Barriers to Early Progress in Adult Cochlear Implant Outcomes

Abstract

Objectives: Adult cochlear implant (CI) recipients obtain varying levels of speech perception from their device. Adult CI users adapt quickly to their CI if they have no peripheral "bottom-up" or neurocognitive "top-down" limiting factors. Our objective here was to understand the influence of limiting factors on the progression of sentence understanding in quiet and in noise, initially and over time. We hypothesized that the presence of limiting factors, detected using a short test battery, would predictably influence sentence recognition with practical consequences. We aimed to validate the test battery by comparing the presence of limiting factors and the success criteria of >90% sentence understanding in quiet 1 month after activation.

Design: The study was a single-clinic, cross-sectional, retrospective design incorporating 32 adult unilateral Nucleus CI users aged 27 to 90 years (mean = 70, SD = 13.5). Postoperative outcome was assessed through sentence recognition scores in quiet and in varying signal to noise ratios at 1 day, 1 to 2 months, and up to 2 years. Our clinic's standard test battery comprises physiological and neurocognitive measures. Physiological measures included electrically evoked compound action potentials for recovery function, spread of excitation, and polarity effect. To evaluate general cognitive function, inhibition, and phonological awareness, the Montreal Cognitive Assessment screening test, the Stroop Color-Word Test, and tests 3 and 4 of the French Assessment of Reading Skills in Adults over 16 years of age, respectively were performed. Physiological scores were considered abnormal, and therefore limiting, when total neural recovery periods and polarity effects, for both apical and basal electrode positions, were >1.65 SDs from the population mean. A spread of excitation of >6 electrode units was also considered limiting. For the neurocognitive tests, scores poorer than 1.65 SDs from published normal population means were considered limiting.

Results: At 1 month, 13 out of 32 CI users scored ≥90% sentence recognition in quiet with no significant dependence on age. Subjects with no limiting peripheral or neurocognitive factors were 8.5 times more likely to achieve ≥90% score in quiet at 1 month after CI switch-on ( p = 0.010). In our sample, we detected 4 out of 32 cases with peripheral limiting factors that related to neural health or poor electrode-neural interface at both apical and basal positions. In contrast, neurocognitive limiting factors were identified in 14 out of 32 subjects. Early sentence recognition scores were predictive of long-term sentence recognition thresholds in noise such that limiting factors appeared to be of continuous influence.

Conclusions: Both peripheral and neurocognitive processing factors affect early sentence recognition after CI activation. Peripheral limiting factors may have been detected less often than neurocognitive limiting factors because they were defined using sample-based criteria versus normal population-based criteria. Early performance was generally predictive of long-term performance. Understanding the measurable covariables that limit CI performance may inform follow-up and improve counseling. A score of ≥90% for sentence recognition in quiet at 1 month may be used to define successful progress; whereas, lower scores indicate the need for diagnostic testing and ongoing rehabilitation. Our findings suggest that sentence test scores as early as 1 day after activation can provide vital information for the new CI user and indicate the need for rehabilitation follow-up.

Fig. 1.

Electrically ECAP neural recovery functions. N1-P1 amplitudes were obtained for masker-probe intervals from 100 to 10,000 µs. Exponential functions (lines) were fit with absolute, τ0 and relative, τR recovery time constants, and maximum amplitude. Three total recovery period values were >95% confidence limit of the data, incl. one basal and one apical from the same subject (blue and red open circles respectively to the right). Shaded circles indicate no alternative basal responses for seven subjects with apical responses. Box limits indicate 1st and 3rd quartiles, center line the median, and error bars minimum and maximum values. ECAP indicates evoked compound action potential.

Fig. 2.

SOE functions for apical E16 (left) and basal E8 (right) probe electrode positions. N1P1 amplitudes are normalized with respect to maximum amplitudes. Individual curves are shifted up to one electrode basally (dotted) or apically (dashed) to improve clarity. A reference triangle of area six electrode units is shown. Functions with area under the curve greater than six electrode units are indicated by blue (only one probe position) and red lines (both positions). SOE indicates spread of excitation.

Fig. 3.

Subjects’ sentence recognition scores in quiet (bars) and 10 dB SNR (open circles) at the 1-mo visit, using CI alone. Color code: gray = no limiting factor; blue = top-down, neurocognitive limiting factor; red = bottom-up, peripheral limiting factor. CI indicates cochlear implant.

Fig. 4.

Sentence recognition over time for individual subjects. The lower scale represents percent correct scores in quiet; the upper scale scores in noise for SNR50 expressed in dB. Where two or more fixed SNRs were tested, SNR50 was derived where scores in quiet were ≥90% and in 10 dB SNR ≥60%. Data points for the same subjects are joined by lines. Encircled points indicate nominal 1-mo intervals. Colors indicate identified limiting factors. ECAP indicates evoked compound action potential; MoCA, Montreal Cognitive Assessment; RF, recovery function; SOE, spread of excitation.

Fig. 5.

Example flow diagram for clinical follow-up based on 1-day and 1-mo intervals. CI, confidence interval; MoCA, Montreal Cognitive Assessment; SOE, spread of excitation.