There are several eye conditions which cause the cornea to have an irregular or unusual shape and this sometimes means that contact lenses may be the only way of providing a good level of vision. Fitting these patients is one of the most professionally challenging and rewarding things I do.
The main causes of irregular corneal shape are corneal thinning (or ectasia) from
1) Keratoconus where the central thickness of the cornea a reduced, causing the cornea to bulge out in the shape of a cone.
2) Problematic or unsuccessful corneal surgery to correct long or short sight. You may remember that Russian ship that came to England around twenty years ago. The surgical procedure aimed to correct peoples vision by making a series of radial cuts in their corneas. Some patients did respond well to this butchery otherwise know as radial keratotomy (RK) however many patients are still needing help or treatment today.
3) Marginal pelucid degeneration where only the inferior of the cornea thins giving rise to high against the rule astigmatism where the cornea become rugby ball shaped with the ball standing vertically.
The final common cause of irregular cornea occurs after a corneal graft.
Kerotoconus accounts for at least 60% of challenging contact lens fits and the type of lens indicated is dictated by the degree of progression of the condition. With progression the cornea thins and bulges out in an irregular fashion part driven by intra occular pressure forcing the cone out and part by gravity forcing the cone down.The corneal curvature changes assymetrically towards the centre causing distortion in all directions which cannot be corrected with spectacles. This is known as irregular astigmatism.
Types of possible visual correction in order of simplicity:
Spectacles.
Where the astigmatism is still fairly regular and not too high spectacles are fine.
Soft Contact Lenses
These mold to the surface of the cornea, they correct quite high prescriptions and regular astigmatism up to about 3 dioptres with non custom made lenses. Custom made start getting very pricey and and are not very durable. Because they mold to the corneas shape, they will not correct significant irregular astigmatism and are so only indicated in the early stages of keratoconus progression.
Standard Gas Pearmeable Rigid (hard) Contact Lenses
These rest on the surface of the cornea on a film of tears, effectively replacing the front surface of the cornea with the regular front surface of the contact lens as the chief optical surface. These will correct very high prescriptions and high degrees of astigmatism both regular and irregular, since the tears fill in any irregularity. Unfortunately standard lens types only work on relatatively low degrees of irregularity as there comes a point where these lenses wont fit with stability and the lens rocks on the cone rubbing it and causing discomfort and irritation.
Speciality Gas Permeable Contact Lenses
These are generally smaller than the standard 9.2 mm diameter lenses and have specially made surface curves to accommodate an irregular surface. These lenses are durable and will usually give good stable vision, although fitting them may be quite challenging for the practitioner.
Combination Gas Permeable Lenses with a Soft Periphery
These are hybrid lenses effectively a gas permeable corneal lens with a soft skirt around the edge.
These have the advantage of comfort and optical stability and the disadvantage of cost and durability.
Semi Scleral Lenses
These are quite new on the market and are large gas permeable contact lenses which completely vault the cornea so that there is no corneal touch and they rest on an area about 3 mm wide beyond the cornea on the sclera ( the white of the eye ).The sclera itself is much less sensitve and tougher than the cornea and so these lenses can be very comfortable. Again these lenses are challenging to fit and their size makes them more difficult to insert and remove by the patient. I tried these my-self recently- they are very comfortable, very durable and quite expensive.
Scleral Lenses
These again are gas permeable hard lenses like semi sclerals, but much bigger, up to 2 cm across, difficult to fit, big to insert and are mostly the domain of hospital clinics.
Prices
Where standard lenses will work then prices are as our prices.
With the more difficult fits then the time component of fitting and after care becomes significant so it’s difficult to give general quotes.
As a guide, complex lens fitting fee could be around £200 as modifications, tuition, aftercare etc can take up several hours of clinic time, the lenses themselves vary from £370 for corneal lenses to £550 for semi-sclerals. Expensive I know, but compared to around £3000 for a dental implant its not so bad and we do have care packages.
