Ординатура / Офтальмология / Учебные материалы / Uveitis Text and Imaging Text and Imaging Text and Imaging 2009

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UVEITIS Text and Imaging

INTRODUCTION

A clear concept of the anatomy of the eye is central to the analysis and interpretation of images acquired by a number of newer diagnostic imaging techniques that have emerged in recent years. Fluorescein and indocyanine green dyes are used to analyse flow dynamics of retina and choroid. Techniques like optical coherence tomography and ultrasonography detect fluid/cell collection in the potential spaces and its sequalae. Accurate interpretation of images depends on a basic knowledge of ocular anatomy. The aim of this chapter is to provide an overview of various anatomical characteristics of uveal tract, retina, retinal pigment epithelium, optic disc, and vitreous from a clinical perspective.

GROSS ANATOMY

The eye has three layers or coats, three compartments and contains three fluids (Figures 1A and B).1-3

1.The three coats of the eye are as follows:

a.Outer layer: cornea, sclera and lamina cribrosa.

b.Middle vascular layer—the uveal tract is divided into two parts: anterior (iris and ciliary body) and posterior uvea (choroid).

c.Inner layer or sensory part of the eye—the retina.

2.The three compartments of the eye are as follows:

a.Anterior chamber: The space between the cornea and the iris diaphragm.

b.Posterior chamber: The triangular space between the iris anteriorly, the lens and zonule posteriorly, and the ciliary body.

c.Vitreous chamber: The space behind the lens and zonule.

3.The three intraocular fluids are as follows:

a.Aqueous humour: A watery, optically clear solution of water and electrolytes similar to tissue fluids except that normally aqueous humour has a low protein content.

b.Vitreous humour: A transparent gel consisting of a three-dimensional network of collagen fibres with the interspaces filled with polymerised hyaluronic acid molecules and water. It fills the space between the posterior surface of the lens, ciliary body, and retina.

c.Blood: In addition to its usual functions, blood contributes to the maintenance of intraocular pressure. Most of the blood within the eye is in the choroid.

Clinically, the eye is considered to be composed of two segments:

1.Anterior segment: All structures from (and including) the lens forward.

2.Posterior segment: All structures posterior to the lens.

THE SCLERA

The sclera is the white outer coat of the eye that gives the eyeball its shape and helps to protect the delicate inner structures.1-3 It extends anteriorly from the limbus to the optic nerve posteriorly. The thickness of sclera ranges from 0.3 mm just behind the rectus muscle insertions to 1.2 mm at the macula.1-3

The sclera is formed of white fibrous tissue intermixed with fine elastic fibres; flattened connectivetissue corpuscles, some of which are pigmented, are contained in cell spaces between the fibres. The fibres are aggregated into bundles, which are arranged chiefly in a longitudinal direction. Its vessels are not numerous, the capillaries being of small size, uniting at long and wide intervals. Sclera is innervated by the ciliary nerves. Inflammation, the principal process affecting the sclera, is frequently part of a general inflammatory reaction associated with a systemic immune-mediated collagen vascular disease. Scleritis usually produces inflammation of the uveoretinal structures by their vicinity.1-3

THE UVEAL TRACT

The uveal tract is the main vascular compartment of the eye, and is necessary for its vital functions including, nutritional support, thermoregulation, and control of intraocular pressure. Of the total blood flow to the eye, about 96% of the blood is distributed to the uveal tissues in contrast to 4% blood that is distributed to the retinal vessels. Highest blood flow is seen in choroidal capillaries (800-1200 ml per 100 gm tissue per min) and it is little affected by high intraocular pressures. As a result, the oxygen content of the choroidal venous blood is only 2-3% less than that of the arterial blood.4-6 The uveal tract consists of three parts, namely the iris, the ciliary body, and the choroid.1,2

IRIS

The iris is a thin, contractile, pigmented diaphragm with a central aperture, the pupil. It is a dynamic structure, capable of causing precise and rapid changes in pupillary diameter in response to light. It consists of four layers: the anterior border layer, the stroma with the sphincter muscle, the anterior epithelium with the dilator pupillae muscle, and the posterior pigment epithelium.1,2

The arterial supply of the iris is provided by radial vessels from the major arterial circle which is formed by two long posterior ciliary arteries and the seven anterior ciliary arteries. They converge in a spiral pattern toward the pupillary margin and form the radial ridges seen on the anterior surface of the iris.1,2

On reaching the collarette, the arteries anastomose to form the incomplete minor arterial circle of the iris. The veins follow the arteries and form a corresponding minor venous circle. The radial veins do not drain into a major venous circle but converge and drain into the vorticose veins. The diameter of the capillaries is relatively large.

Iris capillaries are characterised by nonfenestrated endothelial cells that have a high density of endocytotic vesicles and tight junctions. This makes them less permeable than normal somatic vessels constituting an important component of the bloodaqueous barrier. They do not normally leak fluorescein in angiography.4

The basal lamina of the endothelial cells is thickened (0.5–3 μm) and further strengthened by perivascular collagenous/hyalinised layers.4 However, when there is inflammation, inflammatory cells, proteins and other large molecules leak into the anterior chamber which is easily appreciated on slit-lamp examinations as flare and cells. Flare is, however, best quantified by laser flare photometry (see Chapter 2).

There is a dual sympathetic and parasympathetic innervations of the iris.5 The sphincter pupillae is innervated by parasympathetic nerve fibres derived from the oculomotor nerve. The dilator pupillae muscle is innervated by non-myelinated sympathetic fibres whose cell bodies are situated in the superior cervical sympathetic ganglion. The sensory nerves are branches of the long and short ciliary nerves from nasociliary nerve, ophthalmic division of the trigeminal nerve.4

CILIARY BODY

The ciliary body forms a complete ring that runs around the inside of anterior sclera extending from 1.5 mm posterior to the corneal limbus to 7.5 to 8.0 mm posterior temporally and 6.5 to 7.0 mm nasally. Anteriorly, the ciliary body is plicated for about 1.5 mm and is called the pars plicata. It consists of about 70 ciliary processes. The pars plicata is richly vascularised. The zonular fibres of the lens attach primarily in the valleys of the ciliary processes but also along the pars plana. Posteriorly the ciliary body is flat and avascular and is called the pars plana. The vitreous base gains attachment to the epithelium of the pars plana over a band extending forward from the ora serrata.

The ciliary body is made-up of the ciliary epithelium, ciliary stroma, and the ciliary muscle. The ciliary epithelium consists of two layers of cuboidal cells that cover the inner surface of the ciliary body. There is an outer pigmented layer and an inner nonpigmented layer. The ciliary stroma is made of bundles of loose connective tissue, and is rich in blood vessels and melanocytes. It contains the ciliary muscle that consists of smooth muscle fibres. The blood vessels consist of the ciliary arteries, veins and capillary networks. The capillary plexus of each ciliary process is supplied by arterioles from the major arterial circle, formed predominantly by the long posterior ciliary arteries, and is drained by one or two large venules located at the crest of each process. The ciliary body is innerved by posterior ciliary nerves. Parasympathetic fibres come from the Edinger-Westphal nucleus along with the oculomotor nerve, and the ciliary ganglion. Sympathetic fibres come from the cervical sympathetic trunk.1,2 The ciliary body has several key functions: accommodation, production of aqueous humor (protein-free, optically clear), outflow of aqueous humour, and production of zonules, vitreal collagen and hyaluronic acid.

CHOROID

Macroscopic Appearance

The choroid is a soft, thin vascular layer, lining the inner surface of the sclera. It extends posteriorly from the optic disc to the ora serrata anteriorly.

The choroid is extremely vascular and nourishes the outer portion of the retina. It can be divided into four layers: Haller’s layer (outermost layer of large diameter vessels), Sattler’s layer (deeper medium sized blood vessels), choriocapillaris and Bruch’s membrane.6 Within Sattler’s layer, arteries gradually decrease in caliber and form arterioles. The suprachoroid lamina is a pigmented sheet overlying the perichoroidal space, which lies between the sclera and choroid and contains the long and short posterior ciliary arteries and nerves.

The uveal tract is firmly attached to the sclera at three sites, the scleral spur, the exit points of the vortex veins, and at the optic nerve. These attachments account for the characteristic anterior balloons formed in choroidal detachment.6

The thickness of the choroid has been estimated at about 100 to 220 μm, with the greatest thickness noted over the macula (500 to 1000 μm).7

The visibility of the choroid depends on the density and distribution of pigment in the retinal pigment epithelial cells and, to a lesser extent, on the density of the choroidal pigment.

Histology

a. Suprachoroid lamina

The suprachoroid lamina is 10 to 34 μm thick and consists of pigmented (melanocytes) and nonpigmented uveal cells (fibrocytes), a musculoelastic system, and a mesh of collagen fibres forming pigmented bands, which run from the sclera anteriorly to the choroid.7

b. Choroidal stroma

The choroid contains macrophages, lymphocytes, mast cells and plasma cells, under normal circumstances, these immunocompetent cells are in an inactivated state. However, when ever there is an autoimmune (e.g. sympathetic ophthalmia) or infectious uveitis, the immunologic machinery of the choroid is activated and recruitment of additional inflammatory cells from the systemic circulation occurs.6

c. Choriocapillaris

The choriocapillaris shows a lobular organisation of wide lumen capillaries, supplying an independent segment of choriocapillaries and lying in a single plane. The lobular network is well developed at the posterior pole and is less regular anteriorly towards the ora serrata. There is little anastomosis between the lobules, creating vascular watersheds that may lead to occlusive events in the choroid and at the optic nerve.

d. Bruch’s membrane

Bruch’s membrane is a thin (2 to 4 μm), noncellular lamina containing five layers:

1.The inner basal lamina in continuity with the basal lamina of the retinal pigment epithelium.

2.The inner collagenous zone.

3.The elastic zone.

4.The outer collagenous zone.

5.The outer basal lamina.

Vascular Supply of the Choroid

The choroid receives its blood from the posterior ciliary arteries. There are one to five posterior ciliary arteries

arising from the ophthalmic artery, one in 3%, two in 48%, three in 39%, four in 8%, and five in 2%.8

The branches of posterior ciliary arteries include short posterior ciliary arteries and long posterior ciliary arteries. There are 10 to 20 short posterior ciliary arteries, depending on the intraorbital subdivisions of the posterior ciliary artery before it reaches the sclera. There are two long posterior ciliary arteries: one medial and one lateral.

The branches of posterior ciliary arteries run forward along the optic nerve, and each divides into multiple branches before reaching the eyeball. The branches of a posterior ciliary artery pierce the sclera lateral, medial, or, infrequently, superior to the optic nerve. Each of the posterior ciliary arteries break up into fan shaped lobules of capillaries that supply localised regions of the choroid. Each lobule is supplied by a terminal choroidal arteriole in the centre, and its venous drainage is by venous channels situated in the periphery of the lobules12,13 (Figure 2). Choroidal precapillary arteriole enters the choriocapillaris lobule at a right angle, while the postcapillary venule leaves in an oblique angle, an arrangement that is helpful for ICG angiographic interpretation. Although choriocapillaris is a continuous vascular system, ample clinical evidence suggests that, functionally, it acts like an end-arteriole system with no anastomoses between adjacent lobules.5 The various lobules are arranged like a mosaic. The shape and size of the various choriocapillaris lobules vary in different regions of the choroid: polygonal in the posterior part and elongated in the peripheral part. The density of the capillaries is greatest and the bore is widest at the macula.5 The

Figure 2: Diagrammatic representation of choriocapillaris. A: choroidal arteriole; V: choroidal vein (Reprinted with permission Hayreh SS. Segmental nature of the choroidal vasculature13)

choriocapillaris has fenestrated vascular walls with a relatively large luminal diameter.6 Hence, it represents a high-flow, non-tight junction capillary system that leaks fluorescein and ICG dye during angiography. The middle and outer choroidal vessels are not fenestrated. The large vessels, typical of small arteries elsewhere, possess an internal elastic lamina and smooth muscle cells in the media. As a result, small molecules such as fluorescein, which diffuse across the endothelium of the choriocapillaris, do not leak through medium and large choroidal vessels.5

The cells of the retinal pigment epithelium are taller and more heavily pigmented in the region of the macula, less fluorescence is transmitted from the underlying choriocapillaris in this area. Only if there is destruction of the choriocapillaris along with the pigment epithelium then some of the larger vessels in the choroid can be seen clearly during fluorescein angiography.

The corresponding venous lobules drain into the venules and the veins are much larger and converge to join four or five vortex veins that pierce the sclera to join the ophthalmic veins.

A watershed zone is the border between the territories of distribution of any two end arteries. Being an area of comparatively poor vascularity, the watershed zone is most vulnerable to ischaemia. There are watershed zones between the distribution of the various posterior ciliary arteries, between the short posterior ciliary arteries, and between the anterior and posterior ciliary arterial circulations.12

THE SENSORY RETINA

On ophthalmoscopic examination, the retina is seen as a purplish-red colour in living subjects. Fundus is grossly divided into central posterior pole and peripheral fundus (Figures 3A and B). The peripheral retina is further subdivided into the midperipheral (posterior to the equator) and peripheral (anterior to the equator) retina. The optic disc, a circular to oval area measuring about 1.5 mm across horizontally and nasal to the macula is located in the posterior pole of the retina. The optic disc is the site of confluence of the retinal nerve fibre layer (NFL) as it exits the globe. The macula lutea is an oval, yellowish area, measures about 5 mm in diameter and lies about 3 mm to the temporal margin of the optic disc. The yellow

Figure 3A: A fundus photograph of the macular area delineates the (a) foveola (diameter = 0.35 mm), (b) fovea (diameter=1.85 mm), (c) parafovea (a 0.5-mm ring zone), and

(d) perifovea (a 1.5 mm ring zone)

Figure 3B: Schematic representation of ocular fundus periphery. MPR (mid-peripheral retina): located between the temporal vascular arcade and equator. PR (peripheral retina): anterior to the equator

coloration of the macula lutea is caused by deposition of a yellow carotenoid pigment, the xanthophylls consisting of lutein and zeaxanthin which have significant antioxidant properties. This pigment also serves to absorb the harmful short wavelength light and protects the fovea.1-6

The retina consists of an outer pigmented layer and an inner neurosensory layer. The sensory retina is a thin, transparent tissue. Its thickness varies from 0.56 mm near the optic disc to 0.1 mm at the ora serrata. It is thinnest at the centre of the fovea.1,2,15, 16

The internal surface of the retina is in contact with the vitreous body and its external surface is adjacent to the retinal pigment epithelium (RPE) between which is a potential space (the subretinal space). The neurosensory layer of the retina is firmly attached to the RPE only at two points: posteriorly, at the optic disc, and anteriorly, at the ora serrata. Elsewhere, the attachment to the underlying RPE is weak and is maintained by the intraocular pressure, the contact between the photoreceptor outer segments and the RPE villi, the mucopolysaccharide-cementing substance surrounding the photoreceptors, and the active transport from internal to external. The internal surface of the retina is adjacent to the vitreous at the inner limiting membrane.

HISTOLOGY

The sensory retina is composed of 9 layers (Figure 4). The retinal layers are connected to each other by synaptic connections between axons and dendrites in the inner and outer plexiform layers and to the ganglion cells. The neuronal cells are supported by fibres of Müller cells and astrocytes in the inner portion of the retina. These layers from outside inward are: The retinal pigment epithelium (RPE), rods and cones, external limiting membrane, outer nuclear layer, outer plexiform layer, inner nuclear layer, inner plexiform layer, ganglion cells, nerve fibre layer and internal limiting membrane.

THE MACULAR REGION

The area defined by anatomists as the macula is that portion of the posterior retina containing xanthophyllic (yellow) pigment and two or more layers of ganglion cells. It is 5-6 mm in diameter and it is centered between the temporal vascular arcades. The macula is subdivided into the foveola, fovea, parafovea, and perifovea areas (Figure 3). The foveola is a highly specialised region of the retina different from central and peripheral retina.1,2,15,16 Here the outer layers of the retina are displaced concentrically leaving only a thin sheet of retina consisting of the internal limiting membrane and the cone cells. The obliquely oriented axons with accompanying Müller cell processes form a pale-staining fibrous-looking area known as the Henle fibre layer. This explains the petaloid pattern