Mapping Connectivity in Brains with Epilepsy

White matter paths connecting grey matter in the brain, as mapped by a diffusion tensor imaging MRI. Photo courtesy of Wikimedia Commons.
White matter paths connecting grey matter in the brain, as mapped by a diffusion tensor imaging MRI. Photo courtesy of Wikimedia Commons.

by Poncie Rutsch
BU News Service

For years, neuroscientists have defined epilepsy as a case when too many nerve cells in the brain fire at once, provoking a seizure. Researchers could differentiate a few types of epilepsy, based on which parts of the brain were over-firing. Now, researchers have determined that it isn’t just a matter of function – the brains of people with epilepsy  structurally differ from those of people without the disorder.

According to lead researcher Steven Stufflebeam, a neuroradiologist at Massachusetts General Hospital, the differences between the two brains suggest new ways to diagnose and treat epilepsy moving forward.

Stufflebeam and medical student Matt DeSalvo focused on temporal lobe epilepsy, the most common type of epilepsy among adults. Temporal lobes are part of the cerebral cortex, and aid in visual memories, processing the senses, language, and emotion. During a temporal lobe epileptic seizure, neurons in the brain randomly begin firing too frequently, sending those parts of the cortex go into overdrive.

The researchers compared the brains of people with epilepsy and people without to understand the white matter, the tissues that form connections between grey matter, or the brain cells. They used a type of MRI brain imaging called diffusion tensor imaging. Unlike a typical MRI, which shows some brain tissues or inflammation, this type of imaging induced a magnetic field to show the movement of water molecules through the brain, illuminating the paths molecules use to travel the brain.

The researchers induced sixty different magnetic fields to highlight sixty possible directions in the brain, and then compiled the images in one all-inclusive brain scan.

The researchers found that brains with temporal lobe epilepsy have very different structures from those without epilepsy. Patients with epilepsy have less long-range connectivity, and more short-range connectivity that patients without the disorder. The white matter that the researchers mapped connected parts of the brain located closer together, rather than farther apart.  These short range connections in epileptic brains appeared within the brain structures responsible for wakeful rest such as daydreaming or introspective thought.

The study helps neurology researchers move toward the idea that epilepsy could be an issue in the the brain’s network, not necessarily the result of brain damage. Researchers used to blame temporal lobe epilepsy on injuries to the temporal lobes. But more and more studies like DeSalvo’s are showing that that’s not the case. “Injury can still play a role,” says DeSalvo, “but more and more it’s thought of as a network disease.”

Stufflebeam and his colleagues published their findings this week in Radiology. The study is part of the National Institute of Health’s Human Connectome Project, which seeks to map brain connections.Stufflebeam and his colleagues published their findings this week in Radiology. The study is part of the National Institute of Health’s Human Connectome Project, which seeks to map brain connections.

The findings offer insight into which people might benefit from surgery. “With temporal cases,” said Stufflebeam, “about 90% of those patients will benefit from surgery.” The surgery excises the part of the brain from which the seizures seem to start. “But there’s still 10% that fail,” said Stufflebeam. “They still have seizures and aren’t completely cured.”

Both researchers think this may be due to connectivity within the brain. Surgeons can estimate how much brain to remove to decrease epileptic seizures, but it’s an educated guess. In the future, doctors and patients could use the images from this research to determine whether surgery will be effective.

“We may be able to predict outcomes based on the connectivity we see,” said DeSalvo.

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