According to the World Health Organization, in April 2021, 430 million people worldwide (about 5 % of the entire population) were affected by disabling hearing loss (HL), of whom 34 million were children. According to their estimates, by 2050, >700 million people will suffer from HL that requires rehabilitation [1]. The prevalence of severe to profound bilateral permanent HL in newborns is 1.1 per 1000 children and has not changed significantly in recent decades. Furthermore, another 1 to 2 per 1000 newborns have bilateral mild to moderate HL or unilateral HL of some degree [2]. Poor hearing is a well-known risk factor for delays in speech and language development [3]. In severe or complete deafness, cochlear implantation followed by intensive speech therapy can help young children acquire normal speech, language and social skills. In addition, when cochlear implant is placed early in life, it provides the child with early exposure to environmental sounds [4].

Auditory performance is significantly lower when a patient has only one side available to hear (monaural hearing); it occurs in the case of asymmetric HL, single-sided deafness, or single-sided CI in bilateral deafness [5]. Conversely, people with binaural hearing have several benefits, namely the head shadow effect, squelch, summation and localization, which offer advantages especially in difficult listening environments [6]. To date, it is known that unilateral CI treatment in bilaterally deaf children has an important and, in some ways, sufficient impact on language development, but does not provide the child with the benefits of full binaural hearing [7,8]. Language development in pediatric patients is closely related to their hearing abilities, so it seems of paramount importance to give them the best listening chance [9]. Furthermore, children with unilateral CI experience misaligned brainstem activity. This misalignment is not observed in children who received a bilateral CI [10], which can be considered an additional point in favor of contextual placement of bilateral CI in bilaterally deaf children. When the second CI is implanted sequentially, nine months is considered the ideal period for full recovery of the imbalance provided by a single-sided implant; patients who received the second CI after ten months or more appear to show persistent impairment of brainstem balance [[11], [12], [13]]. Furthermore, simultaneous implantation of bilateral CI was found to be more effective and affordable than sequential implantation, in terms of the incremental cost-effectiveness ratio, which considers the cost of CI therapy and the hearing benefits provided to the patient [14,15]. In available investigations comparing the results of simultaneous versus sequential bilateral CI, the former has been shown to be a valuable contributor to spoken communication among its receivers and with very low surgical risk [6]. It is generally suggested that the CI intervention should be scheduled at a time when neural plasticity still allows for proper development of the central auditory and speech pathways, but this is still a matter of debate and requires further research [16]. To date, the best timing has not yet been defined and studies with sufficient follow-up are lacking in the literature to make an appropriate comparison between simultaneous and sequential bilateral CI.

In our institution, simultaneous bilateral CI has been considered the preferred rehabilitation option for bilaterally deaf children for many years. Consequently, long-term follow-up data are now available. The main objective of this investigation has been to evaluate the hearing results of young patients implanted bilaterally, in association with their age at implantation considering the value of pure tone average (PTA) and speech performance, 3, 6, 12 and 24 months after surgery, and at 3, 4, 5, 7 years of age. The hypothesis is that the evaluation of long-term results would contribute to determining the most appropriate timing for surgery in bilaterally deaf children.