The Basics of EEG and Seizure Activity
An electroencephalogram, or EEG, is a non-invasive medical test that measures the brain’s electrical activity. This procedure involves placing small, metal discs, called electrodes, onto the scalp. These electrodes detect the tiny electrical impulses generated by brain cells, which are then amplified and recorded. Technicians typically apply around 16 to 25 electrodes to specific areas of the head to monitor different brain regions.
Normally, the brain exhibits rhythmic electrical patterns, reflecting its continuous activity even during sleep. These patterns, often described as brain waves, vary in frequency and amplitude depending on a person’s state, such as wakefulness or different sleep stages. This baseline activity allows medical professionals to understand typical brain function.
During a seizure, these patterns change dramatically. A seizure, also known as an ictal event, manifests on an EEG as sudden, synchronized, and often exaggerated electrical discharges. These abnormal bursts indicate a temporary disruption in the brain’s normal electrical signaling, appearing as spikes, sharp waves, or rapid spiking patterns on the recording. The EEG captures this activity in real-time, providing a dynamic view of brain function, which is valuable for identifying the seizure’s type and origin.
Detecting Past Seizures with EEG
An EEG primarily captures the brain’s electrical activity as it occurs in the present moment. Therefore, an EEG generally cannot directly “see” or record a seizure event that happened days, weeks, or months ago. Once a seizure concludes, the brain’s electrical activity typically returns to its usual state, leaving no direct trace of the past event.
Immediately following a seizure, an individual may enter a “post-ictal state,” a period where the brain can show temporary abnormalities on an EEG. These post-ictal changes often include a slowing of brain wave activity. Such changes are transient and usually resolve within minutes to hours after the seizure has ended.
These temporary changes can indicate a recent seizure, but they rarely persist for extended periods. The EEG’s ability to detect a seizure is limited to the time of the recording, making it unlikely to capture an event that transpired significantly in the past.
The Role of Interictal Discharges
While an EEG cannot directly record a past seizure event, it can provide indirect evidence through the presence of “interictal epileptiform discharges” (IEDs). These are abnormal electrical signals that occur in the brain between seizures, not during an actual seizure. IEDs appear as brief, sudden bursts of electrical activity, such as spikes or sharp waves, that are distinct from normal brain rhythms.
The presence of IEDs on an EEG suggests an underlying predisposition to seizures, indicating that the brain is hyperexcitable or “irritable.” While IEDs are not seizures themselves, they serve as markers of epilepsy, implying that an individual has had seizures in the past or is at an increased risk of future seizures. Their detection can support a diagnosis of epilepsy, even if a seizure does not occur during the recording.
The detectability of IEDs can vary; they may be chronic in individuals with epilepsy, but their presence can fluctuate. It is important to note that a person can have epilepsy without consistently showing IEDs on an EEG, and conversely, some individuals may show IEDs without ever experiencing a seizure. Therefore, IEDs provide valuable but not always definitive insight into a history of seizures or seizure risk.
Factors Influencing EEG Detection and Other Diagnostic Tools
Several factors can influence an EEG’s ability to detect seizure activity or interictal epileptiform discharges (IEDs). The type of seizure plays a role, as some seizures, particularly those originating deep within the brain, are more challenging to detect on a scalp EEG than others. The precise location of the seizure focus also impacts detectability; activity closer to the scalp electrodes is more easily recorded. Additionally, a patient’s state during the test, such as being awake or asleep, can affect results, with sleep often bringing out abnormalities and IEDs that might not be present otherwise. Anti-seizure medications can also suppress abnormal electrical activity, potentially leading to a normal EEG reading even in individuals with epilepsy.
Given these limitations, diagnosing past seizures or epilepsy often requires a comprehensive approach beyond just the EEG. A detailed patient history, including descriptions of events from the individual and eyewitness accounts, is important. This information helps clinicians understand the nature and frequency of suspected seizures. A neurological examination provides further insights into brain function.
Other diagnostic tools, such as Magnetic Resonance Imaging (MRI) of the brain, are also used. An MRI can identify underlying structural causes of epilepsy, such as brain lesions, tumors, or areas of scarring that might predispose someone to seizures. Ultimately, a diagnosis typically relies on combining EEG findings with clinical history, physical examination, and other imaging studies to form a complete picture of the patient’s condition.