From the Guidelines
Yes, an electroencephalogram (EEG) is helpful in diagnosing seizure disorders even when a patient is not actively seizing. EEGs can detect abnormal electrical activity in the brain that may indicate a seizure disorder, including interictal epileptiform discharges (IEDs) such as spikes, sharp waves, and spike-and-wave complexes that occur between seizures. These abnormalities serve as biomarkers for epilepsy and help neurologists determine seizure type and focus. According to a recent study 1, EEG can also provide evidence of focal or multifocal brain abnormality when MRI is negative, which would support encephalitis over metabolic encephalopathy.
Key Points to Consider
- Standard EEG recordings typically last 20-40 minutes, but longer recordings increase diagnostic yield.
- For patients with suspected seizures but normal routine EEGs, extended monitoring options include ambulatory EEG (24-72 hours) or video-EEG monitoring (3-7 days in hospital), which can capture events during normal activities.
- Sleep deprivation before EEG testing can also increase the likelihood of detecting abnormalities, as can activation procedures like hyperventilation and photic stimulation performed during the test.
- While a normal EEG doesn't rule out epilepsy (since approximately 50% of patients with epilepsy may have a normal first EEG), repeated EEGs can increase sensitivity to about 80-90% 1.
- EEG is also useful to identify non-convulsive or subtle motor seizures, which occur in both HSV encephalitis and other encephalopathies 1.
Recommendations
- EEG monitoring should be used to monitor nonconvulsive seizure activity in adult patients with either known or suspected seizures 1.
- EEG can be helpful in distinguishing whether abnormal behaviour is due to a primary psychiatric disease as opposed to acute encephalitis 1.
- A high index of clinical suspicion is necessary to suspect nonconvulsive status epilepticus, and EEG is the definitive test 1.
From the Research
EEG Diagnostic Role
- The electroencephalogram (EEG) plays a crucial role in diagnosing seizure disorders, even when a patient is not actively seizing 2.
- EEG provides supporting evidence of a seizure disorder, assists with classification of seizures and epilepsy syndromes, and offers prognostic information regarding seizure recurrence 2.
EEG Findings and Seizure Disorders
- Interictal epileptiform activity (IEA) on EEG can be correlated with seizure control in some patients, particularly those with classical absences and 3-s spike wave activity 3.
- However, for other seizure disorders, the relationship between IEA and seizure control is less clear, and antiepileptic drugs (AEDs) may not always result in a clear correlation between control of seizures and IEA 3.
Diagnostic Testing and EEG
- EEG is a necessary extension of the neurologic examination in evaluating patients with possible seizure disorders, and most patients also require a magnetic resonance imaging (MRI) scan to identify potentially epileptogenic lesions 4.
- Long-term EEG monitoring is useful in diagnosing seizure disorders, classifying seizure types, and localizing the epileptogenic region of the brain 5.
Early EEG and Clinical Decision-Making
- Early EEG performed in the emergency room setting can have a high diagnostic yield and impact clinical decision-making, particularly when combined with MRI findings 6.
- Abnormal EEG and MRI findings can contribute to the decision to commence anticonvulsant therapy, and abnormal MRI is associated with a higher risk of seizure recurrence 6.