A seizure represents a sudden, uncontrolled electrical disturbance within the brain. These brief bursts of abnormal activity can affect various brain functions, leading to temporary changes in consciousness, movement, sensation, or behavior. The unpredictable nature of seizures can significantly impact daily life, making accurate diagnosis important for managing the condition and improving overall well-being.
Understanding Seizures and EEGs
Seizures arise from abnormal, synchronized electrical bursts in brain cells, disrupting normal brain communication. An electroencephalogram (EEG) is a non-invasive medical test designed to detect and record this electrical activity.
During an EEG, small electrodes are placed on specific locations across the scalp. These electrodes pick up electrical impulses generated by brain cells, which are then amplified and displayed as wavy lines on a computer screen. The patterns of these brainwaves can provide insights into brain function and help identify abnormalities associated with seizures.
Detecting Seizure Activity After an Event
An EEG identifies two types of electrical activity related to seizures: ictal and interictal. Ictal activity refers to abnormal electrical patterns that occur during a seizure. Capturing ictal activity directly on an EEG can be challenging because seizures are often unpredictable and brief.
Interictal activity consists of abnormal electrical discharges that happen in the brain between seizures. These interictal abnormalities, such as sharp waves or spikes, are indicative of a seizure disorder. The likelihood of detecting these interictal discharges is higher the closer the EEG is performed to a seizure event. Studies suggest that the highest probability of detecting epileptiform patterns is within the first 16 hours after seizure onset.
Following a seizure, a temporary change in brain activity known as post-ictal slowing is observed. This slowing lasts from minutes to hours, and its duration varies. For generalized onset seizures, it averages about 2 hours and 49 minutes, while for focal onset seizures, it averages about 38 minutes. Detecting evidence of past seizures largely relies on identifying these interictal abnormalities rather than solely on capturing a seizure in progress.
Factors Influencing Detection
Several variables can influence an EEG’s ability to detect evidence of a past seizure:
- The type of seizure plays a role, as some seizures, particularly those originating deep within the brain, may be more difficult to detect on a scalp EEG.
- The frequency of seizures can also impact detection; individuals with more frequent seizures may exhibit more persistent interictal activity.
- Each person’s brain characteristics are unique, meaning the visibility of electrical abnormalities can vary widely.
- The timing of the EEG relative to the seizure event is important, with a higher chance of detecting post-ictal or interictal activity if the test is performed sooner.
- The duration of the EEG recording affects its diagnostic yield. Routine EEGs are relatively short, but extended monitoring, such as ambulatory EEGs or video-EEGs, increases the probability of capturing relevant activity.
- Performing an EEG while the patient is sleep-deprived or during natural sleep can enhance the detection of abnormalities, as certain brainwave patterns are more likely to be seen during sleep.
The Broader Diagnostic Picture
A normal EEG result does not rule out a seizure disorder. Interictal activity may not be present during the recording, or the area of the brain where seizures originate might be difficult for scalp electrodes to detect. Many people with epilepsy only show unusual electrical activity during a seizure, with normal brain activity at other times.
Therefore, an EEG serves as one tool among several in the comprehensive diagnosis of seizures. A detailed medical history, including accounts from the patient and eyewitnesses, is important. A thorough neurological examination also provides valuable information. Other diagnostic tests, such as MRI or CT scans, may be performed to identify or rule out any structural brain issues that could be contributing to seizures. Ultimately, the diagnosis of a seizure disorder is a clinical decision made by a neurologist, who considers all available information to determine the most accurate assessment and treatment plan.