The cornea is the eyes outer most layer. It is the clear, dome-shaped surface that covers the front of the eye. It is a highly organized group of cells and proteins and contains no blood vessels to nourish or protect it from infection. Instead, the cornea receives its nourishment from the tears and the aqueous humor (fluid inside the eye) that fills the chamber behind it. The cornea must remain clear to refract light properly. The tiniest blood vessel, cloudiness, or opaque area can interfere with vision.
When light strikes the cornea, it refracts (bends) the incoming light through the anterior chamber and pupil to the lens of the eye. For clear vision, light rays must be focused by the cornea and the lens to fall precisely on the retina. The retina converts the light rays into impulses that are sent through the optic nerve to the brain, which interprets them as images.
The cornea copes very well with minor injuries or abrasions. If the highly sensitive cornea is scratched, healthy cells quickly patch the injury before infection occurs. If the injury penetrates more deeply, the healing process will take longer. Deeper scratches can cause corneal scarring and/or hazing that can affect vision.
Some diseases and disorders of the cornea are:
Corneal infections – if the cornea is damaged from an abrasion, foreign body, or issue with contact lenses it can cause painful inflammation (keratitis) and infection. These infections can reduce visual clarity, produce discharge and possibly erode the cornea (corneal ulcer). Infections can lead to scarring and can impair vision.
Dry eye – the continuous production and drainage of tears is important to eye health. Tears keep the eye moist, help wounds heal and protect against infection. When the eye is dry it produces fewer or less quality tears and is unable to keep the surface lubricated and comfortable. The tear film consists of three layers including:
- Lipid layer – it keeps tears from evaporating too quickly and helps tears remain on the eye.
- Aqueous layer – it nourishes the cornea and conjunctiva.
- Mucin layer – it helps to spread the aqueous layer across the eye to ensure that the eye remains moist.
The symptoms of dry eye include:
- A scratchy or sandy feeling
- Foreign body sensation
- Blurry vision
Artificial tears are the principal treatment for dry eye and ointments can be used at night to help prevent the eye from drying. For some patients prescription eye drops can assist the eye in producing more of its own tears. Another option is having punctal plugs inserted in the tear drainage openings to keep more of the eyes’ natural tears on the surface.
Corneal dystrophies – they are conditions in which one or more parts of the cornea lose their normal clarity due to a build up of cloudy material. They affect vision in different ways that vary in the effect they have on vision and comfort. Some of the most common dystrophies include Fuchs’ dystrophy, keratoconus, lattice dystrophy, and map-dot-fingerprint dystrophy.
The conjunctiva is a protective membrane that lines the inside of the eyelids and covers the exposed sclera (white of the eye).
Conjunctivitis (pink eye) – this term describes a group of diseases that cause swelling, itching, burning, and redness of the conjunctiva. At its onset, conjunctivitis is usually painless and does not harm vision. In most cases the infection will clear without requiring medical care but for some forms, treatment is needed. It can be caused by a bacterial or viral infection, allergy, environmental irritants, or contact lens product. It can spread from one person to another quite easily. Hand hygiene is very important to prevent spreading it, but bath and bed linens need to be changed and not shared.
Pterygium – it is a pinkish, triangular-shaped growth on the white of the eye that extends onto the cornea. Some pterygia grow slowly throughout a person’s life, while others stop growing after a certain point. Pterygia are more common in sunny climates and in the 20-40 age group. Since many people who develop pterygia have spent a significant amount of time outdoors, it is believed that ultraviolet light from the sun is a leading cause. Artificial tears and prescription medications can manage the symptoms, but it does not cure the condition. They can become chronically red, inflamed, and/or threaten the cornea and in these situations it is reasonable to consider having it surgically removed.
Subconjunctival hemorrhage – it occurs when a tiny blood vessel breaks just beneath the clear layer on the surface of the eye. It is can be caused by coughing, sneezing, heavy lifting, rubbing the eye, and or any action that puts strain on the eye. The conjunctiva cannot absorb the blood quickly so the blood is trapped under the transparent surface and can take 10-14 days to clear. Patients taking aspirin or other blood thinners can be more at risk but the benefit of the taking these medications outweighs the risk of developing a conjunctival hemorrhage. Severe complications from a subconjunctival hemorrhage are rare. Artificial tears can soothe the eye if it feels scratchy or if it is bothersome.