Eyelids and the Lacrimal System

Introduction

The eyelids first develop in the seventh week of gestation as definite folds forming a circular palpebral aperture. The mesenchymal portion of the upper eyelid develops from a medial and lateral portion of the frontonasal process. The lower eyelid develops from an upgrowth from the maxillary process. The eyelids proliferate to fuse at the ninth week of gestation, which permits differentiation of the eyelids. Failure of eyelid fusion inhibits the development of the lid margin, which may lead to a coloboma defect of the eyelid. The first structures to develop are the meibomian glands and glands of Zeis. The tarsal glands originate as a solid ectodermal ingrowth from the eyelid margin. The ectodermal ingrowth develops a central lumen and secretes sebum by the end of the fifth month of gestation. The secretion of these glands may facilitate the separation of the fused eyelids during the sixth month of gestation.

The development of the tarsus is coordinated with the invagination of ectodermal cords, which serve as a precursor of the meibomian glands. The eyelid cilia develop shortly after the tarsus and tarsal glands followed by the marginal strip of orbicularis muscle called the muscle of Riolan. By the sixth month of gestation, the eyelids are fully developed and begin to separate.¹

Lacrimal excretory system precursors first develop in the sixth week of gestation along the cleft between the lateral nasal and maxillary processes. The excretory system appears as a cord of epithelial cells. The solid cord extends downward into the mesenchyma to form the nasal optic fissure, and detaches from the surface. The sequestered epithelial cord lies between the precursors of the medial canthus and the nose. During weeks 6 to 12 of the gestational period, the lacrimal ectodermal bud proliferates to extend toward the inner canthus and nasal cavity. The epithelial cord splits as it approaches the medial canthus, conforming to the medial canthal angle. The precursors of the canalicular system are, thereby, established. At approximately the eighth or ninth week of gestation, the epithelial cord reaches the eyelid margin. During this time, the epithelial cells also proliferate, forming the rudiments of the lacrimal sac, and extend inferiorly to approach the nasal cavity.

During the third month of gestation, the solid epithelial cords that compromise the lacrimal excretory system undergo central disintegration. Isolated cavities appear within the system. With time, these cavities become confluent. The canalization process is completed by the sixth month of gestation. However, the puncta do not become patent until the seventh month. The lower end of the duct opening into the inferior meatus is frequently occluded by a thin membrane at birth.

Anatomy

The various surface contours on the mid and upper face, coupled with varying skin thickness, produce characteristic folds in the periocular area. The palpebral fissure generally ranges from 10 mm to 12 mm in vertical dimension and 25 mm to 30 mm in horizontal dimension. In primary gaze, the upper eyelid margin is generally 1 mm to 2 mm below the superior limbus. It is important that the peak of the upper eyelid margin is nasal to the center of the pupil with the eye in primary gaze. The lower eyelid margin is generally at the inferior edge of the corneal limbus. The lowest portion of the lower eyelid margin is slightly temporal to the pupil. The upper eyelid crease is horizontally oriented approximately 7 mm to 12 mm above the eyelashes. It is formed by the subcutaneous insertion of terminal fibers of the levator aponeurosis. These fibers will firmly adhere to the subcutaneous tissue below the eyelid crease to form the pretarsal portion of the eyelid. The skin overlying the orbital septum has no levator aponeurotic subcutaneous extensions and will fold over the eyelid crease in varying portions. The lower eyelid has a poorly defined eyelid crease extending to within 2 mm to 3 mm of the eyelid margin nasally and 5 mm to 6 mm temporally. The nasojugal fold extends inferiorly and laterally from the medial canthal angle along the side of the nose. The malar folds extend from the lateral portion of the lower eyelid toward the inferior lateral extent to the nasojugal fold. The nasojugal and malar folds represent the junction of the thin eyelid skin to the thicker skin of the cheek. In addition, the angular artery and vein will generally be located in the nasojugal fold.

Eyebrow

The eyebrow is extremely important to facial expression. There are four major layers of the eyebrow: skin, muscle, fat, and aponeurosis. The muscular layer of the brow consists of four muscle groups: frontalis, corrugator superciliaris, procerus, and orbital orbicularis oculi. The frontalis muscle originates from the split in the galea aponeurotica. The frontalis muscle has no bony insertions, which potentiates its contraction and enhances brow mobility. Nasally, the frontalis muscle will interdigitate with the vertically oriented procerus muscle, the obliquely oriented corrugator superciliaris muscles, and the orbital orbicularis muscle. This intermixing of fibers supports the brow at this level nasally. The frontalis muscle may not extend to the superior orbital rim laterally, which may contribute to the brow ptosis that is frequently seen.

The frontalis muscle is encapsulated by an anterior/posterior muscular sheath, which is an extension of the galea aponeurotica. The posterior muscular sheath splits as it approaches the periosteum superior orbital rim. The multiple splits in the posterior muscular sheath will produce a potential space for the brow fat pad. The posterior muscular sheath of the frontalis muscle then extends into the eyelid to contribute to the orbital septum. The brow fat generally enhances brow mobility. The brow fat pad contributes to the gender variation in brow anatomy. In men's brows, the brow fat pad is more prominent and extends inferior into the eyelid directly anterior to the orbital septum.

Upper Eyelids

The eyelid consists of six layers: skin, orbicularis muscle, levator aponeurosis, Müller muscle, tarsus, and conjunctiva (Slide 1).² Near the eyelid margin, there are only four layers: skin, orbicularis muscle, tarsus, and conjunctiva. Directly above the tarsal border, five layers of the eyelid exist: skin, orbicularis muscle, levator aponeurosis, Müller muscle and conjunctiva.

The eyelid skin has unique features to enhance its mobility. The epithelial layer is thin and attaches to the orbicularis muscle by a loose, connective tissue devoid of dermal-like tissue. In addition, sebaceous glands are not developed extensively in the eyelid, which would anchor the skin to the subcutaneous tissues. These distinguishing features of the eyelid skin account, in part, for its tendency to become redundant in exaggerated situations and to fold over the eyelid, producing a visual field loss.

The orbicularis muscle is the protractor for the eyelid and is under voluntary control from the facial nerve. The concentrically arranged orbicularis muscle is divided into three sections: pretarsal, preseptal, and orbital. The pretarsal and preseptal orbicularis muscles form the palpebral portion of the orbicularis and overlie tarsus and orbital septum, respectively. The orbital orbicularis overlaps the orbital rim, interdigitating with the eyebrow muscles superiorly. The pretarsal and preseptal orbicularis contribute to the superficial and deep portions of the medial and lateral canthal tendons. The deep extensions of the pretarsal and preseptal orbicularis muscles are referred to as Horner and Jones muscles, respectively, and insert in the fascia overlying the lacrimal sac. Pretarsal orbicularis also contributes to the lateral canthal tendon. Laterally, the preseptal orbicularis muscle joins in the lateral raphe oriented horizontally in the lateral canthal angle. The medial canthal tendon supports the nasal aspect of the eyelids. The medial canthal tendon has superficial and deep components that anchor the eyelid nasally and posteriorly against the globe. A superior branch of the medial canthal tendon extends from the anterior portion of the medial canthal tendon to insert into the frontal bone. The superior limb of the medial canthal tendon anchors the nasal eyelid preventing dystopia of the medial canthal angle.

The tendinous portion of the lateral canthal tendon is from the pretarsal orbicularis muscle and inserts on the inner aspect of the lateral orbital rim (Whitnall tubercle). The ligamentous component of the lateral canthal tendon is a direct extension from the tarsus that slips posteriorly to the orbital septum to insert at the lateral orbital tubercle. This deep extension and attachment to the lateral canthal tendon draws the eyelid laterally and posteriorly approximating the eyelids to the globe. The orbital septum forms the anatomic boundary between the eyelids and the orbit. It originates from the arcus marginalis of the orbital rim. The orbital septum extends toward the tarsus and fuses with the levator aponeurosis and lower lid retractors prior to their insertion to the tarsus. In the upper lid, variations in the level of fusion to the orbital septum and levator aponeurosis will produce changes in the level of the eyelid crease and fold. In the eyelid of a person from Asia, the orbital septum fuses with the levator aponeurosis below the tarsal border, thereby eliminating the eyelid crease and producing a prominent lid fold. The pre-aponeuroticorbital fat is confined by the orbital septum and forms pockets. However, the multi-laminated orbital septum may thin with age, allowing herniation of orbital fat. The nasal fat pockets in the upper and lower eyelids are white instead of yellow. There are two fat pockets in the upper lid. The lateral pocket is occupied by the orbital lobe of the lacrimal gland and can, sometimes, prolapse producing fullness in the lateral portion of the eyelid. The pre-aponeurotic fat acts as a guide for the levator aponeurosis and muscle as it contracts from down gaze to up gaze.

Whereas the orbicularis muscle is the protractor for the eyelids, the levator palpebral superioris muscle is the retractor of the upper eyelid. The muscle arises at the orbital apex directly above the Zinn ring. The levator muscle proceeds anteriorly from the orbital apex in the superior orbit adjacent to the periorbita. As the superior orbital rim is approached, the levator fans nasally and temporally. As the levator muscle extends below the superior orbital rim, the muscle is redirected by the superior transverse ligament of Whitnall (Slide 2). The superior transverse ligament acts as a fulcrum to redirect the muscle from an anterior/posterior to a superior/inferior direction, enhancing its function. At the level of the superior transverse ligament, the levator muscle begins to transform into its aponeurotic extension. This transformation of levator muscle to a white, shiny structure is complete approximately 10 mm to 12 mm above the superior border of the tarsus. The aponeurotic fibers diffusely extend anteriorly through the orbicularis muscle overlying the tarsus, to insert into the subcutaneous tissues. The superior-most subcutaneous extensions of the levator aponeurosis will define the level of the eyelid crease. The posterior portion of the levator aponeurosis attaches to the lower 7 mm to 8 mm of tarsus. The superior 2 mm to 3 mm of tarsus has loose attachments of the levator aponeurosis, whereas the aponeurosis is more strongly adherent to the tarsus approximately 3 mm above the eyelid margin. The diffuse extension of levator aponeurosis produces a smooth, coordinated movement to the upper eyelid.

Müller muscle originates from the under surface of the levator muscle at the level of its transformation to its aponeurosis. It is approximately 10 mm to 15 mm long and the well-defined smooth muscle attaches to the superior tarsal border in the upper eyelid. In the lower eyelid, Müller muscle, or inferior tarsal muscle, is not as well defined and generally terminates before reaching the inferior tarsal border.

The tarsal plate provides a firm posterior support or foundation for the eyelids. The tarsus in the upper eyelid is approximately 10 mm at its greatest vertical height with the tarsus in the lower lid ranging from 3.2 mm to 5.0 mm (average 3.7 mm). The tarsus may taper toward its medial lateral borders and attaches to the orbital rim by the medial and lateral canthal tendons.

Lower Eyelid

The sagittal view of the normal eyelid anatomy shows that the upper and lower eyelids are analogous structures (Slide 3). The main distinction is the eyelid retractors. The upper eyelid has a distinct eyelid retractor, the levator muscle that imparts a high degree of mobility to the eyelid. The levator muscle excursion between downgaze and upgaze is approximately 15 mm to 17 mm. The lower eyelid does not have a distinct retractor; it has a fibrous extension from the inferior rectus muscle. Therefore, the relatively immobile lower eyelid gains its motion through the action of the inferior rectus muscle. The lower eyelid has a range of motion of only 3 mm to 5 mm on extreme degrees of excursion from downgaze to upgaze. The lower eyelid retractor originates from the inferior rectus muscle before its penetration of the posterior Tenon capsule. The fibrous extension forming the lower lid retractors originates as the capsulopalpebral head of the inferior rectus muscle and encapsulates the inferior oblique muscle. Anterior to the inferior oblique muscle, the fibers reunite to form the capsulopalpebral fascia or aponeurosis. Most of the capsulopalpebral fascia inserts on the inferior tarsal border, although some fibers extend anteriorly through the orbital fat contributing to septae fornix. The capsulopalpebral fibers, however, do not extend through the orbicularis muscle to terminate in subcutaneous tissues or extend on the anterior tarsal surface, as does its analogous structure in the upper eyelid, the levator muscle. As a result, the lower eyelid crease is poorly defined. The orbital septum fuses with the capsulopalpebral fascia before reaching the inferior tarsal border.⁴

Lacrimal System

The lacrimal system provides the production, distribution, and drainage of tears. Tear film protects the globe from desiccation and maintains a clear optical surface. The lacrimal excretory system drains tears from the lacus lacrimalis into the inferior meatus. The precorneal tear film has three layers: innermost mucus layer, prominent central aqueous layer, and superficial lipid layer. The mucus layer is produced by the goblet cells of the conjunctival epithelium. The aqueous layer is produced by the main and accessory lacrimal glands. The outermost lipid layer of the tear film is primarily produced by the meibomian gland with a small component produced by the glands of Zeis and apocrine glands of Moll. The main lacrimal gland lies in the superior lateral portion of the anterior orbit. Directly posterior to the orbital rim is a convexity in the frontal bone called the fossa glandular lacrimalis. The lacrimal gland is divided into two lobes by the lateral horn of the levator aponeurosis. The superior orbital lobe is larger than the inferior or palpebral lobe. The orbital lobe of the lacrimal gland is an oval, bean-shaped mass approximately 20 mm long, 12 mm wide, and 5 mm deep. The orbital lobe extends inferiorly to the level of the zygomatical frontal suture. The inferior portion of the orbital lobe lies directly on and is molded by the globe. The anterior portion of the lower pole extends to and rests in the lateral horn of the levator aponeurosis. The palpebral lobe of the lacrimal gland is approximately one-third the size of an orbital lobe. The palpebral lobe lies between the levator aponeurosis and the palpebral conjunctiva. The palpebral lobe is firmly attached to the conjunctiva and the ducts of both lobes of the lacrimal gland exit through the palpebral lobe. If the upper eyelid is everted, the palpebral lobe is seen to extend within a few millimeters of the superior aspect of the tarsus laterally. The lacrimal gland is a glandular-appearing surface that is a pink-gray color. The ducts of the lacrimal gland accumulate secretions that exit on the palpebral conjunctiva. Approximately 12 to 14 ducts exit on the conjunctiva; the ducts from the orbital lobe traverse the palpebral lobe to enter the conjunctival sac. Most of the ducts open 4 mm to 5 mm from the superior border of the tarsus; however, the ducts' orifices may extend into the lateral canthus. All of the ducts from the lacrimal gland extend through the palpebral lobe of the gland. The lacrimal gland is supported in the superior lateral aspect of the orbit by various attachments. This includes the suspensory ligament of Soemmering, which fixes the superior surface of the gland to the periorbita of the frontal bone. However, the major supportive structure for the lacrimal gland is the Whitnall ligament and the lateral horn of the levator aponeurosis. The lacrimal gland, especially the orbital lobe, is enveloped by what appears to be a connective tissue capsule. However, this is not a true capsule, but rather, an extension of the connective tissue that separates the lobules of the gland.

The accessory lacrimal glands consist of the glands of Wolfring, located at the superior tarsal border, and the glands of Krause, positioned in the conjunctiva fornix. The accessory lacrimal glands consist of small masses of lacrimal glands that drain directly to the conjunctiva. There are 20 to 40 glands of Krause in the upper fornix and only 6 to 8 in the lower fornix. The glands of Wolfring are less common; there are 3 to 4 Wolfring glands at the superior tarsal border and only one at the inferior tarsal border. The primary role of the accessory lacrimal glands is maintenance of the basic tear secretion.

The lacrimal excretory system consists of the puncta, canaliculus, common canaliculus, lacrimal sac, and the nasal lacrimal duct (Slide 4). Tears can accumulate in the medial canthal angle at the lacus lacrimalis. The tears then enter the lacrimal excretory system at the punctum or opening of the canaliculus at the posterior eyelid margin. The initial 2 mm of the canaliculus is vertically oriented. The canaliculus then courses horizontally, 8 mm to 10 mm to the lacrimal sac. Before entering the lacrimal sac, the upper and lower canaliculi fuse, forming the common canaliculus. Common canaliculus enters the lacrimal sac at the posterior/superior aspect of the lacrimal sac at the level of the medial canthal tendon. The lacrimal sac enters the bony nasal lacrimal duct and gains entrance into the nose under the inferior turbinate. Multiple valves have been identified in the nasal lacrimal system. However, most valves generally represent mucosal folds and are of no functional significance. One valve of importance is the valve of Rosenmüller at the entrance to the common canaliculus into the lacrimal sac. This mucosal fold prevents reflux of the lacrimal sac content into the canaliculus. The other nasal lacrimal valve of significance is the valve of Hasner, which is implicated in congenital nasal lacrimal duct obstruction.

References

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