Refractory Neuropathic Pain: The Nature and Extent of the Problem

Role of Mechanoinsensitive Nociceptors in Painful Diabetic Peripheral Neuropathy

The cutaneous mechanisms that trigger spontaneous neuropathic pain in diabetic peripheral neuropathy (PDPN) are far from clear. Two types of nociceptors are found within the epidermal and dermal skin layers. Small-diameter lightly myelinated Aδ and unmyelinated C cutaneous mechano and heat-sensitive (AMH and CMH) and C mechanoinsensitive (CMi) nociceptors transmit pain from the periphery to central nervous system. AMH and CMH fibers are mainly located in the epidermis, and CMi fibers are distributed in the dermis. In DPN, dying back intra-epidermal AMH and CMH fibers leads to reduced pain sensitivity, and the patients exhibit significantly increased pain thresholds to acute pain when tested using traditional methods. The role of CMi fibers in painful neuropathies has not been fully explored. Microneurography has been the only tool to access CMi fibers and differentiate AMH, CMH, and CMi fiber types. Due to the complexity, its use is impractical in clinical settings. In contrast, a newly developed diode laser fiber selective stimulation (DLss) technique allows to safely and selectively stimulate Aδ and C fibers in the superficial and deep skin layers. DLss data demonstrate that patients with painful DPN have increased Aδ fiber pain thresholds, while C-fiber thresholds are intact because, in these patients, CMi fibers are abnormally spontaneously active. It is also possible to determine the involvement of CMi fibers by measuring the area of DLss-induced neurogenic axon reflex flare. The differences in AMH, CMH, and CMi fibers identify patients with painful and painless neuropathy. In this review, we will discuss the role of CMi fibers in PDPN.

Pain-relieving effects of Lawsonia inermis on neuropathic pain induced by chronic constriction injury

The aim of this study was to determine the role of Lawsonia inermis (L. inermis) extract in the chronic constriction injury (CCI)-induced neuropathic pain. Following CCI surgery, L. inermis extract (250 mg/kg and 500 mg/kg) and gabapentin (100 mg/kg) were administered intraperitoneally for 14 consecutive days. Heat hyperalgesia and allodynia were assessed by radiant heat, aceton drop, and von frey filament tests, respectively. Rat pain behaviors were evaluated on -1sh, 3rd, 5th, 7th, 10th and 14th days post CCI surgery. At the end of the study, the spinal levels of malondialdehyde (MDA), total thiol, IL1-β, and TNF-α were estimated. Treatment of L. inermis extract reversed the decreased level of thiol and the elevation of MDA level in the spinal cord of CCI rats. Besides, L. inermis extract treatment decreased the elevation of inflammatory markers including IL1-β, and TNF-α in the spinal cord of CCI rats. These results indicated that L. inermis has potential neuroprotective effects against CCI induced neuropathic pain due to its anti-oxidant, and anti-inflammatory effects.

The Prevalence and Characteristics of Pain in Critically Ill Cancer Patients: A Prospective Nonrandomized Observational Study

Context:

Pain is a distressing symptom common to all stages and ubiquitous at all levels of care in cancer patients. However, there is a lack of scientific literature on prevalence, severity, predictors, and the quality of pain in cancer patients admitted to an Intensive Care Unit (ICU).

Objectives:

To elucidate the prevalence of pain, moderate to severe pain, neuropathic pain, chronic pain, and pain as the most distressing symptom in critically ill-cancer patients at the time of ICU admission.

Methods:

We prospectively interviewed 126 patients within first 24 h of admission to a medical ICU. The patients were assessed for the presence of pain, its severity, sites, duration, nature, and its impact as a distressing symptom. Numerical Rating Scale and self-report version of Leeds Assessment of Neuropathic Signs and Symptoms were used to elucidate intensity of pain and neuropathic pain, respectively. Demographic characteristics such as age and sex, primary site, and stage of cancer were considered for a possible correlation with the prevalence of pain.

Results:

Of 126 patients included in the study 95 (75.40%), 79 (62.70%), 34 (26.98%), and 17 (13.49%) patients had pain, moderate-severe, chronic, and neuropathic pain, respectively. The average duration of pain was 171.16 ± 716.50 days. Totally, 58 (46.03%) and 42 (42.01%) patients had at least one and more than equal to 2 neuropathic pain symptoms, respectively. The primary malignancies associated with the highest prevalence of pain were genitourinary, hematological, and head and neck whereas breast and lung cancers were associated with the highest prevalence of neuropathic and chronic pain, respectively.

Conclusion:

The prevalence of pain among critically ill-cancer patients is high. Assessment for pain at the time of ICU admission would ensure appropriate assessment for the presence, type, severity, and the significance imparted to it.

Effect of 1.5% Topical Diclofenac on Clinical Neuropathic Pain

Background:

Neuropathic pain is a condition resulting from injury to the peripheral and/or central nervous system. Despite extensive research over the last several decades, neuropathic pain remains difficult to manage.

Methods:

The authors conducted a randomized, placebo-controlled, double-blinded, and crossover clinical trial to examine the effect of 1.5% topical diclofenac (TD) on neuropathic pain. The authors hypothesized that 1.5% TD would reduce the visual pain score and improve both quantitative sensory testing and functional status in subjects with neuropathic pain. The authors recruited subjects with postherpetic neuralgia and complex regional pain syndrome. The primary outcome was subject’s visual pain score.

Results:

Twenty-eight subjects completed the study (12 male and 16 female) with the mean age of 48.8 yr. After 2 weeks of topical application, subjects in 1.5% TD group showed lower overall visual pain score compared with placebo group (4.9 [1.9] vs. 5.6 [2.1], difference: 0.8; 95% CI, 0.1 to 1.3; P = 0.04) as well as decreased burning pain (2.9 [2.6] vs. 4.3 [2.8], difference, 1.4; 95% CI, 0.2 to 2.6; P = 0.01). There were no statistical differences in constant pain, shooting pain, or hypersensitivity over the painful area between the groups. This self-reported improvement of pain was corroborated by the decreased pain summation detected by quantitative sensory testing. There were no statistically significant changes in functional status in these subjects. There were no complications in both groups.

Conclusion:

The findings indicate that 1.5% TD may serve as an effective treatment option for patients with neuropathic pain from postherpetic neuralgia and complex regional pain syndrome.

Far lateral lumbar disc extrusion: MRI findings and surgical treatment

This case report describes the magnetic resonance imaging (MRI) findings and the treatment of a far lateral extrusion of disc material at the sixth and seventh lumbar vertebrae (L6-L7) in a five-year-old male Alpine Dachsbracke dog referred to our hospital for investigation of the complaint of a one week progressive lameness in the left pelvic limb and poorly localized back pain. An extra-foraminal left lateral disc herniation impinging on the sixth lumbar nerve root was diagnosed by MRI examinations. Due to the far lateral position of the extruded disc material on MRI, surgical opening of the spinal canal was not necessary. Removal of the herniated soft disc material impinging on the L6 nerve root, and fenestration of the L6-L7 disc was performed laterally. To the author's knowledge 'far-lateral' disc herniation beyond the neuroforamen without any spinal canal contact has not been described in dogs until now. A complete recovery with no evidence of pain was achieved only after a couple of weeks after surgery. We acknowledge that it is possible that other pathological mechanisms may have contributed to clinical signs and to a delayed recovery.

Neuropathic pain in the community: more under-treated than refractory?

Best current estimates of neuropathic pain prevalence come from studies using screening tools detecting pain with probable neuropathic features; the proportion experiencing significant, long-term neuropathic pain, and the proportion not responding to standard treatment are unknown. These “refractory” cases are the most clinically important to detect, being the most severe, requiring specialist treatment. The aim of this study was to estimate the proportion of neuropathic pain in the population that is “refractory,” and to quantify associated clinical and demographic features. We posted self-administered questionnaires to 10,000 adult patients randomly selected from 10 general practitioner practices in 5 UK locations. The questionnaire contained chronic pain identification and severity questions, cause of pain, SF-12, EQ-5D, S-LANSS (Self-administered Leeds Assessment of Neuropathic Signs and Symptoms), PSEQ (Pain Self-Efficacy Questionnaire), use of neuropathic pain medications, and health care utilisation. These data were combined to determine the presence and characteristics of “refractory” neuropathic pain according to the defining features identified by a Delphi survey of international experts. Graded categories of chronic pain with and without neuropathic characteristics were generated, incorporating the refractory criteria. Completed questionnaires were returned by 4451 individuals (response rate 47%); 399 had “chronic pain with neuropathic characteristics” (S-LANSS positive, 8.9% of the study sample); 215 (53.9%) also reported a positive relevant history (“Possible neuropathic pain”); and 98 (4.5% of all Chronic Pain) also reported an “adequate” trial of at least one neuropathic pain drug (“Treated possible neuropathic pain”). The most refractory cases were associated with dramatically poorer physical and mental health, lower pain self-efficacy, higher pain intensity and pain-related disability, and greater health care service use.

Towards a definition of refractory neuropathic pain for epidemiological research. An international Delphi survey of experts

Background

Best current estimates of neuropathic pain (NeuP) prevalence come from studies using various screening detecting pain with probable neuropathic features; the proportion experiencing significant, long-term NeuP, and the proportion not responding to standard treatment are unknown. These “refractory” cases are the most clinically important to detect, being the most severe, requiring specialist treatment.

Methods

We report an international Delphi survey of experts in NeuP, aiming for consensus on the features required to define, for epidemiological research: (1) neuropathic pain; and (2) when NeuP is “refractory”. A web-based questionnaire was developed and data collected from three rounds of questionnaires from nineteen experts.

Results

There was good consensus on essential inclusion of six items to identify NeuP (“prickling, tingling, pins & needles”, “pain evoked by light touch”, “electric shocks or shooting pain”, “hot or burning” pain, “brush allodynia on self-examination”, and “relevant history”) and on some items that were non-essential. Consensus was also reached on components of a “refractory NeuP” definition: minimum duration (one year); number of trials of drugs of known effectiveness (four); adequate duration of these trials (three months / maximum tolerated); outcomes of treatment (pain severity, quality of life). Further work needs to validate these proposed criteria in general population research.

Conclusions

This paper presents an international consensus on measuring the epidemiology of refractory neuropathic pain. This will be valuable in reaching an agreed estimate of the prevalence of neuropathic pain, and the first estimate of refractory neuropathic pain prevalence.

Selective thoracic ganglionectomy for the treatment of segmental neuropathic pain

Segmental thoracic neuropathic pain (NeuP) remains particularly difficult to treat. Sensory ganglionectomy was reported to alleviate NeuP. The experience with thoracic ganglionectomy, however, is very limited. Here, we report the results of a prospective pilot study in patients with incapacitating segmental thoracic NeuP treated by selective ganglionectomy. Seven patients were included suffering from refractory NeuP scoring 8 or more on a visual analogue scale (VAS). Every patient had test anaesthesia prior to surgery yielding more than 50% pain relief. The spinal ganglion was excised completely via an extraforaminal approach. Mean preoperative VAS scores were 9.1 (maximum pain); 5.4 (minimum pain); 7.9 (pain on average); 6.9 (pain at the time of presentation); and 7.4 (allodynia). Early post-operatively, there was a marked improvement of mean scores: 1.7; 0.7; 1.2; 1.0; and 0.7, respectively. One patient developed a mild transient hemihypaesthesia. In three patients, substantial pain occurred in a formerly unaffected dermatome within 1 year. Two of these patients had significant pain relief by a second operation. At the time of last follow-up at a mean of 24 months after the first procedure, mean VAS scores were 6.3; 2.1; 4.3; 4.0; and 1.3. Overall, medication was reduced. The patients rated their outcome as excellent (1), good (2), fair (2) and nil (2) with best improvement for allodynia. Selective thoracic ganglionectomy is a safe and partially effective procedure in selected patients albeit there may be partial recurrence of pain. Recurrent pain may affect dermatomes that were not involved initially.

Pharmacological treatment of neuropathic cancer pain: a comprehensive review of the current literature

Neuropathic cancer pain (NCP), commonly encountered in clinical practice, may be cancer-related, namely resulting from nervous system tumor invasion, surgical nerve damage during tumor removal, radiation-induced nerve damage and chemotherapy-related neuropathy, or may be of benign origin, unrelated to cancer. A neuropathic component is evident in about 1/3 of cancer pain cases. Although from a pathophysiological perspective NCP may differ from chronic neuropathic pain (NP), such as noncancer-related pain, clinical practice, and limited publications have shown that these two pain entities may share some treatment modalities. For example, co-analgesics have been well integrated into cancer pain-management strategies and are often used as First-Line options for the treatment of NCP. These drugs, including antidepressants and anticonvulsants, are recommended by evidence-based guidelines, whereas, others such as lidocaine patch 5%, are supported by randomized, controlled, clinical data and are included in guidelines for restricted conditions treatment. The vast majority of these drugs have already been proven useful in the management of benign NP syndromes. Treatment decisions for patients with NP can be difficult. The intrinsic difficulties in performing randomized controlled trials in cancer pain have traditionally justified the acceptance of drugs already known to be effective in benign NP for the management of malignant NP, despite the lack of relevant high quality data. Interest in NCP mechanisms and pharmacotherapy has increased, resulting in significant mechanism-based treatment advances for the future. In this comprehensive review, we present the latest knowledge regarding NCP pharmacological management.

Potassium channels as a potential therapeutic target for trigeminal neuropathic and inflammatory pain

Previous studies in several different trigeminal nerve injury/inflammation models indicated that the hyperexcitability of primary afferent neurons contributes to the pain pathway underlying mechanical allodynia. Although multiple types of voltage-gated ion channels are associated with neuronal hyperexcitability, voltage-gated K⁺ channels (Kv) are one of the important physiological regulators of membrane potentials in excitable tissues, including nociceptive sensory neurons. Since the opening of K⁺ channels leads to hyperpolarization of cell membrane and a consequent decrease in cell excitability, several Kv channels have been proposed as potential target candidates for pain therapy. In this review, we focus on common changes measured in the Kv channels of several different trigeminal neuropathic/inflammatory pain animal models, particularly the relationship between changes in Kv channels and the excitability of trigeminal ganglion (TRG) neurons. We also discuss the potential of Kv channel openers as therapeutic agents for trigeminal neuropathic/inflammatory pain, such as mechanical allodynia.

Differential diagnosis and physical therapy management of a patient with radial wrist pain of 6 months' duration: a case

STUDY DESIGN

Case report.

BACKGROUND

Differential diagnosis for patients with radial wrist pain requires consideration of systemic disease, referred pain to the radial aspect of the wrist, and local dysfunction. The list of possible local dysfunctions should include De Quervain syndrome, as well as entrapment neuropathy of the superficial radial nerve.

CASE DESCRIPTION

The patient was a 57-year-old man with right radial wrist pain of 6 months' duration. The referral diagnosis was De Quervain syndrome, but a previous course of electrophysical agents-based physical therapy management had been unsuccessful. The physical examination ruled out the cervical, shoulder, elbow, and wrist joints as possible sources of pain. In this case, the diagnosis of entrapment neuropathy of the superficial radial nerve, rather than De Quervain syndrome, was primarily based on the symptom provocation resulting from a modified radial bias upper limb nerve tension test. Based on this diagnosis, treatment consisted of active and passive exercises using neurodynamic techniques.

OUTCOMES

After 1 treatment session, the patient noted changes with regard to current pain intensity and function that exceeded the minimal clinically important difference and the minimal detectable change, respectively. After only 2 treatment sessions, the patient reported a complete resolution of symptoms and a full return to work.

DISCUSSION

This case report critically evaluates the diagnostic process for patients with radial wrist pain and suggests neuropathy of the superficial sensory branch of the radial nerve as a differential diagnostic option.

LEVEL OF EVIDENCE

Therapy, level 4.J Orthop Sports Phys Ther 2010;40(6):361-368, Epub 22 April 2010. doi:10.2519/jospt.2010.3210.

Spinal cord injuries containing asymmetrical damage in the ventrolateral funiculus is associated with a higher incidence of at-level allodynia

Approximately 70% of male rats receiving severe T8 spinal contusions develop allodynia in T5-7 dermatomes (at-level) beginning 2 weeks after injury. In contrast, rats having either complete transections or dorsal hemisections do not develop allodynia at-level after chronic spinal cord injury (SCI). In the present study, incomplete laceration and contusion injuries were made to test for neuroanatomical correlates between areas of white matter damage/sparing at the lesion epicenter and the presence/absence of allodynia. After incomplete laceration lesions and 6 weeks of behavioral testing, histological reconstruction and analysis of the lesion epicenters revealed a significant difference (P < .001) in the amount of ventrolateral funiculus (VLF) asymmetry between rats showing pain-like responses evoked by touch (74.5% +/- 8.4% side-to-side difference in VLF damage) versus those not responding to touch (11.3% +/- 4.4% side-to-side difference in VLF damage). A 5-week mean allodynia score for each rat that incorporates a full range of forces that are all innocuous in intact controls revealed that the degree of hypersensitivity at level is related to the extent of VLF asymmetry after SCI. No other damaged spinal white matter or gray matter area was correlated with sensitivity to touch. Similar findings were obtained for rats receiving T8 contusions, a more clinically relevant injury. These data suggest that different extents of damage/sparing between the 2 sides of VLF probably are a requisite for the development of allodynia after SCI.

PERSPECTIVE

A side-to-side lesion asymmetry after chronic SCI in a rodent model was found to be highly correlated with the presence and degree of allodynia. Greater insight of key factors contributing to the development and maintenance of chronic neuropathic pain is important for improving quality of life.

Alteration of primary afferent activity following inferior alveolar nerve transection in rats

Background

In order to evaluate the neural mechanisms underlying the abnormal facial pain that may develop following regeneration of the injured inferior alveolar nerve (IAN), the properties of the IAN innervated in the mental region were analyzed.

Results

Fluorogold (FG) injection into the mental region 14 days after IAN transection showed massive labeling of trigeminal ganglion (TG). The escape threshold to mechanical stimulation of the mental skin was significantly lower (i.e. mechanical allodynia) at 11-14 days after IAN transection than before surgery. The background activity, mechanically evoked responses and afterdischarges of IAN Aδ-fibers were significantly higher in IAN-transected rats than naive. The small/medium diameter TG neurons showed an increase in both tetrodotoxin (TTX)-resistant (TTX-R) and -sensitive (TTX-S) sodium currents (I_Na) and decrease in total potassium current, transient current (I_A) and sustained current (I_K) in IAN-transected rats. The amplitude, overshoot amplitude and number of action potentials evoked by the depolarizing pulses after 1 μM TTX administration in TG neurons were significantly higher, whereas the threshold current to elicit spikes was smaller in IAN-transected rats than naive. Resting membrane potential was significantly smaller in IAN-transected rats than that of naive.

Conclusions

These data suggest that the increase in both TTX-S I_Na and TTX-R I_Na and the decrease in I_A and I_k in small/medium TG neurons in IAN-transected rats are involved in the activation of spike generation, resulting in hyperexcitability of Aδ-IAN fibers innervating the mental region after IAN transection.

Intrathecal ziconotide for neuropathic pain: a review

Neuropathic pain is a considerable burden that affects activities of daily living. The management of neuropathic pain can be challenging because of multiple etiologies and complex manifestations. Ziconotide is a nonopioid intrathecal (IT) analgesic option for patients with neuropathic pain refractory to conventional treatments. The objective of this article is to review the published literature on ziconotide for the treatment of neuropathic pain. Relevant publications were identified through searches of all years of 6 databases, which included PubMed, EMBASE, and CINAHL. Search terms used were ziconotide, SNX-111, MVIIA, Prialt, and neuropathic pain. Publications were included if ziconotide was intrathecally administered (either alone or in combination with other IT agents) to treat neuropathic pain of any etiology and if pain assessment was an outcome measure. Data extracted included study design, IT drug doses, pain outcome measures, and adverse events (AEs). Twenty-eight articles met the inclusion criteria: 5 were preclinical studies and 23 were clinical studies. In the preclinical studies, ziconotide demonstrated antiallodynic effects on neuropathic pain. Data from double-blind, placebo-controlled (DBPC) trials indicated that patients with neuropathic pain reported a mean percent improvement in pain score with ziconotide monotherapy that ranged from 15.7% to 31.6%. A low starting dose and slow titration of ziconotide resulted in an improved safety profile in the aforementioned trials. Common AEs associated with ziconotide include nausea and/or vomiting, dizziness, confusion, urinary retention, and somnolence. Evidence from DBPC trials, open-label studies, case series, and case studies suggests that ziconotide, as either monotherapy or in combination with other IT drugs, is a potential therapeutic option for patients with refractory neuropathic pain. Additional studies are needed to establish the long-term efficacy and safety of ziconotide for neuropathic pain.

Neuropathic pain in dogs and cats: if only they could tell us if they hurt

Neuropathic pain is difficult to diagnose in veterinary patients because they are unable to verbalize their pain. By assuming that neuropathic pain may exist based on the history of events that each patient has experienced, a focused client history and neurologic examination may identify a lesion resulting in persistent or spontaneous pain. Once neuropathic pain is diagnosed, a trial analgesic or acupuncture session(s) should be prescribed with instructions for owners to observe behavior. Dosing of the analgesic can be titrated to the patient's needs while avoiding adverse effects. When a particular analgesic may be ineffectual, an alternate class should be tried. As research into the neurobiologic mechanisms of neuropathic pain continues, specific therapies for its management should eventually appear in the human clinical setting and subsequently be investigated for veterinary clinical use.

Design and microwave-assisted synthesis of 5-trifluoromethyl-4,5-dihydro-1H-pyrazoles: Novel agents with analgesic and anti-inflammatory properties

In this work, we reported the synthesis and evaluation of the analgesic and anti-inflammatory properties of novel 3- or 4-substituted 5-trifluoromethyl-5-hydroxy-4,5-dihydro-1H-1-carboxyamidepyrazoles (where 3-/4-substituent=H/H, Me/H, Et/H, Pr/H, i-Pr/H, Bu/H, t-Bu/H, Ph/H, 4-Br-Ph/H and H/Me) designed in the exploration of the bioisosteric replacement of benzene present in salicylamide with a 5-trifluoromethyl-4,5-dihydro-1H-pyrazole scaffold. Target compounds were synthesized from the cyclocondensation of 4-alkoxy-1,1,1-trifluoromethyl-3-alken-2-ones with semicarbazide hydrochloride through a rapid one-pot reaction via microwave irradiation. In addition to spectroscopic data, the structure of the compounds was supported by X-ray diffraction. Subcutaneous administration of the 5-trifluoromethyl-4,5-dihydro-1H-pyrazoles decreased pain-related behavior during neurogenic and inflammatory phases of the formalin test in mice. Moreover, the more active analgesic compounds (3-/4-=Et/H and H/Me) significantly decreased carrageenan-induced paw edema in mice. The data obtained in this work suggest that the synthesized compounds could be promising candidates for the future development of novel analgesic and anti-inflammatory agents.

pain2: A neuropathic pain QTL identified on rat chromosome 2

We aimed to locate a chronic pain-associated QTL in the rat (Rattus norvegicus) based on previous findings of a QTL (pain1) on chromosome 15 of the mouse (Mus musculus). The work was based on rat selection lines HA (high autotomy) and LA (low autotomy) which show a contrasting pain phenotype in response to nerve injury in the neuroma model of neuropathic pain. An F(2) segregating population was generated from HA and LA animals. Phenotyped F(2) rats were genotyped on chromosome 7 and chromosome 2, regions that share a partial homology with mouse chromosome 15. Our interval mapping analysis revealed a LOD score value of 3.63 (corresponding to p=0.005 after correcting for multiple testing using permutations) on rat chromosome 2, which is suggestive of the presence of a QTL affecting the predisposition to neuropathic pain. This QTL was mapped to the 14-26cM interval of chromosome 2. Interestingly, this region is syntenic to mouse chromosome 13, rather than to the region of mouse chromosome 15 that contains pain1. This chromosomal position indicates that it is possibly a new QTL, and hence we name it pain2. Further work is needed to replicate and to uncover the underlying gene(s) in both species.

Evaluation of the enantioselective antiallodynic and pharmacokinetic profile of propylisopropylacetamide, a chiral isomer of valproic acid amide

Propylisopropylacetamide (PID) is a chiral CNS-active constitutional isomer of valpromide, the amide derivative of the major antiepileptic drug valproic acid (VPA). The purpose of this work was: a) To evaluate enantiospecific activity of PID on tactile allodynia in the Chung (spinal nerve ligation, SNL) model of neuropathic pain in rats; b) To evaluate possible sedation at effective antiallodynic doses, using the rotorod ataxia test; c) To investigate enantioselectivity in the pharmacokinetics of (R)- and (S)-PID in comparison to (R,S)-PID; and d) To determine electrophysiologically whether PID has the potential to affect tactile allodynia by suppressing ectopic afferent discharge in the peripheral nervous system (PNS). (R)-, (S)- and (R,S)-PID produced dose-related reversal of tactile allodynia with ED(50) values of 46, 48, 42 mg/kg, respectively. The individual PID enantiomers were not enantioselective in their antiallodynic activity. No sedative side-effects were observed at these doses. Following i.p. administration of the individual enantiomers, (S)-PID had lower clearance (CL) and volume of distribution (V) and a shorter half-life (t(1/2)) than (R)-PID. However following administration of (R,S)-PID, both enantiomers had similar CL and V, but (R)-PID had a longer t(1/2). Systemic administration of (R,S)-PID at antiallodynic doses did not suppress spontaneous ectopic afferent discharge generated in the injured peripheral nerve, suggesting that its antiallodynic action is exerted in the CNS rather than the PNS. Both of PID's enantiomers, and the racemate, are more potent antiallodynic agents than VPA and have similar potency to gabapentin. Consequently, they have the potential to become new drugs for treating neuropathic pain.

Sex-specific variability and a ‘cage effect’ independently mask a neuropathic pain quantitative trait locus detected in a whole genome scan

Sex and environment may dramatically affect genetic studies, and thus should be carefully considered. Beginning with two inbred mouse strains with contrasting phenotype in the neuroma model of neuropathic pain (autotomy), we established a backcross population on which we conducted a genome-wide scan. The backcross population was partially maintained in small social groups and partially in isolation. The genome scan detected one previously reported quantitative trait locus (QTL) on chromosome 15 (pain1), but no additional QTLs were found. Interestingly, group caging introduced phenotypic noise large enough to completely mask the genetic effect of the chromosome 15 QTL. The reason appears to be that group-caging animals from the low-autotomy strain together with animals from the high-autotomy strain dramatically increases autotomy in the otherwise low-autotomy mice (males or females). The converse, suppression of pain behaviour in the high-autotomy strain when caged with the low-autotomy strain was also observed, but only in females. Even in isolated mice, the genetic effect of the chromosome 15 QTL was significant only in females. To determine why, we evaluated autotomy levels of females in 12 different inbred stains of mice and compared them to previously reported levels for males. Strikingly larger environmental variation was observed in males than in females for this pain phenotype. The high baseline variance in males can explain the difficulty in detecting the genetic effect, which was readily seen in females. Our study emphasizes the importance of sex and environment in the genetic analysis of pain.

Postherpetic Neuralgia in Older Adults

The purpose of this article is to describe current knowledge and standards of care for postherpetic neuralgia (PHN) among older persons. Three influencing factors are considered: cultural implications, quality of life (QOL), and current practice of alternative/complementary therapy. A review of literature published between 2001 and 2006 was conducted. The findings indicate that PHN has debilitating effects on older adults regardless of culture. The impact of PHN on culture and ethnicity, particularly on the relationship between culture and patient's self-report of herpes zoster and/or PHN, has not been well investigated as evidenced in the literature. PHN is found to be associated with decreased health-related QOL among the elderly, with the most affected domains being sleep, mood, and general activity. Alternative and complementary therapy offers many advantages such as ease of use, availability, and low cost. However, due to lack of controlled trials and insufficient evidence, alternative therapy is not currently used widely and recommended. As the US population ages, the incidence of herpes zoster and PHN is expected to rise. Clinical trials that explore the response of the culturally diverse older adults to current treatment guidelines, strategies for prevention of PHN and its corresponding decrease in QOL, as well as controlled trials of alternative/complementary remedies should be considered.