Dacryocystitis

 

The naso-optic fissure is the source of origin of the lacrimal drainage system. The ectoderm in this region thickens and becomes embedded in the mesenchyme between the lateral nasal and maxillary processes. This cord of ectoderm subsequently canalizes and opens into the conjunctival fornix prior to opening into the nasal vestibule. Frequently, this opening into the nasal cavity is incomplete at birth. Canalization of the lacrimal excretory system begins in the superior portion first and is segmental, only later coalescing to form a continuous lumen. The canaliculi, which develop as outpouchings from the solid cord of ectodermal tissue prior to canalization, also canalize prior to the vertical portions of the nasolacrimal duct.

Many variations in the anatomy of the lacrimal drainage system have been noted. Normally, tears drain into the lacrimal system through two puncta, one present in the upper lid and the other in the lower lid. More commonly, the lower punctum lies slightly temporal to the upper punctum.

The connections from the puncta to the lacrimal sac are called canaliculi. These canaliculi have a short vertical segment, averaging 2 mm in length, and a longer horizontal segment, averaging 10-12 mm in length.

An ampulla connects the vertical and horizontal segments. The individual canalicular horizontal segments join to form a common canaliculus in 90% of patients. This common canaliculus dilates, forming the sinus of Maier just lateral to the lacrimal sac.

A fold of mucosa known as the valve of Rosenmüller marks the junction of the lacrimal sac and the common canaliculus. The lacrimal sac lies in the bony lacrimal fossa derived from the lacrimal and maxillary bones. The average width of the sac is approximately 6-7 mm and the length varies from 12-15 mm. The mucosa of the sac is lined by pseudostratified columnar epithelium with substantial amounts of lymphoid and elastic tissue interposed within the connective tissue layer. The sac is normally irregular and flat in shape with a collapsed lumen.

The lacrimal sac is covered on its outer surface by the lacrimal fascia of the periorbita. This fascia splits to envelop the lacrimal sac between the attachments of the lacrimal fascia to the anterior and posterior lacrimal crests. The lacrimal sac mucosa only loosely adheres to the lacrimal fascia. However, posterior to the sac are the deep heads of the pretarsal and preseptal orbicularis muscles. Anteriorly, the medial canthal tendon covers the upper two fifths of the lacrimal sac.

The nasolacrimal duct averages 18 mm in length and 4.5-5 mm in diameter. Multiple valves are present in the nasolacrimal duct, representing analog from the segmental canalization of the ectodermal cord that develops into the nasolacrimal duct. Of these, the most prominent valves are the valve of Taillefer, the valve of Krause, and the valve of Hasner (located at the junction of the duct with the nasal mucosa). Like the lacrimal sac, the nasolacrimal duct is lined by pseudostratified columnar epithelium.

The lacrimal, maxillary, and ethmoid bones form the bony nasolacrimal canal. The bulk of the duct is contributed by the maxilla, anteriorly, laterally, and posteriorly. The lacrimal bone forms the medial wall superiorly, and the inferior concha of the ethmoid bone forms the medial wall of the canal inferiorly. The mucosal opening of the nasolacrimal duct under the inferior turbinate lies 5-8 mm from the anterior tip of the inferior turbinate. The lacrimal bone and the nasal process of the maxilla make up the lacrimal fossa equally. The anterior and posterior lacrimal crests form the anterior and posterior borders of the lacrimal fossa, respectively.

The dimensions of the lacrimal fossa are 4-8 mm in width, 15 mm in height, and 2 mm in depth. Ethmoid air cells in approximately 40-60% of patients separate the lacrimal fossa from the nasal cavity, although considerable variability exists in the number and location of these air cells. The lacrimal sac fossa lies at the level of the anterior tip of the middle turbinate.