• Clinical science

Hearing loss

Summary

Hearing loss can be defined based on the general type of hearing loss (conductive or sensorineural) and the location of the dysfunction. Conductive hearing loss typically occurs due to dysfunction of the outer or middle ear, which prevents transmission of sound waves from reaching the inner ear. Sensorineural hearing loss, on the other hand, occurs due to dysfunction of the inner ear or auditory nerve, which prevents neuronal transmission to the brain. Sometimes hearing loss can have both conductive and sensorineural components, which is referred to as mixed hearing loss. Patients with a complaint of hearing loss should be screened with techniques such as the whispered voice or finger rub tests. Subsequently, tuning fork tests should be performed to differentiate between conductive and sensorineural hearing loss. This may be followed by otoscopy, audiometry, laboratory tests, or imaging, depending on the underlying suspected cause. Treatment depends on the underlying etiology and can include hearing aids or cochlear implants for irreversible conductive or sensorineural hearing loss, respectively.

Overview

Conductive hearing loss Sensorineural hearing loss
Age of Onset
  • More commonly presents in childhood or young adulthood
  • More commonly presents in middle or late age
Etiology
Pathophysiology
  • External or middle ear pathology that disrupts conduction of sound into the inner ear
  • Inner ear, cochlear, or auditory nerve pathology that impairs neuronal transmission to the brain
Clinical features
  • Hearing improves in noisy environments
  • Volume of voice remains normal because inner ear and auditory nerve are intact
  • Sound normally is not distorted
  • Features of external auditory canal pathology (e.g., cerumen impaction)
  • Hearing worsens in noisy environments
  • Volume of voice may be loud because nerve transmissions are impaired
  • Tend to lose higher frequencies preferentially, such that sounds may be distorted
  • Absent features of external auditory canal pathology
Weber Test (unilateral hearing loss)
  • Lateralization to impaired ear (cannot hear ambient room noise well, so detection of vibration is greater)
  • Lateralization to unimpaired ear (sound is not transmitted by damaged inner ear or auditory nerve)
Rinne Test (unilateral hearing loss)
  • Bone conduction > air conduction (vibrations bypass blockage to reach the cochlea)
  • Air conduction > bone conduction (the inner ear or auditory nerve cannot transmit sound information well regardless of how vibrations reach the cochlea)
Speech audiometry
  • No discrimination loss.
  • Discrimination loss is common.
Audiogram
  • Difference between air and bone conduction.
  • Hearing loss for higher frequency sounds.

Impedance audiometry

Tympanometry
  • Normal or abnormal findings
  • Normal findings

Otoacoustic emission measurements
  • Undetectable
  • Absent in cochlear pathology
  • Present in retrocochlear pathology

Brainstem evoked response audiometry (BERA)
  • Normal latency (cochlear pathology)
  • Prolonged latency (retrocochlear pathology)
  • N/A

Tone Decay Test
  • Normal tone decay (0 - 5 Db)
  • Threshold loss of 15-25dB (cochlear pathology)
  • Threshold loss of > 30 dB (retrocochlear pathology)

Mixed hearing loss is a combination of conductive and sensorineural hearing loss!

Hearing loss in the first years of life can cause a delay in speech, language, and social development!

References:[1][2]

Recruitment

Recruitment (loudness perception) is a phenomenon of the inner ear or outer hair cells that allows individuals with normal hearing the ability to perceive quiet sounds at a higher level and loud sounds at a lower level. The diagnosis of recruitment helps to differentiate between cochlear and retrocochlear sensorineural disorders.
  • Principle
    • When the outer hair cells are intact, loud sounds are subdued and quiet sounds are enhanced. If the outer hair cells are damaged, there is decreased sound absorption, which lowers the margin between the auditory threshold and discomfort level. This makes quieter sounds difficult to hear while making louder sounds discomforting. This mechanism is known as recruitment.
  • Positive recruitment: In positive recruitment, loudness perception changes with an increasing level of loudness. If hearing differs in both ears, the increasing sound volume is heard identically in both the ear with a greater degree of hearing loss and the healthy ear.: in cochlear sensorineural hearing disorders (e.g., acoustic trauma)
  • Negative recruitment The ratio of loudness perception between the healthy and affected ear remains identical at each level of loudness. Hence, the affected ear hears less than the healthy ear, regardless of the volume. (e.g., retrocochlear sensorineural hearing disorders)

Perception of sound
Sound Volume
Whispering 30dB
Normal language 50dB
Motor 70dB
Sound becomes unpleasant above 90dB, the pain treshold is at approx. 120dB

Diagnostics

Initial diagnostic tests

Further diagnostic tests

References:[1][2]

Subjective audiometry

Audiogram (Pure Tone Testing)

  • Procedure: The patient is played various frequencies through a headphone (air conduction) and a bone oscillator (bone conduction) placed on the mastoid bones. Various frequencies are played and the patient gives a signal (e.g., raising a finger or pressing a button) as soon as a sound is heard. The auditory threshold is determined using the decibel level at which the patient's signals are 50% correct.
  • Interpretation: The connection of the individual auditory threshold points results in the generation of an auditory threshold curve for air and bone conduction, which allows the degree of hearing loss for individual frequencies to be determined.
    • In conductive hearing loss (damage to the middle or external ear), the auditory threshold is increased in air conduction; however, the auditory threshold is normal in bone conduction
    • In sensorineural hearing loss (cochlear or retrocochlear damage), the auditory threshold for air and bone conduction are proportionally increased.

Speech audiometry

  • Procedure: The patient is played increasingly loud words, which should be repeated by the patient. The speech reception threshold is calculated from the level at which a patient can correctly repeat 50% of words.
  • Interpretation: Increasing loudness eventually leads to a speech comprehension of 100% in patients with conductive hearing loss, but not in patients with sensorineural hearing loss. Loss of word comprehension is referred to as discrimination loss.

Tone decay test

  • Procedure: A sustained tone with a sound level of 5 dB above the previously determined auditory threshold is played in the affected ear. If the tone is no longer heard by the patient (→ hearing fatigue), the sound level is increased by a further 5 dB and is thereby audible to the patient. This procedure is repeated until a reduction in threshold of 30 dB is achieved or a sustained tone is heard for 1 min.
  • Interpretation:
    • Normal hearing: reduction in threshold up to 10 dB (physiological adaptation)
    • Cochlear damage: reduction in threshold of 15–25 dB (pathological adaptation) → positive recruitment
    • Neuronal damage (retrocochlear disorder): reduction in threshold > 30 dB

Békésy audiometry

  • Procedure: The patient is played a sustained sound via an automatic sound audiometer. The patient is required to press a button, once they hear a sound. As long as the button is pressed, the sound becomes continuously softer. If the patient is unable to hear a sound, the button is no longer pressed and the sound automatically becomes continuously louder. In doing so, the patient allows the sound to fluctuate above and below the sound threshold. The amplitude is recorded. The procedure is repeated with an impulse tone (= interrupted tone). As a result, there are two threshold curves: one for impulse tones and one for sustained tones.
  • Interpretation
    • Air conduction is only used. Interpretation of the results can only be made in regards to cochlear and retrocochlear sensorineural hearing disorders.
      • Normal hearing
        • Waves of ∼ 10–15 dB, approximate overlay of the sustained and impulse tone curves
        • Fewer dB of greater volumes are required to hear sustained tones than for pulse tones (by physiological adaptation)
      • Cochlear damage
        • Waves are increasingly smaller < 5 dB –1 dB (evidence of a disturbance in loudness perception → recruitment)
        • Separation of the continuous tone curve from the impulse tone curve to a maximum 30 dB (subsequently followed by a constant gap)
      • Retrocochlear damage
        • No reduction in amplitude
        • Increasingly strong separation (deviation) of the continuous tone curve from the impulse tone curve by ≥ 50 dm (up to the upper limit of the audiometer)

A damaged auditory nerve is unable to maintain excitation in long, sustained tones, which is demonstrated by an elevation in the auditory threshold with increasing time!

High-frequency audiometry
  • Procedure
    • ABLB test (the test has more of a historical value nowadays)
      • Procedure: Sounds of different volumes are played into both ears until the patient has the impression of a constant sound level in both ears. This procedure is repeated with increasingly higher levels of sound.
      • Interpretation
        • In patients with unilateral conductive hearing loss or unilateral retrocochlear sensorineural hearing loss, the ratio of loudness perception is identical in both the healthy and affected ear for each sound level, which means that the patient hears less from the affected ear. → negative recruitment
        • In patients with cochlear sensorineural hearing loss, loudness perception alters with increasing levels of sound, which means that the patient will hear the same loudness from both ears. → positive recruitment.
    • SISI test Sensitive test for measuring an increase in volume
      • Procedure: A sustained sound of 20 dB above the individual hearing threshold is increased every 5 s by 1 dB for an impulse from 0.2 s. The patient should attempt to identify as many impulses as possible.
      • Interpretation
        • Patients with cochlear sensorineural hearing loss of over 40 dB are able to identify the impulses (60–100% test positive: graded as positive recruitment)
        • Patients with retrocochlear sensorineural hearing loss only rarely identify the impulses (0–15% test negative: graded as negative recruitment)
        • Individuals with normal hearing cannot identify small changes in intensity of 1 db.

Objective audiometry

Overview

Objective audiometric methods include:

The individual methods are presented below.

Impedance audiometry

Tympanometry

  • Description: In tympanometry, the reflected sound from the tympanic membrane is measured by applying various pressures to the external auditory canal. This information can be used to estimate mobility of the tympanic membrane and the pressure in the middle ear, which, e.g., may provide evidence of Eustachian tube dysfunction or secretory otitis media.
  • Procedure: Entry to the auditory canal is sealed by a probe and is airtight, which can manipulate the pressure in the external auditory canal and emit various sound frequencies. The sound reflected from the tympanic membrane provides information on the compliance of the tympanic membrane and is measured as a function of the generated positive and negative pressure (between +300 Pa and -300 Pa). Results are recorded on a tympanogram.
  • Interpretation

Stapedius reflex measurement
  • Short description: measurement of the stapedius reflex to assess the mobility of the auditory ossicles. This tests measures if and when the reflex is triggered and at which sound volume.
  • Principle: The inner ear is protected from loud sounds through contraction of the stapedius muscle (stapedius reflex), which modifies the oscillation of the stirrup bone. When the reflex is preserved, a sudden increase in resistance (impedance) can be expected at the tympanic membrane.
  • Procedure: A headphone is placed onto the ear and sound is played (initially 70 db). The sound is increased in 5 db increments. A probe is placed in the auditory canal and the change in impedance is measured using the microphone of the probe in the contralateral ear.
  • Interpretation: An absent stapedius reflex indicates otosclerosis.

Electric response audiometry (ERA)

  • Description: The electric response audiometry is a broader term for methods used in the measurement of auditory evoked potential, which can be recorded in an EEG. Topographic information on the location of the hearing disorder can be made by a comparison of the duration using standard tables. Among the standard procedures of ERA are BERA and CERA.
  • Indications
    • Suspected retrocochlear disorders
    • Objective auditory threshold measurement in children or uncooperative patients (also suitable for newborn hearing screening)
  • Procedure: Auditory stimuli are presented to the patient through a headphone. Early, middle, and late potentials (depending on the procedure) are generated by a lead electrode on the mastoid and vertex.
  • Standard methods
    • Classification of the methods depends on the time of onset of auditory evoked potentials after auditory stimuli.
      • Brainstem evoked response audiometry (BERA)
        • Deflection of the brainstem auditory evoked potentials: 1.5–10 ms after auditory stimulation
        • Acoustic potential of the BERA is represented in the form of five waves. Waves 1 and 2 correspond to the proximal part of the cochlear nerve. Waves 3, 4, and 5 reflect the course of the auditory pathway in the brainstem.
      • Cortical evoked response audiometry (CERA)
        • Deflection of the late and very late auditory evoked potentials: 100–300 ms or 300–1,000 ms after auditory stimulation

Otoacoustic emissions (OAE)
  • Short description: OAE are sound emissions originating from the cochlea that arise spontaneously or in response to acoustic stimulation. They can be measured in the auditory canal. A function test of the cochlear can be made based on the measurements. There are two types of OAE:
    • Spontaneous OAE (SOAE): physiological, acoustic sound emissions that are emitted from the outer hair cells of the ear and retrogradely reach the auditory canal via the auditory ossicles and tympanic membrane
    • Transient evoked OAE (TEOAE): OAE, which are triggered in response to short acoustic stimuli
  • Indication: method of choice in newborn hearing screening (TEOAE)
  • Procedure: OAE measurement in the external auditory canal through recording with extremely sensitive microphones
  • Interpretation
    • If OAE are measurable, there is normal outer hair cell function.
    • OAE are mainly absent in hearing loss >30 dB.

Differential diagnoses

Conductive hearing loss

External auditory canal atresia

Definition: congenitally absent or stenotic external auditory canal

Cerumen impaction

  • Definition: buildup of tightly packed cerumen (earwax) in the outer ear .
  • Risk factors
    • Anatomic deformity and/or increased number of hairs in external auditory canal
    • Barriers to wax extrusion (e.g., use of earplugs, hearing aids)
  • Clinical findings
  • Diagnostics: otoscopy
  • Treatment
    • Irrigation (e.g., warm saline containing a bacteriostatic substance; cerumenolytics can be added as well)
    • Cerumenolytics (e.g., docusate sodium)
    • Mechanical removal (e.g., with forceps, curette, or suction)

References:[3]

Always consider the possibility of cerumen impaction in patients presenting with hearing loss!

References:[4][5][6]

Sensorineural hearing loss

Presbycusis

  • Definition: age-related, sensorineural hearing loss
  • Pathophysiology: progressive damage of the organ of Corti, especially near the basal turn of the cochlea, which impairs high-frequency hearing
  • Epidemiology:
  • Clinical features
    • Progressive bilateral hearing loss, particularly of higher frequencies → Using a low-pitched and clear voice to speak with older patients can improve communication.
    • Usually first noticed in the sixth decade of life
    • Difficulty hearing in noisy, crowded environments.
    • Can cause depression and/or isolation
  • Treatment

References:[7]

Noise-induced hearing loss (NIHL)

  • Definition: hearing loss that occurs due to exposure to loud sound
  • Risk factors: repeated exposure to sounds louder than 85 dB or even a single exposure to sounds greater than 120–155 dB.
  • Pathophysiology
  • Clinical features
    • Slowly progressive hearing loss with loss of high-frequency hearing first
    • Difficulty hearing in noisy, crowded environments
    • As it progresses, difficulty hearing high-pitched voices (eg, women's, children's) occurs
  • Treatment
    • No definitive treatment is available
    • Prophylaxis is essential (hearing protection)

References:[8]

The differential diagnoses listed here are not exhaustive.

Treatment

Hearing aids

  • Definition: devices that amplify sound to assist individuals with impaired hearing
  • Prerequisite: All patients should undergo a thorough ENT-examination to rule out treatable causes and an audiological examination to determine the severity of hearing loss.
    • Children:
      • Causes of hearing loss that can be treated surgically include tumors, cholesteatomas, and stenosis/atresia of the external auditory canal
      • Pressure equalization tubes in the tympanic membrane can improve hearing in children with hearing loss from recurrent otitis media, middle ear effusions, or dysfunction or the eustachian tube.
    • Adults: Most adults experience age-related hearing loss which is not treatable; other etiologies should nonetheless be investigated.
  • Indications
    • Both conductive and sensorineural hearing loss (regardless of severity)
    • Mild to severe hearing loss
      • Children should be fitted with a hearing aid as soon as possible to avoid a developmental delay in speech.

Cochlear implants

Bilateral cochlear implants can improve speech discrimination in background noise.

  • Definition: Prosthetic devices that are surgically implanted and function by electrical stimulation of the auditory nerve (CN VIII).
  • Prerequisite: the auditory nerve and auditory system are intact.
  • Indications

References:[1][2][9]