Corneal diseases are a group of conditions that damage or distort the cornea, the clear front surface of your eye responsible for about two-thirds of the eye’s focusing power. They range from infections and inherited disorders to progressive thinning conditions, and collectively they represent one of the leading causes of vision loss worldwide. Roughly 5.5 million people over age 40 have bilateral vision loss from corneal opacities alone, with millions more affected in just one eye.
What the Cornea Does and Why It’s Vulnerable
The cornea is a five-layer structure, each layer playing a distinct role in keeping your vision clear. The outermost layer, the epithelium, is five to seven cells thick and acts as a barrier against dust, bacteria, and water. Beneath it sits Bowman’s layer, a thin sheet of collagen that helps the cornea hold its shape. The stroma makes up 80 to 85 percent of the cornea’s thickness and gets its transparency from an extremely precise arrangement of collagen fibers. Descemet’s membrane supports the innermost layer, the endothelium, a single layer of cells that pumps excess water out of the stroma to keep it from swelling and turning cloudy.
Because the cornea is exposed to the outside world and has no blood vessels (it gets oxygen directly from the air and tears), it’s especially susceptible to injury, infection, and drying. Damage to any one of its layers can cloud your vision, distort the way light enters the eye, or cause persistent pain.
Corneal Infections
Infectious keratitis, or corneal infection, is the fifth leading cause of blindness globally. The main risk factors are contact lens wear, eye injuries, and existing problems with the eye’s surface. The type of organism involved determines how the infection behaves and how quickly it can damage your vision.
Bacterial keratitis is the most common form in developed countries. It typically causes blurred vision, redness, light sensitivity, and sometimes a visible white or gray spot on the cornea. Staphylococcus and Pseudomonas species are the usual culprits. Pseudomonas infections tend to be aggressive, spreading rapidly through the stroma and creating a hazy, inflamed cornea, while staph infections often produce a more contained, well-bordered ulcer.
Herpes simplex keratitis, caused by the same virus behind cold sores, produces a distinctive branching ulcer on the cornea’s surface. It can recur throughout a person’s life, and repeated episodes may cause deeper scarring, swelling, or blood vessel growth into the normally clear cornea. Fungal keratitis, often triggered by plant matter or soil entering the eye, progresses more slowly and produces a dry, elevated sore with feathery edges and satellite lesions. Acanthamoeba keratitis, linked to contaminated water and improper contact lens hygiene, causes ring-shaped corneal infiltrates and intense pain from nerve inflammation.
Keratoconus and Other Thinning Disorders
Keratoconus is the best known of the corneal ectasias, a group of conditions where the cornea progressively thins and bulges outward into a cone-like shape. It usually starts in the teens or twenties and worsens over the following decade or two. The irregular curvature creates distorted, blurry vision that glasses alone can’t fully correct. In mild cases, soft toric contact lenses may work, but as the cornea becomes more irregular, rigid gas-permeable lenses or specialty scleral lenses are often needed to create a smooth optical surface over the cone.
A treatment called corneal cross-linking can halt or slow the progression of keratoconus. The procedure uses vitamin B2 (riboflavin) drops and ultraviolet light to strengthen the chemical bonds between collagen fibers in the stroma, stiffening the cornea and making it more resistant to further bulging. The strengthening effect occurs primarily in the front 200 micrometers of the stroma, and studies show the cornea’s structural rigidity increases both immediately and months after the procedure. Cross-linking was originally developed to stop ectasia but has also shown promise in treating certain corneal infections and ulcers.
Corneal Dystrophies
Corneal dystrophies are inherited conditions where abnormal material gradually accumulates in one or more layers of the cornea. There are more than 20 types, but Fuchs’ dystrophy is by far the most common. In Fuchs’, the endothelial cells that pump water out of the cornea slowly die off. Since these cells don’t regenerate, fluid builds up in the stroma, causing the cornea to swell and cloud. Early symptoms include glare and halos around lights, especially at night. Vision tends to be worse in the morning (when overnight fluid hasn’t yet evaporated) and improves as the day goes on.
Other dystrophies affect different layers. Some cause recurrent corneal erosion, where the epithelium peels away from the layer beneath it. This produces a sudden, sharp pain, tearing, and the sensation of something stuck in the eye, often upon waking. Episodes can recur for years.
Surface Growths: Pterygium and Pinguecula
A pinguecula is a small, yellowish or white raised bump on the conjunctiva, the thin tissue that covers the white of the eye. It contains deposits of protein, fat, or calcium. It doesn’t grow onto the cornea itself, but in severe cases it can disrupt the tear film and cause dryness, redness, or irritation.
A pterygium (sometimes called surfer’s eye) starts the same way but extends onto the cornea as a wedge-shaped growth. If it grows far enough, it can physically change the shape of the cornea, causing blurred or distorted vision. Both conditions are driven by UV exposure and chronic dryness or irritation, which is why they’re far more common in people who spend long hours outdoors, especially near water, sand, or snow.
How Corneal Diseases Are Detected
Because many corneal conditions change the shape or thickness of the cornea before symptoms become obvious, several imaging tools play an important role in early detection. Corneal topography maps the curvature of the front surface, making it possible to spot the subtle irregularities of early keratoconus or post-surgical changes. Corneal pachymetry measures thickness, which is useful for tracking the progression of dystrophies and ectasias and for evaluating candidates for laser vision correction. Specular microscopy counts and photographs the endothelial cells on the back of the cornea, helping to stage Fuchs’ dystrophy and monitor cell loss after surgery or injury. These measurements are painless, take seconds, and give clinicians precise data to guide treatment decisions.
When a Corneal Transplant Is Needed
When scarring, swelling, or distortion becomes too severe for lenses or other therapies to correct, a corneal transplant may be recommended. The traditional approach, penetrating keratoplasty, replaces the entire cornea with donor tissue. It has a strong track record: one large study found a five-year graft survival rate of 91 percent overall, though survival drops in eyes that have had previous transplants or have deep blood vessel growth in the cornea.
Newer techniques replace only the diseased layers and leave healthy tissue intact. Deep anterior lamellar keratoplasty removes the front layers of the cornea while keeping the patient’s own endothelium, making it a good fit for conditions like keratoconus where the back of the cornea is healthy. For diseases that primarily affect the endothelium, such as Fuchs’ dystrophy, two posterior transplant options exist. One replaces the endothelium, Descemet’s membrane, and a thin sliver of stroma. The other, the most selective version, transplants only the endothelium and Descemet’s membrane. These partial-thickness procedures generally allow faster visual recovery and carry a lower risk of rejection because less foreign tissue is introduced.
Global Burden and Regional Disparities
Corneal diseases do not affect all populations equally. Africa and the Middle East have corneal opacity rates 8 to 14 times higher than other regions, driven by limited access to eye care, higher rates of untreated infection, and greater exposure to environmental risk factors like dust and UV radiation. In wealthier countries, contact lens-related infections and inherited dystrophies account for a larger share of corneal disease, while trauma and infectious keratitis from agricultural injuries remain dominant causes of corneal blindness in low-income settings.