11Activating and Programming the Electric Brainstem

FA first activation with the crash cart ready

An ABI is usually switched on around six to eight weeks after surgery, once the brainstem has settled. But the first activation is unlike any cochlear-implant fitting, because the electrodes sit on the brainstem surface, the very first pulses can in principle reach vasoactive centres and lower cranial nerves. For that reason initial stimulation is done where emergency help and resuscitation equipment are immediately at hand, historically in hospital and now often in a dedicated audiology room with a crash cart and emergency medical access on standby.

The clinical question at switch-on is not simply how loud each electrode sounds. It is whether stimulating a given electrode produces a clean auditory sensation, an unpleasant non-auditory one, or some mixture, and how those change with the stimulation settings. The audiologist is mapping a brainstem whose response is far less predictable than a cochlea, so the first session is exploratory rather than a quick set of measurements.[2008][2012]

TInterrogating each electrode

Programming proceeds electrode by electrode. Each contact is stimulated, usually in monopolar and sometimes bipolar mode, and the patient reports the quality, loudness and pitch of what they perceive together with any tingling, pulling, dizziness or throat sensation. Electrodes that produce a useful auditory percept go into the map; those that produce only or mainly non-auditory effects are switched off. Threshold and maximum comfortable levels are then set for the surviving electrodes, and pulse duration and the reference electrode are tuned to suppress lingering side-effects.

Because the paddle has no fixed relationship to the tonotopic layout of the cochlear nucleus, pitch must be discovered rather than assumed. In pitch ranking, two electrodes are stimulated in turn and the patient says which is higher; repeating this across all usable electrodes builds an empirical tonotopic order. That order rarely matches the physical electrode numbering, so the processor map is assembled from this individually-measured pitch sequence rather than a standard template.[2002][2004]

CWhy ABI fitting is slower and more individual

A cochlear implant can lean on a fairly standard stimulation pattern because the surviving neurons in the cochlea are tonotopically homogeneous and the array position is reproducible. None of that holds for the ABI. Brainstem anatomy varies, paddle placement varies, and tumour and surgery leave their own marks, so each recipient needs an individually built program. The number of usable electrodes can range from nearly all to almost none, and the pitch order is bespoke. All of this takes extra sessions and extra patience.

Follow-up reflects this. Recipients are typically seen soon after activation and then at progressively spaced intervals, because both sound quality and the burden of non-auditory sensations tend to change over months as the patient adapts and as electrodes that were initially intolerable sometimes become usable. Speech testing is deliberately scaled from simple sound detection upward, so that an ABI user, whose expected speech perception is lower than a cochlear-implant user’s, is measured on tasks they can actually succeed at rather than discouraged by tests beyond reach.[2002][2009]

CManaging expectations alongside the map

Even with careful programming, many recipients are initially disappointed by the quality of ABI sound, and a large part of the audiologist’s job is counselling that quality and the tolerability of non-auditory effects generally improve with time and experience. For most NF2 users the realistic gain is robust environmental sound awareness and a powerful boost to lip-reading rather than open-set speech, although some non-tumour users do considerably better.

The penetrating ABI illustrates how individualised this can become. With both a surface and a penetrating array, the processor can run surface electrodes alone, penetrating electrodes alone, or a combination, and each configuration must be tested in the same patient to find what gives the best perception. The penetrating electrodes reach auditory neurons at lower threshold and can broaden the range of pitches, but they add yet more programming time, reinforcing the theme that ABI fitting is a sustained, personalised process rather than a single appointment.[2008][2002]