Corneal disease: A guide to conditions and treatments
- What is corneal disease?
- How is the cornea affected by corneal disease?
- What are the main types of corneal disease?
- Corneal disease due to dystrophies
- Corneal disease due to degenerations
- Corneal disease due to dry eye (keratitis sicca)
- Corneal disease due to infection (microbial keratitis)
- Corneal disease due to trauma
- How to treat corneal disease
- How to prevent corneal disease
What is corneal disease?
Corneal disease is caused by any condition that damages the cornea — the clear, dome-shaped outer layer of the eye. It can result in corneal scarring or clouding and cause decreased vision. Corneal infection, inflammation, dryness, dystrophies, degenerations and injuries are all corneal diseases.
The cornea is a protective barrier that prevents bacteria, viruses and particles from entering the eye. It also filters out some UV radiation. Corneal disease results in a breakdown of this protective barrier, leaving the eye vulnerable to infection, inflammation and injury.
The cornea also provides most of the focusing power of the eye. It does this by bending light rays as they enter the eye so that they are focused on the retina to provide a sharp, clear image. Corneal disease can damage the clear corneal tissue, leading to scarring, haziness and distorted vision.
The cornea is the only part of the body that does not have blood vessels providing it oxygen and nutrients — it is avascular. There are no blood vessels in the cornea so that it is a clear window for light rays to enter. It gets oxygen that is diffused through the tears, and nutrients from the aqueous humor. It is also the fastest healing tissue in our body, regenerating within 24-36 hours!
SEE RELATED: Cystinosis
How is the cornea affected by corneal disease?
The layers of the cornea provide structure and defense against particles and invading pathogens. To understand how different corneal diseases affect the cornea, it is helpful to understand its composition.
The cornea has five layers:
Epithelium – Smooth outer layer. It serves as a barrier and absorbs nutrients and oxygen from tears. It heals rapidly but is rich in nerve endings, making it very pain-sensitive.
Bowman’s layer – Transparent protective layer made of collagen. It can scar as it heals.
Stroma – Middle layer made of water and collagen. It provides 90% of the cornea’s thickness and gives it structure and elasticity. It is important in keeping the cornea transparent.
Descemet’s membrane – Protective layer of collagen fibers. This membrane recovers well from injury.
Endothelium – A single layer of cells that pump excess water out of the stroma. It keeps the stroma from absorbing too much (which would cause corneal edema and haziness).
What are the main types of corneal disease?
The cornea often responds to damage and disease with inflammation. Inflammation of the cornea is called keratitis. Many corneal diseases have “keratitis” in their names because they cause corneal inflammation along with other symptoms. Two examples are keratitis sicca (corneal disease due to dry eye) and microbial keratitis (corneal disease due to infection).
Corneal disease can result from:
Dry eye (keratitis sicca)
Trauma (such as a scratch or a lodged particle)
Corneal diseases are the second leading cause of blindness in the developing world. Numerous volumes of books exist about all of the many corneal diseases. We will cover a few of the more common ones.
SEE RELATED: Corneal deposits
Corneal disease due to dystrophies
Corneal dystrophies consist of slow-progressing, rare genetic disorders that run in families. They are non-inflammatory and cause a buildup of material in the corneal layers of both eyes. This buildup causes the cornea to lose its transparency, resulting in a decrease or loss of vision.
There are over 20 corneal dystrophies that affect the anterior, middle and posterior parts of the cornea. A symptom that a number of people with corneal dystrophies suffer from is recurrent corneal erosion. This is when the corneal epithelium does not adhere to the eye properly and breaks down as soon as the eyes are opened after sleeping. It can lead to sudden onset of pain, blur, tearing and light sensitivity.
Corneal dystrophies are classified by which of the five layers of the cornea are affected. The most common dystrophies include:
Fuchs’ corneal disease or dystrophy
This dystrophy affects the back (posterior) of the cornea — causing damage to the endothelium and Descemet’s membrane.
One function of the endothelium is to pump fluid out of the stroma. This prevents the stroma from swelling and becoming cloudy. In Fuchs’ corneal disease, vision is most blurry in the morning because fluid builds up overnight and clears as the day progresses. As time goes on, the cornea stays swollen the entire day, causing constant blur.
This condition is more common in women. It can be treated by eye drops and ointments or even a hairdryer held at arm’s length to dry the corneal surface. Eventually, Fuchs’ dystrophy may require corneal transplant surgery or an endothelial keratoplasty (EK). An EK is a procedure that transplants healthy endothelial cells on the cornea.
This dystrophy affects the middle, thickest part of the cornea — the stroma. It is due to abnormal overlapping protein fibers in the stroma (giving the appearance of a lattice).
The protein fibers cause the cornea to become cloudy, decreasing vision. It can be treated by drops and ointments, but if symptoms become severe, a corneal transplant surgery may be necessary.
Epithelial basement membrane disorder (EBMD)
This dystrophy affects the front (anterior) of the cornea — the epithelium and Bowman’s membrane. This condition causes an abnormal folding of the epithelium. EBMD is also known as “map-dot-fingerprint dystrophy” because of its appearance.
Although it does not always cause vision loss or pain, it can cause recurrent corneal erosions in some cases. This condition may cause changes in prescription as well.
Treatment includes artificial tears and ointment, pain relievers, or medical and surgical management for severe cases.
Keratoconus (conical cornea) is considered an ectatic dystrophy. This means that the cornea gets progressively thinner and takes on a distorted, cone-like shape.
It is managed by glasses, contact lenses or surgical intervention. An outpatient procedure such as corneal cross-linking surgery can strengthen the bonds between collagen fibers in the cornea. This slows down the progression of the disease.
READ MORE: Keratoconus FAQ
Corneal disease due to degenerations
Degenerations are deterioration of the corneal tissue due to disease or aging. They often only occur in one eye and begin on the edges of the cornea. Degenerations result in deposits, thinning or blood vessels in the cornea (the cornea should be avascular). They typically aren’t due to genetics and don’t run in families.
Corneal diseases due to corneal degenerations include:
Age-related degeneration – Due to fat deposits on the edges of the cornea from aging.
Salzmann's nodular degeneration – Due to excess collagen in Bowman’s layer after trauma.
Band keratopathy degeneration – Due to calcium deposits in Bowman’s layer.
The most common reasons for corneal degenerations are underlying health conditions such as:
Corneal disease due to dry eye (keratitis sicca)
Keratitis sicca is the medical term for dry eye. Millions of people in the U.S. have symptoms of dry eye. It is generally caused by two mechanisms:
Aqueous tear deficiency – This can occur when the lacrimal gland does not produce enough aqueous (watery) tears.
Meibomian gland dysfunction – This can occur when the meibomian glands are not functioning properly and the oily layer above the watery tears is deficient. This causes the tears to evaporate too quickly. It may result in eyelid inflammation or blepharitis.
Keratitis sicca risk factors, symptoms and treatment
People at a higher risk for corneal disease due to dry eye are those who:
Are 50+ years of age
Are contact lens wearers
Have omega-3 or Vitamin A deficiency
Symptoms of keratitis sicca include:
There are several treatment options for corneal disease due to dry eye, but it’s important to talk to an eye doctor to determine the best option for each patient. Some of the treatments include:
Over-the-counter artificial tears and ointments
Lid scrubs and warm compresses
Prescription medications such as Restasis or Xiidra
Limiting screen time
Taking visual breaks
Making overall healthy lifestyle and nutritional choices
READ MORE: Dry Eyes FAQ
Corneal disease due to infection (microbial keratitis)
When bacteria, viruses, fungi or parasites penetrate the cornea, the infection is called microbial keratitis. This may result in a very painful corneal ulcer and become a vision-threatening condition. There are four main types of microbial keratitis:
Staphylococcus aureus and Pseudomonas aeruginosa are two of the bacteria that can cause an infection of the cornea. Not following the proper cleaning regimen and wear times for contact lenses places you at a higher risk of getting bacterial keratitis.
Risk factors for bacterial keratitis include:
Contact lenses that are not handled properly or are worn overnight
Compromised immune system
Underlying eye conditions
Recent eye trauma
Bacterial keratitis symptoms can include:
The treatment for bacterial keratitis is prescription antibiotic medications. An eye doctor may prescribe:
Antibiotic eye drops
Antibiotic eye ointment
READ MORE: How to care for soft contact lenses
The herpes simplex virus-1 (HSV-1) is the most common cause of viral keratitis (although it can also result from HSV-2). HSV-1 is the same virus that causes cold sores.
The cornea is typically infected by another part of the body that has the infection, such as the mouth. Although most cases heal without long-term effects to the cornea, serious cases can result in scarring.
Herpes zoster (chickenpox virus) can have a resurgence later in life. This can cause corneal lesions or chronic keratitis.
Risk factors, symptoms, and treatment:
Risk factors – Being born outside of the U.S., being female and being sexually active
Symptoms – Redness, pain, tearing, discharge, light sensitivity, blur and a gritty feeling
Treatment – Antiviral medications
Fungal keratitis and Acanthamoeba keratitis
Both fungal and Acanthamoeba keratitis are less common than bacterial or viral keratitis.
Fungal keratitis is caused by the Candida, Fusarium or Aspergillus species. If someone has had a recent injury, especially from some type of vegetation, they should watch for the development of keratitis symptoms.
A single-celled organism (amebae) causes Acanthamoeba keratitis. Although it is uncommon, it is very dangerous and can result in permanent vision loss. It is estimated that 85% of cases are contact lens wearers who did not handle the lenses properly or who showered or swam in them.
Risk factors for fungal keratitis and Acanthamoeba keratitis include:
Contact lenses that are not handled properly
Compromised immune system
Other underlying eye conditions
Recent eye trauma
The common symptoms of fungal keratitis and Acanthamoeba keratitis include:
Acanthamoeba keratitis is a serious condition that requires urgent medical or surgical treatment. Fungal keratitis can be treated with antifungal medicines or surgery.
Corneal disease due to trauma
The cornea can sometimes become damaged by accidental trauma from a scratch or fragment that embeds in the eye. Usually, the fragment can be removed and the abrasion treated with artificial tears or medicated drops. The goal after corneal trauma is to prevent infection and scars from forming. If a scratch or a foreign object penetrates the epithelium into Bowman’s layer, the risk of scarring is high.
READ MORE: How to prevent eye injuries
How to treat corneal disease
Since corneal disease encompasses such a large array of conditions, treatment is very specific to the cause and severity of the disease. Eye drops such as artificial tears and ointments can be used for comfort and relief. Medications such as antibiotics and antivirals can be used to treat corneal infections. Glasses and contact lenses can treat blurry vision and distortions that can result from corneal disease.
Treatments for more severe cases of corneal disease include:
Penetrating keratoplasty (PK) – This is a full-thickness cornea transplant. A diseased cornea is replaced with a healthy donor cornea.
Endothelial keratoplasty (EK) – This is a partial corneal transplant that does not need stitches.
Phototherapeutic keratectomy (PTK) – The outermost diseased layers are removed by a laser to allow healthy tissue to regrow.
How to prevent corneal disease
If you wear contact lenses, adhere to the wearing-time guidelines and the recommended cleaning regimen. Infections due to improper contact lens care and wear are a common cause of corneal disease.
Additional measures to prevent corneal disease are:
Using safety goggles when there is a risk of eye injury
Not smoking and limiting alcohol
Eating a healthy diet rich in Vitamin A and omega-3 fatty acids
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Keratoconus. American Academy of Ophthalmology. EyeWiki. April 2021.
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Page published on Wednesday, October 20, 2021