Looking at the Eye

Most of an eye examination happens through a microscope or a lens: the slit lamp, the lenses that let it see the fundus and the angle, the cameras that record what it sees, and the microscope in the operating room. Each solves an optical problem: how to light a transparent tissue so that it shows, how to see a retina whose image lies at infinity, and how to look round a corner that light cannot turn.

#The slit lamp

The slit lamp, or biomicroscope, has two halves that swing independently about a common vertical axis, both focused on the same point:

• The illumination system projects an image of an adjustable slit onto the eye. It uses Köhler illumination: the condenser images the lamp filament into the projection lens, and the projection lens images the slit onto the eye, so the beam is evenly lit with no image of the filament in it. Filters give cobalt blue light for fluorescein, red-free (green) light for vessels and the nerve fibre layer, and heat-absorbing and neutral density filters for comfort.
• The observation system is a binocular compound microscope: an objective, a Galilean magnification changer (small Galilean telescopes on a drum, each usable either way round), prisms to erect the image, and eyepieces. Magnifications run from about 6× to 40×; the two eyepieces see from slightly different angles, giving a stereoscopic view.

Because the two halves share a focal point and pivot about it, the slit stays focused wherever the microscope is focused, whatever angle the beam comes from. That is what lets the examiner cut an optic section through the cornea and lens and judge depth. For some techniques the beam is deliberately decentred, uncoupled from the point the microscope is looking at.

#Ways of lighting the eye

The cornea and lens are transparent, so they show only where light is scattered. Each way of using the beam reveals different things:

• Diffuse illumination: a broad, defocused beam for an overview of lids, conjunctiva and cornea.
• Direct focal illumination: the slit focused where the microscope is focused. A narrow slit at an angle gives an optic section, a slice through the cornea or lens that shows the depth of a scar, a foreign body or a cataract; a wider slit shows a block (a parallelepiped) of tissue. A short, narrow conical beam in a dark room shows cells and flare in the anterior chamber (flare, the beam made visible by protein, is the Tyndall effect, Chapter 19).
• Indirect illumination: the decentred beam falls beside the area of interest, which is lit by light scattered within the tissue: corneal microcysts, the iris sphincter.
• Retroillumination: the structure is seen against light reflected from behind it, from the iris or from the fundus (the red reflex). Cortical and posterior subcapsular cataract and corneal opacities stand out dark against the glow; iris transillumination defects light up where light shines through gaps in the pigment epithelium.
• Specular reflection: with the microscope placed at the mirror angle, so that the angle of observation equals the angle of illumination, the bright reflection from a smooth interface shows its surface. At high magnification the reflection from the back of the cornea shows the endothelial mosaic, and guttae appear as dark spots; the reflex is seen through one eyepiece only.
• Sclerotic scatter: the beam is aimed at the limbus. Light travels across the cornea by total internal reflection, making the limbus glow all round, and any opacity in the clear cornea, such as central oedema or a faint scar, scatters it and lights up.

The beam also estimates the depth of the angle. In the Van Herick method a narrow beam at 60° to the line of sight falls on the temporal limbus, and the depth of the peripheral chamber is compared with the thickness of the cornea beside it; a gap of a quarter of the corneal thickness or less suggests a narrow angle. The test screens; it does not replace gonioscopy.

#Seeing the fundus at the slit lamp

The slit lamp’s microscope focuses about 10 cm in front of its objective, but the eye forms the image of an emmetropic retina at infinity, out of the microscope’s reach. An auxiliary lens brings the image of the fundus within range, in one of three ways:

• Neutralise the eye’s power with a minus lens. The Hruby lens, about −58.6 D (the power of Gullstrand’s schematic eye), held just in front of the cornea, neutralises the eye’s power and forms a virtual, upright image of the fundus inside the eye, which the microscope focuses on. The field is small, and the lens is now little used.
• Neutralise the cornea with a contact lens. A fundus contact lens, coupled to the cornea with a viscous gel, replaces the curved front of the cornea with its own flat front surface. The view is virtual and upright, steady and with few reflections, and mirrors in the lens show the periphery. In the Goldmann three-mirror lens the central lens shows the posterior pole, the 73° mirror the mid-periphery, the 67° mirror the periphery out to the ora serrata, and the small 59° mirror the extreme periphery and the angle.
• Use the eye’s power with a strong plus lens, as in indirect ophthalmoscopy (Chapter 9). A +60, +78 or +90 D lens held in front of the eye forms a real, inverted aerial image of the fundus near its own focal point, between the lens and the microscope. The image is magnified about 60/P times, so a +90 D lens gives about ×0.67 and a +60 D lens about ×1, before the slit lamp’s own magnification. Weaker lenses magnify more, and so are preferred for the disc and macula; stronger lenses of the same diameter give a wider field.

Lenses that use the same principle with a contact surface, such as wide-field laser lenses, also give inverted images, and they change the size of the laser spot on the retina (Chapter 20).

#Seeing the angle

The angle of the anterior chamber cannot be seen through a normal cornea. Light leaving it toward the examiner meets the front of the cornea beyond the critical angle, about 46° for the cornea against air, and is totally internally reflected back into the eye (Chapter 2).

A goniolens eliminates the cornea’s interface with air. Its back surface is placed on the cornea with tears or a coupling gel between them, whose refractive indices are close to the cornea’s, so the light passes into the lens without being reflected. It then leaves the lens in one of two ways:

• Indirect gonioscopy: a mirror inside the lens reflects the light out through a flat front face. Each mirror shows the angle opposite it, reversed across the mirror but not along it: in the 12 and 6 o’clock mirrors the image is upside down but not reversed left to right; in the 3 and 9 o’clock mirrors it is reversed left to right but not upside down. (The fundus mirrors of a three-mirror lens follow the same rule.) The Goldmann lens has a large contact surface and needs a coupling gel; the Zeiss, Posner and Sussman four-mirror lenses have a small contact area, use the patient’s tears, and can be pressed on the cornea: indentation gonioscopy pushes aqueous into the angle and opens a closure caused by apposition of the iris, but not one caused by synechiae. Pressing unintentionally can open a narrow angle falsely; folds in Descemet’s membrane are the warning. To see over a convex iris, the patient looks toward the mirror. The corneal light wedge, where the beams reflected from the front and back of the cornea meet, marks Schwalbe’s line.
• Direct gonioscopy: a dome-shaped lens such as the Koeppe lens lets the light leave nearly perpendicular to its curved front surface, giving a direct, upright view, with the patient lying down. It is used in children under anaesthesia, and surgical goniolenses allow goniotomy.

Gonioscopy is done in dim light with a short, narrow beam kept off the pupil: light constricts the pupil, which pulls the peripheral iris away from the angle and can make a narrow angle look open.

#Photographing the fundus

A fundus camera is an indirect ophthalmoscope with a camera. To keep reflections from the cornea and lens out of the picture, it illuminates the fundus through a ring of light focused at the pupil and views it through the dark centre of the ring, so the illuminating and returning light pass through different parts of the cornea and lens (Gullstrand’s principle). It therefore needs a reasonably large pupil. Standard cameras capture 30° to 50° of fundus, measured at the pupil; mosaics of several fields cover more. Filters give red-free photographs, which show vessels, haemorrhages and the nerve fibre layer in high contrast, and fluorescein and indocyanine green angiography (Chapter 19). Two photographs taken through different parts of the pupil give a stereoscopic view of the optic disc.

A scanning laser ophthalmoscope builds the image point by point: a laser beam is scanned across the retina, and the reflected light reaches the detector through a confocal pinhole conjugate with the plane in focus. The narrow beam lets it image through a small pupil, and the pinhole rejects light scattered from other depths, so the image keeps its contrast even through a hazy lens. Scanning laser ophthalmoscopes take infrared reflectance and autofluorescence images and angiograms, and track eye movements for OCT. The Optos ultra-widefield scanning laser ophthalmoscope uses an ellipsoidal mirror with two focal points: the scanning laser at one and the patient’s pupil at the other, so that the beam enters the eye from a very wide range of angles. It captures about 200° measured from the centre of the eye (roughly 130° to 140° by the pupil-centred convention of fundus cameras), about 80% of the retina, in one image. Its standard colour pictures combine red and green laser images, so they are pseudocolour, and the lashes and lids intrude at the edges; other widefield systems take true-colour images.

#The operating microscope

The operating microscope is a large stereoscopic microscope with coaxial illumination: light is sent down along the viewing path, so it returns from the fundus through the pupil as a bright red reflex, which outlines the capsule and lens during cataract surgery. The object lies at the focal point of the objective, so the light from it leaves the objective parallel, and the working distance is the objective’s focal length, typically 150 to 250 mm. A magnification changer (stepped or a motorised zoom) and a tube lens follow, and the eyepieces magnify the image formed by the tube lens:

A longer objective gives more room to work but less magnification. More magnification shrinks the field of view and the depth of focus.

The microscope’s bright light, focused on the macula by the patient’s own optics, can injure the retina photochemically, mainly through its blue light and without heating it: operating microscope phototoxicity is a risk in long operations, and greater once an IOL is in place and the light is sharply focused. It is reduced by using the lowest light that works, filtering blue and ultraviolet light, tilting the light away from the fovea or shielding the cornea when the red reflex is not needed, and keeping operations short. For vitreoretinal surgery, wide-angle viewing systems place a strong plus lens over the eye, as in indirect ophthalmoscopy; the image is inverted, so an inverter in the microscope turns it the right way up. Endoilluminators cause the same photochemical injury when held close to the macula, so the probe is kept back and its intensity low.

#Recap

• Slit lamp: Köhler illumination (filament imaged in the projection lens, slit on the eye), Galilean magnification changer, both halves focused on a common pivot.
• Optic section for depth; conical beam for cells and flare; retroillumination for cataract and transillumination defects; specular reflection (observation angle = illumination angle) for the endothelium; sclerotic scatter (total internal reflection) for faint corneal opacities.
• Fundus at the slit lamp: Hruby (−58.6 D) and contact lenses give upright virtual images; +60/+78/+90 D lenses give inverted real images, magnified about 60/P.
• The angle is hidden by total internal reflection (critical angle about 46°). Goniolenses remove the air interface; a mirror shows the opposite angle, reversed across the mirror (upside down in the 12 and 6 o’clock mirrors). Four-mirror lenses allow indentation.
• Fundus camera: ring illumination, viewing through its centre. Scanning laser ophthalmoscope: confocal pinhole for contrast. Optos ultra-widefield: ellipsoidal mirror, pseudocolour.
• Operating microscope: working distance = objective focal length; M = (tube f / objective f) × changer × eyepiece. Beware phototoxicity, which is photochemical.

#Test yourself

#Further reading

• American Academy of Ophthalmology, Basic and Clinical Science Course, Section 3: Clinical Optics and Vision Rehabilitation, chapter on optical instruments.
• D. Henson, Optometric Instrumentation, 2nd ed. Butterworth-Heinemann (1996).
• American Academy of Ophthalmology, Basic and Clinical Science Course, Section 10: Glaucoma, on gonioscopy.