Oral ketamine therapy in the treatment of postamputation stump pain

Residual limb pain: An evidence-based review

Acquired limb loss, whether from accident or amputation, occurs with an incidence of greater than 175,000 per year in the United States. Current prevalence is estimated at greater than 1.5 million and is expected to double within 30 years. While many patients with amputations may have no significant pain or sensory issues after healing from the initial loss, one-quarter to one-half of patients may have ongoing difficulties with residual limb pain, phantom limb pain, or phantom limb sensation. This review explores the potential etiologies of those symptoms, as well as a variety of treatment options that a practitioner may consider when approaching this condition.

Perioperative Intravenous Ketamine Infusion for the Prevention of Persistent Post-amputation Pain: A Randomized, Controlled Trial

We hypothesized that perioperative ketamine administration would modify acute central sensitization following amputation and hence reduce the incidence and severity of persistent post-amputation pain (both phantom limb and stump pain). In a randomized, controlled trial, 45 patients undergoing above- or below-knee amputation received ketamine 0.5 mg.kg–1 or placebo as a pre-induction bolus followed by an intravenous infusion of ketamine 0.15 mg.kg–1.h–1 or normal saline for 72 hours postoperatively. Both groups received standardized general anaesthesia followed by patient-controlled intravenous morphine. The surface area of allodynia over the stump was mapped at days 3 and 6. Postamputation pain was assessed at days 3 and 6 and at 6 months postoperatively.

We found no significant difference between groups in the surface area of stump allodynia or in morphine consumption. There was an unexplained, but significant, increase in the incidence of stump pain in the ketamine group at day 3. At six-month review, the incidence of phantom pain was 47% in the ketamine group and 71% in the control group. This did not reach statistical significance (P=0.28) as the power of the study was based on the search for a large treatment effect. The incidence of stump pain at six months was 47% in the ketamine group and 35% in the control group (P=0.72). There were no significant between-group differences in pain severity throughout the study period.

Ketamine at the dose administered did not significantly reduce acute central sensitization or the incidence and severity of post-amputation pain.

Pharmacologic interventions for treating phantom limb pain

BACKGROUND

This is an updated version of the original Cochrane review published in Issue 12, 2011. Phantom limb pain (PLP) is pain that arises in the missing limb after amputation and can be severe, intractable, and disabling. Various medications have been studied in the treatment of phantom pain. There is currently uncertainty in the optimal pharmacologic management of PLP.

OBJECTIVES

This review aimed to summarise the evidence of effectiveness of pharmacologic interventions in treating PLP.

SEARCH METHODS

For this update, we searched the Cochrane Central Register of Controlled Trials (CENTRAL, the Cochrane Library), MEDLINE, and Embase for relevant studies. We ran the searches for the original review in September 2011 and subsequent searches for this update up to April 2016. We sought additional studies from clinical trials databases and reference lists of retrieved papers.

SELECTION CRITERIA

We included randomised and quasi-randomised trials studying the effectiveness of pharmacologic interventions compared with placebo, another active treatment, or no treatment, in established PLP. We considered the following outcomes: change in pain intensity, function, sleep, depression or mood, quality of life, adverse events, treatment satisfaction, and withdrawals from the study.

DATA COLLECTION AND ANALYSIS

We independently assessed issues of study quality and extracted efficacy and adverse event data. Due to the wide variability in the studies, we did not perform a meta-analysis for all the interventions and outcomes, but attempted to pool the results of some studies where possible. We prepared a qualitative description and narrative summary of results. We assessed clinical heterogeneity by making qualitative comparisons of the populations, interventions, outcomes/outcome measures, and methods.

MAIN RESULTS

We added only one new study with 14 participants to this updated review. We included a 14 studies (10 with low risk of bias and 4 with unclear risk of bias overall) with a total of 269 participants. We added another drug class, botulinum neurotoxins (BoNTs), in particular botulinum toxin A (BoNT/A), to the group of medications reviewed previously. Our primary outcome was change in pain intensity. Most studies did not report our secondary outcomes of sleep, depression or mood, quality of life, treatment satisfaction, or withdrawals from the study.BoNT/A did not improve phantom limb pain intensity during the six months of follow-up compared with lidocaine/methylprednisolone.Compared with placebo, morphine (oral and intravenous) was effective in decreasing pain intensity in the short term with reported adverse events being constipation, sedation, tiredness, dizziness, sweating, voiding difficulty, vertigo, itching, and respiratory problems.The N-methyl D-aspartate (NMDA) receptor antagonists ketamine (versus placebo; versus calcitonin) and dextromethorphan (versus placebo), but not memantine, had analgesic effects. The adverse events of ketamine were more serious than placebo and calcitonin and included loss of consciousness, sedation, hallucinations, hearing and position impairment, and insobriety.The results for gabapentin in terms of pain relief were conflicting, but combining the results favoured treatment group (gabapentin) over control group (placebo) (mean difference -1.16, 95% confidence interval -1.94 to -0.38; 2 studies). However, gabapentin did not improve function, depression score, or sleep quality. Adverse events experienced were somnolence, dizziness, headache, and nausea.Compared with an active control benztropine mesylate, amitriptyline was not effective in PLP, with dry mouth and dizziness as the most frequent adverse events based on one study.The findings for calcitonin (versus placebo; versus ketamine) and local anaesthetics (versus placebo) were variable. Adverse events of calcitonin were headache, vertigo, drowsiness, nausea, vomiting, and hot and cold flushes. Most of the studies were limited by their small sample sizes.

AUTHORS' CONCLUSIONS

Since the last version of this review, we identified another study that added another form of medical therapy, BoNTs, specifically BoNT/A, to the list of pharmacologic interventions being reviewed for clinical efficacy in phantom limb pain. However, the results of this study did not substantially change the main conclusions. The short- and long-term effectiveness of BoNT/A, opioids, NMDA receptor antagonists, anticonvulsants, antidepressants, calcitonins, and local anaesthetics for clinically relevant outcomes including pain, function, mood, sleep, quality of life, treatment satisfaction, and adverse events remain unclear. Based on a small study, BoNT/A (versus lidocaine/methylprednisolone) does not decrease phantom limb pain. Morphine, gabapentin, and ketamine demonstrate favourable short-term analgesic efficacy compared with placebo. Memantine and amitriptyline may not be effective for PLP. However, results must be interpreted with caution, as they were based mostly on a small number of studies with limited sample sizes that varied considerably and also lacked long-term efficacy and safety outcomes. The direction of efficacy of calcitonin, local anaesthetics, and dextromethorphan needs further clarification. Overall, the efficacy evidence for the reviewed medications is thus far inconclusive. Larger and more rigorous randomised controlled trials are needed for us to reach more definitive conclusions about which medications would be useful for clinical practice.

Ketamine for pain in adults and children with cancer: a systematic review and synthesis of the literature

OBJECTIVE

Chronic cancer pain is often refractory and difficult to treat. Ketamine is a medication with evidence of efficacy in the treatment of chronic pain.

DESIGN

This article presents a synthesis of the data on ketamine for refractory cancer pain in adults and children.

RESULTS

There are five randomized, double-blind, controlled trials of ketamine use in cancer pain that demonstrate improvement in pain for some patients. There are six prospective, uncontrolled trials in cancer pain that also demonstrate improvement in pain scores for some patients. There are no randomized, controlled trials in children with cancer pain, although there are a few studies reflecting improved pain control with ketamine for children with cancer pain. Adverse events for adults on ketamine are most commonly somnolence, feelings of insobriety, nausea/vomiting, hallucinations, depersonalization/derealization, and drowsiness. However, when ketamine is combined with benzodiazepines, feelings of insobriety, hallucinations, and depersonalization/derealization are not reported. Children on ketamine have had few reported adverse effects, which include sedation, anorexia, urinary retention, and myoclonic movements. Recommended ketamine infusion dosages are from 0.05 to 0.5 mg/kg/h (intravenous or subcutaneous). Recommended oral dosages of ketamine are 0.2-0.5 mg/kg/dose two to three times daily with a maximum of 50 mg/dose three times daily.

CONCLUSIONS

Despite limitations in the breadth and depth of data available, there is evidence that ketamine may be a viable option for treatment-refractory cancer pain.

Continuous brachial plexus blockade in combination with the NMDA receptor antagonist memantine prevents phantom pain in acute traumatic upper limb amputees

BACKGROUND

Hyperexcitability of N-methyl-d-aspartate acid (NMDA) receptors may play an important role in the development of phantom limb pain (PLP).

AIM OF THE STUDY

To investigate whether early treatment with the NMDA antagonist memantine attenuates phantom pain memory formation in traumatic amputees.

METHODS

In a randomized, double-blind, controlled trial 19 patients with acute traumatic amputation of the upper extremity were investigated. All patients received postoperative analgesia by continuous brachial plexus anesthesia (ropivacaine 0.375% 5 ml/h) for at least 7 days. In addition, the patients received either memantine (20-30 mg daily, n=10) or placebo (n=9) for 4 weeks.

RESULTS

Memantine treatment reduced the number of requested ropivacacine bolus injections during the first week and resulted in a significant decrease of PLP prevalence and intensity at 4 weeks and 6 months follow up, but not at 12 months follow up.

CONCLUSIONS

We conclude that memantine can reduce intensity of phantom limb pain and might also prevent the development of PLP. However, despite the very early begin of treatment; no long-term effect on established PLP was evident.

Pharmacologic interventions for treating phantom limb pain

BACKGROUND

Phantom limb pain (PLP) is pain that arises in the missing limb after amputation and can be severe, intractable and disabling. Various medications have been studied in the treatment of phantom pain. Presently there is uncertainty in the optimal pharmacologic management of PLP. 

OBJECTIVES

This review aims to summarize the evidence of effectiveness of pharmacologic interventions in treating PLP.

SEARCH METHODS

We searched the Cochrane Pain, Palliative and Supportive Care Review Group (PaPaS) Trials Register, the Cochrane Controlled Trials Register (CENTRAL, The Cochrane Library), MEDLINE and EMBASE up to September 2011 for randomised and quasi-randomised trials comparing pharmacologic treatment with placebo, another active treatment, or no treatment.

SELECTION CRITERIA

We included randomised and quasi-randomised trials studying the effectiveness of pharmacologic interventions in patients with established PLP. The outcomes considered were change in pain intensity, function, mood, sleep, quality of life, satisfaction and adverse effects.

DATA COLLECTION AND ANALYSIS

Three review authors independently assessed the methodologic quality of the studies and extracted the data. We reported continuous and dichotomous data on change in pain intensity, function, mood/depression scores, sleep, quality of life, satisfaction for each study, where available. Because of the wide variability in the studies, we did not perform a meta-analysis for all the interventions and outcomes but we attempted to pool the results of some studies where possible. We prepared a qualitative description and narrative summary of results and described adverse effects. We assessed clinical heterogeneity by making qualitative comparisons in terms of the populations, interventions, outcomes/outcome measures and methods.

MAIN RESULTS

From 583 references/publications, we selected 13 studies involving 255 participants. Six groups of medications were reviewed, namely, N-methyl D-aspartate (NMDA) receptor antagonists, antidepressants, anticonvulsants, anaesthetics, opioids, and calcitonin. Ten studies were of high quality and three were of moderate quality based on both Jadad and Van Tulder criteria. Because of the wide variation (heterogeneity) in the pharmacologic interventions, outcome measures, analyses, reporting of results, duration of follow-ups and study designs, it was not possible to pool the results for most of the interventions and outcomes. Morphine (oral and intravenous) was effective in decreasing pain intensity in the short-term with reported adverse effects being constipation, sedation, tiredness, dizziness, sweating, voiding difficulty, vertigo, itching, and respiratory problems. The NMDA receptor antagonists, ketamine and dextromethorphan but not memantine, had analgesic effects. The adverse effects of ketamine were more serious and included loss of consciousness, sedation, hallucinations, hearing and position impairment, and insobriety. The results for gabapentin in terms of pain relief were conflicting but combining the results showed a trend towards benefit. Gabapentin, however, did not improve function, depression score and sleep quality. Side effects experienced were somnolence, dizziness, headache and nausea. Amitryptiline was not effective in PLP with dry mouth and dizziness as side effects based on one study. The findings for calcitonin and anaesthetics were variable. Adverse effects of calcitonin were headache, vertigo, drowsiness, nausea, vomiting, and hot and cold flushes. Most of the studies were limited by their small sample sizes.

AUTHORS' CONCLUSIONS

The short- and long-term effectiveness of opioids, NMDA receptor antagonists, anticonvulsants, antidepressants, calcitonins, and anaesthetics for clinically relevant outcomes that include pain, function, mood, sleep, quality of life, satisfaction and adverse effects remains unclear. Morphine, gabapentin and ketamine demonstrate trends towards short-term analgesic efficacy. Memantine and amitriptyline were ineffective for PLP. Results, however, are to be interpreted with caution as these were based mostly on a small number of studies with limited sample sizes that varied considerably and also lacked long-term efficacy and safety outcomes. The direction of efficacy of calcitonin, anaesthetics and dextromethorphan need further clarification. Larger and more rigorous randomised controlled trials are needed to make stronger recommendations about which medications would be useful for clinical practice.

Oral ketamine in the palliative care setting: a review of the literature and case report of a patient with neurofibromatosis type 1 and glomus tumor-associated complex regional pain syndrome

Ketamine, an N-methyl-D-aspartate (NMDA) receptor antagonist, has been shown to be effective not only for its anesthetic properties but also for the analgesic and opiate-sparing effects. However, data on efficacy and safety of oral ketamine for the treatment of neuropathic or cancer pain syndromes is limited with most of the evidence based on small clinical trials and anecdotal experiences. In this review, we will analyze the clinical data on oral ketamine in the palliative care setting. After an extensive search using five major databases, a total of 19 relevant articles were included. No official clinical guidelines for the use of oral ketamine in this patient population were found. Studies on oral ketamine for cancer and neuropathic pain have shown mixed results which could be partially due to significant differences in hepatic metabolism. In addition, we will include a case report of a 38-year-old female with neurofibromatosis type 1 (NF1) with history of chronic, severe pain in her fingertips secondary to multiple glomus tumors which evolved into CRPS resistant to multiple therapies but responsive to oral ketamine. Based on our experience with oral ketamine, this drug should be administered after an intravenous trial to monitor response and side effects in patients with an adequate functional status. However, patients in the palliative care and hospice setting, especially the one at the end of their lives, may also benefit from oral ketamine even if an intravenous trial is not feasible.

A strategy for conversion from subcutaneous to oral ketamine in cancer pain patients: effect of a 1:1 ratio

CONTEXT

No consensus exists about the most appropriate dose ratio for conversion from parenteral to oral ketamine.

OBJECTIVES

To confirm that a 1:1 dose ratio is suitable for converting subcutaneous (s.c.) to oral ketamine in cancer patients.

METHODS

Patients with opioid poorly responsive cancer pain, who responded to 0.4, 0.6, or 0.8 mg s.c. ketamine bolus, were treated with 0.1, 0.15, or 0.2mg/kg/h ketamine infusion, respectively. Switching to the oral route, by applying a 1:1 dose ratio, was carried out in patients who experienced adequate pain relief and continued to need ketamine as a coanalgesic. Pain, somnolence, feelings of insobriety, confusion, and cardiovascular parameters were assessed throughout the process.

RESULTS

Twenty-nine patients were enrolled in the study. Ketamine infusion decreased pain intensity from severe to no pain or slight pain in 23 of 29 and six of 29 patients, respectively. The median of s.c. ketamine doses was 0.2mg/kg/h (range 0.1-0.5). After oral switching, 27 of 29 patients remained as successfully controlled as when receiving s.c. ketamine. The other two patients needed a slight dose ratio readjustment, to 1:1.3 and 1:1.5, to maintain pain control. The median of oral ketamine doses was 300 mg/day (interquartile range 240-382.5). Seven of 29 patients receiving s.c. ketamine developed moderate and transitory side effects, such as feelings of insobriety and somnolence. No side effects were present while receiving oral ketamine. No significant changes were observed in cardiovascular parameters.

CONCLUSION

A 1:1 dose ratio for conversion from s.c. to oral ketamine is safe and effective in cancer pain patients.

[Oral ketamine-midazolam premedication of uncooperative patients in major outpatient surgery]

BACKGROUND

Currently, ketamine is not often used as an anesthetic. Its clinical characteristics and mechanism of action largely depend on antagonism of the N-methyl-D-aspartate receptor.

OBJECTIVE

To evaluate the utility of oral ketamine as a preanesthetic agent to lower surgical stress for patients with mental disability.

MATERIAL AND METHODS

Observational, retrospective study of 112 mentally disabled patients undergoing major dental surgery on an outpatient basis. The study group received oral midazolam, ibuprofen, and 6 mg/kg of ketamine; the control group received only midazolam and ibuprofen. We recorded data concerning demographics, anesthesia, surgery, physiologic variables, Glasgow score, time of onset of anxiolysis, duration of stay in the recovery ward, and adverse events.

RESULTS

Conservative odontologic treatment was provided in 66.3% of the cases. Seventy-one patients (64.4%) were in the control group and 41 patients (36.6%) in the study group. Hemodynamic, respiratory, and neurologic changes were minimal and there were no significant between-group differences. Level of sedation differed significantly between groups (P = .001) at 15 and 30 minutes; differences were also observed within the study group. Mean (SD) duration of surgery was 72.6 (29.7) minutes. Mean duration of stay in the postoperative recovery ward was 140.9 (52.1) minutes (135.8 [54.89] minutes in the study group and 144.2 [50.5] minutes in the control group). The incidence of adverse events did not differ significantly between groups.

CONCLUSIONS

Oral ketamine is an effective premedication for major ambulatory surgery and does not increase the incidence of side effects.

The use of prolonged peripheral neural blockade after lower extremity amputation: the effect on symptoms associated with phantom limb syndrome

BACKGROUND

Phantom limb syndrome (PLS) is common after limb amputations, involving up to 90% of amputees. Although many different therapies have been evaluated, none has been found to be highly effective. Therefore, we evaluated the efficacy of a prolonged perineural infusion of a high concentration of local anesthetic solution in preventing PLS.

METHODS

A perineural catheter was placed immediately before or during surgery in 71 patients undergoing lower extremity amputation. A continuous infusion of 0.5% ropivacaine was started intraoperatively at 5 mL/h using an elastomeric (nonelectronic) pump, and continued for 4 to 83 days after surgery. PLS was evaluated on the first postoperative day and then 1, 2, 3, and 4 weeks, and 3, 6, 9, and 12 months after surgery. To evaluate the presence and severity of PLS while the patient was receiving the ropivacaine infusion, it was discontinued for 6 to 12 hours before each assessment period (i.e., until the sensation in the extremity returned). The severity of phantom limb and stump pain was assessed using a 5-point verbal rating scale (VRS), with 0 = no pain to 4 = intolerable pain, and "phantom" sensations were recorded as present or absent. If the VRS score was >1 or significant phantom sensations were present, the ropivacaine infusion was immediately restarted at 5 mL/h. If the VRS score remained at 0 to 1 and the patient had not experienced phantom sensations for 48 hours, the infusion was permanently discontinued and the catheter was removed.

RESULTS

Median duration of the local anesthetic infusion was 30 days (95% confidence interval, 25-30 days). On postoperative day 1, 73% of the patients complained of severe-to-intolerable pain (visual analog scale >2). However, the incidence of severe-to-intolerable phantom limb pain was only 3% at the end of the 12-month evaluation period. At the end of the 12-month period, the percentage of patients with VRS pain scores were 0 = 84%, 1 = 10%, 2 = 3%, 3 = 3%, and 4 = none. However, phantom limb sensations were present in 39% of patients at the end of the 12-month evaluation period. All patients were able to manage the elastomeric catheter infusion system at home.

CONCLUSION

Use of a prolonged postoperative perineural infusion of ropivacaine 0.5% seems to be an effective therapy for the treatment of phantom limb pain and sensations after lower extremity amputation.

Use of oral ketamine in chronic pain management: a review

The analgesic effect of ketamine is primarily based on the antagonism of the N-methyl-d-aspartate (NMDA) receptor. Activation of NMDA receptors may play a crucial role in the pathogenesis of chronic pain. Little formal research has been performed on the efficacy and safety of ketamine in chronic pain, especially concerning long-term oral administration. This review provides an overview of the available clinical data on the use of oral ketamine in chronic pain management. A literature search was performed in MEDLINE, EMBASE and the Cochrane Library, resulting in 22 relevant articles. Because most retrieved articles were of a descriptive nature (e.g. case reports and case series) a quantitative analysis was not possible. There was no consistent dose-response relation. A recommended starting dosage in ketamine-naive patients is 0.5 mg/kg racemic ketamine or 0.25 mg/kg S-ketamine as a single oral dose. The dosage is increased by the same amount if required. For a continuous analgesic effect it is usually given 3-4 times daily. The injection fluid can be taken orally. When parenteral ketamine is switched to oral administration the daily dosage can be kept equal and, depending on clinical effect and/or adverse effects, is slowly increased. The pharmacologically active metabolite norketamine is believed to contribute to the analgesic effect of oral ketamine. Lack of evidence regarding efficacy, and the poor safety profile, do not support routine use of oral ketamine in chronic pain management. Oral ketamine may have a limited place as add-on therapy in complex chronic pain patients if other therapeutic options have failed.

Oral ketamine for the treatment of type I complex regional pain syndrome

Ketamine has been shown to be effective in the treatment of neuropathic pain. We present a case of severe complex regional pain syndrome type 1 that was treated with oral ketamine. The response and tolerability of this preparation suggest that further study is warranted.

The role of ketamine in pain management

Ketamine is a dissociative anaesthetic; its mechanism of action is primarily an antagonism of the N-methyl-D-aspartate (NMDA) receptor. The role of ketamine, in particular in lower sub-anaesthetic doses, has recently gained increasing interest in pain management. It has been studied in a considerable number of trials and analysed in meta-analyses and systematic reviews. Based on these data, the primary role of ketamine in such low doses is as an 'anti-hyperalgesic', 'anti-allodynic' or 'tolerance-protective' agent. It therefore has a role in the treatment of opioid resistant or 'pathological' pain (central sensitisation with hyperalgesia or allodynia, opioid induced hyperalgesia, neuropathic pain) rather than as an 'analgesic' in its own right. Low dose ketamine also has 'preventive analgesia' properties. Furthermore, in higher doses it provides effective and safe sedation and analgesia for painful procedures. The place of ketamine in the treatment of chronic pain and the effects of long-term medicinal use remain unclear.

New Treatments for Neuropathic Pain

Existing treatments for neuropathic pain deliver inadequate pain relief, unacceptable side effects, or both. The unmet medical need for more effective treatment is driving a large volume of research to discover new drugs. Most existing treatments are drugs introduced to treat other pain conditions or other medical conditions, such as antidepressants and anticonvulsants, which were found empirically to be effective for neuropathic pain. Only recently have drug discovery efforts have become mechanistically driven, addressing targets identified by a molecular neurobiological approach to the pathophysiology of neuropathic states.

Effects of the N-methyl-D-aspartate receptor antagonist perzinfotel [EAA-090; [2-(8,9-dioxo-2,6-diazabicyclo[5.2.0]non-1(7)-en-2-yl)-ethyl]phosphonic acid] on chemically induced thermal hypersensitivity

Perzinfotel [EAA-090; [2-(8,9-dioxo-2,6-diazabicyclo[5.2.0]non-1(7)-en-2-yl)-ethyl]phosphonic acid] is a selective, competitive N-methyl-D-aspartate (NMDA) receptor antagonist with high affinity for the glutamate site. The current study evaluated whether perzinfotel would have antinociceptive effects or block thermal hypersensitivity associated with the administration of chemical irritants in rats. Perzinfotel lacked antinociceptive effects but dose- and time-dependently blocked prostaglandin E(2) (PGE(2))- and capsaicin-induced thermal hypersensitivity in a warm-water tail-withdrawal assay in rats. Doses of 10 mg/kg intraperitoneal or 100 mg/kg oral blocked PGE(2)-induced hypersensitivity by 60 to 80%. The magnitude of reversal was greater than other negative modulators of the NMDA receptor studied, such as uncompetitive channel blockers (e.g., memantine, dizocilpine, and ketamine), a NR2B selective antagonist (e.g., ifenprodil), and other glutamate antagonists [e.g., selfotel, 3-(2-carboxypiperazin-4-yl)propyl-1-phosphonic acid (CPP), D,L-(E)-2-amino-4-propyl-5-phosphono-3-pentenoic acid (CGP-39653)], up to doses that suppressed operant rates of responding. In contrast to other negative modulators of the NMDA receptor studied, which typically decreased operant rates of responding at doses that lacked antinociceptive effects, perzinfotel did not modify response rates at doses that blocked irritant-induced thermal hypersensitivity. Collectively, these studies demonstrate that perzinfotel has therapeutic ratios for effectiveness versus adverse effects superior to those seen with other competitive and uncompetitive NMDA receptor antagonists studied.

Subanesthetic ketamine infusion therapy: a retrospective analysis of a novel therapeutic approach to complex regional pain syndrome

Complex Regional Pain Syndrome (CRPS) is a disorder that can be accompanied by severe pain that is often both chronic and resistant to conventional therapy. Harbut and Correll previously reported the successful treatment of a 9-year case of intractable Type I CRPS with an intravenous inpatient infusion of ketamine in an adult female patient.

OBJECTIVE

The purpose of this study was to ascertain if indeed the use of subanesthetic inpatient infusions of ketamine provide meaningful improvements in pain scores, and thus, quality of life, in patients suffering from CRPS. To achieve this objective we focused our analysis on the relief of pain obtained by patients undergoing this novel treatment option developed at Mackay Base Hospital, Queensland, Australia.

METHODS

Case notes of 33 patients whose CRPS pain was treated by the inpatient administration of a continuous subanesthetic intravenous infusion of ketamine were reviewed. The dose and duration of ketamine therapy and the degree and duration of relief obtained were recorded. Notable side effects were also recorded. The degree of relief obtained (immediately after the infusion) was assessed using pre- and posttreatment numeric pain scores. The duration of relief obtained (throughout the follow-up period) was analyzed using a Kaplan-Meier cumulative survival curve analysis.

RESULTS

A total of 33 patients with diagnoses of CRPS who had undergone ketamine treatment at least once were identified. Due to relapse, 12 of 33 patients received a second course of therapy, and two of 33 patients received a third. The degree of relief obtained following the initial course of therapy was impressive (N=33); there was complete pain relief in 25 (76%), partial relief in six (18%), and no relief in two (6%) patients. The degree of relief obtained following repeat therapy (N=12) appeared even better, as all 12 patients who received second courses of treatment experienced complete relief of their CRPS pain. The duration of relief was also impressive, as was the difference between the duration of relief obtained after the first and after the second courses of therapy. In this respect, following the first course of therapy, 54% of 33 individuals remained pain free for >/=3 months and 31% remained pain free for >/=6 months. After the second infusion, 58% of 12 patients experienced relief for >/=1 year, while almost 33% remained pain free for >3 years. The most frequent side effect observed in patients receiving this treatment was a feeling of inebriation. Hallucinations occurred in six patients. Less frequent side effects also included complaints of lightheadedness, dizziness, and nausea. In four patients, an alteration in hepatic enzyme profile was noted; the infusion was terminated and the abnormality resolved thereafter.

CONCLUSION

This retrospective review suggests that limited subanesthetic inpatient infusions of ketamine may offer a promising therapeutic option in the treatment of appropriately selected patients with intractable CRPS. More study is needed to further establish the safety and efficacy of this novel approach.

Pain Management in Trauma Patients

Trauma is a major cause of mortality throughout the world. In recent years, major advances have been made in the management of trauma, the end result of which has been reduced mortality and enhanced function. One of these areas is pain control. Improved pain management has not only led to increased comfort in trauma patients, but has also been shown to reduce morbidity and improve long-term outcomes. This review focuses on the treatment of pain in the setting of acute injury and on pain management in trauma patients who go on to develop chronic pain. Emphasis is placed on pharmacologic interventions, invasive and noninvasive pain management techniques, analgesia in challenging patients, and pain control in commonly encountered trauma conditions.

Multiple Sclerosis With Severe Pain and Allodynia Alleviated by Oral Ketamine

The objective of this study is to determine the effect of oral ketamine on pain and allodynia associated with multiple sclerosis. A 60-year-old woman with multiple sclerosis was referred to our clinic because of severe pain and allodynia. Oral ketamine was started at a dose of 20 mg once a day and increased to twice a day. Oral ketamine was effective in the treatment of the pain and allodynia associated with multiple sclerosis.

Ketamine in chronic pain management: an evidence-based review

Ketamine has diverse effects that may be of relevance to chronic pain including: N-methyl-D-aspartic acid, alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid, kainate, gamma-aminobutyric acid(A) receptors; inhibition of voltage gated Na(+) and K(+) channels and serotonin, dopamine re-uptake. Ketamine has been in clinical practice for over 30 yr; however, there has been little formal research on the effectiveness of ketamine for chronic pain management. In this review we evaluate the available clinical data as a basis for defining the potential use of ketamine for chronic pain. Literature referenced in this review was obtained from a computer search of EMBASE and MEDLINE from 1966 through August, 2002. Search terms included ketamine, ketalar, pain, painful, analgesic, and analgesia. Abstracts were screened for relevance and publications relating to chronic pain use were obtained. Levels of evidence were stratified according to accepted guidelines (level I-IV). For central pain, there is level II and level IV evidence of efficacy for parenteral and oral ketamine. For complex regional pain syndromes, there is only level IV evidence of efficacy of epidural ketamine. For fibromyalgia, there is level II evidence of pain relief, reduced tenderness at trigger points, and increased endurance. For ischemic pain, a level II study reported a potent dose-dependent analgesic effect, but with a narrow therapeutic window. For nonspecific neuropathic pain, level II and level IV studies reported divergent results with questionable long-term effects on pain. For phantom limb pain and postherpetic neuralgia, level II and level II studies provided objective evidence of reduced hyperpathia and pain relief was usually substantial either after parenteral or oral ketamine. Acute on chronic episodes of severe neuropathic pain represented the most frequent use of ketamine as a "third line analgesic," often by IV or subcutaneous infusion (level IV). In conclusion, the evidence for efficacy of ketamine for treatment of chronic pain is moderate to weak. However, in situations where standard analgesic options have failed ketamine is a reasonable "third line" option. Further controlled studies are needed.

NMDA-mediated mechanisms in cortical excitability changes after limb amputation

OBJECTIVES

The aim of our study was to determine the role of N-methyl-d-aspartate (NMDA)-mediated mechanisms in cortical excitability changes after limb amputation, and their possible relationship to phantom pain.

MATERIALS AND METHODS

Sixteen upper limb amputees who were suffering from chronic phantom pain received the NMDA-antagonist memantine or placebo for 3 weeks. Intracortical inhibition (ICI) and intracortical facilitation (ICF) were determined at baseline and on day 21 using transcranial magnetic stimulation. Simultaneously, phantom pain intensity was assessed.

RESULTS

Memantine reduced ICF and enhanced ICI to roughly the same extent as seen in healthy subjects in a previous study. These changes were not correlated to the reduction of phantom pain.

CONCLUSION

We therefore conclude that NMDA-mediated mechanisms influence changes of ICI and ICF occurring after limb amputation. However, our results suggest that these cortical excitability changes and phantom pain are independent of each other.

Efficacy of the NMDA-receptor antagonist memantine in patients with chronic phantom limb pain--results of a randomized double-blinded, placebo-controlled trial

Phantom limb pain (PLP) associated neuroplastic changes are partly mediated by excitatory amino acids at NMDA receptor sites. This study was undertaken to deduce if NMDA-receptor antagonists may be effective in patients with chronic PLP. Therefore a four week double-blinded, randomized placebo-controlled trial was performed to evaluate the efficacy of 30 mg memantine/day, an orally administrable NMDA receptor antagonist.Thirty-six patients, 18 per group, with a history of at least 12 months PLP and an average pain of at least 4 on the 11-point numeric rating scale (NRS) were enrolled. The patients completed a standardized questionnaire before the trial. PLP intensity and the level of eight complaints were assessed during the trial. Number needed to treat (NNT) was calculated based on the average PLP during the 3rd week (steady state). In both groups, PLP declined significantly in comparison with the baseline (verum: 5.1 (+/-2.1) to 3,8 (+/-2,3), placebo from 5.1 (+/-2.0) to 3.2 (+/-1,46) NRS) without a re-rising of the PLP during the washout period. Mean pain relief was 47% in the memantine group (10 patients reported more than 50% relief), 40% in the placebo group (6>50%): NNT were 4.5 (KI: 2.1-10.6). Analysis of covariance demonstrated a significant impact only on the prior PLP intensity, but no treatment effect. Two patients have demonstrated long-term pain relief under memantine until now (16 months). The total number of slight adverse events were comparable in both groups, but the overall number of severe events was higher in the memantine group (P<0.05). This trial failed to demonstrate a significant clinical benefit of the NMDA-receptor antagonist memantine in chronic PLP. The administration of a higher dosage is probably not tolerable.

Plasma concentration profiles of ketamine and norketamine after administration of various ketamine preparations to healthy Japanese volunteers

Ketamine is known to provide analgesic effects without an anesthetic when administered in a low dose. We previously reported that a tablet containing ketamine had analgesic effects in patients with neuropathic pain. In the present study, we compared the plasma concentration profiles of the enantiomers of ketamine and its active metabolite, norketamine, up to 8 h after the administration of 20 mg of ketamine by injection, after the administration of two tablets containing 25 mg of ketamine, after the administration of two sublingual tablets containing 25 mg of ketamine, after the insertion of a suppository containing 50 mg of ketamine, and after the application of a nasal spray containing 25 mg of ketamine to three healthy volunteers. The plasma concentration of ketamine biexponentially declined after the administration by injection; the value of T(1/2beta) for ketamine was approximately 120 min. The bioavailability of the tablet was estimated to be approximately 20%; the area under the plasma concentration-time curve, (AUC)(0-->8 h), of norketamine was approximately 500 ng h/ml in both enantiomers. The bioavailabilities of the sublingual tablet and the suppository were estimated to both be approximately 30%; the AUC(0-->8 h) of norketamine was 280-460 ng h/ml in both enantiomers. The plasma concentration profiles of the sublingual tablet and the suppository were almost similar to that of the tablet. The bioavailability of the nasal spray was estimated to be approximately 45%, which was the highest value among the preparations tested, and the AUC(0-->6 h) of norketamine was low (approximately 100 ng h/ml) in both enantiomers. These pharmacokinetic findings suggested that all of the ketamine preparations tested in this study may be useful for the alleviation of neuropathic pain. We propose that the type of ketamine preparation should be selected in accordance with the patient's disease condition and the required dosage amount of ketamine.

Effects of low-dose ketamine on neuropathic pain: An electroencephalogram-electrooculogram/behavioral study

The aim of the present study was to clarify the neurophysiological changes associated with analgesic and behavioral effects of low-dose ketamine HCl in patients suffering from chronic neuropathic pain. Ten in-patients with neuropathic pain participated in this single-blind, placebo-controlled study after giving written informed consent. Following intravenous injections of a saline solution (placebo), three bolus injections of 5 mg ketamine HCl were administered at 5 min intervals. Changes in pain perception were assessed using a numerical rating scale for pain. Behavioral changes, including psychotomimetic effects, were assessed using the Brief Psychiatric Rating Scale (BPRS). Electroencephalograms (EEG) and electrooculograms (EOG) were recorded continuously throughout the testing period. One minute EEG and closed-eye eye movements were quantified. The effects of ketamine were evaluated by comparing the neurophysiological and behavioral parameters obtained from the placebo and ketamine trials. Pain reduction was significantly correlated with ketamine-induced changes in hallucinatory behavior and excitement as measured by the BPRS. Ketamine injections caused a significant decrease in the EEGalpha amplitude without an accompanying reduction in EEG frequency. The EEGalpha amplitude reduction at the right central electrode was significantly related to subjective pain relief. Subanesthetic doses of ketamine significantly decreased rapid eye movements, but did not initiate slow eye movements. In conclusion, the present EEG-EOG/behavioral results indicate that ketamine-induced failure of neural integration between cortical-subcortical regions induces psychotic symptoms and alters pain perception on neuropathic pain.

Ketamine Injection among High Risk Youth: Preliminary Findings from New York City

Ketamine, a synthetic drug commonly consumed by high risk youth, produces a range of experiences, including sedation, dissociation, and hallucinations. While ketamine is more typically sniffed, we describe a small sample of young ketamine injectors (n=25) in New York City and highlight risks associated with this emerging type of injection drug use. Our findings indicate that the injection practices, injection groups, and use norms surrounding ketamine often differ from other injection drug use: intramuscular injections were more common than intravenous injections; injection groups were often large; multiple injections within a single episode were common; bottles rather than cookers were shared; and the drug was often obtained for free. Our findings suggest that the drug injection practices exercised by ketamine injectors place them at risk for bloodborne pathogens, such as HIV, HBV, and HCV. We conclude that ketamine injectors represent an emerging, though often hidden, population of injection drug users, particularly among high risk, street-involved youth.

Oral ketamine as an adjuvant to oral morphine for neuropathic pain in cancer patients

To evaluate the role of oral ketamine as an adjuvant to oral morphine in cancer patients experiencing neuropathic pain, 9 cancer patients (5 men, 4 women) taking maximally tolerated doses of either morphine, amitriptyline, sodium valproate, or a combination of these drugs for intractable neuropathic pain, and reporting a pain score of >6 on a 0-10 scale, were studied prospectively to evaluate analgesia and adverse effects. Ketamine in the dose of 0.5 mg/kg body weight three times daily was added to the existing drug regimen. Patients were taught to maintain a pain diary wherein they daily recorded their pain, sedation, and vomiting scores, and other side effects. A decrease of more than 3 from the baseline in the average pain score, or a score of < or =3 was taken as a successful response. Seven patients exhibited a decrease of more than 3. Four patients experienced nausea, of which one had vomiting. Two developed loss of appetite. Eight patients reported drowsiness during the first two weeks of therapy (P = 0.001), and this gradually improved over the next two weeks in 5 of these 8 patients. Three patients withdrew from the study, two owing to excessive sedation and another due to a "feeling of unreality." None of the patients reported visual or auditory hallucinations. This experience suggests that low dose oral ketamine is beneficial and effective in the management of intractable neuropathic pain in patients with advanced cancer. However, its utility is limited in some patients by the adverse effects that accompany its use.

Treatment of central post-stroke pain with oral ketamine

Case report of 68 year old female with central post-stroke pain successfully treated with oral ketamine. The patient's pain was refractory to conventional pain treatments and she had persistent right hemi-body neuropathic pain with allodynia and hyperalgesia. An intravenous ketamine trial, followed by oral ketamine with titration to 50mg three times a day was beneficial in decreasing allodynia and hyperalgesia, as well as improving functional capabilities. Known side effects including dysphoria, hallucinations, and paranoid feelings were attenuated with benzodiazepines.

Comparison of the effects of MK-801, ketamine and memantine on responses of spinal dorsal horn neurones in a rat model of mononeuropathy

Selective ligation of the L5/L6 spinal nerves produces a partial denervation of the hindpaw and has proved to be a useful model for studying the mechanisms underlying neuropathic pain. Two weeks after surgery, in vivo electrophysiological studies were performed in sham operated and nerve injured rats and the responses of spinal dorsal horn neurones to controlled electrical and natural (mechanical and heat) stimuli were recorded. The systemic effects of three N-methyl-D-aspartate receptor (NMDA) antagonists, ketamine (1-10 mg/kg), memantine (1-20 mg/kg) and MK-801 (0.1-5 mg/kg) were compared. Ketamine a clinically available NMDA receptor antagonist, produced greater reductions of the postdischarge, thermal (10 mg/kg, P=0.02), and mechanical evoked responses in spinal nerve ligated (SNL) rats (von Frey 9 g, 1 mg/kg, P=0.04; 5 mg/kg, P=0.01; 10 mg/kg, P=0.05; von Frey 50 g, 5 mg/kg, P=0.02; 10 mg/kg, P=0.003). The inhibition of wind-up was comparable in both animal groups. Memantine produced powerful inhibitions of wind-up after nerve injury with little effect in sham controls (5 mg/kg, P=0.02). The postdischarge, mechanical and thermal evoked responses were reduced to similar extents by memantine in both experimental groups. The effects of MK-801 were comparable between SNL and sham operated rats for all neuronal measures (wind-up, postdischarge, thermal and noxious mechanical evoked responses). The differential blocking abilities of these antagonists on the various neuronal responses may relate to the characteristics of their voltage-dependent blockage of the channel associated with the receptor. The favourable side effect profile of memantine supports its potential use for the treatment of neuropathic pain.

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.

The pharmacological basis of contemporary pain management

Pain management has become an increasingly well researched area in medicine over recent years, and there have been advances in a number of areas. While opioids remain an integral part of pain-management strategies, there is now an emphasis on the use of adjuvant drugs, such as paracetamol and anti-inflammatory agents, which through physiological or pharmacological synergism, both enhance pain control and reduce opioid use. The management of neuropathic pain continues to be a challenge. Anti-epileptics and antidepressants, together with clonidine and ketamine, provide the foundations for treatment. Another area of interest has been the widespread use of patient-controlled analgesia and the administration of some drugs, especially opioids, by means other than traditional oral and parenteral routes. The number of new drugs that have reached the stage of clinical trials has been small, yet they offer exciting possibilities. The epibatidine analogue ABT-594 and zinconitide both offer novel approaches to the management of neuropathic pain states, while selective cyclo-oxygenase-2 inhibitors and nitroaspirins may see advances in the management of nociceptive pain states.

Targeting the N-methyl-D-aspartate receptor for chronic pain management. Preclinical animal studies, recent clinical experience and future research directions

A 1967-1999 MEDLINE search of published reports evaluating the role of the glutamate N-methyl-D-aspartate (NMDA) receptor in pain identified 378 animal studies and 132 human studies. There is convincing evidence in these studies that the NMDA receptor mediates prolonged nociceptive behaviors in animal models and various chronic pain symptoms in the clinical population. Administration of older compounds, such as ketamine, dextromethorphan, and amantadine, which are now known to act as NMDA receptor antagonists, have recently been shown to alleviate chronic pain. For years, the pharmaceutical industry has been attempting to produce novel compounds that modulate NMDA receptor activity; however, the adverse effects associated with this class of drugs have prevented their widespread clinical use. Collaborative studies between basic researchers, clinical scientists, and clinicians are needed to delineate characteristics of NMDA receptor antagonism that predict optimal analgesic activity and an acceptable toxicity profile in patients with chronic pain.

Memantine (a N-methyl-D-aspartate receptor antagonist) in the treatment of neuropathic pain after amputation or surgery: a randomized, double-blinded, cross-over study

Evidence has accumulated that the N:-methyl-D-aspartate receptor system plays a role in continuous and particularly, in stimulus-evoked pain after nerve injury. We examined, in a randomized, double-blinded, cross-over fashion, the analgesic effect of memantine (a N:-methyl-D-aspartate receptor antagonist) in a group of patients with chronic pain after surgery. We randomized 19 patients to receive either memantine or placebo in the first 5-wk treatment period. A washout period of 4 wks was followed by another 5-wk treatment period with the opposite drug. The dosage of drug was increased from 5 to 20 mg/d. Pain was recorded daily, with the use of a 0-10 numeric rating scale. Before and at the end of each treatment period, pain and sensitivity were also assessed by using the McGill Pain Questionnaire, allodynia to touch, brush and cold, wind-up-like pain, and thresholds to mechanical stimuli (pressure and von Frey hair). A total of 15 patients (12 amputees and three patients with other nerve injuries) completed the study. There was no difference between memantine and placebo on any of the outcome measures. We conclude that memantine at a dosage of 20 mg/d does not reduce spontaneous or evoked pain in patients with nerve injury pain.

IMPLICATIONS

In a randomized, double-blinded and cross-over study, the analgesic effect of memantine (a drug which reduces the excitability of sensitized neurons in the dorsal horn) was examined in 19 patients with chronic pain after nerve injury.

Stereoselective high-performance liquid chromatographic determination of ketamine and its active metabolite, norketamine, in human plasma

A stereoselective high-performance liquid chromatographic method for the determination of the enantiomers of ketamine and its active metabolite, norketamine, in human plasma is described. The compounds were extracted from plasma by liquid-liquid extraction three times in a combination of cyclohexane with 2.5 M NaOH, 1 mM HCl and 1 M carbonate buffer. Stereoselective separation was achieved on a Chiralcel OD column with a mobile phase of n-hexane-2-propanol (98:2, v/v). The detection wavelength was 215 nm. The lower limits of the determination of the method were 5 ng/ml for ketamine and 10 ng/ml for norketamine. The intra- and inter-day coefficients of variation ranged from 2.9 to 9.8% and from 3.4 to 10.7% for all compounds, respectively. The method was sensitive and sufficiently reproducible for stereoselective monitoring of ketamine and norketamine in human plasma during pharmacokinetic studies after the administration of ketamine for analgesia.

Brain plasticity and hand surgery: an overview

The hand is an extension of the brain, and the hand is projected and represented in large areas of the motor and sensory cortex. The brain is a complicated neural network which continuously remodels itself as a result of changes in sensory input. Such synaptic reorganizational changes may be activity-dependent, based on alterations in hand activity and tactile experience, or a result of deafferentiation such as nerve injury or amputation. Inferior recovery of functional sensibility following nerve repair, as well as phantom experiences in virtual, amputated limbs are phenomena reflecting profound cortical reorganizational changes. Surgical procedures on the hand are always accompanied by synaptic reorganizational changes in the brain cortex, and the outcome from many hand surgical procedures is to a large extent dependent on brain plasticity.

The effects of ketamine on the temporal summation (wind-up) of the R(III) nociceptive flexion reflex and pain in humans

Animal studies have suggested that the temporal summation of nociceptive inputs might play a significant role in the development of central sensitization (i.e., hyperexcitability of central nociceptive neurons) and hyperalgesia via the activation of N-methyl-D-aspartate receptors. To further analyze these processes in humans, we evaluated the effects of small systemic doses of ketamine on the temporal summation (i.e., wind-up) of both the nociceptive flexion (R(III)) reflex and sensations of pain in six healthy volunteers. The R(III) reflex was recorded from the biceps femoris and was elicited by electrical stimulation of the sural nerve. First, the recruitment (stimulus/response) curve for the reflex was built using stimuli up to the pain tolerance threshold (applied once every 6 s). A series of 15 stimuli was then applied once a second at an intensity of 1.2 times the reflex threshold. These procedures were performed both before and after the randomized IV injection of either 0.15 mg/kg ketamine or a placebo. The R(III) reflex threshold and its recruitment curve were not significantly altered after the injection of ketamine or placebo. By contrast, the significant increases (i.e., wind-up) in both the reflex responses and the sensations of pain observed during the higher frequency stimulation were significantly reduced after the administration of ketamine, but not placebo. This method might be useful for quantifying and analyzing the wind-up phenomenon and, thus, for studying the neurophysiological and pharmacological mechanisms underlying hyperalgesia in humans.

IMPLICATIONS

The wind-up phenomenon (i.e., the progressive increase of the responses induced by repetitive nociceptive stimuli) was characterized in humans by using electrophysiological recordings of the nociceptive flexion reflex. We showed that, as in animals, this phenomenon, which might represent an elementary form of the central sensitization involved in various painful syndromes, depends on the activation of N-methyl-D-aspartate receptors, because it was selectively reduced after the administration of ketamine.

Efficacy of pharmacological treatments of neuropathic pain: an update and effect related to mechanism of drug action

Tricyclic antidepressants and carbamazepine have become the mainstay in the treatment of neuropathic pain. Within the last decade, controlled trials have shown that numerous other drugs relieve such pain. We identified all placebo-controlled trials and calculated numbers needed to treat (NNT) to obtain one patient with more than 50% pain relief in order to compare the efficacy with the current treatments, and to search for relations between mechanism of pain and drug action. In diabetic neuropathy, NNT was 1.4 in a study with optimal doses of the tricyclic antidepressant imipramine as compared to 2.4 in other studies on tricyclics. The NNT was 6.7 for selective serotonin reuptake inhibitors, 3.3 for carbamazepine, 10.0 for mexiletine, 3.7 for gabapentin, 1.9 for dextromethorphan, 3.4 for tramadol and levodopa and 5.9 for capsaicin. In postherpetic neuralgia, the NNT was 2.3 for tricyclics, 3.2 for gabapentin, 2.5 for oxycodone and 5.3 for capsaicin, whereas dextromethorphan was inactive. In peripheral nerve injury, NNT was 2.5 for tricyclics and 3.5 for capsaicin. In central pain, NNT was 2.5 for tricyclics and 3. 4 for carbamazepine, whereas selective serotonin reuptake inhibitors, mexiletine and dextromethorphan were inactive. There were no clear relations between mechanism of action of the drugs and the effect in distinct pain conditions or for single drug classes and different pain conditions. It is concluded that tricyclic antidepressants in optimal doses appear to be the most efficient treatment of neuropathic pain, but some of the other treatments may be important due to their better tolerability. Relations between drug and pain mechanisms may be elucidated by studies focusing on specific neuropathic pain phenomena such as pain paroxysms and touch-evoked pain.