Instrumentation and Polygraphic EEG

CHAPTER 3

INSTRUMENTATION AND POLYGRAPHIC EEG

ELIZABETH WATERHOUSE, MD

INTRODUCTION

Advances in digital EEG technology have improved ambulatory EEG (aEEG) recordings in numerous ways: expanding the number of channels available, lengthening the recording time, allowing recording of multiple physiological parameters, and reducing the size and weight of the equipment for easier portability. Some aEEG systems now include a portable video camera that can be set up in the patient’s home to record video-EEG.

INITIAL AEEG SYSTEMS

The first commercially available aEEG relied on analog technology to record 24 hours of up to four EEG channels on a cassette tape. Single-channel preamplifiers were attached to the scalp close to the electrodes, in an effort to reduce movement artifacts (1). Experienced electroencephalographers may recall devices of the 1980s, when signal multiplexing allowed expansion to 16 channels, improving localization capability and spatial resolution. Due to limitations in cassette tape storage capacity, data reduction was necessary. Combining the recorder with a portable computer enabled real-time processing. Rather than continuous EEG, stored data included spike and seizure detections, EEG sampled at regular intervals, and time sequences surrounding patient event marker activations (2).

Over subsequent decades, instrumentation included digital technology that has overcome the technical and quality challenges of early aEEG systems. Current aEEG recordings are comparable to those recorded onsite at an EEG laboratory or video-EEG monitoring unit. Various commercially available systems now offer high-quality lightweight multifunctional systems for monitoring EEG and polysomnography at home.

CURRENT AEEG SYSTEMS

Currently available aEEG devices allow for 24 to 34 channels, and thus have the capacity to record at least as many channels as are routinely recorded in the hospital EEG laboratory and video-EEG monitoring unit. Studies indicate that 16-channel recordings with computer-assisted seizure detection are better at capturing ictal events than older systems with eight or fewer channels (3,4). Ambulatory EEG montages should include a minimum of 19 standard scalp electrodes and one channel for EKG. Additional electrodes that are desirable include ear electrodes and electrodes to monitor eye movements. Polysomnogram recording requires fewer scalp electrodes and uses additional channels for limb movements, electromyogram (EMG), eye movements, oximetry, thoracic respiratory effort, and nasal air flow.

RECORDING THE AEEG

Patient Instructions

The technologist should ensure the appropriate care of the patient and aEEG device. Patients should not bathe with the device or expose it to water. They should avoid chewing gum, which causes prolonged intervals of phasic myogenic artifacts during chewing. The technologist should demonstrate and test the use of the event marker (“push button”) and advise the patient how to document routine activities as well as seizures or clinical events on the activity diary. If video is used, the patient must know how to turn the camera on and off.

Ambulatory EEG systems record and store a minimum of 24 hours, with some models now offering up to 96 hours of continuous recording. Several factors limit recording time. The integrity of the electrode–scalp interface degrades over time as the electrode paste dries out, leading to decreased conductance and increased number of artifacts. Battery life is a second potentially limiting factor. Patients may need to be instructed in changing or recharging batteries. Those undergoing prolonged studies may need to return to the laboratory for electrode maintenance and a battery change.

Scalp Electrodes

In most cases, a standard 10–20 array of electrodes is applied using collodion, an electrode cap, or other secure technique. Collodion technique is followed by wrapping the scalp with gauze. To reduce traction on the scalp electrodes, the lead wires are gathered and tacked. Home polysomnography systems utilize scalp electrodes and chest strain gauges held in place with adjustable elastic straps.

An aEEG is frequently obtained after a routine or sleep-deprived EEG has failed to capture the desired diagnostic information. The prolonged recording period of an aEEG increases the diagnostic yield, and the addition of special electrodes may further aid in diagnosis and localization. True anterior temporal electrodes are placed 1 cm above the point at 1/3 of the distance between the outer canthus of the eye and the preauricular point. Subtemporal electrodes may also be used (5). Additional options for specific localization include zygomatic or sphenoidal electrodes, although these are not routinely used.

Eye Movements

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