14EAS and Special Maps

CProgramming electric-acoustic stimulation

EAS combines an electric high-frequency map with acoustic amplification of the preserved low frequencies in the same ear, so the fitter programmes two devices that share one cochlea. The crossover frequency, where acoustic stimulation hands over to electric, is commonly set by default to the frequency at which the audiogram crosses 65 dB HL. A use-crossover-as-minimum option lets electric stimulation begin exactly where acoustic amplification ends, for example near 850 Hz, so the two halves abut without a gap or an overlap. Manufacturer software allows the electric start frequency to be nudged in fine steps as small as 10 Hz to fine-tune the seam between acoustic and electric hearing. Because preserved hearing can drift after surgery, the acoustic audiogram is re-entered and the crossover re-checked at follow-up rather than fixed once at activation.[2020][2005][2006]

TFitting after partial or shallow insertion

When only part of the array sits inside the cochlea, the unused or extracochlear electrodes are deactivated and the speech frequencies are redistributed across the contacts that remain. A shallow lateral-wall insertion reaches only the basal turn, so the active electrodes carry a frequency map shifted higher than the contact's natural place, a mismatch the brain gradually accommodates. Fewer active channels reduce spectral detail, so the fitter weighs each questionable electrode for its contribution before disabling it. In hearing-preservation surgery the basal electrodes deliver high frequencies electrically while the apical low frequencies are left to acoustic amplification, the EAS arrangement in miniature.[2020][2014][2013]

TOssified, malformed and split-array cochleae

Post-meningitic ossification or fibrous tissue can block a full insertion, sometimes requiring a split (double) array placed through two drill-outs into the basal turn. With a split array the fitter must reorder the tonotopic assignment of the band-pass filters according to which sub-array lies in the lower-basal versus the upper-basal region. In a common-cavity malformation the array may be inserted through a posterior cochleostomy, which can require inverting the channel-to-electrode order so high frequencies map to the correct contacts. Where the measured pitch order does not match the default (revealed by sweeping a stimulus across electrodes), channels are reassigned to restore an ascending tonotopic percept.[2020][2009][2014]

CManaging tinnitus and non-auditory stimulation through the map

An electrode that evokes facial-nerve twitching, tactile sensation or pain instead of or alongside sound is identified by sweeping and is reduced in level, reprogrammed, or disabled. Facial-nerve stimulation is more likely with monopolar mode and on electrodes with abnormal impedance, so switching to a focused mode or lowering the upper level can rescue a useful channel before deactivating it. An open circuit, usually flagged at an impedance above about 30 kilohms, is disabled and the adjacent frequencies are redistributed, then re-checked after a period of use as impedances often fall. Many recipients find that everyday implant use itself suppresses pre-operative tinnitus, and the map can be tuned to maintain comfortable, continuous low-level stimulation that masks it. Disabling any electrode changes the frequency allocation across the array, so it is done only when an electrode is genuinely faulty, painful or non-auditory rather than routinely.[2020][2014][2014]