What is the Cornea?
The cornea is the protective, clear outer layer of the eye. Together with the white of the eye they serve as a barrier against germs, dirt, and other things that can cause damage to the eye. It also plays a key role in vision by refracting or bending the light that enters the eye.
The structure of the Cornea
The cornea has five layers, the main 3 layers are:
- Epithelium: It is the outermost layer and it prevents outside matter from entering the eye. It also absorbs nutrients and oxygen from tears.
- Stroma: It is the middle and thickest layer and it is found behind the epithelium. It is mostly made up of proteins and water which give it its elastic but solid form.
- Endothelium: This is a single layer of cells located at the very back of the stroma. It is between the stroma and the aqueous humor – the clear fluid in front of the chamber of the eye. It works as a pump, pulling out water as it is absorbed into the stroma. Without this function, the stroma would become waterlogged leading to the cornea becoming hazy and opaque as well as your vision.
Corneal disease is a term used to refer to a variety of diseases that affect the cornea. This can include degenerations, infections, and other disorders that may arise as a result of genetics. It can cause scarring, clouding, distortion, and eventually blindness.
Causes of Corneal Disease
Corneal damage can be caused by a variety of factors such as:
- Infections – fungal, bacterial, or viral
- Age – aging can affect the clarity and health of the cornea
- Contact lenses
- Eye trauma
- Intraocular eye surgery such as cataract implant surgery
- Certain eye diseases such as retinopathy of prematurity, retinitis pigmentosa, and vernal keratoconjunctivitis
- Systematic diseases such as Downs Syndrome
Symptoms of Corneal Disease
The cornea has the ability to heal itself after most types of injury or disease. However, in cases of deep injury to the cornea, the healing process can be prolonged resulting in a variety of symptoms such as:
- Blurred vision
- Corneal scarring
- Extreme sensitivity to light
Conditions that Can Damage the Cornea
This is inflammation of the cornea and can sometimes occur after bacteria, virus, or fungi infections. The microorganisms can enter the eye after a deep injury and cause inflammation, infection, and ulcers. It can also occur after an injury caused by contact lenses.
The symptoms of keratitis include:
- Blurred vision
- Severe pain
- Extreme sensitivity to light
Treatment for keratitis includes antifungal or antibacterial eye drops. Some patients may require steroid eye drops and antiviral drugs.
Also known as herpes of the eye, it is a viral infection of the eye that may continue recurring. It is mainly caused by the herpes simplex virus I (HSV I), the same one that results in sore colds. It can also be caused by sexually transmitted herpes simplex virus II (HSV II) which causes genital herpes.
This condition leads to sores on the cornea and over time the inflammation can spread deeper into the cornea and the eye.
No cure is currently available for ocular herpes, but it can be controlled with steroid eye drops and antiviral drugs.
Herpes Zoster (Shingles)
This is a recurrence of the chickenpox virus in individuals who already had the disease. After being healed from chickenpox, the virus usually stays in the nerves of the body, but it is inactive. It can travel down the nerves later in life and affect specific body parts like the eyes. A shingles rash on the face can lead to having sores on the cornea.
The shingles usually heal by themselves, but topical steroid eye drops and antiviral medication can ease inflammation.
Although anyone who got the chickenpox bout can get shingles, chances are higher for:
- People with a weakened immune system
- Older adults, over 80 years
There are several diseases that can cause problems with the structure of the cornea. The most common are:
This is a progressive disease that thins the cornea and changes its shape. It causes the cornea to steepen and become cone-shaped in the bottom part. It starts as a blurred vision in teenage years and worsens in early adulthood. Changes in the shape of the cornea can create mild to severe distortion, known as astigmatism and nearsightedness. This disease can also cause scars and swelling on the cornea, as well as vision loss.
Keratoconus can be caused by:
- Eye trauma due to extreme eye rubbing
- Genetics , Dawn syndrome
- Certain eye diseases such as retinopathy of prematurity, retinitis pigmentosa, and vernal conjunctivitis and allergic conditions such as eczema and hay fever
The condition is first corrected with soft contact lenses or glasses. However, as the disease progresses, the patient may need to wear rigid gas permeable contact lenses.
When caught early can be stabilized, a procedure known as corneal crosslinking can be performed. It involves an ophthalmologist instilling riboflavin eye drops and exposing the eyes to small amounts of UV light. The procedure can prevent keratoconus from worsening and the need for corneal surgery. Vison can also be improved with insertion of specific plastic ring in the cornea.
Keratoconus can also be treated with a cornea transplant, which involves replacing the damaged cornea with a donated one.
These are abnormalities that cause structural problems within the cornea and there are more than 20 of them. However, the most common are:
Map-dot-fingerprint dystrophy (Anterior Basement membrane dystrophy)
This affects the front part of the cornea , the epithelium and tends to grow irregularly – thick in some areas and thin in others. This results in irregularities in the cornea that resemble maps, dots, and small fingerprints.
Its usually painless, does not affect vision, and usually gets better without treatment. However, sometimes the epithelial layer can be worn down thus exposing the nerves that line the cornea causing severe pain especially when waking up. It can also alter the cornea’s normal curve causing nearsightedness, farsightedness, and astigmatism.
The vision may get blurry as the cornea changes and the patient may also notice:
- Moderate to severe pain
- Increased sensitivity to light
- A sensation of having something in the eye
- Excessive tearing
Treatment for map-dot-fingerprint dystrophy includes soft contact lens, an eye patch, ointments, or removing the loose layer.
This is a genetic condition that causes a slow breakdown of the endothelial cells and the swelling of the cornea. It leads in difficulty to drain water from the stroma which results in the swelling of the eye and the worsening of vision. Small blisters and haze may appear on the surface.
Symptoms usually appear when a person is in his/her 30s and 40s but it takes about 20 years for vision to be affected.
- Hypertonic Ointments or eye drops
- Drying the swollen cornea with a hair dryer
- Full or partial corneal transplant
This is caused by abnormal protein fibers in the stroma. Although it can occur at any age, early symptoms are seen in childhood. It makes the cornea cloudy and reduces vision. Over time, it can cause the epithelial layer to wear down.
- Eye drops
- Eye patches
- Corneal transplant
Diagnosing Corneal Disease
Diagnosing corneal disease involves the ophthalmologist examining the eyes with magnifying medical instruments. A slit lamp and advanced diagnostic technology such as corneal topography , OCT scan, Specular microscopy. All can be used to detect corneal scars, irrrgularity and endothelium counts.