11Troubleshooting the MAP

TNon-auditory facial-nerve stimulation

Facial-nerve stimulation appears as eyelid, cheek or perioral twitching synchronised to sound, most often from basal contacts whose current spreads to the labyrinthine facial nerve. It is over-represented in otosclerosis, where otospongiotic bone adjacent to the labyrinthine facial segment lowers resistance, and in malformed inner ears, so facial-nerve monitoring is advised in these surgeries. First-line management lowers the C level on the offending contact to just below the twitch threshold, which often preserves a usable channel. Widening the pulse width or switching that contact to a more focused coupling mode (bipolar or pseudomonopolar) confines current and can abolish the twitch. If twitching persists at audible levels the contact is deactivated and its frequency band reallocated to neighbouring channels. Sweeping at the C level is the diagnostic step that isolates which single contact provokes the non-auditory response.[2009][2014]

TDead and abnormal electrodes

Impedance telemetry separates a short circuit (impedance below about 1 kilohm, shared voltage) from an open circuit (above about 30 kilohms, often infinite with a broken lead). A contact that elicits a static-like noise, no auditory percept, or a non-auditory side effect is functionally abnormal even when its impedance reads normal. Confirmed shorts are permanent and both coupled contacts are disabled; opens are disabled but re-checked after use, since fresh-activation elevation often resolves. On a voltage-distribution table the stimulated contact should carry far more voltage than its neighbours, with adjacent contacts under about 50% of the stimulated value; near-equal voltages confirm a short. Deactivating a contact requires reallocating its frequency band so the recipient keeps full bandwidth across fewer channels. Needing to deactivate five or more electrodes is itself a warning sign of impending device (soft) failure in both children and adults.[2020][2009]

CPitch reversals and non-monotonic pitch

Sweeping from apex to base should produce a smooth low-to-high pitch progression; a contact that sounds lower than its more apical neighbour is a pitch reversal. Pitch reversals and non-monotonic pitch arise from focal neural-survival gaps, current spread, or electrode contacts that landed out of expected tonotopic order. Where pitch is scrambled, the audiologist can reallocate the offending contact to a different channel so frequency order is restored, or disable it if no orderly place can be found. Pitch scaling is routine and essential in auditory brainstem implants, where the cochlear nucleus surface is not simply tonotopic, but the same logic rescues a disordered cochlear-implant map. Sweeping at the C level has three objectives at once: sound quality, orderly pitch transition, and equal loudness across channels. Loudness imbalance is corrected by channel-by-channel balancing at C, adjusting the second contact of each pair to match the first.[2020][2009]

CDizziness and sound-quality complaints

Stimulation-locked dizziness or a vestibular sensation points to current spread to vestibular structures; lowering C on the implicated (often basal) contact or deactivating it resolves it. A tinny, sharp or over-bright percept usually means the high-frequency basal channels are weighted too loud; lowering C on basal contacts or rebalancing the C profile softens it. Echoey, hollow or muddy sound often reflects excessive low-frequency emphasis or too wide a dynamic range; tilting the profile toward treble or tightening the dynamic range can sharpen clarity. Soft sounds that are too loud or too soft are addressed by sweeping and balancing at the lower portion of the dynamic range, around 50%, not only at C. Datalogging and sound-field detection thresholds (audible across 250 to 6000 Hz) verify that complaints reflect the map rather than microphone deterioration, which first shows as raised high-frequency thresholds. Before reprogramming, faulty external hardware, especially the transmitting cable, the most failure-prone external part, must be excluded.[2020][2014]