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JukkaKokkonen

Interpretation of an Audiogram and Impaired Hearing

Quality requirements of hearing examinations

  • Reliable hearing examinations require adequate equipment and surroundings as well as qualified personnel.
  • The examinations should be performed in acoustically isolated conditions; a diagnostic or clinical audiometer is required.
  • The examiner should be familiar with air and bone conduction measurements and the use of a masking noise. A recorded speech signal for speech audiometry is often needed 1 2 3 4.

Grade of hearing impairment

  • According to the EU working group classification, hearing impairment is classified according to the average hearing level in the better ear in the so called speech frequencies (BEHL0.5-4kHz = better ear hearing level over the frequencies 0.5-4 kHz; table T1) 5.
  • The grade of hearing impairment is usually classified according to the hearing thresholds in the better ear. However, unilateral profound hearing impairment causes a considerable disability even if this is not considered in any of the above classifications. Particularly hearing in a noisy environment, perception of speech and localization of sounds are difficult. According to the Finnish assessment guidelines, unilateral deafness may be defined as a class 3 disability (15% disability). When the difference of the average hearing levels in the frequency range from 0.5 to 4 kHz is at least 35 dB between the ears, the patient is placed one class higher than what the better ear would suggest.
  • For WHO grades of hearing impairment, see http://www.who.int/pbd/deafness/hearing_impairment_grades/en/index.html.

Grade of hearing impairment according to EU working group classification

Grade of hearing impairmentEU working group (1996)
Mild hearing impairment20 dB < BEHL0.5-4 kHz< 40 dB
Moderate hearing impairment40 dB ≤ BEHL0.5-4 kHz< 70 dB
Severe hearing impairment70 dB ≤ BEHL0.5-4 kHz< 95 dB
Profound hearing impairmentBEHL0.5-4 kHz≥ 95 dB

Conductive or sensorineural hearing impairment?

  • Tuning fork tests are often highly valuable in the assessment of the nature of the hearing impairment.
    • In a conductive hearing impairment, the Weber test is lateralizing towards the defective ear (picture 1) and the Rinne test is abnormal (negative; picture 2).
    • In a sensorineural hearing impairment, the Weber test is lateralizing towards the healthy ear and the Rinne test is normal (positive).
  • Conductive and sensorineural hearing impairment can only be defined by audiometry if both air and bone conduction thresholds are measured.
  • The patient has conductive hearing impairment if bone conduction thresholds are normal (20 dB or better) and the difference between air and bone conduction thresholds is at least 15 dB as an average over the frequencies 0.5 to 2 kHz (picture 3).
  • In sensorineural hearing impairment, bone conduction thresholds are worse than 20 dB and the difference between air and bone conduction thresholds is less than 15 dB (picture 4).
  • In combined hearing impairment both components are present. Bone conduction thresholds are worse than 20 dB and and the gap between air and bone conduction thresholds is at least 15 dB.
  • Bone conduction thresholds can never be significantly poorer than air conduction thresholds. Because the maximal output levels of audiometers are always smaller (softer) for bone conduction measurements than for air conduction measurements, the contribution of conductive impairment cannot be assessed in severe or profound hearing impairments.

Causes of conductive hearing impairment

  • Conductive hearing defects are practically always of the stiffness type, with the greatest impairment at low frequencies.
  • Otitis media Acute Otitis Media in Adults
    • The most common cause of transient hearing impairment
  • Large perforation of the tympanic membrane
    • The hearing impairment is usually greatest at low frequencies.
  • Otosclerosis Otosclerosis
    • Initially conductive hearing impairment is evident at low frequencies, later the hearing impairment spreads to higher frequencies.
    • Conductive hearing impairment may reach 60 dB. A sensorineural component may also be present.
  • Mass type conductive hearing impairment (at high frequencies) is rare. It may be caused by, for example
    • an atrophic tympanic membrane
    • a disruption of the ossicular chain.

Causes of sensorineural hearing impairment

  • Presbyacusis
    • Always a sensorineural hearing impairment
    • Usually, the audiogram slopes more or less steeply towards high frequencies.
    • The speed of progression varies.
  • Noise-induced hearing loss (NIHL; see also Acute Acoustic Trauma)
    • In mild NIHL, the hearing impairment is generally greatest at 4 kHz (3-6 kHz; the so-called 4 kHz dip).
    • An advanced NIHL may spread to the mid frequencies, but pure chronic NIHL never affects the low frequencies.
    • No audiometric findings confirm the diagnosis of NIHL without a history of sufficient noise exposure. Other causes of a similar hearing impairment must be excluded. Some gene defects, for example, may be associated with an audiometric finding that resembles very much that of noise-induced hearing loss.
  • Ménière's disease (see also Ménière's Disease)
    • The hearing impairment is usually sensorineural and fluctuates in the early phase.
    • Initially the impairment is seen at the low frequencies, and at times the hearing may be quite normal.
    • A transient hearing impairment at the low frequencies observed during an acute attack may be a valuable clue to the diagnosis.
    • At a later phase, a flat or even a high frequency hearing loss develops. At the latest stage the hearing does not usually fluctuate or the fluctuation is very limited.
  • Sudden deafness
    • The aetiology is unknown. There are probably several causes.
    • The hearing impairment is sensorineural and may have any shape.
    • Mild acute hearing loss confined to low frequencies has the best prognosis. Corticosteroid treatment may be beneficial Oral Steroids in Idiopathic Sudden Sensorineural Hearing Loss 7 but there are varying opinions on the benefits of the different treatments. In a fresh study the benefits of corticosteroid treatment have been questioned 8.
    • An indication for immediate referral
    • Emergency referral warranted: if the symptoms have started during, for example, diving, blowing one's nose, physical exercise or air travel, the patient may have a rupture of the cochlear membranes.
  • Prolonged otitis media (see also Chronic Otitis Media)
    • See the passage 'Perforation of the tympanic membrane and prolonged otitis media' above in the chapter Causes of conductive hearing impairment.
    • If an acute hearing deterioration develops in a patient with chronic otitis media or cholesteatoma, the patient may have a cochlear complication requiring urgent treatment.
  • Vestibular schwannoma (acoustic neuroma, tumour of the 8th cranial nerve)
    • The usual finding is a slowly progressing unilateral sensorineural hearing impairment.
    • The hearing impairment is often greatest at high frequencies.
    • However, any type of unilateral sensorineural hearing impairment (even acute or fluctuating) may be associated with vestibular schwannoma.

Reliability of an audiogram

  • Audiograms are not always totally reliable, particularly if economic profit, e.g., in the form of insurance benefits or job security are associated with a certain grade of hearing impairment.
  • Noise carried from the outside to the examination room, mental alertness of the examined patient and the positioning of the headphones affect the results.
  • As a rule of thumb, differences of 10 dB or less are not significant considering the limits of measuring accuracy.
  • Assessment of audiometry results
    • The more qualified the examiner, the more reliable the results
    • The distance from the examiner during conversation gives a good impression of hearing.
    • The behaviour of the patient gives clues about possible exaggerated hearing impairment.
    • Air and bone conduction thresholds must be in accordance with each other (the hearing impairment entitling the patient to financial support to be reimbursed is usually sensorineural).
    • Speech audiometry is particularly valuable to confirm the results of pure tone audiometry.
  • Despite air and bone conduction measurements it is sometimes difficult to distinguish a conductive component in the hearing impairment. Tuning fork tests are often useful in such cases.

Indications for specialist consultation

  • Principally unilateral or asymmetric hearing defect suggests an ear disease, the cause of which should be investigated.
  • The result of audiometry is an indication for referral in the following cases:
    • there is a need for rehabilitation Technical Rehabilitation of Hearing Impairment. As a rule of thumb, the average threshold in frequencies 0.5-4 kHz is 30 dB or worse in the better ear (20 dB in children)
    • there is a possibility of a serious disease (complication of chronic otitis media, vestibular schwannoma, etc.)
    • the patient has an ear disease requiring treatment (secretory otitis media, otosclerosis, acute loss of hearing).

    References

    • ISO 6189. Acoustics - Pure tone air conduction audiometry for hearing conservation purposes. International Organization for Standardization. Geneva 1983.
    • ISO 8253-1. Acoustics - Audiometric test methods - Part 1: Basic pure tone air and bone conduction threshold audiometry. International Organization for Standardization. Geneva 1989.
    • ISO 8253-2. Acoustics - Audiometric test methods - Part 2: Sound field audiometry with pure tone and narrow-band test signals. International Organization for Standardization. Geneva 1989.
    • ISO 8253-3. Acoustics - Audiometric test methods - Part 3: Speech audiometry. International Organization for Standardization. Geneva 1989.
    • EU Work Group on Genetics of Hearing Impairment. In: Martini A (ed.). European Commission Directorate, Biomedical and Health Research Programme Hereditary Deafness, Epidemiology and Clinical Research (HEAR). EU Work Group 1996, Infoletter 2 http://audiology.unife.it/www.gendeaf.org/hear/infoletters/Info_02.PDF.
    • Nosrati-Zarenoe R, Hultcrantz E. Corticosteroid treatment of idiopathic sudden sensorineural hearing loss: randomized triple-blind placebo-controlled trial. Otol Neurotol 2012;33(4):523-31. [PubMed]

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