Anterior Eye Examination

The slit lamp

The illumination system is called the slit lamp – so called because of its capacity to project a slit of light onto the ocular surface. The light source and optical elements of the slit lamp are classically contained in a vertically oriented housing ( Fig. 1.4 ). A bright light source (generating approximately 600,000 lux) is a fundamental requirement for a slit lamp if subtle conditions are to be seen clearly. Although halogen or xenon lamps are more expensive than tungsten lamps, they are the preferred illumination source as they provide a brighter light, last longer, have better colour rendering and generate less heat. The light is focused vertically into a slit configuration. It then reflects off a mirror mounted at 45° and is projected onto the eye.

Illumination brightness is controlled by a rheostat or multiposition switch such that brightness can be adjusted to obtain the correct balance between patient comfort and optimal visibility of the area of interest. Generally, the broader the slit, the brighter the light, the greater the patient discomfort and the lower the illumination setting must be.

The optical and aperture-masking components within the illumination system are designed such that the emergent slit of light has sharp edges and an even spread of illumination. The slit width and height are continuously variable so that a section of light of any shape can be projected. The ability to vary the slit width has other practical applications, such as forming a reference for estimating the size of features of interest. Furthermore, the slit can be rotated so that, for example, a horizontal rather than a vertical slit can be projected onto the eye. This facility can also be useful for measuring the degree of rotation of soft toric lenses.

A number of filters can be incorporated into the illumination system, and the filters serve to enhance the visibility of certain conditions:

• Green (‘red-free’) filter – enhances contrast when looking for corneal and iris neovascularization, since red vessels appear black if viewed through such a filter. A green filter may be used to increase the visibility of rose bengal staining on both the cornea and conjunctiva.

• Neutral density (ND) filter – reduces beam brightness and increases comfort for the patient.

• Polarising filter – reduces unwanted specular reflections and can be useful for enhancing the visibility of subtle defects.

• Diffusing filter – diffuses the illumination source over a wide area and is used to provide broad, unfocused illumination for low-magnification viewing of the general ocular surface.

• Cobalt blue filter – provides a suitable means of exciting sodium fluorescein for examination of ocular surface integrity. Illumination of fluorescein with cobalt blue light of 460 to 490 nm produces a greenish light of maximum emission 520 nm. Any abraded area will absorb fluorescein and display a fluorescent green area against a general blue background. The filter is occasionally used on its own to aid in the diagnosis of keratoconus. A frequent finding in this corneal ectasia is a Fleischer ring, which is formed by an annular iron deposition within the stroma at the base of the cone. The iron pigment is often difficult to see in white light but will usually appear in greater contrast when viewed through the cobalt blue filter.

• Yellow (Kodak Wratten #12) filter – this is not a filter contained within the illumination system but is used as a supplementary barrier filter that is placed in front of the viewing system. It significantly enhances the contrast of any fluorescent staining observed with the cobalt blue filter as it allows transmission of the green, fluorescent light but blocks the blue light reflected from the corneal surface. Custom-made barrier filters for certain slit lamps are available from some manufacturers. Inexpensive hand-held versions may be constructed by using a cardboard mask and Lee filters #101 Yellow.

The biomicroscope

A biomicroscope of high optical quality is essential if the observer is to achieve a comfortable, clear, focused binocular image of the eye ( Fig. 1.5 ). The optical system contains an objective, typically with × 3 to × 3.5 magnification, and an eyepiece with variable or interchangeable power. The normal range of total magnification is from × 6 to × 40. In some systems, magnification is continuously variable throughout this range. These systems have two key advantages: (1) There is an uninterrupted view of the eye while the level of magnification is changed, and (2) the observer is not constrained to using discrete levels of magnification and can, in effect, choose any level of magnification within the available range. However, such systems usually require additional optical elements to achieve the ‘zoom’ function, and this may slightly compromise the optical quality of the image.

For the purposes of discussion throughout this book, the level of magnification being used can be broadly classified as follows:

• low: less than × 10 magnification

• medium: × 10 to × 25 magnification

• high: greater than × 25 magnification.

In most systems, magnification is changed in steps, with the typical progression being × 6, × 10, × 16, × 25 and × 40; these systems generally afford high optical quality, but there is the disadvantage of momentarily losing sight of the eye while the magnification is being changed. Some systems require the eyepieces to be interchanged to obtain different levels of magnification. Needless to say, these systems are cumbersome and mitigate against a smooth examination procedure. Manufacturers of slit lamp biomicroscopes could produce instruments with higher levels of magnification than × 40, but natural micronystagmoid eye movements make observation at such high magnification levels impractical.

The working distance of the biomicroscope (the distance from the eye to the front surface of the most anterior lens element of the biomicroscope) is typically set at about 10 cm, which is long enough to allow room for manipulating the eye, but not too long so as to require an uncomfortable arm position during such manipulations.

Diffuse illumination

A ground glass filter is placed in the focused light beam of the slit lamp. This will defocus and diffuse the light to give a broad, even illumination over the entire field of view. The angle of the illumination arm is not critical when the diffuser is in place and can be anywhere from 10° to 70° in relation to the observation arm; it is simply convenient to place it at an angle of at least 45° so as to avoid partially obstructing the field of view. The slit is generally opened wide, and high illumination will not cause too much patient discomfort in view of the diffuse nature of the light ( Fig. 1.7 ).

Diffuse illumination is generally used to provide low-magnification views of the opaque tissues of the anterior segment (AS), including the bulbar conjunctiva, sclera, iris, eyelid margins and the tarsal conjunctiva of the everted lids. Unusual signs in these tissues could include dilated blood vessels in the bulbar conjunctiva, pigmented areas in the conjunctiva or eyelids, roughness or opacity of the conjunctiva and abnormal eyelash position or orientation. Such signs could be indicative of some conditions, such as trichiasis, bulbar redness, pterygium or papillary conjunctivitis. In assessing the eyelid margins, the apposition of the lids and puncta against the globe should be considered. Additionally, one should look for clear glands near the base of the lashes and flaking or scaling of the eyelid skin. These may indicate the presence of ectropion, blepharitis or epiphora.

Focal illumination – parallelepiped

This describes any illumination technique where the slit beam and viewing system are focused coincidentally. The illumination is turned up to a reasonably high level of brightness (ensuring that the patient remains comfortable) and the slit beam is placed at a separation of 40° to 60° on the side of the microscope corresponding to the section of the cornea to be viewed. The beam is swept smoothly across the ocular surface and the illumination system moved across to the opposite side as the beam crosses the mid-point of the cornea. Typically, a beam width of 0.1 mm to 0.5 mm is chosen initially and this may be reduced so as to bring more contrast (as a result of less light scatter) to the area of interest. The term ‘parallelepiped’ refers to the geometric shape of the illuminated optical section of the cornea under examination.

A slit width that is wider than 0.5 mm creates a condition known as ‘broad–beam’ illumination, whereby the width of the beam is greater than the depth of the cornea (effectively creating a parallelepiped, which is turned on its side). Whilst scanning the external ocular surface, a low-to-medium magnification is initially chosen, and the magnification is increased if any area of interest needs to be examined more closely.

Direct

The section of the cornea within the illuminated beam is being observed ( Fig. 1.8 ). This permits assessment of the location, width and height of any object within the cornea or adjacent structures. The parallelepiped is the most commonly used direct illumination technique and is employed, for example, to assess corneal scarring, infiltrates and corneal staining.

Focal illumination – optic section

This condition is identical to ‘focal illumination – parallelepiped’, except that a very thin beam of approximately 0.02 to 0.1 mm is used to essentially create a ‘cross-section’ of the corneal tissue. The illumination beam is placed at a separation of 40° to 60° on the side of the microscope corresponding to the section of the cornea to be viewed. Increasing the angle of the illumination arm increases the depth of the optic section in the cornea, but the same amount of light is spread over a greater depth of cornea, and this reduces brightness and contrast and makes the deeper corneal layers, in particular, more difficult to visualise. Because the light beam is so thin, the illumination must be turned up to maximum brightness.

Direct

The section of the cornea within the illuminated beam is being observed ( Fig. 1.9 ). This provides the ability to accurately assess the depth of an object within the corneal layers. Typical uses include assessing the depth of a foreign body, locating a corneal scar and determining whether tissue within an area of staining is excavated, flat or raised.

Retroillumination

This refers to any technique in which light is reflected from the iris, anterior lens surface or retina and is used to back-illuminate a section of the cornea, which is more anteriorly positioned. The illumination and observation systems can be adjusted so that the feature of interest in the cornea is seen against a light background (e.g. light-coloured iris) or a dark background (e.g. dark-coloured iris, or the pupil in the case of indirect retroillumination).

This technique is particularly useful for examining neovascularization, scars, degenerations and dystrophies.

Direct

Direct retroillumination refers to the configuration whereby the retroillumination is directly behind the feature of interest in the cornea ( Fig. 1.10 ). Thus, for example, a corneal scar is viewed against an illuminated iris in the background. With this technique, corneal opacities will appear black against the bright field.

Indirect

Indirect retroillumination refers to the configuration whereby the retroillumination is not directly behind the feature of interest in the cornea but is offset to one side ( Fig. 1.11 ). Thus, the feature of interest is being observed by virtue of back-scattered light that is deflected away from the feature of interest in the cornea into the eye of the observer.

Specular reflection

This is a specific case of a parallelepiped setup, where the angle of the incident slit beam is equal to the angle of the observation axis through one of the oculars ( Fig. 1.12 ). At this angle (typically 40°–50°), the illumination beam is reflected from the smooth surfaces of the cornea and provides a mirror-like (‘specular’) reflection. Such specular images occur at every interface between structures of different refractive indices, the most prominent of which will be anterior epithelial or posterior endothelial surfaces. The technique of specular reflection is typically used to view the endothelium.

To begin with, the lowest magnification setting is selected, and the illumination arm is set at an angle to the normal that is greater than the angle of the observation system to the normal. The illumination arm is then brought back towards the observation system while observing the corneal surface. At the point where specular reflection is achieved, a bright reflex will fill one of the oculars (specular reflection cannot be achieved binocularly). The illumination/observation system should now remain in a fixed position, and the magnification is set to maximum so that the anterior and/or posterior corneal surface can be viewed in specular reflection. A very bright reflection from the anterior surface constitutes a debilitating distraction when trying to observe the endothelium; this situation can be resolved by increasing the angle between the observation and illumination systems, although there is not much room for manoeuvre before the specular reflection is lost.

The size of endothelial cells is such that, even at × 40 magnification, only gross anomalies of the endothelium, such as large guttae, blebs, bedewing, ruptures and deep folds, can be detected. Subtle cellular characteristics of the endothelial mosaic, such as cell density or polymegethism, cannot be assessed. The tear film lipid layer, inferior tear meniscus and anterior surface of the crystalline lens can also be readily examined using specular reflection. If a contact lens is being worn, front surface wetting can be assessed and the post-lens tear film may be observed using specular reflection.

Sclerotic scatter

This technique is used to investigate subtle changes in corneal clarity, such as central corneal oedema, occurring over a large area. The slit lamp is set up for a wide-angle parallelepiped (45°–60°) and the viewing system is focused centrally. The beam is manually offset (‘uncoupled’) and focused on the limbus. The slit beam is totally internally reflected across the cornea, and a bright limbal glow is seen around the entire cornea ( Fig. 1.13 ). Any specific area of abnormality, such as a corneal scar, will interrupt the beam in its passage and produce a light reflection in the otherwise clear cornea; abnormalities in the cornea are especially visible when viewed against a dark pupil in the background.

Tangential (oblique) illumination

This is infrequently used in contact lens practice but is, nonetheless, a useful technique. Oblique illumination is achieved by setting up a parallelepiped and then moving the illumination system away from the observation system until the angle between them is close to 90°. The observation system is positioned at 90° to the facial plane (i.e. straight ahead) and the illumination arm is adjusted until the light beam is almost tangential to the object of interest. Any raised areas cast a shadow, making this technique particularly useful for viewing subtle irregularities on the surface of the iris, epithelium or contact lens in situ.