3Pure-Tone Audiometry and the Air-Bone Gap

FWhat the air-bone gap measures

Pure-tone audiometry is the cornerstone of the ossiculoplasty work-up because it does two things at once: it quantifies how much hearing has been lost, and it localises where the loss arises. The trick is the comparison of two thresholds at each frequency. Air conduction, tested through an earphone, drives sound along the whole pathway — ear canal, drum, ossicular chain, cochlea, auditory nerve. Bone conduction, tested through a vibrator on the mastoid or forehead, sets the skull and its contents vibrating and stimulates the cochlea more or less directly, bypassing the outer and middle ear. The difference between them at a given frequency is the air-bone gap (ABG), and it is the purest bedside measure of the mechanical, conductive component of a hearing loss.

The logic for the ossiculoplasty surgeon follows directly. Bone conduction reports the state of the cochlea — the patient’s sensorineural reserve, and the ceiling on any hearing they can ever recover. The air-bone gap reports the size of the mechanical fault that surgery is being asked to repair. A patient with normal bone conduction and a 45 dB gap has a purely conductive problem behind an intact inner ear: the entire gap is, in principle, recoverable. A patient with raised bone conduction has a mixed loss, and reconstruction can only ever close the conductive part. Reading the audiogram is therefore the first act of prognosis, long before the drum is lifted. The whole reason the middle ear exists — its transformer action overcoming the impedance step between air and cochlear fluid — is what a conductive loss subtracts, and what the gap measures [1992].

FSizing the conductive loss

How big a gap can a purely mechanical problem produce? This question has a precise answer that every otologist should hold in mind. Even when the ossicular chain is completely interrupted, a faint acoustic couplingremains — the small direct difference in sound pressure between the oval and round windows still nudges the cochlea without help from the chain. That residual route sets a floor, so a conductive loss behind a normal cochlea cannot become total: the maximal conductive loss is only about 50 to 60 dB[1998, 1992]. This number is a yardstick. A “gap” larger than 60 dB is not a bigger mechanical lesion — it signals either an added sensorineural component or, as we shall see, a masking error.

For reporting and for judging surgical success, the gap is averaged across frequencies. The American Academy of Otolaryngology–Head and Neck Surgery convention computes a four-frequency pure-tone average and air-bone gap at 0.5, 1, 2 and 3 kHz, so that series can be compared on a common footing [1995]. A postoperative gap closed to within 20 dBis the usual benchmark of a functionally successful ossiculoplasty. As a rough clinical scale, an ABG of 25–40 dB is consistent with ossicular dysfunction such as fixation or a malleus problem, whereas a near-maximal gap approaching the 50–60 dB ceiling behind an intact drum points strongly toward frank discontinuity, classically erosion or separation at the incudostapedial joint[1998].

TConfiguration: inferring the lesion

Magnitude tells you how much is lost; configuration— the shape of the gap across frequency — often tells you what is wrong. Three patterns recur and are worth committing to memory, because each reflects a different mechanical failure.

• Flat, near-maximal gap.A broadband loss of 50–60 dB that is roughly even across frequencies, behind an intact drum, is the signature of complete discontinuity. The chain is broken, ossicular coupling is abolished, and only the weak acoustic route remains. The unloaded drum is hypermobile (type Ad tympanogram) and stapedial reflexes are absent.
• Low-frequency-weighted gap. A gap largest in the bass that tapers as frequency rises is the print of added stiffness— otosclerotic stapes fixation or tympanosclerotic fixation of the malleus head. Stiffness opposes motion most at low frequencies, where the system is naturally stiffness-controlled. The tympanogram tends to be shallow (type As).
• High-frequency-weighted gap. A gap that grows with frequency suggests partial (incomplete) discontinuity: a long process thinned to a compliant soft-tissue bridge that follows slow low-frequency motion but slips under rapid high-frequency motion. This is the pattern most easily mistaken for sensorineural loss.

That last pattern is more than a curiosity; it is a usable test. Defining the high-frequency conductive hearing-loss sign as the air-bone gap at 4 kHz minus the mean gap at 0.25 and 0.5 kHz, a large series found a positive sign predicted incomplete ossicular discontinuity at surgery with about 83% sensitivity and 92% specificity— a real diagnostic yield from an ordinary audiogram [2017].

One bone-conduction finding deserves separate mention because it is so often over-read. In fixation, the bone-conduction thresholds themselves can dip about 10–15 dB at 2 kHz — Carhart’s notch. This is not cochlear damage: bone-conducted sound normally reaches the cochlea partly through inertial motion of the chain near its resonance around 2 kHz, and fixing the chain removes that contribution, so the bone-conduction threshold artefactually worsens there. The proof that it is mechanical is that the notch typically improves after the chain is freed[1950]. Crucially, the notch indicates fixation in general but is not specific to the stapes: it appears at similar rates with malleus or incus fixation and even incudostapedial detachment, so it cannot localise the lesion on its own[2011].

TMasking: when the gap can lie

Every inference above assumes the thresholds are true. The single most important technical reason an audiogram misleads is inadequate masking. Sound presented to one ear can cross the head and be heard by the other cochlea; the loss of energy in crossing is the interaural attenuation. For air conduction it is substantial — roughly 40 dB with supra-aural earphones and more with insert phones — but for bone conduction it is close to 0 dB, because the vibrator shakes the whole skull and reaches both cochleae almost equally [1973]. The non-test ear must therefore be occupied with masking noise whenever it could be answering for the ear under test.

The consequence for ossiculoplasty is direct and dangerous. If the test ear has a true mixed loss but bone conduction is recorded without masking, the better opposite cochlea may “shadow” and answer first, plotting a near-normal bone-conduction threshold that belongs to the wrong ear. The result is a falsely large, falsely pure conductive gap: the sensorineural component vanishes and a mixed loss masquerades as a clean mechanical one. A surgeon who trusts that gap will both over-promise the result and risk missing a cochlear problem. Conversely, the so-called masking dilemma— a large bilateral conductive loss in which the masking needed for one ear crosses back and masks the other — can leave the true thresholds genuinely uncertain, and insert earphones (with their larger interaural attenuation) are the usual escape. The discipline is simple to state: a large air-bone gap is only believable once the bone-conduction threshold has been properly masked.

CMasqueraders of sensorineural loss

Two patterns deserve a clinician’s vigilance because they cross the conductive– sensorineural boundary in opposite directions. The first is the pseudo-sensorineuralear: a down-sloping high-frequency loss that looks cochlear until carefully masked bone conduction reveals a preserved bone line and a high-frequency-weighted air-bone gap. This is the audiometric face of incomplete ossicular discontinuity or laxity. If bone conduction is omitted, or masked sloppily, such an ear is mislabelled as sensorineural and a surgically correctable loss is missed; the loss may even fluctuate or improve briefly after a Valsalva manoeuvre, a further clue to a soft-tissue bridge [2017].

The second runs the other way: an inner-ear lesion that mimics a conductive, ossicular loss. A third-window abnormality — most often superior semicircular canal dehiscence— opens a low-impedance leak in the bony labyrinth that shunts air-conducted energy away (raising air-conduction thresholds) while enhancing bone conduction, sometimes to supranormal, better-than-0 dB values at low frequencies. The audiogram shows a low-frequency air-bone gap that imitates otosclerosis, but with two tell-tale differences: bone conduction is abnormally good rather than merely normal, and the acoustic reflex is usually preserved— whereas genuine stapes fixation abolishes it[2008]. Operating on such an ear as if it were otosclerosis is futile and can be harmful. The lesson is to treat preserved reflexes plus negative bone-conduction thresholds as a red flag for an inner-ear masquerader and to confirm with CT and vestibular-evoked myogenic potentials before any middle-ear surgery.

CFrom audiogram to operative plan

Put together, the audiogram is the surgeon’s first map of the middle ear. The bone-conduction line fixes the cochlear reserve and so the best result that is even theoretically achievable. The sizeof the air-bone gap, judged against the 50–60 dB ceiling, separates probable discontinuity from lesser ossicular dysfunction. The shape of the gap, read alongside the tympanogram and reflexes, narrows the differential to discontinuity, fixation, partial erosion or an inner-ear masquerader before the drum is ever lifted. And the quality control of proper masking decides whether any of those inferences can be trusted at all.

Two cautions temper this power. First, the audiogram describes a mechanical behaviour— lost coupling, added stiffness, added compliance — not a named anatomical lesion; it narrows the differential powerfully but the final answer is found at exploration. Second, and decisively for counselling, the achievable post-operative gap is governed less by the size of the starting gap than by the surviving scaffold— above all an intact, mobile stapes and a preserved malleus handle — together with middle-ear aeration and the absence of active disease. Staging systems built from large ossiculoplasty series show these anatomical and environmental factors predict the post-operative air-bone gap better than the prosthesis chosen [2001]. Read the gap to localise and to plan; read the surviving anatomy to predict what closing it will cost and how completely it can be done.