Unilateral hearing loss

Unilateral hearing loss (UHL) or single-sided deafness (SSD) occurs when the hearing in one ear is within normal limits and the other ear has a hearing loss ranging from mild to profound. By Elizabeth Hough PhD, Mr Neil Donnelly and David Baguley PhD

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.

Otitis media with effusion may cause unilateral hearing loss

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

Prevalence
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

Aetiology
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

External ear

  • Wax
  • Foreign body
  • Otitis externa
  • Neoplasms (benign: exostoses and osteoma; malignant: squamous cell carcinoma)
  • Congenital aural atresia

Middle ear

  • Acute otitis media
  • Otitis media with effusion
  • Chronic secretory otitis media (mucosal or with cholesteatoma)
  • Otosclerosis (may become bilateral, may have sensorineural component)
  • Neoplasm (glomus tumour)

Sensorineural: affecting the cochlea or the VIII cranial nerve

Inner ear or VIII crnaial nerve

  • Trauma (physical, barotraumas, noise) VIII cranial
  • Infection (labarynthitis or meningitis) nerve
  • Neoplasm (vestibular schwannoma)
  • Congenital
  • Autoimmune
  • Neurological (multiple sclerosis)
  • Meniere's disease

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.

Diagnosis
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 MechanismFunctionEffect of Loss
Binaural
summation
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 noiseReduced ability to tune into a conversation and ignore background noise
LocalisationInteraural intensity
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

Management
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

References

1. Plack C J. The sense of hearing. Psychology Press, London, 2005.

2. Newman C W, Jacobson G P, Hug G A, Sandridge S A. Perceived hearing handicap of patients with unilateral or mild hearing loss. Ann Otol Rhinol Laryngol 1997; 106(3): 210-4.

3. Welsh L W, Welsh J J, Rosen L F, Dragonette J E. Functional impairments due to unilateral deafness. Ann Otol Rhinol Laryngol 2004; 113(12): 987-93.

4. Bess F H, Rothpletz A M, Dodd-Murphy J. Children with a unilateral hearing impairment. In: Newton V E (ed) Paediatric audiological medicine. Whurr Publishers, London, 2002.

5. Davis A C. Hearing in adults. Singular Publishers, San Diego, 1994.

6. Al Khabori M, Khandekar R. Unilateral hearing impairment in Oman. Ear Nose Throat J 2007; 86(5): 274, 277-80.

7. Baguley D M, Bird J, Humphriss R L, Prevost A T. The evidence base for the application of contralateral bone anchored hearing aids in acquired unilateral sensorineural hearing loss in adults. Clin Otolaryngol 2006; 31(1): 6-14.

8. Tharpe A M, Sladen D P. Causation of permanent unilateral and mild bilateral hearing loss in children. Trends Amplif 2008; 12(1): 17-25.

9. Burton M. Acquired hearing loss in adults. In: Graham J L, Baguley D M (eds). Ballantyne's Deafness 7th edition. Wiley/Blackwell, Oxford (in press), June 2009.

10. McFerran D J. Tinnitus. In: Graham J L, Baguley D M (eds). Ballantyne's Deafness 7th edition. Wiley/Blackwell, Oxford (in press), June 2009.

11. McLarnon C M, Davison T, Johnson I J. Bone-anchored hearing aid: comparison of benefit by patient subgroups. Laryngoscope 2004; 114(5): 942-4.

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