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. To test for loud noises, a Bárány noise box needs to be used.
• Tuning forks are often used to test at chosen frequencies. Other methods include whisper, rubbed fingers and a ticking watch.

Whispered voice test

• A simple and accurate test for detecting hearing impairment. There is some concern regarding the lower sensitivity in children and the overall reproducibility of the test, particularly in primary care settings.^[1]
• The examiner stands at arm's length (0.6 m) behind (to prevent lip-reading) the seated patient and whispers a combination of 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.
• 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.

Rinne's test

• This uses tuning forks of 512 Hz but those of 256 Hz may be more accurate. A heavy tuning fork is preferable as a light one produces 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 bone immediately superior and posterior to the external canal, 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 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.

Pure-tone audiometry

• 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, then 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, then 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.
• Bone-conduction masking: the preferred method is shadow masking, recording the threshold on a masking chart. This uses a masking signal of a narrow band of noise around the test frequency in the non-test ear.
• Air-conduction masking: although sound does not escape from the headphones, it can be conducted across the skull to reach the cochlea on the other side. It is generally accepted that attenuation of 40 dB occurs with this. So there is a need for air-conduction masking if there is a 40 dB difference between the 2 ears. It may indicate there is a major conducted defect in the test ear.

Speech audiometry

• This is a subjective test in which the subject repeats a standard list of words given through headphones at various 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, 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.

Tympanometry

• 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 cavities.^[2]

Electrophysiological tests

• There is a need to differentiate sensorineural hearing loss due to cochlea dysfunction from that due to 8th 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 8th 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 8th nerve and brainstem auditory dysfunction.
• Electrophysiological tests include:
  • Electrocochleography: this provides measurement of the electrical output of the cochlea and 8th cranial nerve in response to auditory stimuli.
  • Brainstem auditory evoked 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.

Acoustic reflex testing

• Acoustic reflex testing consists of subjecting the ear to a loud sound and determining if it causes the stapedius muscle to tighten the stapes.
• Acoustic reflexes measure the stapedius and tensor tympani reflex generated eardrum movement in response to intense sound.
• They can be helpful in corroborating particular types of hearing loss in situations where patient reliability is questionable. 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.