Corneal degenerations and keratoconus: anterior eye & oculoplastics part 7 C-20076 O/D.
Keratoconus is a progressive condition where there is corneal thinning and protrusion (ectasia). It affects the central two-thirds of the cornea and results in a conical corneal shape. There are various theories regarding the aetiology of keratoconus, although the exact cause remains unknown. Its association with systemic diseases, eg atopic conditions such as asthma, hayfever and eczema, suggest an ectodermal origin, however association with connective tissue disorders, eg Ehlers-Danlos syndrome and osteogenesis imperfecta, suggest a mesodermal origin. Several reports have also indicated microtrauma, eg from rubbing eyes, as a causative factor in inducing keratoconus. This theory is supported by the fact that patients with keratoconus frequently have itchy eyes and ocular irritation. An autosomal dominant inheritance is reported in approximately 10% of cases.
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The prevalence of keratoconus varies widely, but is estimated to be between 50 and 230 per 100,000. (1) The onset is around puberty and there is gradual progression until the third or fourth decades of life when it usually ceases to progress. There is no gender predominance and it almost always presents bilaterally, although clinical features are frequently asymmetrical. The rate of progression varies between individuals and also between the two eyes. The severity at the time progression stops is also highly variable and ranges from mild irregular astigmatism to severe thinning, scarring and protrusion.
Presentation and clinical signs
The majority of patients present early in the course of disease complaining of decreased or distorted vision. In cases where keratoconus is suspected and VA remains relatively normal, a significant decrease in contrast sensitivity may be noted. The hallmark of keratoconus is central or paracentral thinning, irregular astigmatism and apical protrusion; clinical examination and topography aid in the diagnosis. Key clinical signs include:
* Scissoring reflex on retinoscopy
* Oil droplet sign on direct ophthalmoscopy from a distance
* Angulation of the lower lid on down gaze (Munson's sign)
* Slit lamp biomicroscopy reveals corneal thinning and protrusion (Figure 1), deep, vertical striae (Vogt's) that disappear on application of pressure (Figure 2), prominent corneal nerves, iron deposit line surrounding the base of the cone (Fleischer ring) and scarring (in advanced disease from breaks in Bowman's layer or after hydrops).
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Acute corneal hydrops is a stromal oedema caused by aqueous penetration through breaks in Descemet's membrane. The onset of corneal hydrops is classically associated with a sudden decrease in both uncorrected and corrected vision as well as redness, pain and photophobia. The corneal oedema may persist unpredictably for weeks or months, with gradual resolution as the posterior break is replaced with a collagenous scar. Topical osmotic agents such as hypertonic saline may accelerate the recovery of vision; however, this treatment is mostly helpful to alleviate oedema within the epithelium and anterior stroma. Prophylactic topical antibiotics are frequently prescribed and steroids may reduce the risk of corneal neovascularisation.
Corneal topography is invaluable in detecting the presence of keratoconus, especially in patients with little or no signs on slit lamp examination. Furthermore, it allows detection and monitoring of disease progression and treatment response. Various different types of corneal imaging systems are available, eg Placido discs, slit-scan, Schiempflug imaging, and ocular coherence tomography (OCT). In early keratoconus there is a characteristic steepening of the posterior corneal surface, initially occurring mid-peripherally below the midline. This is demonstrated by the close proximity of the keratoscopy (placido) rings to one another. Above the midline, the superior cornea remains relatively normal in curvature. As the condition progresses, individual corneae can take on a wide range of topographical shapes and three types of cones have been described: nipple, oval and globus (Figure 3).
Topographic indices are used to screen patients and distinguish keratoconus from normal corneae. The indices proposed include central corneal power of >47.2D, inferior-superior dioptric asymmetry (I-S value) >1.2, astigmatism >1.5D and skewee radial axes >21[degrees] (Figure 4). White inbuilt software may indicate Che likelihood of ectasia, normal variation and a poor ocular surface (eg dry eyes) may lead to erroneous results. The scans must therefore be interpreted in conjunction with clinical examination and use of artificial tears prior to scanning may improve image quality.
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The aim of management is to [prevent further progression ob corneal changes and simultaneously provide visual rehabilitation. Various options are available, the choice depending on the severity of the condition.
As many patients may suffer from atopic conditions, the eyelids and ocular surface must tie examined carefully. Any allergic eye disease and ocular surface inflammation must: be appropriately treated with topical antihistamines/mast cell stabilisers and lubricants to improve ocular comfort. Where appropriate, preservative free preparations are preferable. Occasionally topical or subtarsal steroids or other immunomodulatory treatments, eg tacrolimus ointment, may be required.
These patients must also be strongly advised to refrain from rubbing their eyes as t here is evidence that this may exacerbate the ectasia.
In the very early stages of keratoconus, spectacles are an option for correction of vision, especially for patients who achieve VA of b/12 or better. However, spectacles do not correct irregular astigmatism and so contact lenses provide a better option in such cases. Contact lenses represent the treatment of choice in 90% of patients with keratoconus. The type of contact lenses used varies depending on the stage of keratoconus. Early in the disease, soft toric lenses may be adequate to correct myopia and regular astigmatism. As the disease progresses, rigid gas permeable (RGP) lenses are used. Tolerability and advanced protrusion of the cornea may necessitate the use of hybrid/piggyback or scleral contact lenses. It is imperative to understand that many patients with keratoconus depend on their contact lens correction for most of their daily activities. Appropriate selection and fitting of contact lenses may delay the requirement of surgical treatments (see later). On the other hand, a proportion of patients who elect to undergo surgical treatment return to contact lenses in order to achieve their best VA.
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Collagen cross-linking (CXL)
CXL is a restively recent treatment that stiffens the cornea and halts progression of the disease. The treatment involves soaking the corneal stroma with riboflavin (vitamin B2) followed by irradiation with ultraviolet (UV) light. Riboflavin acts as a photosensitiser that causes formation of bonds between the collagen fibrils and extracellular matrix, thus strengthening the cornea and reducing progression of ectasia. Most patients only require a single treatment. It is indicated in patients that have evidence of progressive disease and adequate corneal thickness (at least 450um). This revolutionary treatment may prevent patients from requiring a corneal transplant and if applied early in the course of disease, it may allow t hem to maintain good vision with spectacles or contact lenses. It is therefore imperative to refer patients with suspected keratoconus for an early evaluation.
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Intracorneal ring segments
Intracorneal rings are PMMA segments that were initially approved by the US FDA for management of myopia and astigmatism. However, they have been shown to be effective for" the treatment of keratoconus and to stabilise ectasia resulting from corneal refractive surgery. (2) These rings are inserted into channels within the corneal stroma. The channels may be made mechanically or with the use of a femtosecond laser. Once inserted, they cause flattening of the cornea in the corresponding area of insertion and may allow better" contact lens fitting and reduction of myopia. Treatment with intrastromal rings does not eliminate the progression of keratoconus, but it may delay the need for corneal transplantation. This treatment may also be combined with CXL in patients that achieve a good visual outcome in order to prevent a corneal transplant.
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Phakic intraocular lenses
Several studies have reported the successful use of toric as well as phakic intraocular lenses (IOLs) in patients with Ceratoronus. (3-5) Stable keratoconus or high ametropia/astigmatism following corneal transplantation may be treated with IOLs. Various types exists, eg anterior chamber lenses, iris-clip lenses (Artisan) and intraocular collamer lenses (ICLs).
Once patients achieve a good visual outcome eg after intracorneal ring implants or reduction of irregular astigmatism following topography guided surface ablation, the cornea may be treated with CXL in order to prevent any further deterioration. There ore various reports of combination treatments that show promising outcomes. (5)
This is usually reserved for patients which have extensive corneal scarring and/or are contact lens intolerant. Over the last few years there has been a move away from full thinness (penetrating keratoplasty--PKP) to partial thickness (deep anterior lamellar keratoplasty--DALK) for keratofonus, in cases without significant corneal scarring or corneal hydrops. The DALK technique aims to remove all, or nearly all, of the corneal stroma down to Descemet's membrane. DALK is a more challenging procedure compared to PKO and may result in a slightly reduced visual outcome. It does, however, offer significant advantages over PKP including reduced risk of 'open sky' surgery. Also, since the patient's endothelium is preserved, there is no risk of graft failure from endothelial rejection figure 5 shows the pre-and post-operative clinical appearance of a patient with keratoconus who underwent DACK. Figure 6 shows the corresponding topography.
Other Corneal Degenerations
Degenerative changes may be classified as involutional (primarily related to age) or non-involutional (related to local or systemic conditions). Generalised involutional degeneration of the cornea includes decreased corneal thickness, increased rigidity, thickening of Descemet's membrane, and reduction of endothelial cells. Table 1 lists some of the common corneal degenerations and these are discussed below.
Corneal arcus (Arcus Senilis)
This common degeneration is characterised by a yellow/grey peripheral arc, which has a sharp outer edge and a more diffuse inner edge. There is a translucent zone between the outer edge and the limbus (Figure 7). The prevalence increases with age and it is more common in men than women. It represents deposition of lipids at Descemet's membrane and Bowman's layer with extensions into the stroma. The lipids are thought to permeate through the capillaries at the limbus. Corneal arcus is not visually significant, but young patients (under 40 years of age) may have underlying hyperlipoproteinemia and a higher risk of coronary artery disease. Unilateral arcus may represent vascular abnormality (eg carotid stenosis) on the contra lateral side. Thus young patients and those with unilateral arcus must re referred for further investigations.
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Corneal farinata appears as tiny grey-white 'flour' like opacities in the deep stroma, just anterior to Descemet's membrane. These opacities do not interfere with vision and are best visualised by retroillumination. They are usually an incidental finding.
This is characterised by a mosaic polygonal pattern that: resembles crocodile skin (Figure 8). This appearance results from irregular arrangement of collagen (due to relaxation of tension within the cornea. The opacities may be in the anterior or posterior stroma and a re most often bilateral and central. The posterior form appears similar to central cloudy dystrophy of Francois. They may occasionally roe found in the peripheral cornea mimicking corneal arcus. Crocodile shagreen does not generally affect vision and no treatment is necessary.
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This is represented boy peripheral corneal thinning (between corneal arcus and the limbal vessels). The thinning is usually shallow and the patients are asymptomatic. There is absence of inflammation and vascularisation.
A grey-white opacity results from deposition of calcium at Bowman's layer. It typically beg ins in the periphery at the 3 and 9 o'clock positions and gradually progresses centrally. The peripheral edge is sharply demarcated, while the central edge is diffuse and blends into the normal cornea. Lucent holes are frequently seen within the band and represent areas of penetrating corneal nerves (Figure 9). Band keratopathy can occur from a variety of local and systemic causes (Table 2) but is commonly seen in chronic uveitis and hypercalcaemic states. It is believed that alteration of the pH of tears and evaporation (on the exposed interpalpebral area) leads to precipitation of calcium. Early band keratopathy does not require any intervention. Later stages may cause irritation, grittiness or intense pain (from sharp exposed edges of calcium plaques) and/or reduction of vision as it encroaches the visual axis. In such cases, surgical removal with application of ethylenediaminetetracetic acid (EDTA) may be performed. Alternatively, excimer laser phototherapeutic keratectomy maybe used to clear the visual axis.
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Salzmann's nodular degeneration
This is characterised by white-blue elevated lesions on the corneal surface. The lesions may be singular or multiple and are often found in the mid-periphery (Figure 10). They are more common in women than men and may be unilateral or bilateral. It appears in eyes with inflammatory disease e.g. vernal keratoconjunctivitis phlycten, trauma, and trachoma, or may be idiopathic. The lesions are generally asymptomatic, but may cause irritation or decrease in vision if involving the visual axis. Topical lubrication is usually effective in controlling the mild irritation. A superficial keratectomy or excimer laser ablation may be performed to remove the lesions. Deep stromal lesions may rarely require a lamellar or penetrating keratoplasty. The lesions may recur following treatment.
Terrien's marginal degeneration
This is a rare condition of unknown aetiology. It is most commonly seen between the ages of 20-40 years and is three times more common in males than females. It is a peripheral inflammatory condition that begins an area of superonasal corneal thinning. The central edge is steep and a yellow-white zone of lipid may be seen. The peripheral edge gently slopes towards the limbus and the overlying epithelium is intact (Figure 11). The lesion may progress centrally or circumferentially. It is frequently bilateral and symmetric but may tie asymmetric. Younger patients may get episodes of inflammation, episcleritis or scleritis and may be treated with topical steroids. The superior location frequently leads to against-the-rule astigmatism, which may tie managed with spectacles or contact lenses. Progressive disease may cause corneal perforation (usually precipitated by trauma) and necessitates corneal transplantation.
Spheroidal degeneration (Climatic droplet keratopathy)
Yellow-gold spherules form within the interpalpebral aperture, usually at the limbus, but may start centrally. They may also form on the conjunctiva and are frequently to found in association with pingueculae. The spherules darken with age and may become elevated, leading to irritation and reduced vision. Risp factors include UV light and microtraum a (from dust,wind, or drying). It is thus more common in dry and warm climates with a higher incidence in men. Spheroidal degeneration may also occur secondary to local disease e.g. herpetic keratopathy and glaucoma. Protection from UV light and irritants and the use of lubricants forms the mainstay of treatment. Advanced cases may require excision (conjunctival lesions) or corneal transplant (corneal lesions).
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1. Which of the following stetements regarding! the diagnosis of keratoconus is TRUE?
a) Unaided and best corrected VA may be good in the early stages of disease
b) Vogt's striae are always present on slit lamp) examination
c) Diagnosis can be made solely on corneal topography
d) It never manifests after the age of 30years
2. Which of the following statements about keratoconus is FALSE?
a) There is thinning of the peripheral cornea
b) Multiple breaks can be seen in Bowman's membrane
c) Hydrops results from a break in the Descemet's membrane
d) Iron deposition can be seen at the base of the cone
3. Which of the following is the LEAST appropriate management option for patients suspected of (saving keratoconus?
a) Advise them against rubbing their eyes
b) Vision correction through RGP contact lenses
c) Referral to ophthalmology for collagen cross-linking
d) Referral to ophthalmology for penetrating keratoplasty
4. Which of the following statements about collagen cross-linking is TRUE?
a) It is indicated for advanced keratoconus
b) It aims to prevent further progression of keratoconus
c) It results in flattening of the cornea back to the normal shape
d) Most patients require several treatments for full (effect
5. Which of the following statements regarding] corneal arcus is TRUE?
a) It often encroaches the central cornea
b) It is uncommon in the elderly
c) Occurrence in young people requires further investigation
d) It consists of protein deposits
6. Which of the following statements regarding band keratopathy is FALSE?
a) It is only associated with systemic disease
b) It usually originates at the 3 and 9 o'clock positions
c) It can be removed by chelation with EDTA
d) It consists of calcium deposits in Bowman's layer
See www.optometry.co.uk/clinical. Click on the article title and then on 'references' to download.
Amit Patel, MB BCh, FRCOphth
Sunil Shah MBBS, FRCOphth, FRCS(Ed), FBCLA
Mr Amit Patel is a consultant ophthalmologist at the Heart of England NHS Trust and Midland Eye Institute in Birmingham. He has a special interest in corneal, cataract and refractive surgery. He is a treasurer of the British Society for Refractive Surgery (CSRS) and a council member of the Medical Contact Lens and Ocular Surface Association.
Prof Sunil Shah is n 0onsultant ophthalmologist at the Mid land Eye Institute, Birmingham and Midland Eye Centre, and honorary consultant at Birmingham Children's Hospital. He is also an honorary professor at the University of Ulster and visiting professor at Aston University, Birmingham. He specialises in complex corneal and refractive surgery. Professor Shah is past president of the British Society for Refractive Surgery (BSRS) and is a specialist advisor to the National Institute for Clinical Excellence and British Standards Institute.
Table 1 Classification of corneal degenerations Involutional Non-involutional Corneal arcus Band keratopathy Farinata Salzmann's nodular degeneration Furrow degeneration Spheroidal degeneration Crocodile Shagreen Terrien's marginal degeneration Table 2 Local and systemic causes of band keratopathy Local Systemic Chronic uveitis Hyperparathyroidism Intraocular silicone oil Renal failure Interstitial keratitis Excessive vitamin D intake Topical medications (phosphate containing) Sarcoidosis Juvenile dopathic arthritis Milk-alkali syndrome with uveitis Phthisis bulbi Hypophosphatasia
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|Title Annotation:||CONTINUING EDUCATION & TRAINING|
|Author:||Patel, Amit; Shah, Sunil|
|Date:||Nov 2, 2012|
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