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

Figure 4: Organisation of layers of the retina NFL–Nerve fibre layer, G–Ganglion cell layer, IPL–Inner plexiform layer, INL–Inner nuclear layer, OPL–Outer plexiform layer, PCB-Photoreceptor cell bodies, POS–Photoreceptor outer segments, RPE–Retinal pigment epithelium, ILM–Inner limiting membrane, R–Rods, C–Cones, OLM–Outer limiting membrane, H–Horizontal cells, B–Bipolar cells, Am–Amacrine cells, M– Müller cells, CH–Choroid, RE–Retina, PP–Parsplana, L–Lens, CO–Cornea, IR–Iris, PU–Pupil, and CB–Ciliary body

wavelengths of light. They are used for vision under dark-dim conditions at night. Cones contain cone opsins as their visual pigments and, depending on the exact structure of the opsin molecule, they are maximally sensitive to either long wavelengths of light (red light), medium wavelengths of light (green light) or short wavelengths of light (blue light). Central retina close to the fovea is considerably thicker than peripheral retina. This is due to the increased packing density of photoreceptors and bipolar cells. A remarkable difference is also seen in the number of ganglion cells. The greater number of ganglion cells results in more synaptic interaction and in a thicker inner plexiform layer and greater number of nerve fibres coursing to the optic nerve in the nerve fibre layer. In central retina, the photoreceptors have oblique axons. These oblique axons with accompanying Müller cell processes form the Henle fibre layer. The latter layer is absent in peripheral retina.

Cystoid Macular Edema (CME) and

Central Serous Chorioretinopathy (CSCR)

Leaking perifoveal capillaries lead to the formation of CME. Fluid collects in the loosely arranged outer plexiform layer. The outer plexiform layer of Henle is oriented obliquely in the fovea, in contradistinction to its perpendicular orientation in the perifovea which explains the petaloid pattern of cystoid macular oedema in fluorescein angiography. CSCR occurs when a small focal area of the retinal pigment epithelium becomes compromised and allows serous fluid, from the choroidal vessels to leak underneath the retina, forming a sub-retinal fluid collection.

of cystoid macular oedema on fluorscein angiography. The xanthophyllic pigment in the macula contributes to hypofluorescence as seen on the fluorescein angiography. In the centre of the macula is the foveal avascular zone (FAZ), a small, slightly concave area devoid of retinal capillaries and occupied by cones. On fluorescein angiography the FAZ is an important landmark because its geometric centre usually corresponds to the centre of the macula.

Central Versus Peripheral Retina

Central retina is cone-dominated, whereas peripheral retina is rod-dominated.1,2,15,16 The rods contain the visual pigment—rhodopsin and are sensitive to bluegreen light with peak sensitivity around 500 nm

VASCULARISATION

The retinal circulation extends into the retina as far as the inner portion of the inner nuclear layer. Consequently, the inner portion of the retina derives its nutrition from the retinal arterial system, and the outer layers, including the outer portion of the inner nuclear layer derive their nutrition from the choriocapillaris of the choroid.

As it enters the retina, the central retinal artery divides into four main branches: the upper and lower nasal and the temporal arteries. These arteries divide into smaller arterioles and the circulation further includes precapillary arterioles, capillaries, postcapillary venules, and veins. The capillaries drain

into venules that form a pattern similar to the arteries, joining larger venules distributed parallel to the main arteriolar branches and finally the central retinal vein at the optic disc. The diameter of the vein is about onethird to one fourth larger than that of the corresponding artery. The pattern of the veins, although similar, is not identical to that of the arteries.2 It should be noted that the arteries tend to lie superficial to the veins and thus cross superficial to the veins.

The retinal blood vessels are lined by an endothelium with tight cellular junctions, the site of the inner retinal blood barrier.

THE RETINAL PIGMENT EPITHELIUM

The Retinal pigment epithelium (RPE) is a monolayer of cuboidal shaped cells of neuroectodermal origin. It extends from the margin of the optic disc to the ora serrata, and it is continuous with the pigment epithelium of the ciliary body.

The apical portion of the RPE lies adjacent and is intimately related to the photoreceptor cell layer. Each cell has an optical portion with villous processes that envelop the outer segment of both rods and cones.17 The basal portion is attached to Bruch’s membrane, the innermost layer of which is formed by the basement membrane of the RPE.

In the posterior part, the pigment epithelial cells are low cuboidal cells approximately 16 μm in diameter, fairly uniform in size and shape but denser in the posterior region. The lateral surfaces of adjacent cells are closely apposed and are joined by tight junctional complexes (zonulae occludentes) near the apices.5 These junctional complexes form the outer retinal blood barrier. The cells contain melanin pigment within lancet-shaped or spherical granules.

The RPE has numerous functions including:4,5

1.Visual pigment regeneration

2.Maintaining adhesion of the neurosensory retina: this maintenance is achieved by passive hydrostatic forces, interdigitation of photoreceptors outer segments and RPE microvilli, active transport of subretinal fluid, and the complex structure and binding properties of the interphotoreceptor

matrix.

3.Phagocytosis and degradation of outer segments of rods and cones.

4.Selective permeable barrier action between the choroid and neurosensory retina. The barrier function of the RPE blocks the passage of water and ions and limits diffusion of large toxic molecules from the choriocapillaris to the photoreceptors of the neural retina. A break in the integrity of this barrier results in intraretinal and sub-RPE fluid accumulation.

5.Storage of vitamin A and its conversion to a form that can be utilised by photoreceptors for synthesis of rhodopsin.

6.Production of glycosaminoglycans that envelope the photoreceptors.

7.Absorption of scattered light, hence improving image resolution.

8.The RPE is particularly rich in microperoxisomes, active in detoxifying the large number of free radicals and oxidized lipids that are generated in highly oxidative and light-rich environment.

The RPE has low regenerative capacity in the normal eye, so cells loss in disease process is accommodated by hyperplasia of adjacent cells.4

The RPE cells and pericytes in the retinal vessels can synthesize different cytokines including TGF-beta. RPE can be induced to express class II MHC molecules and thus may also interact with T cells. Both T and B cells are absent from the normal retina and choroid. However, in several forms of uveitis, T cells, B cells, macrophages and polymorphonuclear cells infiltrate the retina and choroid.

The functional integrity of the RPE can be assessed by several clinical techniques, including fluorescein angiography to evaluate the RPE component of the outer blood-retinal barrier and electrophysiologic testing. Because the RPE is related so intimately to the photoreceptor layer and the choriocapillaris, diseases involving one layer frequently affect the others.

THE OPTIC DISC

The optic disc corresponds to the zone in the back of the eye where the optic nerve, consisting of a bundle of about one million nerve fibres, enters the globe. It

is known as the “blind spot” because there are no rods or cones in this location. The lamina cribrosa is a connective tissue “sieve” consisting of a fibrocollagenous weave of holes that “bundles” the million axons as they cross into the retrobulbar optic nerve.

The retrolaminar nerve (behind the lamina cribrosa) is where the axons are myelinated by oligodendrocytes. Occasionally myelination occurs in the prelaminar disc or retina.

The prelaminar disc is supplied with blood from the choroidal vessels, which are branches of the ciliary artery, and the very superficial vessels from the Central Retinal Artery. Lamina cribrosa is supplied by the short ciliary arteries. Posteriorly, there are recurrent branches from the ophthalmic artery and pial vessels.

On ophthalmoscopy, the optic disc is about 1.5 mm in diameter and has a pink neuroretinal rim and a central white depression called the physiologic cup. The central retinal artery and vein pass through the disc and into the optic nerve. A cilioretinal artery exits the disc temporally in 30% of eyes.

THE VITREOUS

The vitreous is a transparent structure which occupies a volume of about 4.5 ml. It is surrounded and adherent to the retina, pars plana and lens of the eye. A number of anatomical regions have been defined including the central vitreous, the basal vitreous, the vitreous cortex, the vitreoretinal interface, and zonule. The vitreous base gains attachment to the epithelium of the pars plana over a band extending forward from the ora serrata.

Vitreous humour has mainly the following composition: water (99%), a network of collagen fibrils, large molecules of hyaluronic acid and peripheral cells (hyalocytes).

The vitreous is normally acellular except in the vitreous cortex and the basal vitreous. Both of these structures contain a low concentration of hyalocytes. Other cells which morphologically resemble macrophages and fibroblast-like cells have been observed within the basal vitreous.18

The gel structure is maintained by a dilute network of thin unbranched collagen fibrils, comprising collagen types II, V/XI and IX. The spaces between

these collagen fibrils are filled by the glycosaminoglycan (GAG) hyaluronan.19

Vitreous gel serves as a reservoir for accumulation of antigens, protein substances and inflammatory mediators and also serves as a substrate for leukocyte cell adhesion. It contains type II collagen that could act as an autoantigen in some forms of arthritis related uveitis.

THE BLOOD-OCULAR BARRIERS

The blood-ocular barriers system is formed by two main barriers: the blood-aqueous barrier and the blood-retinal barrier (Figures 5A and B). The boodaqueous barrier is created by endothelium of vessels of the iris and non-pigmented ciliary body epithelium. The blood-retinal barrier may be sudivided into inner blood-retinal barrier, created by endothelium of retinal vessels, and outer blood-retinal barrier, created by retinal pigment epithelium cells. Blood-ocular barriers are a physical barrier between the local blood vessels

Figure 5A: The blood-ocular barriers are constituted anteriorly by the blood-aqueous barrier and posteriorly by the blood-retinal barriers. The blood-aqueous barrier (top of the Figure 5A) is located at the level of the iris vessel, endothelial cells which have very tight junctions between them and at the level of the ciliary body nonpigmented epithelium (Figure 5B). Posteriorly the blood retinal barrier is located at the level of the retinal vessel endothelial cells that have very tight junctions as well as tight junctions between RPE cells (Courtesy Carl P Herbort, Lausanne, Switzerland, after GOH Naumann)

Figure 5B: Blood-aqueous barrier at the level of the ciliary body. Tight junctions between nonpigmented ciliary epithelial cells are marked in red. PC–Pigmented epithelial cell; NPC– Nonpigmented epithelial cell; MG–Melannine granule (Courtesy: Prof Krstic, Lausanne, Switzerland)

and most parts of the eye itself, stopping many substances from travelling across it.1,2,3

Breakdown of blood-ocular barriers may occur due to inflammation or other noninflammatory mechanisms, resulting in influx of proteins and cells into the anterior chamber or vitreous. Furthermore, breakdown of blood-retinal barrier may result in extravasation and accumulation of fluid in the chorioretinal tissue or space.

Assessment of blood-ocular barrier breakdown and its consequences can be made by clinical examination, photography, fluorescein angiography, indocyanine green angiography, optical coherence tomography, ultrasonography, etc.

KEY POINTS

UVEA

1.The uveal tract is the main vascular compartment of the eye necessary for vital functions such as nutritional support, thermoregulation and control of intraocular pressure.

2.Iris capillaries have nonfenestrated endothelial cells with tight junctions, making them less permeable than normal somatic vessels constituting the blood-aqueous barrier.

3.The choriocapillaris shows a lobular organisation of wide lumen capillaries, supplying an independent segment of choriocapillaries and lying in a single plane. 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 lobules.

4.The choriocapillaris has fenestrated vascular walls with a relatively large luminal diameter. Hence, it represents a non-tight junction capillary system that leaks fluorescein and ICG dye during angiography. The middle and outer choroidal vessels are not fenestrated.

5.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.

THE RETINA

6.The retina consists of an outer pigmented layer and an inner neurosensory layer. 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).

7.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 macula is subdivided into the foveola, fovea, parafovea and perifovea areas.

8.In the centre of the macula is the foveal avascular zone (FAZ), a small, slightly concave area devoid of retinal capillaries and occupied by cones.

9.The cells of the RPE are taller and more heavily pigmented in the macular area, thus less fluorescence is transmitted from the underlying choriocapillaris in this area.

10.The inner portion of the retina except the foveola derives its nutrition from the retinal arterial system. The foveola and the outer layers of the retina derive their nutrition from the choriocapillaris.

THE VITREOUS

11.The vitreous is a transparent gel which occupies cavity of the posterior segment.

12.Vitreous humour has mainly the following composition: water (99%), three-dimensional network of collagen fibrils, large molecules of hyaluronic acid and peripheral cells (hyalocytes).

13.Vitreous gel serves as a reservoir for accumulation of antigens, protein substances and inflammatory mediators and also serves as a substrate for leukocyte cell adhesion.

THE BLOOD-OCULAR BARRIERS

14.Blood-aqueous barrier (= anterior blood-ocular barrier) situated at the level of tight-junction iris vessels and at the level of the nonpigmented ciliary body epithelium.

15.Blood-retinal barrier (= posterior blood-ocular barrier) situated at the level of tight-junction retinal vessels and the retinal pigment epithelium (RPE).

REFERENCES

1.Wolff E, Bron AJ, Tripathi RC. Wolff’s Anatomy of the Eye and Orbit Ed. 8, London: Chapman and Hall Medical 1997:308-487.

2.Snell RS. Clinical Anatomy of the Eye Ed. 2, Tokyo: Blackwell Science,1998:157-92.

3.Marieb EN, Hoehn K. Human Anatomy and Physiology (7th ed) San Francisco. Benjamin Cummings 2007.

4.Forrester J. Eye: Basic Sciences in Practice Ed. 2, Edinburgh: W B Saunders, 2002: 447.

5.Basic and Clinical Science Course/Vol: 2-Fundamentals of Ophthalmology: 2004-2005, San Francisco: AAO, 2004; 528.

6.Shiuey Y, Jakobiec FA, and Friedman E. Uveal physiology and circulatory abnormalities. In Albert, Daniel M, Jakobiec, Frederick A, (Eds): Principles and Practice of Ophthalmology/Vol: 2-Clinical Ophthalmology; Conjunctiva; Cornea; Sclera; Uveitis; Lens Ed. 2, Philadelphia: WB Saunders, 2000;1175-87.

7.Foster CS, Vitale AT. Diagnosis and treatment of uveitis. The Uvea: Anatomy, Histology, and Embryology. Ed. Philadelphia: W B Saunders, 2002:3-15.

8.Hayreh SS. Posterior ciliary artery circulation in health and disease. The Weisenfeld Lecture. Invest Ophthalmol Vis Sci 2004;45:749-57.

9.Zhang HR. Scanning electron-microscopic study of corrosion casts on retinal and choroidal angioarchitecture in man and animals. Prog Ret Eye Res 1994;13:243-70.

10.Risco JM, Grimson BS, Johnson PT. Angioarchitecture of the ciliary artery circulation of the posterior pole. Arch Ophthalmol 1981;99:864-68.

11.Olver JM. Functional anatomy of the choroidal circulation: methyl methacrylate casting of human choroid. Eye 1990;4:262-72.

12.Hayreh SS.In vivo choroidal circulation and its watershed zones. Eye 1990;4:273-89.

13.Hayreh SS. Segmental nature of the choroidal vasculature. Br J Ophthalmol 1975;59:631-48.

14.Yoneya S, Tso MO. Angioarchitecture of the human choroid. Arch Ophthalmol 1987;105:681-87.

15.Yamada E. Some structural features of the fovea centralis in the human retina. Arch Ophthalmol 1969;82:151-9.

16.Hendrickson AE, Youdelis C. The morphological development of the human fovea. Ophthalmology 1984; 91:603-12.

17.Hagerman GS, Johnson LV. The photoreceptor-retinal pigmented epithelium interface. In Heckenlively JR, Arden GB (Eds): Principles and Practice of Clinical Electrophysiology of Vision: Mosby Year Book, St. Louis, 1991;53-68.

18.Grabner G, Boltz G, Forster O. Macrophage-like properties of human hyalocytes. Invest Ophthalmol Vis Sci 1980;19:333-40.

19.Bishop PN. Structural macromolecules and supramolecular organisation of the vitreous gel. Prog Retin Eye Res 2000;19:323-44.

INTRODUCTION

Slit lamp photographs are visual records to document structural abnormalities and pathological processes. Although anterior segment photography is possible with suitably equipped hand-held 35-mm camera, a photo slit lamp, however, is preferred as it provides comfortable working distance, adjustable source of illumination and a wide range of magnifications. A photo slit lamp is essentially the same as the conventional slit lamp, with a few additions like a camera body (manual or digital), background illuminator and a light diffuser. The structures that can be photographed with the slit lamp camera include the eyelids, eyelashes, sclera, conjunctiva, cornea, tear film, anterior chamber, iris, lens, and anterior vitreous.

HISTORY

More than 1000 years ago, an Arab scientist observed an inverted image of the outside objects, when light rays shone through a small hole in a darkened room. That laid the principle for ‘camera obscura’ that was widely used by the Renaissance artists for improving their drawings.1 In 1826, Nicephore Niepce, a French inventor loaded the camera obscura with a bitumencovered pewter plate and obtained the first photograph of the view outside his window. The plates used in that era were coated with wet, light sensitive material that had to be exposed and processed before the coating dried. Further innovations helped in overcoming these hurdles and by 1888, the Kodak Model No. 1 was marketed.

In 1911, Alvar Gullstrand introduced the first rudimentary slit lamp illuminator. By 1916, Henker made a major advancement in external ophthalmic photography by combining Gullstrand’s illuminator and Czapski’s corneal microscope. In 1936, Comberg was able to establish the copivotal and isocentric relationship between the slit lamp illuminator and a microscope. In 1936, Goldmann produced first parfocal instrument in collaboration with Haag-Streit. The shift to Köhler illumination by Goldmann greatly improved the efficiency of the slit lamp. The slit lamp, in its different stages of development, represented a major advance in examining anterior segment structures. It led to the description of many of the signs classically taught nowadays such as cornea guttata, etc. one of the pioneers being Alfred Vogt from Zurich who published three volumes of his slit lamp atlas in the nineteen, twenties and thirties. Imaging of anterior segment pathology was, however, still performed by representing the slit-lamp findings in the form of drawings.

SLIT-LAMP PHOTOGRAPHIC BIOMICROSCOPE

The photo slit lamp has all the basic features of a slit lamp including the optical part, illumination systems, and the rotating arms upon which both the optical and illumination systems are mounted (Figure 1).

The optical part of the biomicroscope consists of the objective lens system and an eyepiece. The objective assembly possesses either individual lenses of varying powers (that are rotated into position) or a

Figure 1: Labeled photograph of the slit lamp biomicroscope for photography

zoom lens system. The image formed by the objective lens is projected to the eyepiece head. The eyepiece head comprises of a binocular fixture with 2 eyepiece lenses. Both the interpupillary distance of the binoculars and the individual focus of each eyepiece need to be adjusted for capturing a well-focused image.

The illumination system is the second major component of the biomicroscope that contains a system of condensers and aperture diaphragm, the modelling lamp. It projects highly collimated and focused rays onto the patient’s eye. The optical part and the illuminator are parfocal, i.e. simultaneously focused at the same point in space at all angles and magnifications.

Photographic biomicroscope, in addition, provides a general diffuse illumination from a second light source. This supplemental lighting may be a complete fixture with modelling lamp and a separate flash tube, or the lighting may come from the main flash tube via a fiberoptic cable. Photographic biomicroscopes also have a beam splitter that transmits a percentage (50% or 30%) of light to the viewing oculars and reflect the remainder of the light to the camera. Some photo biomicroscopes have the ability to rotate the beam splitter out of the viewing path to allow 100% of the reflected light to be seen by the examiner. This procedure helps to reveal all the subtle details of the patient’s eye without interruption of the light. The camera accurately records the details simultaneously. The images can be acquired on a manual or a digital system.

When producing slit lamp photographs, the sequence should begin with a general overview of the eye and then proceed with increasingly emphatic use of magnification and lighting effects. Various illumination techniques are applicable to a variety of anterior segment conditions, although not all of them are useful for every patient.2

SLIT LAMP PHOTOGRAPHIC MODES (TYPES OF ILLUMINATION)

1.Direct illumination

2.Indirect illumination

DIRECT ILLUMINATION

In case of opaque, crystalline, or opalescent lesion, a direct form of illumination delineates the areas of interest better. Direct illumination can further be done by using diffuse, fine slit beam or direct broad tangential beam illumination.

Diffuse Illumination

Diffuse illumination is achieved when the main light has a diffuser filter in front of the beam and the fill illuminator is on. The fill illuminator is directed opposite the main light illuminator with an aim to fill in the shadows. This photograph is taken in all cases to allow orientation of the eye being examined, and is typically taken at low magnification (10X and 16X) (Figure 2).

Figure 2: Anterior segment photograph of the eye under diffuse illumination giving a gross overview of cornea and anterior segment

Figure 3: Anterior segment photograph showing direct focal slit beam illumination focused in anterior chamber

Direct Fine Slit Beam Illumination

Fine slit beam illumination is created by reducing the beam width to a fine slit with no diffuser filter from the main light illuminator. The fill light is also employed to provide the photograph with the surrounding details of the slit’s position. The camera or observer oculars should be positioned at approximately 60° to the fine slit beam’s direction. This angle will allow more of the beam depth or optical section to be in focus to the observer and the photograph. Unlike diffuse illumination, concentrated focal light penetrates the transparent structures. This examination is used to show corneal pathologies in detail. It can also give topographic information in cases of iris lesions, corneal or conjunctival masses, cataract and fluid-filled cysts (Figure 3).

Direct Broad Tangential Beam Illumination

Broad tangential beam illumination is done by increasing the beam width and increasing the angle of incidence for the beam to spread across the cornea. This type of illumination is more effective without the fill illuminator. Broad tangential illumination helps to increase contrast and show varying degrees of texture in the ocular tissues. This lighting is helpful for showing slight corneal scarring, pseudoexfoliation of the lens, and small bullae in a bullous keratopathy (Figure 4).

INDIRECT ILLUMINATION

Direct light can overwhelm the subtle lesions. In case the lesion on the eye is transparent, refractile, almost invisible, indirect forms of illumination are more

Figure 4: Anterior segment photograph showing direct focal broad beam illumination focused on the lens surface showing retracting fibrin

successful at enhancing the fine details contained in subtle lesions. Indirect illumination includes:

Sclerotic Scatter

Sclerotic scatter is an indirect illumination that is created by decentering the slit beam and directing a broad beam to the temporal sclera. With the fill illuminator off, the bright light directed to the sclera transmits across the cornea like a fibre-optic light pipe. A gentle ‘glow’ from the distal sclera is seen when the illumination is properly aligned. The beam will always be severely overexposed in the photograph because of the broad beam reflecting off the white sclera. The soft indirect light shows very subtle corneal changes such as corneal scarring, vortex dystrophies, and crystalline keratopathy (Figure 5).

Figure 5: Anterior segment photograph showing sclerotic scatter demonstrating band shaped keratopathy

Figure 6: Anterior segment photograph using retroillumination from the iris demonstrating corneal changes

Figure 7: Anterior segment photograph using retroillumination from the retina demonstrating keratic precipitates

Retro-illumination

Retro-illumination is seen in profile, when a source of light illuminates an area of interest from behind. It can be further divided into one from the iris or from retina, which is used to outline the shape of a lesion in the patient’s cornea or lens.

a. Retro-illumination from iris

Retro-illumination from the iris is created by making a moderately thin slit beam and directing the beam to the iris at a 45-degree angle and keeping the plane of focus on the cornea. The soft reflected beam from the iris would enhance transparent subtle corneal irregularities (Figure 6).

b. Retro-illumination from retina

Retro-illumination from the retina is used to view the lesions of the iris and lens. The patient must be dilated for this retro-illumination because light has to pass through the pupil. This is achieved by first bringing the illuminator arm to a position adjacent to the microscope, creating a semicoaxial illumination and then decentring it. The observer must then look for a bright orange reflex that appears when the light is just axial enough to reflect off the patient’s retina and glows from behind the lens and cornea. However, the retina is not uniformly reflective. The brightest area of the retina is the optic nerve head, and the brightest reflection (or red reflex) is achieved when the illuminating beam strikes the nerve. To achieve this, slit lamp illuminator has to be positioned to the temporal side of the biomicroscope. This lighting

situation enhances the observation of cataracts, corneal scars, transparent cysts, etc. (Figure 7).

Iris Transillumination

Iris transillumination requires an undilated pupil and is created by making an axial light beam shine into the small pupil and reflect off the retina. The beam aperture should match the pupil size or be smaller than the pupil to avoid iris reflections. Essentially the technique is also retro-illumination, light being however, transmitted through structures that usually are opaque to the transmission of light such as the iris. The beam direction is critical for proper alignment of the light beam. This lighting situation is used to illustrate the degree of iris thinning either due to ocular albinism or pigmentary glaucoma (Figure 8) or iris

Figure 8: Anterior segment photograph showing iris transillumination

defects including iris holes, or atrophy and iridotomies, etc.

Photography with Three-Mirror Contact Lens or 90 Diopter Lens

The posterior pole and the anterior chamber angle can be documented with the three-mirror contact lens. The slit lamp illuminator is kept in almost coaxial position. If the slit beam is too wide, disturbing light reflections may occur. Diagnostic contact lenses are not used in immediate post-intraocular surgery eyes. In such cases the use of 90-diopter lens or any other lens used to view the posterior pole with the slit lamp are useful tools that give good results with a moderate slit beam in the almost coaxial position. The photographer holds

Figure 9: Fundus photograph with a 90 D lens demonstrating optic disc

Figure 11: Slit lamp photograph with a 90-diopter lens showing vitreous snow balls

the lens in his right hand and left hand is used to rotate the joystick of slit lamp in order to focus the image. The image is clicked with the help of footswitch.

These instruments induce lot of reflexes and thus it is better to take these images without the background illumination. The lenses should have a clean surface as any scratches or damaged surfaces would increase the artifacts (Figures 9-11)

INDICATIONS IN UVEITIS

CONJUNCTIVA

Conjunctival hyperemia is a common sign of anterior inflammation. It represents ciliary body inflammation that is uniform in the perilimbal region (Figure 12).

Figure 10: Photograph of the inferior angle with three-mirror contact lens showing angle neovascularisation

Figure 12: Anterior segment photograph of the left eye using diffuse illumination showing perilimbal congestion