Tympanometry
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Direct answer
Tympanometry measures how the admittance of the middle ear system changes as air pressure in a sealed ear canal is swept from negative through ambient to positive, using a 226 hertz probe tone in older children and adults. The result is a single curve classified by Jerger: type A peaks near ambient pressure (normal, or stiff with low compliance As, or hypermobile with very high compliance Ad); type B is flat (fluid, or a perforation if the ear canal volume is abnormally large); type C shows a negative-pressure peak, indicating eustachian tube dysfunction. Because the test requires no behavioural response, it is the objective workhorse behind every paediatric hearing screen and every "is this effusion or not" clinic decision, and its two numbers — peak pressure and compliance — separate most middle ear problems in under a minute.
What you must remember
- Probe frequency: 226 hertz for anyone over about six months; neonates and young infants need a 1000 hertz probe because their canal and middle ear mechanics make 226 hertz traces misleading.
- Type A: peak compliance within the normal range at pressure near zero (roughly between -100 and +50 decapascals) — normal middle ear.
- Type As (shallow): reduced compliance at normal pressure — stiffness pathology: otosclerosis, tympanosclerosis, malleus fixation; the otosclerosis classic.
- Type Ad (deep): excessively high compliance — hypermobile or atrophic drum, ossicular discontinuity; think incus necrosis in chronic ear disease.
- Type B (flat): no measurable peak — middle ear effusion, glue ear, or a perforated drum; the discriminator is ear canal volume: normal volume (about 1 millilitre in adults, around 0.5 in children) with a flat trace means fluid behind an intact drum, whereas a large volume (2-3 times the other ear) means the probe is measuring through a perforation or a patent ventilation tube.
- Type C: intact, compliant system with the peak displaced to, commonly, beyond -150 to -200 decapascals — eustachian tube dysfunction or resolving effusion.
- Combining with acoustic reflexes: absent reflexes with type As supports otosclerosis; present reflexes essentially exclude significant stapes fixation.
Worked example: three tracings, one algorithm
A four-year-old brought for not listening shows a type B tracing with ear canal volume 0.4 millilitres, matching the other side, and a mild conductive loss on audiometry: fluid behind an intact drum — glue ear, managed by watchful waiting first, since many effusions resolve, with ventilation tubes for persistence beyond about three months with hearing or speech impact. A seven-year-old after treated otitis media returns with a type C curve peaking at -250 decapascals, normal compliance and normal hearing — negative pressure without fluid, rechecked rather than operated.
The third child has a flat trace on the left with a canal volume of 3.2 millilitres against 0.6 on the right: the probe is reading through a dry anterior-inferior perforation, confirmed on otoscopy. Same letter on the report, three different ears, and the number students skip — canal volume — did all the discriminating. Read peak pressure, compliance and volume together, never the Jerger letter alone.
Where students slip
The classic error is calling every type B effusion without checking canal volume, missing a perforation that changes counselling. Remember the neonate rule — a 226 hertz probe in young infants often gives artefactually flat traces, so 1,000 hertz probes are used and judged by the presence of a clear peak. Candidates also reverse the letters: As is low compliance (stiffness — otosclerosis, tympanosclerosis), Ad is abnormally high (discontinuity, flaccid drum). Finally, a perfect type A with a failed hearing screen still demands hearing assessment, because tympanometry says nothing about the cochlea.
Frequently asked questions
Which tympanogram type suggests otosclerosis?
Type As — a shallow peak at normal middle ear pressure, reflecting increased stiffness of the ossicular chain; acoustic reflexes are typically absent or elevated.
How is a perforated eardrum identified on tympanometry?
A flat type B trace with an abnormally large ear canal volume, because the probe measures the canal, middle ear and mastoid air spaces as one cavity.
What does a type C tympanogram indicate?
An intact, mobile middle ear system ventilated at persistently negative pressure — eustachian tube dysfunction, commonly seen with or after upper respiratory infections.
Why is a 1000 hertz probe tone used in neonates?
The neonatal ear canal and middle ear mechanics make 226 hertz admittance traces unreliable, so high-frequency probes are used, with the presence of a demonstrable peak taken as normal.
What ear canal volume is expected in a healthy adult?
Roughly 1 millilitre in adults and about 0.5 millilitre in young children; bilateral symmetrical values support proper probe seal and intact drums.