Adolescents who received cochlear implants in both ears at the same time in early childhood had better long-term sound localization than peers who received implants one after the other, according to a new study published in Pediatrics.

The study included 37 adolescents and young adults aged 12-21 years in Sweden. Researchers compared 17 participants who received simultaneous bilateral cochlear implants with 20 who received implants sequentially. All participants received their first implant before age 2.5 and had used bilateral implants for about 15 years on average. In the sequential group, the average delay between implants was 1.2 years.

The simultaneous group was significantly more accurate at identifying where sounds came from than the sequential group (P < .001; Cohen d = 0.58). The estimated difference in localization error index was 0.071 (95% CI, 0.031-0.110), which translated to about a six-degree improvement in sound localization. Both cochlear implant groups, however, performed worse than adolescents with typical hearing.

When it came to understanding speech, the two groups performed similarly, even in noisy settings. At a standard speech presentation level of 65 dB, the difference between groups was 1.4 rationalized arcsine units (RAUs; 95% CI, -7.4 to 10.1; Cohen d = 0.064). At a softer 50 dB level, the difference was 6.5 RAUs (95% CI, -15.2 to 2.3; Cohen d = 0.31). In speech-in-noise testing, differences also were minimal, including a 0.4-dB difference with separated maskers (95% CI, -7.4 to 10.1; Cohen d = 0.07) and 0.04 dB with co-located maskers (95% CI, -2.0 to 2.1; Cohen d = 0.006).

The findings suggest that timing may matter more for spatial hearing — knowing where sound is coming from — than for speech understanding alone.

“If there is no contraindication, there should be no time between implantations and activations of the right and left ear,” said study author Filip Asp, PhD, associate professor at the Karolinska Institutet in Stockholm, Sweden. “One key finding was that even brief periods (months) of hearing with one implant seemed to inhibit later optimal functioning with two.”

According to Kevin Brown, MD, otolaryngologist and executive director of the Children’s Cochlear Implant Center at the University of North Carolina School of Medicine in Chapel Hill, North Carolina, the findings generally align with clinical experience, especially when delays stretch longer.

“For younger patients, I generally advocate that we do simultaneous cochlear implantation,” said Brown, who was not involved in the study. Early input from both ears is important for how the brain learns to process sound.

“The whole concept of binaural hearing is something that ultimately has to occur, we think, by 3 or 4 years of age,” he said.

When that input is delayed, Brown said, “[children] may not develop capacity to either localize sound or understand speech in a noisy environment the same way.”

The findings also align with a broader body of research on early brain development, according to Dana Suskind, MD, founder and director of the University of Chicago’s Pediatric Cochlear Implant Program and co-author of the study’s accompanying editorial.

“The environmental inputs are the instruction guide to the brain,” said Suskind in an interview. “Implanting and allowing that environmental input at the same time clearly makes a long-term difference.”

Consequences of Delaying Implantation

While speech recognition is often the primary benchmark for cochlear implant success, Asp said that it does not fully capture real-world hearing.

“Spatial hearing plays a role in safety, navigation, and group conversation,” Asp said, adding that even modest deficits can matter in daily life.

Adolescents with sequential implants often can still make “reasonably correct judgments” about where sounds come from, Asp said. But reduced accuracy “holds clinical and practical significance,” especially in group settings where identifying who is speaking is essential.

Brown said he sees similar patterns in practice, especially when there are longer gaps between surgeries.

“If there are multiple years between a patient receiving a first implant and a second implant, they lose the capacity to develop that binaural additive effect,” he said.

In addition to developmental considerations, practical factors also shape decision-making. Brown said most families opt for simultaneous implantation when possible, in part to avoid a second surgery and recovery.

“It’s easier for them. It’s easier for their child,” he said, noting that families otherwise must go through surgery, device activation, and rehabilitation twice.

However, broader systemic issues such as insurance barriers and disparities in access can delay treatment during critical developmental windows, Suskind said.

Long-Term Effects Still Unknown

Questions remain about how these differences may affect outcomes later in life. A previous study suggests similar mental health outcomes between adolescents with cochlear implants and those with typical hearing, though the research did not examine differences based on timing of implantation.

Asp said ongoing research using the same cohort will explore broader outcomes, including longer-term impacts.

The study has limitations, including its small sample size and cross-sectional design, which limits causal conclusions. Participants also had to be enrolled in a standard school curriculum, excluding some children with more complex conditions.

“Despite ‘a lifetime of experience’ with input from both ears, the spatial hearing gap resulting from a period of one-sided input during a sensitive period is in a sense resistant to later correction,” he said.

Asp reported previously receiving funding from the Cochlear Corporation and from MED-EL GmbH. They did not fund this research and had no role in the design and conduct of this study. Brown and Suskind reported having no relevant conflicts of interest.

Lara Salahi is a health journalist based in Boston.