What is keratoconus?
Keratoconus is a disease of the central zone of the cornea, the front surface of the eye.
Keratoconus is progressive and almost always occurs in both eyes; however one eye is usually more advanced than the other. Keratoconus is characterized by thinning of the stroma (which constitutes 90% of the corneal thickness).
As a result of this thinning, the regular shape of the cornea is distorted and a cone-like bulge develops, usually inferiorly and increasing in area as the keratoconus advances. As the corneal irregularity increases, it reduces the quality of vision by creating multiple images and distortions. Patients report poor quality vision with multiple images, shadowing and ghosting and flaring, especially at night. This can affect all ranges of vision; driving – especially at night and prolonged computer work and reading.
As one eye is more advanced than the other, often the images from each eye will be quite different and difficult for the brain to integrate. A normal cornea will give a precise image like a magnifying glass focusing at a pinpoint, however the image from a keratoconic cornea will be more like the tail of a comet, with no one point precisely in focus.
Vision corrections for keratoconus
In early keratoconus there is only slight distortion of the cornea and spectacles are usually able to provide good quality vision. These may be required on a part-time or regular basis and regular monitoring is recommended to ensure that the correction is current. If the patient would rather not wear spectacles, soft contact lenses can be another option for regular use or just for social or sporting activities. Soft lenses are usually disposable lenses often incorporating a correction for astigmatism. These lenses usually give good vision, are very comfortable, stable and easy to handle. Disposable lenses need to be replaced on a regular basis and usually only need one solution to store and disinfect the lenses. Your optometrist will advise if spectacles and disposable soft lenses are an option for you.
As keratoconus progresses, the corneal irregularity increases and consequently the vision quality with spectacles or soft lenses decreases. Placing a smooth rigid surface over the cornea allows the tears to fill in the space between the rigid surface and the corneal irregularity and neutralise 90% of the corneal distortion. The substantial reduction in image distortion gives better definition and contrast, reduced ghosting and consequently much clearer vision.
Rigid Gas Permeable Lenses for keratoconus
Rigid gas permeable (RGP) contact lenses are the most common form of contact lenses used to improve vision in keratoconus. Most patients can wear RGP lenses all day wear with good comfort, excellent vision, good stability and clear eyes. A keratoconic cornea has an irregular shape and a “perfect fitting” is often impossible to achieve and as the cornea is the most sensitive part of the body, RGP lenses can initially be uncomfortable.
It may require a 1 or 2 visits to arrive at an initial contact lens design that can be ordered. Even with an experienced contact lens expert it can take numerous visits to alter the RGP lens fitting to an acceptable level.
We give a commitment to providing the best quality contact lens fittings for patients with keratoconus and will strive to ensure that we deliver the best outcome. Unfortunately despite the practitioners and patients best efforts, RGP lenses may be unsuccessful.
Larger Diameter Rigid Gas Permeable Lenses for keratoconus
Advanced computer programs and digtal lathes have facilitated the development of complicated larger diameter RGP lens designs. These lenses have the advantage of improved comfort, better centration and a larger area of vision. These designs are at the forefront of RGP contact lens technology. These lenses require the same maintenance and handling as standard RGP lenses, but occasionally can be more difficult to insert and remove due to the larger diameter.
Miniscleral Lenses for keratoconus
Miniscleral lenses are also manufactured in a rigid gas permeable material but fit the eye in quite a different way. These lenses are much larger and vault the whole cornea, resting on the white part of the eye. Miniscleral lenses rest behind the upper and lower eyelids and as the lids are not hitting the edges of the lens, the lenses are very comfortable. The lenses totally seal on the white part of the eye and cannot be dislodged making them ideal for dusty environments and for active sports where conventional RGP lenses may be dislodged. Miniscleral lenses can be more difficult to insert as they need to be inserted full of solution and more difficult to remove due to the large diameter.
Hybrid Lenses for keratoconus
Hybrid lenses (SynergEyes) have a hard RGP centre with soft plastic on the periphery. The transition from the RGP centre to the soft periphery is seamless and provides a smooth surface for the eyelids, giving excellent comfort. The RGP centre gives excellent vision and the soft periphery gives excellent centration. The RGP centre vaults the cornea minimising irritation and the soft periphery wraps onto the white part of the eye ensuring stability and preventing irritation from foreign bodies. Hybrid lenses require a simple peroxide disinfection system. Hybrid lenses can be more difficult to insert as they need to be inserted full of solution and more difficult to remove due to the adhesion of the soft periphery.