Ambulatory EEG

Clinical indications

Routine electroencephalography (EEG) suffers from the same problem as a standard 12 lead electrocardiogram (ECG): it is limited in time. A standard ECG captures 10 seconds of the heart’s electrical activity while a standard EEG captures perhaps 10 to 20 minutes of the brain’s electrical activity. While these studies have their respective places in diagnostic medicine, they are limited in their ability to detect sporadic events such as paroxysmal atrial fibrillation in the former case, or a seizure event in the latter. Just as cardiologists have adopted Holter monitoring into clinical practice, so too have neurologists turned to ambulatory EEG for certain diagnostic purposes. We discuss the clinical indications for ambulatory EEG.

Ambulatory EEG is used to diagnose seizure disorder

A routine EEG is an essential part of the workup for the first seizure. However, the diagnostic yield of this procedure is relatively low. For example, interictal EEG abnormalities appear in just one in four adults presenting with their first unprovoked seizure.¹ One approach to improve diagnosis is video EEG monitoring—patients are hospitalized in a specialized unit for up to a week where they undergo constant EEG monitoring and video surveillance. The main advantage of this approach is the ability to detect simultaneous seizure and convulsive activity and to rule out pseudoseizures and psychogenic nonepileptic seizures.^2,3 The obvious disadvantage of this approach is that it is prohibitively expensive in most cases. While inpatient video EEG monitoring is still considered to the gold standard, a far more cost-effective and quite effective approach to achieve the same ends is ambulatory EEG.

Ambulatory EEG can increase the yield of interictal epileptiform detection compared to routine EEG. In a retrospective study of 324 outpatient ambulatory EEGs, Faulkner and colleagues identified 219 (68%) studies with positive data, 116 (36%) exhibiting interictal epileptiform discharges, 167 (52%) with events, and 105 (32%) studies that were normal.⁴ In a separate study, the same group reported the median latency to first IED was 316 min (IQR 70-772 min) in 180 patients with epilepsy.⁵ The authors conclude that 48 hours of ambulatory EEG monitoring was sufficient for electro-clinical classification of 95% of patients in the cohort.⁵

Importantly, ambulatory EEG without video monitoring does not allow neurologists to rule out psychogenic nonepileptic attacks. However, newer technology is allowing in-home video monitoring combined with disposable EEG electrode caps and portable EEG recording and analytic devices.

Ambulatory EEG is used to classify seizure type

Ambulatory EEG is also a useful means to classify the type of seizure in patients with presumed epilepsy. The same principles of sporadic event diagnosis apply; specifically, a longer period of EEG recording (compared to routine EEG) is more likely to capture an event. When an event does occur during ambulatory EEG recording, neurologists are able to distinguish between focal and generalized seizures.⁶ Moreover, ambulatory EEG may allow neurologists to identify the epileptogenic focus or foci.

While the diagnostic yield can be increased with certain provocative measures, such as sleep deprivation, hyperventilation, and photic stimulation, these approaches should be used with caution during unsupervised ambulatory EEG monitoring.

Ambulatory EEG is used to localize an epileptogenic region prior to neurosurgery

Health insurers also reimburse for ambulatory EEG when it is used to localize the epileptogenic focus for neurosurgical planning. The technology is probably more valuable for identifying surgical candidates (demonstrating seizure activity consistently originates from a single focus, demonstrating the frequency and severity of attacks, etc.) than it is for anatomical location; however, this localizing information may be valuable in certain circumstances.

Practical considerations with ambulatory EEG

Numerous ambulatory EEG devices are commercially available. From the patient’s perspective, adherence to testing depends largely on the ease-of-use of the medical device. For example, it is less likely that patients will adhere to multi-day ambulatory EEG testing if they must use a device with dozens of separately attached electrodes and a large, cumbersome recording device. Conversely, patients are more likely to agree to and complete testing if they receive user-friendly devices. As such, neurologists should consider ease-of-use when choosing an ambulatory EEG system.