Lacrimal caruncle

The lacrimal caruncle is a small, fleshy, pinkish mound of tissue located in the medial corner of the eye. It consists of skin, sebaceous glands, sweat glands, and some conjunctival tissue. The lacrimal caruncle often contains fine hair follicles and can sometimes become irritated or inflamed. While small, it is a visible structure at the inner corner of the eye and contributes to the overall health of the ocular surface.

Location

The lacrimal caruncle is located at the medial canthus (inner corner) of the eye, adjacent to the plica semilunaris and near the lacrimal drainage system (including the lacrimal puncta and lacus lacrimalis). It sits between the eyelids at the junction where tears collect before entering the tear drainage system.

Structure and Anatomy

The lacrimal caruncle is a small but complex structure located at the inner corner of the eye (medial canthus). It is part of the ocular adnexa and consists of a variety of tissues that give it a unique appearance and function. Below is a detailed description of the lacrimal caruncle’s anatomy.

Size and Shape

The lacrimal caruncle is a small, oval-shaped, fleshy prominence that is approximately 5 to 8 mm in diameter. It is visibly raised above the surface of the surrounding conjunctiva, giving it a distinct appearance. Its surface may have a slightly rough texture due to the presence of small hair follicles and glandular openings.

Composition

The lacrimal caruncle is made up of several different types of tissue, making it unique among the structures of the eye.

• Skin and Conjunctiva: The surface of the lacrimal caruncle is covered by stratified squamous epithelium, similar to skin, but also contains elements of the conjunctiva. This epithelial layer is continuous with the conjunctiva that lines the inner surface of the eyelids and covers the sclera.
• Glands: Embedded within the lacrimal caruncle are:
• • Sebaceous Glands: These glands produce oil (sebum) that helps lubricate the eye and prevent tear evaporation. They are similar to the glands found in the skin.
  • Sweat Glands: These small glands produce sweat, though their function in the lacrimal caruncle is minimal compared to their role in the rest of the body.
  • Accessory Lacrimal Glands: In some individuals, small accessory lacrimal glands may be present within the caruncle, contributing to tear production.
• Hair Follicles: The surface of the lacrimal caruncle may contain fine hair follicles, which are not present in the conjunctiva. These hairs are usually very small and soft, and their presence is one of the distinguishing features of the caruncle.

Surrounding Structures

The lacrimal caruncle is located at the medial canthus, where it is closely associated with other structures involved in tear drainage and ocular surface health.

• Plica Semilunaris: Located directly adjacent to the lacrimal caruncle, the plica semilunaris is a crescent-shaped fold of conjunctival tissue. It allows for greater movement of the eye and is a vestigial structure similar to the third eyelid found in some animals.
• Lacrimal Puncta: Situated near the lacrimal caruncle are the lacrimal puncta, small openings that drain tears from the surface of the eye into the lacrimal canaliculi. The close proximity of the caruncle to the puncta ensures efficient tear drainage.
• Lacrimal Lake (Lacus Lacrimalis): The lacus lacrimalis (tear lake) is the small pool of tears that collects at the medial canthus, just below the lacrimal caruncle. This structure ensures that excess tears are directed toward the lacrimal puncta for drainage.

Vascular Supply

The lacrimal caruncle has a rich blood supply, which contributes to its reddish-pink appearance.

• Arterial Supply: The caruncle is supplied by small branches of the ophthalmic artery and the facial artery, which also supply blood to the surrounding eyelids and medial structures of the eye.
• Venous Drainage: Blood from the lacrimal caruncle drains into the ophthalmic veins and the facial vein, contributing to the overall venous drainage system of the orbit.

Innervation

The lacrimal caruncle is innervated by sensory fibers that provide sensation to the medial part of the eye and its surrounding structures.

Sensory Nerve Supply: Sensory innervation is provided by the infratrochlear nerve, a branch of the ophthalmic division of the trigeminal nerve (cranial nerve V). This nerve supplies sensation to the medial canthus, the lacrimal sac, and the caruncle.

Lymphatic Drainage

The lymphatic drainage of the lacrimal caruncle is directed toward the submandibular lymph nodes and the preauricular lymph nodes. These nodes help filter lymphatic fluid and prevent infections from spreading from the eye to other areas.

Histology

The histological structure of the lacrimal caruncle reveals the diversity of tissue types within this small structure.

• Epithelial Lining: The surface of the caruncle is covered by stratified squamous epithelium, which is similar to the skin. This layer is keratinized in some individuals, although it is typically less so than the skin of the surrounding eyelids.
• Glandular Tissue: Sebaceous and sweat glands are embedded within the deeper layers of the caruncle, contributing to the oily secretion that helps maintain tear film stability.
• Connective Tissue: Beneath the surface epithelium, the lacrimal caruncle contains connective tissue that provides structural support to the glands and hair follicles within it.

Function

The lacrimal caruncle plays several key roles in maintaining the health and function of the ocular surface. Though small, its anatomical structure allows it to contribute to the tear film, support the immune defense of the eye, and aid in the proper drainage of tears. Below are the specific functions of the lacrimal caruncle.

Contribution to Tear Film Stability

The lacrimal caruncle contributes to the stability of the tear film, which is essential for maintaining a smooth and hydrated ocular surface. The presence of sebaceous and sweat glands in the caruncle helps produce secretions that mix with the tear film.

• Sebaceous Gland Secretion: The sebaceous glands within the lacrimal caruncle secrete a small amount of oily substance (sebum) that contributes to the lipid layer of the tear film. This lipid layer helps reduce tear evaporation, maintaining moisture on the surface of the eye.
• Sweat Gland Secretion: Although minimal, the sweat glands present in the lacrimal caruncle may also help maintain the hydration and lubrication of the tear film by contributing to its composition.

By ensuring that the tear film remains stable, the caruncle helps protect the cornea and conjunctiva from drying out, which can lead to discomfort or irritation.

Support for Tear Drainage

The lacrimal caruncle is situated near key components of the tear drainage system, including the lacrimal puncta and lacus lacrimalis (tear lake). Its anatomical positioning allows it to facilitate the efficient drainage of tears.

• Tear Collection: The lacrimal caruncle is located at the medial canthus, where tears collect in the tear lake before draining into the lacrimal puncta. Its proximity to this region ensures that it plays a supporting role in guiding excess tears into the puncta for drainage.
• Mechanical Support: The structure of the caruncle helps maintain the positioning and patency of the lacrimal puncta, ensuring that tears are efficiently drained away from the ocular surface and into the lacrimal canaliculi. This prevents the accumulation of tears on the surface of the eye, reducing the risk of overflow (epiphora).

Protection of the Ocular Surface

The lacrimal caruncle contains various structures that contribute to the immune defense of the eye, helping to protect against infections and irritation.

• Immune Defense: The caruncle produces small amounts of mucus, which can help trap dust, debris, and microorganisms, preventing them from spreading across the ocular surface. Additionally, the immune cells in the surrounding tissues contribute to the eye’s defense mechanisms, preventing infections or inflammation in this region.
• Barrier Function: By sitting at the medial canthus, the lacrimal caruncle acts as a barrier that helps prevent external particles and irritants from entering the lacrimal puncta. This reduces the likelihood of blockages in the tear drainage system and ensures proper tear flow.

Role in Eye Movement and Flexibility

The lacrimal caruncle works in conjunction with the plica semilunaris, a crescent-shaped fold of tissue located adjacent to the caruncle. This fold allows for greater flexibility of the conjunctiva during eye movements, especially in lateral directions.

Support for Eye Movement: The flexibility provided by the lacrimal caruncle and the plica semilunaris allows for the smooth and unrestricted movement of the eye, ensuring that the conjunctiva does not become stretched or damaged during eye rotation. This helps maintain ocular comfort during regular eye movements.

Role in Aesthetics and Structure

Though a minor function, the lacrimal caruncle also contributes to the overall aesthetics of the eye by maintaining the appearance of the medial canthus. Its presence helps give structure to this area of the eye, which would otherwise appear sunken or incomplete.

Maintaining Medial Canthus Anatomy: The lacrimal caruncle provides structural support to the medial canthus, helping to maintain the normal anatomy of this region. Without the caruncle, the appearance of the inner corner of the eye would be altered, potentially affecting eyelid function and tear drainage.

Clinical Significance

The lacrimal caruncle can be involved in several clinical conditions due to its location and composition:

• Inflammation (Carunculitis): The lacrimal caruncle can become inflamed due to infection, irritation, or allergic reactions, leading to redness, swelling, pain, and discomfort at the inner corner of the eye. Carunculitis is often caused by bacterial or viral infections and may require treatment with antibiotics or anti-inflammatory medications.
• Benign Growths: The caruncle may develop benign lesions, such as papillomas, nevi, or cysts, which can affect tear drainage or cause cosmetic concerns. In some cases, surgical removal may be necessary.
• Malignant Tumors: Though rare, malignant tumors can develop in the lacrimal caruncle, requiring careful evaluation and possible biopsy for diagnosis.