What is the Cornea?
The cornea is the eyes transparent outer layer covering the front part of the eye. It helps shield the rest of the eye from germs, dust and other harmful matter. The cornea shares this protective task with the eyelid, eye socket, tears and the sclera. It contains no blood vessels to protect against infection however it receives nourishment from the aqueous humour and tears. The cornea must remain transparent to refract light properly as even the tiniest of blood vessels across the front of the eye will interfere with this process. It controls and focuses the entry of light into the eye and contributes between 65% - 75% of the eye's total focusing power. There are five basic layers of the cornea known as the Epithelium, Bowman's Layer, Stroma, Descemet's Membrane and the Endothelium. Each of these layers has an important function and must be free of any cloudy areas to allow adequate light to enter the eye.

1. Epithelium
This is the outermost layer and makes up approximately 10% of the cornea thickness.  The primary function is to block any foreign material entering the eye, such as dust and bacteria and providing a smooth surface to absorb oxygen and provide cell nutrients to the rest of the eye. As the epithelium is made up of thousands of tiny nerve endings they make the cornea extremely sensitive to pain when rubbed or scratched.

2. Bowman's Layer
Is comprised of strong layered protein fibres called collagen. If injured the Bowman's Layer can form a scar as it heals. If these scars are large and centrally located, some vision loss can occur.

3. Stroma
Beneath Bowman's layer is the stroma, which comprises about 90% of the cornea's thickness and primarily consists of 78% water,16% collagen and does not contain any blood vessels. The collagen gives the cornea its strength, elasticity and form. The collagens unique shape, arrangement and spacing are essential in producing the corneas light-conducting transparency.

4. Descemet's Membrane
Located under the stroma and is the protective barrier against infection and injuries. It is composed of different collagen fibres than those located in the stroma that are readily regenerated after injury.

5. Endothelium
It is the innermost layer of the cornea that is extremely thin. Endothelial cells are essential in keeping the cornea clear. Fluid will naturally leak from the inside of the eye into the middle corneal layer (stroma). The endothelium's primary task is to pump this excess fluid out of the stroma. Without this pumping action the stroma would swell with water, become hazy and ultimately opaque. In a healthy eye, a perfect balance is maintained between the fluid moving into the cornea and the fluid being pumped out of the cornea. Once the endothelial cells are destroyed by disease or trauma, they are lost forever. If too many cells are destroyed, corneal oedema and blindness occurs. Corneal transplantation is the only available therapy. Refractive Errors occur when the curve of the cornea is irregularly shaped (too steep or too flat). When the cornea is of normal shape and curvature it bends or refracts light on the retina with precision, however when irregularly shaped, the cornea bends light imperfectly onto the retina.

Infections of the Cornea

  • Microbial infections (Keratitis)

In the event of a foreign body penetrating the tissue, bacteria or fungi can pass into the cornea causing deep infection and inflammation. This may cause severe pain, reduced visual clarity, produce a corneal discharge and in some cases erode the cornea. In most cases the deeper the corneal infection the more severe the symptoms and complications are. Microbial infections are relatively infrequent however it is the most serious complication of contact lens wearers.

  • Conjunctivitis "Pink Eye"

Describes a group of highly contagious inflammatory diseases of the conjunctiva (the protective membrane lining the eyelids and covers exposed areas of the sclera).  These diseases can be caused by a bacterial or viral infection, drug allergy, environmental irritants or contact lens products. It is usually painless and does not affect the vision in mild cases. However in more severe cases of 'Pink Eye', such as epidemic keratoconjunctivitis, immediate treatment is required, that if delayed, the vision may worsen and cause corneal inflammation and a loss of vision.

  • Ocular Herpes:

Ocular herpes is caused by the same Herpes Simplex Virus1 (HSV1) responsible for cold sores. It produces a painful sore on the surface of the cornea and prompt treatment stops the virus spreading and destroying epithelial cells. If left untreated the infection may spread to the stroma, causing the body's immune system to attack and destroy stroma cells. Like other herpes infections, herpes of the eye remains a controllable, but incurable problem.

  • Herpes Zoster (shingles):

Produced by varicella-zoster virus; the same virus that causes chicken pox. It will remain dormant within the nerve cells of the central nervous system. In some patients it may reactivate later in life. The virus may infect the cornea if it travels to the head, neck, around the eyes, part of the nose, mouth, cheek and forehead. The virus may affect the cells deep within the tissue causing inflammation and scarring of the cornea. Unlike herpes simplex 1, the varicella-zoster virus does not usually flare up in adults with a normally functioning immune system.

Corneal Dystrophies
There are over 20 corneal dystrophies that affect all parts of the cornea. Some of the most common are:

  • Keratoconus
  • Map-Dot-Fingerprint Dystrophy
  • Fuchs' Dystrophy
  • Lattice Dystrophy

Keratoconus is a degenerative disorder of the eye in which structural changes within the cornea cause it to reduce in thickness and change shape to have a more conical appearance than its normal gradual curve. Patients often notice a distortion of vision with multiple images, streaking and sensitivity to light. Typically diagnosed in the adolescent years however becomes more aggressive in a patient in their 20's – 30's. Further progression may require surgery. However despite the diseases unpredictable course, Keratoconus can be successfully managed with little to no impairment to the patient's quality of life. The exact cause of Keratoconus is uncertain but it has been associated with detrimental enzyme activity within the cornea. The progression of Keratoconus is rapid in patients with Down syndrome. 

Map-Dot-Fingerprint Dystrophy is also known as epithelial basement membrane dystrophy and is the most common form of corneal dystrophy. During an eye examination structures can be viewed on the cornea resembling contour lines on a map. Less frequently concentric circles are seen resembling a target board; circles within a larger circle.  It occurs when the epitheliums basement membrane develops abnormally (epithelial cells anchor themselves to this allowing them to absorb nutrients from tears). The cells are unable to adhere correctly causing recurrent epithelial erosions. These erosions cause the outermost  layer of the cornea (epithelium) to swell slightly causing it to separate from the other layers of the cornea and exposing a small gap. Map-dot-fingerprint dystrophy tends to be observed in both eyes and usually affects adults between the ages of 40-70 years; although it may develop in childhood if inherited from the parents. Map-dot-fingerprint dystrophy is usually painless and causes no vision loss, and may clear without treatment.

Fuchs' Dystrophy is a slow, progressive disease that usually develops in both eyes and is observed to be slightly more common in women. Signs can be observed in a patient in their 30's however the disease rarely affects vision until a patient reaches 50 – 60 years. Fuchs' dystrophy is characterised by the deterioration of the cells inside the stroma. As more cells are lost the endothelium becomes less efficient at pumping water out of the stroma causing the cornea to swell and distort vision. This swelling is very painful and causes severe visual impairment as tiny blisters form on the cornea and rupture causing severe pain. Patients may also have the sensation of a foreign body within the eye and become abnormally sensitive to light and see a glare or halo around lights.

Lattice Corneal Dystrophy is a common form of stromal dystrophy. It is a result of abnormal protein fibres forming through the middle and anterior section of the stroma. These deposits in the early stage resemble microscopic cross-cross patterns. As they become more advanced they increase in size they become opaque and involve more of the stroma gradually giving the cornea a cloudy appearance that results in reduced vision. Although Lattice Dystrophy can occur at any stage in a patient's life, the condition usually arises in children between the ages of 2 – 7 years.

Corneal Transplant
A corneal transplant is a surgical procedure which replaces the central section of an unhealthy cornea with a similar shaped healthy donor cornea. Only human donor tissue can be used in corneal transplantation surgery.  In Australia, nearly 2000 corneal transplants are performed each year. Apart from blood transfusion, corneal transplants are the oldest and most common form of human transplantation.