People with epilepsy typically experience recurrent seizures. Despite the diverse causes of seizures, the common mechanism linking many types of epilepsy is the disruption of the brain’s normal electrical activity, which temporarily halts communication between neurons.
About 60% of epilepsy cases have a cause, a lesion, or abnormality in the brain, detectable by neuroimaging methods [1,2]. Another class of pathogenesis of numerous epileptic symptoms is an abnormal expression of specific receptors in the brain, which leads to increased excitation and decreased inhibition resulting in enhanced neural activity.
Because epilepsy can only be diagnosed based on electrophysiological evidence (detection of two independent epileptic events by EEG tests) the use of EEG is mandatory for epilepsy diagnosis and management. Furthermore, based on the EEG evidence a trained epileptologist can determine the type of seizure and diagnose the type of epilepsy syndrome of the patients. The exact diagnosis can help to provide effective antiepileptic medication and prognosis.
Let’s find out the role of EEG in diagnosis, classification, and management in more detail. But first, let’s discuss what an EEG is.
What is EEG?
Electroencephalogram (EEG) is non-invasive research and diagnostic tool used to measure the changes of the brain’s electric potential over time, commonly called brain waves. This electric potential is generated by the discharges of millions of neurons. Although EEG does not have the spatial resolution of detecting the discharges of individual neurons, it can discern levels of activity associated with the major lobes of the human cerebral cortex. In other words, the EEG is a test that helps to detect electrical activity and abnormalities in a patients’ brain and localize them with a certain precision sufficient to make a diagnosis. An EEG equipment uses small sensors (electrodes) made of a conductive material attached to the scalp or they contact the skin. Often these electrodes are preconfigured inside an EEG headset to speed up the positioning.
Typically, specialists, clinical neurophysiologists, neurologists, and researchers carry out an EEG recording. Traditionally it has been done in clinics or academic laboratories and also has been adapted for home monitoring. While EEG has numerous research applications from basic research to Brain-Computer-Interface (BCI), in the field of clinical neurology it is mainly used to diagnose and monitor epilepsy and sleep disorders.
Diagnosis, Classification, and Patient Management
Diagnosis and treatment of epilepsy are often challenging. However, modern therapy provides many patients with multiple treatment options and often complete control of the seizure. After the first two seizures, evaluation should concentrate on:
1. Ruling out any non-epileptic medical or neurological condition that may generate seizures (e.g., psychogenic seizures)
2. Determine the type and location of seizures (e.g., focal, generalized, convulsive, non-convulsive)
3. Evaluating the relative risk of a seizure episode
4. Evaluating treatment options (e.g., diet, pharmacological treatment, surgical intervention, implanted control device)
The Use of EEG in Diagnosis of Epilepsy
Regardless of technological advancements, the first seizure episode typically is not captured in EEG. Numerous paroxysmal events can be confused with epileptic seizures, such as movement disorders, syncope, psychogenic seizures, etc. Probably, the most common event confused with epileptic seizures is syncope. To rule out non-epileptic seizures one needs to record abnormal activity from the brain as primary evidence. This is done by EEG equipment because all other methods to record brain activity are more expensive. At the same time, it is generally recommended to carry out a brain imaging study, such as magnetic resonance imaging (MRI). The MRI can reveal underlying cerebral lesions such as a tumor, stroke, vascular malformation, that could explain the seizure and also help localize it. However, not all epileptic seizures are associated with morphological differences in the brain that can be resolved by MRI. The class of epilepsy associated with electrographic seizures without visually observed MRI evidence is called non-lesional epilepsy.
The Vital Role of EEG in Epilepsy Diagnosis
Why does EEG play a central role in epilepsy diagnosis and treatment? Because EEG can:
● detect epileptiform activity,
● strengthen the putative diagnosis,
● identify the focal cerebral abnormalities, which may indicate a focal structural anomaly such as brain tumor, hemorrhage, vascular malformation and
● document particular epileptiform activity patterns linked to specific epilepsy syndromes
Trained clinicians can recognize a particular type of epilepsy based on their signature waveforms and distribution using an EEG device. Each type of epilepsy diagnosis entails specific treatment strategies. Typical EEG results provide a multiaxial diagnosis of epilepsy describing whether the seizure disorder is generalized or focal, symptomatic or idiopathic (unknown cause), or part of a particular epilepsy syndrome. Because no two epilepsy cases are identical, providing a detailed description of the type of epileptic waveforms, the topography (location in the brain), the frequency of occurrences, the triggering stimulus if there is any, and the effect of seizure on the cognitive and motor functions are all important aspects shaping the treatment strategy.
One critical aspect of epileptic seizures that can be captured by an EEG study is whether it is generalized or focal. The two require completely different medication and treatment strategies. In the case of generalized seizures, abnormal synchronized discharges quickly spread to both cerebral hemispheres, while in focal seizures the abnormal discharges remain localized to a certain area or areas. To capture these events, one needs to spend hours or days with a continuously recording EEG because these events are rare unless it is triggered by a known stimulus (light, sound, touch, anxiety, hyperventilation, etc.).
Because of the scarcity and unpredictable nature of epileptic seizures, these events may not be captured in the clinic during the EEG. However, the description of a seizure by a witness combined with the patient’s self-report can complement the information available from EEG. Abnormal EEG activity patterns that indicate the potential for seizures are called inter-ictal events (sharp waves and spike and waves). These events play an important role in localizing and seizures. Today, a lot of attention is paid to interictal events as potential biomarkers of an impending seizure. One of the biggest machine learning challenges in medicine is to predict seizures based on the types and occurrences of these interictal events.
The role of EEG in Classification of Epilepsy
The classification of epilepsy and the recognition of diagnostic categories based on EEG is an ongoing, evolving process. The categories we use today are not the same as the ones we used 30 years ago, and they change as we understand the disease better. We tend to overclassify epilepsy syndromes as each is associated with particular EEG features. Therefore, it is the task of an internationally elected committee of experts ”International League Against Epilepsy (ILAE” to update the classification systems from time to time, based on consensus and published empirical evidence . Because the classification is evidence-based, and evidence is subject to technological advances, the EEG and other methods, such as neuroimaging, molecular biology, and genomics have a great impact on the classification progress. And will be informed as times go on by developments in imaging, molecular biology, and genetics.
The role of EEG in Management of Epilepsy
The main objective for treating epilepsy patients is to control seizures entirely without causing undesirable side effects. Therefore, besides EEG being an indispensable part of diagnosis, it is also necessary for epilepsy management. Until today the primary measure of the efficacy of epileptic drugs was the extent it reduces seizure frequency. This assessment was often based on self-reports, diary, and caretakers’ notes. With the widespread availability of EEG, this is expected to change and EEG could be utilized for quantifying the efficacy of any treatment, from drug therapy to special diets.
Patients diagnosed with epilepsy have more therapeutic options available to them today than yesterday. To maximize the benefit of these options, clinicians must make an accurate diagnosis of epilepsy syndrome, select and use medications effectively, and promptly refer patients where necessary.
Among the broad range of available diagnostic methods, EEG is still the most versatile and affordable research and diagnostic tool that helps study the brain’s electrical activity and recognize patterns associated with epilepsy. Most importantly EEG provides detailed information about the type and localization of epilepsy.
While it has a very limited spatial resolution and is prone to misinterpretation, EEG remains the gold standard of epilepsy diagnosis. It is and it will remain in the equation to provide better care for patients and to feed our curiosity about the inner workings and communications of brain tissue.
1. Nguyen DK, Mbacfou MT, Nguyen DB, Lassonde M. Prevalence of nonlesional focal epilepsy in an adult epilepsy clinic. Can J Neurol Sci. 2013 Mar;40(2):198-202. doi: 10.1017/s0317167100013731. PMID: 23419568.
2. Téllez-Zenteno JF, Hernández Ronquillo L, Moien-Afshari F, Wiebe S. Surgical outcomes in lesional and non-lesional epilepsy: a systematic review and meta-analysis. Epilepsy Res. 2010 May;89(2-3):310-8. doi: 10.1016/j.eplepsyres.2010.02.007. Epub 2010 Mar 15. PMID: 20227852.