Obstructive sleep-disordered breathing, including obstructive sleep apnea (OSA), represents a significant yet underdiagnosed condition in the pediatric population, particularly among adolescents with obesity. The tongue is a central anatomical contributor to upper airway collapsibility; however, its biomechanical properties have been investigated almost exclusively in adult populations, leaving a critical gap in knowledge regarding younger individuals.

This study aims to assess tongue biomechanical and echographic characteristics using quantitative ultrasound modalities - shear-wave elastography (SWE), acoustic attenuation, and echo intensity - in adolescents aged 12 to 18 years with obesity and obstructive sleep disorders. The study adopts a mixed design combining (1) an observational cross-sectional comparison between participants with obesity and OSDB and age- and sex-matched healthy controls at baseline, and (2) a prospective longitudinal cohort component evaluating changes in tongue ultrasound characteristics over the course of a 30-week multidisciplinary inpatient weight-loss program delivered as standard care at the Zeepreventorium (De Haan, Belgium). An additionnal methodological objective consists in determining intra-rater and inter-rater reliability of the ultrasound acquisition protocol providing a reproducible framework for future studies.

Scientific Background and Rationale Amongst obstructive sleep disorders, Obstructive sleep apnea (OSA) is characterized by repetitive complete (apnea) or partial (hypopnea) collapse of the upper airway during sleep. Upper airway collapsibility in OSA results from the interaction of multiple contributing mechanisms, including craniofacial morphology (retrognathia, maxillary constriction, reduced posterior airway space), adenotonsillar hypertrophy - a predominant anatomical risk factor in the pediatric population -, obesity-related peripharyngeal fat deposition, neuromuscular control deficits of upper airway dilator muscles, and environmental exposures such as prenatal or passive tobacco smoke exposure, associated with increased upper airway inflammation and altered respiratory control.

Among the anatomical structures implicated in upper airway collapsibility, the tongue plays a central role as the largest muscular structure of the oropharynx. Morphological and biomechanical alterations (incl. increased volume, fat infiltration at the base, posterior positioning, and modified tissue stiffness) have been identified as relevant contributors to airway obstruction and are increasingly investigated as potential screening markers.

In the pediatric population, OSA remains substantially underdiagnosed. Access to polysomnography is limited, parents seldom spontaneously report nocturnal symptoms, and prevalence estimates vary widely in the literature (1.2% to over 60% in children with obesity), partly due to heterogeneous diagnostic criteria. This epidemiological uncertainty underscores the need for accessible, non-invasive tools capable of characterizing tongue biomechanical properties in younger populations - a gap that the present study directly addresses.

Ultrasound is non-invasive, non-ionizing, widely accessible, and well tolerated, including in adolescents. Existing studies applying these modalities to the tongue in OSA have been conducted exclusively in adult populations and report heterogeneous, sometimes contradictory findings. Methodological variability in acquisition parameters, probe positioning, and region-of-interest definition substantially limits reproducibility and inter-study comparability. No study to date has applied this multimodal quantitative ultrasound approach to adolescents with obesity and obstructive sleep-disordered breathing, nor examined the longitudinal trajectory of these parameters in response to a structured weight-loss intervention. The present study is therefore the first to address this population and this clinical question.

Tongue morphology and tongue fat in patients suffering from OSA have been assessed several times with ultrasound, a non-invasive, widely accessible imaging modality that provides real-time visualization of tissues and is generally well tolerated, with minimal to no associated discomfort. Ultrasound echo intensity has been validated with MRI to estimate tongue fat percentage. Advanced ultrasound modalities, such as shear wave elastography (SWE) and attenuation imaging (AI), provide quantitative measures of tissue stiffness and its attenuation properties, respectively Existing studies on tongue SWE and echo intensity (EI) have been conducted exclusively in adults and report heterogeneous, and sometimes contradictory, findings in patients suffering from OSA. Raw data in EI values would increase in the case of high lipidic impregnation or fibrous accumulation and decrease when the amount of muscle is higher, but the results are still variable depending on the location. Methodological variability in acquisition parameters, probe positioning, region-of-interest definition, and data analysis substantially limit reproducibility and comparability. Arbitrary region-of-interest sizing that does not account for individual tongue morphology may further introduce systematic bias.

Another gap in current research is the limited focus on younger populations. While studies have primarily targeted middle-aged and older adults due to the higher prevalence of OSA in these groups, the biomechanical properties of the tongue may differ significantly in younger individuals, due to growth and hormonal factors. In particular, heterogeneous distribution of elasticity values between anterior and posterior regions of the tongue indicate that there is a need for a better understanding The objectives of this study and anticipated new evidence are as follows below. Primary objective: to compare tongue biomechanical properties, assessed using shear-wave elastography and quantitative ultrasound, between children and adolescents with obesity and obstructive sleep disorders and age-matched healthy controls.

Secondary objectives: (1) to assess longitudinal changes in tongue echographic characteristics over the course of a 30-week multidisciplinary weight-loss program delivered as part of standard care and (2) to evaluate the influence of anthropometric parameters, sex, and pubertal status on these measurements.

Methodological objective: to determine the intra-rater and inter-rater reliability of ultrasound-based tongue measurements, thereby providing a reproducible framework for future clinical and comparative studies.

Inclusion Criteria:

• Aged 12-18 years
• Already enrolled in Zeepreventorium weight-loss program as Standard Care Diagnosed Obstructive Sleep Apnea (AHI ≥ 1)1 or any Obstructive sleep disordered breathing detected by any of the specific sleep questionnaire (OSA-18, ESS, SSS-Ad, PDSS)

Exclusion Criteria:

• Any other orthopedic, traumatic, or rheumatologic condition affecting the cervicocephalic region
• Syndromic or neuromuscular disorders
• Cognitive impairment limiting compliance with study procedures
• Inability to provide informed consent
• Self-reported alcohol or recreational drug consumption within 48 hours preceding the experimental session

Individual participant data (IPD) may not be shared publicly, given the sensitive nature of the data collected (health data from minors) and the applicable data protection requirements under the General Data Protection Regulation (GDPR, EU 2016/679). Pseudonymized data will be stored on the secure VUB research data platform (Pixiu) and accessible only to authorized members of the research team. Aggregate and summary data supporting the findings of this study will be made available upon reasonable request to the corresponding author, subject to ethical and institutional approval.