C. The Nasolacrimal System and Dacryocystorhinostomy (Figures 43-47)
In select cases, an endoscopic dacryocystorhinostomy may be indicated when epiphora is due to nasolacrimal duct obstruction (61-66). The nasolacrimal sac is found anterior to the most anterior attachment of the middle turbinate in the lateral nasal wall (67,68). The nasolacrimal duct courses slightly diagonal and parallel to the uncinate process in a posteroinferior direction toward the inferior meatus. The nasolacrimal duct may be divided into two parts: 1) a bony nasolacrimal duct and 2) a membranous nasolacrimal duct. The bony nasolacrimal duct has a bony wall circumfer-entially and is typically identified as a prominent convexity in the lateral wall of the nose running adjacent to the anterior middle meatal margin. The bony wall may be very thin posteriorly, adjacent to the uncinate process or maxillary natural ostium. The membranous nasolacrimal duct is located in the inferior meatus and consists of a membranous medial wall and a bony lateral wall.
The membranous medial wall collapses into the lumen and functions as a one-way valve (Hasner's valve) to minimize retrograde flow of secretions or air into the nasolacrimal duct and sac. Hasner's valve may be seen traversing the inferior meatal wall in its anterior one-third. With a small probe, a mucosal canal can be followed superiorly to identify Hasner's valve and the lacrimal ostium. Occasionally, Hasner's valve is absent. In these cases a patulous opening looking into the bony nasolacrimal duct may be seen in the superior recess of the inferior meatus adjacent to the inferior turbinate lamella.
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