10Objective Measures at the Fitting

FImpedance telemetry: the first thing measured

Healthy intracochlear contacts usually sit between roughly 1 and 15 kilohms in monopolar mode, with a typical ground-path impedance near 1 kilohm. A value below about 1 kilohm flags a short circuit, where two contacts are unintentionally coupled and share an identical voltage; shorts are persistent and the linked contacts are permanently disabled. A value above roughly 30 kilohms marks an open circuit from a broken lead, air bubble, protein build-up or ossification; the open-circuit limit is lower (about 20 kilohms) on some array families. Impedances are lowest at surgery because contacts sit in perilymph, then climb over the first weeks from protein, macrophage and fibrous-tissue deposition before stabilising. Contacts at modestly high values of 15 to 25 kilohms should not be disabled at activation, since stimulation typically drops them substantially within days or even minutes of use. Because impedance, current and voltage obey Ohm's law in a series circuit, a recipient with high impedance and a high C level often needs a wider pulse width to deliver the required charge.[2020][2008]

TECAP (NRT/NRI/ART) as a starting profile

The evoked compound action potential is the synchronous whole-nerve response recorded through the implant's own electrodes; it is recordable in about 95% of recipients, the main exceptions being cochlear nerve deficiency and ossification. Each manufacturer brands it differently: neural response telemetry (Cochlear), neural response imaging (Advanced Bionics) and auditory response telemetry (MED-EL). Because it is a large near-field response, it needs only about 32 to 50 sweeps and is unaffected by sleep, so a child need only be still rather than awake and cooperative. Threshold is read off an amplitude growth function from suprathreshold points; the stimulus is a single biphasic pulse at 30 to 80 pulses per second, far slower than the 250 to 5000 of a live speech-coding strategy. Profile-based mapping sets T and C to follow the ECAP threshold contour, then globally raises the whole profile in live-speech mode to behavioural threshold and comfort; scaling the profile improves the C-level estimate because loudness growth flattens the profile at high levels. The ECAP threshold is stable from activation onward, so a new absence or a marked shift over time is an objective red flag for a device or neural problem.[2000][2020][2013]

CThe stapedial reflex and eABR: cross-checking the C level

The electrically evoked stapedial reflex threshold is a simple, recordable-at-any-age objective anchor for the upper-stimulation level and is the recommended method for setting C/M in young children. C is generally placed just below the reflex threshold, typically 5 to 10% below it; if the reflex is at 200 charge units the estimated M level lands around 180 to 190 unless the child shows distress. The reflex tracks behavioural C levels closely, falling within roughly 10 to 15 clinical units of C in Nucleus recipients. The reflex is unrecordable through patent pressure-equalisation tubes or middle-ear disease, so it cannot always substitute for behaviour. The electrically evoked auditory brainstem response is a poor predictor of general implant benefit but retains value in auditory neuropathy spectrum disorder, where a normal response predicts better speech recognition than an abnormal one. Used together, the stapedial reflex corroborates the upper level and the brainstem response confirms central neural transmission, both serving as checks on, not replacements for, behaviourally set levels.[2020][2018]

CWhy the correlation is only modest

The ECAP-to-behavioural-threshold correlation is moderate at best and worsens as the behavioural rate rises toward the fast rates used in modern strategies, because ECAP must be measured at a slow 30 to 80 pulses per second. There is no one-to-one relationship: in the same recipient the ECAP threshold may sit near T on some contacts, near C on others, and even above C elsewhere along the array. Adding one behavioural anchor electrode to the ECAP profile only nudges the correlation from weak to weak-moderate. Amplitude growth function slope reflects neural survival and is steeper with better-preserved nerve and with monopolar stimulation, but it has not been shown to predict speech recognition or MAP levels. Objective measures are therefore most valuable as a starting profile and a safety net for the untestable patient, never as the sole basis for a finished map when reliable behavioural responses can be obtained. The full physiological toolkit and its mechanisms are developed in the Objective Measures chapter (Chapter 27), to which the fitting links.[2020][2013][2014]