Single Sided Deafness Options--Great article about Cros, Bicros, BAHA

Fitting Options for Adult Patients with Single Sided Deafness (SSD)
Michael Valente, Ph.D., Maureen Valente, Ph.D., & Karen Mispagel, M.S.

Michael Valente, Ph.D.
Audiology Online Contributing Editor for Adult Amplification
Director of Adult Audiology
Washington University School of Medicine

Maureen Valente, Ph.D.
Director of Audiology Studies
Washington University School of Medicine

Karen Mispagel, M.S.
Research/Clinical Audiologist
Washington University School of Medicine

Introduction

Adult patients with single sided deafness (SSD) oftentimes have difficulty a) localizing; b) understanding speech when arriving at the poorer ear; and c) understanding speech in background noise, especially when the noise arrives at the better ear. For these reasons, it is clear that patients with SSD can present a challenge to the audiologist. For adult patients with SSD, audiologists will typically provide counseling related to communication problems likely to occur and/or recommend Contralateral Routing Of the Signal (CROS) amplification to the better ear. In the past few years, however, several new fitting options have become available for patients with SSD. This paper will provide information for those audiologists exploring new fitting options for these patients.

Wired-programmable with directional microphone

The wired-analog aid gave way to a wired programmable CROS with omnidirectional and directional microphone options available (Figure 1). In Figure 1, the buttons on the remote could be programmed so that one button would activate omnidirectional performance for listening in quiet, while the second button could be programmed to activate the directional microphone for better performance in noisy listening situations. A third button could activate a telecoil for improved listening over the telephone. The remaining buttons serve as a volume control as well as an on/off switch. The presence of a directional microphone provides the possibility for greater speech recognition in noise than the omnidirectional microphone available in all current wireless models (Valente et al., 1995a).


Figure 1. Example of a wired CROS hearing aid with remote control. The hearing aid on the left is an off-microphone placed over the poorer ear. The hearing aid (with dual-microphones) on the right is placed over the better ear.

While CROS amplification effectively eliminates the head shadow effect by amplifying signals from the poorer side, localization and speech intelligibility in noise still remain a problem. Some CROS users report improved localization based on differences of sound quality from the two ears. If the signal appears “natural” it may be judged to be arriving from the better side. If the sound appears “tinny” or “metallic” it may be judged to be arriving from the impaired side (Harford, 1969). Also, if the level of ambient noise is high, few users of CROS amplification report any significant benefit regardless of the side from which the signal or noise may be arising. In these environments, it may be best to counsel the patient to reduce the volume control setting or remove the hearing aids (Hodgson, 1986; Harford, 1969; Pollack, 1988).

Wireless-analog

One of the major drawbacks of the earlier CROS systems was the need for a wire connecting the output from the microphone on the impaired ear to the receiver on the better ear. To solve this problem, one manufacturer introduced a wireless BTE to ITE CROS (Figure 2). Other models included a BTE to BTE version. This wireless CROS used an amplitude-modulated carrier frequency to transmit signals from the microphone on the side of the impaired ear to the receiver placed in the better ear. Distance between the transmitter and receiver is critical (approximately 6.5 inches). For every half-inch increase in distance between the transmitter and receiver, there is a 3-4 dB decrease in gain. Several years ago the manufacturer of this wireless technology was purchased by Phonak, who decided to continue the wireless CROS line.


Figure 2. Example of a wireless BTE to ITE CROS hearing aid. The hearing aid on the right is the transmitter side (placed over the poorer ear) and the hearing aid on the left is the receiver side (placed in the better ear). Note. Permission to use this photo provided by Phonak, Inc.

Clinically, a major drawback of these wireless CROS systems was the limited ability to shape the frequency-gain response to provide the prescribed gain to the aided ear. Most of these models were nonprogrammable, delivered with only a low frequency tone control or low and high frequency tone controls as a means to shape the frequency response.

Wireless-digital

In the past year, several manufacturers (e.g., Interton, Phonak and Unitron) re-introduced wireless BTE to BTE (Figure 3), BTE to custom (Figure 4), and custom to custom (Figure 5) CROS aids with multichannel digital signal processing. Due to significant advances in digital signal processing (DSP) over the past decade, these models have eliminated most of the shortcomings cited above for the original wireless models.


Figure 3. Example of a wireless BTE to BTE CROS hearing aid. The image to the left is a CROSLink FM transmitter, while the image to the right is a FM receiver attached to a direct auditory input (DAI) boot that can be coupled to BTEs from over 30 different manufacturers. Note. Permission to use this photo provided by Phonak, Inc.


Figure 4. Example of a wireless BTE to ITE CROS hearing aid. This figure illustrates how the CROSLink BTE (right) can be ordered to communicate with a custom hearing aid with an FM receiver (left). Note. Permission to use this photo provided by Phonak, Inc.

In Figure 3, the image to the left is a CROSLink FM transmitter, while the image to the right is an FM receiver attached to a direct auditory input (DAI) boot that can be coupled to BTE’s from over 30 different manufacturers. The reader can go www.phonak.com/croslink for BTE compatibility, audio shoe order numbers, CROSLink pin orientation and DPAI (designated programmable audio input) status. Figure 4 illustrates how the CROSLink BTE (right) can be ordered to communicate with a custom hearing aid with an FM receiver (left). Figure 5 is an example of an FM transmitter placed in a custom ITE aid (upper right). The transmitter sends the signal to an FM receiver (with volume control) within a custom product placed in the better ear (lower left).

Figure 5. Example of a FM transmitter placed in a custom ITE (upper right) that sends the signal to an FM receiver (with volume control) within a custom product placed in the better ear (lower left). Note. Permission to use this photo provided by Interton, Inc.

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