Tympanometry
Tympanometry measures how well the eardrum and middle ear move, by recording the compliance of the system as the air pressure in the ear canal is changed. It is a quick objective test of middle ear function. It detects fluid, negative pressure, perforation, and ossicular problems, whatever the patient can report.
A probe tip is placed in the outer ear canal to form an airtight seal. The probe has three parts:
- A loudspeaker that sends a tone (usually 226 Hz in adults and children, and 1000 Hz in babies under about 6 months).
- A microphone that measures the sound level in the canal.
- A pump that varies the air pressure.
The pressure is swept from about +200 to -400 daPa (decapascals). The eardrum moves most easily, and absorbs the most sound, when the pressure on both sides is equal. The machine records the compliance (mobility) against the pressure, producing a curve called a tympanogram. The test takes less than a minute for each ear.
Three main values are read from the curve:
- The peak compliance (static admittance, normally about 0.3 to 1.5 mL).
- The pressure at which the peak occurs (the middle ear pressure, normally between about +50 and -100 daPa).
- The equivalent ear canal volume (about 0.6 to 1.5 mL in adults, and 0.3 to 0.9 mL in children).
Tympanometry detects middle ear effusion with a sensitivity of about 80 to 90%. With pneumatic otoscopy, it is the best single test for otitis media with effusion, which affects up to 80% of children at some point before school age.
Indications.
- Hearing loss or suspected middle ear disease, together with pure tone audiometry, to separate conductive from sensorineural loss.
- Suspected otitis media with effusion, in children with hearing difficulty, speech delay, school problems, or after repeated ear infections.
- Eustachian tube dysfunction, ear fullness and pressure, and barotrauma after flying or diving.
- Suspected eardrum perforation, grommet (ventilation tube) function and patency, and check after surgery.
- Suspected ossicular chain problems (otosclerosis, discontinuity after trauma or infection, cholesteatoma).
- Part of newborn and infant hearing assessments, to interpret otoacoustic emissions, using a high-frequency probe tone.
- Part of the full audiological test battery, with reflex tests, in the assessment of facial palsy and retrocochlear disease.
- Monitoring of middle ear disease over time and after treatment. Not a test of hearing, and cannot be used alone to diagnose or exclude hearing loss.
Contraindications and precautions.
- Blocked ear canal (wax, foreign body, debris, or severe swelling): the probe cannot seal, and the canal is cleared first.
- Active ear discharge, painful otitis externa, or an acutely painful ear: the test is delayed, since pressure may cause pain.
- Recent ear surgery (particularly stapes surgery or a recent tympanoplasty): the test is delayed or done only on advice from the surgeon, since pressure change can displace the prosthesis or graft.
- Recent head or ear trauma with a suspected perforation or cerebrospinal fluid leak: caution.
- Patients with very small or collapsed ear canals, and babies with soft canals, need a suitable probe tip.
- Patients who cannot keep still, speak, cry, or swallow during the test: the result is not reliable, and the test is repeated.
- Disposable probe tips are used for each patient to avoid cross-infection.
Preparation and technique.
The ear canal is examined with an otoscope, and wax is cleared if it blocks the view. The patient sits quietly and is asked to stay still and not speak or swallow during the test.
The ear canal is straightened by gently pulling the pinna up and back (in adults, and down and back in infants). The probe tip is placed to give an airtight seal.
The test starts, the pressure sweeps from positive to negative, and the tympanogram is displayed and printed. The test is repeated if the curve is irregular. Both ears are tested.
Results are recorded as the curve type, peak compliance, middle ear pressure, and ear canal volume. The test is interpreted with pure tone audiometry and otoscopy.
