Tympanogram types A, As, Ad, B, and C explained

A tympanogram plots the admittance of the middle ear system as air pressure in the ear canal is swept from positive to negative (or the reverse), usually with a 226 Hz probe tone. The result is a curve with a peak where the pressure in the canal matches the pressure in the middle ear, because that is the point where the eardrum moves most freely.

Jerger's classification sorts that curve into five everyday types by two features: where the peak sits on the pressure axis, and how tall the peak is. Getting comfortable with all five, and with the one case that needs a third piece of information, covers the vast majority of tympanograms seen in a clinic.

Type A is the normal pattern: a distinct peak at a normal pressure, roughly negative 100 to positive 50 daPa, with a normal peak admittance, about 0.3 to 1.6 mmho in an adult. Two variants of Type A share the normal pressure but not the normal height.

Type As keeps the normal pressure but the peak is shallow, below the lower limit of normal admittance. It points to a stiffened system: otosclerosis, tympanosclerosis, or a thickened eardrum are the classic causes. Type Ad also keeps the normal pressure but the peak is unusually tall, above the upper limit of normal admittance, sometimes off the scale entirely. It points to a system that is too mobile: ossicular discontinuity, a flaccid or monomeric eardrum, or a healed perforation.

Type B is flat: no identifiable peak appears anywhere in the pressure sweep, so a peak pressure cannot even be read. Type C has a real peak, so the system is mobile, but the peak sits at a pressure more negative than about negative 100 daPa, which points to eustachian tube dysfunction or developing or resolving middle ear fluid.

Type B is the one case where the shape of the curve is not enough. A flat line can mean three completely different things, and the equivalent ear canal volume, the physical volume of air between the probe tip and whatever it is sealed against, is what tells them apart.

A flat tympanogram with a normal ear canal volume points to middle ear effusion: the probe is sealed against an intact but immobile eardrum, because fluid behind it prevents any pressure-driven movement. A flat tympanogram with an abnormally large volume, clearly bigger than the other ear, points to a tympanic membrane perforation or a patent pressure equalization tube, because the probe is now measuring the canal plus the middle ear space beyond the hole. A flat tympanogram with a volume near zero usually means a technical problem: cerumen against the probe tip, or the probe wedged against the canal wall, rather than a diagnosis at all.

A Type A tympanogram with normal acoustic reflexes and normal audiometry describes a normal ear. A Type As or Type B tympanogram in a conductive loss typically comes with absent reflexes on the probe side, because the stiffened or fluid-filled system cannot transmit the small admittance change the reflex produces. A Type Ad tympanogram can appear with a conductive loss too, since a discontinuous ossicular chain is both hypermobile and poor at transmitting sound. A Type C tympanogram by itself is common and often resolves without treatment; it becomes more clinically relevant paired with a mild conductive loss or reduced reflexes than as an isolated finding.