Clinical Comment: Hassall-Henle

Bodies and Guttata

The endothelium can produce mounds of basement membrane material, which are seen as periodic thickenings in Descemet’s membrane that bulge into the anterior chamber. Those located near the corneal periphery are called Hassall-Henle bodies. These bodies are a common finding, and their incidence increases with age. Such deposits of basement membrane in the central cornea are called corneal guttata and are indicative of endothelial dysfunction. The endothelium that covers these mounds is thinned and altered, and the endothelial barrier may be compromised. Both Hassall-Henle bodies and guttata are visible as dark areas when viewed with specular reflection with the biomicroscope. These may be interpreted as holes in the endothelium, but the endothelium is merely displaced posteriorly from the plane of reflection (Figure 2-16).

Clinical Comment: Effects

of Contact Lenses

Clinical studies indicate that epithelial thinning, stromal thinning, and a decreased number of keratocytes are associated with long-term extended wear of contact lenses.70,71 Numerous studies show that contact lens wear can induce changes in the regularity of the endothelial mosaic.72-76 Pleomorphism and polymegathism have been documented after only six years of either rigid gas-permeable or soft contact lens wear, although cell density remained normal.77,78 Endothelial stress resulting from contact lens wear, disease, surgery, or age can lead to endothelial remodeling, including change in size and shape or both.

CORNEAL FUNCTION

The cornea has two primary functions: to refract light and to transmit light. Factors that affect the amount of corneal refraction include (1) the curvature of the anterior corneal surface, (2) the change in refractive index from air to cornea (actually the tear film), (3) corneal thickness, (4) the curvature of the posterior corneal surface, and (5) the change in refractive index from cornea to aqueous humor. The total refractive power of the eye focused at infinity is between 60 and 65 diopters (D), with 43 to 48 D attributable to the cornea.5

In the transmission of light through the cornea, it is important that minimal scattering and distortion occur. Scattering of incident light is minimized by the smooth optical surface formed by the corneal epithelium and its tear film covering. The regular arrangement of the surface epithelial cells provides a relatively smooth surface, and the tear film fills in slight irregularities between cells producing negligible scatter of incident light. The absence of blood vessels and the maintenance of the correct spatial arrangement of components account for minimal scattering and distortion as light rays pass through the tissue. The cornea scatters less than 1% of the visible incident light6,49 and the majority of that scatter as determined by examination with the confocal microscope occurs due to the epithelium and endothelium.48 The epithelial and endothelial cell cytoplasm contain large amounts of water-soluble proteins, which enable the cytoplasm to appear homogenous and help to diminish light scattering. These proteins are now