Ambulatory EEG Cases

CHAPTER 12

AMBULATORY EEG CASES

JOSEPH DRAZKOWSKI, MD and EJERZAIN ANILES-RENOVA, BS, R. EEG T., CLTM

Advances in technology have expanded the capabilities to record electroencephalography (EEG) in the last two decades. The improved use of technology to digitalize EEG increases the versatility of recording. This has led to more reliability in interpretation of EEG recordings by prolonging traditional routine scalp EEG. This allows more clinical neurophysiological techniques including ambulatory EEG (aEEG) to be available to the clinician who desires to supplement his or her traditional skills of a comprehensive history and physical exam in the diagnosis of epilepsy and seizure-like events (aka spells). Ambulatory EEG is typically utilized if the routine scalp EEG obtained awake and asleep has not been diagnostic or if uncertainty persists (1,2). The aEEG is superior to the routine scalp sleep-deprived EEG in recording typical seizures (15%) but had similar results when recording interictal epileptiform discharges (2). The obvious advantage to support the use of aEEG is the extended recording time that has become possible through the advent of large memory storage capacity and extended battery life with reliable recording devices that approach the sophistication of inpatient video-EEG monitoring systems.

As with any diagnostic tool, the yield of useful information regarding individual cases depends on the questions that are asked for an individual patient (3). In addition to EEG, other physiological parameters can now be acquired easily and reliably. These additional measurements often include ECG, eye movements, respirations, oxygenation, and digital video. Another major advantage of aEEG is the relatively lower cost as compared with inpatient video-EEG monitoring in the epilepsy monitoring unit (EMU). The three main indications for ordering an aEEG are similar to the use of other forms of EEG and include (a) differential diagnosis of paroxysmal events (including identifying and quantifying epileptiform discharge to support a clinical diagnosis), (b) spell classification, (c) seizure quantification, and (d) rarely characterizing seizures for epilepsy therapy.

Older studies using four- and eight-channel aEEG machines reported utility in recording seizures and seizure quantification in the adult and pediatric population (4,5). In adults this concept of utility of aEEG in epilepsy and seizure quantification was also confirmed (2). The use of aEEG had substantial impact in the management of enrolled subjects with a yield of 67% and approximately 80% respectively in two studies involving aEEG (6,7). These studies also confirm that many of the subjects were unaware of seizures and did not reliably report a substantial number of recorded seizures (2). The fact that the seizures may occur without awareness should be remembered by ordering clinicians in that the patient may under report the actual number of seizures. The clinician should not be discouraged from ordering aEEG based on reported seizure counts alone.

The history and physical exams on neurology patients have been suggested to be the cornerstone of the evaluation of neurology patients. This truism is probably applicable to many neurological diseases. However, an evaluator of seizure and spells patients should know that accurate diagnosis has significant treatment implications. For example, the divergent treatment of an epilepsy patient with antiseizure drugs (ASDs) versus a nonepileptic seizure patient treated by psychiatric interventions versus a convulsive syncope patient with cardiovascular implications underscores the considerable diversity of treatments and serious clinical implications. How good are we at making an accurate diagnosis in hard to classify patients with transient neurological spells? The answer is that with our current level of accuracy there is room for improvement. Studies that try to answer this question suggest that we make the wrong diagnosis 20% to 25% of the time (8) and that self-reported seizures are typically under reported (9, 10).

A significant limitation associated with the aEEG is that we cannot provide ancillary testing during event monitoring, immediate technical support, or reduce medications safely in the outpatient setting. There is no ability without supportive staff to execute a rescue plan in the event of an emergency. This could be especially problematic in the outpatient setting. The National Association of Epilepsy Centers requires a rescue plan in the event of an emergency such as status epilepticus or other event-related complications (www.naec-epilepsy.org). These series of cases were specifically chosen and included to emphasize important issues when considering the clinical use of aEEG.

CASE 1

CB is a 50-year-old male with a 9-month history of spells reported with a sudden onset anxiety and hyperventilation. The spells began without clear etiology and occurred at a frequency of one to three times per day. The spells lasted 30 to 60 seconds. The patient never lost awareness and was able to communicate with family members. The spells increased in frequency to nine times a day. Neuroimaging and a routine scalp EEG were unrevealing. Psychiatry was consulted with a preliminary diagnosis of panic disorder, and appropriate anxiolytic medications were started. Unfortunately, the spells continued, and a 48-hour computer-assisted ambulatory EEG (CAA-EEG) was ordered. Figure 12.1A demonstrates the EEG prior to a typical event captured representing one of six that were recorded. The EEG during the event illustrates evolving left hemispheric rhythmic ictal theta activity that originated from the left temporal head region (Figure 12.1B). The spells were delineated during the aEEG recording utilizing the push-button event that was activated by the patient and confirmed to represent his typical attack. The spell lasted approximately 20 seconds and terminated with an abrupt offset and a rapid return of the baseline electrocerebral activity (Figure 12.1C).

FIGURE 12.1 (A) The event marker initiated by patient just prior to EEG changes. Parameters of recording: time base 30 mm/sec, filters 1 to 70 Hz, and sensitivity 7 μV/mm.
(B) Subtle ictal theta developing in the left hemisphere that emanated from the temporal derivations. Parameters of recording: time base 30 mm/sec, filters 1 to 70 Hz, and sensitivity 7 μV/mm. (C) The end of the ictal period and a return to baseline EEG after about 21 seconds of clinical and EEG changes. Parameters of recording: time base 30 mm/sec, filters 1 to 70 Hz, and sensitivity 7 μV/mm.

Key Points for the Use of AEEG in Case 1

• The spells were frequent enough to facilitate the rapid assessment with outpatient CAA-EEG to capture a typical event.

• Although the EEG changes were subtle, the aEEG was technically adequate to interpret the ictal EEG during the patients’ typical events to make the diagnosis of focal epilepsy.

• The patient was ultimately diagnosed as having voltage-gated potassium channel auto antibodies, which were successfully treated with immune modulation (steroids then mycophenylate).

CASE 2

TR is a 6-year-old boy with a normal birth and development until 1 month prior to evaluation when he experienced two convulsive seizures in a 1 week period of time. An MRI and neurological examination were normal. The preschool teacher noted he did not seem to be recalling things that he had mastered 2 months prior. Because of the seizures, a routine 30-minute EEG was completed showing a normal waking pattern and a pattern that was suspicious for bi-frontal spike and wave during one brief period of sleep (Figure 12.2A). The child was placed on an antiseizure medication, and a follow-up examination was established for reevaluation after several weeks. Unfortunately the antiseizure medication was ineffective, and he continued to have seizures. A second opinion was then sought, and a 24-hour CAA-EEG was obtained. The following EEG sample shows essentially a continuous bilateral fronto-central spike-and-wave discharge (Figure 12.2B).

Key Points for Case 2

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