Anatomical and histological study of the alar fascia

Introduction: The alar fascia remains one of the most variably described fascial structure in the human body. Much disagreement persists in the literature and mainstream anatomical texts about its anatomy, function, and clinical significance. It is generally described as a coronally oriented fascial sheet separating the retropharyngeal space anteriorly from the danger space posteriorly. The current study aimed to confirm the presence of the alar fascia and delineate its anatomical characteristics, connections, and potential function through gross dissection and microscopic analysis. Possible clinical and surgical implications are considered. Methods: Twelve (12) cadaveric necks were dissected and examined histologically. Smooth muscle (αSMA), nerve (S100 protein), and myosin proteins were identified immunohistologically to characterize the composition and possible functions of the alar fascia. Results: The alar fascia was found in all specimens spanning between the carotid sheaths. Morphologically, it was not a delamination or derivative of the prevertebral fascia. It extended from the base of the skull to the upper thoracic level (T2) where it fused with the visceral fascia. No midsagittal connection was found between the alar and visceral fasciae. Immunohistochemically, the alar fascia was positive in focal areas for αSMA and S100 proteins but negative for fast and slow myosin. Conclusion: The alar fascia is an independent and constant coronal fascial layer between the carotid sheaths. It contains neurovasculature and may limit the spread of retropharyngeal infections into the thorax as well as facilitate normal physiological functions of the cervical viscera.