UHL may be congenital or present as a sudden or progressive hearing loss. It can be associated with other aural symptoms such as otalgia, tinnitus and vertigo.
Effect of UHL
Binaural hearing provides us with a number of advantages.1
When there is a hearing loss in one ear of greater than 20dB many of these advantages are lost (see box above right). This results in difficulty hearing in adverse listening situations such as in noise, group conversations, listening at a distance or in a reverberant environment.
Difficulty with localisation may compromise safety. In adults this can lead to reduced confidence and well-being.2,3 The implications are more serious for congenital or childhood UHL, which can lead to educational and psychosocial problems for some children.4
Most epidemiological studies have focused on bilateral hearing loss. In the UK around 10 per cent of adults have some degree of UHL.5
Studies investigating purely UHL in other countries have found a much lower prevalence of 3 per cent in adults, with conductive losses being more common than sensorineural hearing loss.6
Single-sided sensorineural hearing loss has an estimated incidence of 9,000 cases a year.7
The aetiology of UHL includes conditions that are not exclusively unilateral and may represent a bilateral condition with unilateral onset.
Conductive: affecting the transmission of sound through the external or middle ear
Sensorineural: affecting the cochlea or the VIII cranial nerve
Inner ear or VIII crnaial nerve
As with all types of hearing loss they can be classified as congenital or acquired, genetic or non-genetic and conductive or sensorineural. In children, up to 60 per cent of sensorineural UHL has unknown aetiology.8 The more common aetiologies in both adults and children are shown in the box below.
UHL requires a detailed medical history and clinical examination. Otoscopy may reveal the cause of a conductive hearing loss.
Tuning fork tests can help diagnose whether any loss is conductive or sensorineural. Pure tone audiometry measuring hearing thresholds will confirm whether there is unilateral loss.
When there is a large difference between the two ears it is necessary to use a masking noise. This prevents the good ear hearing the sound that has been presented to the bad ear, which may be transmitted across the skull and heard in the good ear.
The use of bone conduction with appropriate masking allows the type of hearing loss, conductive or sensorineural, to be identified in most cases. Tympanometry may also provide further information on middle ear status.
In view of the possible pathophysiological basis of the UHL, referral to ENT for investigation is almost always indicated.
Where there is a unilateral sensorineural hearing loss, it is vital to exclude the presence of a vestibular schwannoma or other neoplasm of VIII nerve or brainstem. The gold standard investigation is a gadolinium enhanced MRI scan.
Persistent unilateral middle ear effusion (see image) is uncommon in adults and a thorough examination of the nasopharynx is required to exclude a neoplasm.
|Effect of hearing loss greater than 20dB in one ear|
|Binaural Hearing Mechanism||Function||Effect of Loss|
|Sound is perceived louder when it is presented in both ears||Small loss in|
sensitivity, reduced, ease of listening
|Head shadow||The head attenuates sounds |
from the other side
|Difficulty hearing high|
frequency sounds on
affected side, especially
detail in speech
|Binaural squelch||The auditory system compares the two ears to reduce noise||Reduced ability to tune into a conversation and ignore background noise|
and timing differences are
used to localise sound
|Difficulty working out where speech and other sounds are coming from|
and their separation from other sounds
Most sensorineural UHL is permanent, but some degree of recovery may occur in idiopathic sudden sensorineural hearing loss.
There is evidence to support the role of systemic or intratympanic corticosteroid use in acute cases of sudden sensorineural hearing loss.9
Some conductive hearing losses can be treated surgically, which can often lead to improvement in hearing. In some cases the hearing loss is progressive and its impact can become more severe with the additive effects of age or noise-related loss.
Rehabilitation is the cornerstone of management in UHL and should aim to address both technical and psychosocial aspects of the problem.
A range of rehabilitation options exist to aid listening ability but none can restore true binaural hearing. For many patients counselling and communication advice will suffice, in others a device is indicated.
A conventional digital hearing aid may be suitable if there is good speech discrimination in the 'deaf' ear. If speech discrimination is poor, it will detract from the useful signals in the good ear. If the UHL is associated with tinnitus then a hearing aid may have some inhibitory benefit.10
With a contralateral routing of signal (CROS) hearing aid, a microphone worn at the bad ear sends a signal to a hearing aid worn on the good ear via a cable or, more recently, a wireless connection.
The use of a CROS aid overcomes head shadow. However, if an unwanted noise is on the poor side, this is also transmitted and can make speech on the good side harder to hear.
The use of a bone anchored hearing aid (BAHA) for SSD has been taken up by some clinicians.11 A vibrational hearing aid attached to an implanted titanium abutment behind the poor ear transmits sound via bone conduction to the good ear.
The quality of sound received is good, but the auditory benefit over the use of a CROS aid is slight. In particular, no benefit in localisation abilities has been demonstrated.
Patients with UHL require diagnostic and management expertise. The journey begins in primary care, with a careful approach to diagnosis and patient support, but is optimally provided in the multidisciplinary setting of an ENT and audiology department.
- Dr Hough is an audiological scientist, Mr Donnelly is a fellow in skull base surgery and Dr Baguley is a consultant clinical scientist and head of audiology at Cambridge University Hospitals NHS Trust Causes of unilateral hearing loss
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