Epilepsy is a neurological disorder that involves a tendency to experience recurring seizures, which are disruptions of the electrical activity in the brain. This may be the result of an underlying medical condition or injury, which makes a person susceptible to the seizures.

Having a seizure does not necessary mean a person has epilepsy. Epilepsy is not usually diagnosed until a person has had at least two seizures. People who have seizures that are caused by factors that can be controlled or avoided (e.g., certain medications or alcohol or drug use) are not considered to have epilepsy.

Epilepsy is one of the most common disorders of the brain and can occur in people of all ages. More than 2 million people have had an unprovoked seizure or have been diagnosed with epilepsy, according to the National Institute of Neurological Disorders and Stroke (NINDS). Most new cases of epilepsy are diagnosed in children under the age of 2 or in adults over the age of 65, according to the Epilepsy Foundation.

Epileptic seizures are the result of a sudden change in electrical activity in the brain. In normally functioning brains, neurons (nerve cells) produce electrochemical signals that instruct other parts of the body to do certain things, such as move or feel emotions or sensations.

A seizure occurs when something disturbs the pattern of electrochemical signals and causes the neurons to fire at a much faster rate than normal. This storm of electrical activity in the brain can result in changes in mood or behavior, hallucinations, involuntary muscle contractions or loss of consciousness. The symptoms experienced during epileptic seizures depend on the area of the brain affected.

Some people with epilepsy may experience unusual sensations or movements minutes or hours before they have a seizure. This sensation, called an aura, is actually a simple partial seizure. The exact nature of the aura is often unique to each individual, although it may include noticing a strange taste in the mouth, twitching in one limb or an inexplicable feeling of fear or apprehension. Because most people experience the same progression of events during each seizure, auras can serve as a warning that a seizure is going to occur.     

Epilepsy is not a sign of brain damage – many people with epilepsy can function well between seizures. However, people with brain damage are more likely to develop epilepsy.

A serious complication of epilepsy is status epilepticus, a condition whereby a person has a longer than average seizure or never regains consciousness between seizures. People with epilepsy have an increased risk of status epilepticus, although it can also occur in people with no prior history of seizures. Seizures lasting more than 5 minutes are considered a medical emergency.

Having one seizure does not always mean that a person will have another seizure or has epilepsy. Many seizures are classed as first seizures. First seizures may be caused by a reaction to anesthesia or certain medications. They may also be the result of withdrawal from alcohol or drugs. However, in many cases, there is no known trigger for a first seizure. Only a small proportion of people who have first seizures develop epilepsy, although the risk of developing epilepsy is increased in people who have experienced a seizure.

People may have seizures that are unrelated to epilepsy. For example, young children with high fevers may develop febrile seizures. Pregnant women may develop a serious condition called eclampsia that involves seizures. Some people appear to have seizures, but there is no evidence of abnormal electrical activity in the brain. These are called pseudoseizures and they may be psychological in nature, resulting from stress or other medical conditions. Some people with epilepsy may experience pseudoseizures in addition to actual seizures.