Ординатура / Офтальмология / Английские материалы / Clinical Ophthalmology A Systematic Approach 7th Edition_Kanski, Bowling_2011

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Corneal ectasias

Keratoconus

Pathogenesis

Keratoconus is a progressive disorder in which the cornea assumes a conical shape secondary to stromal thinning and protrusion (Fig. 6.40A). Both eyes are affected, at least on topographical imaging, in almost all cases. The role of heredity has not been clearly defined and most patients do not have a positive family history. Offspring appear to be affected in only about 10% of cases and autosomal dominant transmission with incomplete penetrance has been proposed.

Fig. 6.40 Keratoconus. (A) Histology shows central stromal thinning; (B) ‘oil-droplet’ red reflex; (C) Vogt striae; (D) Fleischer ring; (E) advanced thinning; (F) Munson sign

(Courtesy of J Harry and G Misson, from Clinical Ophthalmic Pathology, Butterworth-Heinemann 2001 – fig. A; M Leyland – fig. C; S Fogla

– fig. D; R Bates – fig. E)

Presentation

Presentation is typically during puberty with unilateral impairment of vision due to progressive myopia and astigmatism, which subsequently becomes irregular. As a result of the asymmetrical nature of the condition, the fellow eye usually has normal vision with negligible astigmatism at presentation. Approximately 50% of normal fellow eyes will progress to keratoconus within 16 years; the greatest risk is during the first 6 years of onset.

Diagnosis

The hallmark of keratoconus is central or paracentral stromal thinning, accompanied by apical protrusion and irregular astigmatism. It can be graded by keratometry according to severity as mild (<48 D), moderate (48–54 D) and severe (>54 D).

1Signs

•Direct ophthalmoscopy from a distance of one foot shows a fairly well delineated ‘oil droplet’ reflex (Fig. 6.40B).

•Retinoscopy shows an irregular ‘scissoring’ reflex.

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•Slit-lamp biomicroscopy shows very fine, vertical, deep stromal stress lines (Vogt striae – Fig. 6.40C) which disappear with pressure on the globe.

•Epithelial iron deposits may surround the base of the cone (Fleischer ring) and are best seen with a cobalt blue filter (Fig. 6.40D).

•Progressive corneal thinning (maximal at the apical zone) (Fig. 6.40E) associated with poor visual acuity resulting from marked irregular myopic astigmatism with steep keratometry (K) readings.

•Bulging of the lower lid in downgaze (Munson sign – Fig. 6.40F).

2Corneal topography shows irregular astigmatism and is the most sensitive method of detecting early keratoconus and for monitoring progression (Fig. 6.41).

Fig. 6.41 Relative scale corneal maps showing advanced keratoconus in the right eye and an early paracentral cone in the left

(Courtesy of E Morris)

Acute hydrops

Acute hydrops is caused by a rupture in Descemet membrane that allows an influx of aqueous into the cornea (Fig. 6.42A and B). Although the break usually heals within 6–10 weeks and the corneal oedema clears, a variable amount of stromal scarring may develop (Fig. 6.42C). Acute episodes are initially treated with cycloplegia, hypertonic (5%) saline ointment and patching or a soft bandage contact lens. Healing may sometimes result in improved visual acuity as a result of scarring and flattening of the cornea.

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Fig. 6.42 Advanced keratoconus. (A) Histology shows oedema of basal epithelial cells and partial loss of Bowman layer; (B) severe corneal oedema; (C) apical scarring

(Courtesy of J Harry and G Misson, from Clinical Ophthalmic Pathology, Butterworth-Heinemann 2001 – figs A and C)

Associations

1Systemic disorders include Down, Turner, Ehlers–Danlos and Marfan syndromes, atopy, osteogenesis imperfecta, mitral valve prolapse and mental retardation.

2Ocular associations include vernal keratoconjunctivitis, blue sclera, aniridia, ectopia lentis, Leber congenital amaurosis, retinitis pigmentosa and persistent eye rubbing.

Treatment

1Spectacles or soft contact lenses are generally sufficient in early cases.

2Rigid contact lenses are required for higher degrees of astigmatism to provide a regular refracting surface. Advances in both lens design and material have increased the proportion of keratoconus patients who can satisfactorily use contact lenses.

3Keratoplasty, either penetrating or deep anterior lamellar (DALK), may be necessary in patients with advanced disease, especially those with significant corneal scarring. Prior hydrops indicates the presence of a Descemet membrane discontinuity which is a contraindication to DALK. Although clear grafts are obtained in around 90% of cases, optical outcomes may be compromised by residual astigmatism and anisometropia, necessitating contact lens correction for best acuity.

4Intracorneal ring segment (Intacs) implantation using laser or mechanical channel creation is relatively safe, and typically provides at least a moderate visual improvement, facilitating contact lens tolerance in advanced cases.

5Corneal collagen cross-linking, using riboflavin drops to photosensitize the eye followed by exposure to ultraviolet-A light, is a newer treatment which offers promise of stabilization or reversal of ectasia in at least some patients. It can be combined with Intacs insertion.

Pellucid marginal degeneration

Pellucid marginal degeneration is a rare progressive peripheral corneal thinning disorder, typically involving the inferior cornea. Occasionally it may co-exist with keratoconus and keratoglobus (see below). Like keratoconus, pellucid marginal degeneration is bilateral, although often asymmetrical.

Diagnosis

1 Presentation is in the 4th–5th decades with reduced visual acuity due to increasing astigmatism.

2Signs

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•A bilateral, slowly progressive, crescentic 1–2 mm band of inferior corneal thinning extending from 4–8 o’clock, 1 mm from the limbus (Fig. 6.43A).

•The epithelium is intact, and the cornea above the thinned area is ectatic and flattened.

•In contrast to keratoconus, Fleischer rings and Vogt striae do not occur and acute hydrops is rare.

3Corneal topography shows a ‘butterfly’ pattern, with severe astigmatism and diffuse steepening of the inferior cornea (Fig. 6.43B).

Fig. 6.43 (A) Pellucid marginal degeneration; (B) topography shows severe astigmatismand diffuse steepening of the inferior cornea

(Courtesy of R Visser – fig. A; S Fogla – fig. B)

Treatment

1Spectacles usually fail early as irregular astigmatism increases.

2Contact lenses. In early disease soft toric lenses are adequate but more advanced cases require rigid gas permeable lenses.

3Surgical options, none of which are ideal, in patients intolerant to contact lenses include large eccentric penetrating keratoplasty, thermocauterization, crescentic lamellar keratoplasty, wedge resection of diseased tissue, epikeratoplasty and intracorneal ring implantation. Results of collagen cross-linking are encouraging.

Keratoglobus

Keratoglobus is an extremely rare congenital condition in which the entire cornea is abnormally thin. Possibly genetically related to keratoconus, it may be associated with Leber congenital amaurosis and blue sclera.

1Onset is at birth.

2Signs

•In contrast to keratoconus the cornea develops globular rather than conical ectasia.

•Corneal thinning is generalized rather than at the apex of the protrusion (Fig. 6.44A).

•Acute hydrops (Fig. 6.44B) occurs less commonly than in either keratoconus or pellucid marginal degeneration but the cornea is more prone to rupture on relatively mild trauma.

3 Corneal topography shows generalized steepening (Fig. 6.44C).

4Differential diagnosis is from congenital glaucoma (oedematous cornea) and megalocornea (not thinned). The corneal diameter is normal in keratoglobus.

5Treatment is with scleral contact lenses because the results of surgery are poor, though large-diameter grafting can be attempted. Special care should be taken, particularly in childhood, to protect the eyes from trauma.

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Fig. 6.44 (A) Keratoglobus; (B) acute hydrops; (C) topography shows generalized steepening

(Courtesy of S Fogla – fig. C)

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Corneal dystrophies

The corneal dystrophies are a group of progressive, usually bilateral, mostly genetically determined, non-inflammatory opacifying disorders. Based on biomicroscopical and histopathological features corneal dystrophies are classified into (a) epithelial, (b) Bowman layer, (c) stromal and (d) endothelial. Recent advances in molecular genetics have identified the responsible gene defects for most.

Epithelial dystrophies

Cogan epithelial basement membrane dystrophy

Epithelial basement membrane dystrophy is the most common dystrophy seen in clinical practice. Despite this it is frequently misdiagnosed, principally due to its variable appearance.

1Inheritance. The condition is usually sporadic, and rarely autosomal dominant (AD) with incomplete penetrance.

2Histology shows thickening of the basement membrane with deposition of fibrillary protein between the basement membrane and Bowman layer (Fig. 6.45A). There is also absence of hemidesmosomes of the basal epithelial cells, which is responsible for the typical recurrent corneal erosions.

3Onset is in the 2nd decade. About 10% of patients develop recurrent corneal erosions in the 3rd decade and the remainder are asymptomatic throughout life. Simultaneous bilateral recurrent erosions suggest epithelial basement membrane dystrophy.

4Signs. The following lesions may be seen in isolation or combination and are best visualized by retroillumination or scleral scatter. Over time one pattern frequently changes to another; the distribution of the lesions may also vary. Features may be variable, absent or only subtle in a fellow asymptomatic eye.

•Dot-like opacities (Fig. 6.45B).

•Epithelial microcysts (Fig. 6.45C).

•Subepithelial map-like patterns surrounded by a faint haze (Fig. 6.45D).

•Whorled fingerprint-like lines (Fig. 6.45D).

•Similar features can be seen in an eye suffering recurrent erosions from any cause.

5 Treatment is that of recurrent corneal erosions as described above.

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Fig. 6.45 Cogan epithelial basement membrane dystrophy. (A) Histology shows intraepithelial extension of the basement membrane above the intraepithelial cyst – toluidine blue stain; (B) dots; (C) microcysts; (D) map-like pattern; (E) fingerprint lines seen on retroillumination

(Courtesy of Krachmer, Mannis and Holland, from Cornea, Mosby 2005 – fig. E; J Harry and G Misson, from Clinical Ophthalmic Pathology, Butterworth-Heinemann 2001 – fig. A)

Meesmann epithelial dystrophy

Meesmann dystrophy is a very rare non-progressive abnormality of corneal epithelial metabolism, underlying which mutations in the genes encoding corneal epithelial keratins have been reported.

1Inheritance is AD.

2Histology shows irregular thickening of the epithelial basement membrane and intraepithelial cysts (Fig. 6.46A).

3Symptoms are variable. Patients may be asymptomatic, or ocular irritation may begin in the first few months of life.

4Signs

•Myriad tiny intraepithelial cysts of uniform size but variable density are maximal centrally and extend towards but do not reach the limbus (Fig. 6.46B).

•The cornea may be slightly thinned and sensation reduced.

5Treatment other than lubrication is not normally required.

Fig. 6.46 Meesmann dystrophy. (A) Histology shows thickening of the epithelial basement membrane and intraepithelial cysts – PAS stain; (B) myriad of intraepithelial cysts

(Courtesy of J Harry and G Misson, from Clinical Ophthalmic Pathology, Butterworth-Heinemann 2001 – fig. A; S Fogla – fig. B)

Lisch epithelial dystrophy

Lisch dystrophy was previously suspected to be a variant of Meesmann, but is now believed to be genetically distinct.

1Inheritance is AD or X-linked dominant (XLD) with the gene locus on Xp22.3 in the latter in at least some patients.

2Signs

•Grey bands with a whorled configuration (Fig. 6.47A).

•Retroillumination shows densely packed microcysts (Fig. 6.47B).

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Fig. 6.47 Lisch dystrophy. (A) Grey, bands with a whorled configuration; (B) retroillumination shows clear, densely crowded microcysts

(Courtesy of W Lisch)

Bowman layer/anterior stromal dystrophies

Reis–Bücklers dystrophy (corneal dystrophy of Bowman layer, type I, CDB1, GCD type III)

This so-called ‘true’ form of Reis–Bücklers dystrophy may also be categorized as a form of granular stromal dystrophy (GCD type III).

1Inheritance is AD with the gene locus on 5q31 (gene TGFB1).

2 Histology shows replacement of Bowman layer and the epithelial basement membrane with fibrous tissue (Fig. 6.48A). 3 Onset is in the 1st or 2nd decade with severe recurrent corneal erosions.

4Signs

•Grey-white, fine, round and polygonal subepithelial opacities similar to those of granular dystrophy type I, most dense centrally (Fig. 6.48B).

•The changes increase in density with age resulting in a reticular pattern due to the laying down of irregular bands of collagen replacing Bowman layer.

•Corneal sensation is reduced and visual impairment may occur due to scarring at the level of Bowman layer.

5Treatment is directed at the recurrent erosions. Excimer laser keratectomy achieves satisfactory control in some patients.

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Fig. 6.48 Reis–Bücklers dystrophy. (A) Histology shows replacement of Bowman layer and epithelial basement membrane by fibrous tissue; (B) clinical appearance

(Courtesy of J Harry and G Misson, from Clinical Ophthalmic Pathology, Butterworth-Heinemann 2001 – fig. A)

Thiel–Behnke dystrophy (corneal dystrophy of Bowman layer, type II, CDB2, honeycomb-shaped corneal dystrophy)

1Inheritance is AD with gene loci on 10q24 and 5q31 (gene TGFB1).

2 Histology shows ‘curly fibres’ in Bowman layer on electron microscopy.

3Onset is at the end of the 1st decade with recurrent erosions.

4Signs. Subepithelial opacities in a honeycomb morphology involving the central cornea (Fig. 6.49).

5Treatment may not be necessary because visual impairment is less than in Reis–Bücklers dystrophy.

Fig. 6.49 Thiel–Behnke dystrophy

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