Localization of the epileptogenic focus is a prerequisite for surgical treatment of intractable epilepsy. If the epileptogenic focus cannot be identified, it is impossible to talk about the exact purpose of surgical treatment and will not have the expected efficacy. Due to the leap forward development of new diagnostic techniques in recent years, especially the application of non-invasive functional brain examinations in clinical practice, the diagnostic ability of epileptogenic foci has been effectively improved. However, there is no single examination method that can provide decisive localization information, and the ideal examination means should be low-risk and highly sensitive and specific. The location and extent of the epileptogenic focus needs to be determined by a combination of tests and a comprehensive clinical analysis (with separate conclusions drawn by physicians from different disciplines in a double-blind setting), along with an evaluation of whether damage to the area causes unacceptable neurological deficits. Most tests are fairly safe or have a small risk, but some carry some risk, so the order of selection should start with the method that has a high degree of safety and, of course, for some patients, affordability should be taken into account.
According to the different risks of examination means, they can be simply divided into traumatic and non-traumatic examinations, and the development trend in recent years is gradually replacing traumatic examinations by non-traumatic examinations. One type of examination often reflects only one aspect of the origin of seizures, and the combination includes clinical information of seizures, electrophysiological examination, functional brain examination, and anatomical structure examination. Several common methods are described as follows.
1. Clinical information of seizures
Most patients are no different from normal people when they are not having a seizure, and it is difficult for the doctor to see the patient’s state during a seizure with his own eyes, so it is extremely important to take a careful medical history. In particular, the patient’s performance before the seizure with or without aura and loss of consciousness can often provide direct information about the origin of the seizure. For patients with loss of consciousness at the beginning of the seizure, the first observer should be asked for a detailed and objective description (to exclude descriptions with subjective judgment).
2. Electroencephalography (EEG)
Epilepsy is a paroxysmal brain dysfunction caused by excessive firing of hyperexcitable neurons in the brain, and the EEG can show specific seizure waves during the interictal period. The scalp EEG is the most basic and important test for the diagnosis of epilepsy, and it is also an essential tool for localization. The spike wave is confined or asymmetric and has localization significance, especially in partial seizures with aura or without loss of consciousness. In some patients, the EEG can be normal in the interictal period, and there is a 10%-20% presumed localization rate of interictal EEG spikes. To detect more EEG abnormalities for lateralization and localization, methods of seizure induction can be used, which are commonly used: hyperventilation; flash stimulation; sleep or sleep deprivation; and drug-induced. Since scalp EEG has some shortage of signal acquisition in the basal cortical area, some special electrodes can also be used. nasopharyngeal electrodes, used to record EEG activity in the temporal pole and medial side; nasoethmoidal electrodes, used to record EEG activity in the frontal pole, medial side of the cerebral hemisphere, especially the supplementary motor area and cingulate gyrus; supraorbital electrodes ), for recording EEG activity in the frontal lobe and frontal orbital region; ⑤ foramen ovale electrodes, for recording EEG activity near the medial base of the temporal lobe and the limbic lobe, with the advantage of avoiding artificial interference from the pterygoid and nasopharyngeal electrodes.
Due to the short duration of conventional EEG examination, it often does not correctly reflect the patient’s discharge. In recent years, the advent of 24-hour dynamic EEG (active electroencephalography, AEEG) has greatly improved the diagnostic and localization value, and the patient can carry a recording box with him/her for easy movement and replay of the electrical signal after the examination. Video electroencephalography (VEEG), which can retrospectively analyze the performance of seizures and synchronous EEG discharges simultaneously, is also available for simultaneous video monitoring, and has received increasing attention in recent years as a non-invasive examination tool. The scalp electrodes have been developed from traditional 20-lead to 64-lead and 128-lead, and computerized EEG information is calculated and analyzed.
The current intensity recorded by scalp EEG is extremely weak and susceptible to more interference. The current intensity recorded intracranially can reach more than ten times that of the scalp, so abnormal discharges can be caught earlier and more sensitively, which is an important reference value for the localization of epileptogenic foci. However, because of the invasive nature, it is generally used for cases that have been roughly localized by other means. The recording methods are: epidural; subdural, cortical soft membrane; intraoperative brain surface or epileptogenic foci excision trauma; deep brain nuclei.
3.magnetic resonance imaging (MRI)
Protons and neutrons are collectively called nucleons, and nucleons have spin properties and can produce spin magnetic fields. Because the arrangement of nucleons is irregular, the spin magnetic fields of nuclei with an even number of nucleons cancel each other out, and only nuclei with an odd number of nucleons can produce spin magnetic fields when they spin. The hydrogen atom, the most abundant substance in the human body, contains only one proton in the nucleus, and the magnetic field generated during spin is disordered. If the body enters a strong uniform static magnetic field, the magnetic moment of each proton will be parallel to the direction of the external magnetic field. At this time, when another RF magnetic field is added in the direction perpendicular to the external magnetic field, which is equivalent to the resonant frequency of hydrogen protons, the hydrogen protons will absorb energy and resonate, and the magnetic vector will deviate from the original arrangement direction, and some nuclei will not only change their phase, but also jump to higher energy levels. Protons in different physical and chemical states have different jumping and recovery times, so they will distinguish different tissues, and the imaging after computer reconstruction is known as magnetic resonance imaging.
The image resolution of MRI is much higher than that of X-ray or CT, especially it can avoid the interference of bone and can clearly show the structure of brain tissue. For secondary epilepsy caused by obvious structural changes such as tumors, vascular malformations, developmental malformations, softening foci, and cysts, MRI can be well localized. It is important to note that imaging structural abnormalities and foci of epileptic origin do not coincide exactly and vary in size, and a comprehensive analysis with other examinations is necessary to localize them.
Jack believes that MRI is a sensitive and specific method for measuring the hippocampus in non-occupying lesions with atrophy on one side of the temporal lobe. The temporal lobe epilepsy was accurately localized, and hippocampal volume measurements were performed in 50 epilepsy surgery patients.
4. Other newer methods of localization
(1) Magnetic resonance spectroscopy (MRS)
MRS is the only non-invasive technique to determine the chemical composition of tissues. In a high-field magnetic resonance device, the strong magnetic field applied to the nucleus has an effect on the electrons around the nucleus and the electrons in the adjacent atoms. This results in different peaks in the MRS spectrum.
Currently, it is mainly used for the diagnosis of hippocampal sclerosis. Although MRI can effectively diagnose hippocampal sclerosis by measuring hippocampal volume, it cannot effectively confirm the diagnosis in patients with mild hippocampal sclerosis or severe pathological changes with insignificant volume changes and in cases where glial cell proliferation after hippocampal neuron loss results in little change in hippocampal volume. Studies have demonstrated that almost all nitrogen-acetylaspartate (NAA) is present in neurons, that mature glial cells do not contain NAA, and that creatine (Cr) and choline complexes (Cho) are located mainly in glial cells. MRS can detect the level of these substances and can be calculated to detect hippocampal sclerosis at an early stage, and MRS and MRI can reflect the characteristics of hippocampal sclerosis from different angles and complement each other to improve the diagnostic sensitivity of hippocampal sclerosis.
(2) Magnetoenophalography (MEG)
MEG has been used in clinical practice since 1987 and is a non-invasive test. Scalp EEG can only reflect electrical phenomena on the surface of the skull and requires a location to be selected as a reference point, whereas MEG is an absolute measurement of the magnetic field generated by intracellular axial currents, and in addition the skull is transparent to the brain magnetic field and the magnetic field is less affected by other factors, MEG can improve the localization of epileptiform activity. sutherling et al. applied MEG with scalp Sutherling et al. applied MEG together with scalp EEG or ECoG for localization analysis of epileptogenic foci, which significantly improved the confirmation rate.
(3) Magnetic source imaging (MSI)
The magnetic field information of the brain detected by MEG was processed by computer software and fused with MRI images, which could show the location of the epileptogenic foci on the anatomical structure and facilitate the preoperative evaluation by surgeons.