The increased life expectancy for patients with Down Syndrome (DS) has elicited renewed interest in the factors that may contribute to their characteristic poor linguistic development, overall intellectual disability, and the developmental trajectory associated with long-term cognitive decline in this population (1, 2). In adult population with DS, sleep quality, changes in emotional/behavioral functioning, gait dyspraxia, and language skills havechanges in emotional/behavioral functioning, gait dyspraxia, and language skills have been identified as important predictors of cognitive decline (2–6). In parallel, various factors have been observed to affect cognitive function in pediatric population with DS, such as obstructive sleep apnea (OSA) and the history of cardiac interventions (HCI) (7–9). Whereas we know too little about the overall developmental trajectories of cognitive decline in DS, it has been recently shown that intelligence, adaptive functioning, and motor skills in adolescents with DS are strongly correlated with their mental age in early childhood and that congenital heart defects may have a negative impact on language outcomes (10, 11). Therefore, understanding cognitive functioning early in life may be important in charting cognitive decline over time (1, 12).

A possible detrimental factor for language abilities is hearing impairment (HI) (13), present in up to 90% of patients with DS (14). It is unknown, however, whether the HI may have a direct impact also on the cognitive performance and on mental age growth in these patients. The aim of the present study is to investigate the attested influence of HI on auditory and receptive language skills in young children with DS, prior to their audiological care/rehabilitation. Furthermore, the influence of HI on mental age in the sample was explored, as well as other two factors measured in the sample that are known to affect cognitive abilities in pediatric population with DS, obstructive sleep apnea (OSA) (7) and the history of cardiac interventions (HCI) (8).

Nine out of 17 children in the dataset were affected by a SHI. Children with and without SHI did not differ in chronological age, mental age and IQ, the factors that otherwise affect auditory and language skills (Table 1). Conversely, children with SHI had significantly lower results in auditory performance (CAP) and marginally significantly lower results in receptive language scores (BVL) (Table 1). BVL scores, however, showed significant positive correlation with the mental age in children without SHI [R = 0.7, t (6) = 2.399, p = 0.05], but not in children with SHI [R = 0.422, t (7) = 1.233, p = 0.258]. These results confirm that the presence of SHI may crucially affect auditory and language abilities in children with DS. Chronological age, mental age, IQ, CAP, and BVL scores were furthermore compared when children were split based on the presence or absence of OSA or HCI, but the differences were not significant.

To assess the potential role of SHI to the mental age growth, we have run separate correlation analyses between mental and chronological age for children with and without SHI. In children without SHI, mental age is positively correlated with the chronological age [R = 0.75, t (6) = 2.752, p = 0.033], indicating that mental age progresses steadily with growing. Conversely, children with SHI exhibited no significant correlation between the mental and chronological age [R = 0.227, t (7) = 2.752, p = 0.556], indicating the lack of progress in intellectual performance relative to the chronological age (Figure 1A). Mental age is also positively correlated with the presence of HCI[R = 0.723, t (8) = 2.961, p = 0.018], but not with the presence of OSA (Figures 1B,C). However, there is little overlap between SHI and HCI or OSA; in fact, five of nine children with SHI have been identified with OSA and there is no difference between the PSQ scores in children with and without SHI (Table 1); furthermore, only three of nine children with SHI have had HCI. We therefore conclude that their relation to the mental age may be independent.

The present study highlights the significant impact of SHI not only on auditory and receptive language abilities, but also on mental age growth in young patients with DS.

Our results confirm data from the previous DS cohort report in which children with DS and a history of HI had lower receptive and productive language skills than children without such a history (13). They also partially confirm previous reports on the impact of OSA and heart diseases on neurocognitive development of children with DS (7, 8). Possibly, co-morbidity of these factors leads to further decline in neurocognitive functions, as indicated by the case with all three conditions (Figure 1). The novel contribution of the present study, however, is the result indicating that the growth trend of mental age in children with DS may be reduced also by the effect of SHI. These, albeit very preliminary data could add a new perspective on the management of intellectual disability in DS, emphasizing the role of early diagnosis and treatment of HI in order to minimize the factors that may have a detrimental impact on mental and intellectual abilities in DS later in life. This is particularly relevant considering the high prevalence of HI (14) and the progeroid course in DS characterized by early dementia (19).

Growing literature emphasizes the impact of HI on the outset and progression of dementia, but very little is known about its role in DS. The impact of SHI to mental age growth at an early age could have long-lasting consequences on autonomy and quality of life, and possibly also on the outset and progression of dementia in DS. Due to the numerically very limited cohort, the current results should be considered preliminary. Further research is needed to clarify whether hearing deprivation due to SHI represents a causal factor or a mediator of cognitive functioning in individuals with DS. Future longitudinal studies could concurrently evaluate the impact of audiological treatment on improving functional outcomes and progression of cognitive decline in individuals with DS (1).

HI is a modifiable condition, and effective audiological treatment and rehabilitation are today feasible since the first months of life, minimizing the hearing deficit and its consequences. Yet, audiological assessment of patients with DS is frequently complex and severely delayed as a consequence of multiple life-threatening conditions that influence the clinical management in the first years of life. Nonetheless, professionals involved in the healthcare management of these patients should consider the possible presence of HI as a red-flag not only of a language disorder but possibly also of reduced mental age growth.

The datasets presented in this study can be found in the OSF repository at the following address: https://osf.io/chw8n/.

Ethical review and approval was not required for the study on human participants in accordance with the local legislation and institutional requirements. Written informed consent to participate in this study was provided by the participants' legal guardian/next of kin.

AS, DB, GP, and EO: study concept and design. EB, SB, AS, DB, and GP: acquisition, analysis, and interpretation of data. DB, AF, MB, and GP: drafting of the manuscript. EO: study supervision. All authors had full access to all the data in the study and take responsibility for the integrity of the data and the accuracy of the data analysis.

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

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