Retrocochlear pathology is disease of the auditory nerve or its central connections — beyond the cochlea. The prototype is the vestibular schwannoma, a benign tumour of the eighth nerve. This module is where the special tests of the earlier chapters come together, because retrocochlear disease is exactly what they were designed to detect.

The audiometric signature

The characteristic finding is an asymmetric sensorineural hearing loss, usually worse in the high frequencies, in one ear, often with unilateral tinnitus. There is no air–bone gap — it is a sensorineural loss — and the key feature is the asymmetry rather than any particular shape.

Normal hearing

Vestibular schwannoma

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

Signature: Asymmetric high-frequency sensorineural loss in the affected ear; normal hearing on the other side.

Figure 1. Retrocochlear loss versus normal hearing. An asymmetric, high-frequency sensorineural loss confined to one ear, with the other ear normal — the audiometric pattern that should raise the retrocochlear question.

Trainee The audiogram alone cannot distinguish a retrocochlear loss from a cochlear one of the same shape — and that is the central problem. A patient with a sloping unilateral sensorineural loss may have a benign cochlear cause or a schwannoma. Several findings shift suspicion toward retrocochlear disease: speech discrimination far poorer than the pure-tone loss would predict, unilateral tinnitus, and the special-test pattern below.

The special-test pattern

The classic retrocochlear signature on the behavioural battery is the mirror image of the cochlear one:

SISI negative. A retrocochlear lesion spares the cochlear amplifier, so there is no recruitment and the patient detects few of the 1 dB increments.³
Tone decay marked. The diseased nerve fatigues abnormally fast, so a large rise in level is needed to keep the tone audible.¹
Acoustic reflex decay abnormal and speech discrimination disproportionately poor — both pointing the same way.

Clinician An honest account has to be clear about the limits of these tests. The behavioural site-of-lesion battery — SISI, tone decay and the rest — was the best available tool before modern imaging, but its sensitivity for small tumours is limited, and it has been largely superseded. Today the investigation of an unexplained asymmetric sensorineural loss is auditory brainstem response testing and, definitively, MRI. The behavioural tests retain real teaching value — they make the cochlear/retrocochlear distinction concrete — but a normal SISI and modest tone decay do notexclude a schwannoma. The audiogram’s job is to raise the question; imaging answers it.

The take-home rule

The single most useful habit from this module is simple: an unexplained, genuinely asymmetric sensorineural hearing loss deserves further investigation. The audiogram cannot diagnose a retrocochlear lesion, and neither can the behavioural special tests with certainty — but together they are very often what first prompts the imaging that does.