Understanding Unilateral Hearing Loss: Causes, Impact, and Treatment

There is an increasing global recognition of the negative impact of hearing loss, and its association to many chronic health conditions. Hearing loss is the third leading cause of disability globally, with approximately 466 million people living with disabling hearing loss around the world. The societal impact is high. Disabling hearing loss has long been associated with bilateral age-related changes.

However, disability and handicap in those with unilateral hearing loss have been shown to match or exceed those that are reported in individuals with bilateral hearing loss, despite hearing with only one normally functioning ear. These estimates, however, do not take into account the proportion of adults with undiagnosed or unaddressed hearing loss, such as those with unilateral hearing impairment.

Unlike other paired systems, such as vision, where the impact of unilateral impairment is readily acknowledged, hearing is subject to an invisibility factor, where the disability itself is less overt and subsequently underappreciated. Over time, such additional stressors result in auditory fatigue and reduced performance at work. As such, the impact of SSD is often underestimated. Increased effort is required to compensate for unilateral hearing loss in complex listening environments.

Accumulating evidence points to the loss of spatial perception as the driving factor in functional disability for those with unilateral hearing loss. Profound unilateral sensorineural hearing loss, often termed single-sided deafness (SSD), refers to clinically-unaidable hearing, as defined by severe-to-profound hearing thresholds with a poor word recognition ability. Acquired unilateral hearing loss occurs in 12-27 per 1,000,000 persons annually.

The etiology of SSD is widespread, including such pathologies as cochleovestibular abnormalities, temporal bone trauma, Meniere’s disease, vestibular schwannoma, vascular ischemia, autoimmune disorders and infections, although it is commonly idiopathic in nature. Often, this loss can be sudden in onset, leaving the patient extremely debilitated. Despite this, prevailing misperceptions that one normal hearing ear is sufficient for daily communication persists. Long overlooked are the deficits and disability associated with SSD, which have substantial but differential impacts on the affected.

The Impact of Unilateral Hearing Loss

Asymmetry in... Most people with hearing loss have what's known as "binaural" hearing loss, meaning both ears are affected. This is known as unilateral hearing loss or single-sided deafness, depending on the severity of the hearing loss. Having diminished or no hearing in one ear can present unique challenges that can be more pronounced the more severe the hearing loss is.

You can't always pinpoint where a sound is coming from: Your brain knows where a sound is coming from by which ear receives the sound first, known as sound localization or directional hearing. You may struggle to hear in noisy environments: Your brain is in charge of selective listening, meaning, filtering out noises that aren't useful. This is tougher to do without the aid of a second ear. You may have a harder time telling how loud a sound is: The brain “hears” a sound more loudly when it’s perceived through both ears than if the same sound at the same decibel were only perceived through one ear. You may struggle to multi-task: Because of all of the above, single-sided deafness increases cognitive load on the brain, leading to listening fatigue. The more noise there is, the longer it takes your brain to focus on the tasks at hand.

Yes. Patients with hearing loss in one ear, and specifically those with single-sided deafness, commonly experience tinnitus in one ear. This occurs frequently because the brain receives little to no sound input from the affected ear and may generate “phantom” sounds in response. Tinnitus is also commonly associated with hearing loss in general, so people with mild to moderate hearing loss in one ear may experience ringing or buzzing as well.

The functional deficits of profound unilateral hearing loss have a substantial impact on socialization, learning and work productivity. In recognition of this, rehabilitative solutions such as the rerouting of signal and hearing implants are on the rise. The increased hearing handicap in this group is most strongly linked to deficits in spatial perception.

Spatial Perception and Hearing

Spatial Hearing Explained

Spatial perception is multisensory and multifaceted. The auditory system plays a particularly important role, helping to map where we are in space by continuously sensing auditory events. The constructs of auditory space are complex, depending on our interaction with signals that are dynamically changing in terms of frequency spectrum, level and time. The more complex the listening environment, the more these signals interact and overlap. Therefore, hearing is not just about sound detection or awareness, but also about managing these complex interactions to provide meaning to those signals.

Our auditory system is quite sophisticated in wholly managing the acoustic information presented to our ears in dynamically changing environments. Human listeners are able to rapidly process this information to identify and orient to acoustic stimuli, selectively attending to signals of importance while suppressing competing signals. Spatial hearing is dependent on the processing of monaural and binaural hearing cues (Figure 1A,B). While monaural spectral-shape cues provide important information regarding elevation and contribute to our ability to determine the distance of a sound source, binaural hearing cues play a much larger role in spatial hearing abilities (Figure 1D).

The integration of acoustic information from both ears is essential for spatial hearing, and serves to provide critical information for speech processing, localization, the segregation of auditory streams and the perception of fused sounds. Binaural hearing gives rise to a wide array of auditory phenomena due to the integration and processing of differences in arrival time and intensity between the signals at the two ears. The interaural timing difference (ITD) is the difference in arrival time for a stimulus to reach both ears (Figure 1D); it is greatest for low frequency signals below 1000 Hz.

Sounds presented directly to the front of a listener have an ITD of 0 μs, increasing in time as the signal moves laterally in the horizontal plane, with the largest ITD occurring for signals presented ±90° azimuth, reaching ~600 μs. Due to the extreme binaural sensitivity of this cue, when a sound is presented to a listener, the ear closest to the signal of interest will detect that sound before the ear farthest from the signal. Likewise, the interaural level difference (ILD), or the difference in the intensity of a stimulus reaching both ears, dictates that the ear closer to a stimulus will receive a more intense signal compared to the contralateral ear (Figure 1B).

As the signal deviates away from 0° azimuth in either direction, the ILD increases. As with ITDs, ILDs are a frequency-dependent cue, with level differences increasing as a function of frequency, reaching values of 20 dB of attenuation or more. This is due to the acoustic shadow created by the head. The head acts as a physical barrier to sounds, resulting in an attenuation of the signal in the ear not directed at the source.

Sounds arriving at the ears from different locations in space allow listeners to take advantage of spatially separated sounds to improve the signal-to-noise ratio (SNR). For example, the ear closest to the signal benefits from the shadow created by the head to block noise that would otherwise mask the signal. Furthermore, the integration of the input from the two ears results in a summation of the signals leading to perceived enhancement. In addition to boosting the target signal, a reduction of competing noise occurs with binaural hearing. This is known as the squelch effect, where the processing mechanism takes advantages of amplitude and phase differences of the inputs as they arrive at the two ears to suppress competing noise. Collectively, these binaural processes provide listeners with a 4-10 dB benefit in processing speech in complex environments.

Challenges for Individuals with Single-Sided Deafness (SSD)

These essential interaural hearing cues are unavailable to those with SSD. The reduced ability to discriminate ILDs and ITDs results in primary deficits in speech understanding in noise and localization. As monaural listeners, they are left to rely on the normal hearing ear to process all of the incoming acoustic information, thereby losing the ability to segregate spatially separated streams of sound or take advantage of spatially separated signals in complex listening environments. This is further complicated by reduced access to high frequency speech cues as a result of the acoustic head-shadow.

For monaural listeners, access to sound at the normal hearing ear is both frequency- and direction-dependent. As can be observed in Figure 1C, low frequency information can be well detected even when directed at the deafened ear. Low frequency signals are characterized by long wavelengths, which allow them to easily wrap around the head to stimulate the normal hearing ear. Conversely, high-frequency pinna cues are diffracted by the head, making them virtually undetectable when presented at the side of the deafened ear.

The auditory system’s ability to adapt to the lack of binaural cues to preserve spatial hearing abilities as much as possible is remarkable. This process is achieved by reweighting the available cues, exploiting the location-dependent monaural cues. However, in a normal every-day acoustic scenario, sounds are constantly changing in level, location and frequency, which makes it impossible to have accurate spatial hearing based only on monaural cues.

Losses in gain for deaf-ear listening progressively increase as a function of frequency, reducing access to high-frequency phonemes that give rise to speech intelligibility and discrimination. The resulting outcome is a disruption in speech perception, which is further amplified in the presence of competing noise. Specifically, the parts of speech that provide listeners with the ability to distinguish one word from another are inconsistently available to monaural listeners.

Normal hearing listeners are highly accurate in their ability to localize sound in space, whereas this is largely disrupted in monaural listeners due to the lack of available ITD and ILD cues. This hinders the ability of a monaural listener to accurately and rapidly orient themselves to signals of interest or importance. A number of functional deficits arise from the inability to localize, such as issues regarding safety as they navigate the world around them, the inability to locate the talker, target confusions in multi-talker situations, and an overall sense of uncertainty in complex listening environments.

Treatment Options for Unilateral Hearing Loss

Depending on the cause, hearing loss in a single ear can sometimes be temporary. Common reversible causes include earwax blockages, middle-ear fluid from an infection, or sudden pressure changes (like when flying). Hearing loss in one ear can develop quickly, and when it does it's known as sudden sensorineural hearing loss (SSHL). It’s important to act fast if you or a loved one notice any change. The sooner you seek treatment, the better your chances of a full recovery. Steroid treatment, when started early, can often restore at least some hearing.

Treatment options for mild to moderate unilateral hearing loss will be different than those for severe hearing loss or single-sided deafness.

• CROS: A contralateral routing of sound (CROS) hearing aid system helps people with almost total hearing loss in one ear and normal hearing in the other. It picks up sounds on the deaf side and sends them to the functional ear. This requires two devices: a transmitter on the non-functional ear and a receiver on the functional ear.
• BiCROS: BiCROS hearing aids work the same way, except they're designed for people whose functional ear has moderate-to-severe hearing loss. It's important for patients to realize these devices re-route sound, but do not "restore" hearing to the deaf ear.
• Bone-anchored hearing systems: Some people don’t find CROS hearing aids helpful. An alternative is a bone-anchored hearing system, which requires surgical implantation. These devices, also called bone-anchored auditory implants, transmit sound vibrations through the skull directly to the inner ear. This is especially useful when ear canal or middle-ear problems block sound, making standard hearing aids ineffective.
• Cochlear implants: A cochlear implant is a small electronic device that can provide a sense of sound for people with severe to profound hearing loss. It bypasses the inner ear and directly stimulates the auditory nerve.

If you notice hearing loss in one ear, the first step is to get a thorough evaluation.

Sudden Unilateral Hearing Loss (SUHL)

Sudden unilateral hearing loss (SUHL) is an abrupt and unexplained hearing loss in one ear. The loss of hearing is usually severe and occurs within hours to days. It can be caused by several factors:

1. Viral infections such as mumps, measles, and influenza can cause sudden hearing loss.
2. Autoimmune diseases like lupus, rheumatoid arthritis, and multiple sclerosis can cause sudden hearing loss.
3. Sudden hearing loss can be attributed to inner ear disorders, such as Meniere’s disease and acoustic neuroma.
4. Trauma to the head or ear can also cause sudden hearing loss.
5. Various medications, including antibiotics, diuretics, and chemotherapy drugs, have been known to potentially result in sudden hearing loss.

The manifestation of symptoms in patients with SUHL may exhibit variability based on its severity. However, common symptoms include an abrupt onset of hearing loss in one ear and a sensation of fullness or pressure in the ear.

The treatment of SUHL depends on the underlying cause. In most cases, the cause of SUHL is unknown, and the treatment is supportive.

1. Antiviral drugs may be recommended in cases where the aetiology of SUHL is attributed to a viral pathogen.
2. Hyperbaric oxygen therapy entails inhaling pure oxygen within a chamber that is pressurised.
3. If hearing loss is permanent, hearing aids can improve hearing.
4. A cochlear implant may be recommended if hearing loss is severe. This is a medical apparatus invasively inserted into the cochlea of the inner ear.

Sudden unilateral hearing loss (SUHL) is a medical emergency characterised by a sudden and unexplained loss of hearing in one ear. It can be caused by viral infections, autoimmune diseases, inner ear disorders, trauma, or certain medications.

Do you need help hearing conversations clearly or constantly asking people to repeat themselves? Get the expert hearing care you need at Country Hearing Care. If you’re having trouble hearing, our professional team can provide you with a hearing test and a customised treatment plan to help.