Keratoconus is a progressive ectasia of the cornea, in which the cornea thins, protrudes and becomes conical in shape. This will result in irregular astigmatism, causing distorted vision manifesting as blur, haloes and shadows.1 In this article, optometrist Jessica Chi describes the interventions available, in practice, to manage keratoconus and optimise vision.
In the early stages of keratoconus, spectacles or soft contact lenses may suffice to manage the induced distortion, however as the disease advances they may be of little assistance. Instead, contact lenses with a rigid surface may be required to neutralise the induced higher-order aberrations.
Traditionally, when we refer to rigid gas permeable (RGP) contact lenses, we refer to corneal RGP lenses, i.e. lenses that fit within the limbus. However, they are not well tolerated by all patients, and have necessitated the development of other contact lens modalities to improve patient comfort and convenience including piggyback, hybrid and scleral systems.
Scleral lenses are the fastest growing specialty lens category for keratoconus due to the enhanced comfort compared to corneal RGP lenses
Figure 1. A low riding lens.
Corneal RGP lenses have historically been the preferred contact lens choice for keratoconics, and have a long history of success with low rates of complications. With a diameter between 7.0 to 12.0mm, they have the least corneal coverage and as a result, have the least disruption to corneal oxygenation.
However, corneal RGPs typically have the longest adaptation period as the lens lands on the surface of the highly innervated cornea, and many will be aware of the lidlens interaction upon blink. Most wearers become desensitised with time, however some may fail to adapt. Corneal RGP lenses can also be prone to spontaneous mislocation, ejection and to foreign particles beneath lenses, so wearers who have active lifestyles or work in windy and dusty environments may find them inconvenient. Additionally, as the most common form of cone in keratoconus is the sagging cone, lenses may end up decentred, which can result in glare and flare if the optic zone does not centre over the pupil (Figure 1).
The common accepted keratoconic fitting philosophies for corneal RGPs are apical clearance and ‘three-point-touch’. Apical clearance involves vaulting the apex of the cornea, with the lens landing on the more normal paracentral cornea (Figure 2). This will avoid insult on the thinner, weaker apex of the cone, however it can result in poorer vision, three and nine o’clock staining, and corneal warpage.2,3
Figure 2. A lens with apical clearance.
The ‘three-point-touch’ philosophy involves sharing the weight of the lens between the para-central cornea and the corneal apex (Figure 3). However, bearing on the cone should be avoided at all cost – because excessively flat fitting lenses will cause corneal insult, which will result in progression of the keratoconus and eventual scarring (Figure 4). The Collaborative Longitudinal Evaluation of Keratoconus (CLEK) study recommended fitting a lens which was the ‘first definite apical clearance lens’, i.e. the flattest lens that is minimally clearing the apex. Fluorescein is not visible in the tear film if it is less than 20 microns of thickness, so the ideal fit would exhibit ‘feather-touch’ which, in fact, is minimally clearing the apex, with the lens supported by the more normal paracentral cornea.
In the past there has been the misconception that lenses should be fit flat to ‘push’ the cornea into a more regular shape to retard progression, however it has been shown that flat fitting lenses lead to corneal insult, keratoconus progression and eventual scarring.3
Corneal RGP lenses are the most economical (provided the wearer can take care of them) and are also arguably the easiest to insert and remove, and may be the only option for patients with small apertures or dexterity problems.
Fitting has improved dramatically over the past couple of decades with improved designs and lathing abilities – there are now numerous commercial designs available, which may be produced in multi-curve, aspheric or reverse geometry designs. They can also be manufactured to be quadrant specific – i.e. specific meridians of the lens can be flattened or steepened in particular quadrants. This is useful in the case of decentred sagging cones, where a lens can be ordered with an inferior ‘tuck’, i.e. steeper in the inferior portion of the lens to reduce inferior lift off.
Figure 3. The three-point touch fitting pattern.
Despite advances in manufacture, it may be too challenging to achieve a fit that avoids all apical insult, especially in highly irregular corneas or highly decentred apexes.
PIGGYBACK SYSTEMS
Piggyback systems involve a soft contact lens worn beneath an RGP contact lens (Figure 5) to act as a shield for the eye. This can improve comfort and stability for patients with sensitive corneas, epithelial disruption and epithelial nodules. However, this system will be of little help to a patient who is truly intolerant, as it does not eliminate the lid-lens edge interaction. Many wearers will be averse to this modality as they will find the handling and maintenance of two separate lenses per eye inconvenient. As the soft contact lens contributes very little to optical correction, this may be particularly frustrating for those who have impaired unaided vision as they may struggle to appreciate whether the soft contact lens is on the eye.
If this system is prescribed, both the soft and RGP lenses should have high oxygen transmissibility as there are now two barriers to the eye. Note, this system should be distinguished from a hybrid system, which joins both rigid and soft elements together as one lens.
HYBRID CONTACTS
Hybrid contact lenses incorporate the ‘best of both worlds’; i.e. the optical properties of a rigid centre and the comfort and stability of a soft contact lens. Presently, there are two options commercially available – SynergEyes, which has the ClearKone and UltraHealth lenses specific for keratoconus, and the EyeBrid Silicone lens which is supplied as a button to be lathed by custom contact lens laboratories.
Figure 4. A flat fitting lens.
The ideal fitting relationship between the hybrid contact lens and the eye would exhibit a rigid portion that is elevated above the cornea with the weight on the outer soft skirt (Figure 6). As this lens is bound between the rigid and the soft portion, this significantly reduces the lid-lens interaction and is a viable option for patients who are RGP intolerant. These lenses offer greater stability and centration compared to corneal RGP lenses, and are not prone to foreign particles beneath them.
CORNEO-SCLERALS
Corneo-scleral lenses share the bearing of the lens between the cornea and the sclera – predominantly on the limbus. These result in greater comfort, better lens centration and stability. They differ from scleral lenses in that they exhibit some movement and tear exchange.
SCLERAL LENSES
Scleral lenses are the fastest growing specialty lens category for keratoconus due to the enhanced comfort compared to corneal RGP lenses. They completely vault the distorted cornea, landing on the sclera (or more correctly, the conjunctiva), which is more regular and far less innervated than the sensitive cornea. These lenses are fit with minimal to no movement, and are a ‘sealed-lens’ system, affording greater comfort, faster adaptation and greater stability compared to corneal RGP lenses. They also avoid foreign particles beneath the lenses.5
Figure 5. The piggyback system.
However, these lenses allow for less oxygen as the lenses are thicker, cover a greater area of the ocular surface and exhibit minimal tear exchange.6 Oxygen transmissibility (Dk/t) is defined by the oxygen permeability of the material (Dk) divided by the thickness (t) of the lens. Additionally, the post-lens tear film presents another barrier to oxygen. Care should be taken to maximise oxygen availability – this can be achieved by selecting higher Dk materials, prescribing lenses with reduced centre thicknesses and reduced clearance over the cornea. However, a balance is required between trying to maintain a thinner lens profile for oxygen availability and ensuring the lens is not too fragile, and between minimal clearance and avoiding any corneal touch. Scleral lenses settle back onto the ocular surface with wear, which varies from individual to individual but is approximately 100–200 microns.7
Fitting these lenses has been made easier with anterior-segment optical coherence tomography (OCT) (Figure 7), however assessing these lenses can be performed without OCT – using a cross-section through the lens, clearance and cornea to ascertain clearance (Figure 8).
As there is minimal tear exchange beneath the lens, the post-lens tear film stagnates after prolonged wear, which can result in fogging. This may be tolerable for some, while others may require removal, cleaning and reinserting throughout the day.5
Many patients have difficulty handling these lenses – they are physically larger and patients with smaller apertures or strong blink reflexes may struggle to insert them. As they completely vault the cornea, lenses must be inserted with fluid to avoid bubbles beneath the lens, which can impact comfort and vision. Removal may also present a challenge – as these lenses are a sealed-lens system they may feel ‘suctioned’ after prolonged wear. From clinical experience, issues with handling and the increased cost are this lens’ greatest perceived downfalls from a patient perspective.
Figure 6. A well-fitting hybrid lens.
However, as they can bypass most irregular and steep corneas, they allow a viable option for patients who were otherwise deemed ‘unfittable’, which may delay or even eliminate the need for surgery.
CONCLUSION
Historically, when patients suffered significant distortion from their keratoconus the options were; corneal RGP contact lenses, corneal grafting, or simply no rehabilitation and subsequent visual impairment. Fortunately, in recent years, there have been advances – not just in contact lens options, but also in more sophisticated technology for metrology of the eye including corneal topography, corneo-scleral topography, tomography, aberrometry, and OCT.
Many factors need to be considered in selecting a lens modality, and these factors are not all purely corneal. Indubitably, the cornea may dictate the options available – i.e. advanced cones or particularly displaced cones may require more specialised lenses. However, it is also important to consider the many patient factors including visual expectations, occupation and hobbies, other comorbidities, aperture size, previous lens experience and dexterity.
Ascertaining motivation and setting realistic expectations is key to successful contact lens wear – there will be some patients who are 6/6 in spectacles who will appreciate a significant improvement in perceived vision with RGP lenses, and there will be some who are 6/12 and want to avoid lenses at all costs. A lens modality may be perfect in terms of function, however if the wearer cannot or does not want to handle the lens, then it is useless to them. It is important to counsel patients that wearing lenses is not a cure, as some patients will feel compelled to wear RGP contacts despite discomfort and/or inconvenience, with the belief that no lens wear will exacerbate the condition.
Figure 7. OCT of a scleral lens.
Understanding your patient is key – it is important to avoid adopting a ‘one size fits all approach’ – just as no cone is the same, no individual is the same.
Jessica Chi is the director of Eyetech Optometrists, an independent speciality contact lens practice in Melbourne. She is the current Victorian, and a past national president of the Cornea and Contact Lens Society, and an invited speaker at meetings throughout Australia and beyond. She is a clinical supervisor at the University of Melbourne, a member of Optometry Victoria Optometric Sector Advisory Group and a Fellow of the Australian College of Optometry and the British Contact Lens Association.
Figure 8. A slit lamp cross-section of a scleral lens.
References
- Rabinowitz YS. Keratoconus. Surv Ophthalmol. 1998;42:297-319.
- McMonnies CW. Keratoconus fittings: apical clearance or apical support? Eye Contact Lens, 30 (2004), pp. 147-155
- Zadnik K. Barr JT, Steger-May K, et al. Comparison of flat and steep rigid contact lens fitting methods in keratoconus Optom Vis Sci. 2005;82: 1014–1021.
- Barr J, Zadnik K. Collaborative Longitudinal Evaluation of Keratoconus (CLEK) Study: methods and findings to date. Cont. Lens Anterior Eye, 30 (2007), pp. 223-232
- van der Worp E. A guide to scleral lens fitting. 2nd ed. Books and Monographs. United State of America: Pacific University Common Knowledge; 2015.
- Michaud L, van der Worp E, Brazeau D, Warde R, Giasson CJ (2012) Predicting estimates of oxygen transmissibility for scleral lenses. Contact Lens & Anterior Eye; 35(6), 266-71
- Caroline PJ, Andre MP (2012) Scleral lens settling. Contact Lens Spectrum; 5, 60
