The lacrimal canaliculus is a small duct that serves as part of the tear drainage system, responsible for carrying tears from the surface of the eye into the lacrimal sac. Each eye has two lacrimal canaliculi, one located in the upper eyelid (superior canaliculus) and one in the lower eyelid (inferior canaliculus). These canaliculi collect tears from the lacrimal puncta, tiny openings on the eyelid margins, and channel them toward the lacrimal sac. The canaliculi play an important role in tear drainage, ensuring that tears are efficiently moved away from the eye’s surface.
Location
The lacrimal canaliculus is located in the medial portion of the upper and lower eyelids. Each canaliculus begins at the lacrimal puncta, small openings near the inner corner of the eyelids (medial canthus). The canaliculi travel medially, curving slightly, and then converge to form a single common canaliculus before draining into the lacrimal sac, which is situated in the lacrimal fossa near the nose. The superior and inferior canaliculi work together to transport tears from the eye to the lacrimal sac.
Structure and Anatomy
The lacrimal canaliculi are small ducts responsible for transporting tears from the surface of the eye to the lacrimal sac. Each eye has two canaliculi, one located in the upper eyelid and one in the lower eyelid. Below is a detailed description of their anatomy:
Number and Division
There are two lacrimal canaliculi per eye:
- Superior Canaliculus: Located in the upper eyelid.
- Inferior Canaliculus: Located in the lower eyelid.
These canaliculi are initially independent but typically unite to form a single common canaliculus before draining into the lacrimal sac. In some cases, the canaliculi remain separate and drain directly into the sac.
Structure and Shape
Each canaliculus has a tubular structure with a diameter of approximately 0.5 to 1 mm and a length of about 8-10 mm. The walls of the canaliculi are lined with stratified squamous epithelium and are surrounded by fibrous connective tissue that provides flexibility and structure. The inner lumen of the canaliculi is lined with epithelial cells that help facilitate the movement of tears toward the lacrimal sac.
The canaliculi can be divided into two segments:
- Vertical Segment: The initial portion of each canaliculus rises vertically for about 2 mm from the lacrimal punctum, a small opening on the eyelid margin.
- Horizontal Segment: The canaliculus then curves medially and runs horizontally for about 8 mm along the eyelid, parallel to the lid margin, toward the lacrimal sac.
Lacrimal Puncta
The lacrimal puncta are the small openings where tears enter the canaliculi. There are two puncta in each eye:
- Superior Lacrimal Punctum: Located on the upper eyelid margin, near the medial canthus.
- Inferior Lacrimal Punctum: Located on the lower eyelid margin, also near the medial canthus.
These puncta are located on the inner (medial) edge of the eyelids, near the junction of the eyelids, and serve as the entry point for tears into the canaliculi.
Common Canaliculus
The superior and inferior canaliculi usually unite to form a short common canaliculus before entering the lacrimal sac. This common canaliculus passes through the valve of Rosenmüller, a small mucosal fold that prevents the backflow of tears from the lacrimal sac.
In some cases, the superior and inferior canaliculi remain separate and enter the lacrimal sac independently, but this is less common.
Blood Supply
The blood supply to the lacrimal canaliculi is derived from branches of the ophthalmic artery and the facial artery:
- Ophthalmic Artery: Provides blood to the upper portions of the canaliculi.
- Facial Artery: Supplies blood to the lower regions of the canaliculi.
These arteries provide nourishment to the surrounding tissues and ensure proper functioning of the lacrimal drainage system.
Innervation
The lacrimal canaliculi are innervated by branches of the trigeminal nerve (cranial nerve V), which provides sensory innervation to the eyelids and associated structures:
Infratrochlear Nerve: A branch of the ophthalmic division of the trigeminal nerve that supplies sensory fibers to the medial portion of the eyelids and the lacrimal apparatus, including the canaliculi.
Adjacent Structures
The lacrimal canaliculi are located near several important anatomical structures:
- Orbicularis Oculi Muscle: Surrounds the canaliculi and plays a role in tear drainage by compressing the canaliculi during blinking, which helps propel tears into the lacrimal sac.
- Lacrimal Sac: Located medially to the canaliculi, in the lacrimal fossa. The canaliculi drain into the lacrimal sac, forming the first part of the lacrimal drainage system.
- Medial Canthus: The area where the upper and lower eyelids meet, and where the lacrimal puncta are located.
Lymphatic Drainage
The lymphatic drainage of the lacrimal canaliculi is directed toward the submandibular lymph nodes, which help clear excess fluids and debris from the tissues in this region.
Function
The lacrimal canaliculi play a critical role in the tear drainage system by ensuring the efficient collection and transport of tears from the surface of the eye to the lacrimal sac. Below is a detailed explanation of the specific functions of the lacrimal canaliculus.
Tear Collection
The primary function of the lacrimal canaliculi is to collect tears from the surface of the eye. Tears are produced by the lacrimal glands and spread across the eye during blinking. These tears accumulate at the inner corner of the eye near the medial canthus, where they enter the lacrimal puncta—small openings at the beginning of the superior and inferior canaliculi.
Lacrimal Puncta: Each eye has two lacrimal puncta (one in the upper eyelid and one in the lower eyelid) that act as the entry points for tears into the canaliculi. The superior and inferior lacrimal canaliculi receive the collected tears and transport them toward the lacrimal sac for further drainage.
Tear Transport
Once tears enter the lacrimal puncta, they travel through the lacrimal canaliculi and are transported toward the lacrimal sac. The canaliculi act as conduits, carrying tears through their tubular structures. The efficient transport of tears is crucial to prevent tear overflow and ensure that the eye remains clear and well-lubricated.
Vertical and Horizontal Segments: The vertical segment of each canaliculus rises from the punctum, followed by a horizontal segment that travels medially toward the lacrimal sac. This arrangement allows for a smooth flow of tears from the puncta toward the lacrimal sac.
Coordination with Blinking
The lacrimal canaliculi work in coordination with the blinking mechanism to facilitate tear drainage. During blinking, the orbicularis oculi muscle contracts, exerting pressure on the canaliculi. This action helps propel tears from the canaliculi into the lacrimal sac and prevents stagnation of tears in the canaliculi.
Blinking Mechanism: Blinking not only spreads tears evenly across the surface of the eye but also assists in draining the tears by compressing the canaliculi. This compression acts as a pump, helping to move tears efficiently through the canaliculi and into the lacrimal sac.
Prevention of Tear Backflow
The lacrimal canaliculi contain mechanisms to prevent the backflow of tears, ensuring that tears are transported in a one-way direction toward the lacrimal sac. The presence of valves within the tear drainage system helps maintain this unidirectional flow.
Valve of Rosenmüller: Located at the junction of the common canaliculus and the lacrimal sac, the valve of Rosenmüller prevents tears from flowing backward into the canaliculi from the lacrimal sac. This ensures that tears are effectively directed downward into the nasolacrimal duct for eventual drainage into the nasal cavity.
Protection Against Tear Stasis
The proper functioning of the lacrimal canaliculi helps prevent the accumulation and stasis of tears on the eye’s surface, which could otherwise lead to irritation, blurred vision, or infection. By continuously transporting tears to the lacrimal sac, the canaliculi help maintain a healthy tear film and clear the eye surface of debris, microorganisms, and excess moisture.
Prevention of Epiphora: Efficient drainage of tears through the canaliculi helps prevent epiphora (excessive tearing), a condition where tears overflow onto the face due to improper drainage. The canaliculi’s role in draining tears efficiently is crucial in preventing this condition.
Connection to the Lacrimal Sac
The lacrimal canaliculi serve as the first part of the tear drainage system, connecting the lacrimal puncta to the lacrimal sac. After collecting tears, the canaliculi transport them to the lacrimal sac, where they are stored temporarily before being funneled into the nasolacrimal duct.
Common Canaliculus: The superior and inferior canaliculi typically unite to form a common canaliculus, which empties into the lacrimal sac. This common canaliculus acts as a single pathway for the tears, ensuring a streamlined flow of tears into the lacrimal sac.
Tear Regulation and Control
The canaliculi are essential in maintaining the proper flow of tears from the eye to the nasal cavity. This continuous flow prevents tear buildup on the eye’s surface and ensures that the tear film is refreshed regularly. By regulating the transport of tears, the canaliculi help maintain clear vision and eye comfort.
Tear Volume Control: The canaliculi work as a part of a larger system that manages tear volume and flow. By effectively draining tears, they help maintain the delicate balance of tear production and drainage necessary for optimal eye health.
Clinical Significance
The lacrimal canaliculi are crucial in the drainage of tears, and any dysfunction or obstruction can lead to various clinical issues:
- Canaliculitis: Inflammation or infection of the lacrimal canaliculi, often caused by bacterial or fungal infections, can result in symptoms such as redness, swelling, tenderness near the inner corner of the eye, and discharge. It can lead to persistent tearing (epiphora) and irritation.
- Canalicular Obstruction: Blockage of the lacrimal canaliculi can prevent proper tear drainage, leading to excessive tearing (epiphora) and discomfort. This can result from trauma, infection, or scarring.
- Trauma or Injury: Damage to the canaliculi, often due to eyelid trauma or surgery, can disrupt tear drainage, requiring surgical repair to restore normal function. Procedures such as canaliculodacryocystorhinostomy may be performed to bypass obstructions and restore tear flow.
