Previous epidemiological surveys for chronic pain noted that the most prevalent location of the pain is lower back. It is known that in the chronic low back pain patients, the quality of life, especially mental health, is poor. In addition, the group in which a discrepancy arose between disability and severity of low back pain found some associated factors, such as feeling stress, less satisfaction of their job and relationship with coworkers. Social and psychological factors are involved in the pathogenesis of chronic low back pain, therefore, evaluating the condition of chronic low back pain patients should be conducted from various directions and is not a simple task.
In recent years, analysis of brain activity which is associated with chronicity of pain has been focused on. In addition, brain imaging techniques as a non–invasive tool have developed and play an active role for pain research using functional magnetic resonance imaging (fMRI), MR spectroscopy (MRS), etc. MR spectroscopy (MRS) is used for evaluating metabolites in the chosen brain area. After performing MRI, relative or absolute concentrations of metabolites in the brain are measured. One of the metabolites is N–acetyl aspartate (NAA), which is an amino acid specifically localized in neurons, using as a marker of neuronal function.
Comparison between chronic low back pain (CLBP) patients and healthy control subjects was performed using MRS. The relationship between patients’ social and psychiatric background and metabolites in the anterior cingulate cortex (ACC) were evaluated. In the CLBP patients, the NAA level was lower, whereas glutamate + glutamine ⁄ creatine (Glx ⁄ Cr) and Glx ⁄ myoinositol (Ins) ratios were higher in the ACC compared with the control subjects. In addition, the Ins level was found to have a negative correlation with depression and anxiety using the questionnaire of Hospital Anxiety and Depression Scale (HADS), but no correlation with severity of pain was found. These results suggested that myoinositol, as a marker of glial cells, is found in low level in those with depression and anxiety. In contrast, evaluation of MRS in the chronic pain patients without psycho–social background has been reported. Brain metabolites were compared between the lumbar spinal disease patients with unilateral pain and the healthy control subjects. The NAA ⁄ Cr and NAA ⁄ choline ratios in the thalamus for the lumbar spinal disease patients were significantly lower compared with the control patients, and have a negative correlation with severity of pain. These results suggested that NAA might be useful for evaluation of presence ⁄ degree of pain objectively. Some chronic low back pain patients have brain disorders, and the causal relationship between brain function and chronic pain has not been clarified in cross–sectional studies, therefore this fact should be kept in mind when evaluating brain imaging for interaction between brain function and chronic pain.
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