In the present study, we investigated differences in the structural co-variance and functional networks between the ipsilateral and contralateral hemispheres of migraine pain. Our results indicated that structural volume and cerebral blood flow did not significantly differ between the ipsilateral and contralateral hemispheres. However, significant differences in the structural co-variance network and functional network were observed between the two hemispheres. In addition, we successfully demonstrated the feasibility of functional network analysis using ASL perfusion MRI in patients with migraine.
Our results demonstrated that, in the structural co-variance network, the betweenness centrality of the thalamus was significantly lower in the ipsilateral hemisphere than in the contralateral hemisphere. Alterations in the structural co-variance network may reflect alterations in dendritic complexity, changes in the number of synapses, or brain plasticity, thereby resulting in connectivity changes.[17] Betweenness centrality is a measure of centrality in a graph based on the shortest paths, a measure widely used to detect the amount of influence a node has over the flow of information in a graph.[18] The measure quantifies the number of times a node acts as a bridge along the shortest path between two other nodes.[18] Thus, the present findings suggest that the structural connectivity of the thalamus in the ipsilateral hemisphere is lower than that in the contralateral hemisphere during the interictal state. In the pathogenesis of migraine, the thalamus may play a role as a relay center for ascending nociceptive information from the brainstem to cortical regions, via the trigemino-vascular pain pathway.[19] The thalamus is therefore most likely involved in the allodynia, central sensitization, and photophobia associated with migraine.[19] Previous studies using functional MRI[20] or diffusion tensor imaging[21] have also observed abnormal thalamocortical network connectivity in patients with migraine. In addition, we recently demonstrated that patients with migraine exhibit significant alterations in thalamic nuclei volumes when compared with healthy controls, especially in the anteroventral, medial geniculate, and parafascicular nuclei.[22] Together, these findings suggest that alterations in thalamic connectivity contribute to the pathogenesis of migraine.
In the functional network analysis based on cerebral blood flow, we observed significant alterations in betweenness centrality in the anterior cingulate/paracingulate gyrus. The cingulate gyrus is involved in pain processing, modulation, and associated symptoms such as emotional disturbances in patients with migraine.[5, 23] Previous functional MRI studies have consistently reported atypical brain responses to sensory stimuli, absence of the normal habituating response between attacks, and atypical functional connectivity of sensory processing regions in patients with migraine.[4] The alterations in the betweenness centrality of the cingulate gyrus observed in our functional network analysis are in accordance with the results of previous studies, supporting the notion that sensory hypersensitivities in patients with migraine may be induced by a combination of enhanced sensory facilitation and reduced inhibition in response to sensory stimuli.[24, 25] Furthermore, the cingulate gyrus is one of the regions of the default mode network (DMN), which plays a relevant role in adaptive behaviors other than those associated with cognitive, emotional, and attentional processes.[26] Several studies have identified disrupted DMN connectivity during the interictal period in patients with migraine.[27] Pain has a widespread impact on overall brain function, modifying brain dynamics beyond pain perception, which may produce alterations in DMN connectivity.[28] Based on the amplitude of low-frequency fluctuations, another functional MRI study demonstrated reduced DMN connectivity in the anterior cingulate cortex, prefrontal cortex, and thalamus in patients with migraine.[29] Furthermore, functional DMN changes are negatively correlated with disease duration.[29] Taken together, these results indicate that the cingulate gyrus may be involved in pain processing in patients with migraine.
In the present study, we also observed that betweenness centrality was higher in the inferior frontal gyrus of the ipsilateral hemisphere than in that of the contralateral hemisphere. The frontal cortex is one of the most important areas associated with brain abnormalities in patients with migraine. The role of the frontal lobe in pain processing has been established in previous studies, including those involving patients with chronic back pain, fibromyalgia, phantom pain syndrome, and medication overuse headache.[30, 31] A previous meta-analysis demonstrated that patients with migraine exhibited concordant decreases in gray matter volume in the inferior frontal gyrus.[5] Increased activation in the inferior frontal gyrus may reflect increased effort due to disorganization of these areas or the use of compensatory strategies involving pain processing in migraine.[5] Additionally, one functional MRI study reported increased neural activation in the frontal gyrus in response to fearful faces when compared to neutral faces in patients with migraine, relative to findings observed in healthy controls. Thus, an enhanced response to emotional stimuli may explain the triggering effect of psychosocial stressors on migraine.[32]
This is the first study to investigate differences in the structural and functional networks between the hemispheres according to the side of migraine. We successfully demonstrated significant differences in the structural/functional networks of some regions between the ipsilateral and contralateral hemispheres. Furthermore, our findings highlight the feasibility of functional network analysis based on cerebral blood flow determined using ASL MRI in patients with migraine. However, there were several limitations in this study. First, the sample size was relatively small. However, we only enrolled patients who had unilateral migraine pain that always occurred on the same side. In addition, all patients were newly diagnosed with migraine without aura and underwent MRI during the interictal state. Second, we did not obtain ASL perfusion MR images from healthy controls. Thus, we could not investigate differences in the structural and functional networks between patients with migraine and healthy controls. Third, we could not analyze the correlation between clinical factors and network measures, as we obtained network measures at the group rather than individual level. Fourth, most patients with migraine in our study were taking medications for migraine. These medications may have impacted the structural or functional networks in our patients. Further studies with larger sample sizes may be required to confirm our findings.