8Mind the Gap: The Inter-Implant Interval

FWhat the interval is, and why it matters

The interimplant interval is the time between the first and second cochlear implant operations in a sequential bilateral case. In children the interval is developmentally loaded: while one ear is driving the brain, the unimplanted ear remains deprived. The deprived second ear and its pathways fall behind, so the later the second implant arrives the more catching-up it must do. Adults are far less interval-sensitive than children because their auditory cortex is already organised before either implant.[2010][2011]

TSensitive periods and aural preference

Central auditory pathways are maximally plastic for roughly the first three and a half years of life, the window in which input is most readily encoded. Continued unilateral implant use lets the first ear's pathway strengthen and reorganise the cortex to prefer it—an aural preference that disadvantages the late ear. Cortical activity becomes abnormally weighted toward the side of the first implant once unilateral use extends beyond roughly a year and a half. A prolonged single-ear period also distorts brainstem timing, eroding the binaural cues that bilateral hearing depends on.[2002][2013][2008]

TWhat the outcome data show

A shorter interimplant interval and a younger age at the second operation are associated with better and faster second-ear development. The bilateral advantage emerges after sequential implantation and grows with continued bilateral experience rather than appearing all at once. Even with a long interval, a second implant can still add measurable benefit: in older children with a median gap of about eight years, bilateral listening still improved speech in quiet (around 96% vs 91%) and in noise (around 65% vs 54%). Long-term use of the second device, not just its placement, is a strong determinant of how much that late ear contributes.[2022][2011][2017]

CCounselling families about a late second ear

The honest message is twofold: shorter is better, but later is not hopeless. Expectations for a long-delayed second ear should be tempered—it may serve more as a support to the established ear than as an equal partner. Families should understand that consistent wear of the late device is essential for it to develop at all. Where a short interval is achievable it should be pursued; where it is not, the second ear is still usually worth implanting with realistic goals.[2022][2010]