People with unilateral hearing loss face challenges in localizing sounds and understanding speech in noise, but various treatments can improve sound awareness and potentially even restore the benefits of binaural hearing.
The loss of hearing in one ear occurs more often and poses more problems than most outside the hearing healthcare community appreciate. Every year, about 60,000 people in the United States acquire single-sided deafness, a condition in which they have nonfunctional hearing in one ear and do not clinically benefit from amplification in that ear, while the other ear functions normally.
The causes of single-sided deafness vary. They include acoustic neuroma, a benign tumor developing on the nerve that connects the ear to the brain; sudden idiopathic hearing loss, which is commonly due to viral infection; blunt trauma to the head; vascular insults that damage the auditory pathway; congenital loss of hearing; and Ménière's disease, a disorder that affects balance and hearing, resulting from the buildup of fluid in part of the inner ear.
In people with normal binaural hearing, the auditory system compares timing and intensity signals from the two ears to help locate sounds. Sounds that come from the left side of space reach the left ear sooner than the right ear and are higher in amplitude in the left ear than the right ear due to the obstruction of sound waves by the head.
Because of the so-called head-shadow effect, higher-frequency sounds with shorter wavelengths tend to be reflected by the head. Given that consonants are higher-frequency sounds that contain much of the meaning of speech, the head-shadow effect makes it difficult for people with single-sided deafness to understand speech in the presence of background noise.
“Normal binaural hearing reflects a remarkably complex interaction between our two ears, with highly intricate brain computations and representations of their inputs to compose what we might consider a soundscape,” said John K. Niparko, MD, professor and chair of the Department of Otolaryngology–Head & Neck Surgery at the Keck School of Medicine of the University of Southern California.
“However, multiple talkers in large-room environments challenge the discrimination of speech sounds. This is a commonly encountered situation where we want our speech-recognition abilities to be at their best, but they actually function at their worst, especially in single-sided deafness.”
Unilateral hearing loss not only can mean that it's difficult to carry on a conversation at a cocktail party, but it can also make crossing a busy street dangerous.
Still, many people with single-sided deafness try to manage without the benefit of available devices, while others seek help because problems with spatial hearing and speech recognition affect their quality of life.
“We know that some patients have adapted to their environment and their activities such that single-sided deafness is not a major impairment,” Dr. Niparko said.
“For the majority of single-sided deafness patients, however, there is measurable disability.”
ASSESSING THE IMPACT
For those patients who experience a significant decrease in quality of life, there are various treatment options.
“The best first step is to understand fully the cause of the single-sided loss by combining the findings from a full otologic and head-and-neck examination with audiologic testing and radiographic studies of the temporal bone and skull base,” Dr. Niparko said.
“Once the etiology of the single-sided deafness is established, the impact of the loss should be determined,” he said, adding that it's important to understand the needs of the patient with respect to the challenging listening environments he or she encounters.
In addition to a complete audiologic assessment, evaluation for single-sided deafness should include laboratory testing to rule out inflammatory or infectious causes, and an MRI scan of the internal auditory canal and brain using the contrast agent gadolinium to exclude a vestibular schwannoma, said Anil K. Lalwani, MD, professor and vice chair for research in the Department of Otolaryngology/Head & Neck Surgery at Columbia University Medical Center.
If sudden hearing loss is the cause of single-sided deafness, oral and intratympanic steroids should be considered, he added.
“Clinicians should not underestimate the consequences of single-sided deafness, which is associated with significant functional impairment and psychological consequences,” he said.
In cases of unilateral hearing loss, patients should receive speech-in-noise testing, education for their families on currently available treatment options, and questionnaires to assess the impact of the hearing loss, said Lori A. Garland, MS, an audiologist at Cincinnati Children's Hospital Medical Center.
“Most people with mild unilateral hearing loss or single-sided deafness don't really know how to describe or determine the impact of their hearing loss.
“Putting the questions into real-life terms helps prod one's memory of difficult situations, such as listening in noise and hearing from a distance. Some questionnaires can also open a dialogue about the social and emotional effects of living with single-sided deafness.”
TAKING A DIFFERENT ROUTE
One way to treat single-sided deafness is by rerouting sounds from the bad ear to the good ear.
For example, in the Phonak CROS (contralateral routing of signal) system, a transmitter placed behind or inside the poorer ear picks up sound and transmits it wirelessly to the normal hearing ear. For those who have hearing loss in the better ear, the receiving hearing aid in the Phonak BiCROS system serves as an amplifier to improve hearing in that ear.
Bone-conduction devices also can reroute signals from one side of the head to the other. For example, Ear Technology Corporation's TransEar consists of a custom shell that fits in the ear canal and contains a miniature oscillator, which makes contact with the bony portion of the ear canal. When sound is received on the side of the nonfunctioning ear, it's converted to mechanical energy that drives the oscillator, and those signals are then transferred via the bones of the skull to the cochlea in the opposite ear.
Patients who are candidates for surgical options can also consider bone-anchored implants. The underlying principle behind bone-anchored hearing systems is osseointegration, the process by which bone cells attach to the titanium implant to form a firm and permanent anchor.
For example, Oticon Medical's Ponto implant and abutment are placed behind the ear to anchor the sound processor, which captures sound waves and converts them into vibrations. These signals are then transmitted through the bone via the abutment and implant, and carried directly to the inner ear, thus bypassing any problems in the outer or middle ear.
In Cochlear Baha Systems, a sound processor converts sound into vibrations and transfers them through an abutment or magnet to an implant, which transfers the vibrations through the skull to the inner ear, bypassing the non-hearing side.
Bone-anchored implants also can be used in patients who have a medical contraindication to the use of traditional amplification, such as microtia, atresia, or chronically draining ears.
ROLE OF COCHLEAR IMPLANTS
Bone-anchored hearing systems, as well as specialized hearing aids that enable the contralateral routing of signals or sounds, can restore sound awareness in patients with single-sided deafness, Dr. Lalwani said.
“However, they provide no actual benefit in sound localization and limited improvement in speech perception. Neither the CROS/BiCROS hearing aids nor the bone-anchored hearing systems restore hearing in the deafened ear.
“In contrast to technologies for contralateral routing of signal, cochlear implantation offers the possibility that the deaf ear could again hear and restore the benefit of binaural hearing.”
Currently, cochlear implants are indicated for bilateral sensorineural hearing loss, but they are not approved by the U.S. Food and Drug Administration for people with single-sided deafness. However, recent studies have suggested that cochlear implantation can benefit those with single-sided deafness by improving speech understanding and sound localization, and suppressing tinnitus.
In addition, cochlear implants can reduce the negative emotional effects caused by unilateral hearing loss, Dr. Lalwani said.
“Each of these [benefits] alone offers a sufficient rationale for implantation. There is now a small body of literature that provides the initial justification for further study of the impact of cochlear implants in patients with single-sided deafness.
“However, prospective studies are needed to overcome the shortcomings of published retrospective studies.”
It's likely too early to tell which patients with single-sided deafness cochlear implants would help.
“Although cochlear implants may provide the most significant improvement for sound localization and listening in noise, the outcomes are variable, and the benefit is still difficult to predict for children,” Ms. Garland said.
CHALLENGES IN YOUNG PATIENTS
Children with single-sided deafness require special considerations. For example, untreated single-sided hearing loss in this patient population can lead to functional and developmental problems.
These children can experience speech and language delays, and difficulty paying attention in school. Kids with unilateral hearing loss are more likely to repeat a grade than their normal-hearing peers.
However, it is not clear why some children with unilateral hearing loss have significant academic difficulties and others do not. Studies are currently investigating the reasons for these performance differences, both in adults and children.
“The known variability in each individual's ability to adapt makes it challenging to define the impact beyond listening in noise and loss of sound localization,” Ms. Garland said.
For children with single-sided deafness, current treatment options include a bone-anchored auditory implant, such as Baha or Ponto; a CROS system; and the use of a frequency-modulation system in school.
Baha indications for children are similar to those for an adult, although children younger than five years can be fitted with the Baha Softband until they are old enough to receive an implant. Similarly, the Ponto Softband is a potential solution for children who are too young to have an implant or whose hearing problems are temporary.
Very young children usually do not receive implants because their bone structure is immature and osseointegration may not take place in the same time frame as it would in adult bone.
Personal frequency-modulation systems are like miniature radio stations operating on special frequencies. The system consists of a microphone that the speaker wears and a receiver that the person with hearing loss wears.
The speaker's voice is picked up by the transmitter and is wirelessly sent to the receiver, helping to minimize unwanted noise and maximize the teacher's voice in the classroom. This device can be used with a CROS system or a bone-anchored implant in older children.
From a developmental perspective, waiting to find solutions for children with single-sided deafness does not make sense. The brain's ability to hear in noise is not present at birth and develops over time during childhood when both ears are providing auditory input to the brain.
“Since early identification and intervention have been proven to benefit the brain's ability to adapt, it is important to determine when and what intervention is most appropriate for babies and young children,” Ms. Garland said.
Recent advances and successes in the management of single-sided deafness include improved wireless CROS systems, better options for bone-anchored devices, and the consideration of cochlear implants, according to Ms. Garland.
She also noted that clinicians are now paying more attention to the single-sided deafness population as a whole, and are becoming better at assessing the needs of these patients and their treatment options.
David Baguley, PhD, MBA, head of the Audiology Department at Addenbrooke's Hospital in England, expressed a similar view.
“I think there is very much greater awareness of the problems with single-sided deafness amongst both audiologists and otologists, and amongst the patient community,” he said. “This means that many more patients are getting good advice and appropriate interventions.”
But there still is a need for more accurate evaluation tools to assess sound localization and other hearing abilities, which would help determine the consequences of various treatments, and additional research on the relative efficacy of currently available devices, Ms. Garland noted.
“Right now, there is no perfect solution. But we continue to forge ahead to dispel the old belief that one ear is good enough, and continue to seek improved solutions for the families we serve.”
LINK TO EXTRA INFORMATION:
* American Speech–Language–Hearing Association (ASHA): Unilateral Hearing Loss http://bit.ly/ASHA-Unilateral
* ASHA: Unilateral Hearing Loss in Children http://bit.ly/ASHA-Children
* Cochlear: Baha System http://bit.ly/Cochlear-Baha
* Ear Technology Corporation: TransEar http://bit.ly/TransEar
* MED-EL: Maestro http://bit.ly/MED-EL-Maestro
* Oticon Medical: Ponto http://bit.ly/Oticon-Ponto
* Phonak: CROS http://bit.ly/Phonak-CROS