4Tympanometry and Acoustic Reflexes

FWhat immittance measures

Tympanometry sweeps the ear-canal pressure while a probe tone (classically 226 Hz) is played, then plots how much sound the eardrum admits at each pressure. The peak of that curve tells you the pressure at which the middle ear is stiffest-free — ideally near atmospheric — and the height tells you how mobile the tympanic-ossicular system is. Unlike pure-tone audiometry, it needs no behavioural response, so it objectively separates a conductive problem from a sensorineural one.

This matters in tinnitus because a conductive lesion changes what the patient hears in two ways: it attenuates ambient masking sound (unmasking internal noise) and, for some pathologies, it physically generates sound. A normal study does not exclude tinnitus — most subjective tinnitus has a normal tympanogram — but an abnormal one redirects the work-up toward a potentially treatable middle-ear cause [2013].

TReading the tympanogram: A, As, Ad, B, C

The Jerger classification names the common shapes. Type A is a normal peak near 0 daPa with normal admittance. Type As (‘shallow’) keeps a normal peak pressure but a reduced height — a stiff system, the signature of otosclerosis or a fixed ossicular chain. Type Ad (‘deep’) is abnormally compliant, seen with ossicular discontinuity or a monomeric/flaccid drum. Type B is flat with no clear peak — effusion, a perforation, or impacted cerumen, distinguished by the simultaneously measured ear-canal volume. Type C peaks at markedly negative pressure, indicating Eustachian-tube dysfunction and a retracted drum.

Each pattern points to a tinnitus mechanism: an As otosclerotic ear may report a low buzzing tinnitus that improves after stapedotomy; a type C ear with chronic fullness frequently carries a low-pitched tinnitus that tracks the Eustachian-tube state [2014].

TThe acoustic reflex and reflex decay

A loud sound (roughly 70–100 dB above threshold) triggers the stapedius muscle to contract bilaterally, stiffening the ossicular chain and producing a measurable drop in admittance — the acoustic (stapedial) reflex. Because the reflex arc runs cochlea → auditory nerve → brainstem → facial nerve → stapedius, its presence and threshold report on the integrity of that whole loop, adding information neither audiometry nor tympanometry gives alone.

In retrocochlear disease the reflex is the classic early casualty. With a vestibular schwannoma the ipsilateral reflex may be absent or elevated despite only mild threshold loss, and reflex decay — the amplitude falling to less than half within 10 seconds of a sustained 500 or 1000 Hz tone — is a recognised sign of an eighth-nerve lesion. These findings should lower the threshold for imaging in the patient with unilateral tinnitus, although MRI has now displaced reflexes as the definitive screen [2025].

CThe stapedial reflex in middle-ear myoclonus

Objective tinnitus — sound a clinician can also detect — is rare, and middle-ear myoclonus is one of its mechanical causes. Rhythmic involuntary contraction of the stapedius or tensor tympani produces a clicking, fluttering or buzzing tinnitus, often irregular and sometimes triggered by sound or eye-blink. On immittance the spasm appears as spontaneous rhythmic deflections of the admittance trace synchronised with the patient’s reported click, effectively a continuous read-out of the muscle’s activity.

Recognising this pattern matters because the condition is treatable — reassurance, botulinum toxin, or surgical tenotomy of the offending tendon in refractory cases — and confirming the muscle origin on immittance avoids an unnecessary vascular imaging cascade [2013].

CWhat immittance adds to the tinnitus work-up

Immittance is cheap, fast and objective, so it earns its place as a second-tier test once audiometry and otoscopy are done. Its yield is highest in three situations: a conductive or mixed loss needing a mechanical explanation, unilateral tinnitus where an absent reflex or reflex decay flags retrocochlear risk, and a clicking or fluttering tinnitus where rhythmic immittance changes confirm a myoclonic source.

Equally important is what a normal study tells you: a type A tympanogram with present, normally-decaying reflexes makes a treatable middle-ear or proximal eighth-nerve cause unlikely, supporting a confident move toward a subjective, sensorineural-driven model and away from over-investigation [2013].