What is Epilepsy? What is a Seizure?

Some simplified terms.

Neuron: Also called brain cells. These are the basic building blocks of our brain. They signal (both chemically and electrically) to one another, resulting in our behaviors.

Behavior: What we do.

What is epilepsy?
Epilepsy is a disorder characterized by a wide spectrum of seizures. I will describe the seizure types later, but they range from very brief staring spells to the much stereotyped jerking and flexing of the arms and legs. A diagnosis of epilepsy is made after more than one unprovoked seizure. About 1% of the population has epilepsy.

What is a seizure?
The definition of "seizure" is not completely agreed upon. What we do know is that a seizure consists of a change in brain activity that results in a disruption of our normal behavior. If you give any more detail than that, then people start to argue. Generally speaking, however, people agree that a seizure has elements of "synchrony" and "hyper-excitation".

Normally, brain activity is "chaotic". When you hook someone up to an electroencephalogram (EEG, a machine that measures brain waves) you see very random looking, unpredictable activity (does anyone else find it ironic that chaotic activity is required to generate "normal" behavior**). Synchrony occurs when groups of neurons (called "networks") begin to fire together. "Why" and "how" our neurons become synchronous is not known. It is one of the many mysteries of a very complex system that is being studied by epilepsy researchers.

Seizures are also said to involve hyper-excitation. Brain activity consists of excitatory and inhibitory signals. The analogy often used is that of a car. A car has both an accelerator (excitatory) and brakes (inhibitory). For normal driving, a combination of accelerating and braking is necessary. During a seizure, however, this balance is lost and the brain becomes hyper-excitable. This is where the analogy falls apart because someone having a seizure isn't the same as someone driving off the road and crashing. Rather, they show the behavior of whatever brain region is over-excitable. If the seizure is in a motor area of the brain, then they have a seizure involving motor actions- such as arm and leg movements. If the seizure is in the visual area of the brain, then they may see flashing lights or odd visual patterns.


Types of Seizures.
Seizures types are divided into one of two categories: "generalized seizures" or "partial seizures". A generalized seizure is thought to involve all brain regions at once. One example of a generalized seizure is a "tonic-clonic" seizure (formerly called grand mal seizures). Tonic-clonic seizures are what many people imagine when they hear the word "epilepsy". They involve falling to the ground and shaking of the arms and legs. This movement is the result of the contraction and relaxation of several muscle groups. I know I said that seizures occur in the brain, but these brain cells activate nerve cells in our spinal cord that cause our muscles to contract or relax. Another type of generalized seizure is an absence seizure (formerly called petit mal seizures). Absence seizures involve staring spells, and sometimes rapid blinking of the eyes. Children with absence seizures are often accused of being inattentive or day-dreamers before they are diagnosed with this type of seizure. People are unconscious during all generalized seizures.

A partial seizure, on the other hand, is confined to a specific brain region and is often described as being "focal". An example of a partial seizure is a "simple partial" seizure. Individuals are conscious during simple partial seizures. They may sense flashing lights, feelings of anxiety, hear strange noises or smell a bad smell - depending on the area of the brain involved in the seizure. A partial seizure, however, may spread to other regions of the brain and become generalized. An example of this type of seizure is a "complex partial" seizure. If the simple partial seizure causes a sensation before the seizure generalizes, that individual is said to have an "aura". Auras can be helpful in alerting the individual that they are about to have a generalized seizure. During a complex partial seizure the individual is said to be in an altered state of consciousness. They may appear conscious and responsive, but they will have no recollection of seizure or the events that occurred during the seizure. There are several reports of people doing elaborate activities during a complex partial seizure. One of my favorite stories is of a woman who, during a complex partial seizure, prepared place-settings with her china and cutlery -- on the kitchen floor.

What Causes a Seizure?
Seizures are classified either as "cryptogenic" or "symptomatic". About 60-70% of seizures are cryptogenic- meaning they have no apparent cause. In reality, this means that we don't have the imaging technology to see where the problem is yet. Some of the cryptogenic seizures are genetic, meaning a genetic abnormality changes the way the brain works, making it more susceptible to seizures. This would not show up on an imaging test - and we need to develop better functional tests to screen for these seizure types.



About 30-40% of seizures are symptomatic. This means they have a known underlying problem (eg, brain injury, tumor, known chemical imbalance, etc.). These problems are thought to lower the "seizure threshold" in the brain. The seizure threshold can be defined as the minimal stimulus required to elicit a seizure. Don’t think of "stimulus" in the sense of an electrical shock. Rather, think of it in the sense of whether a cup of coffee, a stressful drive, or poor night's sleep will cause you to have a seizure. People with epilepsy are thought to have a chronically low seizure threshold. This is to say, there is something about the wiring or chemical milieu of their brains that predisposes them to seizure activity.

OK, that's a head-full for now. More to come ...

Kirk.

**I say "normal" behavior because my wise Mother always reminds me that "normal" is just a cycle on your washing machine.

Reference:
Much of the above information was distilled from Dr.W.M.Burnham's excellent article entitled "Epilepsy" from the Encyclopedia of Cognitive Science. (2006) John Wiley & Sons, Ltd.