Definition of Cornea
The cornea is essentially the window through which we see the world. It is a fascinating structure because, despite the fact that it is devoid of blood vessels, it has an amazing ability to regenerate.
The cornea can heal a superficial abrasion in about 24 hours without any scarring. In fact, it heals faster than any other part of the body.
The cornea is a group of cells that you can see through that provide 70 percent of the eye’s focusing power (cells that focus light – how cool is that?). This group of cells is comprised of five layers:
- The Epithelium
- Bowman’s Layer
- The Stroma
- Descemet’s Membrane
- The Endothelium
The Five Layers of the Cornea
A Corneal Abrasion
The epithelium is the first layer of the cornea and contains all the nerve endings. The foreign body sensation, or feeling of something in your eye, is caused by these very sensitive nerve endings. These nerve endings also stimulate the tear ducts when the eye becomes dry.
The endothelium is the most likely layer to be injured by foreign bodies or abrasions and is only protected by the tear film which keeps the epithelium clean and nourished. It fills in irregularities as well, providing a shiny, smooth surface which provides clear vision.
The tear film also plays a part in the cornea’s amazing healing ability. It acts as a lubricant that prevents friction and keeps the eyelid from sloughing off any loose epithelial tissue. With an abrasion, this is very important.
Artificial Tears for Dry Eyes
An abrasion is essentially a scratch on the corneal epithelium. More specifically, it is a piece of the epithelium that is missing due to injury. It is similar to chipped paint, the more you rub the paint, the more the paint comes off.
It’s the same with an abrasion. The more the eyelid is allowed to rub against the abrasion, the worse the abrasion gets. Some people have this condition chronically, and it is known as recurrent corneal erosion.
Abrasions and foreign bodies are quite painful, especially when the eye blinks. Injury to the cornea also causes sensitivity to light. Even if the cornea is merely dry, the person can become quite photophobic (light sensitive).
For those with chronic dry eyes, artificial tears provide relief and prevent corneal damage when used regularly. Keeping the cornea moist also helps abrasions to the epithelium heal faster.
The second layer of the cornea is known as Bowman’s layer. Unlike the other layers of the cornea, this layer is not composed of individual cells; it is made of randomly placed collagen fibrils.
Collagen is a protein most commonly found in connective tissue. In fact, it is the most abundant protein found in mammals. It typically takes the form of elongated filaments called fibrils.
Bowman’s membrane has no regenerative properties, which means it will scar if injured. Its actual function is currently unknown in ophthalmology. Interestingly, Bowman’s membrane is only found in the corneas of primates. It is absent in the cornea of dogs, cats, and other carnivores.
The third and thickest layer of the cornea is called the stroma. This layer gives the cornea its stability. Interestingly, the stroma is 78% water, and it does not regenerate. It typically scars if injured and depending on the severity of the injury may require a corneal transplant to restore visual clarity.
The stroma is the layer of the cornea that is targeted during refractive surgery procedures such as LASIK. This is because it also gives the cornea its shape. Refractive errors such as myopia, hyperopia and astigmatism can be permanently fixed by making the cornea either flatter or steeper (depending on the patient’s refractive error). This is achieved by using a laser to ablate (remove) a portion of the stroma, allowing light to focus properly on the retina.
The fourth layer is Descemet’s membrane which is very elastic and snaps like a rubber band if it is cut. This layer is the basement membrane of the endothelial cells. If it is injured, the eye will usually require a transplant since the endothelial cells depend on Descemet’s membrane for support.
This layer is the target of many new transplant procedures such as:
- Descemet’s Stripping Endothelial Keratoplasty (DSEK)
- Descemet’s Stripping Automated Endothelial Keratoplasty (DSAEK)
- Descemet’s Membrane Endothelial Keratoplasty (DMEK)
- Descemet’s Membrane Automated Endothelial Keratoplasty (DMAEK)
These procedures involve removing Descemet’s membrane and the endothelium of the cornea and replacing them with donor tissue (using the same layers).
This is a huge improvement from the original procedure known as penetrating keratoplasty, or PKP, which removed a central “button” that was the full thickness of the cornea (see photo).
Although PKPs are still performed when needed, the Descemet’s membrane procedures have become the gold standard.
Descemet's Stripping Automated Endothelial Keratoplasty (DSAEK) Procedure
Corneal Endothelium Specular Microscopy
The final layer of the cornea is the endothelium. This layer maintains corneal deturgescence. In English, this means it maintains the relative dehydration that is necessary to keep the cornea transparent.
The cells of the endothelium act like little pumps to maintain the fluid in the cornea so that it looks like a clear window. They are hexagonal in shape and they are limited in number. There are only about 500,000 of them.
Because it cannot regenerate, the endothelium can be afflicted with endothelial dysfunction in which the cells cease to work properly, or die off altogether, causing a corneal dystrophy such as:
- Fuchs Dystrophy
- X-Linked Endothelial Corneal Dystrophy
- Posterior Polymorphous Dystrophy
- Congenital Hereditary Corneal Dystrophy
Fuchs dystrophy is by far the most common of the endothelial dystrophies. With Fuchs dystrophy, the endothelial cells deteriorate and eventually die off. This causes corneal edema and excess fluid to accumulate, causing the vision to become cloudy and blurred.
One of the most common issues with the cornea is astigmatism. In fact, just about everyone has astigmatism, but some do not require correction for it.
Astigmatism is an irregularity in the cornea, and can be likened to a speed bump. It’s a raised portion of tissue in a specific area of the cornea that causes images to look as if they have a doubled edge. It is easily corrected with glasses, contacts or laser corrective surgery, such as LASIK.
Astigmatism can easily be detected with various tests including:
- Corneal Topography
A refraction is the test used to determine a patient's glasses prescription. The technician will normally find astigmatism during a refraction by performing retinoscopy, or using an astigmatic clock.
Keratometry manually looks for astigmatism by aligning a set of mires. This test is typically only performed by technicians with extensive experience in the field and when corneal topography can’t be performed, such as with severe scarring.
Corneal topography is a map that shows corneal elevation in different colors, allowing the doctor to see the thickest and thinnest part of the cornea along with any irregularities. The test is usually performed prior to any ocular surgery such as cataract surgery or LASIK.
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