KERATOCONUS
Dr. Sujatha Mohan,
Associate Medical Director, Rajan Eye Care Hospital,Chennai
Director, Rotary Rajan Eye Bank,Chennai
Introduction:
Keratoconus is a degenerative non-inflammatory disease of the cornea. It presents generally at puberty with progressive corneal steepening & thinning, most typically inferior to the center of the cornea. Keratoconus is characterized by progressive Myopia, both regular and irregular astigmatism in mild cases and vision compromising ectasia in advanced disease. It is usually bilateral and asymmetrical. The incidence of keratoconus is 1 in 2000. Patients often present with complaints of distorted, blurred vision, glare, light sensitivity and history of frequent change of glasses.
PATHOGENESIS:
Despite extensive research the etiology and exact pathogenesis of keratoconus is still not clearly established. It has been associated with atopic disease and chronic eye rubbing like in patients with Down’s Syndrome & Atopic dermatitis.
There has been a possibility of genetic and hereditary disposition. It has been associated with systemic diseases like the Ehler-Danlos syndrome, osteogenisis imperfect leading to a speculation that it may be part of a connective tissue disorder. Long term contact lens wear with resultant corneal warpage has also been implicated as a possible cause for keratoconus.
Biochemical abnormalities:
Studies in patients with keratoconus have demonstrated an increased activity by proteases that breakdown the collagen cross-linkages in the corneal stroma. There is a simultaneous reduced expression of protease inhibitors. The ratio of keratin sulfate to dermatan sulfate is altered in the stromal matrix when compared to normal corneas.Patients with keratoconus have also shown enzymatic changes in the epithelium with increased expression of lysosomal and proteolytic enzymes.
DIAGNOSIS OF KERATOCONUS:
Early signs
- Scissoring of retinoscopic reflex
- Distortion of placido disc mires
- Keratometry- shows irregular astigmatism where the principal meridian are not 90* apart and mires cannot be superimposed.
- Direct ophthalmoscopy from a distance shows an oil droplet reflex
- Typical inferior steepening on topography
These early signs occur even before the appearance of classic corneal signs .
Clinical characteristics
- Munson’s sign-Angulation of the lower lid margin when the patient looks down
- Vogt’s striae- fine, vertical deep stromal striae which disappear with external pressure
- Fleischer’s ring –Epithelial iron deposits at the base of the cone
- Stromal scarring in advanced keratoconus
Acute Hydrops results from rupture of the descemet’s membrane and the overlying endothelium in a keratoconic eye. Aqueous enters the corneal stroma through the ruptured DM and causes a stromal swelling and a spongy elevated area of with overlying bullous edema of the epithelium. Visual acuity drops considerably with congestion of adjacent conjunctiva .Spontaneous resolution occurs in few months with resultant scarring.
Management would consist of supportive therapy in the form of lubricant drops, hypertonic saline and soft contact lens .Steroid drops can be used if the eye is congested.
Thermokeratoplasty has been tried by Aquavella & group. They believe shrinkage of the collagen with resultant flattening of the hydrops helps in early healing of the ruptured DM.
Systemic associations
Down’s syndrome
Ehlers-Danlos syndrome
Osteogenesis Imperfecta
Mitral valve prolapsed
Ocular associations
Retinitis Pigmentosa
Leber’s Congenital Amaurosis
Microcornea
Aniridia
Corneal Degeneration
Congenital Cataract
Ectopia lentis
Lenticonus
Macular coloboma
Retinal dysplasia
Floppy eyelid syndrome
MANAGEMENT OF KERATOCONUS:
The management of keratoconus depends on the stage of the disease the patient presents with. The various modalities available today can be broadly divided into nonsurgical & surgical.
NON-SURGICAL MODALITIES:
Spectacles can be used in early stage of the disease before irregular astigmatism sets in.
Contact lenses are the main stay of nonsurgical management in moderate & advanced keratoconus. Contact lenses used in keratoconus can be broadly classified into-
Rigid gas permeable lenses: They are the main stay of optical management of keratoconus. The contact lens resurfaces the irregular cornea & the intervening fluid lens corrects the irregular astigmatism to provide good quality of vision. However the disadvantage with these lenses is that they have standard lens design with fixed optical zones and do not give an ideal fit in patients with keratoconus
Piggy-back lenses: they are known as piggy back because a RGP Lens is fitted on top of a soft contact lens. These lenses provide excellent comfort and good vision. The down side to this fit is the need to have two lenses care systems.
Rose-K Contact lenses: As the cornea becomes more conical, the base curve steepens only in the area of the cone and hence the difficulty in fitting contact lenses. The Rose-k lenses are made with complex geometry to fit any type of cone. The optical zone is reduced to snuggly fit the cone & the peripheral curve is computer designed to fit the rest of the irregular cornea. It demonstrates the benefits of a smaller optical zone to fit the cone contour. The design results in little tear pooling at the base of the cone and show an even distribution of tears under the lens. It has a large inventory and is easy to fit. . The Rose K™ system has set optical zones to maximize vision while maintaining good corneal health.
Contact lens design can be made in any diameter with spherical back surface, toric back, front, bitoric or peripheral toric design. In patients with very steep and sagging cones, fitting is very difficult since the cornea steep in the periphery and flat in the area away from the cone. With the Advanced Corneal Technology (ACT) available in the Rose K2 system the lens can be steepened at the area of the cone. With ACT, 3 grades of steepening can be made at the 6 0’ clock positions which provide good stability to the lens. Rose K lenses are made of Boston ES, a special material with high oxygen permeability.
Scleral Contact Lenses: They are made of a special polymer called Itaflurocon. They characteristically vault over the cornea and limbus and are supported entirely by the sclera. The fluid lens smoothens the irregular cornea and provides good vision. It also prevents dessication due to its very high oxygen permeability. The small pores on the lens provide adequate exchange of tears. The disadvantage with these lenses is that they are difficult to use. They are large lenses which are cumbersome to the patients .
Boston Scleral Contact lenses(BSLP): BSLP lenses are made of Flurosilicone Acrylate Polmers with DK values of 87 and 130. Its diameter ranges from15.5mm to 20mm.the space created over the cornea is filled with non-preserved, buffered sterile saline. The shape of each lens is based on a master design template. The shape of the haptic scleral bearing is controlled by spline a function which is adjusted to create the desired fitting relation ship even in advanced cones. The limitation to the use of these lenses are that the fitting process is skill intensive and time consuming.
