Keratoconus

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 ( ).

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.