5Duration of Deafness and Age

FDuration of deafness as the leading negative predictor

Across multiple cohorts, the duration of severe-to-profound deafness before implantation shows a consistent negative correlation with postoperative speech recognition: longer deprivation predicts poorer outcomes. In one influential single-centre analysis of postlingual adults, duration of deafness was the single most important predictor, and together with preoperative aided sentence scores accounted for roughly 80% of the variance within that cohort. Duration is measured as time in the severe-to-profound range, not time since first hearing loss; the audiologist often estimates it from when telephone use or uncaptioned television comprehension was lost. The likely mechanism is sensory-deprivation-driven change in the central auditory pathway and cortex, not merely peripheral loss, since the deprived brain reorganises over time.[2020][2013][1999]

FAge at onset: prelingual, perilingual, postlingual

Postlingual onset (after spoken language is established) carries the best adult prognosis; prelingual onset implanted in adulthood generally yields markedly poorer open-set speech, with perilingual onset intermediate. Prelingually deafened adults implanted late show much lower average scores, in one series mean sentence-in-quiet scores around 20% at six to twelve months, yet individual scores ranged from about 40 to 100%, so some gain real benefit. The poor average in late-implanted prelingual adults reflects a critical-period effect: the auditory cortex never received patterned acoustic input during early development. Age at onset and duration of deafness are distinct axes; a patient can be postlingual yet have a long duration, or prelingual yet recently lose residual hearing.[2020][2009]

TAge at implantation in adults: a weaker, derivative effect

Chronological age at implantation correlates negatively with outcome in some studies, but much of this effect is carried by its correlation with longer duration of deafness rather than by age itself. More recent work suggests duration of deafness and preoperative residual hearing are stronger predictors than the age at which an adult receives an implant. Analyses of large adult and adolescent cohorts found that duration of deafness alone was insufficient; the ratio of duration of deafness to age (the proportion of life spent severely-to-profoundly deaf) predicted better. A smaller proportion of life spent deaf preserves a foundation of acoustic and auditory processing that lets the patient exploit the implant signal.[2005][2020]

CThe special weight of long durations

The relationship between duration and outcome is not strictly linear; the penalty becomes clinically conspicuous once severe-to-profound deafness has lasted on the order of two to three decades. Very long durations should temper, but not by themselves contraindicate, implantation, since outcomes remain variable and many long-duration patients still gain useful benefit. Because duration is a leading timing-modifiable predictor, it argues against watchful waiting once a patient meets criteria: every additional year of deprivation worsens the expected outcome. In counselling, long-duration patients should be given a realistic, somewhat lower expected range and longer expected adaptation time rather than a refusal.[2013][2020]