In partial-onset epilepsy that starts with a seizure focus in one hemisphere, after several years of seizures, about 1/3 of patients develop a new seizure focus in the contralateral brain, which is medically referred to as a “secondary focus. The appearance of secondary foci may be related to the frequency of seizures in the primary foci. Animal experiments have shown that secondary foci need to go through 3 stages from generation to maturation: Stage 1: All abnormal discharges are dependent on the contralateral primary foci. At this point, the discharge of the secondary foci stops immediately after removal of the primary foci. Stage 2: The secondary foci are able to discharge independently, but reversibly. In other words, the secondary foci will continue to discharge for a period of time after the primary foci are removed at this time, but will eventually disappear slowly. Stage 3: The secondary foci are completely independent. After the primary foci are removed, the secondary foci will continue to discharge abnormally. Generally, in the presence of secondary foci, the patient’s EEG and clinical seizure performance will appear very different. That is to say: if the patient’s EEG shows a new site of abnormal discharge foci, or if the patient’s seizure presentation is significantly different from the previous one, the possibility of secondary foci should be thought of and more aggressive treatment measures, including surgical treatment, should be taken as soon as possible. As long as the secondary foci do not reach the third stage, they will not cause too much harm to the patient.