Pure Tone Audiometry

Pure tone audiometry measures the quietest sound that a person can hear at different pitches. It is the basic test of hearing. It separates conductive hearing loss (a problem in the outer or middle ear) from sensorineural hearing loss (a problem in the cochlea or auditory nerve), and measures how severe the loss is.

The patient wears headphones or insert earphones and presses a button each time they hear a tone, however faint.

Air conduction thresholds are measured at frequencies from 250 to 8000 Hz (octave steps, with 3000 and 6000 Hz added when needed). Results are recorded in decibels hearing level (dB HL) on a graph called an audiogram. The x axis is frequency and the y axis is intensity, with the best hearing at the top.

Bone conduction thresholds are measured with a vibrator placed on the mastoid. This sends sound directly to the cochlea and bypasses the middle ear.

When the two ears differ markedly, masking noise is given to the better ear. This stops it responding to the sound being tested in the worse ear.

The test takes 15 to 30 minutes, in a sound-treated room. It is done by an audiologist, a trained technician, or an automated and tablet-based system. Hearing loss affects about 1 in 5 people worldwide, and about 1 in 3 over the age of 65 years. In adults, a threshold of 20 dB HL or better is usually normal.

Indications.

• Any hearing difficulty, in adults and in children old enough to cooperate (from about 5 years, and with play methods from about 2 to 3 years).
• Tinnitus, vertigo, ear fullness, ear pain, discharge, and after head injury.
• Sudden hearing loss, and asymmetric hearing loss, to find those needing imaging.
• Occupational and noise exposure screening, and monitoring of hearing in workers at risk.
• Before and after ear surgery and treatment of middle ear disease, and monitoring after ototoxic drugs (aminoglycosides, cisplatin, loop diuretics, some antimalarials).
• Hearing aid, cochlear implant and bone-anchored device assessment.
• Meningitis, congenital cytomegalovirus infection, and other conditions that carry a risk of hearing loss, for follow-up.
• Medico-legal assessment, and school or work screening. Not suitable for infants or patients who cannot cooperate, for whom behavioural tests or objective tests (otoacoustic emissions, auditory brainstem response) are used.

Contraindications and precautions.

• No absolute contraindications.
• Otoscopy is done first. Wax that blocks the canal, foreign bodies, active otitis externa with a painful canal, and a discharging ear need treatment or care in placing earphones. Insert earphones are avoided with canal infection, and bone conduction or sound field testing is used.
• A collapsing ear canal (soft canals, especially in infants and older people) closes with headphones, giving false conductive loss, and insert earphones are used.
• Recent loud noise exposure causes temporary threshold shift, and the patient is asked to avoid loud noise for 12 to 16 hours before the test.
• A noisy room, a patient with poor attention, fatigue, or illness, and a recent ear infection affect results.
• Patients with dementia or learning difficulty need adapted methods.
• Equipment is calibrated regularly, and the test environment meets noise limits.

Preparation and technique.

The patient is asked about hearing symptoms, noise exposure, ear disease, ototoxic drugs, and family history. The ears are examined with an otoscope.

Headphones or insert earphones are placed, and the patient is told to press the button when they hear a tone, even a faint one.

Testing starts at 1000 Hz in the better ear. The tone is presented at a level that is heard, then lowered in 10 dB steps until it is no longer heard. It is then raised in 5 dB steps until it is heard again. The threshold is the lowest level at which the patient responds in at least half of the ascending presentations.

Other frequencies follow, and then the other ear. Bone conduction is measured with the vibrator on the mastoid, with masking in the other ear.

Results are plotted on the audiogram, using agreed symbols for each ear and for air and bone conduction.

Normal appearances.

Air conduction thresholds are 20 dB HL or better (25 dB in some definitions) at all frequencies from 250 to 8000 Hz, in both ears. Bone conduction thresholds are within 10 dB of air conduction (no air-bone gap). The difference between ears is less than 10 to 15 dB.

The pure tone average (the mean of thresholds at 500, 1000, 2000 and 4000 Hz) is under 20 dB HL. Children have normal thresholds of 15 dB HL or better.

Interpretation.

• Degree of hearing loss (based on the pure tone average): slight 16 to 25 dB, mild 26 to 40 dB, moderate 41 to 55 dB, moderately severe 56 to 70 dB, severe 71 to 90 dB, and profound over 90 dB HL.
• Conductive hearing loss: raised air conduction thresholds with normal bone conduction, with an air-bone gap of over 10 dB. The loss is up to about 60 dB. Causes include wax, otitis media with effusion, perforation, ossicular disease, and otosclerosis.
• Sensorineural hearing loss: air and bone thresholds are both raised and match (no gap). Causes include ageing, noise, ototoxic drugs, genetic disease, infection, Meniere disease, and tumours of the auditory nerve.
• Mixed loss: raised bone conduction thresholds with an additional air-bone gap.
• Presbycusis: bilateral, symmetrical, high-frequency loss that slopes downward.
• Noise-induced loss: a notch at 3000 to 6000 Hz (most often at 4000 Hz), with recovery at 8000 Hz.
• Meniere disease: a low-frequency loss that fluctuates, in the early stages. Otosclerosis: a conductive loss, with a dip in bone conduction at 2000 Hz in many patients.
• Asymmetric loss: a difference between the ears of 15 dB or more at two or more frequencies, or 10 dB at three frequencies, needs imaging to exclude a vestibular schwannoma or other retrocochlear lesion.
• Sudden sensorineural hearing loss: a loss of 30 dB or more at three adjacent frequencies, developing over 72 hours or less.
• Non-organic hearing loss: inconsistent thresholds, poor agreement with other tests, and thresholds that vary from test to test.

Critical values and findings needing immediate action.

• Sudden sensorineural hearing loss (a new loss of 30 dB or more at three frequencies over 72 hours or less): an emergency. Urgent ear, nose and throat assessment on the same day, and treatment within days improves recovery, with an MRI to exclude a tumour.
• Asymmetric sensorineural hearing loss, with or without tinnitus, vertigo, or facial weakness: MRI of the internal auditory canals.
• Conductive hearing loss with severe ear pain, discharge and fever, facial weakness, or vertigo: urgent assessment for complicated otitis media and mastoiditis.
• A rapid fall in hearing during treatment with ototoxic drugs: urgent review with the prescriber.
• Hearing loss with neurological signs, or after head injury: urgent assessment.
• A child with a new hearing loss: referral for further assessment and early help, since delay affects speech and language.

Limitations and artefacts.

• Background noise, poor earphone placement, collapsed canals, wax, and a middle ear problem on the day change results. Bone conduction is less accurate at 250 Hz and at 4000 Hz and higher, because of vibrotactile responses and limits of the device.
• Failure to mask when needed causes a false shadow curve (crossover), which makes a dead ear look as if it hears, and over-masking causes a false loss. Masking is applied when the difference between the ears in air conduction is 40 dB or more, or when there is a possible air-bone gap.
• Patient factors: poor attention, fatigue, slow response, tinnitus that masks the tone, unreliable responses, and misunderstanding.
• The test measures only the ability to detect sound and does not test speech understanding, so the real hearing handicap is not shown, and speech audiometry is needed.
• Test-retest variation is about 5 dB, and differences of 10 dB or more between tests show a real change.
• Results do not give the cause, and need tympanometry, otoscopy, and other tests.
• Thresholds at extended high frequencies (over 8000 Hz) are not routinely measured, but are affected first by noise and ototoxic drugs.

Special groups.

• Children: conditioned play audiometry from about 2 to 3 years, visual reinforcement in infants of 6 months to 2 years, and objective tests for babies. Early identification of hearing loss avoids delays in speech and language.
• Elderly patients: age-related hearing loss is common, and is linked to social isolation, depression, and cognitive decline. Hearing aids improve communication.
• Workers exposed to noise: baseline and yearly tests, to detect early notching and protect hearing.
• Patients taking ototoxic drugs: baseline tests, and regular follow-up with high-frequency testing.
• Patients with learning difficulty or dementia: adapted tests, and observation of behaviour.
• Regions: chronic suppurative otitis media, ear infections, meningitis, measles, malaria, and noise from work and entertainment cause avoidable hearing loss. A portable audiometer, a quiet room, and smartphone and tablet-based audiometry extend testing to primary care and schools. Many people have no access to hearing aids.

Follow-up tests.

• Speech audiometry and tympanometry.
• Acoustic reflex testing and otoacoustic emissions.
• Auditory brainstem response in asymmetric loss and in people who cannot be tested by behavioural methods.
• MRI of the internal auditory canals and the brain in sudden or asymmetric loss.
• CT of the temporal bones for conductive loss with chronic ear disease, and suspected otosclerosis or ossicular problems.
• Blood tests: glucose, lipids, thyroid, syphilis and autoimmune screening in unexplained loss.
• Genetic testing in early onset or familial loss, and assessment for hearing aids or implants.