2Impedance & the electrode interface
Impedance is the first number every cochlear-implant session produces — at the end of surgery, at switch-on, and at every follow-up. It is cheap, fast, and entirely objective, and it reports on the one part of the system that sits between the electronics and the neurons: the electrode-tissue interface. Read well, impedance tells you whether each contact is intact, whether the array is where you think it is, and whether the device can deliver the current you ask of it.
FWhat impedance measures
Electrode impedanceis the opposition to current flow at a contact, reported per channel — conventionally in kilohms (kΩ). It is governed by everything in the current's path: the electrode surface itself, the fluid and tissue immediately around it, and, over time, the fibrosis and new bone that the cochlea lays down in response to the implant.[2013]
Because impedance reflects the interface rather than the nerve, it answers a different question from every other measure in this atlas. The ECAPasks “does the nerve respond?”; impedance asks “can current get out of this electrode at all, and is the contact electrically normal?” The two are complementary, and a competent session always starts with impedance — there is no point chasing an absent ECAP on an open electrode.
Total impedance is often decomposed into two components: the access resistance (the resistive path through the fluid and tissue, sensitive to the bony/fibrous environment and to electrode position) and the polarisation impedance (the capacitive behaviour at the electrode surface itself). Some fitting software reports these separately; the distinction matters because a rise driven by access resistance points to the tissue environment, whereas a polarisation change points to the electrode surface.
Impedance and current spread also depend on where the array sits in the cochlea. Lateral-wall arrays lie against the outer wall; perimodiolar arrays curl toward the modiolus, closer to the spiral-ganglion neurons; and mid-scala arrays float centrally between the two. Toggle between them below — the proximity trade-off recurs throughout the atlas (it shapes thresholds and the spread of excitation in Module 4).
FTHow it is recorded
The implant delivers a small, charge-balanced current pulse on a contact and measures the resulting voltage; impedance follows from Ohm's law (Z = V / I). The measurement runs in seconds across the whole array and requires no response from the patient, which is why it is the universal opening move — feasible on a sleeping child, in theatre, or in an uncooperative adult.
Impedance depends on the stimulation mode, and comparing like with like matters:
- Monopolar — current returns to an extracochlear ground (the case and/or a remote ball electrode). This is the clinical default and gives the lowest impedances.
- Bipolar / common-ground — current returns to a neighbouring intracochlear electrode (or all others tied together). Useful diagnostically, but produces different (usually higher) values, so never compare a bipolar reading to a monopolar baseline.
TNormal ranges & the value of trends
Typical monopolar impedances fall in roughly the 1–10 kΩ range, device-dependent, with each manufacturer publishing its own acceptable window. The exact number matters less than two habits:
- Read the profile, not the single value. A flat profile across the array is reassuring; one electrode standing far above or below its neighbours is the signal.
- Read the trend over time. Impedances are characteristically high immediately after surgery, fall after a few weeks of stimulation (electrical activation reduces the interface impedance), and then stabilise. A contact that climbs steadily over follow-ups is more concerning than one that has always sat slightly high.
| Pattern | Typical interpretation |
|---|---|
| Flat, mid-range profile | Normal, intact array. |
| Diffuse high impedances at switch-on, falling by week 4 | Expected post-operative settling with stimulation. |
| Single very high contact | Open circuit — broken wire, extracochlear contact, or encapsulation. |
| Single very low contact / paired low contacts | Short circuit between contacts. |
| Rising impedances across many basal electrodes over months | Progressive fibrosis/ossification; consider in the context of falling performance. |
TCOpen and short circuits
Two fault signatures sit at opposite ends of the impedance scale, and both are usually handled by deactivating the offending contact in the MAP.
An open circuit shows abnormally high impedance — current cannot flow. Causes include a fractured electrode wire, a contact that has ended up outside the cochlea, or dense fibrous/bony encapsulation. An short circuit shows abnormally low impedance because two contacts are electrically connected, which blurs the spatial selectivity the array depends on. Shorts are detectable on impedance telemetry and corroborated by the transimpedance matrix.[2013]
A normal impedance only means current can flow into the tissue — it says nothing about whether neurons are there to receive it. A fibrotic but conductive interface can show acceptable impedance while the underlying spiral ganglion population is poor. This is exactly why the toolbox is layered: impedance clears the interface, the ECAP then asks the nerve.
CThe transimpedance matrix
The transimpedance matrix (TIM) generalises the single-electrode impedance into a grid: each electrode is stimulated in turn while the voltage is recorded at every other electrode. The resulting pattern of voltage spread is highly structured in a normal array — voltage falls off smoothly with distance from the stimulating contact.[2004]
Departures from that smooth pattern are diagnostic. Shorts show as anomalously high coupling between specific contacts. Most usefully, a tip fold-over — where the array tip doubles back on itself during insertion — produces a characteristic distortion in the matrix, because two electrodes that should be far apart along the array end up physically adjacent. TIM can therefore flag a fold-over from the recording booth, prompting a confirmatory radiograph, without exposing the patient to imaging as a first step.
CVoltage compliance
Impedance also sets a practical ceiling on stimulation. The implant can only develop so much voltage (its compliance limit); on a high-impedance contact, the current needed to reach a comfortable level may demand a voltage the device cannot supply. The electrode is then out of compliance, the delivered current is clipped, and loudness growth on that channel is compromised. Recognising this prevents a fruitless search for a behavioural problem when the limitation is electrical — the fix is usually a wider pulse width (which delivers the same charge at lower current) or deactivating the contact.[2014]
What does the impedance finding tell you, and what is the immediate programming step?
Which stimulation mode normally gives the lowest impedance values and is the clinical default?
An abnormally LOW impedance on a contact most suggests:
Which objective measure can flag an electrode-array tip fold-over without imaging?