Epilepsy is a disorder in which nerve cells of the brain from time to time release abnormal electrical impulses. These cause a temporary malfunction of the other nerve cells of the brain, resulting in alteration of, or complete loss of consciousness. When there is a known cause for a person's epilepsy starting it is called symptomatic epilepsy. The causes can include head injury; infections of the brain such as meningitis; a stroke; or because of a scar on the brain. Images from scans of the brain may show what the cause is. The response to drug treatment can vary from person to person. Once epilepsy is diagnosed, it is important to begin treatment as soon as possible. For about 80 percent of those diagnosed with epilepsy, seizures can be controlled with modern medicines and surgical techniques. Some antiepiletic drugs can interfere with the effectiveness of oral contraceptives. In 1997, the FDA approved the vagus nerve stimulator for use in people with seizures that are not well-controlled by medication.