Objective Measures in Cochlear Implants (Core Clinical Concepts in Audiology)

1 The Basics of a Cochlear Implant INTRODUCTION As children receive cochlear implants at increasingly younger ages, the use of objective measures for clinical management becomes ever more important. For the purposes of this book, “objective measures” encompass two general areas: (1) nonphysiological measures (i.e., device function and current fields), and (2) physiological (neural) measures. Objective measures are used to serve a number of purposes: To verify device function, To identify malfunctioning electrodes, To verify the integrity and function of the auditory pathway, To obtain a baseline of neural function for tracking potential changes over time, To assist in programming the cochlear implant sound processor, To measure discrimination of different stimuli, and To measure the plasticity of the auditory system. The first step in learning about evoked potentials with cochlear implants is to gain a solid understanding of: (1) the limitations of the impaired auditory system, (2) how
2 Signal Delivery   This chapter begins with a brief overview of the differences between channels and electrodes, followed by more in-depth descriptions of the various types of signals, stimulus timing, electrode configurations, and electrode designs that are used both clinically and experimentally with cochlear implant recipients. It is essential to have a clear understanding of the types of signals that cochlear implants can employ so that one can better understand how manipulating these signals can affect both perceptual and objective measures with electrical stimulation. CHANNELS VERSUS ELECTRODES The terms “channels” and “electrodes” are often used interchangeably, but they actually refer to two different things. An electrode is the physical structure that injects current into the tissue, whereas a channel is the resulting distinct field of stimulation. Some people further define “channel” as the resulting field of stimulation that produces a distinct percept. An example is shown
3 Electrode Impedance INTRODUCTION The next three chapters describe different types of tests that are primarily used to assess the function of the internal device, including that of the individual electrodes: Chapter 3 describes electrode impedance, Chapter 4 describes electric field imaging, and Chapter 5 describes averaged electrode voltages. These assessment tools are designed to measure electrode-specific voltage, impedance, and electrical field (voltage) patterns across the array. As such, these tests can also be used to gain insight into properties of the surrounding tissue, the electrode-tissue interface, and the path of current flow. In contrast to the measures described in Chapters 6 to 10, the tests described in Chapters 3 to 5 are not used to assess the physiological functioning of the auditory pathway. Depending on the specific tools available to the clinician, one or more of these tests may be used to assess device function for the primary purposes of making decisions abou
4 Electrical Field Potentials   Chapter 3 discussed the clinical importance of electrode impedance measures for assessing the function of intra- and extracochlear electrodes. Although the access resistance and reactance components can provide some insight into the properties of the electrode-nerve interface and surrounding medium, clinical impedance measurements do not provide comprehensive information about these specific components or about spatial spread of current throughout the cochlea. This chapter describes a tool that is presently used with Advanced Bionics (AB) devices (called Electrical Field Imaging and Modeling, or EFIM) and MEDEL devices to assist with calculating impedance for monopolar stimulation, and to generate voltage tables for constructing intracochlear electrical field potentials. In this chapter, electrical field potentials are defined, the method for measurement is described, and the clinical uses are discussed. BASIC DESCRIPTION Electrical field potentials are
5 Averaged Electrode Voltages   As with impedance measures (Chapter 3) and electrical field imaging (Chapter 4), averaged electrode voltages (AEVs) can be used to assess the function of the internal device and individual electrodes. Because impedance measures and electrical field imaging use intracochlear electrodes to measure voltages, these measures can only be made with devices that have reverse telemetry capabilities. Reverse telemetry allows for transmission of the measured voltage or impedance information back across the skin to the processor, then to the processor interface, and finally to the computer. AEVs are far-field measurements (recorded with scalp electrodes) of the artifact associated with stimulating an intracochlear electrode; therefore, AEVs can be measured with devices that either do or do not have reverse telemetry capabilities. This chapter begins with a basic description of what AEVs are, how they are measured, and what normal patterns should look like. Examples
6 Electrically Evoked Stapedial Reflexes INTRODUCTION TO PHYSIOLOGICAL OBJECTIVE MEASURES The final five chapters of this book each describe physiological measures from different levels of the auditory system in response to electrical stimulation through a cochlear implant. Electrically evoked stapedial reflexes (ESRs) are described in Chapter 6, electrically evoked compound action potentials (ECAPs) in Chapter 7, electrically evoked auditory brainstem responses (EABRs) in Chapter 8, electrically evoked auditory middle latency responses (EAMLRs) in Chapter 9, and electrically evoked auditory cortical potentials in Chapter 10. These assessment tools are designed to measure various aspects of auditory responses through an implant, which can provide information regarding behavioral thresholds and comfort levels, spread of excitation within the cochlea, channel interaction, binaural interaction, neural maturation, and objective measures of stimulus discrimination. This chapter begins with
7 Electrically Evoked Compound Action Potential   Of all the physiological potentials covered in this book, the electrically evoked compound action potential (ECAP) is probably the most widely used in the clinical setting. Reverse telemetry, which allows for ECAPs to be recorded quickly and easily without the need for surface/scalp electrodes, was first commercially available in the United States in 1998 when the Nucleus CI24M was introduced. Since that time, all cochlear implant manufacturers with FDA approval in the United States have introduced devices that are equipped with reverse telemetry systems. This chapter begins with a basic description of what ECAPs are and how they are measured. Common artifact reduction methods are explained, and a discussion of the different types of measurements that can be made with ECAPs is included. Finally, some of the challenges associated with measuring ECAPs are discussed, and a summary of the clinical uses for ECAPs is provided. BASIC DESCRIPTI
8 Electrically Evoked Auditory Brainstem Response   Although the clinical use of the electrically evoked auditory brainstem response (EABR) has waned with the advent of telemetry systems for measuring the electrically evoked compound action potential (ECAP), the EABR offers several advantages over the ECAP (Miller et al., 2008). First, EABRs can be obtained in a wider population of implant users because the measures are not dependent on the implant having telemetry capabilities. Second, EABRs can provide information about the auditory pathway up to the level of the brainstem. Third, EABRs can often be recorded in cases when excessive stimulus artifact precludes successful acquisition of ECAPs, such as in ossified cochleae. Finally, the primary wave of interest of the EABR, wave V, occurs at a later latency than the ECAP, and is therefore easier to isolate from the stimulus artifact. This chapter begins with a basic description of what EABRs are and how they are measured. Next, challeng
9 Electrically Evoked Auditory Middle Latency Response   Although more central physiological responses are not used as widely in clinical applications with cochlear implant recipients, these potentials are useful for providing information about physiological maturation at higher levels of the auditory system. Compared to the ECAP (see Chapter 7), EABR (see Chapter 8), and cortical potentials (see Chapter 10), relatively little has been published on the use of the electrically evoked auditory middle latency response (EAMLR) in cochlear implant recipients. This chapter begins with a basic description of what EAMLRs are and how they are measured. Next, different types of measurements with the EAMLR are described, and challenges associated with measuring these potentials are discussed. Finally, a summary of the clinical uses for the EAMLR is provided. BASIC DESCRIPTION The EAMLR is a synchronous physiological response from the upper brainstem, thalamus, and auditory cortex (Pratt, 2007). T
10 Electrically Evoked Auditory Cortical Potentials INTRODUCTION Central (cortical) physiological responses are useful for providing information about central auditory pathways, stimulus detection, perceptual discrimination, and/or physiological maturation at higher levels of the auditory system. One advantage that auditory cortical potentials have over more peripheral measures (ECAP, EABR) is that a wider range of stimuli can be used to elicit responses. The benefit is that it is possible to objectively evaluate the brain\'s ability to detect or discriminate different stimulus characteristics, such as loudness differences, temporal changes, or speech tokens (for example, differentiation of /ba/ versus /da/). The term “cortical auditory evoked potentials” (CAEPs) is a relatively generic term that encompasses several specific types of auditory responses that differ based on how stimuli are presented and whether or not the listener attends to the stimuli. These potentials include the elec