Alar fascia

The alar fascia is a thin fibroareolar membrane separating the (anterior) true retropharyngeal space from the (posterior) prevertebral or "danger" space.

Notably, in the well patient, the alar fascia is not usually visible on cross-sectional imaging, and the two spaces are typically identified as a single space. The alar fascia may however become apparent on contrast-enhanced CT or MRI as a thin membrane in the retropharyngeal space in patients presenting with extensive regional edema or a collection. 

Summary

• location: between the visceral and prevertebral fascia, dividing radiological retropharyngeal space in a coronal plane
• boundaries:
  • superior: level of C1, where it may fuse posteriorly with the prevertebral fascia ¹ or transition to loose areolar tissue ²
  • inferior: level of T2, where it fuses anteriorly with visceral fascia
  • lateral: carotid sheaths bilaterally

Gross anatomy

Boundaries

Superiorly, the prevertebral fascia extends until the level just above the C1 anterior tubercle, where it either fuses with prevertebral fascia posteriorly¹, or narrowed to a median raphe before transitioning to loose areolar tissue ². It does not connect to the skull base as a distinct layer.

Inferiorly, there is progressive fusion anteriorly with the visceral (buccopharyngeal) fascia between the T2 and T4 vertebral levels.

Laterally, the alar fascia fuses with the lateral wall of each carotid sheath. Additionally, some fibers are contributed by the medial raphe extensions of the posterior middle pharyngeal constrictor ².

Relations

Anteriorly, the alar fascia delimits the posterior margin of the true retropharyngeal space.

Posteriorly, the alar fascia delimtits the anterior margin of the prevertebral or "danger" space.

Radiographic features

Alar fascia is not usually resolved on imaging; however, it may be visible on contrast-enhanced CT as a thin membrane in the retropharyngeal space in patients with extensive regional edema e.g. post-radiation therapy ².

Related pathology

• retropharyngeal abscess
  • alar fascia represents the anterior boundary of the danger space, itself a potential space through which an infective process from the pharyngeal region can extend into the mediastinum