Infections from the lateral pharyngeal space can descend via the fascial planes and deep neck spaces into the posterior mediastinum through the retropharyngeal space, which extends from the base of the skull down to the posterior mediastinum. This anatomical continuity facilitates the spread of deep neck infections into the thorax, potentially causing life-threatening mediastinitis. Understanding these pathways is critical for prompt surgical drainage and management to prevent mediastinal complications.

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