4Audiologic Patterns: Discontinuity Versus Fixation

FTwo ways a chain can fail

A conductive hearing loss tells you that sound is reaching a normal cochlea inefficiently; the audiogram’s job is to suggest why. When the tympanic membrane is intact and dry, the fault almost always lies in the ossicular chain, and it can fail in two mechanically opposite ways. The chain can be interrupted— eroded or disarticulated so that vibration no longer crosses from drum to footplate. Or it can be fixed— continuous but immobilised, anchored by tympanosclerosis or bony ankylosis so that it cannot move. Discontinuity and fixation are the two poles of ossicular pathology, and the great practical value of preoperative audiology is that these poles leave different fingerprints on the audiogram and tympanogram[1998].

The reason they differ comes straight from middle-ear mechanics. The normal ear delivers sound to the cochlea mainly through ossicular coupling— the drum and chain together drive the footplate, providing the bulk of the roughly 50–60 dB of gain that the middle ear contributes. A much weaker acoustic couplingroute, the small pressure difference the sound field creates directly across the two cochlear windows, always survives in the background. Break the chain completely and ossicular coupling vanishes; only the feeble acoustic route remains, so the conductive loss climbs to its ceiling of about 50–60 dB [1998]. Stiffenthe chain instead and coupling is impaired but not abolished — the system simply resists the large, slow excursions of low-frequency sound more than the small, fast excursions of high frequencies. The result is a smaller loss, weighted toward the bass. Loss of coupling and stiffening of coupling therefore produce gaps of different size and different shape, and learning to read both is the heart of this module.

FSize and shape of the air-bone gap

The single most useful number is the air-bone gap (ABG)— the difference between air-conduction and (masked) bone-conduction thresholds, which isolates the conductive component. Its magnitude alone narrows the differential. A gap of 25–40 dB behind an intact, mobile drum is the range of mild ossicular dysfunction: partial erosion, scarring, early fixation. A gap that climbs above 50–60 dB with a normal-looking, mobile drum is the classic signature of complete ossicular discontinuity, most often erosion of the long process of the incus or frank separation at the incudostapedial joint[1998, 2001]. The logic is simply the coupling ceiling in reverse: only a totally interrupted chain can drop hearing all the way to the acoustic floor while the drum still moves freely.

The configuration of the gap across frequencies adds a second layer of information. A flatconductive loss — a similar gap at every frequency — fits a complete discontinuity, because once coupling is abolished the residual acoustic route is broadly frequency-independent. A loss that is largest in the low frequencies and narrows toward the highs is the hallmark of a stiffness lesion such as stapes fixation: added stiffness most opposes the long-wavelength, large-displacement motion of low-frequency sound [1998]. (Added mass, by contrast, bites at the high frequencies — relevant when a heavy prosthesis or a sclerotic mass loads the chain.) A third, down-sloping pattern points to partial disruption and is discussed below. The widget makes the contrast concrete: tap between the three curves to see how a flat, near-maximal gap, a low-frequency stiffness gap and a high-frequency partial gap differ in both height and slope.

Two bedside caveats keep this honest. First, the otoscope frames the audiogram: the “intact drum” condition matters because the same large gap in an ear with a perforation is more likely to mean fixation— the perforation explains a modest loss, and any excess gap above it implicates a fixed malleus head or stapes footplate [2001]. Second, the bedside Rinne test is only a coarse screen for the gap. A 512 Hz Rinne becomes reliably negative once the gap exceeds about 20 dB and is highly sensitive for gaps of 30 dB or more, but it cannot grade the loss — the audiogram is indispensable for sizing and shaping the gap[1988].

TTympanometry: hypercompliant versus shallow

Where the pure-tone audiogram measures the cochlear consequence of ossicular pathology, tympanometry measures the mechanical state of the drum-and-chain system directly, and it sharpens the discontinuity-versus-fixation distinction beautifully. The 226 Hz probe sweeps ear-canal pressure and plots admittance (compliance): the height and shape of the resulting peak report how readily the system accepts sound energy. A normal, mobile chain gives a type A peak of ordinary height near 0 daPa.

The two poles deflect this peak in opposite directions. An interrupted chain no longer loads and stiffens the drum, so the membrane becomes floppy: the peak grows abnormally tall — a type Ad (hypercompliant) trace that is the immittance correlate of discontinuity. A fixed chain does the reverse: the stiff, anchored system resists displacement, so the peak barely rises — a shallow, low-admittance type As trace pointing to fixation or tympanosclerosis. Reflectance and tympanometry studies in surgically confirmed ears bear this out: fixed (otosclerotic) ears show reduced low-frequency energy absorbance and a low, shallow peak, while a mobile but disrupted chain runs hypercompliant[2009, 2012]. Note the type B and type C traces sit outside this axis: a flat type B usually means effusion or a perforation (read alongside the ear-canal volume), and a type C, negative-pressure peak means Eustachian dysfunction — neither is a primary ossicular pattern.

One trap deserves emphasis. A type A peak of normal height does not exclude discontinuity. Tympanometry interrogates the lateral chain and drum; a chain broken medialto the point the drum loads — or a stiff scar that happens to offset the floppiness — can leave a deceptively ordinary peak. So a large air-bone gap with a merely “normal” tympanogram should still raise suspicion of a disrupted chain[1998]. Acoustic-reflex testing adds a corroborating, not deciding, vote: stapedial reflexes are typically absent in both discontinuity and fixation, so an absent reflex with normal bone thresholds simply confirms a mechanical problem without telling you which one.

TThe Carhart notch and other bone-conduction clues

Bone conduction is usually treated as the “cochlear reserve” line — the floor beneath the air-conduction trace. But the bone-conduction line is not purely cochlear: part of it depends on the ossicular chain’s own inertia and on the osseotympanic contribution of the drum, so middle-ear mechanics can artefactually depress it. The classic example is the Carhart notch: in stapes fixation the measured bone-conduction threshold dips, maximally around 2 kHz, by perhaps 10–15 dB [1950]. This is not true inner-ear damage. It is a mechanical consequence of fixation near the middle-ear resonance, and it characteristically recoversonce the fixation is relieved — an important point when counselling, because the bone line will appear to “improve” after surgery even though the cochlea was never the problem.

For our purposes the notch is a directional clue: a 2 kHz bone-conduction dip accompanying a low-frequency-weighted gap tilts the diagnosis toward fixation rather than discontinuity. It is most classically described in otosclerosis but is mechanistically applicable to any stapes-fixing process, including tympanosclerotic footplate fixation. The wider lesson is methodological: bone-conduction thresholds must be properly maskedand measured before they are trusted. Sloppy bone testing is the commonest way to misjudge the size of a gap — and, as the next section shows, the way a real conductive loss can be mistaken for a sensorineural one.

CPartial discontinuity and the down-sloping gap

Not every disrupted chain is cleanly broken. In chronic otitis media the long process of the incus is often eroded yet held in loose contact with the stapes by a bridge of fibrous tissue or granulation — an incomplete (partial) discontinuity. Mechanically this soft-tissue bridge behaves like a low-pass filter: it is compliant enough to carry the large, slow excursions of low-frequency sound but too lax to transmit the small, fast excursions of high-frequency sound. The audiometric result is a distinctive down-sloping conductive loss— a small or absent gap in the bass that widens steadily toward 4 kHz, the opposite slope of the low-frequency gap of fixation.

This pattern is diagnostically powerful when sought deliberately. In a series of 328 ears with non-cholesteatomatous chronic suppurative otitis media, a high-frequency conductive loss — defined as the 4 kHz air-bone gap minus the mean of the 0.25 and 0.5 kHz gaps — predicted incomplete ossicular discontinuity with about 83% sensitivity and 92% specificity, a likelihood ratio of roughly ten[2017]. The clinical payoff is real: an ear with a partial bridge may need only the eroded joint reinforced or reconstructed, a different operation from the total prosthesis a complete break demands. The matcher widget below lets you set the drum status, gap size, gap shape and tympanogram and watch the inference swing between discontinuity and fixation — including the equivocal middle ground where partial lesions hide.

The same down-sloping mechanics carry a warning. Because the loss is largest exactly where clinicians expect sensorineural loss — the high frequencies — a partial discontinuity can be mistaken for cochlear loss if bone-conduction thresholds are not measured carefully at every frequency with correct masking. The give-away is that an air-bone gap persists at the high frequencies: the loss is conductive, not sensorineural, and the chain, not the cochlea, is at fault [1998].

CPutting the pattern to work — and its limits

Synthesised, the audiologic patterns give the otologist a working hypothesis before the ear is ever opened. A large, flat gap behind an intact, hypercompliant (type Ad) drum, with absent reflexessays discontinuity — expect incus erosion or incudostapedial separation, and plan for interposition or a total prosthesis. A smaller, low-frequency-weighted gap with a shallow type As trace, tympanosclerotic plaques and perhaps a 2 kHz Carhart dipsays fixation — expect a stiff or ankylosed footplate or malleus head, and plan for mobilisation, plaque removal or stapes surgery, sometimes staged. A down-sloping, high-frequency gap says partial disruption. These inferences sharpen counselling, set the consent discussion and shape the prosthesis tray.

The discipline lies in holding these patterns as hypotheses, not verdicts. Every association above is probabilistic. A type A tympanogram does not exclude discontinuity; a large gap can occasionally reflect a combined lesion; a tympanosclerotic ear may harbour both a fixed malleus and an eroded incus at once. This is why the audiogram is paired with high-resolution CT, which shows a bony gap, a sclerotic oval-window niche or a fixed footplate directly, and why neither test displaces the final arbiter: intraoperative palpationof each ossicle for mobility and continuity once the drum is elevated. The audiologic pattern earns its keep not by being always right, but by making the surgeon walk into theatre expecting the correct lesion — and prepared for the moment the chain tells a different story.