EEG remains the most widely used investigation when epilepsy is suspected, but how and when does one use EEG in the diagnosis of epilepsy?
When not to use EEG to diagnose epilepsy
Epilepsy diagnosis is not always straightforward. Diagnosis depends on precise history taking, general medical and neurological examination, and prudent use of diagnostic tools. Depending on symptoms and examination a physician may order a variety of investigations including EEG, QEEG, ECG (to rule out heart problems like syncope), imaging techniques (including MRI, CT scan, and PET scan), lumbar puncture, prolactin study, blood tests and neuropsychological screening.
When considering an EEG, it is sometimes helpful to think of the cases when EEG should NOT be used. According the 2012 NICE Clinical Guidelines, “an EEG should be performed only to support a diagnosis of epilepsy in adults in whom the clinical history suggests that the seizure is likely to be epileptic in origin.” This places EEG in a supporting role, rather than a definitive test. Indeed, clinical history should be the driver of the diagnosis—not the EEG results.
Moreover, EEG should not be used when syncope is the most probable cause of abnormal behavior. In other words, EEG should not be part of the workup of syncope unless there is strong evidence to support of epileptiform activity as the root cause of the event. Lastly, if clinical evidence supports a non-epileptic event, EEG should not be used to exclude the diagnosis of epilepsy.
The role of EEG in making a diagnosis of epilepsy
Assuming that EEG use is appropriate to the clinical circumstances, NICE guidelines also recommend:
- EEG remains the primary investigation in diagnosing epilepsy.
- Time is of the essence and EEG should be performed as soon as possible, when clinically indicated. Portable EEGs with disposable caps ensure that physicians can perform the EEG promptly and in a variety of locations.
- An EEG should be performed if the clinical history suggests that the seizure is caused by epilepsy. In children they recommend in most instances EEG is performed after a second seizure, due to lower specificity and sensitivity in children.
- For adults at least 21 electrodes should be used, and for children at least 9.
- Care should be taken to removed artefacts, especially eye movement.
- EEG should be recorded during hyperventilation and photic stimulation, to determine if abnormal activity can be activated.
- EEG is useful in determining the type of epilepsy and this in turn can help give a correct prognosis and guide treatment.
- Repeated EEGs are recommended when a diagnosis remains elusive.
- In some circumstances EEG should be performed when asleep or sleep-deprived.
- EEG monitoring at home or long-term video EEG may be needed in some patients where diagnosis is challenging.
Taking direction from the AAN Guidelines
In 2015 the American Academy of Neurology and the American Epilepsy Society released an Evidence-based guideline on the Management of an unprovoked first seizure in adults. The guidelines explain that 21 to 45 percent of adults who have a first seizure will have a recurrence in the following two years, and it is important that patients are informed of this risk. Risk can be stratified based on several factors that correlate with higher risk including prior brain insult or lesion, an EEG with epileptiform abnormalities, a significant brain-imaging abnormality, or a nocturnal seizure. Indeed, EEG is important in determining the level of risk of recurrence. An EEG with epileptiform abnormalities is Level A evidence of seizure recurrence within 1 to 5 years (relative risk of 2.16; 95% CI 1.07–4.38). This asserts the central importance of EEG in the workup of patients with first, unprovoked seizure. The trick, as in all epilepsy diagnosis, is to determine if the behavioral manifestation being evaluated was actually a seizure, and fir this, clinical correlation and judgment is essential.
EEG remains the investigation of choice in diagnosing epilepsy
When used judiciously, EEG remains an essential component in the evaluation of epilepsy. It provides crucial information about background EEG and epileptiform discharges and is required for the diagnosis of specific syndromes. The development of portable EEG devices and disposable caps has improved the accessibility and speed of availability of this indispensable test.