What is Keratoconus
Keratoconus is an inherited condition known as dystrophy, that usually first becomes apparent between the ages of 10 and 25 years. It is a recessive condition requiring genetic factors from both parents, so the chance of it occurring in your children is extremely slight. However, if you have the condition it is imperative that your children be checked regularly from the age of 8 years. The incidence is about 1 in 8,000 generally, but in some groups (South sea Islanders for example) the incidence can reach 1 in 2,000. Keratoconus is sometimes, but not always, associated with conditions such as allergies, infantile eczema and asthma, reduced night vision and a maternal age over 28. On the credit side, some 60% of people with keratoconus go on to tertiary education compared with 12% of the population as a whole. Keratoconus does not cause blindness but, if untreated, it can lead to a significant loss of vision.
Keratoconus (literally conical cornea) is a thinning of the central zone of the cornea, the front surface of the eye. As this happens, the normal pressure within the eye makes the cornea bulge forward to a microscopic degree. The initial effect is to induce a myopic or shortsighted error and indeed, early keratoconus is often not diagnosed for this reason. In early or mild cases, quite reasonable vision can be obtained with spectacles alone. In most cases however, the condition deteriorates slightly, the cornea becomes more distorted and spectacles become increasingly less effective. Consequently, for the majority of those with keratoconus, correction of vision is by means of contact lenses.
The contact lens fitting concept is to match the shape of the periphery of the lens to that of the stronger, peripheral cornea and then to vault across the top of the thinner conical area, aiming to make sure that the lens does not come into contact with the top of the cone. Great care, skill, and experience is required to fit a lens that is neither too tight nor too loose in the peripheral bearing area, and to avoid excessive pressure on the cone apex. Excessive pressure on the cone apex will cause permanent scarring within a few months. For this reason, follow up visits are advisable at least every six months, but sooner if discomfort or visual problems occur. Refitting or lens modification must always be undertaken if advised by your practitioner or if discomfort, intolerance, or loss of vision occurs. In some instances, refitting can actually reduce vision slightly. This is because the cone has moved forward and is contacting the back of the lens and is then moulded into a more regular shape. Reducing pressure on the cone apex allows the cornea to return to a more irregular shape again. Nevertheless, failure to eliminate pressure on the cone will lead to irreversible scarring and visual deterioration.
It should be emphasised that whilst contact lenses can very significantly improve your vision, it may not always be possible to improve it to the level of a normal sighted person. In particular, problems may occur in situations of glare and poor lighting due to the fact that there is still some refraction at the irregular corneal surface beneath the contact lens.
Keratoconus contact lens wearers are strongly advised to maintain extras private health care cover since frequent lens replacements are often necessary. In some instances, the Health Insurance Company may require evidence of you special need for more than average lens replacement. Because of the number of lens shapes for the management of keratoconus, several lenses may have to be supplied to optimise the initial fitting. For this reason the lenses are ordered with a six months warrantee to cover these early fitting changes.
Keratoconus patients are advised to have spectacles for emergency use. However it should be emphasized that typically these will provide only 25-50% of the vision achieved with the contact lenses. In approximately 85% of keratoconus the condition gradually stabilizes by the age of 35, although exceptions are possible. In the remaining 15%, the condition progresses and tolerance of the contact lenses and vision continue to deteriorate. For this group, a corneal graft is necessary.
Corneal Graft (Penetrating Keratoplasty).
The success rate of corneal grafting is extremely high. Nevertheless, surgery should not be contemplated lightly and not until all other avenues have been explored. The average stay in hospital is normally one day followed by a week at home. Heavy or sudden lifting should be avoided for the first few weeks, as should all contact or active sports. Careful follow-up is necessary for several months and you should always report back to your surgeon when advised. If a minor rejection episode is suspected, that is, if the eye is red or painful and/or you experience sudden visual deterioration, you should report back immediately. These episodes can normally be controlled very easily with modern drugs and the symptoms generally disappear Vision is usually blurred immediately after the operation but improves over the first few weeks. Spectacles can sometimes be prescribed to help after the first month or two but this may be impractical if the eye is continually changing as the cornea heals. Optimum vision should be expected at one month to six weeks after the stitches are removed, which is sometimes as soon as six months after surgery but usually one or two years afterwards. Approximately two thirds of graft cases require spectacles for optimum vision and one third require contact lenses (soft lenses can often now be prescribed.)
Corneal Crosslinking with Riboflavin.
This procedure is the first treatment that can stop or slow down the progression of Keratoconus.
If you have progressive keratoconus, then it is highly likely that you will require refitting of your contact lenses on a regular basis. However, the great benefit of corneal cross-linking is that the progression can be halted or slowed down to such an extent that the need to change contact lenses on a regular basis is no longer required. This can significantly reduce the associated costs of contact lens wear for people with keratoconus.
The procedure is performed by a Corneal Specialist. A local anaesthetic is applied to the surface of the eye and the surface layer of skin removed. Riboflavin (Vitamin B2) is then applied to the cornea for 30 minutes and then exposed to Ultraviolet A light. This causes the riboflavin to “crosslink” or stabilize the corneal cells that cause keratoconus, and over time, results in a cessation of progression and in some cases, a mild reversal of the condition.
If the results of your initial examination indicate that you have progressive keratoconus, then you will be referred to a Corneal Specialist to discuss the pros and cons of corneal crosslinking.
Contact lens options for Keratoconus.
There are four types of contact lenses that can be used to enable vision correction for keratoconus.
Soft lenses mold themselves to the shape of the front surface of the eye, and are therefore not suitable for most patients with keratoconus unless it is extremely mild and good vision is still possible with spectacles. If the keratoconus is more advanced, then thick soft contact lenses (Kerasoft) may be useful for those patients with extremely sensitive eyes that cannot tolerate rigid corneal lenses. However, the relative thickness of these lenses reduces the amount of oxygen that can travel through the lens and supply the cornea with the amount it requires for normal functioning. This leads to corneal oedema or swelling and reduced wearing times. Also, the quality of vision is not as good as that from a rigid lens.
Rigid Gas Permeable (RGP) Corneal lenses.
As the name suggests, the lenses are made of material that allows oxygen to pass through the material direct to the cornea. The cornea is the only part of the body that gets its oxygen directly from the atmosphere.
They have been the “gold standard” for keratoconus fitting for over 60 years, due to the high quality of vision they can supply.
They are small (8.50mm to 10.50mm) diameter lenses that rest directly on the cornea itself. These lenses require a high degree of technology (corneal topography) and skill in fitting, as it is imperative that the centre of the lens does not touch the centre of the cornea as this will lead to scarring and a reduction of vision. The fitting process itself can be time consuming, as the lens itself can alter the shape of the cornea and require re-designing after a few weeks. These remakes are always covered by a warranty for the first 6 months.
The lenses are uncomfortable initially, but adaptation usually takes 2 weeks. The common problems associated with small diameter rigid lenses are:
- They can dislodge and move off to the white of the eye.
- Dust can get caught under them and cause discomfort.
- They are small and can easily be accidentally washed down the sink.
- Some patients simply cannot adapt to them due to discomfort.
This is a combination of a rigid gas permeable lens centrally surrounded with a soft “skirt”, combining the clear vision created by a rigid lens with the comfort of a soft lens. They also require the same level of skill in fitting as rigid lenses. However, due to the nature of the material, the life span of the lens is not as good as a rigid lens. Hybrid lenses can be the ideal solution for people with sensitive eyes that cannot adapt to the smaller diameter rigid lenses.
Mini-Scleral lenses have been the greatest single development in contact lens correction for people with keratoconus in the last 20 years.
They are large diameter lenses that are designed to rest on the white of the eye (the sclera) and totally vault or clear the cornea. The sclera is not as sensitive as the cornea, so these lenses are extremely comfortable to wear. The advantages are:
- They cannot fall out, even when swimming.
- They cannot get dust or dirt under them.
- They cannot dislodge.
- They can be designed to correct for residual astigmatism more easily than RGP lenses.
- They are too big to wash down the sink, so the loss rate is greatly reduced.
- The useful life span is greater because they are fitted with much greater clearance than a normal RGP lens.
Mini-Scleral lenses are the ideal correction for those who work in industrial or rural environments as it is impossible for dust and other particles to get under the lens and cause discomfort. They will not dislodge under any working conditions.
However, they do have the following drawbacks:
- The insertion and removal is more complex.
- The initial cost is greater.