It's usually caused by lack of oxygen or blood flow inside the brain. This means that brain function has stopped. The EEG shows no electrical activity in the brain. This may mean that there is a brain injury or brain illness.
The EEG shows delta waves or too many theta waves in adults who are awake. These problems change the chemical balance in the body, including the brain. This includes drug intoxication, infections ( encephalitis), and metabolic disorders (such as diabetic ketoacidosis). A problem that affects the whole brain may cause these kinds of changes. The EEG records changes in the brain waves that may not be in just one area of the brain. An EEG by itself does not diagnose or rule out epilepsy or a seizure problem. In many people with epilepsy, the EEG may appear normal between seizures. When a person has epilepsy, the location and exact pattern of the abnormal brain waves may help show the type of epilepsy or seizures. These changes may be caused by a brain tumor, infection, injury, stroke, or epilepsy. Or the test shows sudden slowing of brain waves in the brain. The EEG shows sudden bursts of electrical activity called spikes. This may mean that there's a problem in one area or side of the brain. The two sides of the brain show different patterns of electrical activity. If flashing lights are used during the test, one area of the brain (the occipital region) may have a brief response after each flash of light. There are no abnormal bursts of electrical activity and no slow brain waves on the EEG tracing.
The two sides of the brain show similar patterns of electrical activity. In adults who are awake, the EEG shows mostly alpha waves and beta waves.
A machine can show the activity as a series of wavy lines on a piece of paper. The electrodes are hooked by wires to a computer that records the electrical activity in the brain. In rare cases, the electrodes may be attached to the scalp with tiny needles. Instead of separate electrodes, you may wear a cap with several fixed electrodes. A sticky paste is used to hold them in place. The EEG technologist will attach several flat metal discs (electrodes) to different places on your head. Or you may sit in a chair with your eyes closed. You will be asked to lie on your back on a bed or table. The EEG record is read by a doctor who is trained to diagnose and treat problems that affect the nervous system ( neurologist). An EEG may be done in a hospital or in a doctor's office.