Dextromethorphan for the treatment of neuropathic pain: a double-blind randomised controlled crossover trial with integral n-of-1 design

The aim was to compare the analgesic effectiveness and adverse effect incidence of oral dextromethorphan (DM) with placebo in patients with neuropathic pain. The first 10-day treatment period was a multiple-dose double-blind randomised controlled cross-over comparison of 13.5 mg of DM 3 times a day (t.d.s.) with placebo t.d.s.: 5 treatment pairs, each pair 1 day DM and 1 day placebo. The second 10-day treatment period used 27 mg of DM t.d.s. vs. placebo, with the same design. The study incorporated a 5 pair n-of-1 design for each of the 2 doses of DM. Patients took the study medication in addition to any pre-existing analgesic regime. Patients who reported benefit could continue with DM after the study. Nineteen patients with chronic neuropathic pain were studied over two 10-day treatment periods. Outcome measures were pain intensity, pain relief, adverse effects, mood, sleep and global rating of treatment. These were recorded by daily patient diaries and by clinic assessments before and after each treatment period. There were no significant differences between DM and placebo on any of the clinic assessment outcome measures. Two patients had significantly better analgesia on more than one outcome measure on within-patient testing. One had better analgesia with DM. The other had better analgesia with placebo. Ten patients had no adverse effects on either dose of DM. Two patients withdrew during the first treatment period because of adverse effects (which included increased pain intensity), and 5 during the second period. Five patients continued with DM after the study for 1-3 months. No long-term clinical benefit was apparent in those who continued with open DM. Dextromethorphan at either 40.5 or 81 mg daily did not relieve neuropathic pain.

Cross-tolerance between mu opioid and alpha-2 adrenergic receptors, but not between mu and delta opioid receptors in the spinal cord of the rat

Intrathecal administration of morphine, Tyr-D-Ser(otbu)-Gly-Phe-Leu-Thr or dexmedetomidine for 5 to 10 days rendered rats tolerant to the test drug as measured by both behavioral and electrophysiological tests. Tolerance to the alpha-2 adrenergic agonist dexmedetomidine required a longer induction time and was not as pronounced as the tolerance to the opioid agonists, probably because lower doses of dexmedetomidine relative to the ED50 dose were used to avoid sedation. In the behavioral studies we used the tail-flick test and in the electrophysiological studies recordings were made from dorsal horn nociceptive neurons under halothane anesthesia. After completion of the behavioral testing the same animals were then used in the electrophysiological study. Cross-tolerance developed clearly between the mu opioid agonist morphine and the alpha-2 adrenergic agonist dexmedetomidine, whereas no cross-tolerance was seen between the delta opioid agonist Tyr-D-Ser(otbu)-Gly-Phe-Leu-Thr and either morphine or dexmedetomidine. This is further evidence to support the assumption that in the dorsal horn the mu opioid and the alpha-2 adrenergic receptor are linked functionally, whereas the delta opioid receptor operates independently. These results have also important clinical implications indicating the potential of delta opioid agonists to restore analgesia in a morphine-tolerant patient.

A multiple dose comparison of ibuprofen and dihydrocodeine after third molar surgery

The objectives were to compare the relative merits of ibuprofen 400 mg and dihydrocodeine 30 mg or 60 mg taken up to four times daily for up to 6 days in the treatment of pain after third molar removal. A randomised, double-blind, multiple dose, crossover study was undertaken in 68 patients undergoing two-stage bilateral lower third molar removal. The results showed that ibuprofen produced significantly greater analgesia than either of the dihydrocodeine treatments on the day of surgery. Ibuprofen and dihydrocodeine 60 mg produced significantly greater analgesia than dihydrocodeine 30 mg on the day after surgery, and on days 4 and 5 ibuprofen was again significantly superior to dihydrocodeine 60 mg. Roughly half the patients taking dihydrocodeine stopped the study on the day after surgery, because of adverse effects and/or inadequate relief, compared with 6 out of 44 taking ibuprofen. Dihydrocodeine 60 mg produced four times the number of patients affected by adverse effects compared with ibuprofen, and dihydrocodeine 30 mg three times as many. The principal adverse effects were nausea, vomiting and drowsiness. In conclusion, ibuprofen produced better analgesia than dihydrocodeine with significantly fewer adverse effects and is therefore a better choice for pain relief after oral surgery.

Dose-response for analgesic effect of amitriptyline in chronic pain

A randomised, double-blind, multiple dose, crossover study with three 3-week treatment periods was set up to compare the analgesic efficacy and adverse effects of amitriptyline in oral doses of 25, 50 or 75 mg. Patients used diaries to assess their pain, and clinic assessments were made at the end of each treatment period. It was found that in 29 patients with chronic (more than 2 months) pain, amitriptyline 75 mg provided significantly greater efficacy than amitriptyline 25 or 50 mg. There was no significant difference in mood scores between the different doses of amitriptyline, but sleep was judged significantly better with 75 mg compared with 25 mg. The incidence of adverse effects was significantly higher with the 75 mg dose, and the principal adverse effects were dry mouth and drowsiness. In the context of chronic pain, the analgesic effect of amitriptyline was shown to have a dose-response unrelated to mood elevation, but there was a dose-response for the incidence of adverse effects.

Lack of effect of morphine-3-glucuronide on the spinal antinociceptive actions of morphine in the rat: an electrophysiological study

Previous studies have shown that morphine-3-glucuronide (M3G), a metabolite of morphine, may functionally antagonize the antinociceptive action of morphine. The interaction between morphine and M3G was therefore investigated in the halothane-anaesthetised rat. Extracellular unit recordings were made of the innocuous A-beta fibre and noxious C-fibre evoked responses of convergent dorsal horn neurones. Intrathecal M3G (5 micrograms, 100 micrograms and 500 micrograms) alone did not show any antinociceptive effect. There was a slight, but not statistically significant, decrease in the antinociceptive effect of 5 micrograms morphine in the M3G-pretreated groups. However, M3G pretreatment (5 micrograms, 100 micrograms and 500 micrograms) had no effect on the higher dose of morphine (50 micrograms) used. We conclude that M3G has, at most, a minor effect on the spinal antinociceptive effects of morphine.

A high-yield strategy to identify randomized controlled trials for systematic reviews

OBJECTIVE

To compare the sensitivity, precision, and the costs in time of searching by hand and by MEDLINE to identify randomized controlled trials (RCTs) for systematic reviews.

DESIGN

Nine anesthetic and pain journals were searched by both MEDLINE and by hand for the years 1970, 1980, and 1990, recording number of publications and time taken to identify randomized double-blind controlled trials in pain research.

RESULTS

Thirty-four volumes, containing 5583 full publications (24,417 pages) and 2889 abstracts and letters (1755 pages) were hand searched; 142 eligible (definite RCTs) full papers and 171 eligible abstracts and letters were identified. The MEDLINE search strategy yielded 274 reports of which 138 were eligible; 125 of these were full papers, 1 was a letter and 12 were abstracts. Two full papers which were identified by the MEDLINE search strategy were missed by hand search. The overall sensitivity of the MEDLINE search strategy for full papers was 87% ([125/144] x 100) with a precision of 52% ([125/242] x 100). This is the best combination of sensitivity and precision reported to date. Abstracts were mostly in supplement issues which were not indexed. Combining the MEDLINE search strategy with selective hand search of abstracts and letters gave a sensitivity of 94%.

CONCLUSIONS

Hand search of entire journals remains the most accurate method for identification of the eligible reports, but it is the most time-consuming. The MEDLINE search was faster, but it failed to identify at least 13% of the indexed eligible reports. Ideally, both hand search and MEDLINE should be used. The combination of MEDLINE with hand search restricted to letters and abstracts might be an acceptable alternative for reviewers with insufficient funds to support a full hand search process.

Opioid sensitivity of chronic pain: a patient-controlled analgesia method

Twenty-two patients with chronic pain of malignant or nonmalignant origin were given intravenous morphine by patient-controlled analgesia. A prestudy judgment was made from the characteristics of the pain as to whether it was nociceptive or neuropathic. Analgesic efficacy was assessed by a nurse-observer; adverse events were noted and plasma morphine and metabolitie concentrations measured. Three categories of opioid response were distinguished. Good responders obtained > 70 mm relief on the visual analogue scale, with minimal or manageable adverse events. Moderate responders obtained < 70 but > 30 mm relief with more problematic adverse events, and poor responders had < 30 mm relief with troublesome adverse events. This method for the study of opioid sensitivity allowed a wide dosage range to be studied. The simultaneous analgesic and adverse event measurements showed that the spectrum of observed response was wide, and response category could be judged for the majority by 4 h. In those with poor or moderate response, adverse event severity limited further dose increment. The relationship between pain characteristics and response showed that some pains judged to be neuropathic had a good response to opioid (5/13), and some pains judged to be nociceptive did not (5/14). The study suggests that the pattern of response is not as black and white as the prediction of good response from nociceptive pain and poor from neuropathic pain would suggest, although nociceptive pain was more likely than neuropathic pain to show a good response. For the moderate responders opioid titration may, in the absence of other effective treatments, be useful, but the analgesic endpoint may not be totally satisfactory. The method provides an operational definition of opioid sensitivity.

A multiple dose comparison of combinations of ibuprofen and codeine and paracetamol, codeine and caffeine after third molar surgery

In a randomised, double-blind, double-dummy, multiple dose, crossover study in 30 patients we compared an ibuprofen/codeine combination (400 mg ibuprofen/25.6 mg codeine phosphate) with a paracetamol/codeine/caffeine combination (1 g paracetamol/16 mg codeine phosphate/60 mg caffeine) for pain relief over 6 days after two-stage bilateral lower third molar removal. The ibuprofen combination produced significantly greater analgesia than the paracetamol combination, both on single-dose analysis of the first and second days and on multiple-dose measures for days 1, 2, 3 and 4. The mean incidence of adverse effects over the 6 days was 20% for both combinations. This trial design (crossover with multiple dosing in outpatients) is a sensitive way of testing for analgesia, and is potentially more predictive of adverse effect problems than single-dose studies. It confirms that multiple dosing may show increased efficacy.

Low dose amitriptyline in the treatment of chronic pain

The analgesic efficacy of amitriptyline 25 mg was compared with placebo in 41 patients with chronic (more than 3 months) nonmalignant pain, using a double-blind randomised multiple-dose 3-week treatment period crossover design. Amitriptyline 25 mg provided significantly greater efficacy than placebo, with significant differences evident within the first week. There was no significant difference on mood scores between amitriptyline and placebo. The results suggest that surprisingly low doses of amitriptyline may be effective without substantial adverse effects, that the effect is evident early, and that the effect is distinguishable from any effect of the amitriptyline on mood.

Morphine responsiveness of chronic pain: double-blind randomised crossover study with patient-controlled analgesia

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.

Spinal antinociception by Tyr-D-Ser(otbu)-Gly-Phe-Leu-Thr, a selective delta-opioid receptor agonist

The spinal antinociceptive potency of the delta-opioid receptor agonist, Tyr-D-Ser(otbu)-Gly-Phe-Leu-Thr (DSTBULET), was studied in rats. The tail flick test was used as nociceptive stimulus and the rotarod test was used to detect any motor or sedative effects. A dose-response curve was also made for the mu-opioid receptor agonist, morphine. The ED50 for DSTBULET was 0.3 micrograms (0.4 nmol) and a near 100% maximum effect was achieved with 5 micrograms (7.5 nmol). No motor or sedative effects were detected. Antinociception by DSTBULET was antagonized by s.c. naltrindole (1 mg/kg), a selective delta-opioid receptor antagonist, and naloxone (1 mg/kg), a non-selective opioid receptor antagonist. The ED50 for morphine was 0.5 micrograms (1.0 nmol) and the antinociceptive effects were not antagonized by naltrindole (1 mg/kg). The results evidence further the important role of the delta-opioid receptor in spinal nociceptive processing.

Evidence for the involvement of the μ but not δ opioid receptor subtype in the synergistic interaction between opioid and α2 adrenergic antinociception in the rat spinal cord

The interaction between the spinal antinociceptive effects of selective mu or delta opioid agonists morphine and DSTBULET (Tyr-D-Ser(OtBu)-Gly-Phe-Leu-Thr), respectively, and the selective alpha 2 adrenergic agonist dexmedetomidine was examined on convergent dorsal horn neuronal responses in the intact anaesthetized rat. The coadministration of intrathecal morphine (0.5 microgram, 2.5 micrograms) and dexmedetomidine (0.5 microgram) produced a greater than additive inhibition of C fibre-evoked responses. Inhibitions were reversed by either the opioid antagonist naloxone or the alpha 2 adrenergic antagonist atipamezole. The coadministration of intrathecal DSTBULET (1 microgram, 2.5 micrograms) and dexmedetomidine did not result in a supra-additive inhibition of C fibre-evoked responses. The results suggest that mu rather than delta opioid receptors are involved in the synergism of spinal opioid and alpha 2 adrenergic antinociception.

The antinociceptive actions of dexmedetomidine on dorsal horn neuronal responses in the anaesthetized rat

The actions of the selective alpha 2-adrenoceptor agonist dexmedetomidine were examined on the nociceptive C and innocuous A beta fibre-evoked responses of dorsal horn neurones to transcutaneous electrical stimulation in the intact anaesthetized rat. C fibre-evoked responses were dose dependently reduced by intrathecal dexmedetomidine--to a maximum 86 +/- 6% inhibition by 10 micrograms of the agonist. The ED50 for inhibition of C fibre responses was estimated to be 2.5 micrograms. A beta-evoked responses were inhibited to a lesser degree--a maximum 54 +/- 8% inhibition after 10 micrograms dexmedetomidine. The antinociceptive effects of dexmedetomidine were reversed by the alpha 2-adrenoceptor antagonist atipamezole and the opioid antagonist naloxone. The results are discussed with reference to adrenergic and opioid mechanisms in the spinal cord.

Dextromethorphan and levorphanol on dorsal horn nociceptive neurones in the rat

Intrathecal administration of dextromethorphan and levorphanol and intravenous injection of dextromethorphan were tested on the electrophysiological response of deep multireceptive dorsal horn neurones to peripheral stimuli. Both blockade of C-fibre input to the cells and wind-up, the increase in C-fibre firing with repeated stimulus, were recorded. Intrathecal injection of levorphanol (0.25-100 micrograms) had a typical opioid effect, blocking the C-fibre input. Its affect on wind-up was dose-dependent, paralleled precisely the blocking effect on the C-fibre input and both effects were reversed by naloxone. Unlike levorphanol and other opiates, intrathecal administration of dextromethorphan (50-500 micrograms) blocked the C-fibre input and A beta response in parallel and was not reversed by naloxone. Wind-up was reduced by a maximum of 56% at the largest dose tested. Intravenous injection of dextromethorphan (5 mg/kg) also produced a reduction in wind-up but not in the C-fibre response.

Double-blind testing fails to confirm analgesic response to extradural morphine

We report two patients with chronic non-malignant pain in whom morphine given intravenously via a patient-controlled analgesia system produced partial pain relief but was accompanied by severe side effects. Open administration of epidural morphine resulted in complete pain relief with minimal side effects and the patients were considered as candidates for implanted opioid delivery systems. However, when the epidural morphine was given in a double-blind and placebo-controlled manner, morphine did not produce greater analgesia than placebo and no dose-response relationship was seen. These cases show that careful investigation is necessary before proceeding to implanted systems and that changing the route did not improve the analgesia:side effect balance for morphine in these patients.

FLFQPQRF-amide modulates alpha 2-adrenergic antinociception in the rat dorsal horn in vivo

The interaction between FLFQPQRFamide and alpha 2-adrenergic spinal antinociception was examined in an electrophysiological study in the intact anaesthetised rat. The inhibition of C fibre-evoked neuronal responses by the selective alpha 2-adrenergic agonist dexmedetomidine was significantly reduced by intrathecal FLFQPQRFamide pretreatment. The results suggest a modulatory role of FLFQPQRFamide in spinal alpha 2-adrenergic antinociception.

Opioid clinical pharmacology and routes of administration

Opiate prescription is based on titration to effect. This principle is supported by the difference between the laboratory and clinical pharmacologies of opiates. Clinically the presence of nociceptive pain appears to act as a counter to the respiratory depressant effect of opiates, and perhaps the dependence, which are such features in the laboratory. Factors in choosing between opiates are described; these include onset speed, duration of effect, toxic and active metabolites and specific side-effects. Side-effect comparison between opiates is only satisfactory when the drugs are compared at equianalgesic doses. The kinetic and clinical logic of alternative routes is explored. Overall there is probably more difference between the effect of the same opiate given by different routes than between the effects of different opiates given by the same route.

32-gauge spinal catheters through 26-gauge needles

Small diameter intrathecal catheters potentially combine the certainty of intrathecal injection and the advantage of repeatability, without the risk of a high incidence of headache after dural puncture. We report problems placing such catheters.