Connective tissue

Canthal tendons

The configuration of the palpebral fissure is maintained by the medial and lateral canthal tendons in conjunction with the attached tarsal plates (see Figs 9-18, 9-22). The 2 origins of the medial canthal tendon from the anterior and posterior lacrimal crests fuse just temporal to the lacrimal sac; they then split again into an upper limb and a lower limb that attach to the upper superior and lower inferior tarsal plates. The attachment of the tendon to the periosteum overlying the anterior lacrimal crest is diffuse and strong. The attachment to the posterior lacrimal crest is more delicate but is important in maintaining apposition of the eyelids to the globe, allowing the puncta to lie in the tear lake.

Figure 9-22 Tarsal plates and suspensory tendons of the eyelid.

(Illustration courtesy of Mark Miller.)

The lateral canthal tendon attaches at the lateral orbital tubercle 2–5 mm inside the lateral orbital rim. It splits into superior and inferior limbs that attach to the respective tarsal plates. Stretching or disinsertion of the medial canthal tendon may cause cosmetic or functional problems such as telecanthus (Fig 9-23). Horizontal eyelid instability is frequently the result of lateral canthal tendon lengthening. The lateral canthal tendon usually inserts 2 mm higher than the medial canthal tendon, giving the horizontal palpebral fissure a natural upward slope from medial to lateral. Insertion of the lateral canthal tendon inferior to the medial canthal tendon causes the horizontal palpebral fissure to slant downward.

Figure 9-23 Traumatic telecanthus, right side.

(Courtesy of Cat N. Burkat, MD.)

Figure 9-24 Eyelid margin gray line (arrow).

(Courtesy of Cat N. Burkat, MD.)

Eyelid margin

The eyelid margin is the confluence of the mucosal surface of the conjunctiva, the edge of the orbicularis, and the cutaneous epithelium. Along the margin are eyelashes and glands, which provide protection for the ocular surface. The gray line is an isolated section of pretarsal orbicularis muscle (Riolan) just anterior to the tarsus (Fig 9-24; also see Fig 9-14). The mucocutaneous junction is located posterior to the meibomian gland orifices on the eyelid margin. The horizontal palpebral fissure is approximately 30 mm long. The main portion of the margin, called the ciliary margin, has a rather well-defined anterior and posterior edge. Medial to the punctum and in the lateral quarter of the eyelid, the eyelid margin is thinner.

Eyelashes

There are approximately 100 eyelashes, or cilia, on the upper eyelid and 50 on the lower eyelid. The lashes originate in the anterior lamella of the eyelid margin just anterior to the tarsal plate, forming 2 or 3 irregular rows. A few cilia may be found in the caruncle.

Meibomian glands

The meibomian glands are sebaceous glands that contribute to the lipid layer of the tear film via modified holocrine secretion. They originate in the tarsus, numbering approximately 30–40 in the upper eyelid and 20 in the lower eyelid. During the second month of gestation, both the eyelashes and meibomian glands differentiate from a common pilosebaceous unit. This dual potentiality explains why, following trauma or chronic irritation, an eyelash follicle may develop from a meibomian gland (acquired distichiasis). Similarly, an extra row of eyelashes arising from the meibomian orifices may be present from birth (congenital distichiasis).

Vascular and lymphatic supply

The extensive vascularity of the eyelids promotes healing and helps defend against infection. The arterial supply of the eyelids comes from 2 main sources: (1) the internal carotid artery by way of the ophthalmic artery and its branches (supraorbital and lacrimal) and (2) the external carotid artery by way of the arteries of the face (angular and temporal). Collateral circulation between these 2 systems is extensive, anastomosing throughout the upper and lower eyelids and forming the marginal and peripheral arcades.

The marginal arterial arcade should not be confused with the peripheral arterial arcade. In the upper eyelid, the marginal arcade lies 2 mm superior to the margin, near the follicles of the cilia and anterior to the tarsal plate. The peripheral arcade lies superior to the tarsus, between the levator aponeurosis and the Müller muscle (see Figs 9-10, 9-20). The lower eyelid often has only 1 arterial arcade, located at the inferior tarsal border.

Eyelid venous drainage may be divided into a preseptal system, in which the preseptal tissues drain into the angular vein medially and into the superficial temporal vein laterally, and a postseptal system, in which drainage flows into the orbital veins and the deeper branches of the anterior facial vein and pterygoid plexus. Lymphatic vessels serving the medial portion of the eyelids typically drain into the submandibular lymph nodes. Lymphatic channels serving the lateral eyelids drain first into the superficial preauricular nodes and then into the deeper cervical nodes.

Excerpted from BCSC 2020-2021 series: Section 10 - Glaucoma. For more information and to purchase the entire series, please visit https://www.aao.org/bcsc.