Keratoconus is a progressive, non-inflammatory, ectatic disorder of the cornea and is characterised by decreasing corneal thickness and increasing corneal curvature (quantified by steepest keratometry reading).6 This coincides with increasing corneal irregularity and higher-order aberrations that manifest as decreasing optical quality.7 Most cases are bilateral and asymmetric but unilateral cases do occur.8 The keratoconic patient may report simple blur or be more specific and describe ghosting, haloes or monocular diplopia. High contrast visual acuity with spectacles may be well preserved, but this measure is a relatively poor indicator of the stage of the disease and poorly reflects the optical quality or symptoms experienced by the patient.9
Diagnosis and epidemiology
Keratoconus should be suspected in cases involving increasing astigmatism, frequently changing refractive error and reduced visual acuity presenting in children and young adults when the condition typically presents.6 Retinoscopy of keratoconic patients will reveal an irregular reflex (described as classically split or oil droplet reflex). Corneal irregularity can also be detected during keratometry as distorted mires. Slit-lamp signs of keratoconus vary with disease stage and range from subtle early signs (eg prominent corneal nerves) to the emergence of an iron ring demarcating the base of the cone (Fleischer’s ring) and posterior corneal vertical striae (Vogt’s striae) that become evident as the condition progresses.4 In advanced cases corneal steepening, apical thinning and scarring become obvious (Figure 1).4
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Sub-clinical (or forme fruste) cases show little or no sign of keratoconus and may go undiagnosed until detailed topography and pachymetry maps are performed. Attempts have been made to classify keratoconus into distinct stages9,10 (Table 1). It should be remembered, however, that keratoconus is a spectrum; individual cases may not fit neatly into one stage of the disease.
The reported incidence and prevalence of keratoconus varies greatly within the literature; this is likely to represent differences in the ethnic groups studied. Two UK-based studies reported the prevalence to be 4.4 and 7.5 times greater for those of Asian descent compared with Caucasians.2,3 UK prevalence of keratoconus among those of Asian descent is 229/100,000 and 57/100,000 among Caucasians.2 Prevalence is higher among men,3 Down’s patients, those with atopy (eczema, hayfever and asthma) and those who report a history of abnormal eye rubbing.4 Keratoconus has also been linked to certain immune disorders (including rheumatoid arthritis, autoimmune chronic active hepatitis and irritable bowel syndrome).20 A positive family history is also considered a risk factor as it is reported by 10 per cent of keratoconic patients compared to 0.05 per cent of age-matched controls.5
Aetiology
Keratoconus is almost certainly a multifactorial disease with both genetic and environmental causes. Several genes have been identified as potential key players, but the exact aetiology of the condition continues to be elusive and may even differ in different patient groups.19 Research into the genetic basis of keratoconus is ongoing.
Monitoring for progression
Once a diagnosis has been given, efforts should be made to monitor the patient for progression. Those with advancing disease or a significant risk of further progression may be suitable for collagen cross-linking (a surgical intervention proven to halt progression of the condition).13 Those who have previously been treated should be reviewed periodically to monitor ongoing stability. Measurement of corneal topography (including corneal thickness maps), refraction and best spectacle corrected visual acuity would ideally be performed at each visit along with ocular examination. The Oculus Pentacam instrument is comprised of a coupled slit-illumination system and Scheimpflug camera that rotates in front of the cornea while taking a series of images. The resultant sectional images are then compiled to create a three-dimensional model of the anterior segment that provides both anterior and posterior topography and corneal thickness maps. This instrument is considered the gold standard in keratoconus monitoring.
Optometrists have long been responsible for the non-surgical visual rehabilitation of keratoconus patients and with little additional training can gain the skills required to diagnose keratoconus and monitor patients for progression. As such, modern models of keratoconus care in some factions of the hospital eye service are predominately optometrist-led. Patients with suspected keratoconus referred to Moorfields Eye Hospital, for example, are seen by an optometrist-led early keratoconus monitoring clinic where diagnosis, refraction, monitoring and listing for collagen cross-linking are performed. Onward referral to the contact lens service or external disease ophthalmology clinic are then arranged as needed.
There is no single measure that is definitive of progression and various tests and values are considered when monitoring keratoconus. An increase in refractive astigmatism by -1.50DC or a loss of one line of best corrected visual acuity are both considered suggestive of progression. When reviewing Pentacam scans an increase in anterior K readings by 1.50D or more is suggestive of progression as is a 0.50D increase in posterior K reading. Corneal thinning of 13 microns or more is also suggestive of progression. Care should be taken to compare like for like scans and various factors such as recent contact lens wear, recent installation of fluorescein sodium or poor fixation can all cause erroneous results.
Preventing progression
In the absence of any treatment to arrest progression, approximately 20 per cent of keratoconic patients have previously gone on to require corneal graft surgery,11 making keratoconus the most frequent reason for keratoplasty in the last three decades of the 20th century.12 The corneal collagen cross-linking (CXL) procedure is the only proven treatment to stabilise progressive keratoconus and has been recommended by NICE since 2013.13 The particulars of the procedure have been refined since first described but it still involves three main steps – corneal epithelial debridement followed by the addition of riboflavin eye drops and subsequent exposure to ultraviolet (UV) A radiation.14 In its latest reincarnation the UV A is delivered over 8 minutes in a pulsed (one second off, one second on) pattern; this prevents the cornea from becoming oxygen depleted during treatment and increases the efficacy of the treatment. The UV A interacts with the riboflavin within the corneal tissue and cause the release of reactive oxygen species that facilitate the formation of bonds between collagen fibres. This stiffens the cornea, making further progression much less likely (only 7.8 per cent continue to progress)17 and in many cases causes a subtle flattening of the cone.15,16
Epithelium-off CXL is a relatively low risk procedure with a 2.8 per cent risk of persistent stromal haze and 7.6 per cent risk of sterile infiltrates.17 The removal of the corneal epithelium causes pain during recovery 18 and is associated with several of the potential complications. As a result, attempts have been made to develop techniques that do not require epithelial debridement, so called ‘trans-epithelial CXL’. These techniques have relied upon chemical agents (eg ethylenediaminetetraacetic acid or EDTA) to increase permeability of the epithelium to riboflavin. Until recently the effectivity of these procedures has been reported in case reports and non-randomised comparative studies with up to two years follow-up. The results of the latter have been inconclusive. A recent randomised controlled study with one year follow-up has reported ‘epi-on’ CXL with EDTA riboflavin to be less effective than the epi-off CXL used as a control group (23 per cent showing progression at one year after ‘epi-on’ CXL while 0 per cent showed progression in the ‘epi-on’ group).1 Further research into the effectivity of ‘epi-on’ CXL techniques is needed.13
Table 2 lists the current inclusion and exclusion criteria for CXL used by Moorfields Eye Hospital.
Spectacle correction
Although advancing keratoconus is often associated with an increasing reliance on contact lenses, many keratoconic patients are satisfactorily corrected with spectacles and it is surprising how well some keratoconic patients tolerate high astigmatic prescriptions in spectacles. At baseline, approximately 16 per cent of the keratoconus cohort included in the collaborative longitudinal evaluation of keratoconus (CLEK) study were wearing spectacles alone to correct both eyes, while approximately 47 per cent swapped between contact lenses and spectacle correction.21 In addition to finding a possible means of vision correction, refraction is also useful while monitoring patients for progression. Despite finding an irregular reflex retinoscopy is still a valuable tool in providing a starting point for subjective refinement. When performing subjective refraction on keratoconic patients the author advises starting with large steps that are appropriate to the patient’s visual acuity and ±1.00DC and ±0.50DC cross-cyls are advised. It should also be borne in mind that a diagnosis of keratoconus does not guarantee a myopic refractive error. Many keratoconics (especially among the mild to moderate group) will be found to have mixed astigmatism (eg hyperopic sphere with moderate/large vertical cylinder).
Contact lens correction
While there may be no clinical indication to fit a visually asymptomatic, well corrected spectacle wearing keratoconic with contact lenses, such patients may desire spectacle independence for cosmetic or lifestyle reasons just like any other spectacle wearer. These patients are free to explore that option within community practice and with the introduction of corneal cross-linking the prevalence of such cases will likely increase. The result is that more keratoconics will be optically managed within standard community practice.
As keratoconus progresses, high prescriptions (specifically high cylinders) and significant anisometropia become more common. As a result spectacle intolerance becomes more likely. Keratoconics suffering from spectacle intolerance despite achieving acceptable spectacle visual acuity (and visual quality) can do very well with standard soft toric contact lenses. Those with poor spectacle acuity may require a speciality custom-made soft lens or more likely a lens with rigid optics (ie corneal RGP, hybrid, corneoscleral or scleral). Those who are unable to tolerate spectacle correction or have poor spectacle visual acuity are eligible for NHS contact lens care and so referral to an appropriate service may be indicated. When faced with the challenge of fitting a keratoconic the following step-by-step approach may be useful.
- Step 1 — Identify the goals of fitting
It is important that both practitioner and patient have a clear understanding of the goals of contact lens fitting. It is not uncommon for patients appearing in the hospital contact lens clinic, for example, to be under the impression that their diagnosis of keratoconus means they must wear contact lenses despite being well corrected and completely happy with spectacles. Knowing the problems currently experienced by the patient will help set realistic expectations from the outset and will guide lens choice. Explaining to the patient at the outset that some night time glare may remain despite contact lens wear, for example, will help avoid disappointment further down the line.
- Step 2 – Assess best corrected spectacle acuity
Whether the patient desires lenses as a simple alternative to spectacles or is unsatisfied with the visual performance of spectacle correction, it is vital to perform a good refraction before beginning to fit the patient with contact lenses. Table 3 highlights the three commonest outcomes to this step and how this will guide further management and lens choice.
It is also worth considering how likely the patient is to wear back-up spectacles between periods of lens wear. Those with reasonable spectacle vision will be more likely to wear glasses after contact lens wear and so extra effort should be made to avoid lens fits that cause considerable spectacle blur on lens removal. In contrast those without useful spectacles are unlikely to notice fluctuations in their vision following removal of their contact lenses.
- Step 3 – Consider the patient
It is important that the patient is considered as a whole (rather than a pair of eyes) before embarking upon contact lens fitting. The contact lens fitter should consider the patient’s ability to handle the lenses being fitted and the environment in which they need to wear them. Those with atopy and prone to papillary conjunctivitis or those working in very dusty environments may do poorly with corneal RGP contact lenses. The presence of co-morbidities is also an important consideration. A patient heavily reliant on the use of medicated eye drops, for example, may be better off wearing a rigid lens to avoid accumulation of the drug or preservative within a soft lens.
- Step 4 – Consider the cornea
Cone size, steepness and placement are vital considerations when fitting corneal and hybrid contact lenses for a keratoconic patient. Low cones for example will often retain good spectacle visual acuity (albeit often with high vertical cylinders) and can make corneal RGP and hybrid lens fitting problematic. When larger lenses are fitted corneal sagittal height (or ‘sag’) becomes the most important consideration rather than corneal curvature; a deep cornea may not necessarily be very steep, while a steep cornea may not necessarily have a high sag. When fitting corneo-scleral or scleral contact lenses, a useful starting point is to observe the cornea in profile from side on, gauge the depth of the cornea and use this to guide initial lens choice.
Contact lens types
The past 10 years has seen the emergence, re-invention or renewed interest in several contact lens types (namely corneosclerals, hybrids and sclerals respectively). These, along with corneal RGP and soft toric contact lenses, are of great importance to the contact lens management of keratoconus and are the main weapons in the contact lens practitioner’s arsenal.
- Corneal RGPs — It is still a common misconception that RGPs are fitted to squash the irregularity out of the cornea and so improve vision. While some degree of corneal moulding is unavoidable with all contact lens wear, rigid lenses should fit harmoniously with the corneal profile while correcting the majority of corneal irregularities via the tear lens.7 There is no evidence that a compressive rigid lens will halt progression, rather such a fit is likely to be poorly tolerated and cause abrasions. Corneal RGP contact lenses remain a fantastic and relevant option in the optical rehabilitation of keratoconus and come with relatively low complication rates (Figure 2). Corneal RGPs designed specifically for the keratoconic cornea differ from those for normal corneas; typically they feature a greater degree of peripheral flattening, steeper central curves and greater sagittal depth. The shape, size and location of the cone as well as the profile of the peripheral cornea should all be considered when selecting a corneal RGP. Small, steep central cones, for example, may be best suited to a lens with smaller back optic zone diameter and steeper back optic zone radius while large or low cones may require a lens with a larger back optic zone to facilitate adequate centration. Corneal RGP contact lenses provide excellent optical correction for the irregular cornea and when well fitted can be very well tolerated once the patient has adapted. In some instances it becomes necessary to fit soft contact lenses under RGP lenses, a system known as piggybacking. This is done to improve contact lens tolerance by providing the corneal epithelium with additional protection or to improve the RGP fitting characteristics.
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- Hybrid contact lenses — Hybrid contact lenses combine a rigid central portion bonded to a soft skirt, the aim being to provide the superior optics of RGP lenses and the comfort associated with soft lenses. They are available with both a high Dk RGP centre and a silicone hydrogel skirt and their design means that wearers are at little risk of environmental foreign bodies getting under the lenses. Hybrid designs aimed at the keratoconus patient are generally fitted with shallow central clearance, so it is often necessary to insert the lens filled with saline to prevent bubble formation. Hybrid lenses are best suited to early or moderate cases of keratoconus with central or near-central cones.
- Corneoscleral contact lenses — Corneoscleral contact lenses are defined as RGP lenses that bear on both the cornea and conjunctiva and are typically around 14mm in diameter. Their large size and peripheral alignment on the conjunctiva means they move very little and in most cases the lens edge will sit under the patient’s lids when in the habitual position. This makes them very comfortable with minimal risk of environmental foreign bodies becoming trapped under the lens. Depending upon the fitting characteristics it may be necessary to insert corneoscleral contact lens filled with saline to avoid insertion bubbles. Like corneal RGP lenses they provide excellent optical correction but benefit from better stability on blink.
- Scleral contact lenses — Scleral contact lenses are designed to vault the cornea and rest entirely on the conjunctiva while trapping a sealed fluid reservoir between the lens and cornea; this can make them an excellent choice for those with irregular corneas and especially when corneal desiccation and dry eye have been a problem with other lens types. The trapped fluid reservoir, however, is also associated with potential problems, such as the accumulation of debris behind the lens that may decrease vision or hypoxia associated with reduction of oxygen supply to the cornea. The risk of the latter can be minimised through the use of high Dk materials and avoiding excessive corneal clearance. Sclerals range in size, but those most widely available are 16.5mm, 18mm or 23mm in diameter; whatever their size they must be inserted filled with saline or alternative suitable solution and this must not leak during wear. Made of an RGP material, scleral contact lenses provide excellent optics and like corneosclerals they tend to be very comfortable due to the lack of interaction between the eyelid and lens edge. Scleral lenses have recently had a resurgence in popularity due to their unique properties and increased availability and affordability.
Conclusion
With the introduction of corneal collagen cross-linking it is hoped that more patients will avoid the need for corneal transplantation and more will be suitable for visual correction within the primary care/community setting. Optometrists are ideally placed diagnose and monitor keratoconic patients and this is a natural expansion of their traditional role of providing optical rehabilitation for these patients. By managing expectations, ensuring practitioners and patients are acquainted with the benefits and limitations of contact lenses and by adopting a logical and systematic approach to contact lens fitting for keratoconus, the optometrist can maximise their chances of success.
Model answers
Which of the following is not known to be associated with early keratoconus?
C Corneal opacification
Where in the cornea are Vogt's striae to be found?
C Stroma
Which of the following systemic conditions is associated with keratoconus?
A Rheumatoid arthritis
Which of the following would exclude treatment with collagen cross linking?
D Corneal thickness of 350 microns
Which of the following is not part of collagen cross-linking technique?
B UVC exposure
Which of the following statements about scleral lenses for keratoconus is false?
D They remould the corneal surface to a regular profile
References
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Matthew Carter is an optometrist based at Moorfields Eye Hospital and Linklater & Warren Opticians, Chislehurst