Audiologists realize that the loss of hearing in one ear, single sided deafness (SSD), is more common and more troublesome than most people realize. SSD can be caused by a number of things, from viral infections or thrombi to brain tumors and is currently incurable and difficult to treat. While the resulting hearing symptoms of SSD vary from one patient to another, people with hearing loss on one side generally experience difficulty with localization or determining the direction of sounds.
Poor localization can make it dangerous to cross a street or otherwise navigate in traffic or elsewhere. For example, if the person is deaf in the left ear, sound from the left side will be perceived by the person as coming from the right, and sound from the right ear will seem as it comes from the right. So it is easy to be confused as to where a sound is coming from, especially when crossing a street or attempting to localize other sounds. Additionally, there is substantial research indicating those with SSD have significant difficulty hearing in noisy situations, especially when the noise is coming from their hearing side. The noise becomes part of the speech and very difficult to separate.
While there are no exact statistics on the number of individuals world wide with SSD there are about 60,000 people in the U.S. acquire single-sided deafness (SSD) every year and many of these individuals attempt to manage their hearing deficit without the benefit of a number of beneficial and readily available devices. People that use these devices report that listening is easier and life is more comfortable with a device than without one.
Years ago audiologists had to use a hearing aid attached to a bone conduction oscillator on a headband (A) to treat these individuals but technology. Technology has evolved to the point that this is not necessary and in most countries as we now have sophisticated Contralateral Routing of Signals (CROS) Bilateral Contralateral Routing of Signals (BICROS) (B) there is a hearing loss on the good hearing side). These are manufactured by traditional hearing aid companies and they collect the signal from the deaf side and broadcast that signal to the a device that presents that sound to the patients good ear. If there is normal hearing on the good hearing side the CROS version is used , if there is impaired hearing on the good side then the BICROS version is used. While the person still does not hear from the deaf ear they are aware of sound on that side. Another option in 2016 could be the TransEar (C) which works on the principle that loud sound will stimulate the other ear by bone conduction if it is loud enough and, thus, the person can be aware of the sound on the deaf side. A popular surgical option is a bone anchored hearing aid (D) where a post is implanted into the mastoid process of the deaf ear and a device is mounted on that post. This concept stimulates the auditory system on the good side via bone conduction and allows the person to be aware of sound on the deaf side.
Another option is a device that was introduced a few years ago called Sound Bite that offered a behind the ear transmitter coupled to a dental appliance that seemed to beneficial, but the company went out of business in 2015. While there is always someone that could bring this novel technology back into the mx of treatments there is also some speculation that there may be a use of cochlear implants for SSD as they develop in the future.
New Research in Single Sided Deafness
A new discovery could help people with SSD find a treatment quicker. quicker treatment that those that are traditionally proposed for the disorder. When looking at the various treatments for SSD, a major obstacle to discovering the best treatment has been the current lack of biomarkers against which to measure the efficiency of the treatment. A group of scientists from the University of California, San Francisco (USA) headed by Dr. Sikantan Nagarajan and Dr. Steven Cheung have been studying brain plasticity in response to SSD. Brain plasticity is the ability of the brain to modify its own structure and function in response to changes, such as disease, within the body or external factors. It is at the base of normal brain function: it helps us to learn and change our behavior as children, and as adults can help us to overcome brain injuries, use prosthetic limbs and of course, as well as many other things – including hearing devices. With the development of new methods for studying brain plasticity more is being learned about how the brain works and changes as well as how to use this knowledge to our advantage. Their study used multi-modal imaging combining several brain imaging techniques to review the changes in the brain allowing the scientists to review how the brain reorganizes itself during learning and disease.
In their study, Drs. Nagarajan and Cheng and the group used magnetoencephalographic (MEGI) and fMRI scans to study the auditory cortices of 26 subjects, 13 individuals with SSD and 13 normal hearing in both ears. Within the brain there are two auditory cortexes, one for each hemisphere and each is arranged according to a tonotopic map, i.e., they are arranged according to the frequencies of sound to which they are the most sensitive. So, neurons activated by one frequency of sound, are found near to neurons activated by a similar frequency.
The 26 subjects in the University of California study were exposed to sounds of different frequencies as the researchers monitored the areas of activation in their auditory cortexes. The researchers observed the spread of neuron activation across the tonotopic maps in both brain hemispheres, and noticed significant differences between the two subject groups.
The spread of cortical activation was symmetrical across the hemispheres of the brain in normal-hearing subjects, while in those with the single sided deafness the spread of neuron activation was extended in one hemisphere and reduced in the other. This discovery demonstrates plasticity in both hemispheres of the brain in SSD sufferers, and is an important step toward the possible development of biomarkers that will assist in the guidance of treatment choices. Ultimately, it may even be possible to use this plasticity to develop therapies to cure the condition: by using brain stimulation as a process to restore a normal interhemispheric relationship. Additionally, it may be that scientists may be able to restore normal auditory processing, returning SSD sufferers to a life less affected by their hearing and communication handicap.
Chang, J., Pross, S., Findlay, A., Mizuiri, D., Henderson-Sabes, J., Garrett, C., Nagarajan, S., & Cheung, S. Spatial plasticity of the auditory cortex in single-sided deafness. The Laryngoscope, Vol 10 (1002).
Hear-it.org (2004). Single sided deafness-SSD-Single sided hearing loss. Retrieved September 27, 2016.
Humphries, C., Liebenthal, E. & Binder, J. (2010). Tonotopic organization of the human auditory cortex. Neuroimage, 50(3), pp. 1202-1211. Retrieved September 28, 2016.
Bernafon (2016). Images from Web Site. Retrieved September 28, 2016.
Neuroplasticity (2016). Image of Brain. Retrieved September 28, 2016.
Phonak (2016. Images from CROS/BICROS. Retrieved September 28, 2016.
TransEar (2016). Images from Web Site. Retrieved September 28, 2016.
Widex (2016). Image from Cros brochure. Retrieved September 28, 2016.