Your horse’s eye is surprisingly tough yet frustratingly fragile. It can withstand an amazing number of challenges—exposure to microbes, ultraviolet light, dust and debris—but it is still vulnerable to injury and disease. One of the telltale signs that something’s wrong with it is a sudden opaque cloudiness over the cornea.
This is where your veterinarian comes in, as a number of things—some benign, some serious—can cause a cloudy eye. And any horse, from the senior with the scar on his eye due to an old corneal ulcer to the Paint with squamous cell carcinoma, can fall victim to one of these issues.
Anatomy of the Eye Surface
“Having some knowledge about the anatomy of the eye, specifically the cornea, can help us understand some of these eye issues,” says Ann Dwyer, DVM, of Genesee Valley Equine Clinic, in Scottsville, New York.
The cornea—the eye’s clear, outer covering—is a metabolic tissue made up of several layers. The entire transparent layer of tissue is only about 1 mm in diameter. From the outside in, you have:
The epithelium, which is about 0.1 mm thick and makes up less than 15% of the cornea’s thickness;
The stroma, which constitutes most of the rest of the cornea’s thickness.
The very thin and elastic Descemet’s membrane; and
The endothelium, which is just one cell layer thick.
Dwyer likens the epithelium’s cellular composition to a stone wall with fatter stones/cells on the bottom and thinner ones toward the top.
“At the bottom of the layers of epithelial cells there is a basement membrane, which you could liken to a piece of Saran Wrap below the stone wall,” she says.
The stroma is primarily made up of long, thin fibers that are organized in layers, with just a few cells, called keratocytes, peppered among them. “With so few cells, it’s a challenge if there’s a problem there, because the body needs cells to repair injury,” says Dwyer.
The underlying Descemet’s membrane is like another piece of Saran Wrap beneath the stroma, followed by the even thinner endothelium. “Unfortunately, (the endothelium’s) cells hardly reproduce and, thus, don’t regenerate very well,” says Dwyer. “The epithelium can completely regenerate and does this continually, like your skin. A healthy cornea will turn over all its cells for new ones in about a week. But the endothelium has very little capacity to regenerate, and the density of these cells declines with age. They play a critical role in maintaining transparency, however, and if the endothelium gets damaged, the eye is in trouble.”
The cornea is transparent because these layers, especially the stroma, remain relatively dehydrated. “The endothelium (innermost layer) pulls fluid out of the stroma,” says Dwyer. “If the endothelium is damaged, old, or does not function properly, the cornea ‘steams up’ in the affected area (similar to your windshield if the defroster isn’t working), becoming cloudy.”