Diffusion MRI reveals abnormalities in DMN of epilepsy patients

Alterations in brain structure in the default mode network (DMN), assessed with diffusion tensor imaging, could provide a noninvasive biomarker for diagnosis and treatment monitoring in patients with temporal lobe epilepsy (TLE), according to a study published online Nov. 19 in Radiology.

“This study suggests that TLE involves altered structural connectivity in a network that reaches beyond the temporal lobe, especially in the [DMN],” wrote Matthew N. DeSalvo, MD, of the Athinoula A. Martinos Center for Biomedical Imaging in Charlestown, Mass., and colleagues.

TLE results in seizures emanating from the temporal lobes, and was previously blamed on isolated injuries to structures within the lobes. However, the authors noted that previous research has implicated the DMN, a set of brain regions more active during a resting, task-free state.

DeSalvo and colleagues further tested this idea by performing diffusion tensor imaging on 48 study participants, half of whom were patients with left TLE slated for surgical treatment and half of whom served as healthy controls. The authors segmented magnetization-prepared rapid acquisition gradient-echo volumes into more than 1,000 regions of interest across the whole brain. They performed white matter tractography and compared theoretical measures of connectivity networks using linear models with permutation testing.

Results showed that patients with TLE had decreases in long-range connectivity of 22-45 percent among areas of the DMN compared with healthy controls.

Epileptic patients also demonstrated an 85-270 percent increase in local connectivity in areas beyond the DMN, possibly an adaptation to the deficit in long-range connections.

“We hypothesize that an increase in local structural connectivity may represent a compensatory, perhaps maladaptive, mechanism by which overall neural connectivity is maintained despite the loss of connections through important hub areas and explain the apparent contradiction between increased local connectivity and reduced long-range DMN connectivity,” wrote DeSalvo et al.

The results of the current study echo the previous research into the abnormalities associated with epilepsy, though researchers remain uncertain whether the structural changes cause the functional changes, or vice versa.

The study is part of the five-year Human Connectome Project, funded by the National Institutes of Health, which aims to study connectomics, the functional and structural connections in the brain.

Next steps for the authors include using structural and functional MRI with electroencephalography and magnetoencephalography to track diffusion changes and look at real-time brain activity.