Adenoid facies is a characteristic facial appearance caused by chronic mouth breathing due to nasal airway obstruction, commonly from enlarged adenoids. Persistent mouth breathing leads to an open-mouth posture, elongated face, and a dull expression. It affects normal maxillofacial growth, resulting in a narrow maxilla, high-arched palate, and dental malocclusion. This altered facial development is directly linked to the inability to breathe effectively through the nose, making mouth breathing the key clinical factor in adenoid facies.

Reference: Gray's Anatomy, Susan Standring, 42nd Edition.