A sensorineural hearing loss is a failure of the cochlea or the auditory nerve. Sound reaches the inner ear normally, but it cannot be transduced and carried onward as it should. This is the larger of the two families of hearing loss, and the special tests exist largely to subdivide it.

The defining feature: no air–bone gap

Because the fault lies in the cochlea or beyond, it impairs sound arriving by either route equally. Air conduction and bone conduction are both abnormal and track together — there is nosignificant air–bone gap. That single observation separates a sensorineural loss from a conductive or mixed one.

Normal hearing

Presbycusis

O Right ear, air
X Left ear, air
< Right ear, bone
> Left ear, bone

Signature: Symmetrical, gently sloping high-frequency sensorineural loss; air and bone conduction track together.

Figure 1. Sensorineural loss versus normal hearing. In the sensorineural ear, air and bone conduction are equally depressed and overlap — there is no air–bone gap.

Cochlear versus retrocochlear

The audiogram tells you a loss is sensorineural but not where in the sensorineural pathway the lesion sits. The critical division is between a cochlear lesion — in the hair cells of the inner ear — and a retrocochlear lesion — in the auditory nerve or its central connections. They can produce identical audiograms yet demand very different management.

Trainee This is where the special-test battery earns its place. A cochlear lesion damages outer hair cells, producing recruitment: the SISI is typically positive, and tone decay is minimal. A retrocochlear lesion spares the cochlear amplifier but fatigues abnormally fast: the SISI is typically negative, and tone decay is marked. The same contrast shows in other tests — acoustic reflex decay, speech discrimination disproportionate to the pure-tone loss, and auditory brainstem responses.

Configuration as a clue to cause

The shape of a sensorineural audiogram narrows the differential considerably:

Gently sloping, high-frequency, symmetrical — the pattern of presbycusis.
Notched at 3–6 kHz — suggests noise-induced hearing loss.¹⁶
Low-frequency, rising, unilateral and fluctuating — suggests Ménière’s disease.
Asymmetric, especially high-frequency — raises the possibility of a retrocochlear lesion and should prompt further investigation.

Clinician Asymmetry is the audiometric red flag that matters most. A genuine, unexplained between-ear difference in a sensorineural loss — particularly with poor speech discrimination on the worse side, or unilateral tinnitus — warrants exclusion of a retrocochlear lesion, today chiefly with MRI. The audiogram cannot make that diagnosis, but it is very often the test that first raises the question. The individual disease modules that follow take each configuration in turn.

Sudden sensorineural hearing loss

One pattern is an emergency rather than a configuration. A rapid, unexplained, usually unilateral sensorineural loss developing over hours to days is sudden sensorineural hearing loss. It is treated as urgent because timely intervention may improve the outcome, and because it can occasionally be the presenting feature of a retrocochlear lesion. An audiogram that documents a new, marked, one-sided sensorineural loss should never be filed away without action.