Uvea

The uvea (also called uveal tract) is the main vascular layer of the eye. It consists of 3 parts (Fig 2-19):

• iris

• ciliary body (located in the anterior uvea)

• choroid (located in the posterior uvea)

These structures are discussed separately in the next 3 sections.

Figure 2-15 Relationship between the juxtacanalicular (JCT) meshwork and the Schlemm canal (SC). Inset: The endothelial meshwork (ECM) within the juxtacanalicular meshwork. Note the vacuole along the inner wall of the Schlemm canal (black arrow). TM = trabecular meshwork.

(Modified with permission from Levin LA, Nilsson SFE, Ver Hoeve J, Wu SM. Adler’s Physiology of the Eye. 11th ed. Philadelphia: Elsevier/Saunders; 2011:285.)

Figure 2-16 A, Low-magnification electron micrograph of the endothelial lining of Schlemm canal (SC), showing that most of the vacuolar configurations (V) at this level have direct communication (arrows) with the subendothelial extracellular spaces, which contain aqueous humor (×3970). B, Electron micrograph of a vacuolar structure that shows both basal and apical openings, thus constituting a vacuolar transcellular channel (arrow). Through this channel, the fluid-containing extracellular space on the basal aspect of the cell is temporarily connected with the lumen of the Schlemm canal, allowing bulk outflow of aqueous humor. N = indented nucleus of the cell (×23,825).

(Reproduced with permission from Tripathi RC. The functional morphology of the outflow systems of ocular and cerebospinal fluids. Exp Eye Res. 1977;25(Suppl):65–116.)

The uvea is firmly attached to the sclera at only 3 sites:

• scleral spur

• exit points of the vortex veins

• optic nerve

These attachments account for the characteristic anterior dome-shaped choroidal detachment.

The classification of uveitis, established by the 2005 SUN (Standardization of Uveitis Nomenclature) Working Group anatomical classification system, is based on the primary site of inflammation within the zones of the uvea:

• anterior: anterior chamber

• intermediate: vitreous

• posterior: choroid (primary or secondary from the retina)

• panuveitis: anterior chamber, vitreous, and retina or choroid

Uveitis is discussed extensively in BCSC Section 9, Uveitis and Ocular Inflammation.

Figure 2-17 Schematic representation of the Schlemm canal and relationships of the arteriolar and venous vascular supply. For clarity, the various systems have been limited to only parts of the circumference of the canal. Small, tortuous, blind diverticula (so-called Sondermann channels) extend from the canal into the trabecular meshwork. Externally, the collector channels arising from the Schlemm canal anastomose to form the intrascleral and deep scleral venous plexuses. At irregular intervals around the circumference, aqueous veins arise from the intrascleral plexus and connect directly to the episcleral veins. The arteriolar supply closely approximates the canal, but no direct communication occurs between the two.

(Reproduced with permission from Tripathi RC, Tripathi BJ. Functional anatomy of the anterior chamber angle. In: Jakobiec FA, ed. Ocular Anatomy, Embryology, and Teratology. Philadelphia: Harper & Row; 1982:236.)

Figure 2-18 Aqueous vein (arrow). Collector channels from the Schlemm canal drain into the episcleral venous plexus. With high magnification of the slit-lamp biomicroscope, they are visible near the limbus. Laminar flow and the mixing of aqueous and blood are visible.

(Reproduced with permission from Thiel R. Atlas of Diseases of the Eye. Amsterdam: Elsevier; 1963.)

Figure 2-19 The uvea consists of the iris, ciliary body, and choroid. The classification of uveitis, established by the SUN (Standardization of Uveitis Nomenclature) Working Group, is based on the primary site of inflammation. Anterior uveitis (red) involves the iris and anterior ciliary body; intermediate uveitis (blue) involves the posterior ciliary body and the pars plana and/or the peripheral retina; posterior uveitis (green) involves the choroid, either primarily or secondarily from the retina.

(Illustration by Paul Schiffmacher.)

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.