Keratoconus is something of a misnomer, along with many other medical conditions. The name suggests a conical shape, but there are many variants, few of which resemble a cone. The cornea becomes thinner than normal and distends, a process referred to as ectasia. As a result, the front surface of the cornea is more protrusive than normal, and becomes irregular at the visual axis. So a more appropriate name for keratoconus is primary corneal ectasia, which describes the whole range of 'keratoconus' profiles. Some clinical publications use this expression.
Why not spectacles?
Spectacles may give a reasonable acuity in some mild cases of keratoconus, but if moderate or advanced, spectacles do not help much because of the irregular corneal surface. Even if a spectacle lens could be made to match the irregular surface, nothing can be done to affect the chaotic refraction at the corneal surface.
Contact lens management of keratoconus
However, most people with keratoconus can see reasonably well with contact lenses of some kind.
Rigid corneal lenses
The latest information suggests that just somewhere around 6 or 7% of new contact lens fittings are corneal lenses these days. Corneal defines the approximate size, ie fitting just on the cornea. Sadly, the corneal lens industry is in decline, but I believe there will always be contact lens companies who will continue to cater for keratoconus.
Rigid corneal contact lenses are the mainstream option for keratoconus. They offer functional vision because a fluid reservoir is trapped between the lens and the corneal surface. The refractive index is similar to that of the cornea so only a minimal amount of bending of rays of light (refraction) takes place at the corneal surface, thereby neutralising the irregular surface and astigmatism.
There may also be a contact zone which compresses slightly on the cornea, so the cornea takes up the regular shape of the back surface of the lens, further improving the refraction of light rays at the corneal surface. The front surface of the contact lens is regular and usually spherical, so in effect, an artificial cornea is created.
Rigid lenses are also sometimes called hard lenses, but this term is not in vogue now, or is used in reference to polymethyl methacrylate (PMMA). PMMA was the original material from which lenses were made in the 1950’s. Rigid gas permeable (RGP) materials have greatly improved corneal physiology and wearing tolerance over PMMA, accounting for the great majority of corneal lens fittings these days. PMMA, although transmitting no oxygen, still has a role from time to time: for example, some people who have worn PMMA for many years often prefer not to make the change. If so, and if there are no serious problems with PMMA, there is no pressing clinical need to switch to RGP..
Rigid gas permeable corneal lens on the eye, photo with blue light to excite the fluorescein stained tears showing areas of corneal contact and clearance.
Most people who wear lenses for conditions other than keratoconus, simple myopia, hyperopia and even regular astigmatism wear soft (hydrogel)
Hydrogel lenses which are used for regular myopia, such as daily disposables, are very thin; a piece of clingfilm is a term often used to describe them. They drape over the surface, so the front surface of the lens assumes the same irregular surface as the cornea without trapping a fluid reservoir, and with minimal compression of the corneal surface. The refractive index difference between air and the front surface of the lens is much as the difference between air and the cornea, so the effective refracting surface is not much of an improvement over the original corneal surface, limiting their usefulness in keratoconus. Sometimes a degree of myopia associated with low grade keratoconus may be partially corrected with a thin soft lens. If so, they are usually comfortable to wear, so there is no reason for not taking advantage if there is some visual gain. In more advanced cases, the shape of the cornea does not allow sufficient draping, so soft lenses designed for normal topography fold while on the cornea or fall out.
Keratoconus designed hydrogel lenses
Some hydrogel lenses have been designed specially for keratoconus in recent years. They are thicker than regular soft lenses so they retain a rigid shape to some extent. For this reason they compress the central cornea and partially trap a tear pool, as with RGP corneal lenses. In effect, they are rigid lenses made from hydrogel materials, and they give a reasonable visual result with some types of keratoconus. When they do perform well, they are stable on the eye, and they often position themselves well in the corneal centre.
There is often some residual regular astigmatism, that is, when not fully corrected by a contact lens on its own. Astigmatic spectacle lenses are surfaced or moulded with two different optical powers at right angles to eachother. The spectacle prescription defines the axis at which the lens has to be orientated, and a spectacle frame keeps the lens in that position. It is possible to work an astigmatic component onto a contact lens, but for it to function, a means of stabilisation on the eye is necessary. There would be more likelihood of residual astigmatism with hydrogel than with RGP lenses, but the stabilisation on the eye is more likely to be successful with hydrogel than with RGP.
The astigmatism recorded during a refraction assessment, ie a sight test for spectacles, does not always convert fully to a soft lens correction for astigmatism, because the flexure on the eye may introduce unexpected curvature changes. But there have been good technical developments in recent years, and these are still ongoing. If the astigmatic correction does not come up as well as predicted from the refraction, a soft lens may still be an worthwhile way forward, but with the addition of a spectacle lens to ‘top up’ the contact lens correction.
Combination rigid / hydrogels
Two possibilities exist.
'Piggy-back' is a rigid corneal lens fitted on top of a hydrogel. It is worth trying if discomfort from corneal lens is due primarily to corneal contact rather than lid sensation. Piggy-backing may also be useful as a temporising measure to help lens wearers through difficult times, or to extend lens wear later in the day. Some people gain a major relief from persistent corneal erosions associated with RGP lens wear.
RGP / hydrogel fused lenses are also available, and may be applicable if the discomfort is more due to lid sensation. The oxygen permeability is on the low side compared to usual RGP corneal lenses, but fusing the two polymers is a notable manufacturing achievement. There may be some problems arising from corneal hypoxia, but with careful monitoring, it has been shown to be a successful option and is the lens of choice for many people with keratoconus who struggle with RGP corneals.
The name suggests, correctly, that the lens fits on the white sclerotic coat of the eye. Sclerals were the first contact lenses developed in 1888 and were the only lens available until the mid 1950s. From the mid 1940s PMMA replaced glass. Most of the fittings for keratoconus was by taking an eye impression from which a tailor made lens was fabricated. Those were the days.
The size, as much as 23-25mm, may be off-putting for some people, but sclerals have many advantages. They are surprisingly comfortable even for the unadapted eye because although there is a feeling of bulk, there is not contact between the edge of the lens and the sensitive eyelid margin. They do not fall out and foreign bodies do not lodge behind them during wear. Rigid gas permeable (RGP) materials have transformed scleral lens fitting from a procedure perceived as cumbersome and outmoded into a readily available option when appropriate. RGP materials have enabled relatively straightforward fitting procedures are in most instances with a much reduced need to resort to eye impressions.
RGP sclerals are optimally fitted with full corneal clearance, thereby having the potential to also alleviate contact lens sensation which is caused by corneal contact. The development has also allowed the application of scleral lenses for much lower levels of pathology than were previously possible, so they can be considered for low, moderate or high grade keratoconus. An increasing use of scleral lenses recently has been by way of enhancing rigid corneal lens wear. Quite a few people alternate between the two types, thereby gaining respite from the lid sensation or the effects of corneal contact with regular RGP lenses.
This has described a brief outline of lenses available. There is a degree of healthy disagreement among practitioners, and there is no reason why you should not input into the discussion. There is one point about which most practitioners are as one: it is difficult at times to keep motivated to wear lenses, but keep trying with lens wear if you can.