Central pain in a hemispherectomized patient

Pain measurement and brain activity: will neuroimages replace pain ratings?

Arguments made for the advantages of replacing pain ratings with brain-imaging data include assumptions that pain ratings are less reliable and objective and that brain image data would greatly benefit the measurement of treatment efficacy. None of these assumptions are supported by available evidence. Self-report of pain is predictable and does not necessarily reflect unreliability or error. Because pain is defined as an experience, magnitudes of its dimensions can be estimated by well-established methods, including those used to validate brain imaging of pain. Brain imaging helps to study pain mechanisms and might be used as proxy measures of pain in persons unable to provide verbal reports. Yet eliminating pain ratings or replacing them with neuroimaging data is misguided because brain images only help explain pain if they are used in conjunction with self-report. There is no objective readout mechanism of pain (pain thermometer) that is unaffected by psychological factors. Benefits from including neuroimaging data might include increased understanding of underlying neural mechanisms of treatment efficacy, discovery of new treatment vectors, and support of conclusions derived from self-report. However, neither brain imaging nor self-report data are privileged over the other. The assumption that treatment efficacy is hampered by self-report has not been shown; there is a plethora of treatment studies showing that self-report is sensitive to treatment. Dismissal of patients' self-reports (pain ratings) by brain-imaging data is potentially harmful. The aim of replacing self-report with brain-imaging data is misguided and has no scientific or philosophical foundation.

PERSPECTIVE

Although brain imaging may offer considerable insight into the neural mechanisms of pain, including relevant causes and correlations, brain images cannot and should not replace self-report. Only the latter assesses the experience of pain, which is not identical to neural activity. Brain imaging may help to explain pain, but replacing self-report with brain-imaging data would be philosophically and scientifically misguided and potentially harmful to pain patients.

Adaptive neuroplastic responses in early and late hemispherectomized monkeys

Behavioural recovery in children who undergo medically required hemispherectomy showcase the remarkable ability of the cerebral cortex to adapt and reorganize following insult early in life. Case study data suggest that lesions sustained early in childhood lead to better recovery compared to those that occur later in life. In these children, it is possible that neural reorganization had begun prior to surgery but was masked by the dysfunctional hemisphere. The degree of neural reorganization has been difficult to study systematically in human infants. Here we present a 20-year culmination of data on our nonhuman primate model (Chlorocebus sabeus) of early-life hemispherectomy in which behavioral recovery is interpreted in light of plastic processes that lead to the anatomical reorganization of the early-damaged brain. The model presented here suggests that significant functional recovery occurs after the removal of one hemisphere in monkeys with no preexisting neurological dysfunctions. Human and primate studies suggest a critical role for subcortical and brainstem structures as well as corticospinal tracts in the neuroanatomical reorganization which result in the remarkable behavioral recovery following hemispherectomy. The non-human primate model presented here offers a unique opportunity for studying the behavioral and functional neuroanatomical reorganization that underlies developmental plasticity.

Illusion of pain: pre-existing knowledge determines brain activation of 'imagined allodynia'

Allodynia means that innocuous tactile stimulation is felt as pain. Accordingly, cerebral activations during allodynia or touch should markedly differ. The aim of this study was to investigate whether the imagination of allodynia affects brain processing of touch in healthy subjects. Seventeen healthy subjects divided into 2 subgroups were investigated: The first group (n = 7) was familiar with allodynia, based on previous pain studies, whereas the second group (n = 10) had never knowingly experienced allodynia. Using functional magnetic resonance imaging, 2 experimental conditions were investigated. In one condition the subjects were simply touched at their left hand, whereas during the other condition they were asked to imagine pain (allodynia) during tactile stimulation of the right hand and to estimate the imagined pain on a numeric rating scale. Data processing and analysis were performed with the use of SPM5. The group analysis of all subjects revealed that tactile stimulation activated contralateral somatosensory cortices (S1 [primary] and S2 [secondary]), but the imagination of allodynia led to an additional activation of anterior cingulate cortex and bilateral activation of S2, insular cortex, and prefrontal cortices. Subgroup analysis using rating-weighted predictors revealed activation of the contralateral thalamus, anterior cingulate cortex, and amygdala and a bilateral activation of S1, S2, and insular cortex and prefrontal cortices in allodynia-experienced subjects. In contrast, allodynia-inexperienced subjects only activated contralateral S1 and bilateral S2. Just the imagination that touch is painful is able to partly activate the central pain system, but only when the subject has previous experience of this. According to our results, the medial pain system is involved in the encoding of imagined allodynia.

PERSPECTIVE

This article reports that pain experience is able to alter central processing of sensory stimuli. Pain knowledge appears to be able to shift "normal" tactile processing to a different quality, resulting in modified brain activity. Therefore, our study may contribute to the current understanding of human pain and will promote future research on this field.

Brain imaging of neuropathic pain

Many studies have focused on defining the network of brain structures involved in normal physiological pain. The different dimensions of pain perception (i.e., sensory discriminative, affective/emotional, cognitive/evaluative) have been shown to depend on different areas of the brain. In contrast, much less is known about the neural basis of pathological chronic pain. In particular, it is unclear whether such pain results from changes to the physiological "pain matrix". We review here studies on changes in brain activity associated with neuropathic pain syndromes-a specific category of chronic pain associated with peripheral or central neurological lesions. Patients may report combinations of spontaneous pain and allodynia/hyperalgesia-abnormal pain evoked by stimuli that normally induce no/little sensation of pain. Modern neuroimaging methods (positron emission tomography (PET) and functional MRI (fMRI)) have been used to determine whether different neuropathic pain symptoms involve similar brain structures and whether these structures are related to the physiological "pain matrix". PET studies have suggested that spontaneous neuropathic pain is associated principally with changes in thalamic activity and the medial pain system, which is preferentially involved in the emotional dimension of pain. Both PET and fMRI have been used to investigate the basis of allodynia. The results obtained have been very variable, probably reflecting the heterogeneity of patients in terms of etiology, lesion topography, symptoms and stimulation procedures. Overall, these studies indicated that acute physiological pain and neuropathic pain have distinct although overlapping brain activation pattern, but that there is no unique "pain matrix" or "allodynia network".

Thermal and Tactile Sensory Deficits and Allodynia in a Nerve-Injured Patient: A Multimodal Psychophysical and Functional Magnetic Resonance Imaging Study

OBJECTIVE

A case study was conducted to examine a patient with chronic neuropathic pain of the right foot following peripheral nerve injury and characterize associated sensory abnormalities.

METHODS

Multimodal psychophysical examination of the patient's affected and nonaffected foot included thermal sensibility, dynamic touch, and directional sensibility. In addition, we used functional magnetic resonance imaging to study cortical representation of brush-evoked allodynia.

RESULTS

Detailed psychophysical examination revealed substantial deficits in warm, cool, and tactile perception on the injured foot. These findings indicated severe dysfunction of perceptual processes mediated by A beta, A delta, and C fibers. Despite reduced tactile perception, light touch evoked a deep burning pain in the foot. Functional magnetic resonance imaging during brushing of the patient's injured foot showed that tactile allodynia led to activation of several cortical regions including secondary somatosensory cortex, anterior and posterior insular cortex, and anterior cingulate cortex. Brushing of the patient's nonaffected foot led to fewer activated regions.

DISCUSSION

The profound sensory disturbances suggest a possible deafferentation type of tactile allodynia mediated by changes within the central nervous system, such as a disruption of normal tactile or thermal inhibition of nociception. The functional magnetic resonance imaging data suggest that tactile allodynia is represented in similar brain regions as experimental pain.

Sumatriptan (5-HT1B/1D-agonist) causes a transient allodynia

Unpleasant sensory symptoms are commonly reported in association with the use of 5-HT1B/1D-agonists, i.e. triptans. In particular, pain/pressure symptoms from the chest and neck have restricted the use of triptans in the acute treatment of migraine. The cause of these triptan induced side-effects is still unidentified. We have now tested the hypothesis that sumatriptan influences the perception of tactile and thermal stimuli in humans in a randomized, double-blind, placebo-controlled cross-over study. Two groups were tested; one consisted of 12 (mean age 41.2 years, 10 women) subjects with migraine and a history of cutaneous allodynia in association with sumatriptan treatment. Twelve healthy subjects (mean age 38.7 years, 10 women) without migraine served as control group. During pain- and medication-free intervals tactile directional sensibility, perception of dynamic touch (brush) and thermal sensory and pain thresholds were studied on the dorsal side of the left hand. Measurements were performed before, 20, and 40 min after injection of 6 mg sumatriptan or saline. Twenty minutes after injection, sumatriptan caused a significant placebo-subtracted increase in brush-evoked feeling of unpleasantness in both groups (P < 0.01), an increase in brush-evoked pain in migraineurs only (P = 0.021), a reduction of heat pain threshold in all participants pooled (P = 0.031), and a reduction of cold pain threshold in controls only (P = 0.013). At 40 min after injection, no differences remained significant. There were no changes in ratings of brush intensity, tactile directional sensibility or cold or warm sensation thresholds. Thus, sumatriptan may cause a short-lasting allodynia in response to light dynamic touch and a reduction of heat and cold pain thresholds. This could explain at least some of the temporary sensory side-effects of triptans and warrants consideration in the interpretation of studies on migraine-induced allodynia.

Regional cerebral blood flow correlates of the severity of writer's cramp symptoms

Writer's cramp is a type of idiopathic focal dystonia with incompletely understood pathophysiology. Recent studies provide evidence that one element might be a sensory processing defect. We performed a PET study with O(15) H(2)O to find out in which brain areas activity correlates with the severity of writer's cramp symptoms.

METHODS

We studied 10 patients with writer's cramp and 10 age- and gender-matched control subjects. There were seven conditions, each repeated twice: rest, writing, tapping with index finger for 2, 3, 4, and 5 min. For each scan, we obtained EMG recordings from the flexor digitorum superficialis (FDS), extensor indicis proprius (EIP) muscles, and a subjective score of severity of dystonia. Scans were realigned, normalized, smoothed, and analyzed using SPM99. Analysis included both intra- and intergroup comparisons and a correlation analysis where we used EMG recordings and subjective dystonia score as covariates.

RESULTS

Random effect analysis of the writing task showed overactivity of the primary sensory cortex and no significant underactivity. Correlation analysis of dystonia patients showed activation of SI when we used the subjective dystonia score as a covariate, and activation of both the SI and primary motor cortex when the normalized EMG score of FDS was used.

CONCLUSION

While some overactivity of MI is not surprising, overactivity of SI is more dramatic and suggests a primary deficit in processing sensory feedback. Writer's cramp may arise in part as a dysfunction of sensory circuits, which causes defective sensorimotor integration resulting in co-contractions of muscles and overflow phenomena.

Pain mechanisms: labeled lines versus convergence in central processing

The issue of whether pain is represented by specific neural elements or by patterned activity within a convergent somatosensory subsystem has been debated for over a century. The gate control theory introduced in 1965 denied central specificity, and since then most authors have endorsed convergent wide-dynamic-range neurons. Recent functional and anatomical findings provide compelling support for a new perspective that views pain in humans as a homeostatic emotion that integrates both specific labeled lines and convergent somatic activity.

Central pain

PURPOSE OF REVIEW

As a result of its accompanying co-morbidity, our lack of understanding regarding its mechanisms, and its resistance to conventional treatment, central pain is one of the most formidable challenges pain physicians are faced with. The objective of this review is to summarize recent advances in our understanding of the etiology, clinical presentation, and treatment of central pain, with special emphasis being placed on studies published within the past year.

RECENT FINDINGS

Recent evidence suggests that not only injuries commonly associated with central pain, such as strokes and spinal cord lesions, but also disorders such as fibromyalgia, phantom limb pain and tension-type headaches may involve central phenomena. Perhaps because of the lack of clinical trials, treatment is still largely based on traditional prescribing methods and anecdotal evidence. Recent studies have indicated possible roles for tricyclic antidepressants, anti-seizure medications, and motor cortex stimulation in the treatment of central pain.

SUMMARY

Injury to the spinothalamocortical pathways is a necessary, but not sufficient factor in the pathogenesis of central pain. Perhaps because of the similarities in mechanisms, there is considerable overlap between effective treatments for central pain and those for peripheral neuropathic pain. Our poor understanding of the etiology of central pain and the relative lack of effective treatments emphasize the need for further research into this elusive disorder.

Functional imaging of pain perception

The application of functional imaging techniques has revolutionized the field of human pain physiology and has elaborated the understanding of mechanisms involved in pain processing at the cortical and subcortical levels. With these insights, new therapeutic interventions are being developed in the treatment of acute and chronic pain conditions.