An attempt to attenuate experimental pain in humans by dextromethorphan, an NMDA receptor antagonist

Human surrogate models of central sensitization: A critical review and practical guide

Background

As in other fields of medicine, development of new medications for management of neuropathic pain has been difficult since preclinical rodent models do not necessarily translate to the clinics. Aside from ongoing pain with burning or shock‐like qualities, neuropathic pain is often characterized by pain hypersensitivity (hyperalgesia and allodynia), most often towards mechanical stimuli, reflecting sensitization of neural transmission.

Data treatment

We therefore performed a systematic literature review (PubMed‐Medline, Cochrane, WoS, ClinicalTrials) and semi‐quantitative meta‐analysis of human pain models that aim to induce central sensitization, and generate hyperalgesia surrounding a real or simulated injury.

Results

From an initial set of 1569 reports, we identified and analysed 269 studies using more than a dozen human models of sensitization. Five of these models (intradermal or topical capsaicin, low‐ or high‐frequency electrical stimulation, thermode‐induced heat‐injury) were found to reliably induce secondary hyperalgesia to pinprick and have been implemented in multiple laboratories. The ability of these models to induce dynamic mechanical allodynia was however substantially lower. The proportion of subjects who developed hypersensitivity was rarely provided, giving rise to significant reporting bias. In four of these models pharmacological profiles allowed to verify similarity to some clinical conditions, and therefore may inform basic research for new drug development.

Conclusions

While there is no single “optimal” model of central sensitization, the range of validated and easy‐to‐use procedures in humans should be able to inform preclinical researchers on helpful potential biomarkers, thereby narrowing the translation gap between basic and clinical data.

Significance

Being able to mimic aspects of pathological pain directly in humans has a huge potential to understand pathophysiology and provide animal research with translatable biomarkers for drug development. One group of human surrogate models has proven to have excellent predictive validity: they respond to clinically active medications and do not respond to clinically inactive medications, including some that worked in animals but failed in the clinics. They should therefore inform basic research for new drug development.

Dextromethorphan Analgesia in a Human Experimental Model of Hyperalgesia

Editor’s Perspective

What We Already Know about This Topic

What This Article Tells Us That Is New

Background

Central pain sensitization is often refractory to drug treatment. Dextromethorphan, an N-methyl-d-aspartate receptor antagonist, is antihyperalgesic in preclinical pain models. The hypothesis is that dextromethorphan is also antihyperalgesic in humans.

Methods

This randomized, double-blind, placebo-controlled, crossover study explores the antihyperalgesic effect of single and repeated 30-mg dose of oral dextromethorphan in 20 volunteers, using the freeze-injury pain model. This model leads to development of primary and secondary hyperalgesia, which develops away from the site of injury and is associated with central sensitization and activation of N-methyl-d-aspartate receptor in the spinal cord. The primary outcome was antihyperalgesia calculated with the area under the curve of the percentage change in mechanical pain threshold (electronic von Frey) on the area of secondary hyperalgesia. The secondary outcomes were mechanical pain threshold on the area of primary hyperalgesia and cognitive (reaction time) effect.

Results

Single 30-mg results are reported. Antihyperalgesia (% · min) is significantly higher on the area of secondary hyperalgesia with dextromethorphan than placebo (median [interquartile range]: 3,029 [746; 6,195] vs. 710 [–3,248; 4,439], P = 0.009, Hedge’s g = 0.8, 95% CI [0.1; 1.4]). On primary hyperalgesia area, mechanical pain threshold 2 h after drug intake is significantly higher with dextromethorphan (P = 0.011, Hedge’s g = 0.63, 95% CI [0.01; 1.25]). No difference in antinociception is observed after thermal painful stimuli on healthy skin between groups. Reaction time (ms) is shorter with placebo than with dextromethorphan (median [interquartile range]: 21.6 [–37.4; 0.1] vs. –1.2 [–24.3; 15.4], P = 0.015, Hedge’s g = 0.75, 95% CI [0.12; 1.39]). Nonserious adverse events occurrence (15%, 3 of 20 volunteers) was similar in both groups.

Conclusions

This study shows that low-dose (30-mg) dextromethorphan is antihyperalgesic in humans on the areas of primary and secondary hyperalgesia and reverses peripheral and central neuronal sensitization. Because dextromethorphan had no intrinsic antinociceptive effect in acute pain on healthy skin, N-methyl-d-aspartate receptor may need to be sensitized by pain for dextromethorphan to be effective.

A literature review on the pharmacological sensitivity of human evoked hyperalgesia pain models

AIMS

Human evoked pain models can be used to determine the efficacy of new and existing analgesics and to aid in the identification of new targets. Aspects of neuropathic pain can be simulated by inducing hyperalgesia resulting from provoked sensitization. The present literature review aimed to provide insight into the sensitivity of different hyperalgesia and allodynia models of pharmacological treatment.

METHODS

A literature search was performed to identify randomized, double-blind, placebo-controlled studies that included human hyperalgesia pain models and investigated the pharmacodynamic effects of different classes of drugs.

RESULTS

Three hyperalgesia models [ultraviolet B (UVB) irradiation, capsaicin and thermode burn] have been used extensively. Assessment of hyperalgesia/allodynia and pharmacological effect are measured using challenge tests, which generally comprise thermal (heat/cold) or mechanical stimulation (pin-prick, stroking or impact). The UVB model was sensitive to the antihyperalgesic effects of nonsteroidal anti-inflammatory drugs (NSAIDs) and opioids. The capsaicin model was partially sensitive to opioids. The burn model did not detect any antihyperalgesic effects when NSAIDs or local anaesthetics were administered but responded to the effects of N-methyl D-aspartate (NMDA) receptor antagonists by moderately reducing mechanical hyperalgesia.

CONCLUSIONS

Based on pharmacological sensitivity, the UVB model adequately reflects inflammatory pain and was sensitive to NSAIDs and opioids. Findings from the capsaicin and burn models raised questions about the translatability of these models to the treatment of neuropathic pain. There is a need for a reproducible and predictive model of neuropathic pain, either in healthy subjects or in patients.

Investigational drugs for treating agitation in persons with dementia

INTRODUCTION

Agitation is common and distressing in persons with dementia, but safe, effective treatments remain elusive. In this review, the authors describe investigational compounds in ongoing or recently completed clinical trials for this indication and provide an opinion on how they may meet current therapeutic needs.

AREAS COVERED

Phase II and phase III clinical trials for agitation in persons with dementia were searched in US and EU clinical trial registries and in the medical literature for the period January 2013-February 2016 EXPERT OPINION: The authors searches identified 24 recent clinical trials investigating new treatments for agitation in persons with dementia. Candidate drugs in phase III development included the antipsychotic brexpiprazole, the antidepressant citalopram, the novel compound AVP-786 (deuterated-dextromethorphan/quinidine combination) and the cannabinoid nabilone. Of the compounds in phase II clinical trials, ELND005 (scyllo-inositol) is intended to progress into phase III development, based on evidence from a subgroup analysis and biomarker data. After many years without an FDA/EMA (Food and Drug Administration/European Medicines Agency) approved medication to treat agitation in persons with dementia, we may see the arrival of the first approved drug in the near future.

Interaction between dextromethorphan and norpethidine in rats

With increasing interest in the application of dextromethorphan in pain control, it is probable that patients will receive this drug in combination with analgesics such as opioids, giving rise to the potential for previously unobserved drug interactions. The interaction between dextromethorphan, and its pharmacologically active metabolite dextrorphan, and norpethidine, a toxic metabolite of pethidine, was examined in rats. Rats were assigned to receive dextromethorphan (0, 20 or 40 mg/kg) or dextrorphan (0, 15 or 30 mg/kg) combined with norpethidine (0, 28 or 42 mg/kg). The occurrence of seizures, myoclonic jerks and shivering was recorded for 60 min after drug administration. Norpethidine produced dose-related increases in the incidence of seizures, myoclonic jerks and shivering. Dextromethorphan, but not dextrorphan, increased the incidence of these behaviours. It is recommended that extreme caution be exercised if dextromethorphan and pethidine are to be used together.

Preoperative oral dextromethorphan does not reduce pain or morphine consumption after open cholecystectomy

BACKGROUND

Dextromethorphan, the D-isomer of the codeine analog levorphanol, is a weak, noncompetitive N-Methyl-D-Aspartate (NMDA) receptor antagonist. It has been suggested that NMDA receptor antagonists induce preemptive analgesia when administered before tissue injury occurs, thus decreasing the subsequent sensation of pain.

MATERIALS AND METHODS

The study was conducted in the Dr. Ali Shariati Hospital, Tehran University of Medical Sciences, Tehran, Iran, between February 2005 and December 2006. In this study, 72 patients scheduled for elective cholesyctectomy were randomized into three groups to receive either oral dextromethorphan 45 mg (Group D45 = 24), dextromethorphan 90 mg (Group D90 = 24), or placebo (Group C, n = 24), as premedication, 120 minutes before surgery. A visual analog scale (VAS) for pain of each patient was measured at arrival in the ward and six and 24 hours after surgery.

RESULTS

The demographic characteristics of patients, ASA physical status class, duration of surgery, and the basal VAS pain score were similar in the two groups. There was no significant difference in the mean of the VAS pain scores measured over time or morphine consumption among the three groups.

CONCLUSION

Dextromethorphan 45 mg and 90 mg, administrated orally, two hours before surgery, had no effect on postoperative morphine requirement and pain intensity.

Co-administration of dextromethorphan and morphine: reduction of post-operative pain and lack of influence on morphine metabolism

We investigated co-analgesic effect of dextromethorphan in adolescent post-operative patients with idiopathic scoliosis. In a double-blind study, 60 patients with ASA physical status I-II were randomised into two groups. Group dextromethorphan (n = 30; age: 15.9 +/- 2.4 years) was given oral dextromethorphan 30 or 45 mg 1 hr before surgery and 8, 20 and 32 hr after operation. Group placebo (n = 30; age: 16.5 +/- 2.7 years) received placebo at identical times. Post-operative analgesic requirements were assessed using nurse-controlled analgesia system. Pain was assessed using numeric rating scale before first administration of morphine and at 2, 3, 4, 6, 24 and 48 hr after operation. Blood samples were taken 20 min. after the first use of morphine (within 1 hr after operation). The total use of analgesics during surgery was lower in the dextromethorphan group. The dose of morphine providing relief immediately after surgery, as well as total analgesic requirements in the first and second day after surgery did not differ between groups. Subjectively evaluated pain intensity score (numeric rating scale) was lower for the dextromethorphan patients in the first 4 hr, but not later after surgery. Plasma levels of morphine, morphine-6-glucuronide and morphine-3-glucuronide did not differ between groups. Dextromethorphan did not influence morphine glucuronidation, in terms of promotion of formation of any morphine glucuronides. In conclusion, in young patients subjected to spine surgery, addition of dextromethorphan to morphine reduced pain only in early post-operative period. In such patients, co-analgesic action of dextromethorphan was not associated with significant changes in plasma levels of morphine metabolites.

Effect of metabolic blockade on the psychoactive effects of dextromethorphan

OBJECTIVE

Variation in the activity of cytochrome P450 2D6 (CYP2D6) affects the pharmacokinetics and effectiveness of dextromethorphan (DM), because it controls the production of dextrorphan, an active metabolite, with higher affinity for the NMDA receptor than the parent compound. This study examined whether pharmacological inhibition of CYP2D6 activity with quinidine would mimic the genetic mutation and thus also alter the psychoactive effects of DM.

METHODS

In a single-blind, within-subjects study, eight healthy volunteers (all homozygous for the wild type allele for CYP2D6) received placebo and varying doses of DM, both with and without quinidine pre-treatment. Pharmacokinetic and pharmacodynamic measures were assessed at baseline and every hour post-drug for 6 h.

RESULTS

Compared to the no quinidine condition, quinidine pre-treatment decreased the area under the dose-response curve on subjective measures of positively reinforcing effects (e.g., euphoria, p < 0.04; drug liking, p < 0.05), and was significantly greater for measures of dysphoria (e.g., unpleasantness, p < 0.02). These changes corresponded to increased DM and decreased dextrorphan plasma concentrations.

CONCLUSIONS

Compared to DM alone, quinidine pre-treatment inhibited DM metabolism and changed its subjective effects, demonstrating that the psychoactive properties of DM are a function of drug metabolism. These results demonstrate the relationship between CYP2D6 activity, plasma drug levels, and psychoactive drug effects, and have implications for both the abuse liability and therapeutic utility of DM.

Efficacy of pre- and postoperative oral dextromethorphan for reduction of intra- and 24-hour postoperative morphine consumption for transabdominal hysterectomy

OBJECTIVE

We studied the effect of dextromethorphan (DEX), an N-methyl-D-aspartate receptor antagonist, on analgesic consumption and pain scores after abdominal hysterectomy. We aimed to compare the analgesic effectiveness and incidence of adverse side effects of oral DEX with placebo (P).

DESIGN

This was a double-blinded, randomized, placebo-controlled study.

SETTING AND PATIENTS

One hundred patients were randomized to two groups. Group DEX was given 30-mg tablets of oral DEX with their premedication and three more times in the first 24 hours after surgery. Group P received the placebo following the same schedule. Postoperative analgesic requirements were assessed using a patient-controlled analgesia system. Pain was assessed at rest using a visual analogue scale in the post-anesthetic care unit (PACU), 6 and 24 hours after surgery.

RESULTS

Mean pain scores were significantly lower at the PACU (62.6 vs 75.7) as was the mean sum of all resting pain scores (144.5 vs 173.1). Mean morphine consumption was greatest in the DEX group (35.1 vs 33.0 mg; P < 0.05). The sedation scores, postoperative analgesic requirements, and incidence of side effects were similar between the P and DEX groups as were the mean pain scores.

CONCLUSIONS

During PACU, however, there were two differences between the DEX and placebo groups: 1) a lower pain score at PACU; and 2) a prolonged time to the first use of morphine at PACU in the DEX group as compared with the placebo group. A low-dose DEX had a weaker though still measurable effect in this clinical trial compared with trials using higher doses.

Effects of Paracetamol Combined with Dextromethorphan in Human Experimental Muscle and Skin Pain

By combining drugs with different mechanisms of action, synergistic effects may be achieved. The aim of the present experimental pain study was to combine paracetamol with dextromethorphan for synergistic effects. Furthermore, the reproducibility of the pain assessment methods was evaluated. Eighteen volunteers completed all periods in a three-way cross-over study. Pain stimuli were assessed at baseline and 1, 2 and 3 hr after dosing. The aim was to compare the pain-alleviating effect of 1 g paracetamol, 1 g paracetamol plus 30 mg dextromethorphan and placebo in response to a number of different stimuli in a human experimental volunteer model of skin and muscle pain. Repeated electrical stimulation of skin and muscle (temporal summation) modelled central integration and intramuscular hypertonic saline mimicked musculoskeletal pain. The method provided statistically stable pain recordings between repetitions on the same day and between days (all P > 0.05). Between repetitions on the same day, all tests were reproducible within the participants (intra-class correlation > 0.60). Between days all tests, except muscular pain pressure threshold, were reproducible within the participants (intra-class correlation > 0.60). There were no statistical differences (all P > 0.05) between paracetamol compared to placebo, and between the effect of paracetamol and dextromethorphan compared to placebo. The acute pain models were not sufficiently sensitive to detect an analgesic effect of paracetamol or the combination with dextromethorphan. The selected dose of dextromethorphan was low as the aim was to use commonly used doses, and a higher dose of dextromethorphan is most likely needed to attenuate the selected pain measures.

Preoperative oral dextromethorphan vs. clonidine to prevent tourniquet-induced cardiovascular responses in orthopaedic patients under general anaesthesia

BACKGROUND AND OBJECTIVE

Preoperative oral dextromethorphan and intravenous clonidine attenuate arterial pressure and heart rate increases during tourniquet inflation under general anaesthesia. The effect of preoperative oral clonidine on these variables has not been investigated.

METHODS

We designed this study to compare the effect of preoperative oral dextromethorphan or clonidine on haemodynamic changes during tourniquet inflation in 75 patients undergoing lower limb surgery under general anaesthesia. Patients were randomly assigned into three groups: dextromethorphan 30 mg (n = 25), clonidine 3 microg kg(-1) (n = 25) and placebo (n = 25). Anaesthesia was maintained with isoflurane 1.2% and N2O 50% in oxygen with endotracheal intubation. Dextromethorphan, clonidine or placebo was given orally in a double-blinded fashion 90 min before induction of anaesthesia. Systolic, diastolic and mean arterial pressure and heart rate were measured at 0, 30, 45, 60 min after the start of tourniquet inflation, before tourniquet release and 20 min after tourniquet deflation.

RESULTS

Systolic, diastolic and mean arterial pressure were significantly lower in the clonidine group compared with control after 45, 60 min tourniquet inflation and before tourniquet release (P < 0.05). Twenty minutes after deflation, diastolic and mean arterial pressure in the control group were still increased and significantly higher compared with the clonidine group (P < 0.05). Development of more than a 30% increase in systolic arterial pressure during tourniquet inflation was more frequent in the control group than in the other groups.

CONCLUSIONS

Preoperative oral clonidine 3 microg kg(-1) significantly prevented tourniquet-induced systemic arterial pressure increase in patients under general anaesthesia better than oral dextromethorphan.

Placebo-Controlled Comparison of a Morphine/Dextromethorphan Combination With Morphine on Experimental Pain and Hyperalgesia in Healthy Volunteers

In this double-blind, placebo-controlled, crossover study we compared the analgesic effect of a single oral dose of 30-mg dextromethorphan and 30-mg morphine combination (MS/DM) to 30 mg morphine (MS) alone and either placebo or 30 mg dextromethorphan (DM) on cutaneous sensitization induced by heat/capsaicin (topical) sensitization on the forearm and the brief thermal sensitization model on the thigh in 22 healthy volunteers. Outcome measures were areas of secondary hyperalgesia to brush and von Frey hair stimulation in both sensitization models and the painfulness of acute thermal noxious stimulation on the upper arm. Compared with placebo, both MS/DM and morphine had some effect on the secondary hyperalgesia and reduced the painfulness of a noxious thermal stimulus. The analgesic effect of MS/DM was not superior to that of morphine on any outcome measure. These results differ from preclinical studies with animal experimental pain models in which DM markedly potentiated the analgesic effects of opioids, but they are in accordance with recent clinical trials for chronic pain.

PERSPECTIVE

Adding dextromethorphan to morphine (1:1 ratio) did not enhance analgesia on measures of experimental cutaneous sensitization and acute noxious thermal stimulation in healthy volunteers. The results differ from preclinical studies but agree with clinical trials. Human experimental models of pain and neuronal sensitization, which are responsive to oral opioids, allow efficient study of opioid combination analgesics and simplify the process for determining the optimal dose and/or dose ratio.

Preoperative Oral Dextromethorphan Attenuated Tourniquet-Induced Arterial Blood Pressure and Heart Rate Increases in Knee Cruciate Ligament Reconstruction Patients Under General Anesthesia

The precise mechanism of tourniquet-induced arterial blood pressure increase is unknown. We determined the effect of preoperative oral dextromethorphan (DM) on arterial blood pressure and heart rate changes during tourniquet inflation in knee cruciate ligament reconstruction patients under general anesthesia. Patients in the DM group (n = 38) received oral DM 30 mg, and patients in the control group (n = 38) received oral placebo 2 h before the induction of anesthesia. Anesthesia was maintained with sevoflurane 2.0% and N(2)O in 33% oxygen, and the trachea was intubated until the end of surgery. Arterial blood pressure and heart rate were measured at 0, 30, and 60 min after the start of tourniquet inflation. Systolic arterial blood pressure and heart rate at 60 min in the control group were significantly more than those in the DM group (131.1 +/- 15.8 mm Hg versus 123.6 +/- 15.9 mm Hg [P < 0.05] and 74.1 +/- 11.3 bpm versus 67.8 +/- 8.5 bpm [P < 0.01], respectively). The percentage increase in systolic arterial blood pressure and heart rate in the DM group was also attenuated when compared with that in the control group (P < 0.05). In conclusion, preoperative oral DM 30 mg significantly attenuated arterial blood pressure and heart rate increases during tourniquet inflation under general anesthesia.

IMPLICATIONS

We demonstrated that preoperative oral dextromethorphan 30 mg significantly attenuated arterial blood pressure and heart rate increases at 60 min during tourniquet inflation in patients undergoing knee cruciate ligament reconstruction under general anesthesia.

Dextromethorphan and intrathecal morphine for analgesia after Caesarean section under spinal anaesthesia

BACKGROUND

Dextromethorphan is an N-methyl-D-aspartic acid antagonist which can attenuate acute pain with few side-effects. In this prospective, randomized, double-blind study of dextromethorphan and intrathecal morphine, we investigated postoperative pain, pruritus, nausea and vomiting in women undergoing Caesarean section under spinal anaesthesia.

METHODS

Women were allocated randomly to one of six groups, to receive intrathecal morphine 0.05, 0.1 or 0.2 mg plus oral dextromethorphan 60 mg or placebo.

RESULTS

The addition of dextromethorphan did not reduce postoperative pain scores (P=0.83). Compared with women receiving intrathecal morphine 0.05 mg, women receiving higher doses had a significantly higher incidence of nausea and vomiting [odds ratio for intrathecal morphine 0.1 mg, 4.0 (95% confidence interval 1.2-14.1); for intrathecal morphine 0.2 mg, 7.9 (2.3-27.1)]. Compared with women receiving intrathecal morphine 0.05 mg, women receiving higher doses also had a significantly higher incidence of pruritus [odds ratio for intrathecal morphine 0.1 mg, 3.2 (95% confidence interval 1.3-8.2); for intrathecal morphine 0.2 mg, 3.7 (1.4-9.5)]. Women receiving dextromethorphan had a lower incidence of nausea and vomiting [odds ratio 2.6 (1.1-6.3)].

CONCLUSIONS

Postoperative pain after Caesarean section under spinal anaesthesia was not reduced by the addition of oral dextromethorphan to a multimodal approach including intrathecal morphine.

Dextromethorphan mitigates phantom pain in cancer amputees

BACKGROUND

Hyperexcitability of N-methyl-D-aspartate (NMDA) receptors may play a role in the persistence of phantom pain. Dextromethorphan (DM) blocks NMDA receptors.

METHODS

Eight cancer and two noncancer amputees with established, disabling phantom pain received oral DM 60 or 90 mg twice daily (BID) in a three-period double-blind crossover placebo-controlled trial. This followed an open-phase trial in which either dose was given three times daily if pain relief during the double-blind phase was <50% of pretreatment intensity. Patients then underwent a 3-month phase of treatment with the best regimen and a subsequent 1-month posttreatment follow-up.

RESULTS

All patients reported a >50% decrease in pain intensity, better mood, and lower sedation in each treatment phase. Four individuals reported this level of pain relief with the 60-mg and one with the 90-mg BID regimen during the double-blind phase, whereas two amputees benefited from the 60-mg and three from the 90-mg thrice-daily regimen in the open-phase trial. One reported exacerbation of pain with the 90-mg BID regimen, and three reported pain rebound at the 1-month posttreatment follow-up phase. Three patients stopped all previous analgesic use during the study.

CONCLUSIONS

Persistent phantom pain probably involves NMDA receptor hyperexcitability because DM 120 to 270 mg/day mitigated the pain satisfactorily.

Analgesic effects of dextromethorphan and morphine in patients with chronic pain

N-methyl-aspartate (NMDA) receptor antagonists have been shown to improve opioid analgesia in the animal model. The cough suppressant dextromethorphan is a clinically available NMDA-receptor antagonist. In this randomised, double-blind, placebo-controlled study 20 patients with chronic pain of several years duration were given 100 mg of oral dextromethorphan or matching placebo 4 h prior to an intravenous infusion of morphine 15 mg. Pain intensity and adverse effects were assessed at 0, 4, 5 and 7 h. Dextromethorphan had no effect on morphine analgesia: the mean (+/-SEM) visual analogue scores for pain relief (VAS, 0-100 mm) at the end of the morphine infusion were 38 (+/-6) for dextromethorphan+morphine and 38 (+/-7) for placebo+morphine. VAS scores for pain intensity were comparable both at rest and at movement at all time points. The most common adverse effects reported were dizziness, nausea and sedation. There were no significant differences in either the incidence or severity of adverse effects. In conclusion, oral dextromethorphan 100 mg had no effect on pain relief by intravenous morphine 15 mg in patients with chronic pain.

The effect of the preemptive use of the NMDA receptor antagonist dextromethorphan on postoperative analgesic requirements

Both central sensitization after peripheral tissue injury and the development of opiate tolerance involve activation of N-methyl-D-aspartate receptors. In this double-blinded, randomized study, we investigated the preemptive versus postincisional effects of dextromethorphan, an N-methyl-D-aspartate receptor antagonist, on postoperative pain management. Sixty ASA I and II patients undergoing elective upper abdominal surgery were randomly allocated to three equally sized groups. The Preincisional group patients received dextromethorphan (120 mg) IM 30 min before skin incision and a placebo (isotonic saline) 30 min before the end of surgery. The Postincisional group received the same dose of dextromethorphan 30 min before the end of surgery and a placebo 30 min before skin incision, and the Control group received a placebo both 30 min before skin incision and 30 min before the end of surgery. A standard general anesthetic technique including fentanyl, propofol, isoflurane, and atracurium was used. Postoperative meperidine patient-controlled analgesia (PCA) was used. There were no significant group differences in the median pain scores except in the visual analog scale at 6 h both at rest and on movement; these were significantly lower in the Preincisional group than the other two groups (P < 0.05). The mean time to initiation of PCA was significantly longer in the Preincisional than in the Postincisional and Control groups (mean [SD]: 10.7 [2.2 h], 5.4 [2.1 h], and 3.7 [1.6 h], respectively; P < 0.001]. The 24-h PCA-meperidine consumption was significantly less in the Preincisional than in the Postincisional and Control groups (mean [SD]: 140 [60 mg], 390 [80 mg], and 570 [70 mg], respectively; P < 0.001]. The incidence of postoperative hypoxemia (SpO(2) < 90%) and nausea was significantly less in the Preincisional group (P < 0.05). In conclusion, preincisional IM 120 mg dextromethorphan compared with the same postincisional dose significantly reduced postoperative meperidine consumption.

IMPLICATIONS

IM administration of preincisional dextromethorphan (120 mg), allowing the use of a larger dose sufficient to block the central sensitization caused by activation of the N-methyl-D-aspartate receptors, provides preemptive analgesia and has a supportive role in postoperative pain relief, as shown by a significant decrease in 24-h meperidine consumption.

Dextromethorphan attenuation of postoperative pain and primary and secondary thermal hyperalgesia

PURPOSE

To determine the effect of 90 mg dextromethorphan (DM) p.o. vs placebo 90 min preoperatively, on the immediate and delayed postoperative course.

METHODS

Thirty patients undergoing laparoscopic cholecystectomy or inguinal hernioplasty under general anesthesia were studied. Half (DM) received 90 mg dextromethorphan and half received placebo 90 min before anesthesia. Intravenous Patient Controlled Aanalgesia with morphine was available for two hours within a six-hour observation period; 75 mg diclofenac i.m. prn was given later in PACU and on-ward (24 hr). Pain was assessed using the visual analogue scales. Thermal thresholds for cold and hot sensation and for pain (by limit method) were evaluated at the site of skin incision (primary-) and distantly (secondary hyperalgesia). Von Frey filaments were applied testing touch sensation. Sedation level and morphine consumption were also assessed in PACU.

RESULTS

Demographic, surgical and perioperative parameters were similar; no untoward effects were encountered. Pain intensity and sedation were lower, and the feeling of well-being was greater, in the DM patients: one vs five (median), two vs five, five vs two, respectively, P <0.01 (90 min time-point). Thermal application revealed absence of primary and secondary hyperalgesia only in the DM patients; von Frey filaments induced similar pain sensation in both groups. Mean morphine/group, morphine/weight and diclofenac injection rates were approximately 55% lower in the DM group: 2.1 +/- 1.2 (SD) vs 4.7 +/- 2.3, 0.03 +/- 0.02 vs 0.07 +/- 0.03, 1.0 +/- 0.3 vs 2.4 +/- 0.2, respectively, P <0.01.

CONCLUSIONS

Compared with placebo, DM enabled reduction of postoperative analgesics consumption, improved well-being, and reduced sedation, pain intensity and primary and secondary thermal hyperalgesia.

Large-dose oral dextromethorphan as an adjunct to patient-controlled analgesia with morphine after knee surgery

Dextromethorphan is a weak N-methyl-d-aspartate (NMDA) receptor antagonist that inhibits spinal cord sensitization in animal models of pain and also inhibits the development of cutaneous secondary hyperalgesia after tissue trauma. Perhaps coadministration of an NMDA antagonist with an opioid would lead to better pain relief, particularly with movement and an opioid-sparing effect. This has been shown for ketamine, but previous studies with dextromethorphan that have used small doses have shown only a modest reduction in morphine requirements with no or minimal changes in the postoperative pain experience. We sought to determine whether a large dose of this drug, just below the maximum tolerated dose, could potentiate morphine analgesia while simultaneously causing a significant improvement in the management of the postoperative pain experience. Sixty-six patients undergoing knee surgery were enrolled in the study. The study design was a prospective, randomized double-blinded comparison with placebo of 200 mg of dextromethorphan given eight hourly. Postoperative pain experiences were assessed by postoperative morphine usage. Visual analog and verbal rating scales were used to assess pain with movement as well as side effects. Dextromethorphan treatment led to a significant but modest reduction in morphine requirements (29.3% P < 0.05) but no reduction in postoperative pain levels. We conclude that increasing orally administered dextromethorphan to near maximum tolerated doses does not provide greater morphine sparing than 20-40 mg given 6-8 hourly as in previous studies. Furthermore we conclude that dextromethorphan does not improve pain scores in a manner expected of a drug with NMDA antagonist properties.

Glutamate receptors and nociception: implications for the drug treatment of pain

Evidence from the last several decades indicates that the excitatory amino acid glutamate plays a significant role in nociceptive processing. Glutamate and glutamate receptors are located in areas of the brain, spinal cord and periphery that are involved in pain sensation and transmission. Glutamate acts at several types of receptors, including ionotropic (directly coupled to ion channels) and metabotropic (directly coupled to intracellular second messengers). Ionotropic receptors include those selectively activated by N-methyl-D-aspartate, alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid and kainate. Metabotropic glutamate receptors are classified into 3 groups based on sequence homology, signal transduction mechanisms and receptor pharmacology. Glutamate also interacts with the opioid system, and intrathecal or systemic coadministration of glutamate receptor antagonists with opioids may enhance analgesia while reducing the development of opioid tolerance and dependence. The actions of glutamate in the brain seem to be more complex. Activation of glutamate receptors in some brain areas seems to be pronociceptive (e.g. thalamus, trigeminal nucleus), although activation of glutamate receptors in other brain areas seems to be antinociceptive (e.g. periaqueductal grey, ventrolateral medulla). Application of glutamate, or agonists selective for one of the several types of glutamate receptor, to the spinal cord or periphery induces nociceptive behaviours. Inhibition of glutamate release, or of glutamate receptors, in the spinal cord or periphery attenuates both acute and chronic pain in animal models. Similar benefits have been seen in studies involving humans (both patients and volunteers); however, results have been inconsistent. More research is needed to clearly define the role of existing treatment options and explore the possibilities for future drug development.

Acute pain and pain control: state of the art

Recognition and treatment of pain in the emergency department has undergone an evolution in the past decade. Emergency clinicians, educators, and researchers have begun to address the undertreatment of pain as well as challenge the long-standing dogmas concerning pain treatment. Well-described barriers, both psychological and educational, contribute to our providing inadequate pain relief. This state-of-the-art update describes the current perception of our practice with regard to pain relief and how it can be modified. Pain and pain control is such a broad and complex topic that only new advances and important principles relevant to the practice of emergency medicine are presented. Headache, pediatric pain, and procedural sedation and analgesia are not covered in this article as they will be addressed in future state-of-the-art articles.

The role of dextromethorphan in pain control

PURPOSE

To review the clinical benefits of dextromethorphan (DM) in pain management, describe its neuropharmacological properties.

SOURCE

A Medline search was made for experimental and clinical data on DM use from 1967 to date using keywords nociception, acute and chronic pain control, N-methyl-D-aspartate, antagonists, dextromethorphan.

PRINCIPLE FINDINGS

The 930 DM citations mostly described its antitussive, metabolic and toxicological aspects, animal studies and its possible role in minimizing post-brain ischemia complications in humans. The use of DM in acute pain revealed eight original studies involving 443 patients, as well as two preliminary reports and our own unpublished data on 513 patients. Most of the 956 patients had general anesthesia. Eight studies (154 patients) and one case report dealt with chronic pain management. This N-methyl-D-aspartate (NMDA) receptor antagonist binds to receptor sites in the spinal cord and central nervous system, thereby blocking the generation of central acute and chronic pain sensations arising from peripheral nociceptive stimuli and enabling reduction in the amount of analgesics required for pain control. DM attenuated the sensation of acute pain at doses of 30-90 mg, without major side effects, and reduced the amount of analgesics in 73% of the postoperative DM-treated patients. Studies in secondary pain models in healthy volunteers and in various types of chronic pain showed DM to be associated with unsatisfactory pain relief.

CONCLUSION

DM attenuates acute pain sensation with tolerable side effects. Its availability in oral form bestow advantages over other NMDA antagonists.

Premedication with dextromethorphan provides posthemorrhoidectomy pain relief

PURPOSE

Previous studies have shown that N-methyl-D-aspartate receptor antagonists provide a preemptive analgesic effect in humans. This study was designed to examine whether premedication with dextromethorphan, an N-methyl-D-aspartate antagonist, also provided a preemptive analgesic effect that improved postoperative pain management.

METHODS

Sixty patients who were American Society of Anesthesiologists status I and II scheduled for hemorrhoidectomy (modified Whitehead procedure) were included in the study. Patients were randomly assigned to the control and study groups. For the control group patients received chlorpheniramine maleate (20 mg), a component of the injection form of dextromethorphan, intramuscular injection 30 minutes before skin incision. In the study group dextromethorphan 40 mg containing 20 mg chlorpheniramine maleate (intramuscular) was given as premedication 30 minutes before skin incision. Pethidine (1 mg/kg, intramuscular) was given for pain relief as required postoperatively. The time to first pethidine injection, total pethidine consumption, worst pain score, and pethidine-related side effects were recorded for 48 hours postoperatively.

RESULTS

The times to first pethidine injection (mean +/- standard error of the mean) were 5.2 +/- 3 and 19.6 +/- 6 hours in the control and study groups, respectively. Total pethidine consumption was 140 +/- 11.3 and 63.5 +/- 11.8 mg in the control and study groups. The worst visual analog scale pain scores were 7.4 +/- 0.2 and 5.6 +/- 0.3 in the control and study groups during the two-day observation. The numbers of patients who required pethidine injection were 29 and 20 in the control and study groups, respectively. Two patients suffered pethidine-related side effects, such as nausea, vomiting, dizziness, and headache, in the control group, and no patient complained of any side effect in the study group.

CONCLUSION

We found that dextromethorphan premedication provided a preemptive analgesic effect, thus producing reduced postoperative pain and pethidine requirement and improved recovery from hemorrhoidectomy.

Preincisional dextromethorphan decreases postoperative pain and opioid requirement after modified radical mastectomy

PURPOSE

To examine whether preincisional dextromethorphan (DM) improved analgesia after modified radical mastectomy (MRM).

METHODS

Sixty patients (ASA I-II) scheduled for MRM were included and randomly allocated into two groups. Patients in the treatment group (DM) received 40 mg DM and 20 mg chlorpheniramine maleate (CPM) i.m., and those in the control group received 20 mg CPM i.m. alone 30 min before skin incision. Meperidine, 1 mg x kg(-1) i.m., was given for postoperative pain relief as required. The time to first meperidine injection, total meperidine consumption, worst pain score, bed-rest time, and side effects were recorded every 24 hr for 48 hr after surgery by a resident anesthesiologist on a double-blind basis.

RESULTS

A longer time to first meperidine injection (19.2 +/- 1.6 vs 1.5 +/- 0.23 hr, P < 0.001) and lower meperidine consumption (0[10] vs 75[50] mg, median [interquartile range], P < 0.001) were observed in the DM group than in the control group. The bed-rest time was shorter in the DM than in the control group (18.0[4] vs 23.0[19] hr, P < 0.001). No difference was noted in worst VAS pain score. Meperidine-related side effects (nausea, vomiting, pruritus, dizziness, headache) were more frequent in the control (10/30) than in the DM group (3/30, P < 0.05). The number of patients who required meperidine injection for pain relief was lower in the DM (7/30) than in the control group (25/30, P < 0.005). No DM- or CPM-associated side effects were observed.

CONCLUSION

Preincisional IM. DM treatment decreased postoperative pain and opioid requirement after MRM surgery.

Dextromethorphan and diabetic neuropathy

OBJECTIVE

To review the use of dextromethorphan for the treatment of painful diabetic neuropathy.

DATA SOURCES

MEDLINE and EMBASE searches for studies using dextromethorphan to manage diabetic neuropathy were conducted from 1966 to 1999 and 1980 to 1999, respectively.

DATA SYNTHESIS

Peripheral neuropathy is a common manifestation of diabetic patients. Many classes of medications have been investigated to treat this condition, including N-methyl-D-aspartate inhibitors. A review of studies using dextromethorphan to manage diabetic neuropathy was performed.

CONCLUSIONS

There are insufficient safety and efficacy data to justify the use of dextromethorphan for treating painful diabetic neuropathy. Further clinical trials are needed.

The preoperative administration of intravenous dextromethorphan reduces postoperative morphine consumption

We evaluated the effect of dextromethorphan on postoperative pain management. Sixty ASA physical status I-III female patients undergoing major abdominal surgery underwent standardized general anesthesia. Thirty patients received an i.v. infusion of dextromethorphan 5 mg/kg before anesthetic induction (Pre group), whereas the remaining 30 patients received the same volume of isotonic sodium chloride solution, followed by a postoperative i.v. infusion of dextromethorphan 5 mg/kg (Post group). Patients in the Pre group received the same volume of isotonic sodium chloride solution postoperatively. All patients were then treated with patient-controlled i.v. analgesia, which administered a 0.6-mg bolus of morphine on demand (maximal 4 h dose 20 mg). The mean visual analog pain score during cough or movement and at rest were similar in the two groups in the first 3 days postoperatively. However, Post group patients consumed more morphine than Pre group patients during the first 2 days (P < 0.01). The sedation scores, patient satisfaction, and the incidence of morphine-related side effects were similar between the two groups. We conclude that the preoperative administration of dextromethorphan 5 mg/kg reduces postoperative morphine consumption compared with postoperative administration.

IMPLICATIONS

In this double-blinded study, we found that the preoperative administration of i.v. dextromethorphan 5 mg/kg, compared with postoperative administration, reduces postoperative morphine consumption, which may provide clinical evidence of preemptive or preventive analgesic effects of dextromethorphan.

Perioperative dextromethorphan reduces postoperative pain after hysterectomy

We studied the effect of dextromethorphan, an N-methyl-D-aspartate antagonist, on analgesic consumption and pain scoring after abdominal hysterectomy. In this double-blinded study, 50 patients were randomized into two groups. Group DM was given oral dextromethorphan 40 mg with their premedication, then 40 mg three times per day for the next 2 days. Group P received placebo at identical times. Postoperative analgesic requirements were assessed using a patient-controlled analgesia system and subsequent oral analgesic intake using a set protocol. Pain was assessed at rest and on movement using a visual analog scale 4, 24, 48, and 72 h after the operation. Median pain scores at rest were significantly lower at 48 and 72 h and also for the sum of all resting pain scores. Mean morphine consumption was less in Group DM (1.1 vs 1.5 mg/h; P = 0.054). Usage of oral diclofenac, given every 8 h as needed, did not differ between groups, but consumption of codydramol (paracetamol 500 mg and dihydrocodeine 10 mg) was significantly less in Group DM. We conclude that the use of oral dextromethorphan has an analgesia-sparing effect and some beneficial effects on pain scoring at rest after abdominal hysterectomy.

IMPLICATIONS

Patients given dextromethorphan before and after surgery had a significant reduction in some pain scores at rest, but not on movement. There was a trend to lower morphine requirements in the first 24 h. Over the next 48 h, oral analgesic usage was significantly reduced.

Preincisional dextromethorphan treatment decreases postoperative pain and opioid requirement after laparoscopic cholecystectomy

In the present study, we examined whether preincisional treatment with dextromethorphan (DM) provides preemptive analgesia. Ninety patients scheduled for laparoscopic cholecystectomy were included. Patients receiving chlorpheniramine maleate (CPM) 20 mg via an IM injection 30 min before skin incision were designated as the control group. Patients in Group A received DM 40 mg (containing CPM 20 mg) IM after removal of the gallbladder, whereas in Group B, DM 40 mg (containing CPM 20 mg) was administered IM 30 min before skin incision. Meperidine (1 mg/kg IM) was given for postoperative pain relief as required. Times to first meperidine injection, total meperidine consumption, worst pain score, bed rest time, and side effects were recorded for 48 h after surgery. Times to first meperidine injection were 9.3+/-15.9, 17.4+/-3.4, and 28.6+/-3.9 h for the control group and Groups A and B, respectively. The total meperidine consumption was 90.7+/-11.9, 77.5+/-12.7, and 20.0+/-4.4 mg for the control group and Groups A and B, respectively. The worst visual analog pain scores were 6.0+/-0.2, 6.0+/-0.2, and 4.0+/-0.4 for the control group and Groups A and B, respectively. The bed rest times were 21.0+/-0.5, 20.0+/-0.5, and 19.0+/-0.4 h for the control group and Groups A and B, respectively. The number of patients who required meperidine injection was 26, 22, and 12 for the control group and Groups A and B, respectively. We conclude that DM is more effective in producing postoperative analgesia when it is administered preincision rather than after the gallbladder removal treatment, which suggests a preemptive analgesic effect.

IMPLICATIONS

Preincisional dextromethorphan (40 mg IM) treatment offers a preemptive analgesic effect, thus improving the postoperative pain management.

Preoperative oral dextromethorphan does not reduce pain or analgesic consumption in children after adenotonsillectomy

In this randomized, double-blinded, placebo-controlled, prospective study, we evaluated the analgesic efficacy of dextromethorphan 0.5 mg/kg or 1.0 mg/kg p.o. 1 h before adenotonsillectomy in 57 children 6-12 yr of age. Anesthetic management was standardized. Morphine 0.075 mg/kg i.v. and acetaminophen 25-35 mg/kg p.r. were administered after anesthetic induction but before the start of surgery. A 4-point behavioral score (1 = asleep, 2 = awake and calm, 3 = awake and crying, 4 = thrashing) was recorded on admission to and discharge from the postanesthesia care unit (PACU). In the PACU, pain was assessed with Children's Hospital of Eastern Ontario Pain Scale (CHEOPS) and recorded every 15 min until the patient was transferred to the day surgery unit (DSU). In the DSU, patients rated their pain using a 10-cm baseline 0-10 visual analog pain scale (VAS) every 30 min until they were discharged home. A 24-h VAS was obtained by phone interview, and parental satisfaction was scored (yes/no) regarding their child's postoperative analgesia. Morphine 0.025 mg/kg i.v. was administered to children with CHEOPS score >6, who verbalized pain, or who were crying in any consecutive 5-min observation periods in the PACU. Total morphine consumption was recorded. The study groups were comparable with respect to demographic variables. We were unable to detect any differences between study groups with respect to postoperative morphine consumption, CHEOPS, behavior scores, VAS, or parental satisfaction.

IMPLICATIONS

Premedication with dextromethorphan 0.5 or 1.0 mg/kg p.o. does not improve postoperative analgesia in school-aged children who receive preemptive morphine 0.075 mg/kg i.v. and acetaminophen 25-35 mg/kg p.r. during nitrous oxide and desflurane anesthesia for adenotonsillectomy.

Antihyperalgesic effect of the N-methyl-D-aspartate receptor antagonist dextromethorphan in the oral surgery model

Peripheral neuronal barrage from tissue injury produces central nervous system hyperexcitability through the activation of N-methyl-D-aspartate (NMDA) receptor sites by excitatory amino acids and neuropeptides. This study evaluated if attenuation of NMDA receptor activation with dextromethorphan (DM) suppresses the postoperative development of hyperalgesia. Seventy-five patients undergoing oral surgery in a parallel-group, double-blind study randomly received either a placebo or the maximally tolerated dose of DM administered orally prior to and continuing for 48 hours following surgery. Pain as measured by category, visual analog, and verbal descriptor scales was not significantly different between groups during the first 6 hours following surgery. However, pain at 48 hours was decreased in the DM group as measured by scales for pain intensity and unpleasantness. Subjects in the DM group also self-administered fewer acetaminophen tablets for unrelieved pain over 24 to 48 hours postoperatively. The results suggest that DM at maximally tolerated doses does not produce an analgesic effect in the immediate postoperative period but reduces pain at 48 hours. This may be related to antagonism of NMDA receptors necessary for the expression of hyperalgesia associated with noxious afferent input postoperatively.

Psychotropic effects of dextromethorphan are altered by the CYP2D6 polymorphism: a pilot study

Dextromethorphan is a nonopioid antitussive metabolized by cytochrome P450 2D6 (CYP2D6) to an active metabolite, dextrorphan. CYP2D6 is polymorphically expressed in humans, with 5 to 10% of Caucasians being homozygous deficient for the active form of the enzyme. In a pilot study, the authors investigated the pharmacologic effects of dextromethorphan in individuals phenotyped and genotyped as extensive metabolizers (EMs, N = 4) and poor metabolizers (PMs, N = 2) of CYP2D6 substrates. Dextromethorphan doses ranged from 0 to 6 mg/kg based on individual subject tolerance. All EMs tolerated 3 to 6 mg/kg dextromethorphan, whereas PMs barely tolerated 3 mg/kg dextromethorphan and therefore received lower doses. As shown in previous studies, plasma kinetics show profound differences in dextromethorphan metabolism between EMs and PMs. Dextromethorphan produced qualitatively and quantitatively different objective and subjective effects in the two groups. Objectively, PMs had greater psychomotor impairment, as measured by a joystick tracking task, compared with EMs on 3 mg/kg dextromethorphan (mean performance +/- SE, 95+/-0.5% for EMs vs. 86+/-6% for PMs; p < 0.05). At this dose, EMs also reported greater abuse potential compared with PMs (p < 0.05), and PMs reported greater sedation and dysphoria compared with EMs (p < 0.01). These data provide preliminary evidence that dextrorphan contributes to dextromethorphan abuse liability, and therefore PMs may be less likely to abuse dextromethorphan.

Differential effects of noxious conditioning stimulation of the cheek by capsaicin on human sensory and inhibitory masseter reflex responses evoked by tooth pulp stimulation

In this study, we investigated whether selective activation of nociceptive primary afferent fibers by capsaicin would induce modulations on tooth-pulp-evoked sensory or inhibitory masseter reflex responses in healthy human subjects. The contribution of central N-methyl-D-aspartate (NMDA) receptor mechanisms in capsaicin-induced effects on sensory or reflex responses was evaluated by dextromethorphan, an NMDA-receptor antagonist. The inhibitory masseter reflex was evoked by electrical stimulation (constant current, single pulses) of the upper incisor while the subject was biting at 10% of his maximal force. The sensation of the tooth pulp stimulation was evaluated by visual analogue scale (VAS). The magnitude, duration, and the the latency of the reflex were determined by bite force measurements. The inhibitor masseter reflex could be induced by non-painful tooth pulp stimulation, and the inhibition was enhanced as a function of increasing stimulus intensity. Capsaicin (1%) applied topically to the skin of the cheek produced a spontaneous burning pain sensation. During capsaicin treatment, the VAS ratings for the sensation induced by tooth pulp stimulation were significantly reduced, whereas no significant changes were found in the tooth-pulp-induced masseter reflex responses. Double-blind treatment with dextromethorphan at a dose of 100 mg (= the highest does without side-effects) had no effect on sensory or reflex responses. These data indicate that noxious stimulation of the facial skin by capsaicin induces differential effects on tooth-pulp-evoked sensory and inhibitory masseter reflex responses: Sensory responses are strongly attenuated, while masseter reflex responses are not significantly changed. Dextromethorphan at a clinically applicable dose does not influence tooth-pulp-evoked sensory or reflex responses or their modulation by capsaicin. Furthermore, the lack of modulation of the masseter reflex response by capsaicin differs from the capsaicin-induced enhancement of a nocifensive limb flexion reflex described earlier.