CHAPTER 2
Examination Techniques for the External Eye and Cornea
Evaluation of Vision in the Patient With an Abnormal Cornea
A patient with an abnormal cornea and reduced vision may require special testing to determine whether the loss of vision is from irregular astigmatism or stromal scarring. Use of a rigid gaspermeable (RGP) contact lens during testing will neutralize irregularity caused by an abnormal ocular surface or corneal curvature. The average keratometry reading is used in selection of the base curve of the lens; the refractive power of the lens is based on the spherical equivalent. After placing the lens, the examiner can perform an overrefraction or use a pinhole occluder to measure the best visual acuity. If the vision does not improve, then corneal scarring may be the cause of the decreased vision. See BCSC Section 3, Clinical Optics, to learn more about various methods of evaluating visual function.
External Examination
The external examination should begin as the patient enters the room. A careful look at the patient may reveal signs of associated systemic disease (eg, rheumatoid arthritis), indications of personal habits (eg, smoking), signs of associated ocular conditions (eg, pseudoptosis, blepharospasm), or conditions of the skin (eg, rosacea). The position and action of the eyelids and the globes can be assessed as well.
Slit-Lamp Biomicroscopy
The slit-lamp biomicroscope has 2 rotating arms—1 for the slit illuminator and the other for the biomicroscope—mounted on a common axis. The illumination unit is essentially a projector with a light beam that is adjustable in width, height, direction, intensity, and color. The biomicroscope is a binocular Galilean telescope with multiple magnifications. The illumination and microscope arms are parfocal, arranged so that both focus on the same spot, with the slit beam centered in the field of view. This setup provides direct illumination, and purposeful shifting of alignment allows for indirect
illumination.
Direct Illumination Methods
Diffuse illumination
With diffuse illumination, the light beam is broadened, reduced in intensity, and directed at the eye from an oblique angle. Swinging the illuminator arm to produce highlights and shadows can enhance the visibility of raised lesions of the ocular surface and iris.
Focal illumination
With focal illumination, the light and the microscope are focused on the same spot, and the slit aperture is adjusted from wide to narrow. Broad-beam illumination, using a slit width of around 3 mm, is optimal to visualize eyelid lesions as well as the corneal opacities seen in dystrophies or scarring. Slit-beam illumination, using a beam width of about 1 mm or less, gives an optical section of the cornea (Fig 2-1) that is essential for evaluation of corneal thinning, edema, stromal infiltrates, and endothelial abnormalities. The examiner can use a very narrow slit beam to help identify refractive index differences in transparent structures as light rays pass through the cornea, anterior chamber, and lens. The examiner can also reduce the height of a narrow beam to determine the presence and amount of cell and flare in the anterior chamber.
Figure 2-1 Slit section of normal cornea. 1, Tear film. 2, Epithelium. 3, Anterior stroma with high density of keratocytes. 4, Posterior stroma with lower density of keratocytes. 5, Descemet membrane and endothelium. (Reproduced with permission from
Krachmer JH, Mannis MJ, Holland EJ, eds. Cornea. 2nd ed. Vol 1. Philadelphia: Elsevier/Mosby; 2005:201. © CL Mártonyi, WK Kellogg Eye Center, University of Michigan.)
Specular reflection
Specular reflections are normal light reflexes bouncing off a surface. An example is the bright round or oval spot seen reflected from the ocular surface in a typical flash photograph of an eye. These mirror images of the light source can be annoying, and it is tempting to ignore them during slit-lamp examination. However, the clarity and sharpness of these reflections from the tear film give clues to the condition of the underlying tissue.
A faint reflection also comes from the posterior corneal surface. The examiner can enhance this specular reflection by using a light beam at an appropriate angle, revealing the corneal endothelium (Fig 2-2). Following are the steps for examining the corneal endothelium with specular reflection:
1.Begin by setting the slit-beam arm at an angle of 60° from the viewing arm and using a short slit or 0.2-mm spot.
2.Identify the very bright mirror image of the lightbulb’s filament and the paired epithelial and endothelial Purkinje light reflexes.
3.Superimpose the corneal endothelial light reflex onto the filament’s mirror image, giving a bright glare.
4.Use the joystick to move the biomicroscope slightly forward in order to focus the endothelial reflex.
Specular microscopy is monocular, and 1 eyepiece may require focusing. A setting of ×25 to ×40 is usually needed to obtain a clear view of the endothelial mosaic. Cell density and morphology are noted; guttae and keratic precipitates appear as nonreflective dark areas.
Figure 2-2 A, Corneal endothelium seen with specular reflection using the slit-lamp biomicroscope at ×40 magnification. B, Fuchs endothelial dystrophy seen in specular microscopy showing guttae. (Part A reproduced with permission from Krachmer JH,
Mannis MJ, Holland EJ, eds. Cornea. 2nd ed. Vol 1. Philadelphia: Elsevier/Mosby; 2005:208. © CL Mártonyi, WK Kellogg Eye Center, University of