The cornea is probably the one ocular structure that can have the most varied appearance in disease. While this can be a daunting structure to examine, changes to the cornea can be simplified for ease of classification into 3 changes in color: haze, red, and brown.
Haze to the cornea is quite common, and is caused by edema, infiltrate, scarring, or corneal deposits. Corneal edema can be focal or diffuse and presents in varying degrees of severity depending upon the underlying cause. Focal corneal edema is most often secondary to epithelial disruption, such as corneal ulceration. Diffuse corneal edema occurs secondary to endothelial disease, such as with glaucoma, uveitis, or endothelial degeneration. Dense corneal edema appears as a sky-blue haze and can prevent thorough examination of the internal structures. Corneal infiltration with leukocytes and/or bacteria will produce a haze. This haze is denser than edema and can appear white or creamy. Infiltrate is usually focal or multifocal, and often incites a vascular response. Corneal scarring after injury or ulceration will appear white or hazy and can be present at variable depths within the cornea. Deposition of lipid, cholesterol, or mineral within the cornea produces haze that is often sparkly or crystalline in appearance.
A Red Cornea
A red color change to the cornea is most often produced by the ingrowth of blood vessels but can also be from the formation for granulation tissue or a blood clot associated with a ruptured corneal ulcer. Vascularization of the cornea is a helpful exam finding in that it is most pronounced or progresses towards the primary disease. Focal corneal disease, such as a corneal ulcer, will incite long, branching blood vessels to grow towards the ulcer. More severe corneal disease or intraocular disease, such as uveitis, will produce a dense brush border of vessels to grow from the limbus towards the central cornea. This is called “ciliary flush” as is indicative of more severe pathology within the eye. Granulation tissue will form on the cornea to variable degrees during healing of ulceration, often associated with chronicity or persistent trauma (chronic entropion, for example), and appears as a bright red, slightly elevated lesion. A deep corneal ulcer that ruptures can also present as a red color change. As the ulceration progresses, fibrin, leukocytes, and red blood cells leak from the uvea into the anterior chamber of the eye. Once a perforation develops, a red clot will fill the defect with variable degrees of iris involvement. This clot will appear as a focal, red lump present within the center of a deep corneal defect.
A brown color change to the cornea can be pigmentation, sequestrum formation, iris prolapse, or foreign material. Corneal pigmentation is very common in dogs secondary to chronic surface diseases such as dry eye or Pannus, and appears as splotchy to wispy brown areas with variable density. A triangular shaped wisp of corneal pigmentation in the medial cornea is exceedingly common in Pugs. In cats, a brown color change to the cornea is more often a corneal sequestrum. A corneal sequestrum is a focus of dead corneal tissue that can appear auburn, brown, or black. Corneal ulceration is common surrounding sequestra, and vascularization is present to varying degrees. If penetrating trauma occurs to the cornea acutely, the iris will often fill the defect as a fibrin and blood clot has not had time to form. An iris prolapse appears as an elevated, dark brown to black tissue in the center of a corneal defect. Finally, many corneal foreign bodies are brown given that they are usually organic debris from the environment (shards of wood, leaves, sand).
Glowing Green (after Fluorescein stain!)
Corneal ulceration is a very common condition and also has a significantly variably appearance. In addition to the ulcer, the cornea may have edema, vascularization or granulation, and even pigmentation. After instillation and rinsing of fluorescein stain, the corneal ulcer is examined using a cobalt blue light. Corneal ulcers should be described based upon their location, size, shape and depth. Superficial corneal ulcers involve the epithelial cell layer and may extend to the anterior 1/3 – 1/2 of the corneal stroma. Deep corneal ulcers extend beyond 1/2 of the stromal thickness, and descemetoceles result when the ulceration has extended to the level of Descemet’s membrane. Ruptured corneal ulcers are often filled with fibrin, blood, or iris tissue. Based on the appearance of the ulcer, it should be classified as infected or not. Infection of a corneal ulcer will produce more pronounced discomfort, moderate to severe edema, reflex uveitis, and possibly corneal melt, which appears as a mushy, creamy cornea. An attempt should be made to identify and underlying cause of the ulcer by using the location and appearance of the ulcer in conjunction with examination of the adjacent structures.