Abnormal EEG Findings and Their Clinical Indications
An abnormal electroencephalogram (EEG) indicates underlying brain dysfunction that may be caused by epilepsy, encephalitis, metabolic encephalopathy, structural brain lesions, or other neurological disorders requiring further investigation and treatment. 1, 2
Types of Abnormal EEG Patterns and Their Clinical Significance
Epileptiform Abnormalities
- Spikes, sharp waves, and spike-and-wave complexes: Strong indicators of epilepsy or seizure disorders
- Focal epileptiform discharges: May indicate a localized seizure focus, seen in 16.4% of abnormal EEGs 3
- Generalized epileptiform discharges: Suggest generalized epilepsy syndromes, observed in 5.6% of abnormal EEGs 3
- Extreme delta brush: Occasionally seen in NMDAR-antibody encephalitis 1
- Lateralized periodic discharges: May indicate autoimmune encephalitis 1
Non-Epileptiform Abnormalities
- Focal slowing: Suggests a structural lesion in a specific brain region
- Generalized slowing: Indicates diffuse encephalopathy, which may be metabolic, toxic, or infectious
- Triphasic waves: Highly suggestive of metabolic encephalopathy, particularly hepatic encephalopathy 1, 2
- Periodic lateralizing epileptiform discharges (PLEDs): Often seen in herpes simplex encephalitis, typically with a temporal focus 1
Clinical Conditions Associated with Abnormal EEG
Epilepsy and Seizure Disorders
- Abnormal EEGs support a clinical diagnosis of epilepsy, though only when an electrographic seizure is recorded is the diagnosis confirmed 4
- Sensitivity of interictal EEG for epilepsy diagnosis ranges from 29% in possible epilepsy to 53% in probable epilepsy 5
- EEG is essential for classifying seizure types and epilepsy syndromes 2, 4
Encephalitis and Encephalopathy
- Autoimmune encephalitis: May show focal slowing/seizures, lateralized periodic discharges 1
- Infectious encephalitis: Herpes simplex encephalitis typically shows temporal lobe PLEDs in >80% of cases 1
- Metabolic encephalopathy: Often shows generalized slowing and triphasic waves 1, 2
- Hepatic encephalopathy: Characteristic triphasic waves; EEG changes correlate with severity 1
Status Epilepticus
- EEG is crucial for detecting nonconvulsive status epilepticus in encephalopathic patients 1, 2
- Continuous EEG monitoring is recommended to detect cyclic seizure patterns in ICU patients 2
Other Neurological Conditions
- Post-cardiac arrest: EEG helps with prognostication and detection of nonconvulsive seizures 2
- Disorders of consciousness: EEG patterns help distinguish between different states 2
- Brain death assessment: EEG shows electrocerebral silence 2
Risk Factors for Abnormal EEG Findings
- Age: Older patients have 3.48 times higher risk for EEG abnormalities 3
- Focal seizures: 2.24 times higher risk of EEG abnormalities 3
- Low protein levels: Associated with increased risk of EEG abnormalities 3
Clinical Applications and Limitations
When EEG is Most Useful
- Suspected seizures or epilepsy
- Altered mental status of unknown etiology
- Suspected encephalitis or encephalopathy
- Monitoring treatment response in seizure disorders
- Prognostication in comatose patients
When EEG is Less Useful
- Headaches (low yield unless specific concerns for seizures)
- Attention-deficit hyperactivity disorder (only 1% show slowing) 5
- Syncope (rarely shows significant abnormalities) 5
- Tic disorders 5
Interpretation Pitfalls
- Normal variants can mimic epileptiform patterns, leading to overinterpretation 6, 4
- Environmental electrical noise can interfere with recordings 2
- Accurate interpretation requires expertise and experience 2, 4
- A normal EEG does not exclude neurological disease, including epilepsy 1, 4
Conclusion
An abnormal EEG requires correlation with clinical presentation for proper interpretation and management. While highly sensitive for detecting brain dysfunction, EEG findings must be interpreted in the appropriate clinical context to avoid overdiagnosis or underdiagnosis of neurological conditions.