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An electroencephalogram (EEG) is a test used to detect electrical activity in the brain. It is one test used to help better understand a patient’s epilepsy. The EEG signal recorded at the scalp is generated by the summation of neuronal excitatory postsynaptic potentials and inhibitory postsynaptic potentials. Scalp EEG is able to detect radial dipoles from cortical gyri.
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DIAGNOSTIC UTILITY OF ELECTROENCEPHALOGRAPHY
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Epileptiform activity on EEG is specific but not sensitive for the diagnosis of epilepsy following a paroxysmal event concerning for seizure. The yield of epileptiform abnormalities on initial EEG after a first-time unprovoked seizure or new epilepsy diagnosis in children ranges from 32% to 59%. It has been shown in adults that the sensitivity increases with serial EEGs up to 92% by the fourth EEG.
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Interictal epileptiform discharges can be seen in 0.2% to 0.5% of healthy adults and 2% to 4% of healthy children. In one longitudinal study of healthy children, 3.5% were found to have epileptiform discharges. They were followed over 8 to 9 years, and only 5% of them went on to develop epilepsy. All cases were idiopathic generalized epilepsies.
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INDICATIONS FOR ELECTROENCEPHALOGRAPHY
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Evaluation of First Unprovoked Seizure
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Multiple studies have shown that epileptiform discharges or focal slowing on EEG are predictive of seizure recurrence in children who have had their first unprovoked seizure. In one study of children with a single seizure of unknown etiology, 54% of 103 children with an abnormal EEG had seizure recurrence versus 25% of 165 children with a normal EEG. EEG results should be used in combination with history, neurologic exam, and imaging results to determine the risk for seizure recurrence. EEG can be used to support a diagnosis of seizure and epilepsy and also to help determine seizure type, epilepsy type (focal or generalized), epilepsy syndrome, and seizure recurrence risk. This information can then aid in management and treatment decisions. Therefore, a routine EEG is recommended by the American Academy of Neurology, Child Neurology Society, and American Epilepsy Society after all first unprovoked seizures. EEG abnormalities must be interpreted within the context of each individual patient’s presentation, and an EEG should be used to support the clinical diagnosis of seizure, epilepsy, or epilepsy syndrome. An EEG abnormality alone is not sufficient to confirm that an event was an epileptic seizure, nor should a normal EEG be used to exclude the diagnosis. A diagnosis of epilepsy is made either when a patient has had 2 or more unprovoked seizures or when a patient has had a single seizure and has at least a 60% chance of seizure recurrence over the next 10 years.
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Determining Epilepsy Type: Focal or Generalized
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Generalized interictal epileptiform discharges are frequently seen during a routine EEG in patients who have genetic (idiopathic) generalized epilepsies. Generalized ...