By: Joan D’Alessandro, Au.D.
Doctor of Audiology
Advanced Hearing & Balance Care
30 South Valley Rd., Ste. 208
Paoli, PA 19301
(610) 296-5857
www.hearingcare4u.com
Hearing loss in one ear can result from several disorders or injuries:
- Bacterial infections
- Head injury
- Vascular (blood supply) problem
- Meniere’s disease
- Viral infection (herpes)
- Acoustic or 8th nerve tumor
- Acoustic trauma (sudden loud noise to one ear)
Patients who suffer a severe to profound hearing loss in one ear report no comprehension difficulty in quiet, one-one-one conversations. However, understanding conversation in less than ideal listening situations is severely compromised. These include noisy areas such as restaurants, group conversations and reverberant areas such as churches, gyms, and indoor swimming pools.
Why are these areas so challenging for those with unilateral deafness? Your ears individually and collectively gather acoustic information and relay those signals to the auditory reception centers in your brain. With only one ear collecting information, the auditory centers are starved for the complete acoustic picture normally provided by two ears. When this information is lacking, a number of things happen that impair your ability to organize and make sense of speech.
Head Shadow Effect
When a sound occurs toward the deaf ear, the arrival of that sound at the hearing ear is partially blocked by your head, commonly known as the head shadow effect. The sounds most easily blocked are the higher pitches which are absolutely necessary for accurate perception of the voiceless consonant sounds of speech, the s,c,f,t,p,ch and sh sounds. These consonants allow us to tell the difference between words like teach versus feast and distillery versus facility. This word discrimination ability is compromised by hearing with only one ear because of the head shadow effect.
Localization Ability
Your brain needs well-balanced sound information from both ears for you to be able to easily pick out the direction from which a sound originates. Both a sound’s time of arrival (it arrives a few milliseconds faster at the ear closest to the sound) and its intensity or loudness (louder for the ear closest to the sound) are cues that your brain uses to locate the source of a sound. When you have only one ear, all sounds seem to be coming from the side of the good ear, even when originating from the bad ear side. So someone with hearing in only one ear lacks the instantaneous locating cues of time and intensity. If someone calls your name outdoors, you have to turn your head around to locate the source and it may take several repetitions to accurately pinpoint the source.
Sound Summation and Noise Squelch
To your brain, one ear + one ear = three! A sound which is barely audible at 20 feet away with only one ear, is easily audible at 30 feet when two ears are listening. This is called the binaural summation effect and is the result of the two hearing nerves crossing many hundreds of times before the information reaches the cortex. This crossing or decussation results in the enhancement of sound, so softer sounds become audible.
In addition, this neural sharing contributes to an advantage, known as binaural squelch, when listening in background noise. This ‘squelch’ provides 3 decibels more signal than noise, also known as the signal –to- noise ratio or SNR. Even though three decibels may not seem like much, the decibel is a logarithmic function much like the Richter scale for earthquake measurement. For every 3 decibels, sound power actually doubles. So three decibels extra is a significant amount and allows for easier understanding in noisy situations.
Solutions for Those with Only One Hearing Ear
The traditional hearing aid solution for unilateral hearing loss has been the use of a CROS hearing aid. The acronym stands for Contralateral Routing of Sound and employs a unit on the dead ear which contains a microphone to pick up sounds arriving on that dead side. Those sounds are then transmitted wirelessly to a receiver unit on the good side. The sounds from the dead side are then routed into the good ear.
This CROS type aid prevents the head shadow effect but does nothing for localization ability and does not provide binaural summation or squelch.
If the hearing loss is in the severe category, a traditional hearing aid either in a behind-the-ear(BTE) or in-the-ear style can be fit. As with all hearing aids fittings there is an accommodation period during which the patient learns to use the acoustic information from the damaged ear. The sooner the fitting is done, the more likely the patient will be a successful hearing aid user.
A fairly recent surgical solution for single-sided deafness (SSD) is the implantation of a BAHA or Bone-Anchored Hearing Aid. This surgical implant uses bone conduction to send sounds from the bad ear side to the good ear via an implant with a microprocessor that adheres to a magnet under your scalp. The processor is detachable for showering, etc., and is easily hidden underneath your hair. Again, the BAHA does not accomplish true binaural hearing but does lessen the head shadow effect and some recipients do notice improved localization ability after use of the BAHA for some time.
Our brains are wired to receive information from both ears. Loss of hearing in one ear is a significant handicap with which many people cope through the use of CROS type hearing aids, BAHA implant and positioning themselves to maximize the best sound reception to the hearing ear.
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