Purpose Small-world is an attractive model for the description of complex brain networks. In the current study, we aim to investigate the functional network changes of patients suffering from lumbosacral pain (LP) and relate LP pain to small-world properties of brain functional networks.

Methods Functional MRI (fMRI) was performed on 20 LP patients and 17 age and gender-matched normal controls during resting state. Both 3D-T1W and rs-fMRI were acquired by using a 3.0T MR scanner (Philips Achieva TX, Best, The Netherlands). Pain degrees of all the patients were assessed by a mechanical Visual Analog Scale (VAS). Network-Based Statistic (NBS) was performed to in investigate the difference between LP patients and normal controls. Several small-world parameters of the networks were calculated, including: clustering coefficient (Cp), characteristic path length (Lp), local efficiency (Eloc) and global efficiency (Eg) which reflect the network efficiency.

Results Compared to normal controls, significantly decreased functional connectivity especially related to anterior cingulate cortex, middle cingulate cortex, post cingulate cortex, inferior frontal gyrus, middle temporal gyrus, occipital gyrus, postcentral gyrus, precentral gyrus, supplementary motor area, thalamus, fusiform, caudate and cerebellum was found. The brain networks of the LP group demonstrated increased Lp and decreased Cp, Eloc and Eg which denoted that LP had the tendency of a disorder-related network which is unstable and low efficient compared with healthy controls. The ROC demonstrated that Cp, Lp, Eloc and Eg have the prominent ability to discriminate the LP patients from normal controls. Discussion A small-world network was applied in pain for the first time and an altered functional network was found in the brain of LP pain. In particular, a tendency of shift toward random networks was demonstrated in LP and shift away from random networks after treatment for LP pain. And this tendency is increasing as VAS increases. Random networks have less modularized information processing or fault-tolerance as compared to small-world networks³. Our findings suggested that the structural brain networks in LP patients tend to have a more randomized configuration while lidocaine therapy could benefit in reconstruct the brain networks into a high efficient functional network. Besides, we also found the significant correlation between the topological measures of the efficient small-world attributes in some brain regions and VAS in LP, including ACC, striatum and parahippocampal gyrus showed the trend of correlation with VAS. It may suggest these brain areas were significantly affected by LP pain.

Conclusion An altered functional network was found in the brain of LP here represents a less optimal network organization in the LP. It has been suggested that the small-world structure reflects an optimal balance between local processing and global integration. And the small-world structure of LP brain networks maybe destroyed due to the chronic LP.