CI Atlas · Audiological Evaluation · Module 10

10ASSR, ECochG & cortical responses

The brainstem response is the workhorse of objective audiometry, but it cannot do everything, and a family of related electrophysiologic tests fills its gaps. When the question is whether a profoundly impaired ear has any usable hearing at all, the steady-state response can be driven louder than the brainstem response and resolve a threshold the ABR can only call 'absent.' When the question is whether the inner ear is swollen by hydrops, electrocochleography reads the cochlea's own potentials. And when the question is whether amplified sound is actually reaching the cortex of a deaf child, the slow cortical response can answer where threshold tests cannot. This module covers these adjuncts — their distinct strengths and their pitfalls — and the pre-operative electrical probes used when no behavioural hearing exists. The through-the-implant versions belong to the Objective Measures chapter.

TASSR — separating severe from profound

The auditory steady-state response uses amplitude/frequency- modulated tones with automated statistical detection, gives simultaneous multi-frequency and binaural thresholds, and can be driven to ~120 dB nHL — so it can separate severe from profound loss where tone-burst ABR saturates (~95 dB). Its pitfalls (broad cochlear spread; myogenic responses masquerading as a present ASSR) mean it is used as a supplement confirmed by manual tone-burst ABR.[2006]

CElectrocochleography

Electrocochleography records three near-cochlear potentials: the cochlear microphonic (outer-hair-cell AC potential that reverses with polarity), the summating potential (a DC distortion of basilar-membrane motion), and the compound action potential (N1, coinciding with ABR wave I). An enlarged SP/AP ratiosupports endolymphatic hydrops / Ménière's disease, though SP variability limits it.

CMiddle-latency & cortical responses

The middle-latency response (Na-Pa-Nb) and the 40-Hz response estimate threshold in non-cooperative patients but are sensitive to arousal and anaesthesia. The cortical auditory evoked response (P1-N1-P2) indexes thalamo-cortical activity to speech: an absent aided cortical responseimplies amplification is not reaching the cortex (supporting early implantation), while a present one suggests adequate stimulation — linking back to the plasticity chapter's P1 biomarker.

CPre-operative electrical stimulation

When no behavioural thresholds exist — ossification, head injury — pre-operative promontory or round-window electrical stimulation, with an evoked eABR or a behavioural percept, probes surviving auditory-nerve responsiveness. Its prognostic value is limited and it is now an optional adjunct. The intra-operative and through-the-implant electrically evoked measures are detailed in the Objective Measures chapter (Chapter 27).[2013]

Where the click-ABR gives a single broadband threshold, the ASSR delivers frequency-specific thresholds at 500–4000 Hz and can track very high stimulus levels, so its great value is constructing an estimated audiogram in an infant too young for reliable behavioural testing. Because the electrophysiologic threshold sits a little above the true behavioural one, a frequency-dependent correction is applied, and the estimate is always cross-checked behaviourally as the child develops. It is how a profound loss is documented early enough to act on. Illustrative; schematic.

Case 10.10 · Severe or profound?

Tone-burst ABR shows no response at the equipment maximum (~95 dB), leaving the team unsure whether the child has a severe or profound loss — a distinction that affects device counselling.

Which test best resolves this?

Self-assessment — Module 102 questions

Question 1 · Trainee

What can ASSR do that tone-burst ABR cannot?

Question 2 · Clinician

An enlarged ECochG SP/AP ratio supports which diagnosis?

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