(Pun intended, obviously.)
Chronic mouth breathing can adversely affect normal development in children, resulting in anatomical changes that directly impact the functionality of the upper airway during sleep.
Mouth breathing is a multifactorial problem that can be structural, functional, and even neurological in origin.
For example: a high-vaulted narrow palate, which would predispose a narrower nasal cavity that could impede the intake of air; the tongue and its resting place, swallowing position, and associated ties; the volume of the airway space in the nasopharynx and oropharynx; the epiglottis, correctly folding over the opening of the larynx, and conditions like stridor and laryngomalacia.
Therapeutic interventions (such as craniosacral therapy) can address the functional and behavioral factors that contribute to mouth breathing.
Oral breathing causes changes in pulmonary mechanics as well as in the pressure of arterial blood gases. In response to increased nasal obstruction oro-nasal breathing occurs. The level of oro-nasal partitioning maintains an adequate level of respiratory resistance. When upper airway resistance is increased, limitation of flow occurs. (Snoring indicates a mild degree of reduced airflow, whereas sleep apnea occurs when upstream pressure falls below a critical pressure.)
Therapeutic interventions (myofunctional, speech and swallowing, occupational, and craniosacral therapy) can address the functional and behavioral factors that contribute to mouth breathing. Progress may sometimes be limited by restrictive lingual and labial frenum which interfere with tongue and lip mobility. In such cases, lingual and maxillary labial frenuloplasty with myofunctional therapy can be effective as a treatment for Mouth Breathing and Snoring.
Gillespie Approach–Craniosacral Fascial Therapy Articles - Dr. Barry Gillespie November 22, 2021
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https://pubmed.ncbi.nlm.nih.gov/8317211/
Govardhan C, Murdock J, Norouz-Knutsen L, Valcu-Pinkerton S, Zaghi S. Lingual and Maxillary Labial Frenuloplasty with Myofunctional Therapy as a Treatment for Mouth Breathing and Snoring. Case Rep Otolaryngol. 2019 Mar 10;2019:3408053. doi: 10.1155/2019/3408053. PMID: 31001444; PMCID: PMC6437727.
https://pubmed.ncbi.nlm.nih.gov/31001444/
Bussi MT, Corrêa CC, Cassettari AJ, Giacomin LT, Faria AC, Moreira APSM, Magalhães I, Cunha MOD, Weber SAT, Zancanella E, Machado Júnior AJ. Is ankyloglossia associated with obstructive sleep apnea? Braz J Otorhinolaryngol. 2021 Nov 5:S1808-8694(21)00181-6. doi: 10.1016/j.bjorl.2021.09.008. Epub ahead of print. PMID: 34895868.
https://pubmed.ncbi.nlm.nih.gov/34895868/