Hearing tests

Hearing impairment may be very variable in severity but can cause severe communication difficulties leading to profound educational, social and psychological problems. As well as ensuring effective screening for problems in children, it is essential to consider and arrange appropriate tests for any child or adult with possible hearing impairment. There is a high rate of occult hearing loss in the elderly, who report sadness, depression, social isolation and insecurity; physical, emotional, mental and social well-being all improve once hearing aids are used¹.

Other relevant separate articles include Hearing Problems and Deafness in Children.

Methods of hearing assessment in infants and young children are discussed in the separate Hearing Testing and Screening in Young Children article.

The hearing level is quantified relative to 'normal' hearing in decibels (dB), with higher numbers of dB indicating worse hearing. Hearing loss can be graded as follows²:

• Normal hearing: less than 25 dB in adults and 15 dB in children³.

• Mild hearing loss: 25-39 dB.

• Moderate hearing loss: 40-69 dB.

• Severe hearing loss: 70-94 dB.

• Profound hearing loss: 95+ dB.

Hearing loss of 100 dB is nearly equivalent to complete deafness for that particular frequency. A score of 0 is normal. It is possible to have scores less than 0, which indicates better-than-average hearing.

Testing of hearing in the surgery

• This may be used for initial assessment but formal audiometry is preferable.

• Masking the other ear: putting a finger in the other ear is insufficient. If the tragus is rubbed at the same time, this will provide sufficient masking in most tests except for use of a loud voice. Alternatively, continuously rubbing a piece of paper between the thumb and index finger produces a consistent broadband sound.

• Tuning forks are often used to test at chosen frequencies. Other methods include whisper, rubbed fingers and a ticking watch.

Whispered voice test¹

• A simple and accurate test for detecting hearing impairment. It is the only test of hearing that requires no equipment.

• There is some concern regarding the lower sensitivity in children and the overall reproducibility of the test, particularly in primary care settings⁴.

• The examiner stands at arm's length (0.6 m) behind the seated patient (to prevent lip-reading) and whispers a combination of three numbers and letters (for example, 4-K-2) and then asks the patient to repeat the sequence.

• The examiner should quietly exhale before whispering to ensure as quiet a voice as possible.

• If the patient responds incorrectly, the test is repeated using a different number/letter combination. The patient is considered to have passed the screening test if they repeat at least three out of a possible six numbers or letters correctly (ie 50% correct).

• Each ear is tested individually, starting with the ear with better hearing. During testing, the non-test ear is masked by gently occluding the auditory canal with a finger and rubbing the tragus in a circular motion.

• The other ear is assessed similarly with a different combination of numbers and letters.

• One source of variability in the test is the loudness of the whisper. One study has shown that experienced practitioners are on average 8-10 dB louder than those without experience and they have shown higher sensitivity and specificity when administering the test⁵.

Weber's test

• A 512 Hz tuning fork is placed in the midline of the patient's forehead.

• If the sound is louder on one side than the other, the patient may have either an ipsilateral conductive hearing loss or a contralateral sensorineural hearing loss. Rinne's test may provide further information which will be useful to distinguish between these possibilities.

Rinne's test

• This uses a tuning fork of 256 Hz or 512 Hz. A heavy tuning fork is preferable, as a light one can produce a sound that fades too rapidly.

• It produces a sound level of 90 dB when struck against the knee or elbow.

• To test air conduction, hold the tuning fork directly in line with the external auditory canal.

• When testing bone conduction, place the flat end of the stem of the tuning fork against the mastoid process, using firm pressure (loudness varies by up to 15 dB with different pressures). Hold the patient's head steady with your free hand.

• When air conduction is louder than bone conduction it is reported as Rinne-positive. Rinne's test will reliably detect a conduction defect with an air-bone gap of at least 30-40 dB. It is no substitute for pure-tone audiometry.

Weber's and Rinne's tests are both, on their own, poor screening tests for hearing loss⁶. However, in acute hearing loss, they can be used to differentiate between conductive hearing loss (which is less urgent) and sensorineural hearing loss which indicates urgent referral⁷.

Pure-tone audiometry⁸

• Hearing is measured over a range of pure tones in each ear. Frequencies vary from low pitches (250 Hz) to high pitches (8,000 Hz).

• It measures the threshold for air and bone conduction and can determine whether it is due to conductive or sensorineural loss, or mixed.

• Each ear is tested at octave intervals from 250-8,000 Hz and plotted on a pure-tone audiogram with the test frequency along the horizontal axis and the thresholds of hearing on the vertical axis. This is in decibels hearing level (dB HL), which ranges from minus 10 (at the top) to 120 (the loudest that most audiometers can generate). The dB HL scale uses 0 dB HL as the normal threshold of hearing.

Practical issues when performing the audiometric test

• Level of background noise: this may significantly affect the results, especially at low frequencies. Sound-attenuating headphones and/or an acoustic booth/special test room need to be used.

• Calibration: the machine needs testing and recalibrating regularly.

• Threshold determination: the patient should be told to respond 'at the least suggestion of a signal' instead of when a signal is definitely heard.

• Test order: usually test the better ear first, starting at 1 kHz then 2, 4, 8, 0.25 and 0.5 kHz for air-conduction thresholds. Then test the other ear. Then test non-masked bone-conduction thresholds at 1, 2, 4, 0.5 and 0.25 kHz in that order with the vibrator on the side with the better air-conduction threshold (if any).

• Air-bone gap:
  

  • If the air-conduction thresholds in both ears together with the non-masked bone-conduction thresholds are within 10 dB of each other at all frequencies then there is no possible air-bone gap in either ear, ie either normal hearing or bilateral, symmetrical, sensorineural impairment.

  • When the non-masked bone-conduction thresholds are lower than the air-conduction thresholds by >10 dB in both ears, there is definitely air-bone gap in one or both ears.

  • When there is a difference between the non-masked bone conduction thresholds and air-conduction thresholds in one ear but not the other, there is potentially an air-bone gap or sensorineural impairment in the poorer ear, so the better ear needs to be masked to establish its bone-condition threshold.

Speech audiometry⁹

• This is a subjective test in which the subject repeats a standard list of words given through headphones at various levels of loudness. It is very useful in assessing a need for hearing aid provision.

• Word recognition tests (also known as speech discrimination tests): this involves reading a list of words to see if patients can discriminate words.

• Inferences can be made about central processing and central hearing deficits.

• Speech reception threshold:
  

  • This determines the lowest intensity level at which the patient can correctly identify 50% of common two-syllable words - eg, airplane and mushroom.

  • It should be in close agreement with pure-tone threshold results. A significant difference between the two thresholds would raise questions about the validity of the pure-tone thresholds, such as in cases of exaggerated hearing loss.

• Complex speech tests:
  

  • These are mainly used in evaluations of central auditory processing (CAP).

  • There may be normal pure-tone thresholds and even normal word recognition ability, but inability to process complex speech signals.

  • One commonly used test presents two different words to each ear simultaneously (a dichotic task).

  • Persons with normal CAP can repeat both words easily, while someone with a temporal lobe problem might be unable to repeat the word presented to the ear contralateral to the lesion.

Tympanometry¹⁰

• Tympanometry is a measure of the stiffness of the eardrum and thus evaluates middle-ear function.

• It can be helpful in detecting fluid in the middle ear, negative middle-ear pressure, disruption of the ossicles, tympanic membrane perforation and otosclerosis.

• A small amount of pressure is applied. The instrument then measures movement of the tympanic membrane in responses to the pressure changes.

• If there is fluid in the middle ear, the tympanic membrane will not vibrate properly and the line on the tympanogram will be flat.

• If there is air in the middle ear but the air is at a higher or lower pressure than the surrounding atmosphere, the line on the tympanogram will be shifted in position.

• The pressure readings produced by tympanometry do not reflect true middle-ear pressure and are subject to substantial errors, especially in persons with small mastoid sinus cavities¹¹.

Electrophysiological tests

• There is a need to differentiate sensorineural hearing loss due to cochleal dysfunction from that due to VIIIth nerve dysfunction or neurological disorder.

• Loudness recruitment (an abnormally rapid increase in loudness resulting from an increase in intensity of the stimulus) is characteristic of disorders of the hair cells of the organ of Corti, but not found in VIIIth nerve abnormalities.

• Abnormal auditory adaptation (a decline in discharge frequency over time following an initial burst of neural activity in response to a stimulus to the organ of Corti) is characteristic of VIIIth nerve and brainstem auditory dysfunction.

• Electrophysiological tests include:
  

  • Electrocochleography: this provides measurement of the electrical output of the cochlea and VIIIth cranial nerve in response to auditory stimuli.

  • Auditory brainstem responses: obtained from time-locked responses produced by major processing centres of the auditory system in response to repetitive sound stimulus.

  • Acoustic reflex threshold: measures the minimum intensity of sound at a given frequency required to produce contraction of the stapedius muscle in the middle ear (and thus tympanic membrane movement). Measures recruitment and abnormal auditory adaptation.

  • Otoacoustic emissions: these can be recorded in the ear canal and are produced by contractile properties of the outer hair cells of the cochlea and provide data on cochlea function. An automated otoacoustic emission (AOAE) test is usually part of a newborn hearing screening programme¹².

Acoustic reflex testing

• Acoustic reflex testing consists of exposing the ear to a loud sound and determining if it causes a reflex contraction of the stapedius and tensor tympani muscles and if so, at what decibel.

• They can be helpful in corroborating particular types of hearing loss in situations where patient reliability may be equivocal. They also occasionally indicate central nervous system pathology.

• Reflexes are usually present for fairly loud sounds, relative to hearing ability. Reflexes that are present at abnormally low sound input levels suggest recruitment with a cochlear site of lesion. Reflexes that fade rapidly suggest a retrocochlear lesion. Reflexes that are bilaterally absent contralaterally suggest a midline brainstem lesion.