Study finds altered brain connections in epilepsy patients
Patients with the most common form of focal epilepsy have widespread, abnormal connections in their brains that could provide clues toward diagnosis and treatment, according to a new study published online in the journal Radiology.
Temporal lobe epilepsy is characterized by seizures emanating from the temporal lobes, which sit on each side of the brain just above the ear. Previously, experts believed that the condition was related to isolated injuries of structures within the temporal lobe, like the hippocampus. But recent research has implicated the default mode network (DMN), the set of brain regions activated during task-free introspection and deactivated during goal-directed behavior. The DMN consists of several hubs that are more active during the resting state.
To learn more, researchers performed diffusion tensor imaging, a type of MRI that tracks the movement, or diffusion, of water in the brain’s white matter, the nerve fibers that transmit signals throughout the brain. The study group consisted of 24 patients with left temporal lobe epilepsy who were slated for surgery to remove the site from where their seizures emanated. The researchers compared them with 24 healthy controls using an MRI protocol dedicated to finding white matter tracts with diffusion imaging at high resolution. The data was analyzed with a new technique that identifies and quantifies structural connections in the brain.
Patients with left temporal lobe epilepsy exhibited a decrease in long-range connectivity of 22 percent to 45 percent among areas of the DMN when compared with the healthy controls.
“Using diffusion MRI, we found alterations in the structural connectivity beyond the medial temporal lobe, especially in the default mode network,” said Steven M. Stufflebeam, M.D., from the Athinoula A. Martinos Center for Biomedical Imaging at Massachusetts General Hospital in Boston.
In addition to reduced long-range connectivity, the epileptic patients had an 85 percent to 270 percent increase in local connectivity within and beyond the DMN. The researchers believe this may be an adaptation to the loss of the long-range connections.
Epilepsy is a disorder of the central nervous system, specifically the brain. In simple terms, our nervous system is a communications network that controls every thought, emotion, impression, memory, and movement, essentially defining who we are. Nerves throughout the body function like telephone lines, enabling the brain to communicate with every part of the body via electrical signals. In epilepsy, the brain’s electrical rhythms have a tendency to become imbalanced, resulting in recurrent seizures.
If you have seen a picture of the brain before, it probably looked like this one, which illustrates the outer surface of the upper brain. This outer surface contains numerous folds that increase the surface area and allow more cerebral cortex to be packed into the skull, giving us more “brain power.”
The brain is an extraordinarily complex organ. When it comes to understanding epilepsy, there are several concepts about the brain you’ll need to learn.
The first is that the brain works on electricity. Normally, the brain continuously generates tiny electrical impulses in an orderly pattern. These impulses travel along the network of nerve cells, called neurons, in the brain and throughout the whole body via chemical messengers called neurotransmitters. A seizure occurs when the brain’s nerve cells misfire and generate a sudden, uncontrolled surge of electrical activity in the brain.
Another concept important to epilepsy is that different areas of the brain control different functions.
“The increase in local connections could represent a maladaptive mechanism by which overall neural connectivity is maintained despite the loss of connections through important hub areas,” Dr. Stufflebeam said.
The results are supported by prior functional MRI studies that have shown decreased functional connectivity in DMN areas in temporal lobe epilepsy. Researchers are not certain if the structural changes cause the functional changes, or vice versa.
“It’s probably a breakdown of myelin, which is the insulation of neurons, causing a slowdown in the propagation of information, but we don’t know for sure,” Dr. Stufflebeam said.
Brain tumors can cause seizures. Seizures can indicate the presence of a brain tumor. In neurology and neurosurgery we evaluate new onset seizures with an MRI scan or CT scan to look for a structural lesion, like a brain tumor, that could be the cause of this seizure. In this article we explore the connection between brain tumors and seizures, and discuss their management.
The presence of a tumor or abnormal cells can irritate the surrounding brain. That irritation can generate abnormal electrical activity. Propagation of abnormal electrical activity manifests as a seizure. Seizures that spread beyond a small area may affect a person’s consciousness. The three primary forms of seizures are simple partial, complex partial, and generalized seizures. A simple partial seizure does not affect a person’s consciousness but can involve muscle twitching or sensation or even emotional symptoms. A complex partial seizure could contribute to an alteration of consciousness. A generalized seizure often results in a convulsion. Brain tumors typically cause simple partial or complex partial seizures but can result in generalized seizures as well.
Seizures occur in 1/3 to 1/2 of brain tumors. Some slow-growing tumors are more likely to present with seizures than others. Certain low-grade tumors will be discovered after a first-time seizure or during the workup of refractory epilepsy. Ganglioglioma, low-grade astrocytoma, and low-grade oligodendroglioma all present with a seizure 70 percent – 83 percent of the time. These seizures are more often focal seizures or complex partial seizures. The exact character of the seizure is related to the location of the tumor in the brain. The frontal, temporal, and parietal lobes are more sensitive to these irritative phenomena, and low-grade tumors in these regions are more likely to present with seizure than mass effect or other deficits.
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By Peter B. Weber, M.D., Neurosurgeon, Surgical Director,
Sutter Pacific Epilepsy Program at California Pacific Medical Center
Dr. Stufflebeam and colleagues plan to continue their research, using structural and functional MRI with electroencephalography and magnetoencephalography to track diffusion changes and look at real-time brain activity.
“Our long-term goal is to see if we can we predict from diffusion studies who will respond to surgery and who will not,” he said.
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The study is part of the Human Connectome Project, a five-year project funded by the National Institutes of Health that uses neuroimaging techniques to study connectomics, or the functional and structural connections in the brain. Dr. Stufflebeam’s colleague, Matthew N. DeSalvo, M.D., initiated the study as a medical student with the help of a 2013 Research Medical Student Grant from the Radiological Society of North America (RSNA).
“Altered Structural Connectome in Temporal Lobe Epilepsy.” Collaborating with Drs. Stufflebeam and DeSalvo were Linda Douw, Ph.D., Naoaki Tanaka, M.D., Ph.D., and Claus Reinsberger, M.D., Ph.D.
Radiology is edited by Herbert Y. Kressel, M.D., Harvard Medical School, Boston, Mass., and owned and published by the Radiological Society of North America, Inc.
RSNA is an association of more than 53,000 radiologists, radiation oncologists, medical physicists and related scientists promoting excellence in patient care and health care delivery through education, research and technologic innovation. The Society is based in Oak Brook, Ill. (RSNA.org)
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Linda Brooks
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630-590-7762
Radiological Society of North America