The lacrimal sac is an integral part of the lacrimal apparatus involved in tear drainage. It is a small, sac-like structure that collects tears from the eye’s surface through the lacrimal canaliculi and passes them into the nasolacrimal duct, which ultimately drains into the nasal cavity. The lacrimal sac is lined with epithelial cells and surrounded by a thin fibrous capsule. It plays a critical role in tear drainage, helping maintain proper moisture and cleanliness on the eye’s surface.
Location
The lacrimal sac is located in the medial part of the orbit, within the lacrimal fossa, which is formed by the lacrimal bone and the frontal process of the maxilla. It sits just below the inner corner (medial canthus) of the eye, near the side of the nose. The sac connects superiorly to the lacrimal canaliculi, which carry tears from the eye, and inferiorly to the nasolacrimal duct, which leads to the nasal cavity.
Structure and Anatomy
The lacrimal sac is an important part of the lacrimal apparatus, which is responsible for the drainage of tears from the surface of the eye into the nasal cavity. The anatomy of the lacrimal sac and its associated structures is detailed below:
Shape and Structure
The lacrimal sac is a small, oblong, sac-like structure approximately 12 to 15 mm in length. It has two distinct parts:
- Fundus: The superior portion of the sac, which is the blind end. It extends slightly above the medial palpebral ligament.
- Body: The main part of the sac, which connects to the nasolacrimal duct inferiorly.
The sac is composed of a thin, distensible wall made of epithelial tissue. It is surrounded by a fibrous layer that provides structural support, and it is covered by the lacrimal fascia, which is part of the periorbita (the connective tissue surrounding the eye).
Surrounding Structures
Several anatomical structures surround and interact with the lacrimal sac:
- Medial Palpebral Ligament: This ligament crosses the lacrimal sac horizontally and divides it into two parts. The ligament plays a role in stabilizing the position of the sac.
- Orbicularis Oculi Muscle: The lacrimal portion of the orbicularis oculi muscle surrounds the sac. When the muscle contracts during blinking, it aids in pumping tears through the lacrimal sac and into the nasolacrimal duct.
- Lacrimal Bone: The lacrimal sac lies within the bony depression of the lacrimal bone, which forms part of the medial wall of the orbit.
- Frontal Process of Maxilla: This structure helps form the anterior boundary of the lacrimal fossa, supporting the sac.
Connections
The lacrimal sac serves as an intermediary structure between the lacrimal canaliculi and the nasolacrimal duct:
- Lacrimal Canaliculi: The lacrimal sac connects superiorly to the lacrimal canaliculi, which consist of two small ducts, the superior and inferior canaliculus, that collect tears from the puncta (tiny openings on the inner edge of the eyelids). These canaliculi converge to form a common canaliculus, which opens into the lacrimal sac.
- Nasolacrimal Duct: The nasolacrimal duct arises from the lower part of the lacrimal sac and descends within the nasolacrimal canal, leading to the inferior meatus of the nasal cavity. This duct is approximately 18 mm long and helps drain tears into the nose.
Lining and Epithelium
The inner surface of the lacrimal sac is lined by a specialized pseudostratified columnar epithelium with goblet cells. The goblet cells secrete mucins, which help maintain the tear film and aid in the smooth passage of tears through the sac. The epithelium is continuous with the lining of both the lacrimal canaliculi above and the nasolacrimal duct below.
Blood Supply
The lacrimal sac receives its blood supply from branches of the ophthalmic artery and the facial artery:
- Ophthalmic Artery: A branch of the internal carotid artery, this supplies blood to the surrounding orbital structures, including the lacrimal sac.
- Facial Artery: This artery, a branch of the external carotid artery, provides additional blood supply to the lacrimal region.
Venous drainage from the lacrimal sac follows the veins of the same names, with blood eventually draining into the ophthalmic veins and the facial vein.
Nerve Supply
The lacrimal sac is innervated by sensory and autonomic nerves:
- Sensory Innervation: The sensory innervation of the lacrimal sac comes from the infraorbital nerve and the infratrochlear nerve, which are branches of the trigeminal nerve (cranial nerve V).
- Autonomic Innervation: Parasympathetic and sympathetic fibers that regulate tear production reach the lacrimal sac via the greater petrosal nerve (parasympathetic fibers) and deep petrosal nerve (sympathetic fibers).
Lymphatic Drainage
The lymphatic drainage from the lacrimal sac and its associated structures leads to the submandibular lymph nodes. These nodes help manage any excess fluid and waste products generated in the region.
Function
The lacrimal sac plays a crucial role in the drainage of tears from the eye to the nasal cavity. It is a key component of the lacrimal apparatus, which helps maintain eye moisture, clear debris, and regulate tear flow. Below are the detailed functions of the lacrimal sac.
Tear Collection
The primary function of the lacrimal sac is to collect tears from the eye’s surface. Tears are produced by the lacrimal glands, located in the upper outer region of the orbit, and spread across the surface of the eye during blinking. These tears serve to lubricate, nourish, and protect the eye by washing away debris and foreign particles.
Lacrimal Canaliculi: Tears are funneled into the superior and inferior lacrimal canaliculi from small openings called puncta, located on the inner edges of the upper and lower eyelids. The canaliculi converge to form a common duct that drains into the lacrimal sac. The lacrimal sac acts as a reservoir for these tears before they are drained into the nasolacrimal duct.
Tear Transport
Once the tears are collected in the lacrimal sac, they are transported to the nasolacrimal duct, which leads to the nasal cavity. This process ensures that excess tears do not accumulate on the eye’s surface, preventing overflow and maintaining clear vision.
Tear Flow Mechanism: The contraction of the orbicularis oculi muscle during blinking compresses the lacrimal sac, which helps propel tears into the nasolacrimal duct. This muscular action functions like a pump, pushing tears from the sac into the duct and, ultimately, into the nasal cavity.
Regulation of Tear Drainage
The lacrimal sac plays an essential role in regulating the drainage of tears to ensure a proper balance between tear production and removal. This balance is necessary for maintaining optimal eye health and preventing conditions such as dry eye or excessive tearing (epiphora).
Dynamic Drainage: As blinking occurs, the lacrimal sac alternates between filling and emptying. During each blink, the lacrimal sac is compressed, forcing tears into the nasolacrimal duct. When the eye is open, the sac refills with tears collected from the canaliculi, ensuring a constant flow of fluid through the tear drainage system.
Tear Drainage into the Nasal Cavity
Once the tears are transported through the nasolacrimal duct, they drain into the nasal cavity at the level of the inferior meatus, located beneath the inferior nasal concha. This drainage pathway helps prevent excessive tear accumulation on the eye’s surface and allows for the efficient removal of excess tears.
Nasal Drainage Pathway: The tears that drain into the nasal cavity help moisten the nasal mucosa and are eventually reabsorbed or evaporate. This connection between the eye and the nose explains why the nose often becomes runny when crying; the excess tears are directed into the nasal passages.
Tear Protection and Filtration
In addition to its role in tear transport, the lacrimal sac also acts as a protective structure, filtering out debris or foreign particles that may be present in the tears. By collecting and temporarily storing tears before they enter the nasolacrimal duct, the sac helps ensure that any particles or debris are flushed away and do not accumulate within the duct system.
Tear Filtration: Tears carry small particles, debris, and potentially harmful microorganisms that can irritate the eye. By passing through the lacrimal sac, these particles are effectively transported away from the surface of the eye and disposed of in the nasal cavity.
Maintenance of Ocular Health
The function of the lacrimal sac in tear drainage is essential for maintaining the overall health of the eye. Proper tear drainage prevents the excessive buildup of tears on the eye’s surface, ensuring that the cornea remains clear for optimal vision. Furthermore, the constant flow of tears helps flush out harmful substances and provides the cornea and conjunctiva with the necessary nutrients and moisture to function properly.
Prevention of Epiphora: If the lacrimal sac or the nasolacrimal duct becomes blocked or damaged, it can lead to a condition known as epiphora, characterized by excessive tearing. This happens because the tears cannot drain effectively through the lacrimal sac, leading to tear overflow on the cheeks. The lacrimal sac plays a crucial role in preventing such conditions by ensuring the smooth flow of tears.
Clinical Significance
The lacrimal sac is clinically significant due to its role in the drainage of tears, and any obstruction or infection in this area can lead to various conditions:
- Dacryocystitis: Inflammation or infection of the lacrimal sac, often caused by blockage of the nasolacrimal duct, can result in dacryocystitis. This condition presents with pain, redness, and swelling over the sac, along with excessive tearing (epiphora) and possibly discharge. Acute cases may lead to abscess formation.
- Nasolacrimal Duct Obstruction: A blockage in the nasolacrimal duct can prevent tears from draining properly, causing tear overflow (epiphora). This can be congenital in infants or acquired in adults due to trauma, infection, or inflammation.
- Dacryocystorhinostomy (DCR): This surgical procedure is often performed to bypass the blockage and restore tear drainage by creating a direct connection between the lacrimal sac and the nasal cavity.
