Randomised double-blind active-placebo-controlled crossover trial of intravenous fentanyl in neuropathic pain. Relief of post-herpetic neuralgia with the N-methyl-D-aspartic acid receptor antagonist ketamine: a double-blind, cross-over comparison with morphine and placebo. Central dysesthesia pain after traumatic spinal cord injury is dependent on N-methyl-D-aspartate receptor activation. The effect of opioids on phantom limb pain and cortical reorganization. Prolonged analgesic effect of ketamine, an N-methyl-D-aspartate receptor inhibitor, in patients with chronic pain. Analgesic effects of intravenous lidocaine and morphine on postamputation pain: a randomized double-blind, active placebo-controlled, crossover trial. Low-dose methadone has an analgesic effect in neuropathic pain: a double-blind randomized controlled crossover trial. Association of pain relief with drug side effects in postherpetic neuralgia: a single-dose study of clonidine, codeine, ibuprofen, and placebo. Morphine, gabapentin, or their combination for neuropathic pain. Cold allodynia and hyperalgesia in neuropathic pain: the effect of N-methyl-D-aspartate (NMDA) receptor antagonist ketamine--a double-blind, cross-over comparison with alfentanil and placebo. Controlled-release oxycodone for pain in diabetic neuropathy: a randomized controlled trial. Concentration-effect relationship of intravenous alfentanil and ketamine on peripheral neurosensory thresholds, allodynia and hyperalgesia of neuropathic pain. Efficacy of oxycodone in neuropathic pain: a randomized trial in postherpetic neuralgia. Effects of IV morphine in central pain: a randomized placebo-controlled study. Oral opioid therapy for chronic peripheral and central neuropathic pain. Both intravenous lidocaine and morphine reduce the pain of postherpetic neuralgia. The response of neuropathic pain and pain in complex regional pain syndrome I to carbamazepine and sustained-release morphine in patients pretreated with spinal cord stimulation: a double-blinded randomized study. Morphine differentially affects the sensory and affective pain ratings in neurogenic and idiopathic forms of pain. Controlled-release oxycodone relieves neuropathic pain: a randomized controlled trial in painful diabetic neuropathy. Morphine responsiveness of chronic pain: double-blind randomised crossover study with patient-controlled analgesia. Opioids versus antidepressants in postherpetic neuralgia: a randomized, placebo-controlled trial. Intravenous infusion of the NMDA antagonist, ketamine, in chronic posttraumatic pain with allodynia: a double-blind comparison to alfentanil and placebo. Lack of analgesic effect of opioids on neuropathic and idiopathic forms of pain.
The effectiveness of opioid analgesics in non-cancer neuropathic pain is unpredictable and can be disappointing. It is not clear whether opioids, when effective, relieve pain by decreasing pain intensity or pain unpleasantness or by their sedative effect. The aim of this prospective randomised double-blind placebo-controlled crossover trial was to assess relief of pain intensity and pain unpleasantness with intravenous infusions of fentanyl.
                We compared the analgesic effect of intravenous dose titration of fentanyl with diazepam (active placebo) or saline (inert placebo) in 53 patients with different types of neuropathic pain. Patients were randomly assigned two consecutive infusions: fentanyl plus diazepam (27 patients) or fentanyl plus saline (26 patients). Study medication was infused at a constant rate for a maximum of 5 h. Pain, sedation, and side-effects were assessed from the start of infusion for 8 h. The primary outcome measure was maximum relief of pain intensity.
                One patient in the fentanyl/diazepam group and two in the fentanyl/saline group were withdrawn. Maximum relief of pain intensity was better with fentanyl than with diazepam (66% [95% CI 53-80] vs 23% [12-35]) or with saline (50% [36-63] vs 12% [4-20]). The beneficial effect of fentanyl was independent of the type of neuropathic pain and the degree of sedation. Fentanyl therapy produced equal relief of pain intensity and pain unpleasantness, whereas diazepam and saline did not reduce either pain index. Patients reported significantly more side-effects while receiving fentanyl than during diazepam or saline infusion (p < 0.0001), but none of the side-effects was severe.
                Fentanyl may relieve non-cancer neuropathic pain by its intrinsic analgesic effect. The clinical characteristics of neuropathic pain do not predict response to opioids. Pain and sensory thresholds were examined before and after intravenous administration of ketamine (0.15 mg/kg), morphine (0.075 mg/kg) or saline in 8 patients with post-herpetic neuralgia. A randomized, double-blind, cross-over study design was used. Post-herpetic neuralgia was associated with impaired sensory function, as shown by reduced tactile and warm sensation in the affected compared with the contralateral non-affected skin area. Neither ketamine nor morphine changed significantly the thresholds for warm, cold, heat pain or tactile sensation. However, ketamine normalized abnormal heat pain sensations in 4 patients, probably due to a central effect. Ketamine, but not morphine, produced significant relief of pain. Pain evoked by non-noxious stimulation of the skin (allodynia) was significantly inhibited by ketamine as well as by morphine. Wind-up-like pain (i.e., pain evoked by repeatedly pricking the affected skin area) was significantly inhibited by ketamine, but significantly aggravated by morphine. Side effects were observed in all the 8 patients after injection of ketamine and in 6 patients after injection of morphine. The present results support the hypothesis that the N-methyl-D-aspartic acid (NMDA) receptors are involved in the control of post-herpetic neuralgia including allodynia and wind-up-like pain. The NMDA receptors also may play a role in the modulation of thermal perception. The role of central N-methyl-D-aspartate (NMDA) receptors in the pathogenesis of central pain was examined in nine patients with central dysesthesia pain after spinal cord injury. The central pain syndrome included spontaneous continuous and intermittent pain as well as evoked pain. Pain was evoked by non-noxious stimulation of the skin (allodynia) and by repeated pricking of the skin (wind-up-like pain). The severity of continuous and evoked pain was examined before and after the intravenous infusion of either the NMDA receptor antagonist ketamine (6 micrograms/kg/min after a bolus dose of 60 micrograms/kg), the mu-opioid receptor agonist alfentanil (0.6 microgram/kg/min after after a bolus dose of 7 micrograms/kg), or placebo (0.9% NaCl). A randomized, double-blind, crossover study design was used. It was found that both continuous and evoked pain were markedly reduced by the blockade of NMDA receptors by ketamine as well as by the activation of mu-opioid receptors by alfentanil. Neither ketamine nor alfentanil significantly changed thresholds for the sensation of heat pain. The reduction of pain was not associated with severe side effects; the most severe side effect of ketamine was bothersome dizziness in one patient, and only modest side effects were caused by alfentanil. The present data provide clinical evidence that the development of central dysesthesia pain after traumatic spinal cord injury is dependent on the activation of central NMDA receptors. The results further indicate that mu-opioid receptors are involved in the control of this type of pain. The efficacy of oral retarded morphine sulphate (MST) was tested against placebo in a double-blind crossover design in 12 patients with phantom limb pain after unilateral leg or arm amputation. Two counterbalanced treatment phases of 4 weeks each were initiated with an intravenous test infusion of MST or Placebo. The titration phase was 2 weeks. The dose of MST was titrated to at least 70 mg/day and at highest 300 mg/day. Pain intensity was assessed hourly on visual analog scales during a 4-week treatment-free phase, both treatment phases and at two follow-ups (6 and 12 months). Reorganization of somatosensory cortex, electric perception and pain thresholds as well as selective attention were measured pre- and post-treatment. A significant pain reduction was found during MST but not during placebo. A clinically relevant response to MST (pain reduction of more than 50%) was evident in 42%, a partial response (pain reduction of 25-50%) in 8% of the patients. Neuromagnetic source imaging of three patients showed initial evidence for reduced cortical reorganization under MST concurrent with the reduction in pain intensity. Perception and pain thresholds were not significantly altered whereas attention was significantly lower under MST. Thus, opioids show efficacy in the treatment of phantom limb pain and may potentially influence also cortical reorganization. These data need to be replicated in larger patient samples. We examined the role of N-methyl-D-aspartate (NMDA) receptors in chronic (pathological) pain in humans by using the NMDA receptor antagonist ketamine as a probe. Thirty patients with neuropathic pain in the trigeminal area were given an i.m. injection of ketamine 0.4 mg/kg combined with midazolam 0.05 mg/kg. Pethidine 1.0 mg/kg served as a control. Three different response patterns were observed. Ketamine caused a long-term (6-24 h) analgesic effect partly dissociated from the mental side effects in 8 of the 26 patients who completed the study; these patients also had a slight analgesic effect of pethidine. In nine patients, ketamine caused a short-lasting (<2 h) analgesic effect closely associated with the mental side effects, whereas pethidine caused little or no analgesia. The remaining nine patients did not experience any reduction of pain after either drug in spite of characteristic side effects. One week after the i.m. challenge the patients received either 4.0 mg/kg ketamine hydrochloride or placebo capsules to be taken orally as a nightly dose for three consecutive nights. Five of the eight patients who had a long-term analgesic effect of the i.m. challenge reported decreased pain on days after ketamine. None of the others reported an analgesic effect. The phenomenon of long-term depression of pain in a subgroup of patients was thus confirmed when ketamine was given p.o. These findings indicate that NMDA receptors are involved in the perception and maintenance of pathological pain in some patients. In others, pain appears to be mediated by NMDA receptor-independent mechanisms. We suggest that NMDA receptor-independent transmission in central pain pathways may contribute to the reduced efficiency of analgesic drugs often seen in chronic pain states. Phantom and stump pains, common sequelae of limb amputations, are significant impediments to rehabilitation of amputees. The pathophysiology and optimal treatment of postamputation pain states are unclear. While stump pain may result from neuromas in the stump, phantom pain is thought to be related to cortical reorganization. The authors hypothesized that morphine and lidocaine may have differential effectiveness on stump and phantom pains.
                The authors conducted a randomized double-blind, active-placebo-controlled, crossover trial to compare the analgesic effects of intravenous morphine and lidocaine on postamputation stump and phantom pains. An intravenous bolus followed by an intravenous infusion of morphine (0.05 mg/kg bolus + 0.2 mg/kg infusion over 40 min), lidocaine (1 mg/kg bolus + 4 mg/kg infusion) and the active placebo, diphenhydramine (10 mg bolus + 40 mg infusion), were performed on three consecutive days. Phantom and stump pain ratings and sedation scores were recorded at 5-min intervals using a 0-100 visual analog scale. Pain measures were initiated 30 min before drug infusion and continued until 30 min after the end of infusion. Subjects' self-reported pain relief and satisfaction were assessed at the end of each infusion.
                Thirty-one of 32 subjects enrolled completed the study. Eleven subjects had both stump and phantom pains, 11 and 9 subjects had stump and phantom pain alone, respectively. Baseline pain scores were similar in the three drug groups. Compared with placebo, morphine reduced both stump and phantom pains significantly (P < 0.01). In contrast, lidocaine decreased stump (P < 0.01), but not phantom pain. The changes in sedation scores for morphine and lidocaine were not significantly different from placebo. Compared with placebo, self-reported stump pain relief was significantly greater for lidocaine (P < 0.05) and morphine (P < 0.01), while phantom pain relief was greater only for morphine (P < 0.01). Satisfaction scores were significantly higher for lidocaine (mean +/- SD: 39.3 +/- 37.8, P < 0.01) and morphine (45.9 +/- 35.5, P < 0.01) when compared with placebo (9.6 +/- 21.0).
                Stump pain was diminished both by morphine and lidocaine, while phantom pain was diminished only by morphine, suggesting that the mechanisms and pharmacological sensitivity of stump and phantom pains are different. The analgesic effectiveness and adverse effect incidence of a daily dose of 10 or 20 mg of oral methadone were evaluated in 18 patients with a diverse range of chronic neuropathic pain syndromes, who had all responded poorly to traditional analgesic regimens. Analgesia was seen after each dose of methadone. As compared with placebo, the 20 mg daily dose (given as 10 mg bd) resulted in statistically significant (P = 0.013-0.020) improvements in patient Visual Analogue Scale ratings of maximum pain intensity, average pain intensity and pain relief, recorded at the same time daily. The analgesic effects extended over 48 hours, as shown by statistically significant (P = 0.013-0.020) improvements in all three outcomes on the rest days instituted between each daily dose. Analgesic effects (lowered maximum pain intensity and increased pain relief, on the day of dosing only) were also seen when the lower daily dose of 10 mg methadone (given as 5 mg bd) was used, but these failed to reach statistical significance (P = 0.064 and 0.065, respectively). Interpatient analysis showed that the analgesic effects were not restricted to any particular type of neuropathic pain. Patient compliance was high throughout the trial. One patient withdrew during the 10 mg and six during the 20 mg methadone treatment periods. This is the first double-blind randomized controlled trial to demonstrate that methadone has an analgesic effect in neuropathic pain. In a randomized, double-blind crossover study, 40 patients with postherpetic neuralgia were given single oral doses of clonidine, 0.2 mg, codeine, 120 mg, ibuprofen, 800 mg, or inert placebo. Pain relief and side effects were recorded for 6 hours. Patients reported significantly more relief after clonidine than after the other three treatments. Codeine and ibuprofen were ineffective. Sedation, dizziness, and other side effects were more frequent after clonidine (74%) or codeine (69%) than after placebo (36%) or ibuprofen (28%). Reported pain relief was greater during trials in which side effects were present. A single, mild side effect was associated with as much additional pain relief as multiple, severe side effects. Clonidine's superiority to codeine, which had a similar incidence of side effects, argues for a specific analgesic effect. In addition, side effects may have contributed to clonidine analgesia, perhaps by suggesting to patients that they had received a potent drug. The available drugs to treat neuropathic pain have incomplete efficacy and dose-limiting adverse effects. We compared the efficacy of a combination of gabapentin and morphine with that of each as a single agent in patients with painful diabetic neuropathy or postherpetic neuralgia.
                In this randomized, double-blind, active placebo-controlled, four-period crossover trial, patients received daily active placebo (lorazepam), sustained-release morphine, gabapentin, and a combination of gabapentin and morphine--each given orally for five weeks. The primary outcome measure was mean daily pain intensity in patients receiving a maximal tolerated dose; secondary outcomes included pain (rated according to the Short-Form McGill Pain Questionnaire), adverse effects, maximal tolerated doses, mood, and quality of life.
                Of 57 patients who underwent randomization (35 with diabetic neuropathy and 22 with postherpetic neuralgia), 41 completed the trial. Mean daily pain (on a scale from 0 to 10, with higher numbers indicating more severe pain) at a maximal tolerated dose of the study drug was as follows: 5.72 at baseline, 4.49 with placebo, 4.15 with gabapentin, 3.70 with morphine, and 3.06 with the gabapentin-morphine combination (P<0.05 for the combination vs. placebo, gabapentin, and morphine). Total scores on the Short-Form McGill Pain Questionnaire (on a scale from 0 to 45, with higher numbers indicating more severe pain) at a maximal tolerated dose were 14.4 with placebo, 10.7 with gabapentin, 10.7 with morphine, and 7.5 with the gabapentin-morphine combination (P<0.05 for the combination vs. placebo, gabapentin, and morphine). The maximal tolerated doses of morphine and gabapentin were lower (P<0.05) with the combination than for each drug as single agent. At the maximal tolerated dose, the gabapentin-morphine combination resulted in a higher frequency of constipation than gabapentin alone (P<0.05) and a higher frequency of dry mouth than morphine alone (P<0.05).
                Gabapentin and morphine combined achieved better analgesia at lower doses of each drug than either as a single agent, with constipation, sedation, and dry mouth as the most frequent adverse effects.
                Copyright 2005 Massachusetts Medical Society. Cold allodynia and hyperalgesia are frequent clinical findings in patients with neuropathic pain. While there have been several clinical studies showing the involvement of central sensitization mechanisms and N-methyl-D-aspartate (NMDA) receptor activation in mechanical allodynia/hyperalgesia and ongoing pain, the mechanisms of thermal allodynia and hyperalgesia have received less attention. The aim of the present study was to examine the effect of the NMDA-receptor antagonist ketamine on thermal allodynia/hyperalgesia, ongoing pain and mechanical allodynia/hyperalgesia in patients with neuropathic pain (11 patients with post-traumatic neuralgia and one patient with post-herpetic neuralgia). All the patients were known to suffer from severe cold allodynia (cold pain detection threshold (CPDT): 23.8 degrees C, median value). The mu-opioid agonist alfentanil was used as an active control. The study design was double-blind and placebo-controlled and the drugs were administered i.v. (bolus dose and infusion). CPDT in the asymptomatic contralateral area was found to be significantly decreased (cold allodynia) compared to CPDT in site- and age-matched normal controls. Heat pain detection thresholds were found to be normal and no consistent heat hyperalgesia occurred. Alfentanil significantly reduced cold allodynia (by increasing CPDT) in symptomatic area (P=0.0076). Ketamine did not significantly increase the threshold. Significant and marked reductions of hyperalgesia to cold (visual analogue score at threshold value) were seen following both alfentanil (4.5 before, 1.4 after, median value) and ketamine (6.8 before, 0.4 after, median value). Alfentanil and ketamine also significantly reduced ongoing pain and mechanical hyperalgesia. It is concluded that NMDA-receptor mediated central sensitization is involved in cold hyperalgesia, but since CPDT remained unaltered, it is likely that other mechanisms are present. Opioid treatment has played a limited role in the management of diabetic neuropathy, in part because of concerns about the responsiveness of neuropathic pain to opioid treatment. This controlled study evaluated the efficacy and safety of controlled-release (CR) oxycodone in subjects with moderate to severe pain due to diabetic neuropathy.
                This multicenter, randomized, double-blind, placebo-controlled, parallel-group study included 159 subjects with moderate to severe pain due to diabetic neuropathy. Treatment began with either one 10-mg tablet of CR oxycodone (n = 82) or identical placebo (n = 77) every 12 hours. Doses could be increased every 3 days to a maximum of 6 tablets (60 mg CR oxycodone) every 12 hours. Treatment lasted up to 6 weeks. The primary efficacy variable was overall average daily pain intensity during study days 28 to 42.
                At an average (SD) dose of 37 (21) mg per day (range 10 to 99 mg/d), CR oxycodone provided more analgesia than placebo (p= 0.002) in the intent-to-treat cohort. From days 28 to 42, overall average daily pain intensity (least squares mean +/-SE), rated in subject diaries on a numeric scale of 0 (no pain) to 10 (pain as bad as you can imagine), was 4.1 +/- 0.3 in subjects given CR oxycodone and 5.3 +/- 0.3 in placebo-treated subjects. Overall, 80 (96%) of 82 subjects given CR oxycodone and 52 (68%) of 77 subjects who received placebo reported adverse events. The most common adverse events in the CR oxycodone group were opioid related.
                In this 6-week trial, CR oxycodone was effective for the treatment of moderate to severe pain due to diabetic neuropathy. Adverse events were typical of opioid-related side effects. Both mu opioid agonists and N-methyl-D-aspartate (NMDA) receptor antagonists are implicated in the regulation of neuropathic pain in post-nerve injury preclinical pain models. This study characterizes the effects of intravenously infused alfentanil (a mu-receptor agonist) and ketamine (an NMDA-receptor antagonist) on human neuropathic pain states, characterized by allodynia and hyperalgesia. Using diphenhydramine as the placebo, alfentanil and ketamine infusions were given in a randomized double-blind fashion 1 week apart via a computer-controlled infusion (CCI) pump that was programmed to target plasma levels of alfentanil at 25, 50 and 75 ng/ml and ketamine at 50, 100 and 150 ng/ml. At the beginning of each infusion and each targeted plasma level, baseline vital signs, neurosensory testing that included thermal thresholds, thermal pain and von Frey filament thresholds, and spontaneous and evoked pain scores were obtained. Moreover, the areas of allodynia or hyperalgesia to stroking and a 5.18 von Frey filament were mapped at the beginning and the end of each infusion. A total of seven males and five females with post-nerve injury allodynia and hyperalgesia were enrolled in the study. Elevations of cold, warm, hot pain and von Frey tactile thresholds were noted. Dose-dependent increases in cold and cold pain thresholds, and reductions in stroking pain scores were noted in both the alfentanil and the ketamine infusions. In addition, alfentanil showed a statistically significant dose-dependent reduction in both spontaneous and von Frey pain scores. Both the alfentanil and ketamine infusions showed a reduction in the stroking hyperalgesic area and ketamine showed a significant reduction in the von Frey hyperalgesia area. No significant CNS side effects and changes in vital signs were noted. A partial deafferentation state was found in the post-nerve injury patients who presented with allodynia and hyperalgesia. The effects of alfentanil on cold and cold pain thresholds and spontaneous pain scores correlates with previous studies suggesting an opiate central analgesic effect. In addition, the reduction of the hyperalgesic area and evoked pain scores with the alfentanil infusion suggests that opioids may have some peripheral effects in the post-nerve injury patients. Therefore, clinical utilization of opioids with careful titration may be beneficial in post-nerve injury patients with partial deafferentation. With the absence of significant CNS side effects, the ketamine infusion not only demonstrated the well-documented spinal cord mechanism of the NMDA receptor, but the result of the current study also suggests that a peripheral mechanism of NMDA receptor may exist. The relationship between central sensitization and regulation of peripheral NMDA-receptor expression requires further investigation. Although opioid analgesics are used in the management of neuropathic pain syndromes, evidence of their efficacy remains to be established. We evaluated the clinical efficacy and safety of oxycodone in neuropathic pain using postherpetic neuralgia as a model.
                Patients with postherpetic neuralgia of at least moderate intensity were randomized to controlled-release oxycodone 10 mg or placebo every 12 hours, each for 4 weeks, using a double-blind, crossover design. The dose was increased weekly up to a possible maximum of 30 mg every 12 hours. Pain intensity and pain relief were assessed daily, and steady (ongoing) pain, brief (paroxysmal) pain, skin pain (allodynia), and pain relief were recorded at weekly visits. Clinical effectiveness, disability, and treatment preference were also assessed.
                Fifty patients were enrolled and 38 completed the study (16 men, 22 women, age 70+/-11 years, onset of postherpetic neuralgia 31+/-29 months, duration of pain 18+/-5 hours per day). The oxycodone dose during the final week was 45+/-17 mg per day. Compared with placebo, oxycodone resulted in pain relief (2.9+/-1.2 versus 1.8+/-1.1, p=0.0001) and reductions in steady pain (34+/-26 versus 55+/-27 mm, p=0.0001), allodynia (32+/-26 versus 50+/-30 mm, p=0.0004), and paroxysmal spontaneous pain (22+/-24 versus 42+/-32 mm, p=0.0001). Global effectiveness, disability, and masked patient preference all showed superior scores with oxycodone relative to placebo (1.8+/-1.1 versus 0.7+/-1.0, p=0.0001; 0.3+/-0.8 versus 0.7+/-1.0, p=0.041; 67% versus 11%, p=0.001, respectively).
                Controlled-release oxycodone is an effective analgesic for the management of steady pain, paroxysmal spontaneous pain, and allodynia, which frequently characterize postherpetic neuralgia. To investigate the effects of IV morphine on central pain syndromes through quantitative sensory testing and to assess the long-term benefit of oral morphine.
                After an initial open titration phase aiming to determine the maximal tolerated dosage of IV morphine, the efficacy of morphine infusion (9-30 mg; mean dosage, 16 mg) was assessed in a double-blind, placebo-controlled and crossover fashion in 15 patients with poststroke- (6 patients) or spinal cord injury- (9 patients) related pain. All of the patients subsequently received sustained oral morphine.
                Morphine significantly reduced the intensity of brush-induced allodynia but had no effect on other evoked pains (i.e., static mechanical and thermal allodynia/hyperalgesia). The effects of morphine on ongoing pain were not significantly different from those of the placebo, but 7 patients (46%) responded to morphine. There was a correlation between the effects of morphine on spontaneous pain and the decrease of the responses to suprathreshold thermal stimuli on the nonpainful contralateral side, suggesting that these effects were related to the general antinociceptive activity of the drug. The effects of IV morphine were correlated with those of oral morphine at 1 month, but only 3 patients (20%) were still taking morphine after 1 year.
                IV morphine induces analgesic effects on some components of central neuropathic pain syndromes, but only a minority of patients may benefit from long-term opioid treatment. Although opioids are commonly used to treat chronic neuropathic pain, there are limited data to guide their use. Few controlled trials have been performed, and many types of neuropathic pain remain unstudied.
                Adults with neuropathic pain that was refractory to treatment were randomly assigned to receive either high-strength (0.75-mg) or low-strength (0.15-mg) capsules of the potent mu-opioid agonist levorphanol for eight weeks under double-blind conditions. Intake was titrated by the patient to a maximum of 21 capsules of either strength per day. Outcome measures included the intensity of pain as recorded in a diary, the degree of pain relief, quality of life, psychological and cognitive function, the number of capsules taken daily, and blood levorphanol levels.
                Among the 81 patients exposed to the study drug, high-strength levorphanol capsules reduced pain by 36 percent, as compared with a 21 percent reduction in pain in the low-strength group (P=0.02). On average, patients in the high-strength group took 11.9 capsules per day (8.9 mg per day) and patients in the low-strength group took close to the 21 allowed (18.3 capsules per day; 2.7 mg per day). Affective distress and interference with functioning were reduced, and sleep was improved, but there were no differences between the high-strength group and the low-strength group in terms of these variables. Noncompletion of the study was primarily due to side effects of the opioid. Patients with central pain after stroke were the least likely to report benefit.
                The reduction in the intensity of neuropathic pain was significantly greater during treatment with higher doses of opioids than with lower doses. Higher doses produced more side effects without significant additional benefit in terms of other outcome measures.
                Copyright 2003 Massachusetts Medical Society We studied the analgesic efficacy of an intravenous infusion of lidocaine and morphine in 19 adults with well-established postherpetic neuralgia in a three-session, randomized, double-blind, placebo-controlled trial. Compared with saline placebo, both lidocaine and morphine reduced pain intensity. Reductions in pain did not correlate with side effects produced by the infusions. For morphine, there was a significant correlation between reductions in pain intensity and blood level achieved. In the majority of subjects who reported definite pain relief, allodynia also disappeared. The results show that neuropathic pain can respond to opioids and to systemically administered local anesthetic drugs. Forty-three patients with peripheral neuropathic pain, exclusively pain reduced by spinal cord stimulation (SCS), were switched into a painful state after SCS inactivation. This mode was used to assess the pain-relieving effect of carbamazepine (CMZ) and opioids in a double-blinded, placebo-controlled trial. In Phase 1, the patients were randomly allocated to receive either CMZ (600 mg/d) or placebo during an SCS-free period of 8 days. In Phase 2, after a CMZ elimination interval of 7 days, 38 patients received either sustained-release morphine (90 mg/d) or placebo for 8 days. In cases of intolerable pain, the patients were authorized to reactivate their SCS. The pain intensity was rated on a numeric analog scale. In 38 patients who completed Phase 1, significant delay in pain increase was observed in the CMZ group as compared with placebo (P = 0.038). In Phase 2, the trend observed with morphine was insignificant (P = 0.41). Two CMZ patients and one morphine patient showed complete pain relief and preferred to continue the medication. Thirty-five patients returned to SCS. We conclude that CMZ is effective in peripheral neuropathic pain. Morphine obviously requires larger individually titrated dosages than those used in this study for results to be adequately interpreted. In a double-blind, placebo controlled crossover study, the effect of morphine on the affective and sensory pain ratings in different forms of chronic pain was investigated. Six patients suffering from central neurogenic pain, 8 from peripheral neurogenic pain and 6 from idiopathic pain participated in the study. Morphine (0.3 mg/kg bodyweight) and placebo (saline) were administered intravenously. Both the affective and sensory dimensions of pain sensation were assessed by means of the 101-point rating scale. From our results it appeared that morphine reduced the affective but not the sensory dimension of pain sensation in both groups of neurogenic pain patients. In the idiopathic pain group, neither the affective nor the sensory dimension of pain sensation were affected. The observed differences in opioid responsiveness were neither the result of differences in opioid consumption nor of differences in baseline pain levels. Painful neuropathy is one of the most common long-term complications of diabetes mellitus and often proves difficult to relieve.
                Patients with diabetic neuropathy with moderate or greater pain for at least 3 months, were evaluated for efficacy, safety and health-related quality of life (QOL) while receiving controlled-release (CR) oxycodone (OxyContin) or active placebo. Patients underwent washout from all opioids 2-7 days before randomization to 10 mg CR oxycodone or active placebo (0.25 mg benztropine) q12h. The dose was increased, approximately weekly, to a maximum of 40 mg q12h CR oxycodone or 1 mg q12h benztropine, with crossover to the alternate treatment after a maximum of 4 weeks. Acetaminophen, 325-650 mg q4-6h prn was provided as rescue.
                Thirty-six patients were evaluable for efficacy (21 men, 15 women, mean age 63.0+/-9.4 years). CR oxycodone resulted in significantly lower (P=0.0001) mean daily pain (21.8+/-20.7 vs. 48.6+/-26.6 mm VAS), steady pain (23.5+/-23.0 vs. 47.6+/-30.7 mm VAS), brief pain (21.8+/-23.5 vs. 46.7+/-30.8 mm VAS), skin pain (14.3+/-20.4 vs. 43.2+/-31.3 mm VAS), and total pain and disability (16.8+/-15.6 vs. 25.2+/-16.7; P=0.004). Scores from 6 of the 8 SF-36 domains and both summary scales, Standardized Physical Component (P=0.0002) and Standardized Mental Component (P=0.0338) were significantly better during CR oxycodone treatment. The number needed to treat to obtain one patient with at least 50% pain relief is 2.6 and clinical effectiveness scores favoured treatment with CR oxycodone over placebo (P=0.0001).
                CR oxycodone is effective and safe for the management of painful diabetic neuropathy and improves QOL. There is controversy about whether the lack of response of some chronic pain to opioid treatment is absolute or relative. It is widely believed that nociceptive pain is responsive to opioids whereas neuropathic pain tends not to be. We have used a method of patient-controlled analgesia (PCA) with simultaneous nurse-observer measurement of analgesia, mood, and adverse effects to address these issues. Ten patients with chronic pain were given morphine at two concentrations (10 and 30 mg/ml) by PCA in two separate sessions in a double-blind randomised crossover study. Before the study a clinical judgment was made as to whether each pain was nociceptive or neuropathic. Seven patients showed good analgesic responses (more than 70 mm pain relief on a visual-analogue scale) of pain at rest, two patients poor responses (less than 30 mm pain relief), and one a moderate response with both concentrations (30-70 mm pain relief). The response to morphine was consistent (greater and faster relief with the higher concentration) in nine patients. Two patients had pain on movement that responded moderately to low-concentration morphine and well to the higher concentration. All patients with pains judged to be nociceptive showed good analgesic responses compared with half of those with neuropathic pain. There was no evidence that analgesic responses in patients with neuropathic pain were due to changes in mood. This PCA method is a quick and efficient tool to determine the consistency of the analgesic response. Such consistency can guide the clinician as to whether continued or higher-dose opioid treatment will produce good analgesia. An inconsistent response points to the use of other pain-relieving strategies. Tricyclic antidepressants (TCA) provide less than satisfactory pain relief for postherpetic neuralgia (PHN), and the role of opioids is controversial.
                To compare the analgesic and cognitive effects of opioids with those of TCA and placebo in the treatment of PHN.
                Seventy-six patients with PHN were randomized in a double-blind, placebo-controlled, crossover trial. Each subject was scheduled to undergo three treatment periods (opioid, TCA, and placebo), approximately 8 weeks' duration each. Doses were titrated to maximal relief or intolerable side effects. The primary outcome measures were pain intensity (0 to 10 scale), pain relief (0 to 100%), and cognitive function. Analyses included patients who provided any pain ratings after having received at least a single dose of a study medication.
                Fifty patients completed two periods, and 44 patients completed all three. Mean daily maintenance doses were morphine 91 mg or methadone 15 mg and nortriptyline 89 mg or desipramine 63 mg. Opioids and TCA reduced pain (1.9 and 1.4) more than placebo (0.2; p < 0.001), with no appreciable effect on any cognitive measure. The trend favoring opioids over TCA fell short of significance (p = 0.06), and reduction in pain with opioids did not correlate with that following TCA. Treatment with opioids and TCA resulted in greater pain relief (38 and 32%) compared with placebo (11%; p < 0.001). More patients completing all three treatments preferred opioids (54%) than TCA (30%; p = 0.02).
                Opioids effectively treat PHN without impairing cognition. Opioids and TCA act via independent mechanisms and with varied individual effect. NMDA antagonists and opioids relieve experimentally produced hyperalgesia in animals and humans, presumably by attenuating a heightened central nervous system response to afferent input. A few small studies in patients have suggested that intravenous boluses or rapid infusions of the N-methyl-D-aspartate (NMDA) antagonist ketamine relieve some neuropathic pains but also produce disturbances of cognition and mood. In a randomized, double-blind, crossover trial, we treated eight patients with chronic posttraumatic pain and widespread mechanical allodynia with 2-h intravenous infusions of the NMDA antagonist ketamine (mean dose, 58 mg), the opioid mu-receptor agonist alfentanil (mean dose, 11 mg), and placebo. The patients were selected because extensive sensory testing suggested that altered central processing contributed to their symptoms. The slow rate of drug infusion was chosen to see if pain relief would precede dose-limiting side effects. Means of the peak effect scores achieved during the 2-h infusion were for pain relief: ketamine, 65%, alfentanil, 46%, and placebo, 22% (p < 0.01 for ketamine and p = 0.08 for alfentanil, each compared to placebo); and for relief of allodynia: ketamine, 71%, alfentanil, 57%, and placebo, 21% (p < 0.01 for both ketamine and alfentanil). Appreciable symptomatic relief developed only after the onset of unpleasant drug side effects. After the infusion was stopped, pain relief disappeared before the side effects resolved. We conclude that NMDA antagonists may have promise for the treatment of neuropathic pain, but strategies are needed to improve their therapeutic ratio, such as intrathecal administration or systemic treatment with more selective drugs. The aim of the present study has been to assess the responsiveness of various types of chronic pain to opioids given i.v. and tested against placebo in a double-blind, randomized fashion. Pain classified as primary nociceptive was effectively alleviated (P greater than 0.001) while neuropathic deafferentation pain was not significantly influenced by morphine or equivalent doses of other opioids. Also 'idiopathic' pain, defined as chronic pain with no or little demonstrable pathology, failed to respond. The results were not related to whether the patients were regular users of narcotic analgesics or not. The outcome of our double-blind opioid test has proved useful to justify a continued, or discontinued, use of narcotic medication in individual patients. It may also support the indication and choice of invasive stimulation procedures (spinal cord or brain). The results of the study illustrate the misconception of chronic pain as an entity and highlight the importance of recognizing different neurobiological mechanisms and differences in responsiveness to analgesic drugs as well as to non-pharmacological modes of treatment. The opioid test has thus become a valuable tool in pain analysis and helpful as a guide for further treatment.
Short-term studies provide only equivocal evidence regarding the efficacy of opioids in reducing the intensity of neuropathic pain, whereas intermediate-term studies demonstrate significant efficacy of opioids over placebo, which is likely to be clinically important. Reported adverse events of opioids are common but not life threatening. Further randomized controlled trials are needed to establish long-term efficacy, safety (including addiction potential), and effects on quality of life.