Network organization of the brain has been a long-standing goal in neuroscience. Through the application of graph theory, we are beginning to unravel the complex network organization of the brain. The human brain is thought to have evolved to optimize the efficiency of information transfer, while minimizing connectivity costs. This phenomenon is similar to other behaviors of complex networks and is thought to occur at all scales of space and time (41). Graph analysis suggests that fibromyalgia is associated with challenges of brain integrity and segregation that may lead to clinical symptoms (31). Neural integration allows different brain regions to quickly incorporate specialized information. The communication and integration among brain regions can be measured using global efficiency and characteristic path length. Conversely, neural segregation refers to the capacity of interconnected clusters of brain regions to support specific processing (31). This can be quantified using measures such as the local efficiency and clustering coefficient (42). Additionally, the brains of individuals with FM exhibit the ability to process information in a low-cost parallel manner, consistent with previous findings in pain studies (43, 44), and further confirmed the viewpoint that small-world characteristics are able to endure the developmental changes and disorders (45, 46).
Patients with FM may have a higher density of myelinated axons, which may impede diffusion orthogonal to white matter fibers. This is indicated by increased FA and decreased radial diffusivity, mean diffusivity, and axial diffusivity (47). The denser packing is likely due to a decrease in surrounding tissue and an increase in the diameter of previously existing white matter fibers with greater directionality (47, 48). One of the symptoms of demyelination is a decrease in axial diffusivity (49). Nonetheless, based on murine studies,it is suggested that rather than demyelination, lower axial diffusivity is related to axonal damage (50). It was found that decreased axial diffusivity values were linked to axonal degeneration, while increased radial diffusivity values were linked to myelin injury (51). In a human study, a reduction was seen in axial diffusivity in initial lesions and areas with subsequent white matter degeneration (52). It is likely that a reduction in axial diffusivity can be used as a biomarker for axonal loss, which indicates a significant loss of axonal integrity in FM (37). White matter tracts with varying axial diffusivity are primarily associated with pain processing and motor control (53).
Decreased clustering coefficient, nodal degree, and efficiency indicated less communication and interaction in the basal ganglia, prefrontal cortex, and thalamic areas with aberrant nodal metrics belonging to “pain matrix” (31, 54). There is an increasing knowledge of the basal ganglia participation in pain processing. Thalamic-cortical-basal ganglia loops integrate various aspects of pain, such as autonomic, motor, cognitive responses, and emotions (55). Patients with lesions in the basal ganglia exhibit decreased pain sensitivity and diminished brain activity related to pain, indicating the significance of this area in pain's sensory component (56).
It is noteworthy that a decrease in FA was discovered in the tracts that surround and connect the thalamus, such as the thalamic radiation and dentato-rubro-thalamic tracts. This suggests that there may have been changes to the mentioned white matter connectivity.
Thalamus is widely recognized as being critically involved in the processing and distribution of nociceptive information (12). There are some abnormalities related to the thalamus in another region called the internal capsule. The internal capsule is made up of axonal fibers that connect the cortical cortex to the spinal cord and thalamus. The fiber bundles in the anterior part of the internal capsule connect the prefrontal cortex and medial thalamus. This tract is thought to be involved in the attentional and cognitive aspects of pain perception processing (57). This reduction in axial diffusivity of the internal capsule may be caused by abnormal processing of somatosensory input or reduced physical activity (37). The cerebral peduncles, anterior corona radiata, and corticospinal tracts, are not involved in the processing of pain perception. These tracts are generally believed to communicate information related to movement. In individuals with FM, pain often worsens with excessive exercise or a sudden increase in physical activity. This can lead to an aversion to physical activity (59).
Research suggests that the dorsolateral prefrontal cortex modulates the cognitive and emotional aspects of pain processing (60). Individuals with FM have been reported to exhibit a decreased activation in the dorsolateral prefrontal cortex (61). Non-invasive stimulation of the dorsolateral prefrontal cortex can alleviate pain and improve daily function in people with FM (62). The discovery of reduced nodal efficiency in the dorsolateral prefrontal cortex in patients with FM provides further evidence of the region's involvement in pain regulation and highlights its role in FM (31).
Motor areas are involved in FM. The supplementary motor area is a critical region of the medial brain and is involved in the generation and/or regulation of motor behavior (63, 64). However, the supplementary motor area could also be hypo-active in cognitive circumstances, such as anticipating pain and completing executive tasks in FM individuals (65–67). Chronic neuropathic pain can affect M1 function. M1 is associated with FM and may be a non-invasive pain relief option (68). This stimulation-induced pain relief may be linked to changes in the functional connectivity between cerebral regions involved in sensory, emotional, and cognitive pain processing as well as serum endorphin levels (69–72). Reduced nodal efficiency in M1 suggests that structural abnormalities may contribute to FM (31).
In FM patients, certain white matter tracts exhibit increased FA. These tracts are located in and around the corpus callosum, which includes the tapetum corporis callosi, forceps major, and forceps minor. This area is strongly connected to the bilateral sensorimotor cortex and is responsible for interhemispheric transfer (73). Therefore, changes in the structure of the corpus callosum may indicate a significant alteration in the connectivity between the two hemispheres (12).
Limitations
The studies mentioned in the text were designed as cross-sectional, which means that they only provide a snapshot of the data at a particular point in time. Therefore, it can be difficult to determine the reasons for and the effects of the detected group differences. Additionally, the informative value of diffusion indices such as FA might be a critical factor to consider. Furthermore, since the number of participants enrolled in the studies was relatively small, the statistical significance of the results might be reduced. It is also important to note that only women with FM were recruited for the studies, so the results cannot be generalized to the entire FM population. Moreover, it is important to consider that white matter changes might be influenced by analgesic or antidepressant medication, which are known to exert neuroplastic changes in brain structure and function. Therefore, the observed changes in white matter might not be solely due to FM, and medication use should be taken into account when interpreting the results.