The Role of Diagnostic Testing in Corneal Diseases

OVERVIEW The cornea is a transparent organ with no blood vessels. It covers the iris, pupil, and anterior chamber of the eye. With a refractive power of about +45.00 diopters, the cornea's primary function is the refraction of light rays. Corneal focusing accounts for approximately 70% of the eye's optical power, while the remainder is provided by the lens. When distortions, irregularities, and diseases of the cornea exist, patients may experience major problems with their vision. Therefore it is important to understand the various abnormalities and diseases of the eye as well as the tools physicians use in diagnosing and treating corneal disease. The cornea is comprised of five layers. The outermost layer, the epithelium, is five cell-layers thick. Because these cells slough and regenerate, the epithelium heals very quickly if injured and does not scar. From the outermost layer inward, the second layer is Bowman's layer. This thin layer consists of acellular collagen. The stroma is the center layer and comprises 90% of the cornea. The highly organized layers of collagen fibers and the level of relative dehydration of the stroma account for most of the transparency of the cornea. After injury, the repaired collagen is usually disorganized and therefore creates a hazy scar. The fourth layer is the Descemet's membrane, which is the basement membrane of the innermost fifth layer of endothelial cells. Together, these two layers regulate the flow of water in and out of the cornea; they are thin and generally viewed together during slit lamp examination unless there is pathology present that affects the Descemet's membrane. This course discusses many of the basic concepts and technologies used in corneal diagnostic testing. Descriptions of common corneal abnormalities are explored, as are the indications, purposes, and limitations of the testing process. Information is also provided on the role of the topographic image in detecting regular and irregular astigmatism and other corneal abnormalities. The following sections of the course describe the diagnostic tools available in assisting the physician in the accurate diagnosis and treatment of corneal diseases. The first step in the evaluation of a cornea is performed with the slit lamp biomicroscope. The slit lamp is a fundamental instrument found in every eye care practice and is invaluable in diagnosing abnormalities of the cornea. This instrument is used to magnify and illuminate all of the anterior structures of the eye, which are those from the anterior portion …