How Does a Brain Develop Absence Epilepsy?

“Do you know why you shouldn’t talk to strangers?” asks a police officer standing at the front of a classroom to a room full of students.

Emily’s hand shoots up and the teacher encourages her to answer¹. Moments pass, but Emily remains still and silent. “Emily, you can answer the question, put your arm down.”

Nothing. Emily is frozen, the only noticeable movement is a slight shift in her gaze upwards.

Emily recounts the experience: “My hand shot up; I knew the answer. I was so excited to talk to a police officer. But then my vision started to blur, and I felt my eyes drifting back. I could hear my teacher say my name to answer the question, but I couldn’t reply. My hand remained up and that was it.”

What Emily is describing is her first memory of an absence seizure.

Absence seizures are typical of childhood absence epilepsy, juvenile absence epilepsy, juvenile myoclonic epilepsy, and in up to 60% of patients with Lennox-Gastaut syndrome. During an absence seizure, which may last up to 30 seconds, the person simply stops moving or speaking; they may stare blankly ahead; and their eyes may drift upwards (as Emily’s did) or appear to flutter.

“My vision came back and I looked around the room, only to find everyone looking at me,” remembers Emily. “A few children were smirking, and my teacher was looking at me confused. The officer smiled to let me know she was listening.”

Nerve cells, or neurons, in the brain communicate with one another using tiny bursts of electricity. During a seizure, the firing pattern of these electrical signals being passed between neurons becomes unusually intense and abnormal.

Absence seizures are thought to, at least in part, be caused by abnormal neuron firing between the thalamus and the cortex. For unknown reasons, neurons in these two brain regions become hypersynchronized, generating intense neural activity that spreads to the rest of the brain along cable-like extensions called axons.

As Emily’s awareness returned, she began to cry. “I was so confused and didn’t know how long I was absent for,” recalls Emily. “How could I explain that I wasn’t being rude, and that I couldn’t control my ‘daydreams?’”

A need exists for a better understanding of how absence epilepsy develops and how it can be treated. As Emily describes, the seizures themselves cause considerable social distress. What’s more, absence epilepsy is linked to other cognitive conditions, 35% of patients do not respond to medical therapy, and seizures return in 20 to 50% of patients when they stop taking antiepileptic medication.

Researchers in the Department of Neurology and Neurological Science at Stanford University are looking outside the thalamus and cortex—the usual suspects when investigating the mechanisms of absence epilepsy—and focusing their attention on those cable-like projections (axons) that generalize the seizure to broader areas of the brain.

Axons are wrapped in myelin—a fatty coating which, upon its initial discovery, was thought to be merely an inert substance whose sole function was to speed electrical signals traveling along cable-like axons. In the last decade, however, scientists have gained a greater appreciation for how dynamic myelin is. It changes in response to neuronal activity.

“Activity-regulated myelination is adaptive in the healthy brain,” write the authors of a new study currently being peer-reviewed for publication. It increases neural network synchrony and contributes to cognitive functions like learning, attention, and memory consolidation.

In response to neuronal activity, oligodendrocytes—the specialized cells producing myelin in the brain and spinal cord—alter the myelin landscape, optimizing myelin thickness and the space between successive wraps to ensure electrical signals transmitted along the axon move as quickly as possible. Increased myelin also shuttles additional energy to the active neuron, allowing it to maintain a higher level of activity.

Oligodendrocytes provide the supplies and work required to remodel myelin. But the ultimate gatekeeper of activity-regulated myelination is, of course, neuronal activity. When coaxed into action, oligodendrocytes are unable to consider the consequences of myelin remodeling on the overall health of the brain.

This non-discriminatory feature of activity-regulated myelination led Drs. Michelle Monje and John Guguenard of Stanford University to consider a simple, yet very important question: what happens to myelin in brains plagued by abnormal patterns of neuronal activity, like epilepsy?

Their hypothesis: activity-regulated myelination due to absence seizures promotes the development of epilepsy. In other words, myelin changes due to increased neuronal activity contribute to a brain environment more sensitive to abnormal, increased activity.

The scientists used two preclinical models of absence epilepsy to test their hypothesis. Both models feature animals genetically predisposed to absence seizures that consistently and reliably develop absence epilepsy across their lifespan. In both models, the researchers focused their attention on a brain structure called the corpus callosum.

The corpus callosum sits in the middle of the brain right beneath the cortex. It represents the largest collection of myelinated fibers in the brain and functions as a means of communication between the two hemispheres. In absence epilepsy, the myelinated tracts of the corpus callosum transmit abnormal oscillations between the thalamus and cortex to the rest of the brain.

The Stanford researchers assessed oligodendrocyte changes in the corpus callosum after absence seizures using a technique called immunohistochemistry. This technique allows you to fluorescently label specific cell types and structures within tissue. When the tissue is viewed under a microscope, the labeled structures can be easily observed and quantified.

Using various combinations of markers in both preclinical models, the Stanford researchers saw more mature oligodendrocytes (potentially capable of producing myelin) and more new oligodendrocytes after absence seizures developed. “These data indicate that oligodendrogenesis increases within the seizure circuit in parallel with epilepsy development in [the model] of absence epilepsy.”

They then looked more directly at myelin using transmission electron microscopy, or TEM. TEM works by shooting a beam of electrons through an extremely thin slice of tissue. The density of the tissue determines how much of the electron beam passes through. Cellular structures, like organelles, cell walls, and myelin obstruct the beam. The part of the beam that makes it through is focused by an objective lens into an image. Portions of the tissue that easily allow the beam to pass through appear as light spots on the image, whereas structures obstructing the beam appear dark.

This type of microscopy is so powerful, structures can be viewed at the atomic level at a resolution of less than a nanometer—that’s one-millionth of a millimeter.

In the corpus callosum the average width of an axon is about half a micrometer, and myelin is approximately an eighth to one-quarter of that size. TEM allows these structures to be viewed and measured. After absence epilepsy develops, the Stanford scientists saw thicker myelin and more myelinated axons. They concluded, “seizures are associated with […] abnormally increased myelination in an anatomical pattern that parallels seizure activity.”

In biological science, mechanisms are determined by identifying relationships and subsequently removing pieces to determine how critical they are. In this study, the researchers identified a relationship between absence seizures and an altered myelin landscape in the corpus callosum. They investigated mechanism by pharmacologically inhibiting absence seizures and by genetically preventing activity-regulated myelination.

Preventing seizures with the antiepileptic drug ethosuximide made the oligodendrocyte profile and myelin look much more like it does in animals without absence epilepsy. Indicating that “seizures increase myelination specifically within the seizure-affected region and suggest a mechanism of aberrantly increased activity-dependent myelination that could be maladaptive, contributing to epilepsy pathogenesis.”

Genetically inhibiting myelination resulted in animals that didn’t develop seizures like those whose myelination was intact. “We found that seizure burden was strikingly reduced in [mice] with impaired activity-dependent myelination,” wrote the researchers. “[Mice] with intact activity-regulated myelination exhibit a marked increase in the number of seizures per hour over the period of epileptogenesis.”

Preventing myelin changes with the antiepileptic drug and the reduction of seizures when myelination is impaired, “indicate that activity-dependent myelination contributes to kindling of absence seizures during epileptogenesis,” concluded the authors. It means increased myelination on tracts carrying signals away from the thalamus and cortex makes the brain more prone to generalizing the seizure to the rest of the brain as epilepsy develops. It could also mean the threshold for an absence seizure lowers.

Of course, activity-regulated myelination is just one piece of the very complex puzzle of epilepsy. “Our findings that seizures are markedly reduced but not entirely prevented by blockade of activity-dependent myelination suggests that multiple mechanisms are responsible,” write the authors. Interneuron dysfunction in the reticular thalamic nucleus and the altered function of calcium channels, GABA receptors, potassium channels, and glucose transport also play a role.

But, activity-regulated myelination may be a large enough piece of this puzzle that targeting it with medications could substantially reduce seizures. The use of chemical compounds called histone deacetylase (HDAC) inhibitors could provide insight into the effectiveness of this approach.

HDAC inhibitors work by altering the accessibility of certain genes, which changes the proteins expressed in a cell, and subsequently the functions and characteristics of that cell. By altering gene expression, HDAC inhibitors have been shown to inhibit oligodendrocyte lineage cells from maturing into the type of oligodendrocytes that produce myelin—a necessary step for activity-regulated myelination.

Valproate, or valproic acid, is one of three antiepileptic drugs used to treat absence seizures (the other two being lamotrigine and ethosuximide). And one of its suspected mechanisms of action, is as an HDAC inhibitor.

To gauge the effectiveness of valproate in treating absence seizures, we first have to look at lamotrigine. In a 1999 study, participants recently diagnosed with absence seizures were treated with the antiepileptic lamotrigine. Participants seizure free after four or more weeks of treatment were then randomly assigned to one of two groups: placebo, or the antiepileptic.

In the initial four or so weeks of treatment, when all participants were getting lamotrigine, 71% became seizure free. In the second phase of the study, the placebo-controlled part, participants were three times as likely to remain seizure free if they continued taking the antiepileptic.

Comparing valproate to lamotrigine in four randomly controlled trials suggests valproate is as good or slightly better than lamotrigine, particularly in the first month of treatment. Whether valproate is influencing myelination is unclear, as researchers have never looked at white matter (tissue in the brain rich in myelin) changes in the brains of patients taking valproate. But the work of Monje and Guguenard provides another mechanism by which valproate may be working.

1. Emily's description of her experience with absence epilepsy can be found here.