CHAPTER 5

CLINICAL USE OF AMBULATORY EEG IN ADULTS

DONALD L. SCHOMER, MD

INTRODUCTION

The use of ambulatory EEG (aEEG) should be thought of as a part of a comprehensive clinical neurophysiology laboratory’s available technologies. This technology is complementary to the inpatient epilepsy monitoring unit (EMU) and should be integrated with it. These techniques have evolved as an essential part of the diagnostic and treatment programs for people suffering from seizures and/or epilepsy. Clinicians wanting to use either of these technologies should ask themselves a series of questions before ordering their patient to undergo either procedure. A logical approach, summarized in Table 5.1, is discussed in the following.

The field of aEEG monitoring has evolved tremendously over the last 10 to 15 years secondary to the significant technological advances in EEG monitoring in general (see Chapters 1–3) (1,2), and the demand for its use has grown even more rapidly secondary to the ever significantly increasing costs related to inpatient evaluations. There is also a reasonable and somewhat rational approach to the dilemma of, “Should I use aEEG or go for an inpatient (EMU) workup?” (Table 5.2). Since enhanced technology has brought inpatient and outpatient aEEG capabilities to be on fairly equal footing, the choice relates more to the individual patient’s related idiosyncratic concerns and the requesting physician’s goals. I discuss the pros and cons of inpatient versus outpatient EEG telemetry in more detail in the following since the choice of technologies is more realistically based on the clinician’s intended goal, modified by specific patient needs and requirements, than it is on costs for services or on technological sophistication. If used thoughtfully, aEEG can be used successfully for both diagnostic and treatment or management purposes. As a diagnostic tool, aEEG can certainly be used in the differential diagnosis of behavioral events. There is also a significant role for its use in determining seizure classification or identifying a specific epilepsy syndrome in a patient with known epilepsy. It may be used successfully as a part of a comprehensive evaluation of patients for possible epilepsy surgery, when inpatient (EMU) studies may have failed for reasons noted in the following. In the long-term treatment/management of patients with epilepsy, aEEG is useful in reassessing patients with changes in behavior or cognition to see whether seizures are contributing to their more newly acquired problem. Also, aEEG is a reasonable method of identifying whether or not drug withdrawal causes an increase in either subtle sustained EEG events or in the frequency of interictal epileptic activity. Such knowledge may be critical in assessing whether or not to continue a drug taper or to evaluate if the patient is at high risk for recurrences of clinically symptomatic seizures. In like fashion, aEEG has become a more frequently considered method for identifying ictal events during either clinical or experimental drug trials. Each of these areas will be dealt with separately (Table 5.3).

TABLE 5.1  The Clinician’s Approach to EEG Monitoring: Think First!

TABLE 5.2  Inpatient (EMU) Versus Outpatient (aEEG) EEG Monitoring

TABLE 5.3  Role for EEG Telemetry

fMRI, functional magnetic resonance imaging; PET, positron emission tomography; SPECT, single-photon emission computed tomography.

DIFFERENTIAL DIAGNOSIS

When approaching a patient with a possible diagnosis of epilepsy and based on a history of events and likelihood that those events might have an epileptic origin, clinicians must ask themselves several questions before considering either aEEG or inpatient EMU-based video-EEG monitoring (VEM). First, if the patient’s symptoms are epileptic in origin, where is it likely to have origin? If the answer is that it is likely a focal onset seizure, the follow-up question should be where in the brain might that be? If the answer is from a site where routine EEG electrode arrays will see it electrically if it does have an epileptic origin, then aEEG might be helpful. If it is from deeper regions of the brain or areas that are difficult to see using the routine 10–20 electrode arrays, for example, hippocampi, insula, or mesial frontal regions, are there other electrodes that could be incorporated into the design of the recording montage that would address those specific needs? For example, it is reasonable to design a montage that records from parasagittal and midline regions where a mesial frontal or mesial parietal focus is under suspicion clinically or use a subtemporal electrode or chain of subtemporal electrodes where a mesial or inferior temporal focus is considered.

An equally important question, relevant for either the inpatient EMU or outpatient aEEG evaluation is, “How often and under what circumstances do the symptoms occur?” If symptoms occur in relationship to others events, for example, the menstrual cycle, or following sleep deprivation, one needs to consider the timing of such external events to maximize the possibility of recording a patient’s symptoms. The clinical behavior associated with the patient’s events may also need special consideration too, if one is going to successfully record them. For example, if the patient becomes violent or aggressive during or following an event, perhaps he or she should be studied under a more controlled condition such as in an EMU capable of handling such behaviors. The proper decision provides for both patient safety and good recording conditions. On the other hand, some events, such as those associated with life stressors, are more likely to be recorded in the outpatient environment due to the “anticonvulsive” effects of hospitalization (3).

A somewhat related concern regards exactly what level of evidence is acceptable to the clinician in order to satisfy the clinical diagnosis and hence the therapeutic approach to the patient. Will the clinician require that the patient have an “actual” event or will indirect evidence suffice, for example, the presence of focal or generalized interictal epileptiform discharges, periods of rhythmic focal slowing, or bursts of generalized frontal-central predominant rhythmic delta activity? If events occur relatively rarely or unpredictably, then more indirect evidence may not only be sufficient but may be the best one could get.

An equally important question to ask is, “What else could this be?” If the differential diagnosis includes cardiac or respiratory disorders, then is the recording system that one is using capable of doing good ECG monitoring or linking nighttime audio/video recording with SaO₂ monitors to assess sleep and snoring for potential sleep-related disorders and apnea? As we adapt more devices for monitoring other physiological variables to the basic EEG recorder systems (see Chapter 10), we are increasing our abilities to study and diagnose other possible disease states that may mimic seizures and epilepsy.

Generalized Seizures/Epilepsy

In the EEG workup of someone suspected of “absence seizures,” one would anticipate that the aEEG would be an ideal instrument for many reasons. The EEG discharges seen in this disorder are frequent and very distinctive. The disorder usually presents in childhood or early adolescence. For many reasons, this age group prefers an outpatient workup where appropriate. In fact, the vast majority of such patients with absence epilepsy can be diagnosed by the astute clinician during a routine clinical exam or by routine scalp EEG where a classical interictal 3-Hz spike-and-wave discharge can often be provoked (see Figure 5.1A). Ambulatory EEG still offers additional information that neither of the earlier two procedures can provide. With aEEG, one is able to quantify the frequency of events over the course of the day- or nighttime and, if coupled with take home audio/video recording, assess how often these brief discharges appear to be symptomatic. As shown in Figure 5.1B, the child was clinically diagnosed with absence epilepsy, and the treating physician had started her on appropriate medication. However, the family felt the young child was still having events. The mother was in attendance during long portions of this aEEG study and, in fact, was proven correct when she activated the recorder with a time locked audio/video recording multiple times for virtually every subtle event. She proved that she was an excellent observer who could recognize even minor events. However, during the recording session, the event shown in Figure 5.1B occurred when the mother looked away to sip some coffee, and the seizure detection algorithm captured this brief event and the clinical behavior, which strongly supported that the child was symptomatic during it. Thus, continuous aEEG monitoring detected and quantified the events that occurred independently of the parent’s close observations. This child had further medication adjustments based on these findings.

FIGURE 5.1 (A) This is a classic 3-Hz generalized spike-and-wave discharge and is the interictal marker for absence epilepsy.
(B) This was a “captured” seizure from the seizure detection algorithm on the patient where the mother was very attentive but yet still missed a symptomatic event.

In a related disorder, juvenile myoclonic epilepsy (JME), which usually presents in adolescence, routine EEG often does not capture the classical interictal markers, that is, generalized 4- to 6-Hz polyspike and wave (Figure 5.2A). For similar reasons to the absence seizure disorder mentioned previously, aEEG is more likely to capture at least the interictal discharges, which tend to occur more often at night while sleeping. Since clinically symptomatic seizure and myoclonus tend to occur during the first several hours following waking, aEEG coupled with home audio/video recording can often see evidence for subtle myoclonic jerks that have a distinctive EEG correlate but are often subconsciously incorporated into movements by the patient as noted in Figure 5.2B.

Related posts:

Reimbursement Issues in Ambulatory EEG The History of Ambulatory EEG—A Personal Perspective Ambulatory Sleep Monitoring Short-Term Ambulatory EEG Chronic Ambulatory EEG With Implanted Electrodes Artifact and Ambulatory EEG

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