The Uveal Tract

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9 The Uveal Tract

NORMAL ANATOMY

The uveal tract is a pigmented, vascular layer lying between the retina and sclera. It consists of the iris, ciliary body and choroid lying in continuity with one another. Apart from the specialized muscular structures of the iris and ciliary body, the uveal tract is concerned with nutrition of the eye through the secretion of aqueous humour by the epithelium of the ciliary body, and with the maintainence of the outer retina from the choroidal circulation.

Melanocytes, derived from the neural crest, contain melanin in melanosomes and are scattered throughout the tract; there is both an inter-individual and inter-racial difference in their relative concentration which accounts for the colour of the iris and the degree of fundus pigmentation. In contrast, the pigment epithelium of the iris and retina is derived from the neuroectoderm of the optic cup. Pigmentation appears here (apart from the inner layer of the ciliary epithelium, which remains nonpigmented throughout life) at 6–8 weeks of gestation whereas pigmentation of the iris and choroid is not complete until about 9 months of age.

THE CILIARY BODY

A precise knowledge of the position of the ciliary body is important in the positioning of surgical incisions for vitreous surgery. The surface markings of the ciliary body from the corneal limbus are 1.5–8 mm on the temporal side and 1.5–7 mm on the nasal side. The anterior third (2 mm) contains the ciliary muscle and ciliary processes, and is known as the pars plicata. The posterior two-thirds—the pars plana—extends posteriorly to the ora serrata where it merges with the retina. There is a dense attachment of the vitreous base over this area and on to the anterior equatorial retina (see Ch. 12).

The ciliary muscle is triangular in transverse section and controls accommodation. The outermost fibres run longitudinally inserting into the scleral spur; more internally the muscle fibres are radial with the innermost fibres running circumferentially. Contraction of the external longitudinal fibres transfers tension indirectly to the trabecular meshwork through the scleral spur and may explain the mechanism of IOP lowering by pilocarpine.

Overlying the ciliary muscle the epithelium and stroma are thrown up into about 80 ciliary processes. These have a vascular stroma and are covered by two layers of epithelium which are continuous with the iris pigment epithelium anteriorly and with the retinal pigment epithelium and neurosensory retina posteriorly. The inner or superficial epithelial layer is nonpigmented and has tight intercellular junctions. Aqueous is secreted through these cells (see Ch. 7). As in the choroid, the capillaries in the ciliary processes are fenestrated.

BLOOD SUPPLY OF THE UVEAL TRACT

The uveal tract, and especially the choroid, has an exceptionally high blood flow; for this reason, only about 3 per cent of the oxygen carried is extracted. The choroid supplies oxygen to the retinal pigment epithelium and photoreceptors by diffusion. Metabolites are transported through the pigment epithelium to and from the retina by active transport processes.

The vascular supply of the uveal tract comes from the posterior ciliary circulation anastomosing anteriorly with the anterior ciliary arteries. The short posterior ciliary arteries leave the ophthalmic artery posteriorly in the orbit (see Ch. 20) and run forwards to penetrate the sclera circumferentially around the optic disc, usually in two major horizontal trunks that divide to supply the optic disc, retrobulbar optic nerve (see Ch. 17) and the choroid. At the disc, two long posterior ciliary branches from these run forward medially and laterally in the lamina suprachoroidia to anastomose with the anterior ciliary arteries adjacent to the major circle of the iris. These long posterior ciliary arteries can frequently be seen in the horizontal meridians of a normal eye if the retinal pigmentation is not too dense. The anterior ciliary arteries are also derived from the ophthalmic artery. They lie on the external ocular muscles (two arteries on the medial, inferior and superior recti, and one on the lateral) and penetrate the sclera at the muscle insertions, and may contribute to the supply of the iris, ciliary body and anterior choroid (although under normal circumstances in a healthy eye the flow is retrograde). The choroidal venous return drains into the orbital veins by the vortex veins, of which there is usually one, but sometimes two, lying in each quadrant of the sclera at the equator.

CONGENITAL ANOMALIES OF THE UVEAL TRACT

ALBINISM

Albinism results from a defect in the synthesis of melanin. There are at least ten differently inherited forms of albinism; most forms are inherited recessively and many types are extremely rare. Albinism can be classified into oculocutaneous forms, in which there is both eye and skin involvement and ocular forms, in which hypopigmentation is limited to the eye. Ocular albinism is usually X-linked. Oculocutaneous forms can be subdivided by hair follicle analysis into those that have a complete absence of pigmentation (tyrosinase negative) and those that are tyrosinase positive. Tyrosinase-positive subjects can be more difficult to diagnose; these patients usually have reddish or light brown hair and paler skin pigmentation than other members of the family although their pigmentation increases with age. All ocular albinos have translucent irides on retroillumination. Purely cutaneous albinos have no ocular complications.

Apart from increased iris transillumination and hypopigmented fundi, albinos with ocular involvement have congenital nystagmus, macular hypoplasia, a high incidence of squint and amblyopia, and an anomaly of the chiasm in which the majority of optic nerve fibres from each eye decussate. This is thought to be caused by the absence of pigmented cells in the chiasm during embryogenesis; these cells ‘direct’ the ingrowing axons. Ocular albinism is a common cause of congenital nystagmus and it is important to examine all such patients for increased iris translucency by iris retroillumination. Excessive pigmentation (melanosis oculi) is discussed in Chapter 3.