Since the pathophysiological basis of seizures is the abnormal discharge of neurons in the brain, electroencephalography (EEG) is an essential laboratory test for patients with epilepsy.
Principle of EEG
EEG is an electroencephalographic activity in which the spontaneous bioelectrical activity of neurons in the brain is amplified 1 million times by placing appropriate electrodes and converting pulsed direct current into alternating current and recording it with the help of electronic amplification technology. It has a high temporal resolution.
The human brain EEG activity recorded by scalp electrodes usually has a frequency between 1 and 60 Hz, a very weak current, and a voltage between 5 and 300 μV. It is now believed that the potential changes in the EEG arise from postsynaptic potentials in the parietal dendrites of the large cortical pyramidal cells, and that the rhythmic changes in brain potentials are the result of the interaction of the thalamus and brainstem reticular system with the cerebral cortex.
Selection of the type of EEG monitoring
Currently, there are three main types of scalp EEG monitoring: video EEG, dynamic EEG and conventional EEG.
Conventional EEG: Because of the great randomness of epileptiform discharges, conventional EEG generally takes about 20-40 minutes to record, and it is often difficult to capture epileptiform discharges, so the current usage rate is decreasing year by year.
Ambulatory EEG monitoring: Ambulatory EEG monitoring (ambulatoryEEG monitoring), AEEG or portable EEG monitoring, can usually be recorded continuously for about 24 hours, so it is also called 24-hour EEG monitoring. Since there is no video equipment, it is mainly used for people with relatively infrequent seizures, whose seizures are not easily captured by short-range EEG recordings; or people whose seizures have been controlled and who are ready to review the EEG before or after complete withdrawal of antiepileptic drugs (monitoring time is long and does not require sleep deprivation).
Video EEG monitoring: Video-EEG (VEEG), also known as video EEG monitoring, is an EEG device that adds synchronized video equipment to the EEG device so that the patient’s clinical situation can be filmed simultaneously. The monitoring time can be flexible according to the condition of the equipment and the needs of the disease, ranging from a few hours to a few days, but in view of the extended monitoring time leading to increased costs, limited resources and long waiting time for patient appointments, such as EEG monitoring purposes are used for epilepsy diagnosis and drug treatment and does not involve surgery, general monitoring for a few hours and recorded to a more complete wake-sleep-wake process of EEG can The EEG is usually monitored for a few hours and records a complete wake-sleep-wake process to meet the needs of clinical diagnosis and treatment.
At present, the length of EEG monitoring time is relatively fixed in each hospital according to the actual situation, and most patients can record a complete wake-sleep-wake cycle during the time period (sleep deprivation is often required before monitoring, and chloral hydrate is given to induce sleep if necessary), and its positive rate is similar to that of 24-hour ambulatory EEG. The most reliable screening method.
Applications of EEG in the field of epilepsy
Application values.
1, Epileptiform discharges found by EEG support the diagnosis of epilepsy when the clinical data suggest epilepsy.
2, It can better reflect the origin and propagation of abnormal discharges.
3, Most of the seizures and epileptic syndromes have characteristic EEG features, and EEG helps in the diagnosis of seizure type and epileptic syndrome type.
4.It helps to evaluate the possibility of reoccurrence of seizures after the first occurrence of seizures.
5. It helps to judge the response to treatment and serves as a reference for drug reduction and discontinuation.
Application limitations.
1. Epilepsy cannot be diagnosed based on EEG findings of epileptiform discharges alone, as few “normal” people have epileptiform discharges.
2. A normal EEG does not mean that epilepsy can be ruled out. When the discharge is hidden or sparse, it is difficult to record abnormal discharges on EEG.
In most cases, the frequency of abnormal epileptic discharges is not consistent with the clinical severity.
4, The presence of typical epileptiform discharges is accompanied by a large number of atypical EEG manifestations, which require careful screening.
Evoked test procedure
Evoked test is a special procedure in EEG tracing, the purpose of which is to enhance or elicit abnormal EEG activity by certain methods when performing clinical EEG recordings.
Evoked tests during routine EEG examination include eye opening and closing, flash stimulation, hyperventilation, sleep evocation, etc., which should be performed one by one during tracing.
Open-eye and closed-eye test: 3 open-eye and closed-eye tests are performed when the baseline of standard unipolar leads is smooth, with eyes open for 3 seconds each time on the basis of quiet eyes closed and repeated at 10-second intervals. It is mainly used to observe the response of occipital rhythm, to understand the functional state of the brain, and also to induce epileptiform discharges, to identify hysteria, fraud and other conditions.
Rhythmic flash stimulation: When recording, it is recommended to use the reference lead, the room light should be dimmed, and the patient’s eyes should be closed. The flash stimulator is placed at 30 cm in front of the subject’s eyes, with a flash duration of 0.1 – 10 ms. The duration of a series of stimuli is usually 5 – 10 seconds with an interval of 10 seconds, and the stimulation frequency is generally varied from low to high.
The light pulse signal should be recorded simultaneously underneath the EEG. If clinical seizures occur during stimulation, the stimulation should be discontinued promptly. Flash stimulation can be used to detect photosensitive responses and photosensitive epilepsy.
Hyperventilation: Hyperventilation should be routinely performed on EEG recordings except under specific limiting conditions (e.g., recent history of intracranial hemorrhage, significant cardiopulmonary disease, acute cranial hypertension, sickle cell disease, or when the patient is unwilling or unable to cooperate). Hyperventilation should be preceded by at least 1 minute of tracing in the same leads, hyperventilation traced for at least 3 minutes at 15 – 20 breaths per minute, and continued for 2 minutes after aborting hyperventilation. The technician should pay attention to the quality of the hyperventilation of the subject and make the child blow a feather, piece of paper or windmill if he/she cannot cooperate. Hyperventilation can be effective in inducing an anhedonic episode.
Sleep evocation: Sleep recording is one of the most common methods used in EEG evocation tests in epileptic patients. Natural sleep is most desirable, and to obtain satisfactory sleep recordings, sleep duration should be limited the night before the subject. If natural sleep cannot be obtained, drug-induced sleep can be used, choosing drugs that have a rapid onset and short duration and that have little effect on sleep architecture (e.g., pentobarbital, chloral hydrate, etc.).
Sleepiness and sleep are a good trigger for both focal and generalized epileptiform discharges. For anxious individuals or uncooperative children, sleep recording may be the only way to obtain reliable records. In cases of clinical suspicion of epilepsy without positive findings on awake EEG, a sleep-evoked test is routinely recommended.
The purpose of EEG testing is to clarify the diagnosis and determine the effectiveness of treatment. Therefore, routine EEG examination of patients who are taking antiepileptic drugs should generally not be reduced or discontinued to avoid leading to recurrent disease and possible persistent status epilepticus. When performing precise localization of the epileptic source before surgical procedures, drug reduction or even drug discontinuation is required to induce clinical seizures to obtain seizure-phase EEG changes.
The application of central stimulants such as meprobamate for drug provocation testing can result in more false positives and false negatives, as well as the risk of inducing a persistent status epilepticus, and has been largely unused clinically except in special circumstances.