Effect of intravenous magnesium on pain and secondary hyperalgesia associated with the heat/capsaicin sensitization model in healthy volunteers

We investigated the effects of i.v. magnesium on secondary hyperalgesia following heat/capsaicin stimulation in human volunteers. Twenty-five volunteers were included in this double blind, randomized, crossover study. Sensitization was induced in the volunteers, who were then subjected to either i.v. saline or magnesium sulphate. No analgesic or antihyperalgesic effect could be demonstrated in sensitized skin during infusion of magnesium. In contrast, painfulness of thermal stimulation was increased in normal skin. These results suggest that i.v. magnesium has no important analgesic effects in clinically relevant doses.

Effect of remifentanil on pain and secondary hyperalgesia associated with the heat--capsaicin sensitization model in healthy volunteers

BACKGROUND

The heat--capsaicin sensitization model was developed as a noninvasive and noninjurious human experimental pain model. The sequential application of moderate intensity thermal and topical chemical stimuli produces stable and long-lasting areas of cutaneous secondary hyperalgesia. The aim of the present study was to validate the heat--capsaicin sensitization model as a tool for testing analgesic drug efficacy. Responsivity of model-associated measures was tested with remifentanil, a potent and ultrashort acting mu-opioid agonist.

METHODS

Sensitization was induced by heating forearm skin with a thermode at 45 degrees C for 5 min, immediately followed by application of 0.075% capsaicin cream for 30 min. Sensitization was rekindled four times at 40-min intervals with the thermode at 40 degrees C for 5 min. After each rekindling, areas of secondary hyperalgesia were measured, and the painfulness of thermal stimulation in normal skin with 45 degrees C for 1 min (long thermal stimulation [LTS]) was rated. Before and during the second rekindling, remifentanil 0.10 microg x kg(-1) x min(-1) or saline-placebo was infused for 35 min.

RESULTS

Infusion of remifentanil reduced the areas of secondary hyperalgesia to 29--30% of baseline size compared with 75--83% during placebo. Similarly, remifentanil reduced the painfulness of LTS to 29% of baseline severity compared with 84% during placebo. Areas of secondary hyperalgesia and LTS painfulness returned to baseline levels by the time of the third rekindling, demonstrating rapid disappearance of remifentanil analgesia and possibly transient spontaneous opioid withdrawal hyperalgesia.

CONCLUSION

Using the heat-capsaicin sensitization model, opioid analgesia and suppression of secondary hyperalgesia was reliably demonstrated without skin injury.

Capsaicin evoked pain and allodynia in post-herpetic neuralgia

The hypothesis that the pain and allodynia associated with post-herpetic neuralgia (PHN) is maintained by a combination of input from preserved primary afferent nociceptors and sensitization of central pain transmitting neurons was examined in 17 subjects with PHN. Pain, allodynia, thermal sensory function, cutaneous innervation, and response to controlled application of 0.075% capsaicin were measured. Compared to mirror-image skin, applying capsaicin on a 9 cm(2) area of PHN skin significantly increased overall PHN pain and allodynia in 11 of 17 subjects. These 'capsaicin responders' were characterized by higher average daily pain, higher allodynia ratings, and relatively preserved sensory function at baseline compared to the non-responders. In three of the 'capsaicin responders' the area of allodynia expanded into previously non-allodynic and non-painful skin that had normal sensory function and cutaneous innervation. These observations support the hypothesis that allodynia in some PHN patients is a form of chronic secondary hyperalgesia maintained by input from intact and possibly 'irritable' primary afferent nociceptors to a sensitized CNS.

Lack of efficacy of riluzole in the treatment of peripheral neuropathic pain conditions

OBJECTIVE

To assess the efficacy, tolerability, and safety of riluzole in the treatment of peripheral neuropathic pain conditions.

BACKGROUND

Both basic and clinical research has demonstrated that drugs with sodium channel and NMDA antagonism can be effective in alleviating neuropathic pain. Riluzole, a drug currently used for treatment of ALS, possesses these properties. It was hypothesized that riluzole would be effective in reducing the pain in subjects with peripheral neuropathic pain.

METHODS

Two randomized, placebo-controlled, crossover studies were performed at two sites. Study 1 compared 100 mg/day of riluzole (the currently recommended dosage for treatment of ALS) versus placebo, and Study 2 compared 200 mg/day of riluzole versus placebo. Each treatment phase (both studies) was 2 weeks long, separated by 2-week wash-out periods. Outcome measures included change in the score on a 100-mm pain intensity visual analog scale, the Neuropathic Pain Scale, allodynia, hyperalgesia, and preference for study treatment phase.

RESULTS

Twenty-two subjects completed Study 1, and 21 subjects completed Study 2. Four subjects (two from each study) discontinued the study because of intolerable side effects. No statistical difference was found for any study outcome measure between riluzole and placebo for either study. In Study 1, pain intensity was more likely to increase than decrease with riluzole (mean treatment difference 8.7 mm; 95% CI -19.5 to +2.1 mm). In Study 2, very slight pain reduction was observed with riluzole compared with placebo (mean treatment difference 1.4 mm; 95% CI -5.1 to +8.0 mm). In both studies, the majority of subjects chose "no change" in pain on the category relief scale after placebo and riluzole treatment phases. On study completion, no treatment preference was reported by 76% of the subjects in Study 1 and by 61% of the subjects in Study 2.

CONCLUSIONS

Doses of riluzole at (100 mg) or above (200 mg) those used for the treatment of ALS were not effective in alleviating peripheral neuropathic pain.

The effect of systemic lidocaine on pain and secondary hyperalgesia associated with the heat/capsaicin sensitization model in healthy volunteers

Although effective in neuropathic pain, the efficacy of systemic lidocaine in non-neuropathic pain remains uncertain. We investigated the analgesic effect of systemic lidocaine on the heat/capsaicin sensitization model of experimental pain in 24 volunteers. Sensitization was produced by heating the skin to 45 degrees C for 5 min, followed by a 30-min application of 0.075% capsaicin cream, and maintained by periodically reheating the sensitized skin. Subjects received IV lidocaine (bolus 2 mg/kg, then infusion 3 mg. kg. h), or saline for 85 min. Areas of secondary hyperalgesia, heat pain detection thresholds, and painfulness of stimulation with 45 degrees C for 1 min (long thermal stimulation) were quantified. Systemic lidocaine reduced the area of secondary hyperalgesia to brush, but not to von Frey hair stimulation. Lidocaine did not alter heat pain detection thresholds or painfulness of long thermal stimulation in normal skin. We conclude that, at infusion rates in the low- to mid-antiarrhythmic range, lidocaine has no effect on acute nociceptive pain but does have a limited and selective effect on secondary hyperalgesia.

IMPLICATIONS

The efficacy of systemic lidocaine in nonneuropathic pain remains uncertain. This study investigates the effect of systemic lidocaine on experimental-induced hyperalgesia in 25 volunteers. Hyperalgesia was induced by using an experimental pain model that uses heat and capsaicin in combination. Systemic lidocaine showed a selective effect on secondary hyperalgesia.

Quantitation of levorphanol in human plasma at subnanogram per milliliter levels using capillary gas chromatography with electron-capture detection

A gas chromatographic method for the determination of levorphanol in human plasma is described. The method utilizes extractive alkylation with tetrabutylammonium cation as the phase-transfer catalyst and pentafluorobenzyl bromide as the alkylating agent, and employs a structural analog, d-3-hydroxy-N-ethylmorphinan, as the internal standard. The pentafluorobenzyl ethers formed are separated by capillary gas chromatography and detected by electron capture. The method has good precision and accuracy for concentrations ranging from 0.25 ng/ml to 100 ng/ml and has been used to measure plasma concentrations as part of a study to evaluate the management of chronic neuropathic pain with levorphanol.

A new human experimental pain model: the heat/capsaicin sensitization model

The heat/capsaicin sensitization model is a new human experimental pain model that synergistically combines non-invasive physical and chemical methods of nociceptor stimulation to produce stable and long-lasting hyperalgesia with a low potential for skin injury. In 10 healthy volunteers the forearm was stimulated with a 45 degrees C thermode for 5 min to produce an area of secondary hyperalgesia. Applying capsaicin cream for 30 min further expanded the area of secondary hyperalgesia. Periodically heating the treated skin with a previously non-painful temperature of 40 degrees C re-kindled the sensitization enough to maintain stable areas of secondary hyperalgesia for 4h. The evoked pain was moderate and well tolerated. The heat/capsaicin sensitization model should be well suited for studying pain mechanisms and testing new analgesics.

Topical lidocaine patch relieves postherpetic neuralgia more effectively than a vehicle topical patch: results of an enriched enrollment study

This study compared the efficacy of topical lidocaine patches versus vehicle (placebo) patches applied directly to the painful skin of subjects with postherpetic neuralgia (PHN) utilizing an 'enriched enrollment' study design. All subjects had been successfully treated with topical lidocaine patches on a regular basis for at least 1 month prior to study enrollment. Subjects were enrolled in a randomized, two-treatment period, vehicle-controlled, cross-over study. The primary efficacy variable was 'time to exit'; subjects were allowed to exit either treatment period if their pain relief score decreased by 2 or more categories on a 6-item Pain Relief Scale for any 2 consecutive days. The median time to exit with the lidocaine patch phase was greater than 14 days, whereas the vehicle patch exit time was 3.8 days (P < 0.001). At study completion, 25/32 (78.1%) of subjects preferred the lidocaine patch treatment phase as compared with 3/32 (9.4%) the placebo patch phase (P < 0.001). No statistical difference was noted between the active and placebo treatments with regards to side effects. Thus, topical lidocaine patch provides significantly more pain relief for PHN than does a vehicle patch. Topical lidocaine patch is a novel therapy for PHN that is effective, does not cause systemic side effects, and is simple to use.

Effect of EMLA pre-treatment on capsaicin-induced burning and hyperalgesia

Capsaicin, which has been studied extensively as a treatment for itch and several chronic pain disorders, induces burning during the first week of therapy, causing a substantial percentage of patients to discontinue treatment prematurely. We examined whether pre-treatment with the topical anesthetic EMLA reduces the burning sensation induced by capsaicin and alters capsaicin effects on thermal sensation and pain thresholds. Healthy adult volunteers participated in the single-blind, 6-day study. After baseline measurement of warmth, cold pain and heat pain thresholds with a computerized thermal sensory analyzer, subjects applied EMLA thrice daily on one forearm and vehicle placebo on the other forearm, 60 min before applying capsaicin 0.075% on both forearms. Subjects rated burning sensations 3 times a day throughout the study. After 1 and 5 days of thrice daily application of EMLA or vehicle followed by capsaicin, thermal sensory testing was repeated. Subjects rated burning sensations to the significantly less on the EMLA pre-treated forearm compared with the placebo pre-treated forearm during all 5 days of treatment (p < 0.01). Capsaicin with and without EMLA produced significant heat pain hyperalgesia and cold pain hypoalgesia after 1 day of treatment. After 5 days of treatment, heat pain hyperalgesia persisted on both forearms; however, it was significantly less on the EMLA-treated forearm vs the vehicle-treated site (p < 0.03). Cold pain hypoalgesia persisted in both forearms. The warmth sensation threshold was significantly higher on the EMLA-pre-treated forearm after 1 and 5 days of treatment. In conclusion, pre-treatment with EMLA significantly reduced the burning sensation from capsaicin and attenuated heat hyperalgesia during treatment.

Cocaine metabolite kinetics in the newborn

The study goal was to determine the half-life elimination of cocaine and benzoylecgonine (BZE) in the newborn. Three 0.3-mL blood samples were collected during the first day of life. Urine was collected once daily. Cocaine and BZE concentrations were determined by gas chromatography-mass spectrometry. An extraction method was developed for measuring low concentrations of cocaine and BZE in small (0.1 mL) blood samples. Cocaine had a half-life of 11.6 h in one subject. The half-life of BZE during the first day of life, based on blood data in 13 subjects, was 16 h (95% confidence interval [CI], 12.8 to 21.4 h). The half-life of BZE during the first week of life, based on urine data in 16 subjects, was 11.2 h (95% CI, 10.1 to 11.8 h). The novel extraction method for small blood sample volumes should be applicable to other basic drugs.

Pain caused by herpes zoster infection

Postherpetic neuralgia (PHN) is a neuropathic pain disorder that occurs most often in the elderly. This painful condition is uniquely suited for clinical research, resulting in an emerging understanding of the pathophysiology of the persistent pain. Until recently, only the tricyclic antidepressants proved effective for PHN. Controlled trials of a wide variety of therapeutic strategies are in progress or have been recently completed.

Cocaine, nicotine, caffeine, and metabolite plasma concentrations in neonates

The objective of this study was to measure the umbilical cord plasma levels of cocaine, nicotine, caffeine, and their metabolites. Thirty-six neonates at risk for prenatal cocaine exposure were prospectively enrolled. Umbilical cord plasma was analyzed by gas chromatography-mass spectroscopy for cocaine, cocaethylene, benzoylecgonine (BZE), nicotine, cotinine, and caffeine. Eighteen neonates were plasma positive for BZE, and 50% of these were also positive for cocaine. Cocaethylene was not found. The maximum plasma cocaine concentration was 88 ng/mL (mean, 39 ng/mL). The maximum plasma BZE concentration was 3880 ng/mL (mean, 844 ng/mL). Among BZE-positive babies, the mean plasma drug levels were as follows: nicotine, 1.8 ng/mL; cotinine, 94 ng/mL; and caffeine, 1205 ng/mL. Among the BZE-negative babies, the mean plasma drug levels were as follows: nicotine, 5.2 ng/mL; cotinine, 97 ng/mL; and caffeine, 1440 ng/mL. These cocaine levels raise the possibility of pharmacological effects of cocaine in the early neonatal period.

Effect of adrenergic receptor activation on post-herpetic neuralgia pain and sensory disturbances

Patients with acute herpes zoster, and to a lesser extent post-herpetic neuralgia (PHN), have been reported to respond to local anesthetic blockade of the sympathetic nervous system. In animal models of nerve injury, local injection of adrenergic agonists after nerve injury, but not before, excites nociceptors. In some patients with chronic neuropathic pain, local application of norepinephrine evokes pain. In 15 subjects with PHN, the role of adrenergic receptors in PHN pain was assessed in a two-session double blind study comparing the response to cutaneous infiltration of epinephrine or phenylephrine (30 micrograms in 3 ml) with the response to normal saline in both the painfully affected skin and mirror-image normal skin. Two adjacent sites were studied on each side of the body, one site for injection and the other for measuring sensory effects of the injection. In the morning part of each session, mirror-image normal skin was injected. In the afternoon portion of each session, skin in the most painful area affected by PHN was injected. Injection of saline or the adrenergic agonist in normal skin produced mild and transient pain without development of allodynia and without affecting overall PHN pain intensity. In PHN skin, injection of saline and the adrenergic agonist produced an equivalent degree of transient pain that was slightly greater than injection into mirror-image normal skin. After injection of the adrenergic agonist into PHN skin, both overall PHN pain and allodynia severity were significantly greater than after saline injection, peaking at 10-15 min post-injection. Even when PHN has been present for years, adrenergic receptor stimulation in PHN skin increases pain, most likely through direct activation of C-nociceptors in the painful skin. Increased allodynia is most likely mediated centrally and driven by the increase in C-nociceptor input.

Response to intravenous lidocaine infusion predicts subsequent response to oral mexiletine: a prospective study

The local anesthetic sodium-channel blockers lidocaine and mexiletine reduce spontaneous and evoked activity in experimental neuroma models and have been reported to relieve a variety of clinical neuropathic pain conditions. The predictive value of relief from an intravenous lidocaine infusion (IVL) for subsequent relief from a 4-week trial of oral mexiletine was assessed in a prospective study of nine subjects with chronic neuropathic pain of peripheral origin. Subjects received IVL, 2 mg/kg and 5 mg/kg, over 45 min during separate sessions in random order under double-blind conditions. One week later, a 4-week titrating trial of oral mexiletine was initiated. Both doses of IVL significantly reduced pain visual analogue scores (VAS) scores. Although IVL 5 mg/kg produced significantly higher pain relief scores than IVL 2 mg/kg, subjects responded in a highly consistent manner to the two IVL. Subsequent response to oral mexiletine was significantly correlated with the average response to the two IVL. Mexiletine dose and blood levels were not correlated with pain relief. The results suggest that IVL may be a valuable tool in selecting patients for oral therapy with analogous drugs.

Cutaneous innervation density in the allodynic form of postherpetic neuralgia

The relationship between deafferentation, sensory function, and pain was explored in 18 subjects with chronic postherpetic neuralgia (PHN). Subjective thresholds for warmth, cooling, and heat pain were measured quantitatively in painful skin areas and compared with normal contralateral skin. The severity of allodynia was graded in the affected area. Two 3-mm punch biopsies were taken from the most painful skin area and one from unaffected contralateral mirror-image skin. Immunofluorescence with the axonal marker PGP 9.5 revealed a reduction in density of innervation of the epidermis, the dermal-epidermal junction, and the eccrine sweat glands in PHN skin. In painful PHN skin, the reduction in innervation density was positively correlated with the magnitude of the thermal sensory deficits. However, loss of cutaneous innervation was inversely correlated with allodynia, indicating that surviving cutaneous primary afferent nociceptors that are spontaneously active and/or sensitized contribute to PHN pain and allodynia.

The relationship of pain, allodynia and thermal sensation in post-herpetic neuralgia

In the syndrome of post-herpetic neuralgia (PHN), the nature of the sensory disturbance and its relationship both to the severity and cause of the pain is controversial. To address these issues, sensory mapping and quantitative thermal sensory testing was carried out four times in separate sessions on 35 subjects with established PHN. All subjects had pain affecting the torso or extremities and brush-evoked allodynia. Each session included rating of ongoing pain, mapping of the area of any sensory disturbance and the area of greatest pain, grading of allodynia severity within the area of greatest ongoing pain, and quantitative testing of thermal sensation in both the painful and the contralateral unaffected mirror-image skin. The severity of allodynia was positively correlated with reported ongoing pain severity. As a group, subjects had a sensory deficit to thermal stimuli in PHN skin compared with unaffected mirror-image skin. However, the magnitude of the heat pain sensory deficit was inversely correlated with both pain intensity and severity of allodynia. In fact, 12 subjects had heat hyperalgesia in their region of maximum pain. Compared with the 23 subjects with heat hypoalgesia, the group of 12 heat hyperalgesic subjects had significantly higher pain ratings and allodynia severity. Sensory loss was less strongly, but still inversely related to pain severity for the thermal modalities of innocuous warming, cooling and cold pain. This implies that there is no simple relationship between loss of peripheral nerve function and spontaneous or evoked pain. Rather, the preservation of several sensory modalities in their area of maximal pain suggests that in some PHN patients, activity in primary afferent nociceptors that remain connected to both their peripheral and central targets contributes significantly to ongoing pain. Although other mechanisms are likely to contribute to the pain, the demonstrated responsivity of PHN to topical agents including local anaesthetics, capsaicin, and non-steroidal anti-inflammatory drugs, supports this proposed mechanism of pain generation.

Lidocaine patch: double-blind controlled study of a new treatment method for post-herpetic neuralgia

Post-herpetic neuralgia (PHN) is a common and often intractable neuropathic pain syndrome predominantly affecting the elderly. Topical local anesthetics have shown promise in both uncontrolled and controlled studies. Thirty-five subjects with established PHN affecting the torso or extremities completed a four-session, random order, double-blind, vehicle-controlled study of the analgesic effects of topically applied 5% lidocaine in the form of a non-woven polyethylene adhesive patch. All subjects had allodynia on examination. Up to 3 patches, covering a maximum of 420 cm2, were applied to cover the area of greatest pain as fully as possible. Lidocaine containing patches were applied in two of the four 12-h-long sessions, in one session vehicle patches were applied, and one session was a no-treatment observation session. Lidocaine containing patches significantly reduced pain intensity at all time points 30 min to 12 h compared to no-treatment observation, and at all time points 4--12 h compared to vehicle patches. Lidocaine patches were superior to both no-treatment observation and vehicle patches in averaged category pain relief scores. The highest blood lidocaine level measured was 0.1 micrograms/ml, indicating minimal systemic absorption of lidocaine. Patch application was without systemic side effect and well tolerated when applied on allodynic skin for 12 h. This study demonstrates that topical 5% lidocaine in patch form is easy to use and relieves post-herpetic neuralgia.

Chronic pain: from theory to practical management

The neurologist is an important part of the pain management team. Factors that can alter presentation and complicate establishing a diagnosis are reviewed. A multidisciplinary approach to evaluation is advocated, particularly the inclusion of independent psychiatric or psychological evaluation. Treatment planning consists of addressing potential sources of failure of pain management, setting appropriate goals, and using the diagnostic assessment to plan pharmacologic and nonpharmacologic interventions based on pain mechanisms. Even if pharmacologic interventions do not alter pain, an education-oriented behavioral pain program integrated with physical therapy can improve function and foster self-reliance in controlling pain.

Topical lidocaine gel relieves postherpetic neuralgia

Postherpetic neuralgia (PHN) following herpes zoster is a common and disabling neuropathic pain syndrome. In a double-blind, three-session study, 5% lidocaine gel or vehicle was applied simultaneously to both the area of pain and to the contralateral mirror-image unaffected skin. In the local session, lidocaine gel was applied to the painful skin area. In the remote session, lidocaine gel was applied to mirror-image skin. In the placebo session, vehicle was applied bilaterally. For cranial PHN, gel was applied without occlusion for 8 hours. For limb or torso PHN, gel was applied under occlusion for 24 hours. The 16 subjects with cranial PHN reported pain relief significantly favoring local drug application at 30 minutes, 2, 4, and 8 hours. The 23 subjects with torso or limb PHN reported significantly lower pain intensity with local drug application at 8 hours and both pain relief and reduced pain intensity at 24 hours. Remote lidocaine application to mirror-image skin was no different from placebo. No systemic adverse effects were reported and blood levels did not exceed 0.6 microgram/ml. Topical application of 5% lidocaine gel relieves PHN pain by a direct drug action on painful skin.

The interpretation of pain relief and sensory changes following sympathetic blockade

A comparative study of the effects of sympathetic blockade by stellate ganglion block (SGB) and intravenous phentolamine infusion (PhI) was carried out in 24 patients with presumed sympathetically maintained pain of an upper extremity. A total of 15 SGBs and 16 PhIs were performed, with seven patients undergoing both procedures. All patients developed a Horner's syndrome with SGB and nasal stuffiness and cardiovascular changes with PhI. Similar pain relief was obtained with SGB and PhI in six of the seven who underwent both procedures. Pre-procedure patient characteristics including age, sex, duration of pain, historical and physical examination features suggestive of the reflex sympathetic dystrophy syndrome, and sensory disturbances such as allodynia and hyperpathia did not predict pain relief from either procedure. Changes in skin temperature following the sympatholytic procedure did not correlate with pain relief. For PhI, pain relief correlated with the magnitude of decrease in systolic blood pressure. After SGB, changes in quantitative thermal sensory testing (QST) suggestive of a partial deficit in thermal sensation correlated with pain relief. In 20 normal controls, water bath immersion to cool the hand passively by 7 degrees C and warm the hand passively by 4 degrees C had small and selective effects on thermal QST thresholds, but did not produce a general impairment in thermal sensation. In conclusion, the diagnosis of sympathetically maintained pain based on the history and physical examination alone cannot be made with confidence and therefore a sympatholytic procedure is necessary. When SGB produces pain relief but PhI does not, systemic absorption of local anaesthetic and/or sensory blockade by spread to somatic nerves may be the reason. Thus, PhI appears to be a less sensitive but more specific test than SGB. These two procedures provide complementary information and both may be needed to establish the diagnosis of sympathetically maintained pain.