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Biology 202
2000 First Web Report
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Vagal Nerve Stimulation and Epilepsy

Rachel Kahn

Epilepsy is a brain disorder characterized by seizures which occur when neurons fire uncontrollably and chaotically. The four most common categories of seizures are: simple partial, complex partial, secondary generalized tonic-clonic, and generalized tonic-clonic. A partial seizure is one in which epileptic activity stays in a focused area of the brain. During a simple partial seizure, the patient maintains full consciousness and experiences hallucinations, illusions and/or muscle jerking. A patient experiencing a complex partial seizure has impaired consciousness and often makes automated movements. A generalized tonic-clonic (GTC) seizure is characterized by complete loss of consciousness, rhythmic jerking, and a period of unresponsiveness. A secondary generalized tonic-clonic (SGTC) seizure is one in which the epileptic activity of a partial seizure spreads to the entire brain, resulting in a generalized seizure.(3).

Epilepsy has a variety of different pathologies. These include genetic factors, poisoning, and brain injury. High levels of excitatory neurotransmitters, low levels of inhibitory neurotransmitters, and abnormal wiring are all thought to be causes of seizures. Possibly, abnormalities in the cell membrane play a part in causing seizures. (3). This makes sense since the cell membrane essentially controls the polarization of the cell, which in turn controls the activation of signaling between cells. Often, seizures are caused by a deficiency of GABA, a neurotransmitter which inhibits the passing of information from one neuron to another. (1).

Seizures which are caused by lack of inhibitor could, in a way, be likened to the example of a chicken which runs around for some time after its head is cut off. The spastic wild activity of the chicken is due to the fact that the inhibitory neurons are all in the head, which was removed. There is nothing to control the excitatory neurons. Similarly, without enough inhibiting GABA, many neurons send signals all at once, and there is nothing to stop them.

One of the more recent forms of treatment for epilepsy is called Vagal Nerve Stimulation (VNS). The first VNS device was implanted into a human in 1988. ((2).) The vagus nerve is a cranial nerve controlling muscles involved with swallowing, speaking and coughing. The nerve is also involved with receiving input from and sending information about the heart, stomach and lungs to the brain. (6). The Vagus Nerve Stimulator is a small generator which sends electrical activity to the brain to prevent seizures. (5).

The VNS generator is surgically implanted into the chest, under the collarbone. (4). It is about the size of a stopwatch. The generator is attached to the left vagus nerve and electrical signals are released 24 hours a day, typically for 30 seconds every 5 minutes.(5). The size of the current is generally between one and three mA.(7). If a patient senses a seizure coming on, he or she can pass a magnet over the device, activating a burst of energy which often stops the seizure.(5).

The operation to implant the generator takes about two hours. The device is not activated until two weeks after surgery, in order to prevent ambiguity about which effects are a result of the surgery and which are a result of the stimulation. Often, if the patients is taking Anti-epileptic drugs (AED's), these will be continued for a significant period of time after the implantation. Patients must try to avoid microwaves and radios, and may experience side effects such as hoarseness and coughing when the stimulation is released. (7).

How does the VNS generator stop the seizures? Many theories have been proposed, yet no single convincing argument has been presented which explains exactly what is happening. Jacob Zabarra showed through animal studies that VNS changed brain wave patterns. These animal studies led him to the idea for the VNS generator. One theory behind how it actually works is that the stimulation increases the action potential threshold of the neuron, making it harder for the cell to pass along a signal to other cells. Another idea is that the stimulation increases inhibitory neurotransmitters such as GABA and decreases stimulatory neurotransmitters. (4).

Presently, VNS is used as a last resort treatment. AED's are commonly the first course of action, followed by either a special diet or evaluation for brain surgery. If AED's and diet changes are ineffective or inadequate, and surgery is either inappropriate or fails to reduce seizure frequency, VNS is considered.(5). (2). Some studies have shown that 40% of patients receiving a VNS implant have 50% fewer seizures after more than one year. (8). Many VNS advocates hope the treatment will become more widespread, as there are far fewer related risks than for brain surgery. Also, VNS could help more epilepsy sufferers than brain surgery since it could be made available in community hospitals. (7).

A related area of research in epilepsy treatment has to do with the development of transcranial magnetic stimulation. Researchers hope to create a strong magnet which can reduce seizures when placed near the head. Another treatment being developed is a device which could release drugs to the specific part of the brain from which the seizure originates. (3).

Epilepsy and Vagal Nerve Stimulation provide fascinating new questions about the I-function and whether brain equals behavior. Human feelings, and thoughts arise from electrical impulses which originate in neurons in the brain. However, what are the implications of a VNS generator, a machine which initiates these electrical impulses? Has the I-function been changed? Is it no longer completely contained within one's nervous system, but rather partly in a piece of metal inside one's chest? In researching for this paper, I read a lot about electroencephalograms (EEG) and other tests of brain function, and I am intrigued by the notion that, in a sense, the I-function can be recorded by a machine and saved on a piece of paper. Also, assuming the I-function is located in the nervous system and is a result of neuronal activity, how can we explain the apparent loss of the I-function when too much neuronal activity occurs at once (a complex partial seizure, GTC, or SGTC)?

WWW Sources

1)Epilepsy and GABA, Excellent background information about what happens to neurons during a seizure.

2)Article: Vagus-nerve stimulation of benefit in epilepsy. (Lancet, 1999), Search Vagus in lexis-nexis.

3)National Institute of Neurological Disorders and Stroke, Good resource for general information about epilepsy.

4)Vagus Nerve Stimulator, From the Internet-Enhanced Journal of Pharmacology and Therapeutics.

5)The Epilepsy Foundation of America.

6)CEP-Vagal Nerve Stimulation, Directed toward patients, but has lots of useful information.

7)Vagal Nerve Stimulation,A page by Cyberonics, the makers of the Pulse Generator.

8)Management of Epilepsy in Adolescents and Adults (Lancet, 2000), Search Vagus.




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