9Hearing-Preservation Surgery and EAS

FWhy preserve residual hearing

Acoustic low-frequency hearing carries fine spectral and temporal detail that a cochlear implant does not convey, so combining it with electric stimulation can improve speech understanding in noise and music perception beyond either alone. EAS targets candidates with usable low-frequency thresholds but severe high-frequency loss; in one major series patients had thresholds better than 60 dB HL at 125, 250, or 500 Hz and worse than 60 dB HL at 1000 Hz and above. In the Iowa/Nucleus Hybrid cohort, mean CNC word scores in the binaural-aided condition rose from 32% pre-implant to 75% at 9 months with combined Hybrid-plus-hearing-aid use. The apex is the site of low-frequency transduction, so the surgical objective is to leave the apical organ of Corti and its delicate structures undisturbed while stimulating the base electrically.[2009][2005][2006][2006]

TShort and flexible arrays

Hearing-preservation arrays are deliberately short so they do not reach or traumatise the most apical spiral ganglion cells; the Iowa/Nucleus Hybrid electrode is only 10 mm long and is designed to enter just the basal turn. Spiral ganglion cells span roughly 13.7 to 16.0 mm of the cochlea (about 1.5 turns), far less than the 33.1 to 35.6 mm spanned by the organ of Corti, so a short array can stimulate basal neurons without crowding the apex. Flexible lateral-wall designs lower insertion force and are paired with a round-window or extended round-window entry to avoid promontory drilling. In one EAS series, actual insertion depths ranged from 18 to 22 mm using atraumatic technique, a deliberate compromise between electrode coverage and apical preservation.[2014][2006][2013][2004]

CThe soft-surgery protocol for preservation

Round-window or modified round-window insertion avoids promontory drilling, reducing acoustic trauma and post-operative vertigo while keeping the entry into scala tympani clean. A drop of sodium hyaluronate on the opened endosteum lowers insertion force, and the array is then advanced immediately, slowly, and steadily, with the cochleostomy or round window sealed with fascia. Perioperative glucocorticoids are used to blunt the cochlear inflammatory response, delivered systemically and increasingly intratympanically before insertion; dexamethasone is the agent most often cited, though its efficacy has not yet been firmly proven. Real-time intraoperative electrocochleography lets the surgeon detect a fall in cochlear response and slow or pause insertion; in one study, complete hearing preservation (loss under 10 dB) reached 85% with electrocochleography feedback versus 33% without.[2014][2009][2013][2012]

COutcomes, delayed loss, and the trade-off

Across centres, hearing can be conserved in roughly 80% of ears in which preservation is attempted, with functional preservation usually defined by a low-frequency threshold shift under 10 dB. In the Iowa Hybrid series of 48 adults, immediate preservation was achieved in 47 of 48, but 3 lost more than 30 dB by about 3 months and 2 more showed progressive loss over 2 to 3 years, illustrating that some patients have intrinsically progressive disease. Delayed hearing loss is a recognised pattern: in one EAS cohort, several patients had initial preservation that progressed to total deafness at 7 to 18 months and a few lost all hearing immediately after surgery. Deeper insertion may improve electric speech perception by covering more of the cochlea but raises the risk of apical trauma and loss of residual hearing, the central trade-off the surgeon balances by choosing array length and target depth. For patients who held a threshold shift of 10 dB or less, word recognition rose from a mean of 13% pre-implant to 75% in electroacoustic mode.[2006][2006][2009]