Keratoconus

Symptoms of Keratoconus

• Reduced vision
• Frequent changes of glasses as the prescription increases
• In advanced keratoconus, the cone of the cornea pressing the eyelid in when looking down

Causes of Keratoconus

• Raised levels of enzymes that break down corneal tissue
• More cell death (apoptosis) in corneas with keratoconus
• A smaller number of collagen layers in the cornea
• Hormones: the fact that it begins in adolescence and becomes active during pregnancy supports a role for hormones in keratoconus.
• Certain diseases: keratoconus is seen more often in people with the following conditions:
• Down syndrome
• Ehlers–Danlos syndrome
• Marfan syndrome
• Hay fever
• Atopic dermatitis
• Asthma
• Vernal conjunctivitis (an eye allergy)
• Retinitis pigmentosa (night blindness)
• Frequent eye rubbing: about 70% of patients with keratoconus report having rubbed their eyes often in the past. Rubbing causes microtrauma.

The Role of Heredity in Keratoconus

Even if the parents and siblings of a person with keratoconus do not have the disease, there are abnormalities in their corneal maps. A person whose first-degree relatives (parents, siblings) have keratoconus is 70 times more likely to develop it than someone without keratoconus in the family. Even so, the total risk for such a person is less than 1 in 20.

Diagnosis

• Measuring visual acuity: as keratoconus progresses, vision decreases.
• Determining the prescription: there is usually high myopia and irregular astigmatism.
• Biomicroscopy: the corneal tissue is examined with an instrument called a biomicroscope (slit lamp), which shows the cornea bulging forward into a cone.
• Pachymetry: corneal thickness is measured with pachymetry. In keratoconus the cornea is thin; normal thickness is 540 microns.
• Corneal topography: mapping of the cornea is called “corneal topography”. The cone in a keratoconic cornea appears on the map as a “red” elevation.

Course of the Disease

Eye examinations at regular intervals give an idea of how fast the disease is progressing. In the past, 20% of patients with keratoconus underwent a corneal transplant. Today, thanks to advanced contact lens designs, cross-linking and intracorneal ring treatment, the need for transplantation has decreased.

Treatment

A step-by-step treatment approach is used in keratoconus.

1. Glasses

Glasses are tried first. In the early stages of the disease, glasses give good vision; however, the prescription changes frequently.

2. Contact Lenses

Various types of contact lens are used in keratoconus:

• Soft lenses: soft lenses are used only in the early stage of the disease, when adequate vision can also be achieved with glasses. Some soft lenses correct both myopia and astigmatism up to a certain degree (3.25 D).
• Rigid lenses: in the advanced stages of the disease, soft lenses are no longer enough to give good vision, because the cornea has become very cone-shaped, its shape is distorted and, as a result, the astigmatism has become irregular. A rigid lens creates a new refracting surface over the cornea. Because it is rigid, it does not take on the shape of the cornea as a soft lens does; it keeps its own shape, and this improves vision. Rigid lenses are not as comfortable as soft lenses. Outdoors in windy weather, dust gets under them, and sometimes the lens can pop out of the eye. Because of complaints such as stinging and a foreign-body sensation, patients with keratoconus may be unable to wear these lenses. Sometimes, in very advanced keratoconus, no suitable lens fitting can be achieved at all.
• Hybrid lenses: hybrid lenses are made of rigid lens material in the centre and soft lens material around the edge. Their advantage is that they combine the visual quality of rigid lenses with the comfort of soft lenses. Their disadvantages are that they cannot be used in advanced cases and are made only in a limited range of curvatures (sizes).
• “Piggy-back” lens fitting (a rigid lens on top of a soft lens): this is not commonly used. It is used in advanced keratoconus in patients who cannot tolerate rigid lenses, or in whom rigid lenses mechanically damage the outermost layer of the cornea. First a thin soft lens is placed on the cornea, and a rigid lens is placed on top of it. The disadvantages of this technique are the difficulty of handling two separate lenses and the cost.

3. Cross-Linking Treatment

Cross-linking has been used since 1998. Its aim is to make the cornea more resistant by strengthening the bonds between the collagen fibres. As the cornea becomes stronger, its forward bulging stops or slows greatly. In about 50% of patients treated with cross-linking, the cornea flattens by 2–3 dioptres.

Cross-linking is performed in the operating room with anaesthetic drops. After the epithelium, the outermost layer of the cornea, has been removed, vitamin B2 (riboflavin) is dropped into the eye, and ultraviolet A light is directed at the eye for 30 minutes.

4. Intracorneal Ring Treatment (INTACS)

Intracorneal rings are transparent half-rings made of a material called PMMA. The results of intracorneal ring treatment in keratoconus were first reported in the USA in 2002.

Intracorneal rings are used in keratoconus in the following situations:

• Mild to moderate keratoconus (corneal steepness should be limited to 55–57 D)
• A clear cornea (there should be no scarring)
• People who cannot wear contact lenses
• A corneal thickness of at least 450 microns where the INTACS will be placed
• People whose keratoconus has stopped progressing or is progressing very slowly

Intracorneal rings are placed under drop anaesthesia or general anaesthesia. The channels in which the rings will sit can be created mechanically or with a femtosecond laser. The laser technique is easier for the surgeon, but there is no difference in the reduction of corneal steepness or the improvement in visual acuity between rings placed with laser and those placed mechanically.

Intracorneal rings do not stop the progression of keratoconus. After rings are placed in an eye with keratoconus, one should not expect the eye to be fully corrected or to see perfectly without glasses. After INTACS the steepness of the cornea decreases, the prescription falls, and vision with and without glasses improves — but this never means the prescription will fall to exactly zero. A contact lens can be worn on an eye with INTACS if desired.

4. Corneal Transplantation

Corneal transplantation is the last treatment option in keratoconus. In keratoconus, a corneal transplant is performed as either “full-thickness” or “lamellar (partial-thickness)”.

It is used as a last resort in people who have scarring of the cornea, who cannot see well with contact lenses (vision below 20%), who cannot wear contact lenses, and whose eyes are not suitable for intracorneal ring treatment.