Eyelids and the Corneal Surface

Key Concepts

The eyelids function to cover, cleanse, and lubricate the eye.
The eyelid skin is the thinnest in the body and allows for unrestricted movement.
The tarsal plates provide structural stability to the eyelids.
The orbicularis oculi is important for eyelid closure and the involuntary blink reflex.
The conjunctiva, meibomian glands, and lacrimal glands produce secretions that make up the tear film.

Introduction

An understanding of eyelid anatomy and function is important to achieve ocular surface health. The eyelids are a thin, complex, and dynamic structure whose primary function is to cover and protect the ocular surface of the eye. With every blink, the eyelids cleanse and lubricate the globe and, in doing so, maintain optical visual clarity of the cornea. They serve as both a physical and immunologic barrier against infection. In addition, the eyelids also serve as an important facial aesthetic subunit, playing an essential role in facial expression. It is for these reasons that the eyelids are not only critical for vision but also for quality of life.

The upper and lower eyelids join at the medial and lateral canthi. The average aperture of the eyelids is approximately 30 mm in horizontal width and 10 mm in vertical height. The peak of the upper eyelid is slightly nasal to the pupillary axis, and the lowest point of the lower lid rests just lateral. In general, the upper lid covers 1–3 mm of the upper cornea and the lower lid rests at or near the inferior limbus. The upper lid crease measures approximately 6–10 mm from the eyelid lash line.

The eyelid is often compartmentalized into the anterior and posterior lamellae. The anterior layer is composed of the eyelid skin and orbicularis oculi. The posterior layer consists of the tarsal plate and palpebral conjunctiva. The gray line is considered the junction of the anterior and posterior lamellae. ^, The eyelid structures starting from the most superficial are skin, eyelid protractors, orbital septum, orbital fat, eyelid retractors, tarsus, and conjunctiva. Pathology affecting the eyelid apposition to the globe can have detrimental consequences for the ocular surface.

Anatomy

Eyelid Skin

The skin of the eyelid is the thinnest and most pliable on the human body, ranging from 500 to 1000 μm in thickness. This allows for movement of the eyelid with minimal resistance. It is thinnest near the lid margin and thickest near the orbital rims. Because of its physical characteristics and constant dynamic movement, the eyelid skin is more prone to laxity than other facial skin, and replacement of eyelid skin may pose a challenge in finding donor sites with comparably thin skin.

Histologically, the skin of the eyelid is composed of keratinized stratified squamous epithelium overlying a basement membrane and subcutaneous connective tissue. The skin becomes mucosa, or nonkeratinized, at the mucocutaneous junction, which is adherent to the posterior tarsus ( Fig. 4.1 ).

Fig. 4.1, Magnified view of the upper eyelid and margin.

The dermis of the eyelid is nearly nonexistent as the eyelid skin is attached to the underlying muscle by way of loose collagenous fibers interspersed with an elastic fiber network. The subcutaneous tissue of the eyelid is unique because it contains no fat. Sparse glands and hair follicles, when present, are contained in the dermis. The skin is more attached at the eyelid crease, canthal angles, and eyelid margin.

The eyelids are relatively hairless. Eyelashes are a relatively sparse, specialized type of hair, which may serve as sensory structures causing a reflex eyelid closure when dust or foreign bodies hit them. There are approximately 100 eyelashes in the upper eyelid and 50 in the lower eyelid. Eyelashes also serve an important role for eyelid esthetics. Their limited length, increased shaft diameter, and unique curvature set them apart from any other hair on the body.

A variety of conditions both dermatologic and systemic, as well as trauma, can result in eyelid scarring, retraction, poor closure, and trichiasis, which can lead to problems with the ocular surface.

Eyelid Muscles: Protractors

Orbicularis Oculi

The main protractor of the eyelid is the orbicularis oculi. Its functions include eyelid closure, blink, drainage of tears, and meibomian gland secretion. It acts as a sphincter, depressing the brow and upper eyelid while elevating the lower lid and cheek. The orbicularis oculi is a striated muscle that lies directly beneath the skin and extends from the lid margin to beyond the superior and inferior orbital rims. The orbicularis originates from and into the canthal tendons. It is densely adherent to the tarsal plates and innervated by the facial nerve.

The orbicularis is divided into three concentric portions: pretarsal, preseptal, and orbital. The orbital is under voluntary control, preseptal under voluntary and involuntary, and the pretarsal orbicularis primarily under the involuntary blink reflex. ^,

The pretarsal deep origins are located on the posterior lacrimal crest, with superficial origins on the anterior limb of the medial canthal tendon. Horner muscle is a branch of the pretarsal orbicularis that encircles both canaliculi and contributes to the lacrimal pump.

The muscle of Riolan consists of marginal fibers of the orbicularis and forms the “gray line” at the eyelid margin. It functions to hold the lacrimal punctum against the sclera for proper drainage of tears, and its contraction facilitates expression of meibomian gland secretions.

Disruption of the orbicularis oculi, whether due to myopathy, facial nerve palsy, trauma, or chemodenervation (botulinum toxin), may lead to eyelid laxity, ectropion, lagophthalmos, and incomplete blink and subsequently ocular surface dysfunction.

Eyelid Muscles: Retractors

The eyelid retractors open the eye. The upper retractors are the levator palpebrae superioris, Müller muscle, and frontalis. The lower retractors are the capsulopalpebral muscle and the inferior tarsal/palpebral muscle.

Upper Lid Retractor: Levator Palpebrae Superioris

The levator palpebrae superioris is the primary retractor of the upper eyelid. It originates on the orbital roof near the apex, in front of the optic foramen, and is located anterior to the superior rectus muscle. It is innervated on its undersurface by the third cranial nerve (CN). The levator muscle passes along with the superior rectus muscle through the posterior orbit and is loosely attached to the rectus muscle by intramuscular septa. At the point where the superior rectus attaches to the globe, the levator muscle transitions to a fascial sheet known as the levator aponeurosis. The levator aponeurosis is a fan-like structure that divides anteriorly and posteriorly. The anterior layer becomes contiguous with the orbital septum and travels through the orbicularis oculi to insert onto subcutaneous tissue, forming the upper eyelid crease. The posterior layer lies anterior to the Müller muscle and attaches to the anterior aspect of the tarsal plate. The levator muscle is 40 mm long, and the aponeurosis is 14–20 mm in length.

The Whitnall ligament is a condensation of elastic fibers of the anterior sheath of the levator muscle. It is located between the levator aponeurosis and muscle. It acts as the main suspensory ligament of the upper eyelid, a support ligament for the fornix, and as a check ligament for the levator complex. The Whitnall ligament is thought to transfer the vector of force of the levator muscle from anterior-posterior to superior-inferior.

The medial horn of the aponeurosis attaches near the trochlea and superior oblique tendon medially, on the posterior lacrimal crest. The lateral horn runs through the lacrimal gland laterally, dividing it into the orbital and palpebral lobes. It attaches 10 mm above the lateral tubercle on the inner lateral orbital wall. The lateral horn is stronger than the medial, and it accounts for lateral flare in thyroid lid retraction.

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