The anatomy and diseases of the equine cornea are described in Chapter 5. To examine the cornea of the horse, diffuse and focal direct illumination (or transillumination) with magnification is used first, followed in most cases by biomicroscopy using a slit-lamp. The corneal examination should be performed with the observer located rostral to the eye. Light directed diagonally across the cornea will reveal opacities of the cornea against the dark background of the pupil.17
The Purkinje-Sanson reflexes are three reflections from the eye produced by the light source during transillumination (Fig. 1-31).17 Disease may alter the sharpness and location of these reflexes. The first, largest, and most anterior originates from the cornea. The second originates from the anterior lens capsule, and the third and most posterior originates from the posterior lens capsule. If a slit-lamp biomicroscope is used, two corneal reflexes are seen, one from the anterior surface and the other from the endothelium.17 The corneal and anterior lens capsule reflexes are virtual and noninverted and will move in the same direction as a change in the light position. The image on the posterior surface is real and inverted and will move in the opposite direction to the light.17 The images are valuable in determining corneal clarity, depth of the anterior chamber, thickness and position of the lens (after mydriasis), and in locating lesions within the lens.17
Biomicroscopy for Corneal Exam
The technique of biomicroscopy, in which a slit-lamp binocular microscope with an external pivoting light source is used, is the same for horses as for humans and small animals and has been well described elsewhere.12,55–58 Slit-lamp biomicroscopy improves visualization and localization of lesions of the cornea, anterior chamber, lens, and anterior vitreous by means of transillumination and retroillumination.12 It can also be used to assess corneal thickness (i.e., pachymetry), anterior chamber depth, and aqueous flare.12
The availability of portable handheld models of the slit-lamp biomicroscope has made biomicroscopy for equine ophthalmology easy and efficient. Portable models are available from Clement-Clark, Kowa, Nippon, Dioptrix, and Zeiss. The Kowa SL-14 or SL-15 (×10 or ×16 magnification) is light and powered by a rechargeable battery, and therefore of excellent use in examination of a horse (Fig. 1-32). However, lack of magnification above ×16 and inherent movement of the examiner and horse limits the ability to see fine structure and lesions. An alternative to a biomicroscope is using magnification (i.e, ×2.3 magnifying head loupes) and the slit beam on the direct ophthalmoscope. Very small “slit-lamps” are also made by Heine (HSL 150 [Heine USA, Dover, NH]; Eidolon Hand Held Slit Lamp Model 510L [Eidolon Optical LLC, Natick, MA]) that resemble a penlight with a magnifier on the end (Fig. 1-33). Although these instruments are inexpensive and portable, their lack of magnification and illumination limits their usefulness.12
Figure 1-33 Use of a small, portable slit-lamp (Heine Handheld Slit Lamp, Heine USA, Dover, NH).
(Photograph courtesy Dr. David Wilkie.)
The light beam of the biomicroscope should be angled at 20 to 45 degrees from the axis of the microscope and thus the visual plane of the observer (Fig. 1-34). The light beam width, length, orientation, and color can then be modified by a series of diaphragms and filters.12 The focal distance of the instrument is 7 to 10cm, and fine focus is achieved by moving either toward or away from the eye within this range.12
The initial examination of the horse should proceed with diffuse illumination: a wide, low-intensity slit beam should be used, and the microscope should be defocused from the light.12 The surfaces of the eyelids, cornea, conjunctiva, and iris should be inspected. With the use of low magnification, a broad slit beam is focused on the cornea, creating a parallelepiped (i.e., a three-dimensional section) of illuminated tissue.12,55 This allows visualization of transparent structures such as the cornea and lens in three dimensions. In the cornea, the anterior surface, stroma, and posterior surface of the cornea can be visualized.55 Nontransparent structures such as the sclera only yield a magnified two-dimensional surface or external view. The slit beam is then narrowed and intensified to reveal a two-dimensional cross-section of the cornea and lens, allowing the examiner to accurately determine lesion depth and axial positioning.55 This is extremely important in evaluating the depth of corneal lesions (e.g., stromal ulcerative keratitis, stromal abscesses) in the horse.
Direct and indirect retroillumination are performed by reflecting the slit beam from deeper structures while focusing on more superficial structures.55 Other techniques that can be performed with slit-lamp biomicroscopy, such as specular reflection, are difficult to impossible in a horse because of continuous slight ocular movements.
