Suppose we have several eyes with the same anterior segment and, therefore, the same anterior focal plane determined by that cornea, anterior chamber depth, and lens. We suppose the eyes differ only in their axial lengths, and for each eye, we place at the anterior focal plane whatever power lens is needed to correct the refractive error of that eye, depending on how long the eye is. Knapp’s rule, illustrated in Figure 1-41 says the size on the retina of the image of a distant object is the same for all those eyes, when each one’s corrective lens is in place. Clinical application of this rule is limited. Eyes may have unequal myopia because of differences in their anterior segments rather than in their axial lengths. The retinal photoreceptors may be spaced farther apart in a longer eye, and spectacles are usually worn closer to the eye than the anterior focal point, which is approximately 17 mm in front of the cornea.
Optical Pachymeter
With the optical pachymeter, the thickness of the cornea or the depth of the anterior chamber is measured by lining up prism-split images in the focused slit lamp’s optical section through the eye (Fig 7-24).
Figure 7-24 In the most common type of optical pachymeter, the cornea is illuminated with a slit beam (a). The image is
viewed through a biomicroscope, half through a glass plate orthogonal to the path of light (b) and half through a glass plate rotated through an angle (c). The beam path through the plate is displaced laterally for a distance (d) that varies depending on the angle of rotation. Through the eyepiece (e), a split image is seen (f) wherein half the image comes from the fixed plate and the other half from the rotatable plate. The endothelial surface of one image and the epithelial surface of the other are aligned by the observer by adjustment of the rotatable plate (c), and the corneal thickness measurement is read
off a calibrated scale (g). (Courtesy of Neal H. Ateb ara, MD.)
Applanation Tonometry
The head of the applanation tonometer contains a prism that splits the image of a fluorescing circle of tears to determine when that circle is precisely a certain size (Fig 7-25). Intraocular pressure is inferred from the amount of pressure required to flatten the cornea just enough to create that size circle of tears.
Figure 7-25 The split prism in the applanation head creates 2 offset images. A, When the area of applanation is smaller than 3.06 mm, the arms of the inner semicircles remain some distance apart. B, When the area of applanation is greater than 3.06 mm, the arms of the inner semicircles overlap. C, When the area of applanation is exactly 3.06 mm, the arms of the inner semicircles just touch each other. This is the endpoint for measuring intraocular pressure. The value of 3.06 mm was chosen to approximately balance tear-film surface tension and corneal rigidity. (Courtesy of Neal H. Ateb ara, MD. Redrawn
b y C. H. Wooley.)
Specular Microscopy
Specular microscopy is a modality for examining endothelial cells that uses specular reflection from the interface between the endothelial cells and the aqueous humor. The technique can be performed using contact or noncontact methods. In both methods, the instruments are designed to separate the illumination and viewing paths so that reflections from the anterior corneal surface do not obscure the weak reflection arising from the endothelial cell surface.
Endothelial cells can also be visualized through a slit-lamp biomicroscope, if the illumination and viewing axes are symmetrically displaced on either side of the normal line to the cornea (Fig 7-26). A narrow illumination slit must be used; hence, the field of view is narrow. Photographic recording has been made possible by the addition of a long-working-distance microscope system on the viewing axis and flash capability to the illumination system. Patient eye motion is the chief problem with this
technique.
Figure 7-26 Specular reflection microscopy. When a beam of light passes through the transparent corneal structures, most of the light is transmitted (a). However, at each optical interface, such as the corneal endothelium, a proportion of light is reflected (b). This light (called specular reflection) can be collected to form a relatively dim image of the corneal endothelium (c), where individual endothelial cells can be counted. (Courtesy of Neal H. Ateb ara, MD. Redrawn b y C. H. Wooley.)
In contact specular microscopy, the illumination and viewing paths traverse opposite sides of a special microscope objective, the front element of which touches the cornea. This reduces eye rotation and effectively eliminates longitudinal motion that interferes with focus. Contact specular microscopy allows for higher magnifications than slit-lamp biomicroscopy, making cellular detail and endothelial abnormalities more discernible.
Video recording of endothelial layer images makes it possible to document larger, overlapping areas of the endothelial layer. Also, it allows for the recording of high-magnification images, despite patient eye motion.
Wide-field specular microscopy employs techniques to ensure that reflections from the interface between the cornea and contact element do not overlap the image of the endothelial cell layer. Because scattered light from edema in the epithelium and stroma can degrade the endothelial image, variable slit widths are sometimes provided to reduce this problem.
Analysis of specular micrographs may consist simply of assessment of cell appearance together with notation of abnormalities such as guttae or keratic precipitates. Frequently, cell counts are
desired; these are often obtained by superimposing a transparent grid of specific dimensions on the endothelial image (photograph or video) and simply counting the cells in a known area. Cell-size distribution can be determined by computer analysis after cell boundaries have been determined digitally. The normal cell density in young people exceeds 3000 cells/mm2; the average density in the older age group susceptible to cataract is 2250 cells/mm2, which suggests a gradual decline with age.
Specular microscopy has been important in studying the morphology of the endothelium and in quantifying damage to the endothelium produced by various surgical procedures and intraocular devices.
Keratometer
The keratometer is used to measure the curvature of the central outer corneal surface by measuring the size of a reflected image in each meridian (or only in the meridians of greatest and least curvature). The measurement is accomplished by lining up prism-doubled images at a distance regulated by sharpness of focus (Fig 7-27). This measurement is performed at only one diameter, 3 mm, in a limited choice of meridians, and is therefore lacking the detail provided by more elaborate topography.
Figure 7-27 Two prisms placed base to base produce doubled images separated by a fixed distance that are not affected by small movements of the eye. The observer varies the object size (ie, the distance between the red and green objects)