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Long-term EEG Monitoring: 'Holter' Monitor for the Brain
Abstract and Commentary
By Padmaja Kandula, MD, Assistant Professor of Neurology and Neuroscience, Comprehensive Epilepsy Center, Weill Medical College. Dr. Kandula reports no financial relationships relevant to this field of study.
Synopsis: The authors in this retrospective study review their experience in long term EEG monitoring (LTM) and utility in the clinical diagnosis and classification of epilepsy.
Source: Yogarajah M, Powell HW, Heaney D, et al. Long term monitoring in refractory epilepsy: the Gowers Unit experience. J Neurol Neurosurg Psychiatry 2009;80:305-311.
Historically, the gold standard in establishing or refuting a diagnosis of epilepsy or seizures has been the electroencephalogram. Advances in neurophysiology, including digital long-term monitoring, have helped increase the yield of capturing and characterizing paroxysmal symptoms. In response to this ever-growing technology, the International League Against Epilepsy (ILAE) released a 2007 position paper outlining the recommended applications for long-term recording, including detection, characterization, and quantification of electroclinical seizures in epilepsy, differentiation of epileptic and non-epileptic events, and identification of subclinical seizures in comatose patients. In this article, the authors scrutinize their own database of long-term monitoring cases over a consecutive one-year period and report their findings in the context of the established guidelines.
From 20052005, 364 patients at the Gowers Center were retrospectively identified and met criteria for inclusion into the study. Patients who had been previously monitored at the center were excluded. Patients were admitted to the unit for either inpatient ambulatory EEG or inpatient video EEG. Both the admitting and discharge diagnoses were recorded for each case. The reason for admission was divided into three distinct categories: diagnostic clarification, medication changes, and presurgical evaluation. For those admitted for diagnostic clarification, pre- and post-admission diagnoses were compared. Patients were then stratified into one of three categories after long-term monitoring: no change in diagnosis, refinement in diagnosis, and change in diagnosis. All epilepsy diagnoses were classified as focal, idiopathic generalized, or symptomatic/cryptogenic generalized according to the International Classification of Epileptic Seizures and Epileptic Syndromes (ILAE) guidelines.
Of the 364 patients, 230 patients were referred for diagnostic clarification (63%), 75 (21%) for medication changes, and 59 (16%) for presurgical evaluation.
Of the 230 patients, 58% of patients had a change in diagnosis and 13% had a refinement in diagnosis. Slightly fewer than one third of patients had no change in diagnosis. In the no-change subgroup, 75% of the patients had an inconclusive study for either lack of habitual events or uninformative interictal data.
In those with a change of diagnosis, long-term monitoring helped distinguish between epileptic and non-epileptic events in 73 out of 133 patients (55%) and between focal and generalized epilepsies in 47 out of 133 patients (35%). In the 29 patients who had a refinement in diagnosis, seizure focus was lateralized to the frontal lobe in 59% of the patients.
The mean duration of video telemetry (VT) and inpatient ambulatory EEG (aEEG) in those patients with change in diagnosis was 69.9 and 59.7 hours. In those patients where monitoring led to a refinement in diagnosis, mean duration of monitoring was 54.5 and 33.8 hours respectively.
Over the years, much debate has ensued regarding the diagnostic yield of routine, 30-minute EEGs. Prior studies have shown that single 30-minute awake EEGs have identified abnormalities in approximately 50% of patients.1 Provocative maneuvers such as sleep deprivation and repeat studies have increased sensitivity to the 70%85% range.2 However, this still leaves a large subgroup of patients with normal routine EEG and no clear clinical diagnosis. What complicates matters further is that a persistently normal EEG does not exclude the diagnosis of epilepsy. On the other hand, epileptiform potentials can occur without a history of seizures, reflecting underlying pathology without a clear clinical correlate. Historically, it is this gap in the ability to support or refute epilepsy that has sparked the development and use of long-term EEG monitoring.
A drawback of this study is a lack of detailed information regarding prior workup (including EEGs, imaging, and clinical history) before entrance into the center. As a tertiary referral center, the high diagnostic yield seen in this study may in part be due to patient selection bias. Perhaps a larger methodological flaw was in the arbitrary selection of inpatient ambulatory monitoring (without video) versus video monitoring. In the case where both ambulatory and video EEG monitoring were used during the admission, the rationale for the combined modalities was not clearly explained. In the group of patients that differentiated non-epileptic events from epilepsy, nearly 40% underwent both modalities versus 14% in those patients where monitoring distinguished between generalized and focal epilepsies. The reader is left to the assumption that perhaps actual video monitoring was required to establish a diagnosis of non-epileptic events. Thus, the authors were not able to clearly demonstrate whether inpatient aEEG may be as effective as video EEG in distinguishing between non-epileptic events and seizures and discriminating between focal and generalized epilepsies.
This paper highlights two scenarios where inpatient monitoring may be clinically useful: 1.) in differentiating non-epileptic events from seizures; and 2.) in differentiating focal (predominantly frontal) versus generalized epilepsy. Currently, long-term EEG monitoring is not recognized by all physician groups, but as per-unit cost of digital EEG and recording equipment continues to fall and availability of a skilled neurophysiology team rises, long-term monitoring may be not just limited to tertiary referral centers, but may become part of a community hospital service.
1. Ajmone-Marsan C, Zivin LS. Factors related to the occurrence of typical paroxysmal abnormalities in the EEG records of epileptic patients. Epilepsia 1970;11: 361-381.
2. Binnie CD, Rowan AJ, Overweg J, et al. Telemetric EEG and video monitoring in epilepsy. Neurology 1981;31:298-303.