The journal of pain : official journal of the American Pain Society
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Neuropathic pain is characterized by persistent, intractable pain following damage or dysfunction of the nervous system. Analgesics that include central, rather than purely peripheral, targets are more effective when treating neuropathic pain, highlighting the spinal and/or supraspinal mechanisms that contribute to this aberrant pain condition. The striatum represents one of the brain regions that have been implicated in pain processing. Release of dopamine in the ventral striatum is normally associated with analgesia. Clinical and human imaging studies suggest that dopamine is disrupted in neuropathic pain patients, although the conclusions drawn from these studies are limited by their noninvasive imaging or pharmacologic approaches. In this study, we used a C57Bl/6 mouse model of neuropathic pain to describe the changes in neurotransmitter content in the striatum and their relationship to evoked pain thresholds. Striatal dopamine content negatively correlated with mechanical thresholds in sham animals. Neuropathic pain animals had reduced dopamine content that was not correlated with mechanical thresholds. In contrast, norepinephrine content was significantly increased and correlated with mechanical thresholds in neuropathic, but not sham, animals. These results describe changes in striatal signaling in neuropathic pain animals and contribute to the literature defining the role of dopamine and norepinephrine in mediating sensory thresholds in healthy and neuropathic pain states. ⋯ Results show significant loss of ventral striatal dopamine in neuropathic pain conditions, and the relationship of ventral striatal catecholamines to pain thresholds is changed in neuropathic pain. These results complement human imaging studies and provide evidence that chronic pain alters the function of reward systems.
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Pain is among the most common symptoms of cancer. Because cancer can occur at any age, it is imperative that pain assessment tools are valid for use across the adult lifespan. The Short-Form McGill Pain Questionnaire-2 (SF-MPQ-2) is a valid and reliable tool for the assessment of the multidimensional qualities of pain in people with chronic nonmalignant pain, but its psychometric properties in people with cancer pain and in older versus younger people require investigation. This study evaluated age differences in the validity, reliability, and use of the SF-MPQ-2 in 244 people with advanced cancer and pain. We confirmed the previously reported 4-factor solution in older (≥ 60 years) and younger (<60 years) patients. Internal consistency reliability and convergent validity were similar across age groups, although the SF-MPQ-2 sensory subscales were correlated with mental health quality of life in older, but not younger, patients. Older and younger patients selected the same words with the same intensity to describe their pain. The most commonly selected words in both age groups were aching, tiring-exhausting, sharp, and dull. These results demonstrate that the SF-MPQ-2 is appropriate for use across the adult lifespan in people with cancer pain. ⋯ This study demonstrated that the SF-MPQ-2 is valid for use in older and younger people with advanced cancer and pain. This measure could improve cancer pain assessment across the adult lifespan, which may lead to improved pain management.
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Patients often fail to attend appointments in chronic pain clinics for unknown reasons. We hypothesized that certain patient characteristics predict failure to attend scheduled appointments, pointing to systematic barriers to accessing chronic pain services for certain underserved populations. We collected retrospective data from a longitudinal observational cohort of patients at an academic pain clinic in Newark, New Jersey. To examine the effect of demographic factors on appointment status, we fit a marginal logistic regression using generalized estimating equations with exchangeable correlation. A total of 1,394 patients with 3,488 total encounters between January 1, 2006, and December 31, 2009, were included. Spanish spoken as a primary language (alternatively Hispanic or other race) and living between 5 and 10 miles from the clinic were associated with reduced odds of arriving for an appointment; making an appointment for a particular complaint such as cancer pain or back pain, an interventional pain procedure scheduled in connection with the appointment, unemployed status, and continuity of care (as measured by office visit number) were associated with increased odds of arriving. Spanish spoken as a primary language and distance to the pain clinic predicted failure to attend a scheduled appointment in our cohort. If these constitute systematic barriers to access, they may be amenable to targeted interventions. ⋯ We identified certain patient characteristics, specifically Spanish spoken as a primary language and geographic distance from the clinic, that predict failure to attend an inner-city chronic pain clinic. These identified barriers to accessing chronic pain services may be modifiable by simple cost-effective interventions.
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Repeated exposure to noxious stimuli changes their painfulness, due to multiple adaptive processes in the peripheral and central nervous systems. Somewhat paradoxically, repeated stimulation can produce an increase (sensitization) or a decrease (habituation) in pain. Adaptation processes may also be body-site-specific or operate across body sites, and considering this distinction may help explain the conditions under which habituation versus sensitization occurs. To dissociate the effects of site-specific and site-nonspecific adaptation processes, we examined reported pain in 100 participants during counterbalanced sequences of noxious thermal stimulation on multiple skin sites. Analysis of pain ratings revealed 2 opposing sequential effects: repeated stimulations of the same skin site produced temperature-dependent habituation, whereas repeated stimulations across different sites produced sensitization. Stimulation trials were separated by ∼20 seconds, and sensitization was unrelated to the distance between successively stimulated sites, suggesting that neither temporal nor spatial summation occurred. To explain these effects, we propose a dynamic model with 2 adaptation processes, one site-specific and the other site-nonspecific. The model explains 93% of the variance in the group-mean pain ratings after controlling for current stimulation temperature, with its estimated parameters showing evidence for habituation for the site-specific process and sensitization for the site-nonspecific process. The 2 pain adaptation processes revealed in this study, and the ability to disentangle them, may hold keys to understanding multiple pain-regulatory mechanisms and their disturbance in chronic pain syndromes. ⋯ This article presents novel evidence for simultaneous site-specific habituation and site-nonspecific sensitization in thermal pain, which can be disentangled (and the direction and strength of each process estimated) by a dynamic model. The dissociation of site-specific and site-nonspecific adaptation processes may hold keys to understanding multiple pain-regulatory mechanisms in both healthy and patient populations.
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The purpose of this systematic review was to examine the outcome of complex regional pain syndrome (CRPS) type 1. We searched MEDLINE, Embase, and PsycINFO for relevant studies and included 18 studies, with 3,991 participants, in this review. The following data were extracted: study details, measurement tools used, and rates or severity scores for the symptoms/signs of CRPS at baseline and follow-up, or in groups of patients with different disease durations. A quality assessment revealed significant limitations in the literature, with many studies using different diagnostic criteria. The 3 prospective studies demonstrated that for many patients, symptoms improve markedly within 6 to 13 months of onset. The 12 retrospective studies had highly heterogeneous findings, documenting lasting impairments in many patients. The 3 cross-sectional studies showed that rates of pain and sensory symptoms were highest among those with the longest duration of CRPS. Additionally, most studies showed that motor symptoms (stiffness and weakness) were the most likely to persist whereas sudomotor and vasomotor symptoms were the most likely to improve. Overall, this suggests that some CRPS patients make a good early recovery whereas others develop lasting pain and disability. As yet little is known about the prognostic factors that might differentiate between these groups. ⋯ We found evidence that many CRPS patients recover within 6 to 13 months, but a significant number experience some lasting symptoms, and some experience chronic pain and disability. The quality of the evidence was poor. Future research should examine the factors associated with recovery and identify those at risk of poor outcomes.