They are associated with epilepsy, a chronic neurological disorder characterized by the occurrence of unprovoked seizures. More than 50 million people have epilepsy worldwide, and 85 percent of those cases occur in developing countries. It is estimated that, globally, there are 2.4 million new cases each year.
Epilepsy can start at any age and be idiopathic — arising from an uncertain cause — or symptomatic — having a known or presumed cause. Most idiopathic epilepsies probably are due to the inheritance of one or more mutant genes, often a mutant ion channel gene. Symptomatic epilepsies result from a wide variety of brain diseases or injuries, including birth trauma, head injury, neurodegenerative disease, brain infection, brain tumor, or stroke.
Epilepsies can be either generalized or partial. Generalized seizures typically result in loss of consciousness and can cause a range of behavioral changes, including convulsions or sudden changes in muscle tone. They occur when there is simultaneous excessive electrical activity over a wide area of the brain, often involving the thalamus and cerebral cortex. Partial epilepsies, however, are characterized by seizures in which the individual maintains consciousness or has altered awareness and behavioral changes. Partial seizures can produce localized visual, auditory, and skin sensory disturbances; repetitive uncontrolled movements; or confused, automatic behaviors. Such seizures arise from excessive electrical activity in one area of the brain, such as a restricted cortical or hippocampal area.
Many antiepileptic drugs are available. Their principal targets are either ion channels or neurotransmitter receptors. Generalized epilepsies often are readily controlled by antiepileptic drugs, with up to 80 percent of patients seizure-free with treatment. Unfortunately, partial epilepsies are generally more difficult to treat. Often, they can be controlled with a single antiepileptic that prevents seizures or lessens their frequency, but sometimes a combination of these drugs is necessary. Identification of the mutated genes underlying epilepsy may provide new targets for the next generation of antiseizure drugs.
Surgery is an excellent option for patients with specific types of partial seizures who do not respond to antiepileptic drugs. Electrical recordings of brain activity from patients allow for precise localization of the brain area from which the partial seizures originate. Once this area has been found, neurosurgeons can then remove it. After surgery, most properly selected patients experience improvement or complete remission of seizures for at least several years.
A new form of epilepsy treatment, electrical stimulation therapy, was introduced as another option for hard-to-control partial seizures. An implanted device delivers small bursts of electrical energy to the brain via the vagus nerve on the side of the neck. While not curative, vagal nerve stimulation has been shown to reduce the frequency of partial seizures in many patients.