Introduction
Congenital or developmental abnormalities of corneal topography include primary corneal ectasias, such as keratoconus and keratoglobus, and cornea plana ( Fig. 25.1 ). These atypical corneal shapes provide significant challenges to clinicians who are attempting to restore vision for patients with these conditions. Keratoglobus and cornea plana are relatively rare, so this chapter concentrates on the contact lens correction of keratoconus.
Keratoconus is classically considered a bilateral, asymmetric, noninflammatory corneal disease that is characterised by progressive thinning and steepening of the central cornea ( ). These pathological changes lead to a relative reduction in vision secondary to the development of irregular astigmatism and/or corneal scarring ( ). Keratoconus is the most highly prevalent primary corneal ectasia. Data from a 48-year clinical and epidemiologic study of keratoconus estimated the condition to affect about 54 per 100,000 people ( ). However, a recent Australian cross-sectional study involving 20-year-old adults reported a prevalence of 1.2% (or 1 in 84 individuals) in this population ( ). Although the aetiology of keratoconus remains uncertain, it is recognised to be a multifactorial condition, involving genetic, biochemical and/or environmental factors ( ).
Contact lens practitioners play a vital role in providing appropriate optical appliances to delay, or even preclude, the need for keratoplasty in people with keratoconus. The first application of contact lenses for keratoconus was described by the German-born physician and physiologist, . In more recent times, advances in contact lens designs and materials have expanded the fitting options for patients with corneal ectasia. These developments are underpinned by the need for contact lenses to not only deliver the desired improvement to vision, but to also provide appropriate levels of comfort and to maintain corneal physiology.
This chapter provides a comprehensive overview of the range of contemporary contact lens modalities, including soft, rigid (i.e. corneal, corneo-scleral and scleral lenses), hybrid and piggyback designs, that are available for the optical management of keratoconus. The application of anterior segment imaging technologies, including corneal topography and optical coherence tomography (OCT), to assist with contact lens fitting is also considered. Furthermore, the importance of monitoring for disease progression in people with keratoconus, particularly children, is discussed.
Keratoconus Clinical Assessment
Robust baseline clinical data are essential for both monitoring disease progression and guiding contact lens management in people with keratoconus. While several keratoconus classification systems have been proposed, based upon various criteria that include corneal morphology, clinical signs, topographical parameters and/or corneal structural changes ( ), there remains a lack of a universally accepted keratoconus severity staging scale for either research or clinical purposes.
Anterior Segment Imaging Technologies
Contemporary imaging technologies have significantly enhanced the capacity to image the cornea in clinical practice (see Chapter 34 ). Compared with traditional keratometry, Placido-disc based corneal topography facilitates relatively earlier diagnosis and enhanced monitoring of keratoconus ( ). A range of quantitative topographic indices exist to identify anomalies of corneal curvature that indicate for corneal ectasia.
More recently, imaging systems that do not rely upon the quality of the surface image, such as OCT, have emerged and have clinical application both for assessing in vivo corneal microstructural changes and guiding contact lens fitting. An OCT-based keratoconus classification system that was reported to also provide a grading of severity has been described ( ). Further clinical studies, over follow-up periods that are sufficient to evaluate the natural history of classification using this OCT system, are still required to validate this method of disease categorisation. Total and sublayer corneal thickness analyses, derived from OCT imaging, have also been proposed to be useful for the early detection of keratoconus ( ). Anterior segment OCT has been used to quantify postlens tear film clearance patterns in rigid corneal-lenses relationships (i.e. three-point touch, apical clearance and apical bearing) ( ). The application of OCT for fitting scleral lenses is discussed in the ‘scleral lens’ section of this chapter.
Keratoconus Cone Morphology
Based upon morphological criteria, three major subtypes of keratoconus are recognised ( Fig. 25.2 ) ( ). Although the prevalence of these subtypes can vary in different demographics, centred (nipple) cones are considered to account for about half of morphologies; this subtype is characterised by a cone diameter of five millimetres or less that is round and positioned centrally or slightly inferior to the geometric centre of the cornea. Oval (sagging) cones are larger in diameter and demonstrate either infero-nasal or infero-temporal displacement of the corneal apex. Since rigid contact lenses tend to align over the corneal apex, achieving adequate lens centration and pupil coverage for oval cones can pose a relative challenge ( ). Commonly regarded as the least common cone morphology is the globus cone, in which the conical area involves at least 75% of the cornea. From a contact lens fitting perspective, these cases are typically the most complex and typically warrant designs with larger diameters to achieve a desired fitting.
Anterior Ocular Health
An additional important element of the clinical assessment of people with keratoconus, particularly in the context of contact lens fitting, is the diagnosis and management of comorbid ocular conditions. Associations between keratoconus and atopic disease, including eczema, asthma and rhinoconjunctivitis, are well known ( ). Allergic and/or dry eye disease can be major impediments to successful contact lens wear and therefore require timely and appropriate ophthalmic care prior to the commencement of contact lens wear ( ).
Refractive Management of Keratoconus
The management of keratoconus will vary depending upon the severity of the disease. Refractive management options are largely nonsurgical, with contact lens correction often being essential to achieving the best functional outcomes ( ). Table 25.1 provides a summary of the major forms of vision correction that are currently available in clinical practice for the nonsurgical management of keratoconus; each of these modalities is elaborated upon in this chapter.
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Spectacle Correction
Spectacle correction may provide adequate visual correction in earlier stages of keratoconus and/or be of value as an adjunct to contact lenses when an appropriate level of functional spectacle acuity can be achieved. Retinoscopic findings can assist the practitioner with determining an appropriate starting point for subjective refraction. Refraction should be undertaken judiciously, as people with keratoconus may have difficulty with subjective determinations of refractive endpoints; this is likely due to the multifocal nature of the cornea ( ) and is an effect that is heightened with advanced disease. In undertaking subjective refraction, relatively large refractive steps (e.g. ±1.00 to ±3.00 D) may need to be presented to the patient to enable subjective differences to be discernible. Whenever practicable, and particularly at baseline, best-corrected spectacle acuity should be recorded to serve as a reference point for longitudinally evaluating significant changes to visual function.
The use of a spectacle correction for the optical management of keratoconus often has, however, some notable limitations. Subjective refraction can be poorly repeatable ( ) and particularly in cases of more advanced keratoconus, simply not feasible to perform. Progressive disease can lead to rapid changes to the refraction, with refractive shifts possible over periods of weeks, which can render a relatively recent spectacle correction suboptimal. Consideration with regard to the likelihood of patient tolerance to high degrees of refractive astigmatism, particularly in the context of anisometropic refractions, is also required. Given these factors and the potential for enhanced irregular astigmatism correction with many contact lens modalities, contact lenses remain the predominant optical corrective modality for keratoconus.
Soft Lenses
Soft contact lenses, typically in disposable form, are a useful visual correction option for forme-fruste and/or earlier stages of keratoconus ( ). Soft lenses may also provide a suitable form of contact lens correction for people with keratoconus who have implanted intra-stromal corneal ring segments (ICRS) ( ). Similar to spectacle correction, a major shortcoming of this modality is the inability to mask irregular corneal astigmatism ( ). One small study involving 13 individuals with different stages of keratoconus showed that spherical hydrogel soft lenses provide poorer high- and low-contrast visual acuities than rigid corneal-lens correction ( ). Similar findings were reported in another clinical investigation, whereby rigid corneal lenses were found to provide relatively enhanced low-contrast acuity and a reduction in higher order aberrations compared with toric hydrogel soft contact lenses ( ). In this study, visual performance with soft toric lenses was reported to be comparable to that measured with a spectacle correction ( ). Refereed studies comparing the visual efficacy of soft contact lenses made from silicone-hydrogel materials with rigid lens designs for keratoconus are currently lacking. It has been suggested that the modulus of elasticity of silicone-hydrogel lenses, being greater than hydrogel materials, may enable these lenses to have relatively improved conformational integrity in situ and thereby provide enhanced/or more stable visual acuity. Studies to clarify whether such an advantage exists are still needed.
Major advantages of soft contact lenses over traditional rigid lens correction are the enhanced on-eye comfort profile and the relative ease of fitting ( ). Compared with spectacles, potential benefits also exist with adopting contact lenses in relation to improved quality of life in young adults ( ). Consideration of on-eye comfort is pertinent in the context of any prior history of rigid lens intolerance and/or when rigid lenses are deemed impractical. Factors that may influence the practicality of rigid lens correction include ocular (e.g. monocular correction), occupational (e.g. dusty working environment) and recreational (e.g. participation in dynamic sporting activities) considerations. From the clinician’s perspective, a soft lens fitting procedure for a person with keratoconus essentially mirrors the process that is routinely applied to eyes without irregular astigmatism (see Chapter 8 ). Depending upon the extent of corneal irregularity, the soft lens parameters may need to be carefully selected to ensure adequate on-eye lens movement. The soft lens material should also be selected so as to minimise the likelihood of corneal hypoxic complications, such as corneal neovascularisation, which may complicate a future keratoplasty procedure.
In recent years, the ability to lathe quadrant-specific curve designs in soft lens materials has supported the development of keratoconus-specific soft contact lenses (e.g. KeraSoft IC, Bausch & Lomb; Soft K, Soflex; NovaKone, Alden Optical). At present, there is a relative paucity of published data regarding the clinical efficacy of these designs. One case series, involving two people, reported on the use of the Soft K lens design for optical correction of mild keratoconus ( ). A retrospective analysis comparing visual acuity outcomes in keratoconus eyes with mild-to-moderate ectasia that had been fitted with either the silicone-hydrogel KeraSoft IC lens ( n = 94) or the Menicon Rose-K2 rigid lens ( n = 94) reported no significant difference between lens types ( ). A case series describing the successful fitting of KeraSoft IC lenses to eyes implanted with ICRS has also been published ( ). More recently, a prospective, randomised, cross-over study, involving 28 individuals with keratoconus and 10 age-matched control participants, evaluated a range of different contact lens designs (a conventional rigid corneal-lens, KeraSoft IC lens, Rose-K2 rigid lens and a scleral lens) for their effects on visual performance ( ). The authors reported that while all contact lens types delivered superior visual acuity, contrast sensitivity and steroacuity relative to spectacles in individuals with keratoconus, the degree of improvement was less for the KeraSoft IC lenses relative to the other contact lens modalities.
Computer-based simulations suggest that customised correction of lower and higher order ocular aberrations, even withstanding contact lens movement in situ, should benefit visual function in keratoconus ( ). As recently reviewed ( ), over the past several years, a number of custom aberration-controlled soft contact lens designs have been developed and have undergone varying degrees of investigation ( ). Although significant reductions to overall ocular aberrations have been demonstrated, the success of these soft lens modalities varies between individuals with keratoconus ( ). Such variability may be attributable to a range of factors, including keratoconus severity, cone morphology and the magnitude and consistency of any lens translation in situ. At present further research is needed to determine the clinical applicability of customised, aberration-controlled soft contact lenses for mainstream keratoconus management.
Rigid Lenses
Rigid contact lenses, both prior to and since the availability of gas permeable materials, have been the primary form of visual correction for keratoconus ( ). In maintaining their on-eye conformation, rigid lenses create a lacrimal lens between the irregular anterior corneal surface and posterior lens surface that neutralises much of the corneal astigmatic error, but will not necessarily normalise higher order aberrations ( ).
Rigid lens classification – historically involving corneal, corneo-scleral, mini-scleral and scleral categories – has previously been based upon differences in lens total diameter (TD). The problem with this classification system is that it does not take into account differences in corneal size between patients. For example, if a patient has a typical corneal diameter of about 11.5 mm, then a rigid lens with a TD of 10.0 mm would be considered a ‘corneal lens’ when placed on the eye of this patient. However, if another patient had microcornea, with a corneal diameter of 9.0 mm, then in this case, a rigid lens with the same TD of 10.0 mm could arguably be considered a corneo-scleral lens in situ.
As outlined in the Preface of this book, rigid lenses are now categorised based on where the lens bears (or rests) on the ocular surface. By definition, rigid corneal lenses only show bearing on the cornea. Rigid lenses resting partly on the cornea and partly on the sclera are known as corneo-scleral lenses, and scleral lenses only show bearing on the sclera. Further, in line with the standardised terminology recommended by the Scleral Lens Education Society, the new terminology to define scleral lenses removes the distinction between ‘mini-scleral’ and ‘scleral’ lenses. All scleral lenses, regardless of their size, are fitted to completely vault over the entire cornea, and so there is no need for this distinction ( ).
Until recently, corneal lenses have been the most common form of rigid lens prescribed for keratoconus. In the preceding three to four decades, the use of scleral lenses became less frequent. However, over the past several years contact lens practice has witnessed the re-emergence of the larger diameter rigid lens modalities, in the form of corneo-scleral and scleral lenses ( ). The prescription of these alternative rigid lens forms for the contact lens management of keratoconus has increased significantly in recent years due to the associated advantages of larger diameter contact lenses, which include enhanced on-eye comfort and stability ( ).
Corneal Lenses
A diverse range of commercial rigid corneal lens designs exist for keratoconus, including spherical multicurve and aspheric designs. Rigid corneal lenses can also be custom designed by the practitioner, on a case-by-case basis. Most studies that have evaluated rigid corneal lens correction in keratoconus have involved retrospective analyses in specific clinical populations. These studies ( ) provide insight into the range of lens designs that have been utilised in practice for the optical correction of keratoconus. However, objective comparisons between lens designs are not possible in the absence of a suitable control group and/or clearly defined criteria regarding the definition of a ‘successful’ lens fitting. There is currently a lack of high-quality, controlled prospective clinical evidence to inform clinical decisions regarding the relative merit of different proprietary keratoconus lens designs.
Fitting Philosophies
As discussed in Chapter 15 , traditionally the clinical evaluation of rigid lens fitting is facilitated by the application of sodium fluorescein to the eye and the subsequent observance of the fluorescence pattern beneath the contact lens. A Wratten 12 barrier filter, or similar, placed in front of the slit lamp biomicroscope objective, can significantly enhance sodium fluorescein pattern assessment. Using sodium fluorescein patterns, three major philosophies for fitting corneal lenses to eyes with keratoconus are recognised ( ), being apical bearing, apical clearance and three-point touch.
Apical Bearing
In an ‘apical bearing’ fitting relationship, the primary support of the rigid lens is directly on the corneal apex. This results in a sodium fluorescein pattern that has a central darkened region ( Fig. 25.3A ). Historically, it was incorrectly hypothesised that this fitting philosophy could retard keratoconus progression by imparting physical resistance to progressive corneal deformation. It is now recognised that any apparent flattening effect on the corneal apex is transient and not of value for attenuating progressive corneal ectasia ( ).
Clinically, a primary concern with this mode of fitting is the potential for axial corneal scarring, subsequent to long-term abrasive contact between the rigid lens and the cornea. It is important to note that central corneal scarring may occur in the absence ( ) ( Fig. 25.3B ) or presence ( ) ( Fig. 25.3C ) of contact lens wear in keratoconus. It is, however, possible that contact lens wear is an exacerbating factor that hastens the pathology ( ). Harsh apical lens bearing can result in significant corneal epithelial disruption, which is clinically identifiable by a ‘whorl’-type staining pattern ( Fig. 25.3D ). This apical epithelial breakdown may progress to corneal scarring in the scenario that an excessively flat-fitting rigid lens is worn for a prolonged period of time ( ).
Apical Clearance
An ‘apical clearance’ rigid lens fitting features complete clearance of the corneal apex, with lens support (landing) on the paracentral cornea ( Fig. 25.3E ) ( ). This approach is achieved by fitting the back optic zone radius (BOZR) of the lens relatively steeper than the apical corneal curvature. The rationale behind this fitting philosophy is that it should minimise direct corneal epithelial trauma and the subsequent long-term risk of corneal scarring ( ). There is, however, still the potential for transient corneal moulding effects, peripheral corneal disruption and lens binding ( ).
The Collaborative Longitudinal Evaluation of Keratoconus (CLEK) study developed a standardised approach to apical clearance rigid corneal lens fitting, whereby the endpoint of the process was the flattest BOZR that showed definite apical clearance using sodium fluorescein assessment. Based upon the results of this study, the conclusion was made, despite absence of a control group, that fitting rigid corneal lenses with apical clearance was preferred for keratoconus ( ). Although the CLEK study reported superior visual acuity with relatively steep-fitting corneal lenses ( ), the association between better visual outcomes and apical clearance fitting remains unclear ( ). Whether apical clearance rigid lens fitting affects keratoconus progression is also uncertain. It has been postulated that apical clearance fittings may actually increase the risk of progression in early keratoconus, especially given that with corneal moulding there is a tendency for the corneal curvature to shift towards the contact lens BOZR ( ).
Three-Point-Touch
For a ‘three-point-touch’ rigid corneal lens fitting, the aim is to have ‘apparent touch’ at the corneal apex (evident as a subtle reduction in the brightness of the sodium fluorescein beneath the lens), with the majority of the lens-bearing pressure residing on the peripheral cornea ( Fig. 25.3F ). The term ‘apparent touch’ intends to reflect the lack of true physical interaction between the lens and the corneal apex; the relative darkening of the sodium fluorescein profile in this zone is the consequence of a tear lens thickness below about 20 μm, being the approximate threshold for fluorescence. This method of fitting rigid corneal lenses to keratoconus eyes has previously been shown to be popular amongst contact lens practitioners ( ). The key factor with this approach is to ensure that the contact lens does not bear heavily on the corneal apex, so as to cause an epithelial abrasion ( ). In practice, a three point touch fitting pattern that tends towards apical clearance is probably optimal, as it allows for a small degree of keratoconus progression ( ).
Lens Designs
Spherical
Many rigid lens designs have been developed for keratoconus. Most of these designs aim to provide a steeper than typical BOZR, to accommodate the conical nature of the central cornea, and then incorporate a series of peripheral curves with progressively flatter radii to clear the relatively normal peripheral cornea ( ). Due to the relatively steeper BOZR, there needs to be relatively more flattening of the peripheral curves to achieve an acceptable degree, about 0.08–0.1 mm, of edge clearance ( ). That a greater degree of flattening is warranted in the lens periphery is reflected by the relatively high corneal eccentricity values, consistent with a relatively prolate corneal shape, being a clinical feature of keratoconus.
The multicurve lens, consisting of multiple spherical radii that are blended together to form the desired flattening contour, is a common rigid lens design for keratoconus ( ). Multicurve lenses have the advantage of readily changeable parameters, including BOZR, TD, BOZD, back peripheral curve radii (BPR) and back peripheral curve widths (BPDs) ( ). Flexibility to customise these parameters is important, as the contact lens practitioner will often need to modify, and typically more than once, some or all of these parameters before determining the optimal contact lens prescription. It is therefore essential for the practitioner to have access to an appropriate keratoconus rigid lens fitting set for this purpose ( ).
Consider the following three typical corneal rigid lens specifications for keratoconus:
Lens 1 C4/6.80:6.40/7.60:7.20/8.40:8.00/10.20:8.80 −7.00 AEL 0.23
Lens 2 C4/6.20:6.00/7.10:6.80/8.20:7.60/10.40:8.40 −11.00 AEL 0.31
Lens 3 C4/5.60:5.60/6.60:6.40/8.10:7.20/10.60:8.00 −15.00 AEL 0.38
Of the three lenses, Lens 1 shows parameters for the relatively earliest stage of keratoconus. Lenses 2 and 3 incorporate parameters for increasingly more severe disease respectively. Note that with increasing keratoconus severity, greater axial edge lift (AEL) of the contact lens is required to achieve an acceptable degree of edge clearance; this is the consequence of the steeper BOZR.
These three rigid lens specifications also demonstrate some other important fitting principles with respect to corneal lenses and keratoconus. First, the TD generally decreases as the keratoconus progresses. Second, the more advanced the keratoconus, the smaller will be the BOZD; this is primarily to facilitate adequate tear exchange around the apex of the cone, which in turn will assist with preventing tear pooling and accumulation of tear debris under the optic zone of the lens. Finally, the required back vertex power (BVP) of the lens will be more negative (i.e. higher minus) as the keratoconus progresses due to the effect of the tear lens becoming more positive when fitting a steeper BOZR.
There are a couple of important points that need to be noted in relation to rigid lens BVPs. First, the higher minus power that is usually associated with a contact lens prescription for keratoconus does not necessarily indicate that the patient is highly myopic, although this mistake is often made in clinical practice. Indeed, for the three lenses previously specified, the BVPs of −7.00 diopters (D), −11.00 D and −15.00 D would actually be more typical of patients with keratoconus who are also relatively emmetropic based on the corresponding BOZRs of 6.80 mm, 6.20 mm and 5.60 mm, respectively. A person with keratoconus can also have associated myopia or hyperopia; in these cases, the BVP in the contact lens prescription would be quite different from what is normally predicted according to the BOZR.
Second, with regard to taking into account the change in tear lens power that will occur as a result of changing the BOZR, it was noted in Chapter 15 that, as an approximation, for every 0.05 mm decrease (steepening) in BOZR, −0.25 D should be added to the BVP of the rigid contact lens. Likewise, +0.25 D should be added to the BVP of the contact lens for every 0.05 mm increase (flattening) in BOZR. However, this approximation does not hold for BOZR steeper than 7.00 mm, even for small changes in BOZR. For keratoconus, it is therefore more accurate to use the tear lens formulae outlined in Chapter 15 to calculate the required BVP following BOZR modification.
For example, let us consider the compensatory change to BVP that is required when a rigid lens BOZR is steepened by a relatively small amount, from 6.70 mm to 6.60 mm. According to the approximation, we would add −0.50 D to the BVP of the rigid contact lens. However, using tear lens formulae, we see that we actually need to add (336/6.70–336/6.60) ≈ −0.75 D to the BVP. The magnitude of this error increases with increasingly steep BOZR. For example, using the approximation for a change in BOZR from 5.90 mm to 5.80 mm gives a suggested increase in BVP of −0.50 D; however, actual calculation shows that the BVP should be adjusted by −1.00 D.
Aspheric
Considerable improvements in manufacturing and technology, such as precision aspheric lathing, mean that rigid aspheric lenses also have application for the contact lens management of keratoconus ( ). Aspheric rigid corneal lenses are typically fitted nominally steeper than multicurve lenses in relation to the corneal curvature, however their sagittal height is usually less than a multicurve lens due to the relationship between the nominal BOZR and the lens eccentricity ( ). TD is an important fitting factor in aspheric designs, as it has to be manipulated, as a result of the base curve-eccentricity value relationship, to maintain the desired degree of edge clearance ( ). Smaller lens diameters will give greater edge clearance, while larger lens diameters will provide relatively less edge clearance.
Toroidal
Due to the highly irregular and asymmetric astigmatism that is characteristic of keratoconus, rigid corneal lenses with toroidal BOZR should be fitted sparingly ( ). A rigid lens with toroidal BOZR will generally locate poorly on an eye with keratoconicus, and show excessive on-eye rotation. Notably, the peripheral corneal curvature in keratoconus is often relatively symmetric and can have significant regular astigmatism; this is most commonly with-the-rule astigmatism. In such cases, the edge clearance will be more generous along the vertical meridian compared with the horizontal meridian ( Fig. 25.4A ); the quality of the peripheral fitting relationship can be enhanced with the use of toroidal peripheral curves ( Fig. 25.4B ).
Let us assume that the rigid lens shown in Fig. 25.4A incorporates the following parameters,
C4/6.70,6.40/7.40,7.20/8.30,8.00/10.10,8.80−7.50
Then, using the notation shown in Chapter 15 for specification of a rigid lens with a spheroidal optic zone and a toroidal back optic zone, the lens shown in Fig. 25.4B might have the following specification,
C4/6.70,6.40/7.107.80,7.20/8.008.70,8.00/9.8010.50,8.80−7.50
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