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Profound sensorineural hearing loss in one ear only, sometimes called single-sided deafness, has numerous causes, including idiopathic sudden hearing loss and the removal of an acoustic tumor. Although single-sided deafness has generally been considered a minor inconvenience when compared to hearing loss in both ears, the ability to hear in one ear only presents a number of listening disadvantages.

First, when the speaker is on the side of the non-hearing ear, the amplitude of high-frequency sounds critical for speech perception and understanding is reduced by as much as 20 dB SPL as they travel through and around the head to reach the hearing ear. This obstruction of sounds by the head, called the head-shadow effect, can also impair sound localization and distance estimation.

Second, the loss of hearing in one ear prevents the perceived doubling of loudness normally experienced through the binaural summation of sound energy coming into the two ears simultaneously.

Third, the ability to squelch background noises in favor of foreground speech is lost or diminished, making listening in noise especially difficult.

The communication difficulty presented by single-sided deafness was recognized more than 40 years ago by Harford and Barry, who proposed in l965 a unique hearing aid configuration, aptly-named CROS, which stands for Contralateral Routing of Signals.¹ In the conventional CROS fitting, a hearing-aid microphone placed on the side of the non-hearing ear transfers sound via wired or wireless means to a hearing-aid receiver and tubing connected to the opposite ear. In this way, the hearing ear receives sounds from both sides of the head. If the better-hearing ear needs aiding as well, a BICROS arrangement may be specified.

Although the CROS aid represents an innovative approach to solving some of the listening difficulties associated with single-sided deafness, the initial disadvantages were numerous and often discouraged prospective users from accepting the aids. Traditionally, the typical CROS aid required the user to wear behind-the-ear (BTE) hearing aids in both ears connected with a wire. The nuisance of wearing an aid in the hearing ear as well as the non-hearing ear was just too cumbersome and cosmetically unappealing for most listeners. According to Valente and colleagues, as few as 10 percent accepted traditional CROS aids even after a 30-day trial period.²

The acceptance rate of CROS aids has increased over the years with the advent of various configurations of digital CROS and BICROS hearing aids. For example, researchers at the Ear Research Foundation conducted a study of 91 patients with severe-to-profound asymmetric hearing loss to assess their satisfaction with such aids.³ Satisfaction was evaluated on the basis of the number of patients who elected to purchase their hearing aid following a 30-day trial and on the results of a subsequent eight-question survey.

Researchers found "overall patient satisfaction was generally high. At the end of the 30-day trail, 66 of the 91 patients (72.5 percent) elected to keep their CROS or BICROS device, a percentage that is far greater than the acceptance rates of 10 to 20 percent that had been previously reported with older models of the CROS and BICROS devices."

Those who kept their devices gave them an overall rating of 3.4 on a scale of 1 (very dissatisfied) to 5 (very satisfied).

In 2002, the Food and Drug Administration (FDA) approved the use of the bone-anchored hearing aid (BAHA) for use in cases of single-sided deafness. BAHA is a registered trademark of Entific Medical Systems of Sweden, owned by Cochlear Limited. The bone-anchored aid is a surgically-implanted system in the temporal bone consisting of a titanium implant, an external abutment and a sound processor. The system works by transferring sound vibrations from the processor through bone conduction to the hair cells of the cochlea, thereby stimulating the auditory nerve and cortex.

By implanting the BAHA within the temporal bone of the non-hearing ear, sound vibrations are transferred through the skull to the opposite, hearing ear by the process of transcranial bone conduction. That is, stimulation of the skull with a sound processor sufficient to vibrate bone on one side of the head will stimulate the other side equally as well.            

A casual reading of the available peer-reviewed medical literature suggests that the BAHA CROS aid has significant advantages over the traditional CROS, as evidenced by the following research conclusions:

• "There was consistent satisfaction with bone anchored hearing aid implantation and amplification, and poor acceptance of contralateral routing of offside signal amplification."⁴
• "Patients reported a significant improvement in speech intelligibility in noise and greater benefit from BAHA compared with the CROS hearing aids."⁵
• "The results showed a significant benefit with the BAHA in situations involving background noise and reverberation and a reduced aversion to loud sounds in comparison to the unaided and conventional CROS conditions."⁶
• "The speech-in-noise measurements reflect the benefit of a BAHA CROS in lifting the head shadow while avoiding some of the disadvantages of a traditional CROS."⁷
• "BAHA amplification on the side of the deaf ear yields greater benefit in subjects with monaural hearing than does CROS amplification."⁸

On the other hand, a recent meta-analysis of four controlled trials of the BAHA CROS versus the traditional CROS found "material shortfalls" in all studies and concluded, "There is a paucity of evidence to support the efficacy of BAHA in the treatment of acquired unilateral sensorineural hearing loss. Clinicians should proceed with caution and perhaps await a larger randomized sample."⁹

Based primarily on the above meta-analysis, the health insurance company Aetna considers the use of BAHA experimental and investigational in persons with single-sided deafness.¹⁰

Audiologists interested in reviewing current fitting options for patients with single-sided deafness, including the BAHA, will benefit from reading "Fitting options for adult patients with single sided deafness (SSD)," by Valente and colleagues.²  

References

1. Harford E, Barry J. (l965). A rehabilitative approach to the problem of unilateral hearing impairment:  The Contralateral Routing of Signals CROS. Journal of Speech and Hearing Research, 30: 121-38.

2. Valente M, Valente, M, Mispagel K. (2006). Fitting options for adult patients with single sided deafness (SSD). AudiologyOnline. Accessed online at www.adiologyonline.com/articles/pf_article_detail.asp?article_id=1629

3. Hill SL 3^rd, Marcus A, Digges EN, Gillman N, Silverstein H. (2006). Assessment of patient satisfaction with various configurations of digital CROS and BICROS hearing aids. Ear Nose and Throat Journal, 85(7): 427-30, 442.

4. Niparko JK, Cox KM, Lustig LR. (2003). Comparison of the bone anchored hearing aid implantable hearing device with contralateral routing of offside signal amplification in the rehabilitation of unilateral deafness. Otology & Neurotology, 24(1): 73-8.

5. Wazen JJ, Spitzer JB, Ghossaini SN, Fayad JN, Niparko JK, Cox K, Brackmann DE, Soll, SD. (2003). Transcranial contralateral cochlear stimulation in unilateral deafness. Otolaryngology- Head and Neck Surgery, 129(3): 248-54.

6. Bosman AJ, Hol MK, Snik AF, Mylanus EA, Cremers CW. (2003). Bone-anchored hearing aids in unilateral inner ear deafness. Acta Otolaryngologica, 123(2): 268-60.

7. Hol MK, Bosman AJ, Snik AF, Mylanus EA, Cremers CW. (2004). Bone-anchored hearing aid in unilateral inner ear deafness:  a study of 20 patients. Audiology & Neurotology, 9(5): 274-81.

8. Lin LM, Bowditch S, Anderson MJ, May B, Cox KM, Niparko JK. (2006). Amplification in the rehabilitation of unilateral deafness:  speech in noise and directional hearing effects with bone-anchored hearing and contralateral routing of signal amplification. Otology & Neurotology, 27(2): 172-82.

9. Baugley DM, Bird J, Humphriss RL, Prevost AT. (2006). The evidence base for the application of contralateral bone anchored hearing aids in acquired unilateral sensorineural hearing loss in adults. Clinical Otolaryngology, 31(1): 6-14.

10. Clinical Policy Bulletin:  Bone-anchored hearing aid. Last review: 12/11/07. Aetna. Accessed online at www.aetna.com/cpb/medical/data/400_499/0403.html

Jess Dancer is professor emeritus of audiology at the University of Arkansas at Little Rock. Contact him at jedancer@ualr.edu regarding your clinical experiences with CROS hearing aids.