Epilepsy is a group of neurological disorders characterized by seizures. Epileptic seizures are episodes that can vary from brief and nearly undetectable to long periods of vigorous shaking. In epilepsy, seizures tend to recur. Some people develop epilepsy as the result of brain injury, stroke, brain tumor, or exposure to toxins. Genetic mutations are linked to a small proportion of the disease. In children epileptic seizures may occur as a result of brain malformations. In about 70 percent of cases seizures are controllable with medication. When medication fails to control the seizures, though, then the epilepsy is called intractable. Intractable epilepsy may sometimes be cured with surgery when the location within the brain that causes the seizures can be identified and removed. Other cases of intractable epilepsy without an identifiable focal source causing the seizure may be improved surgically with neurostimulation technologies.