Continuing Education

Keratoconus is a primary corneal ectasia, meaning the cornea is thin and distended. It is usually bilateral but may be asymmetric and, in some cases, it is virtually undetectable in one eye.

The aetiology is not clearly defined but there is a genetic element and several candidate genes have been identified. Other possible associations include history of atopy such as eczema, eye rubbing (although this may be secondary to atopy) and Down’s syndrome.²

DETECTION OF KERATOCONUS

Early presentation includes poor vision at both distance and near, which can be distinguished from simple myopia in pre-presbyopes, who complain of poor distance vision. Patients also often complain of distortion and ghosting. Progressing astigmatism is a sign of keratoconus, if not secondary to cataract, and can be distinguished from non-pathological astigmatism, which typically occurs at a younger age.

Other signs include distorted irregular keratometry mires (where the principal meridians are not orthogonal) prominent corneal nerves,³ Vogt’s striae in the posterior stroma, Fleisher’s ring which is iron deposition around the base of the cone and/or a scissors-type reflex with retinoscopy.⁴

A ‘scissors’ reflex can be seen in normal astigmatic patients, whereas in keratoconus the reflex is more ‘shadowy’. This is a similar type of reflex to that seen with central posterior pole cataracts and corneal scars – it is not ideal to use this as a diagnostic sign. If a topographer is available, abnormal steep zones are identified by colours in the red end of the range (Figure 1).

Topography is a useful test to aid diagnosis when slit lamp signs are not obvious.

Spectacles may improve vision to an acceptable level and should be prescribed when there is an appreciable gain. This is often the simplest and best option.

Ideally, the lowest cylinder, which maintains visual acuity at an acceptable standard, should be prescribed, as full cylindrical correction may not be tolerated. It is important to discuss management options with patients, and the prognosis should not be overstated. There is no evidence that optically correcting keratoconus retards progression. If the astigmatism is such that it is intolerable in spectacles, causes distortion, or simply that the patient is unhappy with the level of vision attained, then contact lenses are indicated.

It should be emphasised that contact lenses are not a treatment, but simply a way of improving visual acuity to an acceptable level.

Trial lens selection is based on keratometry readings (Ks) and topography⁵ (as a combination or separately) or from inspection of the corneal profile from the side, depending on the lens type and the severity of the keratoconus. Mild keratoconus with a central cone can be fitted based on Ks.

Often there is a large difference in the Ks. The average K can be used as a starting point for the base curve of the initial trial lens. It can be seen from Figure 2 that central Ks in this case would be of little use. A topography plot can be useful to enable the lens of first choice to be selected more easily and the fit to be nearer the end point of the fitting process. It can be seen by Figure 1 that the steep zone identified by the red end of the range is decentred. In this case a large diameter lens could be a good starting point (see ‘corneal lenses’ section).

A detailed slit-lamp examination outlining the apex of the cone is also useful to assess the decentration of the cone. The corneal profile is often enough to select an initial lens when a scleral lens is being fitted. It should be noted that the initial lens is usually a guide for the next step, irrespective of the technique used to select this.

There are various contact lenses for keratoconus on the market. A discussion of lenses with which the author has experience will follow.

Soft lenses

Soft lenses are increasingly offered in the management of keratoconus, although corneal RGP lenses are generally considered to be the mainstream option. Should a patient with mild keratoconus simply wish to wear contact lenses for cosmetic reasons and is happy with spectacles most of the time, a soft spherical or toric lens can be fitted.

Mild keratoconus can be fitted with any type of soft lens should the visual acuity attained be acceptable. Silicone hydrogel lenses (Si Hy) can be relevant here since the increase in rigidity compared to conventional hydrogels often helps to mask the astigmatism and Si Hy torics, such as PureVision Toric (Bausch & Lomb), may be successful. Also, since patients with keratoconus often have long wearing times, Si Hy lenses are ideal.

Fitting conventional design soft contact lenses becomes problematic when the cylindrical component is large or when the corneal irregularity causes these lenses to be less stable. Keratoconic design soft contact lenses such as Kerasoft⁶ (Ultravision) or Soft K⁷ (Acuity Contact Lenses) are designed for keratoconus and post-graft cases. They should be reserved for mild to moderate keratoconus as opposed to advanced cases.

The lenses should be allowed to settle for at least half an hour, assessed for movement and centration, and some types should also be assessed with high molecular weight fluorescein. A sphere-cylinder over refraction must be performed and the lens can then be ordered with this over refraction, taking into account the back vertex distance. Further power adjustment may be necessary if there is a large over refraction. These lenses can be fitted when a patient is experiencing corneal RGP tolerance problems or for keratoconic patients who happily wear corneal RGPs day-to-day but need an alternative for contact sports.⁷

Corneal RGP lenses retain a liquid lens between the lens and the cornea which neutralises the irregular corneal surface. This is not attainable with soft lenses, which is why RGP lenses are usually first choice for keratoconus. These lenses usually resolve the majority of the ghosting and distortion.

In unilateral cases, the more affected eye may only need to be fitted to improve vision and aid binocularity. Corneal RGP lens wear can sometimes be difficult to tolerate unilaterally, so both eyes may need to be fitted to aid adaptation (or alternatively a soft lens should be considered). However, some patients do not find correction of the more advanced eye beneficial, so it may not be necessary. Leaving the eye uncorrected does not cause progression, nor does it overwork the fellow eye but the theoretical risk of secondary exotropia should be considered.

The main factors to consider in lens choice are severity of the keratoconus, size of the cone and location of the cone apex. As a general guide and starting point, early central cones (Figure 3) are suited to lens diameters approximately between 9.00mm-9.90mm, nipple type cones (Figure 4) to a diameter approximately less than 9.00mm and a cone with a low apex (Figure 5) to a diameter approximately greater than or equal to 10.00mm.

For corneal RGP fitting, the lens fit is evaluated on slit lamp with white light initially for centration and movement, and then with cobalt blue illumination with fluorescein for assessment of apical touch, edge clearance and the mid-periphery.

In an ideal situation a lens with apical clearance⁸ would be chosen, but good visual acuity is not always attainable with this fitting technique. In general, a slightly flatter fit with apical contact, while maintaining tear exchange, provides a better level of visual acuity. This usually gives a ‘three-point touch’ fitting pattern (Figure 6) where the corneal lens is in contact with the central apex and in the mid-periphery. This is currently thought to be the most successful fitting technique.⁹ The touch at the apex of the cone should be light¹⁰ and there should be a feathered transition from the central and mid-peripheral contact zones to the areas of clearance. Often the corneal profile means a three-point touch is unachievable and it is often more like a two-point touch (Figure 7). However, when there is a dynamic variation in the fitting pattern on blinking this is acceptable. It can be seen in Figure 7 that the lens post blink is above centre, there is a slight superior pool with light touch below this, and although the edge clearance inferiorly is acceptable, it is not adequate superiorly. When the lens drops between blinks (Figure 8) the opposite is true. There is a slight inferior pool with light touch just superior to this, the edge clearance superiorly is now increased but inferiorly is unlikely to be due to the nature of where the lens is sitting on the eye. This is an acceptable fit as there is a dynamic variation in the central fitting pattern and edge clearance with blinks demonstrating adequate tear exchange.

In some cases a moderate amount of touch on the corneal apex must be accepted, but careful follow-up with slit lamp examination is necessary to ensure there are no detrimental effects. Scarring is part of the natural progression of the disease, but it must be ensured that the fit of the lens does not contribute to this.¹¹

Early keratoconus can be fitted with an RGP contact lens designed for ‘normal’ eyes. An aspheric or a tricurve lens is a good starting point. The cornea is steeper at the apex and the rate of flattening of the cornea can be greater than in normal eyes, therefore such a design may have reduced edge clearance. This can be improved by flattening the peripheral curves if a multicurve lens is fitted or ordering an aspheric lens with an increased axial edge lift (AEL), such as Aspheri-P (Jack Allen, JA) and Quasar lenses (No 7 Contact Lenses). As the cone becomes steeper, a lens design which tries to mimic the corneal profile is preferable, such as 95D (JA), a keratoconic tricurve lens design (for example Woodward KC3, Moorfields Eye Hospital, MEH), Quasar K (No 7) and Ruben Offset (MEH).

The 95D lens design has a spherical centre and an aspheric edge with an option of increasing or decreasing the AEL. The benefit of a keratoconic multicurve design lens is that individual curves can be modified according to the patient’s corneal profile. Quasar K lenses are an aspheric keratoconic lens design. The Ruben Offset lens is a bicurve made with a flat peripheral curve, offset from the geometric axis of the optic zone, so that the optic and peripheral curves share a common tangent, allowing for a smoother transition of the lens. The Offset 1 is a 9.00mm diameter lens useful for central cones, the Offset 2 is a 10.00mm diameter useful for low cones and an Offset 3, which is a 8.50mm diameter for nipple cones. This is similar to the Maguire lens range (Ultravision CLPL) Oval, Globus and Nipple respectively.

Moderate cones are those which cannot be easily managed with spectacles, conventional soft lenses or corneal RGP lenses designed for normal ocular topography. There is an overlap in lens designs suitable for moderate and advanced keratoconus, such as Ruben Offset (MEH), Woodward KC3 (MEH), Profile (JA) and Rose K lenses (Davis Thomas). It is useful to have a comprehensive fitting set with a range of base curves and an ability to vary the AEL and diameter. Quite a few systems do this, such as the Profile and the Rose K.

The Profile system is an aspheric design based on the principle of the axial profile of the lens¹² as opposed to specific measurements of curvature. There are two main variants, these are the ‘N’ range and the ‘R’ range, indicating normal and reduced AEL; these are available in three different diameters. Rose K lenses are a multicurve design where the fitting is based on a good central fit being obtained initially with the appropriate diameter. The edge can then be ordered as an increased or decreased AEL, if necessary. The number of ‘AEL steps’ by which the lens needs to be modified should be specified. Rose K lenses can be custom made. The main difference in these two lens types is that Rose K attempts to match the corneal profile while Profiles bridge the cornea from the apex to the periphery.

All grades of keratoconus may benefit from scleral lenses, from the relatively mild keratoconus (Figure 3) with intolerance to corneal lenses, to advanced protrusive keratoglobus (Figure 9). Specific indications are when there is a particularly low cone or pellucid marginal degeneration, which is thinning of the inferior cornea adjacent to the limbus (Figure 2). Most corneal lenses in these cases will drop (Figure 10), a scleral has less lens mobility and reduced lid sensation. The ideal fit for a sealed RGP scleral lens is total corneal clearance, therefore the visual acuity attainable with corneal lenses is often unachievable with scleral lenses, since there is no corneal contact. They are typically 23mm in diameter and should be assessed with a corneal section on the slit lamp biomicroscope with fluorescein in the pre-corneal tear reservoir.

Other options for patients who have an RGP intolerance are piggy-back lenses¹³ (Figure 11) or hybrid lenses with an RGP centre and soft skirt. These lenses, in addition to keratoconic design soft lenses and sclerals, have a reduction in oxygen permeability compared to corneal RGP lenses.

Using a soft lens underneath an RGP in a piggy back system acts as a cushion to alleviate discomfort as corneal sensation is reduced. Vision is not usually adversely affected by the presence of the soft lens. The ideal soft lens for this system would be a steep daily disposable Si Hy lens with a low modulus. Since these do not yet exist, a lens must be chosen to suit each patient.

A second cleaning regime would be required for the Si Hy lens; a daily disposable removes this additional step so provides an advantage, but also adds the disadvantage of lower oxygen transmissibility. A daily disposable or a Si Hy may not always fit the cone and a thin lens which drapes may be the best choice, such as the Proclear Biocompatible (CooperVision). Although the oxygen transmissibility is reduced compared to Si Hys, this may be the only choice in some cases. The RGP and soft lens should be assessed for fitting separately and then together using high molecular weight fluorescein.

Hybrid lenses such as Softperm¹⁴ (CIBA Vision) and Flex-edge lenses (JA) often give good vision since there is apical contact and the lens is more stable due to the soft skirt. The Flex-edge lens may be ordered with a modified soft skirt. These lenses should be assessed for fitting with high molecular weight fluorescein. Problems can occur secondary to the apical contact, poor tear exchange and low oxygen transmissibility.

References

6 Ultravision CLPL Product Information.

Acknowledgement

The author wishes to thank Ken Pullum, Vijay Anand, Scott Hau and Dan Ehrlich for their help and support. Special thanks to Ken Pullum for Figures 2-10 and Dan Ehrlich for Figure 1.