A randomized, double-blind, placebo-controlled trial of a glycine antagonist in neuropathic pain.

BACKGROUND: Nerve injury results in increases in spinal glutamate, which opens the NMDA ionophore channel, causing an influx of calcium. A glycine-binding site must be occupied for the channel to open. GV196771 is a selective antagonist of the glycine-binding site of the NMDA ionophore. OBJECTIVE: To determine the efficacy of GV196771 in subjects with chronic neuropathic pain in a proof-of-concept study. METHODS: With informed consent, 63 subjects (31 placebo, 32 GV196771) with neuropathic pain (diabetic neuropathy, postherpetic neuralgia, complex regional pain syndrome, or peripheral nerve injury), a visual analogue score averaging > or =30 mm during the screening period, and a well-defined primary area of mechanical allodynia were recruited for the study. A multicenter, randomized, double-blind, placebo-controlled, parallel-group study design was utilized. Subjects came to the research center for a total of five visits over a 21-day period, which consisted of a 14-day treatment period followed by a 7-day washout period. Spontaneous and evoked pain scores, mechanical sensory testing, quantitative sensory testing, Short Form McGill Pain Questionnaire, patient global satisfaction, and safety assessments were made during the study. RESULTS: There was no significant effect of GV196771 on spontaneous or evoked pain, quantitative sensory testing, or patient global satisfaction. There was a significant effect of GV196771 on the area of dynamic and static allodynia on days 7 and 14. The overall incidence of adverse events during treatment was similar for GV196771 (56%) and placebo (71%). The incidence of drug-related adverse events during treatment was higher for placebo (42%) than GV196771 (28%). CONCLUSIONS: Although the glycine antagonists show anti-hyperalgesic action in animal models of neuropathic pain, GV196771 does not appear to be an effective treatment in subjects with chronic neuropathic pain. This may be due to insufficient penetration of GV196771 to central sites of action, differences between the human and animal glycine receptors, or differences between neuropathic pain in animal models and humans.

Anticonvulsants for neuropathic pain syndromes: mechanisms of action and place in therapy.

Neuropathic pain, a form of chronic pain caused by injury to or disease of the peripheral or central nervous system, is a formidable therapeutic challenge to clinicians because it does not respond well to traditional pain therapies. Our knowledge about the pathogenesis of neuropathic pain has grown significantly over last 2 decades. Basic research with animal and human models of neuropathic pain has shown that a number of pathophysiological and biochemical changes take place in the nervous system as a result of an insult. This property of the nervous system to adapt morphologically and functionally to external stimuli is known as neuroplasticity and plays a crucial role in the onset and maintenance of pain symptoms. Many similarities between the pathophysiological phenomena observed in some epilepsy models and in neuropathic pain models justify the rational for use of anticonvulsant drugs in the symptomatic management of neuropathic pain disorders. Carbamazepine, the first anticonvulsant studied in clinical trials, probably alleviates pain by decreasing conductance in Na+ channels and inhibiting ectopic discharges. Results from clinical trials have been positive in the treatment of trigeminal neuralgia, painful diabetic neuropathy and postherpetic neuralgia. The availability of newer anticonvulsants tested in higher quality clinical trials has marked a new era in the treatment of neuropathic pain. Gabapentin has the most clearly demonstrated analgesic effect for the treatment of neuropathic pain, specifically for treatment of painful diabetic neuropathy and postherpetic neuralgia. Based on the positive results of these studies and its favourable adverse effect profile, gabapentin should be considered the first choice of therapy for neuropathic pain. Evidence for the efficacy of phenytoin as an antinociceptive agent is, at best, weak to modest. Lamotrigine has good potential to modulate and control neuropathic pain, as shown in 2 controlled clinical trials, although another randomised trial showed no effect. There is potential for phenobarbital, clonazepam, valproic acid, topiramate, pregabalin and tiagabine to have antihyperalgesic and antinociceptive activities based on result in animal models of neuropathic pain, but the efficacy of these drugs in the treatment of human neuropathic pain has not yet been fully determined in clinical trials. The role of anticonvulsant drugs in the treatment of neuropathic pain is evolving and has been clearly demonstrated with gabapentin and carbamazepine. Further advances in our understanding of the mechanisms underlying neuropathic pain syndromes and well-designed clinical trials should further the opportunities to establish the role of anticonvulsants in the treatment of neuropathic pain.

The effect of continuous morphine analgesia on chronic thermal hyperalgesia due to sciatic constriction injury in rats.

We employed hindfoot withdrawal latencies to radiant heat to assess the analgesic effect of prolonged morphine infusion on thermal hyperalgesia induced by chronic constriction injury (CCI) of the rat sciatic nerve. All CCI rats developed thermal hyperalgesia while sham-operated animals did not. Continuous systemic infusion of morphine dose-dependently reversed the thermal hyperalgesia in the CCI rats. In contrast, thermal hyperalgesia persisted in saline-treated CCI rats. Tolerance to morphine's analgesic effect did not develop over a period of seven days of morphine infusion, which is considered long-term for animal models. These data suggest that morphine acts rapidly and effectively to reduce behavioral signs of hyperalgesia in rats with sciatic CCI, without the concomitant development of tolerance. Scheduled administration of morphine might be an appropriate treatment regimen for relief of neuropathic pain, and the infrequent use of opioids in equivalent human clinical pain syndromes due to fear of opioid unresponsiveness and tolerance might need to be re-evaluated.

Local anesthetics as adjuvant analgesics.

Local anesthetics administered to block nerve conduction for surgical anesthesia and to provide analgesia in management of acute pain have become a standard of anesthesiology practice. These drugs have had an important role in the multimodality management of chronic pain as well, and this role is expanding since the revival of systemic administration. Local anesthetics are analgesics, albeit not in the traditional clinical and pharmacologic sense. Evidence suggests that intravenous administration is an effective treatment in chronic neuropathic pain syndromes. There is also evidence that intravenous local anesthetics can relieve acute pain. Furthermore, the novel idea that acute procedural and postprocedural pain control with local anesthetics could prevent the development of chronic pain syndromes, including chronic neuropathic pain syndromes, adds another important potential dimension to the role of local anesthetics in pain management.

Anticonvulsants (antineuropathics) for neuropathic pain syndromes.

Our knowledge about the pathogenesis of neuropathic pain has grown significantly during last two decades. Basic research with animal models of neuropathic pain and human clinical trials with neuropathic pain have accumulated solid evidence that a number of pathophysiologic and biochemical changes take place in the nervous system at a peripheral or central level as a result of the insult or disease. Many similarities between the pathophysiologic phenomena observed in some epilepsy models and neuropathic pain models justify the rationale for the use of anticonvulsant drugs in the symptomatic management of neuropathic pain disorders. Carbamazepine (CBZ) was the first representative from this class of drugs to be studied in clinical trials. It has been used for the treatment of neuropathic pain syndromes, in particular, trigeminal neuralgia (TN), for the longest time of any of the drugs in this class. Results from clinical trials support the use of CBZ in the treatment of TN, painful diabetic neuropathy, and postherpetic neuralgia. The use of CBZ was not studied for complex regional pain syndrome, phantom limb pain, and other neuropathic conditions, however. Phenytoin was the first anticonvulsant to be used as an antinociceptive agent, but based on clinical trials, there is no evidence for its efficacy in relieving neuropathic pain. Newer anticonvulsants have marked a new era in the treatment of neuropathic pain, with clinical trials of higher quality standards. Gabapentin (GBP) has most clearly demonstrated an analgesic effect for the treatment of neuropathic pain, specifically for the treatment of painful diabetic neuropathy and postherpetic neuralgia. Gabapentin has a favorable side effects profile, and based on the results of these studies, it should be considered a first-line treatment for neuropathic pain. Gabapentin mechanisms of action are still not thoroughly defined, but GBP is effective in relieving indexes of allodynia and hyperalgesia in animal models. It still remains to be seen whether GBP is as effective in other painful disorders. One small clinical trial with lamotrigine demonstrated improved pain control in TN. Evidence in support of the efficacy of anticonvulsant drugs in the treatment of neuropathic pain continues to evolve, and benefits have been clearly demonstrated in the case of GBP and CBZ. More advances in our understanding of the mechanisms underlying neuropathic pain syndromes should further our opportunities to establish the role of anticonvulsants in the treatment of neuropathic pain.

Pain assessment and evaluation of patients who have neuropathic pain.

Pain assessment and physical examination are the first crucial steps in diagnosis of neuropathic pain disorders because these are still solely diagnosed on clinical grounds. The physical examination should be conducted in such a way that all of the positive sensory phenomena, such as allodynia, hyperalgesia, hyperpathia, summation, and after-sensation are elicited. Other physical examination findings should corroborate the diagnostic impression of neuropathic pain. Specific pain diagnosis should then lead to more specific therapy.

Traumatic neuralgias: complex regional pain syndromes (reflex sympathetic dystrophy and causalgia): clinical characteristics, pathophysiological mechanisms and therapy.

Complex regional pain syndromes (CPRS) may develop as a disproportionate consequence of a trauma affecting the limbs without (CRPS I, reflex sympathetic dystrophy) or with (CRPS II, causalgia) obvious nerve lesions. The clinical picture of CRPS consists of asymmetrical distal extremity pain, swelling, and autonomic (sympathetic) and motor symptoms. Changes in the peripheral and central somatosensory, autonomic and motor processing, and a pathologic interaction of sympathetic and afferent systems are discussed as underlying pathophysiologic mechanisms. Therapeutic strategies include pharmacologic pain relief, sympatholytic interventions, and rehabilitation.

Gabapentin monotherapy for the symptomatic treatment of painful neuropathy: a multicenter, double-blind, placebo-controlled trial in patients with diabetes mellitus.

Pain is the most disturbing symptom of diabetic neuropathy. Traditionally this type of pain was treated with tricyclic antidepressants which frequently have many side effects. In the study reported here, gabapentin was administered in escalating doses up to 3600 mg per day to eligible patients with moderate to severe diabetic neuropathy pain in a double blind placebo controlled fashion. Gabapentin provided superior and significant pain relief over placebo. In addition, patients taking gabapentin had improvement of sleep scores and a number of items on mood and quality of life questionnaires. Gabapentin was tolerated well with mild and tolerable side effects.

Vulvar vestibulitis syndrome: a form of reflex sympathetic dystrophy?

OBJECTIVE: Our purpose was to determine whether vulvar vestibulitis syndrome is a form of reflex sympathetic dystrophy syndrome. MATERIALS AND METHODS: Between October 1, 1998, and February 16, 1999, 20 subjects attending a gynecology clinic at a tertiary care center received diagnoses of vulvar vestibulitis syndrome. A history was recorded, and a gynecological examination was performed for each, including a detailed description of the vulva, vaginal Gram's stain, vaginal culture, pH, and wet preparation of any discharge. Vulvar examination included assessing each subject's response to touch, pressure, pinprick, vibration, and cold. Subjects completed the short form of the McGill Pain Questionnaire. RESULTS: Twenty subjects with vulvar vestibulitis syndrome (average age, 30.6 years; range, 20-71 years) had pain present an average of 4 years and 8 months (range, 6 months-33 years). Pain occurred with entry dyspareunia, tampon insertion, or touching of the vestibule. Vulvar vestibular erythema was present in 13 subjects. None of the 20 subjects felt pain to light touch on the vulvar vestibule. Light pressure produced allodynia in all 20. Nine subjects had hyperalgesia to pinprick; five subjects had allodynia in response to vibration; and three subjects experienced allodynia to cold. Summation and aftersensation to test modalities were uncommon. CONCLUSION: Though both vulvar vestibulitis and reflex sympathetic dystrophy seem to be neuropathic pain syndromes, they do not share enough physical characteristics to be considered the same pathological processes.