Imaging central pain syndromes

Brain resting-state connectivity in the development of secondary hyperalgesia in healthy men

Central sensitization is a condition in which there is an abnormal responsiveness to nociceptive stimuli. As such, the process may contribute to the development and maintenance of pain. Factors influencing the propensity for development of central sensitization have been a subject of intense debate and remain elusive. Injury-induced secondary hyperalgesia can be elicited by experimental pain models in humans, and is believed to be a result of central sensitization. Secondary hyperalgesia may thus reflect the individual level of central sensitization. The objective of this study was to investigate possible associations between increasing size of secondary hyperalgesia area and brain connectivity in known resting-state networks. We recruited 121 healthy participants (male, age 22, SD 3.35) who underwent resting-state functional magnetic resonance imaging. Prior to the scan session, areas of secondary hyperalgesia following brief thermal sensitization (3 min. 45 °C heat stimulation) were evaluated in all participants. 115 participants were included in the final analysis. We found a positive correlation (increasing connectivity) with increasing area of secondary hyperalgesia in the sensorimotor- and default mode networks. We also observed a negative correlation (decreasing connectivity) with increasing secondary hyperalgesia area in the sensorimotor-, fronto-parietal-, and default mode networks. Our findings indicate that increasing area of secondary hyperalgesia is associated with increasing and decreasing connectivity in multiple networks, suggesting that differences in the propensity for central sensitization, assessed as secondary hyperalgesia areas, may be expressed as differences in the resting-state central neuronal activity.

Evidence for thalamic involvement in the thermal grill illusion: an FMRI study

BACKGROUND

Perceptual illusions play an important role in untangling neural mechanisms underlying conscious phenomena. The thermal grill illusion (TGI) has been suggested as a promising model for exploring percepts involved in neuropathic pain, such as cold-allodynia (pain arising from contact with innocuous cold). The TGI is an unpleasant/painful sensation from touching juxtapositioned bars of cold and warm innocuous temperatures.

AIM

To develop an MRI-compatible TGI-unit and explore the supraspinal correlates of the illusion, using fMRI, in a group of healthy volunteers.

METHODS

We constructed a TGI-thermode allowing the rapid presentation of warm(41°C), cold(18°C) and interleaved(41°C+18°C = TGI) temperatures in an fMRI-environment. Twenty volunteers were tested. The affective-motivational ("unpleasantness") and sensory-disciminatory ("pain-intensity") dimensions of each respective stimulus were rated. Functional images were analyzed at a corrected α-level <0.05.

RESULTS

The TGI was rated as significantly more unpleasant and painful than stimulation with each of its constituent temperatures. Also, the TGI was rated as significantly more unpleasant than painful. Thermal stimulation versus neutral baseline revealed bilateral activations of the anterior insulae and fronto-parietal regions. Unlike its constituent temperatures the TGI displayed a strong activation of the right (contralateral) thalamus. Exploratory contrasts at a slightly more liberal threshold-level also revealed a TGI-activation of the right mid/anterior insula, correlating with ratings of unpleasantness (rho = 0.31).

CONCLUSION/SIGNIFICANCE

To the best of our knowledge, this is the first fMRI-study of the TGI. The activation of the anterior insula is consistent with this region's putative role in processing of homeostatically relevant feeling-states. Our results constitute the first neurophysiologic evidence of thalamic involvement in the TGI. Similar thalamic activity has previously been observed during evoked cold-allodynia in patients with central neuropathic pain. Our results further the understanding of the supraspinal correlates of the TGI-phenomenon and pave the way for future inquiries into if and how it may relate to neuropathic pain.

Neurological diseases and pain

Chronic pain is a frequent component of many neurological disorders, affecting 20-40% of patients for many primary neurological diseases. These diseases result from a wide range of pathophysiologies including traumatic injury to the central nervous system, neurodegeneration and neuroinflammation, and exploring the aetiology of pain in these disorders is an opportunity to achieve new insight into pain processing. Whether pain originates in the central or peripheral nervous system, it frequently becomes centralized through maladaptive responses within the central nervous system that can profoundly alter brain systems and thereby behaviour (e.g. depression). Chronic pain should thus be considered a brain disease in which alterations in neural networks affect multiple aspects of brain function, structure and chemistry. The study and treatment of this disease is greatly complicated by the lack of objective measures for either the symptoms or the underlying mechanisms of chronic pain. In pain associated with neurological disease, it is sometimes difficult to obtain even a subjective evaluation of pain, as is the case for patients in a vegetative state or end-stage Alzheimer's disease. It is critical that neurologists become more involved in chronic pain treatment and research (already significant in the fields of migraine and peripheral neuropathies). To achieve this goal, greater efforts are needed to enhance training for neurologists in pain treatment and promote greater interest in the field. This review describes examples of pain in different neurological diseases including primary neurological pain conditions, discusses the therapeutic potential of brain-targeted therapies and highlights the need for objective measures of pain.

[Pharmacological fMRI : new possibilities for assessing the efficacy of analgesic agents]

Pain is a complex subjective phenomenon that so far cannot be objectively quantified by any standardized procedure. This fact renders it also difficult to measure the efficacy of analgesic drugs. In recent years the application of functional magnetic resonance imaging (fMRI) has significantly increased our current knowledge about the brain physiological correlates of pain in humans. The technique is non-invasive and detects the increased blood flow into neuronally active brain regions based on the so-called BOLD (blood oxygenation level dependent) effect of T2-weighted MRI. This paper gives an overview of the application of pharmacological fMRI (phfMRI) as an approach to evaluate the efficacy of analgesics. In contrast to EEG- and MEG-based methods phfMRI allows more flexibility in the design of experimental paradigms and stimulus protocols to account for the diversity of clinical pain types (inflammatory pain, tactile allodynia etc.) or their dependence upon psychological circumstances (anxiety, depression, stress) in which pain occurs. However, in order to specifically refer results from phfMRI to the neuronal processes underlying pain, future research needs to increase the understanding of the mechanisms underlying the neurovascular coupling reaction represented by the BOLD technique. The same applies for the influence of cerebrovascular diseases on the BOLD response.

A review of central neuropathic pain states

PURPOSE OF REVIEW

Central neuropathic pain is an important and disabling but often neglected problem following central nervous system lesions. The present review highlights recent advances in the understanding of the underlying mechanisms and in the diagnosis and treatment of central pain.

RECENT FINDINGS

Within the past year, the field of central pain has moved toward a more integrative understanding of central pain. The involvement of nonneuronal cells and interaction between multiple areas of the central nervous system has been recognized as important in the underlying mechanisms. The interest for conditions other than spinal cord injury, multiple sclerosis, and stroke has increased, and continued discussions on clear clinical diagnostic criteria are needed. The treatment of central pain is still a great challenge, but recent evidence points to tricyclic antidepressants, gabapentin and pregabalin, and selective serotonin-noradrenaline reuptake inhibitors as first-line drugs in central pain. An increased understanding of the psychosocial aspects of central pain also has implications for the treatment.

SUMMARY

Increased insight into the mechanisms of central pain will hopefully lead to increased efforts to study mechanism-based treatment of central pain.