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.
| Location | Back of the neck, between the buccopharyngeal fascia and the prevertebral fascia |
|---|---|
| Boundaries |
|
Divides the space behind the pharynx into the retropharyngeal space in front and the danger space behind, which matters for how neck infections spread to the mediastinum.
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.
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