Epilepsy is a neurological condition marked by recurrent, unprovoked seizures, which are sudden bursts of abnormal electrical activity in the brain. Understanding these electrical patterns is important for diagnosis and management. The Electroencephalogram (EEG) serves as a primary tool in neurology to observe and record this brain activity. This article explores how EEG helps in understanding the electrical signatures of epilepsy.
What is an Electroencephalogram (EEG)?
An Electroencephalogram (EEG) is a non-invasive medical test that measures the electrical activity produced by the brain. Small metal discs, known as electrodes, are gently placed on the scalp. These electrodes detect minute electrical charges generated by the communication between brain cells. The collected signals are then amplified and displayed as a continuous wavy line on a computer screen or printed on paper.
The brain’s electrical activity is characterized by different types of brain waves, each associated with distinct states of consciousness. For example, alpha waves, ranging from 8 to 13 Hertz (Hz), are observed during relaxed wakefulness, particularly over the parietal and occipital regions of the brain. Beta waves, with frequencies greater than 13 Hz, are prominent during active mental engagement and seen in frontal areas. Slower waves, such as theta (3.5-7.5 Hz) and delta (3 Hz or less), are associated with sleep in adults, though delta waves can be normal in young children. The frequency, amplitude, and location of these waveforms on the scalp provide information about brain function.
Identifying Epileptic Activity on EEG
Epilepsy manifests on an EEG through abnormal electrical patterns. These patterns can be seen both during a seizure (ictal activity) and in between seizures (interictal activity). Epileptiform discharges represent a disruption of the brain’s usual electrical rhythms.
One common abnormal pattern is a “spike,” which is a single, sharp epileptiform discharge lasting less than 70 milliseconds. A “sharp wave” is similar but has a slightly longer duration, between 70 and 200 milliseconds. Both spikes and sharp waves appear as high-amplitude deflections that stand out from the background brain activity and are followed by a slow wave. These discharges suggest a potential area of seizure activity.
Another characteristic pattern is the “spike-and-wave discharge,” which involves a spike immediately followed by a slow wave component. These discharges are seen in generalized epilepsies, such as absence epilepsy, where they can appear as regular, symmetrical patterns across both sides of the brain. Polyspike-and-wave bursts, characterized by repetitive spikes followed by a wave, are also observed in generalized epilepsies. When these abnormal electrical activities affect both sides of the brain, they are termed “generalized activity.” In contrast, “focal” or “partial” patterns indicate that the abnormal activity is localized to specific regions of the brain, such as the temporal or frontal lobes, suggesting that seizures may originate from a particular area.
Interpreting and Contextualizing EEG Results
An EEG is a valuable diagnostic tool, but its results are only one component of a comprehensive evaluation for epilepsy. An abnormal EEG, showing epileptiform activity like spikes or sharp waves, suggests an increased likelihood of seizures. These abnormalities can indicate that normal brain activity has been interrupted or changed, as seen in epileptic seizures.
A normal EEG does not definitively rule out epilepsy. Approximately half of all EEGs performed on individuals with seizures may appear normal, because the test only captures brain activity during the recording period. Seizures are episodic events, meaning they have a beginning and an end, and abnormal electrical activity may not be present when the EEG is being recorded. Factors such as sleep deprivation or certain medications can influence EEG results.
Healthcare providers interpret EEG findings in conjunction with a patient’s clinical history, observed symptoms, and other diagnostic information. A detailed description of any events from the patient and witnesses is the most significant piece of information. The EEG provides objective evidence of brain electrical activity, but a diagnosis of epilepsy relies on a comprehensive assessment that integrates all available data.
Other Diagnostic Approaches for Epilepsy
Beyond routine EEG, specialized EEG variations are employed to capture infrequent epileptic activity. A sleep-deprived EEG involves the patient having fewer hours of sleep before the test, to increase the likelihood of recording abnormal electrical activity. This approach is particularly helpful for certain types of seizures, such as absence, myoclonic, or focal seizures. Long-term video-EEG monitoring, also known as video-telemetry, involves continuous EEG recording synchronized with video surveillance over several days. This allows clinicians to correlate observed behaviors during an event with the corresponding brain electrical activity.
Other diagnostic tests complement EEG in identifying the cause of seizures. Brain imaging techniques, such as Magnetic Resonance Imaging (MRI) and Computed Tomography (CT) scans, provide detailed pictures of brain structure. An MRI can reveal subtle structural abnormalities like scar tissue, tumors, or areas of cortical dysplasia that might be underlying causes of seizures. CT scans, which use X-rays, can identify broader brain abnormalities, including atrophy or abnormal blood vessels. Blood tests are also conducted to assess overall health, check for infections, or identify genetic conditions. These additional tests aid in accurate diagnosis and guiding treatment strategies.