A rigid gas permeable lens used to correct an irregular cornea [Keratoconus] where the only alternative would be a corneal transplant! [Relatively common in New Zealand. It's very satisfying to provide excellent vision to those who would otherwise suffer severely, sometimes being classified as legally blind if not for contact lenses!]

Compare this to the regular contact lens fitting depicted on the front page. A pool of tears can be seen just above the apex of the 'cone' as depicted by the bright green fluorescein, which is used to 'colour' the tears and make them visible.

Like many medical terms the name KERATOCONUS is a description of what the condition looks like. KERA - meaning the cornea, and CONUS - meaning cone-shaped giving the description of a cone shaped cornea.

At left is a graphic depiction of a normally shaped cornea.

At right is a graphic depiction of a conically shaped, Keratoconic cornea.

What Is It?

The cornea is the clear convex structure that forms the front of the eye. It is the principal refracting surface and is, in effect, the main lens of the eye and does the same job as the front curve on a camera lens. Because the cornea is transparent it is not seen when looking at the eye from the front but is clearly visible when looking at it from the side. In the normal eye the cornea appears rounded like part of a sphere and it focuses light, with help from the internal lens, clearly on the retina at the back of the eye.

Normally the cornea is nearly spherically shaped [in fact slightly elliptical] thus allowing light to be focused clearly on the back of the eye [retina]. However with Keratoconus the cornea begins to thin, and this allows the normal pressure of the eye to make the cornea bulge forward taking on a cone-shape.

What Causes It?

The exact cause is not known. However what is known is that it has acquired and inherited components. It is quite rare with the incidence being about 1 in 3000. Keratoconus is sometimes [but not always] associated with conditions such as allergies, asthma and eczema. There is a strong link to eye rubbing in many sufferers. Generally the onset is between the ages of 10 and 25 years.

What Happens As It Develops?

As the cornea gradually becomes more cone-shaped, the vision blurs and becomes distorted. Monocular diplopia [double vision in one eye] and ghost images are common symptoms. Initially vision may be correctable with spectacles, but as the condition develops, and the cornea becomes more distorted, then spectacles become less effective. Because of this, the majority of people with Keratoconus usually require RGP contact lenses. Although it is difficult to fit a contact lens to a Keratoconus cornea, most people with Keratoconus successfully wear contact lenses, which provide good vision. It is a good idea to consult a practitioner who specialises in Keratoconus fitting and management, for the optimum results.

In about 85% of cases the Keratoconus slowly stabilises by the age of 35. In the other 15% the condition progresses until contact lenses can no longer be used. For this group of people a corneal graft [transplant] is necessary. While there is a risk with this type of surgery [and thus it is not considered as a first option], the success rate for corneal grafts is extremely high, although in many cases a contact lens may still need to be worn.

Contact Lens Fitting for Keratoconus

In the early stages of Keratoconus, as the cornea thins and slowly bulges forward, the initial treatment can simply be with spectacles to correct the slight distortion of vision that is caused. However as the condition progresses, the vision with spectacles becomes less acceptable and contact lenses are the next step.

How Do Rigid Contact Lenses Help?

A Keratoconus contact lens is designed so that its front curve is spherical [similar in shape to the normal cornea] thus allowing less distorted vision. The reason a rigid lens must be used, is so that it can hold its shape, as a soft lens would simply mould to the existing shape and thus not allow complete correction of the problem.

The diagram below graphically shows the contact lens fitting goal.

The goal is to try to distribute the touch of the lens so that most of the weight is borne by the stronger peripheral cornea. Very slight clearance, or very light contact at the tip of the cone is the goal. This fitting is more difficult to attain than it sounds, as the delicate balance between the periphery and the cone is extremely complex. It is important to have a lens that is neither too tight nor too loose. For this reason several initial lenses may need to be tried before the ideal lens is determined.

Follow-up Visits and Refitting

Any excessive pressure of a poorly fitting lens on the cone apex can cause permanent scarring within months or years. [This scarring can also occur naturally]. For this reason it is important for regular follow-up visits to be made so that any corneal changes that have occurred can be compensated for in the design of a new lens. It is quite common for patients to be refitted at irregular intervals as the condition progresses. For this reason it is recommended that Keratoconic contact lens wearers maintain good health insurance to cover the cost of the lenses that are required. In New Zealand there is also a state subsidy that helps cover some of the fees and lens costs.

Generally refitting is required when the cornea has changed shape causing excessive pressure, however this may not mean that the vision has been made worse. In fact in some cases refitting may result in slightly reduced vision, however it must be done for the long term good of the cornea.

A Final Note

It should be emphasised that whilst contact lenses can improve the vision significantly, it may not be possible to improve the sight to that of the average person.

In about 85% of people with contact lenses for Keratoconus the condition stabilises by the age of 35 [although exceptions do occur]. In the other 15% of cases the condition progresses to the stage that tolerance to lenses, or vision deterioration, means that a corneal graft [transplant] is necessary.