Nasolacrimal System Anatomy: Embryology, Puncta, Canaliculi

The nasolacrimal drainage system serves as a conduit for tear flow from the external eye to the nasal cavity. It consists of the puncta, canaliculi, lacrimal sac, and nasolacrimal duct (see the image below).

At 32 days' embryonic gestation, the maxillary and frontonasal prominences appear, and as these processes enlarge, a groove forms between them (see the image below). Ectoderm from the floor of the groove becomes entrapped between the processes and detaches from the surface ectoderm as a cord of epithelium. Simultaneously, cords of the epithelium invaginate at the upper and lower lid margins, eventually forming the canaliculi. These epithelial cords fuse to form the nasolacrimal drainage system. ^[1]

Canalization of the epithelial cords occurs simultaneously throughout their entire length, beginning at 4 months' gestation. Remnants of the epithelium within the cords form inconsistent, valvelike folds. A membranous covering consisting of conjunctival and canalicular epithelium remains over the puncta (see the image below), and a covering consisting of nasal and nasolacrimal epithelium remains over the nasolacrimal duct outlet (i.e., the membrane of Hasner). Punctal membranes open at full term; however, the membrane of Hasner remains imperforate in up to 70% newborns. This usually opens within the first month but may remain imperforate for a longer time, resulting in epiphora and/or mucopurulent discharge.

Understanding these embryological processes is essential for recognizing the congenital anomalies associated with the nasolacrimal system such as congenital nasolacrimal duct obstruction (NLDO). This obstruction is often symptomatic in about 5% of the infants. ^{[2, 3, 4]}

Studies have also highlighted the variability in the nasolacrimal system's formation, with inconsistent epithelial remnants forming valvelike folds throughout the drainage pathway. These developments are critical for understanding both the normal function and potential congenital issues such as dacryostenosis, where the obstruction may lead to epiphora or dacryocystitis. ^{[2, 3, 4]}

Rapid growth of the maxillary bone in relation to the frontal bone results in greater lateral stretching of the inferior canaliculus with a subsequent lateral position of the inferior punctum with respect to the superior punctum. The epithelium at the site of the future lacrimal sac is initially thicker and canalization in this area is more extensive.

Puncta are openings 0.3 mm in diameter located on the medial aspect of the upper and lower eyelid margins. Each punctum sits on top of an elevated mound known as the papilla lacrimalis. The puncta are relatively avascular in comparison with the surrounding tissue, giving them a pale appearance, which is accentuated with lateral traction of the lid. This pallor can be helpful in localizing a stenosed punctum.

Puncta are directed posteriorly against the globe; therefore, they are not usually visible unless the eyelid is everted. Punctal ectropion may lead to inadequate tear drainage and resulting epiphora. The inferior punctum is approximately 0.5 mm lateral to the superior punctum, with distances to the medial canthus of 6.5 mm and 6.0 mm, respectively.

Tears enter the puncta from the medial canthal area and pass into the canaliculi, which consist of vertical and horizontal segments before draining into the lacrimal sac. A proper functioning of the puncta is essential for maintaining ocular surface health by facilitating tear drainage into the canaliculi and subsequently into the lacrimal sac. ^{[5, 6]}

Studies have highlighted various conditions related to punctal anatomy:

Supernumerary puncta - Supernumerary lacrimal puncta are rare congenital anomalies that can lead to epiphora if they do not function properly. These additional puncta may be asymptomatic or require intervention depending on the associated symptoms. ^{[5, 6]}

Punctal stenosis - Punctal stenosis maybe divided into four types based on the shape of occlusion- pinpoint, membranous, horseshoe, and slit type. Common canalicular duct obstruction may occur in association with punctal stenosis. ^[6]

Punctal ectropion - A condition where the punctum turns outward, can lead to inadequate tear drainage, resulting in epiphora due to impaired tear drainage. ^{[5, 6]}

Canaliculi have an initial vertical segment, measuring 2 mm, followed by an 8-mm horizontal segment (see the image below), following the arc of the eyelid. The angle between the vertical and horizontal segments is approximately 90 degrees, and the canaliculi dilate at the junction to form the ampulla. In most individuals, the horizontal portion of the canaliculi converges to form the common canaliculus, which is typically 3-5 mm long. The canaliculi may enter the lacrimal sac independently. ^[7] Canaliculi pierce the lacrimal fascia before entering the lacrimal sac. At its entrance to the lacrimal sac, the common canaliculus may dilate slightly, forming the sinus of Maier.

Canaliculi are lined by nonkeratinized, stratified squamous epithelium and are surrounded by elastic tissue, which permits dilation to two or three times the normal diameter. The oblique entrance of the common canaliculus into the lacrimal sac forms the valve of Rosenmüller, which prevents the retrograde reflux of fluid from the sac into the canaliculi. ^[8] However, the posterior angulation of the upper and lower canaliculi followed by the anterior angulation of the common canaliculus may also block reflux at the canaliculus-sac junction. An incompetent valve of Rosenmüller is observed clinically as air escaping from the lacrimal puncta when the individual blows his or her nose.

Studies have shown variability in how canaliculi connect to the lacrimal sac: ^{[5, 9]}

Common canaliculus - Present in about 94% of the individuals.

Separate openings - In approximately 2% of the cases, upper and lower canaliculi enter separately into the sac without a common opening.

Additionally, congenital conditions such as dacryocystocele, an obstruction causing cystic dilation, and acquired conditions such as dacryocystitis, resulting from tear duct blockage, continue to be common concerns in lacrimal system pathologies. ^{[5, 9]}

The lacrimal sac sits within the lacrimal fossa, which is bound anteriorly by the frontal process of the maxillary bone (anterior lacrimal crest) and posteriorly by the lacrimal bone (posterior lacrimal crest). (See the image below.) Differing proportions of the lacrimal bone and maxillary bone make up the lacrimal fossa; the position of the vertically oriented suture between them is variable.

Research has highlighted the anatomical variations of the lacrimal sac fossa (LSF). A study found that in 45% of the cases, the LSF is formed equally by the lacrimal bone and the frontal process of the maxilla. This contrasts with the findings that suggested a more uniform contribution from either bone type. The study also noted that the shape of the LSF was consistently oval across all specimens examined. ^{[10, 11, 12, 13]} .

The thickness of the lacrimal bone varies; ^[14] however, one study found an average thickness of 0.1 mm. Because the lacrimal bone is generally thinner than the maxillary bone, a perforation of the lacrimal bone can be made during dacryocystorhinostomy (DCR), followed by extension of the osteotomy to include the maxillary bone. The lacrimal bone can be localized intranasally by its position, which is anterior to the uncinate process of the ethmoid bone.

The thickness of the lacrimal bone has been a point of interest in surgical contexts. The variability in thickness of the lacrimal bone has implications for procedures such as DCR, where understanding the thickness is essential for avoiding complications during surgery. ^{[10, 11, 12, 13]}

DCR is a common surgical intervention for addressing obstructions in the nasolacrimal system. Studies indicate that endoscopic DCR (E-DCR) has a success rate of approximately 85%, which is higher than that of laser DCR, which shows success rates of around 62%. ^{[10, 11, 12, 13]} .

The choice between these techniques often depends on patient-specific factors such as anatomical considerations and overall health status. In E-DCR, surgeons utilize endoscopic visualization to create an osteotomy that connects the lacrimal sac to the nasal cavity. This method allows for direct access to lacrimal sac abnormalities, such as stones or tumors, facilitating better outcomes compared with traditional external approaches. ^{[10, 11, 12, 13]}

The lacrimal sac is lined by a double-layered epithelium (superficial is columnar, and deep is flatter). It can be divided into a fundus superiorly and a body inferiorly. The fundus extends 3-5 mm above the superior portion of the medial canthal tendon, and the body extends approximately 10 mm below the fundus to the osseous opening of the nasolacrimal canal.

At the posterior lacrimal crest, the orbital periosteum splits to envelop the lacrimal sac as a covering known as the lacrimal fascia. This periosteum then continues inferiorly to enclose the nasolacrimal duct. The lacrimal fascia is surrounded by fibers of the orbicularis oculi muscle; the superficial head of the muscle travels around the front of the sac to attach to the anterior lacrimal crest, and the deep head of the muscle travels behind the sac to attach to the posterior lacrimal crest. Between the lacrimal fascia and the lacrimal sac lies a venous plexus. The orbital septum attaches to the medial orbital wall at the posterior lacrimal crest, so the lacrimal sac is a preseptal structure.

The lacrimal sac and its surrounding structures receive their blood supply primarily from the branches of the ophthalmic artery and the infraorbital artery. Venous drainage is via the ophthalmic and facial veins. ^[2]

The relationship between the lacrimal sac and adjacent anatomical features — such as the uncinate process of the ethmoid bone — must be carefully considered to minimize complications during procedures such as DCR. ^{[10, 11, 12, 13]}

The nasolacrimal duct consists of a 12-mm superior intraosseous portion and a 5-mm inferior membranous portion. The bony nasolacrimal canal is approximately 1 mm in diameter; the intraosseous part travels posterolaterally through the nasolacrimal canal within the maxillary bone, while the membranous part runs within the nasal mucosa, eventually opening into the inferior meatus under the inferior nasal turbinate. ^[15]

A double layer of epithelium similar to that observed in the lacrimal sac lines the nasolacrimal duct. The venous plexus surrounding the lacrimal sac continues inferiorly to surround the nasolacrimal duct, eventually connecting to the vascular tissue of the inferior turbinate.

Research highlights the functional role of mucins and peptide interactions in the nasolacrimal ducts. These secretions, produced by goblet cells within the ducts, support the tear drainage mechanism and help protect the ocular surface from infections and dryness. The role of the cavernous bodies within the tear ducts has also been linked to efficient tear drainage, particularly under variable pressure conditions such as during nasal congestion or infections. ^{[16, 17, 18, 19]}

Primary acquired NLDO (PANDO) has been linked to anatomical variations and inflammatory processes. Computed tomography (CT) imaging has revealed that a narrow nasolacrimal canal and concurrent nasal inflammation are significant risk factors for PANDO. ^{[16, 17, 18, 19]}

The valve of Hasner is located at the duct's opening into the nasal cavity. It may be imperforate in up to 70% of newborns. Spontaneous resolution typically occurs within 6-12 months as the valve opens naturally.

The lacrimal pump mechanism of the facial muscles, particularly the orbicularis oculi, is essential for tear propulsion through the nasolacrimal system. This mechanism relies on proper innervation from cranial nerves VII and III. ^{[2, 16, 17, 18, 19]}

A study published in July 2023 reviewed anatomical differences in patients with PANDO using CT scans, emphasizing that an expanded lacrimal sac fossa and changes in the surrounding structures could be indicative of pathology. Research has also focused on histological changes associated with PANDO, noting recurrent inflammation and fibrosis as contributing factors to obstruction. ^{[16, 17, 18, 19]}