The alar fascia is a fascial layer within the deep cervical fasciae between the pretracheal/buccopharyngeal fascia anteriorly and the prevertebral fascia posteriorly. It runs from the skull base to approximately T2 where it fuses with the prevertebral fascia. The space between the alar and prevertebral fascia is the danger space (space 4), which extends directly from the skull base to the posterior mediastinum without obstruction.
Separates the retropharyngeal space (between alar and buccopharyngeal fascia anteriorly) from the danger space (between alar and prevertebral fascia posteriorly), theoretically limiting infection spread.
The danger space between the alar and prevertebral fascia is so named because it provides an unobstructed pathway from the skull base to the posterior mediastinum at the diaphragm. Infection penetrating the alar fascia from a retropharyngeal abscess enters the danger space and descends rapidly to the posterior mediastinum, causing posterior mediastinitis and empyema. CT imaging showing air or fluid posterior to the alar fascia in the danger space indicates this catastrophic spread requiring emergency cervicothoracic drainage.
Deep neck infection penetrating the alar fascia enters the danger space where no fascial barriers obstruct descent to the posterior mediastinum, producing descending necrotising mediastinitis with a mortality rate of 30-50% requiring emergency combined cervical and thoracoscopic drainage with IV antibiotics.
