5Tympanometry and Impedance Audiometry

FWhat tympanometry actually measures

Otoscopy tells you what the drum looks like; tympanometry tells you how the whole tympano-ossicular system moves. The principle is simple. A probe sealed in the ear canal delivers a low-frequency tone (classically 226 Hz) while a small pump sweeps the canal pressure from positive, through ambient, to negative. At each pressure the instrument measures how much sound the middle ear admits — its admittance, the reciprocal of impedance. The result is plotted as a curve of admittance against pressure: the tympanogram [1970, 2004].

Why does sweeping the pressure matter? The middle ear transmits sound most efficiently when the pressure on both sides of the drum is equal. Pushing the canal pressure away from the middle-ear pressure stiffens the drum and drops admittance; matching it lets the drum move freely and admittance peaks. The position of the peakon the pressure axis therefore reports the middle-ear pressure, and the height of the peak(the static admittance, in millilitres of equivalent volume) reports how compliant— how mobile — the system is. A third number, the ear-canal volume, is read from the admittance measured at high pressure with the drum pushed stiff; an abnormally large value betrays a hole in the drum, because the probe is then sampling the middle-ear space as well [2004].

The crucial conceptual point — the one that makes this a test of ossicularstatus and not merely of the drum — is that at low frequencies the middle ear is stiffness-dominated. Its admittance is governed far more by how easily the chain can be set in motion than by its mass. Anything that adds stiffness (a fixed footplate, a thickened drum, tympanosclerosis) lowers compliance; anything that removes coupling (a broken chain, a flaccid drum) raises it. Tympanometry thus functionally probes the mobility of the conductive apparatus through an intact eardrum [1998].

FThe tympanogram types: A, As, Ad, B, C

Jerger’s 1970 classification reduced the family of tracings to a handful of memorable shapes, and the scheme has survived essentially unchanged for half a century [1970]. Learn it as a small grid of peak position (where on the pressure axis) crossed with peak height (how compliant):

• Type A— a sharp peak near 0 daPa with normal static admittance. Normal pressure, normal mobility, intact chain.
• Type As (“s” for shallow/stiff) — a peak in the normal pressure position but abnormally low. A stiff, hypocompliant system: stapes fixation (otosclerosis), tympanosclerosis, a thickened drum.
• Type Ad (“d” for deep) — a peak in the normal pressure position but abnormally high. A hypermobile system: ossicular discontinuity, or a flaccid/monomeric drum.
• Type B — a flat tracing with no identifiable peak. With a normal ear-canal volume this means effusion; with a large volume it means a perforation or patent tube.
• Type C — a peak of near-normal height but shifted to significantly negative pressure(typically more negative than −100 daPa), the signature of eustachian-tube dysfunction or a resolving effusion [2004].

The two variants that matter most for the ossicular surgeon are As and Ad, because they sit at opposite ends of the compliance scale and point at the two great mechanical opposites of the chain: fixation versus discontinuity. The explorer below lets you switch between the types and watch the tracing change while the readout names the implied lesion.

A practical caution: the type letter alone is never the diagnosis. A type B is meaningless until you have read the ear-canal volume; a type As is only suggestive until you have the audiogram and the reflex. Tympanometry earns its keep when its three numbers — peak pressure, static admittance and canal volume — are read together and then reconciled with otoscopy and the audiogram.

TThe acoustic reflex as a probe of the chain

Impedance audiometry pairs the tympanogram with the acoustic (stapedial) reflex, and the two are complementary because they interrogate the chain in different ways. The reflex test plays a loud activating sound and watches the probe tone for a brief drop in admittance. That drop is the mechanical footprint of the stapediuscontracting and stiffening the ossicular chain — a transient, sound-evoked version of the same stiffness change the tympanogram maps statically [1970].

For the reflex to be recorded, two things must hold: the brainstem arc must be intact (cochlea → auditory nerve → superior olive → facial nucleus → nerve to stapedius), andthe ossicular chain must be able to translate the muscle’s pull into a measurable admittance change. This second requirement is what makes the reflex a sensitive, if non-specific, probe of ossicular mechanics. Both fixation and discontinuity abolish the reflex— fixation because the locked footplate cannot change its stiffness, discontinuity because the broken chain cannot transmit the contraction to the probe. An absent reflex behind a normal drum is therefore a red flag for an ossicular problem, but it does not by itself say which one [1998].

The reflex carries other clinically useful information. Because the activator is bilateral, a crossed (contralateral) reflex tests the auditory afferent and facial efferent on opposite sides; because the efferent limb is the facial nerve, an absent ipsilateral reflex can localise a facial lesion; and because a reflex elicited at an abnormally low sensation level (recruitment) suggests cochlear loss, the same test contributes to the sensorineural picture. For the ossicular question, though, its single most important behaviour is its tendency to fall silent the moment chain mechanics are disturbed.

TReading impedance in the conductive ear

The reward for learning these patterns is the ability to work out the mechanism of a conductive loss beforethe ear is opened. Consider the common scenario: a purely conductive air-bone gap behind a completely normal-looking drum, with a normal audiogram in the other ear. The audiogram tells you the cochlea is intact and the deficit is mechanical; impedance audiometry tells you where in the mechanism the fault lies. The matrix below lets you combine a tympanogram pattern with a reflex result and reason to the most likely diagnosis.

The three patterns worth holding in working memory:

The elegance is that compliance separates what the reflex cannot. Both fixation and discontinuity give absent reflexes, so the reflex alone is ambiguous; but fixation drives admittance down (type As) while discontinuity drives it up (type Ad), and the tympanogram shape resolves the two. This is the single most useful trick in interpreting impedance audiometry for the ossicular chain [1970, 2009].

CMultifrequency and wideband immittance

Standard 226 Hz tympanometry samples only the low-frequency, stiffness-dominated corner of middle-ear mechanics, and the As/Ad distinction, although useful, is not always clean. Multifrequency tympanometryaddresses this by characterising the system across a range of probe tones and extracting the middle-ear resonance frequency— the frequency at which stiffness and mass effects cancel. The direction of the shift is diagnostically powerful: a stiffening lesion such as otosclerotic stapes fixation raises the resonance frequency, whereas a mass-loading or decouplinglesion such as ossicular discontinuity, atelectasis or effusion lowers it [2012].

Wideband acoustic immittance (energy reflectance/absorbance) takes the same idea further, measuring how much acoustic energy the middle ear reflects across a broad frequency band, often at ambient pressure and in seconds. In confirmed cohorts these methods outperform conventional tympanometry at the very distinction the ossicular surgeon cares about: stapes fixation raises low-frequency reflectance (the added stiffness reflects more energy back), while ossicular discontinuity produces a characteristic deep low-frequency reflectance notch as the decoupled, hypermobile system absorbs energy abnormally well[2009]. When wideband measures are combined with audiometry — or with umbo velocity by laser Doppler vibrometry — the preoperative differential between fixation, discontinuity and superior canal dehiscence behind an intact, aerated drum becomes genuinely tractable [2012].

None of this replaces the audiogram or the operative finding; the definitive ossicular grade is still seen at exploration. But these tools convert an ambiguous “conductive gap, normal drum, absent reflex” into a ranked, mechanism-based differential, which is exactly what is needed when counselling a patient and choosing between a stapes procedure and an ossiculoplasty.

CImplications for ossiculoplasty

Impedance audiometry feeds the ossiculoplasty decision in three concrete ways. First, it helps localise the lesion. Distinguishing fixation (type As, raised resonance frequency, absent reflex) from discontinuity (type Ad, lowered resonance frequency, absent reflex) steers the operative plan toward a stapedotomy on the one hand or a chain-bridging reconstruction on the other, and flags the possibility of combined pathology when the patterns do not fit a single mechanism [2009, 2012].

Second, it characterises the middle-ear environmenton which any reconstruction depends. A type B tympanogram or a persistent type C warns of effusion or chronic negative pressure — an unventilated, high-impedance ear in which even a perfectly placed prosthesis will move poorly and hearing gain will disappoint. Restoring aeration and a healthy mucosa often has to precede, or be staged with, ossicular reconstruction; the tympanogram is one of the cheapest ways to detect that the environment is hostile [1998].

Third, impedance measures give an objective baselineagainst which to judge recovery. Because the reconstructed chain is, in mechanical terms, a re-tuned stiffness-and-mass system, the same principles that explain the preoperative tracings explain the postoperative ones: an over-tight prosthesis behaves like a stiffening lesion and a loosely coupled one like a partial discontinuity. Reading tympanometry and reflexes fluently therefore does more than localise disease — it teaches the surgeon to think about the reconstruction itself as an admittance problem, which is the most useful frame of all [1998].