16Electric-Acoustic Stimulation and Hearing Preservation

FExploiting tonotopy

EAS exploits tonotopy: a short atraumatic array provides electric stimulation for mid-to-high frequencies while preserved native low-frequency acoustic hearing (typically normal-to-moderate loss 125–500 Hz) is amplified acoustically, leaving the apical low-frequency region undisturbed. Functional gains over electric-only include better speech in noise, melody/music, localisation and pitch — because acoustic fine temporal structure carries cues the pulsatile electric signal cannot (cross-ref Ch.8, Module 9).[2020]

TThe short hybrid arrays

Hybrid arrays are deliberately short, thin, flexible lateral-wall electrodes for atraumatic placement. The Nucleus Hybrid lineage ran S8 (10 mm, 6 contacts, Iowa/Gantz) → S12 (10 mm, 10 contacts) → L24 (~16 mm, 22 half-banded contacts over 15 mm, ~230–270°, FDA Nov 2013, first US Hybrid; candidacy >60 dB HL at 500 Hz); lead wires are only ~20 microns.[2005]

CMED-EL's EAS arrays

MED-EL's EAS arrays sit on the FLEX ladder: FLEX24 (formerly FLEXEAS, US 2012, contacts over 20.9 mm) and FLEX20 (~20 mm, ~360°, contacts over 15.4 mm, for considerable low-frequency hearing with high-frequency deafness ≥1500 Hz); the SYNCHRONY EAS system was FDA-approved 2016. EAS processors (Nucleus 7 EAS, AB Naida CI Q90 EAS, MED-EL SONNET EAS) integrate an acoustic receiver — the Nucleus 7 offers receivers of 60/85/100 for losses up to ~60/85/100 dB HL.[2013]

FSoft-surgery principles

Soft-surgery / atraumatic principles are now standard even without preservation intent: slow controlled insertion, round-window or small cochleostomy entry, a ~1 mm diamond burr at 5,000–10,000 rpm, irrigation/suction before opening the cochlea but not after, and perioperative steroids to blunt cochlear inflammation; drug-eluting (dexamethasone) electrodes are in development (cross-ref Ch.15 Surgical Technique).[2007]

TThe risk of delayed loss

Delayed or progressive loss of residual hearing is the major risk: ~90% of Hybrid S8 patients had initial preservation but 17/87 were re-implanted with longer arrays, and ~30% of S12 patients had ≥30 dB low-frequency shifts. This drove the shift from 10 mm to 16–24 mm arrays so electric coverage remains if hearing is lost — and even those who lose residual hearing usually still beat their preoperative bilateral hearing-aid scores.

CMonitoring and mismatch

Intraoperative ECoG/cochlear-microphonic/impedance monitoring can guide atraumatic insertion: in a prospective trial, surgeons given live ECoG feedback who slowed insertion achieved 85% complete preservation (<10 dB loss) versus 33% blinded (cross-ref Ch.23). A short array's basalward frequency mismatch limits speech performance and only partially adapts over a year — the central trade-off of EAS array length (cross-ref Module 10).