Reading the Patterns
Most of the diagnostic value of the acoustic reflex lies not in any single threshold but in the shape the results make across the four-cell grid.
Three grid shapes account for the great majority of localising information. Learning to recognise them — and to know what each implies about the site of lesion — is the core skill of reflex interpretation.
The probe-ear pattern
When reflexes are absent for both conditions recorded with the probe in one ear, but normal with the probe in the other, the pattern is vertical — a probe-ear pattern. It localises the problem to the probe ear itself.
Two mechanisms produce this shape. A conductive lesion in the probe ear blocks measurement of the admittance change. An efferent-limb lesion — facial nerve palsy proximal to the stapedial branch — prevents the muscle from contracting. Tympanometry and the audiogram separate the two.
The stimulus-ear pattern
When reflexes are abnormal whenever one particular ear is stimulated — regardless of probe position — the pattern is horizontal, a stimulus-ear pattern. It localises the problem to the afferent limb of that ear.
A severe cochlear loss or an eighth-nerve lesion produces this shape. When it is accompanied by positive reflex decay, a retrocochlear site is favoured over a cochlear one[3].
The diagonal pattern
When both contralateral cells are abnormal while both ipsilateral cells are preserved, the abnormal cells lie on a diagonal. This crossed-uncrossed dissociation is the signature of an intra-axial brainstem lesion interrupting the midline crossing fibres[3].
This pattern is the strongest argument for always recording both ipsilateral and contralateral reflexes. A lesion confined to the crossing fibres would be entirely invisible on ipsilateral testing alone.
The Metz test for recruitment
Beyond the grid shape, the relationship between the reflex threshold and the pure-tone threshold carries information. The Metz test identifies recruitment — the abnormally rapid growth of loudness typical of cochlear loss — when the reflex appears within 60 dB of the pure-tone threshold[14].
A reduced sensation level of the reflex is an objective cochlear sign, obtainable without behavioural cooperation, and most informative in unilateral or asymmetric losses.