Description of the condition
Keratoconus means conical cornea. It is a rare condition of the eye that affects approximately 50 to 230 per 100,000 people (Li 2009). The cornea is the main focusing surface of the eye. Keratoconus reduces vision by altering the shape of the cornea so that it becomes stretched and thin, making the vision short-sighted, irregular and distorted. The condition can affect one or both eyes, in which case it can progress at varying rates.
Initially, the patient may present with either a spherical cornea or regular corneal astigmatism. Around the onset of puberty, the cornea begins to thin and protrude, resulting in irregular astigmatism with what is usually a steep curvature. Usually, over a period of the next 10 to 20 years, the process continues until the progression gradually stops. The rate of progression is variable. The severity of the disorder at the time progression stops can range from very mild irregular astigmatism to severe thinning and protrusion with scarring (Krachmer 1984).
Keratoconus is usually diagnosed during the second and third decades of life. The ectasia progresses at a variable rate, although it is more rapid at a younger age. Patients usually have myopic astigmatism and are often suspected of having the condition by their ophthalmologist and/or optometrist when a deterioration in visual acuity (that is no longer correctable by spectacles) occurs.
Hydrops is an acute complication of keratoconus where there is severe photophobia (sensitivity to light) and reduction in visual acuity due to sudden stromal oedema. This is caused by breaks in the Descemet’s membrane (deep layer in the cornea) due to progressive ectasia.
Reported ocular associations of keratoconus include vernal keratoconjunctivitis, retinitis pigmentosa and Leber's congenital amaurosis. Systemic putative associations include many of the connective tissue disorders (e.g. Ehlers-Danlos and Marfan syndromes), mitral valve prolapse, atopic dermatitis and Down's syndrome (Rabinowitz 1998). The outcomes of the Collaborative Longitudinal Evaluation of Keratoconus (CLEK) Study showed that keratoconus is not associated with increased risk of connective tissue disease (Wagner 2007).
Predisposing factors identified include atopic history, especially ocular allergies, rigid contact lens wear and vigorous eye rubbing. In 13.5% of cases there is a family history of the disease (Zadnik 1996). The inheritance in these cases is thought to exhibit variable penetrance (Zadnik 1998). There is no sexual or racial predilection although the incidence has been found to be higher or more severe in certain ethnic groups (Georgiou 2004).
There is a general observation that elderly patients with keratoconus are not seen as often in clinics and some have postulated that the co-existing connective tissue disease may be contributory to a decreased life expectancy. However, this has been disputed by a study which has shown that patients with keratoconus do not have an increased mortality rate (Moodaley 1992).
Keratoconus is unique among eye diseases, in that it is typically diagnosed during peak education, income-earning, and child-rearing years (Wagner 2007). Keratoconus is diagnosed based on clinical examination and corneal topographic/tomographic analysis. There are several clinical signs for which the presence or absence of each is determined by the severity of the condition. An acute angulation, made by the ectatic cornea, can be seen in the lower lid on downgaze (Munson’s sign). Fleisher’s ring is a ring of epithelial iron deposition around the base of the cone. Vertical striae (Vogt’s), which are fine stress lines in the Descemet’s membrane, are detectable posteriorly. With time there is progressive thinning of the corneal stroma and the ectasia may be clinically detectable. An 'oil droplet' sign is often visible by direct ophthalmoscopy on viewing the red reflex in a dilated eye. Scissoring and distortion of the reflex can be seen on retinoscopy (Zadnik 1996). After repeated attacks of hydrops, stromal scarring may be visible.
Computer-assisted videophotokeratoscopy or Scheimpflug imaging are sensitive means for detecting subtle changes in topography on the anterior and posterior corneal surface and allow detailed qualitative and quantitative analysis of corneal shape. Corneal topography measures the steepening in terms of a diopteric power map of the cornea. Various topographic indices have been proposed for pre-clinical diagnosis of keratoconus (forme fruste) or the diagnosis of keratoconus and grading of the severity of the disease.
Classification of keratoconus can be based on morphology, disease evolution, ocular signs and index-based systems of keratoconus. Smolek et al have developed the Keratoconus Severity Index (KSI) using a neural network approach to data collected from corneal topography (Smolek 1997). The Krumeich Classification of Keratoconus Severity is another method to classify the disease and depends on mean K-readings on the anterior curvature sagittal map, thickness at the thinnest location, and the refractive error of the patient. This classification is useful in choosing the best approach for keratoconus. The keratoconus percentage index (KISA%), which combines many of the earlier indices, has been shown to have a high sensitivity in the videokeratoscopic identification of keratoconus (Li 2009). Keratoconus has three characteristics seen by videokeratoscopy that are not present in normals: an increased area of corneal power surrounded by concentric areas of decreasing power, inferior-superior power asymmetry, and skewing of the steepest radial axes above and below the horizontal meridian (Rabinowitz 1998). Keratoconus Severity Score (KSS) is another simple tool that was developed using common clinical markers in addition to two corneal topographic indices (average corneal power (ACP) and higher-order first corneal surface wavefront root mean square (RMS) error (HORMSE), resulting in severity score (0 to 5) (McMahon 2006). Belin/Ambrósio Enhanced Ectasia Display is an integrated application in Pentacam system (Oculus Optikgeräte GmbH, Wetzlar, Germany) that combines data from maximal keratometry, tomographic thickness distribution, and enhanced elevation to facilitate the detection of keratoconus (Ambrosio 2003; Belin 2007). Additional metrics, such as epithelial thickness mapping with very high frequency ultrasound or high-resolution optical coherence tomography can detect early keratoconus (Li 2012; Reinstein 2009; Reinstein 2010). Newer techniques include analysing biomechanical properties of the cornea using the Ocular Response Analyzer (Reichert, Buffalo, NY), which so far has limited ability in screening for keratoconus (Fontes 2010).
To date, the best and safest method of screening/diagnosing keratoconus is to use as many data as possible in combination with established clinical parameters.
The differential diagnosis of keratoconus includes keratoglobus, pellucid marginal degeneration and posterior keratoconus.
Aetiology and pathogenesis
Keratoconus has been reported in various clinical settings. It is most commonly an isolated sporadic disorder, or it may be associated with other rare genetic disorders, Down's syndrome and Leber’s congenital amaurosis, connective tissue disorders, atopy, hard contact lens wear and eye rubbing, and a positive family history of the disorder. Several theories have been postulated regarding the aetiology of keratoconus.
The biomechanical characteristics of the normal cornea result from the collagen scaffold and collagen compound and their bonding with the collagen fibrils. The three-dimensional configuration of the collagen lamella fundamentally codetermines the cornea’s resistance. Biochemical and immunohistochemical studies of the matrix’s proteoglycans show differences between normal and keratoconic corneas (Meek 2005; Raiksup-Wolf 2008).
Despite intensive biochemical investigation into the pathogenesis of keratoconus, the underlying biochemical process and its aetiologic basis remain poorly understood. Corneal thinning appears to result from loss of structural components in the cornea; the reason this occurs is not clear. Theoretically, the cornea can thin because it has fewer collagen lamellae than normal, fewer collagen fibrils per lamellae, closer packing of collagen fibrils, or various combinations of these factors. These conditions may result from defective formation of extracellular constituents of corneal tissue, a destruction of previously formed components, an increased distensibility of corneal tissue with sliding collagen fibres or collagen lamellae, or a combination of these mechanisms (Akhtar 2008; Hayes 2008). However some biochemical studies have demonstrated that collagen composition in corneas with keratoconus was unaltered. Biochemical assays and immunohistological studies of corneas with keratoconus suggest that the loss of corneal stroma after digestion by proteolytic enzymes could be caused by increased levels of proteases and other catabolic enzymes (Rabinowitz 1998).
Knowing the natural course of keratoconus is important in order to understand the rate and severity of visual change. However, it is difficult to fully appreciate the natural course of the disorder as usually the corneal changes have begun before the patient is first seen and, after that, treatment may modify the natural course (Krachmer 1984). CLEK study findings revealed that age appears to be a factor in severity-related outcomes in keratoconus (Wagner 2007).
There is a general trend for the disease to progress leading to a gradual increase in corneal curvature and decrease in visual acuity with consequent impact on quality of life (dependency, driving, mental health, near activities and role difficulties) (Wagner 2007). There are no definitive criteria for progression, but parameters to consider are change in refraction (both sphere and cylinder), uncorrected and best-corrected visual acuity, and corneal topographical changes. The increase of the maximum keratometry reading (Kmax) by one dioptre or more (≥ 1 D) remains the most frequently reported index of disease progression (Caporossi 2010; Hersh 2011; Raiksup-Wolf 2008; Wittig-Silva 2008).
It is possible in the early stages to use spectacles to improve vision but as the disease progresses, rigid gas permeable contact lenses often offer the best vision. Various contact lens designs and fittings have been developed to adapt to the challenging needs of this disease, which typically progresses. The presence of corneal scarring, significant thinning and intolerance to contact lens wear are indications for corneal transplantation (keratoplasty). In developed countries, keratoconus is often the most common indication for keratoplasty.
Several new therapeutic options have emerged which include refractive, optical and tectonic interventions, which slow the progression of disease and/or delay more invasive treatment.
There are several methods for corneal transplantation. Penetrating keratoplasty (replacement of the full thickness of the cornea) and more recently deep anterior lamellar keratoplasty (DALK, replacement of the front layers of the cornea only) are the surgical interventions most commonly performed (Shimmura 2006). Epikeratoplasty is a surgical technique where a piece of donor cornea is grafted onto the host after removal of the host epithelium (surface layer). Epikeratoplasty is now performed infrequently as the visual outcomes are poorer than those of penetrating or deep lamellar keratoplasties. However, epikeratoplasty may be suitable in cases where penetrating keratoplasty carries a higher risk (Wagoner 2001).
If the scarring is superficial, phototherapeutic keratectomy (use of laser to remove layers of the cornea) has been found to be useful (Rapuano 1997). In carefully selected cases, photorefractive keratectomy (PTK) has helped to smoothen the corneal surface and allow better contact lens fitting. Laser-assisted in situ keratomileusis (LASIK) has also been used. Both PTK and LASIK carry a high risk of the keratoconus progressing as both procedures weaken the normal corneal biomechanics.
Intrastromal rings (Intacs, Ferrara, Kerarings) are small devices that can be implanted into the cornea in an attempt to flatten the corneal profile to achieve a better uncorrected visual acuity and to enhance contact lens tolerance (Rabinowitz 2007). However, this procedure has its own limitations. Firstly, it does not affect the underlying biochemical properties of the cornea. Secondly, there is a limit to how much corneal flattening can be achieved. Most complications of intrastromal ring implantation can be reversible by removing the segment, but serious complications may occur, such as intraoperative corneal perforation, infectious keratitis (corneal infection), damage to the central visual axis, or corneal melt (Boxer Wachler 2003; Miranda 2003). Conductive keratoplasty has been used in an attempt to reduce the severity of astigmatism (Naoumidi 2005). 'Bioptics' is a sequential method of treating large and complex refractive errors by several methods often involving intraocular lens implants. It has been used in keratoconus with treatment algorithms that involve various combinations of intracorneal rings, phacoemulsification, in-bag implants, iris clipped phakic lenses and posterior chamber phakic lenses (Leccisotti 2006). Bioengineered corneas are likely to be available in the future and may offer superior optical results when transplanted (Carlsson 2003).
Collagen cross-linking (CXL) with ultraviolet A (UVA) light and riboflavin (vitamin B2) is a relatively new treatment that has been reported to slow the progression of the disease in its early stages (Spoerl 1998; Wollensak 2003; Wollensak 2006). When combined with intracorneal ring segments, the improvement in vision has been found to be greater than using the segments alone.
Description of the intervention
Collagen cross-linking with UVA and topical riboflavin is carried out in sterile conditions. There are two methods of performing the procedure: corneal epithelium off or corneal epithelium on.
Corneal epithelium off
In this method (Baiocchi 2009; Hayes 2008), the epithelium of the central 7 mm of cornea is removed after installing topical anaesthesia (e.g. proxymetacaine 0.5%). The surface is then treated by the application of riboflavin (vitamin B2) 0.1% solution (10 mg riboflavin-5-phosphate in 10 ml dextran 20% solution which is iso-osmolar 0.1% riboflavin solution) for 30 minutes starting five minutes before the start of irradiation. UVA radiation of 370 nm wavelength and an irradiance of 3 mW/cm2 at distance of 1 cm from the cornea is applied for a period of 30 minutes, delivering a dose of 5.4 J/cm2 (Wollensak 2006). Antibiotic drops are instilled as prophylaxis and a bandage contact lens is inserted, which is then removed at the follow-up visit once epithelial healing is complete.
Variations of this protocol include the use of pilocarpine 1% pre-operatively, a treatment area of 9 mm and the selective use of steroids in the postoperative regimen to prevent corneal haze (Caporossi 2006).
Another modification of the technique involves an epithelial removal zone of 9 mm diameter followed by riboflavin drops instillation onto the cornea every three minutes for 30 minutes. After confirming that riboflavin has appeared in the anterior chamber on slit-lamp examination, UV irradiation is commenced. During the irradiation time of 30 minutes, the cornea is rinsed with riboflavin solution and topical anaesthetic every five minutes.
Corneal epithelium on
In this method, the corneal epithelium is kept on (Chan 2007; Pinelli 2007). In 2003, Boxer Wachler proposed a slight modification of the treatment using preoperative anaesthetic eyedrops containing benzalkonium chloride to loosen the tight junctions of the corneal epithelial cells (Boxer Wachler 2003). The use of benzalkonium chloride may allow transepithelial cross-linking treatment without removal of the epithelium. This technique was designed to reduce postoperative pain and improve patient comfort. This modification is known as C3-R (Baiocchi 2009; Vicente 2010). In this technique, 30 minutes before the UV-A treatment, one drop of pilocarpine 2% is installed and riboflavin solution is started (one drop every two minutes with minimum 16 drops over 30 minutes). Topical anaesthetic drop is started 20 minutes before the treatment (one drop every four minutes, repeated four times). Treatment with UVA irradiation lasts for a total of 30 minutes, adding one drop riboflavin every five minutes. In both techniques, a bandage contact lens is inserted at the end of the procedure and removed after five days.
A slight modification to the epithelium-on technique is by performing epithelial disruption in the 9 mm zone (unpublished data) using a special disrupter to create pockmarks in the corneal epithelium. The primary goal is to maintain as much live epithelium as possible but also promote riboflavin penetration. The secondary goal is to reduce inflammation in the eye and to get the contact lens out of the patient within 48 hours. Riboflavin eye drops are instilled after disruption and every two to five minutes for at least 30 minutes before UV radiation treatment.
Recent trials aiming to reduce time of procedure to nine minutes have used higher power (up to 30 mw/cm2 compared to 3 mW/cm2 in the standard procedure). The aim is to achieve a rapid treatment protocol by using higher intensity UVA and shorter irradiation time. This technique (also called flash-linking or rapid cross-linking) aims to keep an equivalent energy dose to the standard irradiation of 3 mW/cm2 whilst reducing the treatment time from 30 minutes to nine minutes (Schumacher 2011).
In eyes with a corneal thickness less than 400 microns after epithelial removal, there is a risk of corneal endothelial, lenticular or indeed intraocular UVA damage (Kymionis 2012). In order to counter this, a hypo-osmolar (hypotonic) solution of riboflavin is used to swell the corneal stroma and hence increase the corneal thickness through a denuded epithelium, prior to application of UVA. This technique was tried on corneal thickness (after epithelial removal) between 320 to 400 microns (Hafezi 2009).
How the intervention might work
Collagen cross-linking employs the photosensitiser riboflavin (vitamin B2), which when exposed to longer wavelength ultraviolet light (370 nm UVA), will induce chemical reactions (free radical production) in the corneal stroma and ultimately result in the formation of covalent bonds between the collagen molecules, fibres and microfibrils. This increase in collagen bonding is thought to prevent further thinning and ectasia and as such slow or halt the progression of keratoconus. Some preclinical investigations, including biochemical and biophysical measurements, have demonstrated enhanced corneal rigidity and greater biomechanical stability of the cornea following this treatment (Raiksup-Wolf 2008; Wollensak 2003).
Wollensak et al have further demonstrated a significant increase in collagen fibre diameter as the underlying histopathologic correlate after collagen cross-linking. Increased resistance to pepsin digestion after cross-linking has been found, which might be important for keratoconus as a significantly elevated activity of collagenases has previously been noted (Wollensak 2004).
Why it is important to do this review
As mentioned above, corneal collagen cross-linking treatment is the only treatment that claims to slow down the progression of the disease. The treatment is carried out largely unregulated, with very few standardised criteria available for identification of the ideal patient who would benefit from the treatment. Keratoconus is very asymmetrical and at times a very slowly progressive disease; in particular it is well known that the rate of progression slows with age (Hovakimyan 2012; Krachmer 1984). Robust evidence on long-term efficacy and safety of collagen cross-linking, which is currently unavailable, is needed. Short-term data from trials are available and long-term data are awaited. Keratometric indices are at present the main indicators of treatment effect. Changes in corneal biomechanics, which this treatment purports to induce, have not been studied in vivo (Ashwin 2009). This review will examine evidence from current literature (and future trial results as and when they become available) and provide answers to clinicians on what they can expect of this relatively new treatment.