The sclera covers the posterior five-sixths of the surface of the globe, with an anterior opening for the cornea and a posterior opening for the optic nerve. The tendons of the rectus muscles insert into the superficial scleral collagen. The Tenon capsule covers the sclera and rectus muscles anteriorly, and both are overlain by the bulbar conjunctiva. The capsule and conjunctiva fuse near the limbus.
The sclera is thinnest (0.3 mm) just behind the insertions of the rectus muscles and thickest (1.0 mm) at the posterior pole around the optic nerve head. It is 0.4–0.5 mm thick at the equator and 0.6 mm thick anterior to the muscle insertions. Because of the thinness of the sclera, strabismus and retinal detachment surgery require careful placement of sutures.
The sclera, like the cornea, is essentially avascular except for the vessels of the intrascleral vascular plexus, located just posterior to the limbus, and the episcleral vessels. The episcleral vessels have superficial and deep plexuses (Fig 2-9). The superficial plexus runs beneath the Tenon capsule in a radial pattern; in episcleritis, it is this vascular plexus that is involved. The deep episcleral plexus rests on the surface of the sclera and is the layer involved in scleritis.
Figure 2-9 Episcleral vessels. The sclera is avascular but has overlying episcleral vessels, which are divided into superficial and deep plexuses. The organization of the conjunctival vasculature, which is also depicted, is similar to that of the episcleral vessels, with the addition of lymphatics, shown in green.
(Modified with permission from Levin LA, Nilsson SFE, Ver Hoeve J, Wu SM. Adler’s Physiology of the Eye. 11th ed. Philadelphia: Elsevier/Saunders; 2011:118–119.)
Numerous channels, or emissaria, penetrate the sclera (see Chapter 1, Figs 1-19, 1-20), allowing the passage of arteries, veins, and nerves:
anterior emissaria: penetration of the anterior ciliary arteries anterior to the rectus muscle insertions
middle emissaria: exit of vortex veins
posterior emissaria: lamina cribrosa, penetration of the short and long posterior ciliary vessels and ciliary nerves
Extraocular extension of malignant melanoma of the choroid occurs by way of the middle emissaria.
Branches of the ciliary nerves that supply the cornea sometimes leave the sclera to form loops posterior to the nasal and temporal limbus. These nerve loops, called Axenfeldloops, are sometimes pigmented and, consequently, have been mistaken for uveal tissue or malignant melanoma (Fig 2-10).
Anterior to the rectus muscle insertions, the episclera consists of a dense vascular connective tissue that merges deeply with the superficial sclera and superficially with the Tenon capsule and the conjunctiva. The scleral stroma is composed of bundles of collagen, fibroblasts, and a moderate amount of ground substance.
Collagen fibers of the sclera vary in size and shape and taper at their ends, indicating that they are not continuous fibers as in the cornea. The inner layer of the sclera (lamina fusca) blends imperceptibly with the suprachoroidal and supraciliary lamellae of the uvea. The collagen fibers in this portion of the sclera branch and intermingle with the outer ciliary body and choroid. The opaque, porcelain-white appearance of the sclera contrasts markedly with the transparency of the cornea and is primarily due to 2 factors: the greater variation in collagen fibril separation and diameter, and the greater degree of fibril interweaving in the sclera (see also Chapter 8). In addition, the lack of vascular elements contributes to corneal clarity.
Figure 2-10 External photograph of Axenfeld nerve loops in an arc pattern roughly equidistant from the limbus.
(Reproduced with permission from Jesse Vislisel, MD; EyeRounds.org, University of Iowa. Photograph by Cindy Montague, CRA.)
Excerpted from BCSC 2020-2021 series: Section 2 - Fundamentals and Principles of Ophthalmology. For more information and to purchase the entire series, please visit https://www.aao.org/bcsc.