Functional Brain Imaging of Pain
Remarkable advances in understanding pain and providing improved treatments have come through scientific discoveries, improved training, and access to specialized clinics, organizations, national agendas, industry, and advocacy groups. However, our clinical armamentarium is relatively limited in providing relief in chronic pain conditions. In the past, the basic therapy has included, for the most part:
- Drugs mostly belonging to three classes—opioids, nonsteroidal anti-inflammatory drugs, and local anesthetics;
- Interventional treatments;
- Psychological biobehavioral therapy; and
- Some adjuvant medications, as well as complementary and alternative therapies.
For all of these efforts, the number of outcome studies of nonpharmacological trials is limited, and most pharmacological studies show poor efficacy of treatment in chronic pain. Pain researchers, pharmaceutical companies, and clinicians have struggled to break the barriers of finding treatments for pain that are both specific and efficient and have limited side effects.
Part of the problem we have faced is a new realization that chronic pain is a disease of the brain. Until recently, there has been a lack of ability to measure changes in the brain that are a consequence of chronic pain. Anatomical, functional, and chemical neuroimaging have opened the door to new vistas and new opportunities for a better understanding of chronic pain, for better diagnostic possibilities, and perhaps better drug treatments to be developed. Although genetic and other molecular approaches in the pain field have shown tremendous advances, only in recent years has brain imaging contributed to the revolution in understanding pain and greatly changed the field of pain research.
The major insight that emerged from neuroimaging studies is that chronic pain is a disease of the brain and thus all therapeutic modalities will need to take this into consideration. The ability to explore the human brain in human volunteers or patients has dramatically changed our understanding of pain. Imaging has the ability to define theoretical constructs of numerous thinkers in the field of brain processing in chronic pain in the human condition. Imaging has allowed unprecedented interrogation of brain systems in terms of brain circuitry, the effects of analgesics on neural networks, transition of acute into chronic pain, definition of brain regions that heretofore may not have been considered important, brain plasticity including functional and morphological changes, networks that are involved in the placebo response, and alterations in neurochemistry in chronic pain.
It is now increasingly understood that pain represents a multifaceted process shaped by a multitude of factors (somatosensory, emotional, cognitive, genetic) and, in turn, affecting behavioral responses as well as producing an altered brain state. Functional brain imaging may allow us to provide an objective measure of pain—one that may be complex and require taking into account sensory, emotional, and modulatory processes in the context of expectations and life experiences. Imaging pain has already produced far-reaching changes in the way we think about chronic pain and defining a signature of changes in the brain that contribute or are part of the chronic pain syndrome, which will eventually result in better pain treatments.
The development of a number of noninvasive magnetic resonance imaging (MRI) methods, including morphological/anatomical imaging of gray matter (voxel-based morphometry, VBM), white matter tract connectivity (diffusion tensor imaging, DTI), functional magnetic resonance imaging (fMRI), and magnetic resonance spectroscopy (MRS), has paved the way to an unprecedented boom in brain research. MRI methods, as well as other techniques like magnetic encephalography (MEG) and near-infrared spectroscopy (NIRS), are rapidly evolving as novel analytical methods and more sophisticated equipment become available. Because their noninvasive nature allows in vivo longitudinal studies of the dynamic structural and functional changes in the brain as a result of pain, these approaches have produced a shift in our understanding of chronic pain.
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