Experimental cutaneous pain thresholds and tolerance in clinical analgesia with epidural morphine

Adenosine increases the cutaneous heat pain threshold in healthy volunteers

Adenosine is an endogenously produced substance which in animal experiments exerts anti-nociceptive effects. In humans, algesic effects have been presented following exogenous adenosine administration. A recent study on anaesthetized patients, however, suggested an anti-nociceptive effect during i.v. adenosine. We have studied the pain-reducing effect in healthy volunteers using adenosine 50-80 micrograms.kg-1.min-1 (n = 10), morphine 0.1 mg.kg-1 (n = 5), adenosine 50 micrograms.kg-1.min-1 + morphine 0.1 mg.kg-1 (n = 6), and ketamine 0.1 mg.kg-1 (n = 5); all drugs given i.v., single-blind. Quantitative sensory tests (QST) revealed a significantly increased cutaneous heat pain threshold following adenosine. No effect was seen following ketamine or morphine. Suprathreshold heat pain perception was unchanged in all subjects. Furthermore, warm and cold perception thresholds were not influenced significantly by any drug. Adenosine, morphine and ketamine produced well-known side-effects but of a mild intensity not necessitating any treatment. The present results show that i.v. adenosine can provide a modest but selective increase of cutaneous heat pain thresholds, suggesting a pain-reducing capacity of adenosine in humans.

The hypoalgesic effect of imipramine in different human experimental pain models

In a randomized, placebo-controlled, double-blind, cross-over study, the hypoalgesic effect of a single oral dose of 100 mg imipramine was investigated in 12 healthy volunteers. Test procedures performed before, 3, 6, and 9 h after medication included determination of (1) pain detection and tolerance thresholds to heat and pressure; (2) the thresholds of quadriceps femoris muscle withdrawal reflex to single and repeated electric stimulation of the sural nerve; (3) amplitude of the reflex evoked by 1.5 times the premedication reflex threshold; and (4) continuous pain rating during the cold pressor test. Imipramine significantly increased pain tolerance thresholds to heat (P = 0.03) and pressure (P = 0.01), and both the psychophysical pain tolerance threshold and the reflex threshold to single electric stimulation (P = 0.02 and P = 0.03, respectively). On the repeated stimuli, which consisted of 4 pulses given at 3 Hz, imipramine induced a significant increase in the threshold at which the pain summated through the stimulation series (P = 0.03), whereas the increase in the threshold at which the reflex summated was not significant (P = 0.09). Pain detection thresholds to heat and pressure, the amplitude of the reflex to single suprathreshold stimulation, and pain ratings during the cold pressor test were unaltered by imipramine. It is concluded that imipramine has a differential hypoalgesic effect on different human experimental pain tests. This provides new possibilities of assessing the differential effect of different tricyclic antidepressants on different pain modalities and intensities.

Are poor metabolisers of sparteine/debrisoquine less pain tolerant than extensive metabolisers?

It has recently been shown that O-demethylation of the opioid drug codeine to morphine depends on the sparteine/debrisoquine oxygenase (CYP2D6) which in man exhibits genetic polymorphism. Morphine may be an endogenously formed substance in mammalians. Therefore, it may be hypothesized that the final step in an endogenous synthesis of morphine from codeine also depends on CYP2D6. CYP2D6, which is present in the liver and presumably also in the brain, is not expressed in subjects who are poor metabolisers of the sparteine/debrisoquine type. We have determined sensitivity to painful stimuli in 94 extensive metabolisers and 82 poor metabolisers of sparteine in 2 phasic (pain thresholds to heat and pressure) and 1 tonic (cold pressor test) experimental pain model. Extensive and poor metabolisers did not differ significantly in the 2 phasic pain models neither with respect to pain detection nor pain tolerance thresholds. However, for the cold pressor test, peak pain ratings and area under the pain rating-time curve during 2 min were significantly higher in poor than in extensive metabolisers (P = 0.0024 and 0.044). Furthermore, a substantially higher fraction of poor metabolisers prematurely withdrew their hand from the ice water during the cold pressor test due to intolerable pain (32 vs. 18%, P = 0.0545). We conclude that poor metabolisers of sparteine may be less tolerant to tonic pain than extensive metabolisers, and we hypothesize that this may be related to an inherited defect in endogenous synthesis of morphine via CYP2D6 in the brain.

A two-dose epidural morphine regimen for cesarean section patients: therapeutic efficacy

A single dose of epidural morphine (EM) usually produces 24 h of post-cesarean section (CS) analgesia and patients require supplemental analgesics beyond this period. This study assesses if a second dose of EM administered 24 h after the first one offers superior therapeutic efficacy compared to conventional analgesics. Patients (n = 100) were randomized to receive one or two doses of epidural morphine. In all patients, EM 5 mg was administered after delivery. After 24 h patients received epidurally either normal saline (n = 50, Group 1) or morphine 5 mg (n = 50, Group 2). An independent observer used a visual analogue scale to assess nausea, itching, and analgesia 24 h after each injection. Results were expressed as mean +/- 1 s.e. mean and analyzed using nonparametric methods. The second dose of EM produced a significantly lower incidence and severity of nausea and itching than did the first dose (P < 0.01) in Group 2 with no difference in analgesia. The second day postoperative pain score in Group 1 was significantly greater than the first day score in the same group, and significantly greater than the severity score in Group 2. Only 36% of patients receiving two doses of EM required supplemental analgesics beyond 48 h compared to 76% of those receiving one dose (P < 0.01). No serious complications were noted. In summary, the use of a second dose of EM for post-CS analgesia produces better analgesia and reduces the need for oral analgesics. The second dose produced fewer side-effects, probably due to acute tolerance to morphine.

Hypoalgesia following epidural morphine: a controlled quantitative experimental study

The efficacy, duration, and spread of epidural morphine hypoalgesia were assessed by an experimentally induced pricking pain evoked by laser stimulation. Four mg of plain morphine was injected epidurally in 7 volunteers at the L2-L3 interspace. Thresholds to warmth and pain perception, and pain-evoked potentials were measured. In the first experiment, hypoalgesia was monitored each hour for 7 h at various dermatomes. Hypoalgesia was detected at S1 dermatome after 2 h, but 3 h elapsed before hypoalgesia could be detected at the L1, T12, T10, T8, and T6 dermatomes. No effect was found at C7. No conduction delay was found along the pain pathway during hypoalgesia. Hypoalgesia lasted more than 7 h at S1, whereas hypoalgesia could not be detected after 5 h at other dermatomes. In the second experiment, naloxone (0.8 mg i.v.) was injected 230 min after injection of epidural morphine, and the subsequent recording 10 min later showed that hypoalgesia had been partly reversed. The onset and duration of hypoalgesia are different for experimentally induced pain and clinical pain. Experimentally laser-induced pain has the advantage of being quantitative, and is, as such, useful to assess hypoalgesia, and to test the potency of narcotics.

Hypoalgesia following intrathecal morphine: a segmental dependent effect

The onset phase of hypoalgesia, following intrathecal morphine, was assessed by experimental argon laser-induced pain. A dose of 0.4 mg morphine was injected pre-operatively at the L3-L4 level into nine patients. The thresholds to laser-induced pain and pain-evoked brain potentials were monitored for 2 h at the S1, L1, and C7 dermatomes. Hypoalgesia was detected at the S1 and L1 dermatomes after 5 and 15 min, respectively. No hypoalgesic effect was found at C7. This indicates that hypoalgesia was caused predominantly by segmental spinal mechanisms during the onset phase, and not by a general widespread effect. No latency changes (conduction delay) of the brain potentials evoked from the hypoalgesic dermatomes were found. Cutaneous pain, induced experimentally by laser stimulation, has the advantage of being quantitative and is useful to assess the onset and the segmental spread of hypoalgesia.

The nature of opioid responsiveness and its implications for neuropathic pain: new hypotheses derived from studies of opioid infusions

In recent years, the observation that the response of patients to opioid drugs may be influenced by properties inherent in the pain or pain syndrome, such as its pathophysiology, has evolved into the belief that certain types of pain, e.g., neuropathic pains, may be unresponsive to these drugs. This concept has important implications for both clinical practice and basic understanding of opioid mechanisms. We critically evaluate opioid responsiveness, particularly as it relates to neuropathic pain, and propose a clinically relevant definition and a paradigm for its investigation. The paradigm is illustrated by analgesic responses to opioid infusion in 28 patients with neuropathic pains and by a detailed presentation of the pharmacokinetic and pharmacodynamic relationships in one of these patients, whose central pain responded promptly to an infusion of hydromorphone. From this analysis, we hypothesize that (1) opioid responsiveness in man can be defined by the degree of analgesia achieved during dose escalation to either intolerable side effects or the occurrence of 'complete' or 'adequate' analgesia; (2) opioid responsiveness is a continuum, rather than a quantal phenomenon; (3) opioid responsiveness is determined by a diverse group of patient characteristics and pain-related factors, as well as drug-selective effects; and (4) a neuropathic mechanism may reduce opioid responsiveness, but does not result in an inherent resistance to these drugs. Given the complexity of factors contributing to opioid responsiveness and the observation that outcome cannot be reliably predicted, opioids should not be withheld on the assumption that pain mechanism, or any other factor, precludes a favorable response. Both the clinical use of opioids and paradigms to investigate opioid responsiveness should include dose escalation to maximally tolerated levels and repeated monitoring of analgesia and other effects.

Comparison of subjective and objective analgesic effects of intravenous and intrathecal morphine in chronic pain patients by heat beam dolorimetry

The pain tolerance latencies of 10 chronic pain patients were evaluated by heat beam dolorimetry (stimulus intensity 15.33 mW.cm-2.sec-1) prior to and following administration of morphine by intrathecal (n = 5) or intravenous (n = 5) routes. Patients not undergoing opiate withdrawal evinced increased baseline pain tolerance latencies prior to drug administration compared with normal volunteers. Two patients undergoing the opiate withdrawal syndrome at the time of test experienced reduced pain tolerance latencies compared with normal volunteers, most probably corresponding to the hyperesthesia symptom of the syndrome. Intravenous morphine infusion (30 mg) induced a time-dependent increase in cutaneous pain tolerance with peak effect occurring 1-2 h after administration. This persisted for up to 4 h and thereafter declined. The time course of subjective pain self-report by visual pain analog scale (VPAS) measurements corresponded to the time course of increasing cutaneous pain tolerance latency assessed by dolorimetry. Pain self-reports following intrathecal morphine infusion (2.25 or 1 mg) followed a similar though slower onset to that reported by patients receiving intravenous morphine and was of lesser degree. In contrast, heat beam dolorimetric evidence of increased cutaneous pain tolerance (which was of lesser degree than following i.v. morphine) did not reach its maximum during the 4 h measuring period. A dissociation was noted therefore between the self-reported relief of endogenous pain and dolorimetrically measured cutaneous analgesia following intrathecal morphine administration. Linear regression correlation analysis characterized this phenomenon as a positive correlation between cutaneous pain tolerance and pain relief self-report following intravenous morphine infusion and a negative correlation following intrathecal administration. We propose that the phenomenon may be due to intrathecal morphine acting via two separate compartments: one spinal and one supraspinal.(ABSTRACT TRUNCATED AT 250 WORDS)

Tolerance to morphine microinjections in the periaqueductal gray (PAG) induces tolerance to systemic, but not intrathecal morphine

The acquisition and retention of tolerance to the antinociceptive effect of supraspinal morphine on the tail withdrawal reflex was assessed in rats implanted with unilateral cannulae in the periaqueductal gray (PAG). Development of tolerance to daily microinjections of morphine was indicated by the return of the tail flick response within 4 days, followed by the recovery of analgesic sensitivity one week later. After tolerance had developed, the effect of an acute systemic (1.5-4.5 mg/kg) or intrathecal (5-15 micrograms) morphine injection was determined. 'Cross-tolerance' was observed between systemic and supraspinal morphine but not between intrathecal and supraspinal morphine. The data indicate that tolerance to chronic intracerebral morphine produces the same behavioral consequences as tolerance to systemic morphine.

Antinociceptive effect of systemic and intrathecal morphine in spinally transected rats

The antinociceptive effect of morphine on the tail withdrawal reflex was examined in spinally transected rats. The efficacy of systemically administered morphine was significantly reduced within 24 h after transection, and continued to decline during the first three posttransection weeks. In contrast to the diminished effect of systemic morphine, the efficacy of intrathecal morphine was not reduced during the first three weeks after a spinal transection. These data demonstrate a significant difference in the functional effect of systemic and spinal morphine in spinally transected rats. The results indicate that the direct antinociceptive effect of morphine on the spinal cord is not reduced after spinal transection.

Investigation of tolerance to chronic intrathecal morphine infusion in the rat

Rats received chronic subcutaneous or intrathecal infusions of either saline or 25 micrograms/microliter/hr or 50 micrograms/microliter/hr of morphine sulfate. During five days of infusion individual groups of rats were assessed on either the nociceptive tail flick or hot plate test. After the infusions, the analgesic effects of either subcutaneous or intrathecal morphine test doses were evaluated. Tolerance developed to the analgesic effect of both subcutaneous and intrathecal morphine infusions on the tail flick test. Subcutaneously infused rats were also tolerant to a subcutaneous morphine challenge on this test. However, intrathecally infused rats were not tolerant to either the subcutaneous or intrathecal challenge. In contrast to these results, rats tested on the hot plate were not analgesic in response to either subcutaneous or intrathecal morphine infusions. However, these rats were tolerant when challenged with either subcutaneous or intrathecal morphine. The results are discussed in terms of the relative contribution of spinal and supraspinal sites to opiate tolerance, and the possibility that tolerance does not develop to the antinociceptive effect of spinal morphine on spinally mediated reflexes.

Investigation of "cross-tolerance" between systemic and intrathecal morphine in rats

Animals were implanted subcutaneously with morphine or placebo pellets and assessed daily on the hot plate or the tail flick test. After tolerance developed to morphine-induced analgesia the response to an acute systemic (1-6 mg/kg SC) or intrathecal morphine injection (0-30 micrograms) was determined. "Cross-tolerance" was observed on both the hot plate (6 mg/kg) and the tail flick tests (3 and 6 mg/kg) between the two different routes of subcutaneous administration. "Cross-tolerance" was also observed between systemic and intrathecal morphine on the hot plate test. However, no "cross-tolerance" between systemic and spinal morphine was observed in animals on the tail flick test. Assessment of naloxone-precipitated withdrawal indicated that morphine implanted animals showed more abstinence signs (wet shakes and teeth chattering) than placebo animals. These results suggest that the nociceptive assessment procedure plays a significant role in the expression of "cross-tolerance" between systemic and spinal opiates.

Epidural opiates: long-term experiences in cancer pain

Epidural opiates were administered to 139 patients with pain due to malignant diseases via a chronic indwelling catheter inserted percutaneously. So far, 9,716 days of treatment can be evaluated. In 87% of the patients whose pain previously could not be controlled with conventional analgesic approaches, epidural opiates resulted in remarkable pain relief. With a mean daily dose of 15.6 mg morphine (range 2-290 mg) or 0.86 mg buprenorphine (range 0.15-7.2 mg) half of the patients could be treated as outpatients. The mean duration of therapy was 72 days (range 1-700 days), 26 catheters being in place for more than 100 days and one catheter being in place for 510 days. Two severe side-effects (meningitis) were observed, both patients being free of symptoms after catheter removal and antibiotic therapy. Epidural opiates proved to be a valuable method of pain control in terminal illness. The method should be reserved for those patients, for whom oral opiates fail to produce effective pain relief.

[Analgesia with an implanted device for repetitive intrathecal injections of morphine]

The use of intraspinal narcotics has been widely accepted as pain relief treatment for intractable cancer pain. Intraspinal low doses of morphine induce a potent selective long lasting analgesia. To avoid repetitive lumbar puncture, a drug delivery device was surgically implanted in 41 patients. The surgical procedure is described. The mean amount of morphine needed was 1.48 +/- 0.25 mg per day at time of surgery, rising to 6.86 +/- 1.47 mg per day after a mean survival time of 65 days. Tolerance became a major problem in 18 patients, which nearly all were selected at a late disease stage and previously received narcotics for pain relief. However, no clear-cut prognostic factor had a predictive value for the appearance of tolerance. In some cases, it could be successfully treated by intraspinal injection of local anaesthetics or clonidine. CSF leakage was noted in 11 patients; this was a challenge for us, as no other authors reported such a high rate for this complication. Aseptic meningitis was noted three times. In all cases but one, the symptoms resolved with appropriate treatment.

Differential effects of epidural morphine in the treatment of cancer-related pain

Fifty-five patients with pain associated with cancer were selected for long-term treatment with epidural morphine. Patients who had more than one type of pain within the same region were specially analysed concerning differential analgesic effects of the treatment, i.e. the patients served as their own control. Twenty-eight of the 55 patients became pain free. In 21 patients alleviation of pain was complete only for one or two of several types of pain within the same area with a certain dose of epidural morphine. In six patients the treatment failed. An analysis revealed that the best response was obtained when the pain was continuous and originated from deep somatic structures. In co-existing continuous visceral pain or intermittent somatic pain originating e.g. from a pathological fracture, the outcome of the treatment was variable. Cutaneous pain, pain classified as neurogenic, and intermittent pain due to intestinal obstruction was only occasionally relieved. Ten of the patients had co-existing pain of non-malignant origin and none of them was helped for that pain. The variable response to epidural morphine may indicate that different types of pain-producing stimuli engage different kinds of receptors which differ in affinity to morphine in the spinal fluid; it is also possible that some pain-mediating systems are non-responsive to opiates.