a fast-inactivating tetrodotoxin-sensitive (TTX-S) current that reprimes (recovers from inactivation) slowly. After axotomy, the TTX-R current density was greatly reduced. No difference was observed in the density of TTX-S currents after axotomy, and their voltage dependence was not different from controls. However, TTX-S currents in axotomized neurons reprimed four times faster than control TTX-S currents. These data indicate that axotomy of spinal neurons is followed by downregulation of TTX-R current and by the emergence of a rapidly repriming TTX-S current and suggest that this may be attributable to the upregulation of a sodium channel isoform that was unexpressed previously in these cells. These axotomy-induced changes in sodium currents are expected to alter excitability substantially and could underlie the molecular pathogenesis of some chronic pain syndromes associated with injury to the axons of spinal sensory neurons.

[Review of current pharmacologic treatment of pain]

Pain is the main reason prompting patients to consult their physicians. In acute conditions, pain has a very particular significance as a warning sign, enabling the physician to attempt a diagnosis. Nevertheless, its detrimental effect upon the individual (even in the case of acute pain) and its cost to society are now widely acknowledged. There can be no doubt about the physical component of pain, but the psychological and social aspects should not be ignored, particularly in the case of chronic pain. There is no single therapeutic approach to pain and, more often than not, successful treatment comprises a combination of several. Pharmacological treatments are undeniably the most common approach. In clinical practice, recent advances have been based upon an improved understanding of 'old' substances such as morphine and, at the same time, research continues in the hope of finding the 'ideal' analgesic-effective in most situations but without adverse effects: this appears to be a somewhat utopian arm at present, considering the number of different causes of pain. An improved understanding of the physiological mechanisms of pain has led, within the field of clinical practice, to several methods of differentiating pain. These depend on whether or not pain responds to morphine, or on the type of pain: pain due to an excess of nociception, pain resulting from deafferentation (caused by damage to nerve pathways) in the central or peripheral nervous system and psychogenic (idiopathic) pain. Likewise, there are several different ways of classifying analgesic treatments: according to the intensity of pain, as with use of the WHO ladder (which is based on the notion of steps) for the treatment of cancer pain; according to the presumed physiopathological mechanism and, in particular, the response to morphine, and according to the presumed central or peripheral mechanism of the drugs. In reality, peripherally acting drugs can also have a central mechanism of action, just as drugs known to have a central mechanism of action can also have peripheral activity. As a result, several therapeutic classes have been identified. Firstly NSAIDs, which act by inhibiting the enzymes that synthesise prostaglandins, cyclooxygenases (COX-1, COX-2), but which also act upon lipo-oxygenases: Their efficacy is interesting, although somewhat limited by both their ceiling effect and the frequent adverse gastrointestinal reactions they produce. Specific inhibitors of COX-2 could well reduce the risk of adverse effects. Opioids constitute the first-line treatment for pain, particularly severe pain. There are several classifications for these drugs. Firstly, weak opioids (such as codeine) and strong opioids (such as morphine) are differentiated. Secondly, a distinction is made between pure agonists (such as morphine), partial agonists (such as buprenorphine), agonist-antagonists (such as nalbuphine) and antagonists (such as naloxone). Finally, agents are distinguished on the basis of their chemical structure (synthetic, semi-synthetic or natural derivatives). These molecules act upon different receptors (mu, delta, kappa, sigma) and, although peripheral mechanisms have been described, their activity occurs mainly at spinal and supraspinal levels. They provide a potent analgesic effect but are also responsible for various adverse effects-nausea, vomiting, sedation, constipation and respiratory depression-which seriously limit their use. As long as the indication is appropriate, these drugs should not be withheld because of fear of dependence or abuse. It has been observed that other adjuvant therapeutic approaches, generally used to treat conditions other than pain, provide pain relief in certain situations. These include corticosteroids, which are-widely used in rheumatology and oncology, and antidepressants, which are frequently used to treat chronic pain, especially that with a neuropathic component. Anti-epileptics are also used, particularly for excrutiating

Response to intravenous lidocaine infusion predicts subsequent response to oral mexiletine: a prospective study

The local anesthetic sodium-channel blockers lidocaine and mexiletine reduce spontaneous and evoked activity in experimental neuroma models and have been reported to relieve a variety of clinical neuropathic pain conditions. The predictive value of relief from an intravenous lidocaine infusion (IVL) for subsequent relief from a 4-week trial of oral mexiletine was assessed in a prospective study of nine subjects with chronic neuropathic pain of peripheral origin. Subjects received IVL, 2 mg/kg and 5 mg/kg, over 45 min during separate sessions in random order under double-blind conditions. One week later, a 4-week titrating trial of oral mexiletine was initiated. Both doses of IVL significantly reduced pain visual analogue scores (VAS) scores. Although IVL 5 mg/kg produced significantly higher pain relief scores than IVL 2 mg/kg, subjects responded in a highly consistent manner to the two IVL. Subsequent response to oral mexiletine was significantly correlated with the average response to the two IVL. Mexiletine dose and blood levels were not correlated with pain relief. The results suggest that IVL may be a valuable tool in selecting patients for oral therapy with analogous drugs.

Effective management of chronic pain. The analgesic dilemma

Despite the fact that pain is a common presenting complaint, its effective management remains an elusive goal surrounded by contradictions and controversy. In this article, Dr Stacey discusses the goals of chronic pain management and the treatment options available, with a focus on pharmacologic therapy and prescribing issues involving currently available analgesics.

Advances in comprehensive pain management

Chronic pain is a significant public health problem and frustrating for everyone affected by it. Psychiatrists should take an active role in the care of patients with chronic pain. They no longer should wait to make a psychiatric diagnosis by exclusion in the patient who has failed to respond to multiple treatments over a period of years. Recent advances in the treatment of chronic pain include the diagnosis and treatment of psychiatric comorbidity, the application of primary psychiatric treatments to chronic pain, and the development of interdisciplinary efforts to provide comprehensive health care to the patient suffering with chronic pain.

Pain management in patients with pancreatic carcinoma

Pancreatic cancer has a very poor prognosis and is often associated with severe pain. A variety of pain syndromes and pain pathophysiologies can be identified. Information about the analgesic efficacy of available oncological treatments is very limited, but the available data suggest that pharmacological and non-pharmacological approaches can be effective in the majority of cases. Guidelines have been developed for drug administration that emphasize indications, selection of routes, optimal dosing, and side effect treatment. Celiac plexus block can be considered for a subgroup of patients who fail to benefit from drug therapy. Optimally, pain management should be provided within a broader model of palliative care, which can address the many problems associated with this challenging disease.

Computer-controlled lidocaine infusion for the evaluation of neuropathic pain after peripheral nerve injury

BACKGROUND

Systemic lidocaine has been reported to be effective in treating several neuropathic pain syndromes. Few reports relate plasma lidocaine concentration to analgesia and the available studies have been complicated by labile plasma lidocaine concentrations. We used a computer-controlled infusion pump (CCIP) to target and maintain stable plasma lidocaine concentrations and study the effect of intravenous lidocaine on (1) pain scores, (2) current perception thresholds, (3) side effects, and (4) pain distribution in patients suffering from peripheral nerve injury pain.

METHODS

This study used a randomized double-blind placebo-controlled design. Eleven patients suffering from neuropathic pain after peripheral nerve injury received both a lidocaine and saline infusion in separate study sessions. The order of the study sessions was randomized and separated from each other by 1 week. The CCIP was programmed to target plasma lidocaine concentrations of 0.5, 1, 1.5, 2, and 2.5 micrograms/ml, each held for 10 min. Pain scores and pain distribution were assessed in the painful area, and electrical current perception thresholds (CPT) of the ring finger were measured using a cutaneous perception threshold neurometer (Neurometer CPT, Neurotron, Baltimore, MD). Side effects were recorded at fixed intervals. Plasma lidocaine concentrations were measured at 4 and 9 min after each step increase in infusion and correlated with the observed effects.

RESULTS

Saline infusion had no effect. However, with lidocaine there was a significant plasma concentration-dependent decrease in pain scores starting at 1.5 micrograms/ml. This effect typically corresponded with a decrease in the size of the receptive field to which the pain was referred. For the electrical stimulus, there was no significant effect on cutaneous perception at 2000-Hz stimulation at the highest concentration examined; however, there was a significant increase in thresholds at 250-Hz (starting at 1.5 micrograms/ml) and 5-Hz (starting at 1.0 micrograms/ml) stimulation. There were no serious side effects. In all, 54.5% of patients reported lightheadedness (average plasma lidocaine concentration: 1.5 micrograms/ml) and one patient reported nausea (2.3 micrograms/ml).

DISCUSSION

The computer-controlled delivery of intravenous lidocaine results in relatively stable plasma concentrations which allows a more thorough evaluation of the relationship between plasma concentration and patient response. This administration methodology for intravenous lidocaine may prove to be a valuable clinical and research tool.

Pain and Symptom Management in Palliative Care

BACKGROUND: Pain, dyspnea, and anorexia are common symptoms experienced by patients with cancer and often are poorly managed. METHODS: The incidence and causes of these symptoms are described, as well as factors that exacerbate or ameliorate their impact. RESULTS: Pharmacologic management of cancer pain is based on the use of a sequential "ladder" that incorporates nonopioid, opioid, and adjuvant drugs, depending on the severity of the pain. This approach usually is effective. Other symptoms of advanced disease may be more difficult to control. CONCLUSIONS: Adherence to an adequate pain-control strategy will significantly enhance palliation of pain in patients with cancer.

Determination of the enantiomeric composition of mexiletine and its four hydroxylated metabolites in urine by enantioselective capillary gas chromatography

The enantiomers of mexiletine and four of its hydroxylated metabolites were directly separated by capillary gas chromatography using a heptakis(6-O-tert-butyl-dimethylsilyl-2,3-di-O-methyl)-beta- cyclodextrin column. The method was applied to the analysis of urine samples from cancer patients who were treated with racemic mexiletine as part of a study of the use of mexiletine in the relief of neuropathic pain. Samples analyzed before and after deconjugation of the urine with beta-glucuronidase/arylsulfatase showed a high stereoselectivity in the formation and conjugation of these compounds.

Endometriosis and pelvic pain

The majority of patients with pain associated with endometriosis will obtain benefit from the many and varied therapies available for the treatment of endometriosis. The minority who fail to obtain relief from the conventional therapies may obtain benefit from referral to a pain clinic.

Direct analysis of the enantiomers of mexiletine and its metabolites in plasma and urine using an HPLC-CSP

A high-performance liquid chromatographic assay has been developed for the quantification of the enantiomers of mexiletine and its four major metabolites, in plasma and in urine. Mexiletine and all metabolites were determined, after derivatization of mexiletine and its hydroxymetabolites, p-hydroxymexiletine and hydroxymethylmexiletine, using a Chiralpak AD chiral stationary phase, based on a carbamoyl derivative of amylose. o-phthalaldehyde was chosen as derivatization reagent to increase the sensitivity of detection, to achieve separation of all compounds in one chromatographic system, and to avoid interferences.

Selective opioid receptor agonists modulate mechanical allodynia in an animal model of neuropathic pain

This study evaluated the antinociceptive effects of systemically administered selective opioid agonists of mu (DAMGO), delta (BUBU) and kappa (U 69593) receptors on the vocalization threshold to paw pressure in a rat model of peripheral unilateral mononeuropathy produced by loose ligatures around the common sciatic nerve. DAMGO (0.5-2 mg/kg), BUBU (1.5-6 mg/kg) and U 69593 (0.75-3 mg/kg) injected intravenously (i.v.) produced a potent long-lasting antinociceptive effect on both hind paws. The effects on the lesioned paw were clearly and statistically more potent than for the non-lesioned paw. The selective antinociceptive effect of 2 mg/kg DAMGO, 3 mg/kg BUBU and 1.5 mg/kg U 69593 were completely prevented by prior administration of the appropriate antagonists: 0.1 mg/kg naloxone, 1 mg/kg naltrindole and 0.4 mg/kg MR 2266. The present data clearly show that an acute i.v. injection of these selective opioid agonists induces potent antinociceptive effects in a rat model of peripheral neuropathy. These data are discussed with regard to the classical view that there is opioid resistance in neuropathic pain.