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KERTOCONUS: A CHALLENGE TO OPHTHALMOLOGIST

KERATOCONUS: UPDATE

Keratoconus (from Greek: kerato- horn, cornea; and konos cone), is a degenerative disorder of the eye in which structural changes within the cornea cause it to thin and change to a more conical shape than its normal gradual spherical curve. In keratoconus, there can be substantial distortion of vision, with multiple images, streaking and sensitivity to light, all often reported by the patient. It is typically diagnosed in the patient's adolescent  years and attains its most severe state in the twenties and thirties. If afflicting both eyes, the deterioration in vision can affect the patient's ability to drive a car or read normal print. In most cases, corrective spectacle lenses are effective enough to allow the patient to continue to drive legally and likewise function normally. But with the progress of the disease, the cone increases and the vision gets further deteriorated and the spectacle lenses are of not great help. The cone may take the shape of ‘nipple' or may be ‘oval' or may be ‘globus' in appearance.

Symptoms and Signs

Keratoconus can be difficult to detect, because it usually develops slowly. However, in some cases, it may progress rapidly. As the cornea becomes more irregular in shape, it causes progressive nearsjghtedness(myopia) with high degree of irregular astigmatism, leading to distorted and blurred vision. Glare and light sensitivity also may occur. Often, keratoconic patients experience changes in their eye glasses prescription every time they visit their eye care practitioner. In mild cases, the diagnosis may be delayed.

Placido's disc examination & Keratometry are easiest methods to diagnose keratoconus. Orbscan gives detailed information about changes in the cornea.

What Causes Keratoconus?

Recent research suggests weakening of the corneal tissue due to an imbalance of enzymes within the cornea. This imbalance makes the cornea more susceptible to oxidative damage from compounds called free radicals, causing it to weaken and bulge forward. Genetic disposition leading to weakening of cornea, explains familial incidence.

Keratoconus is also associated with overexposure to ultraviolet rays from the sun, excessive eye rubbing, a history of poorly fitted contact lenses and chronic eye irritation.

Keratoconus Management: A challenge

In the mildest form of keratoconus, eyeglasses or soft contact lenses may help. But as the disease progresses and the cornea thins and becomes increasingly more irregular in shape, glasses and soft contacts no longer provide adequate vision correction.

Rigid Gas Permeable Contact Lenses

When glasses fail to correct visual acuity, the best option is highly gas permeable semi soft contact lenses. The rigid lens material enables GP lenses to vault over the cornea, replacing its irregular shape. RGP lenses are initially more uncomfortable but in due course of time, patient gets well adapted. In moderate cases of keratoconus, contact lenses may provide 6/6 vision. FP 92 (Fluoroperm Siloxinate) manufactured by Paragon VisionSciences can be worn during sleep without much discomfort. BostonXO & BostonO2 (B & L) with Dk Value of 100 & 141 respectively, are highly gas permeable. These lenses are made of Fluorosilicon Acrylate (FSA), having excellent wettability. There is special Keratoconus contact lens set which can help to fit a GP lens. In my practice I have found good acceptance by 90 percent of my early & moderately advanced keratoconus patients

In keratoconus, aspheric designs have proven to not only fit the varied corneal shapes, but also simplify the fitting process by eliminating some of the variables involved with lens design and fitting. These designs provide improved mid peripheral alignment as compared to spherical designs. Optical zone sizes are generally small but will usually accommodate the larger globus cones as well as oval or nipple type cones.

Piggybacking" contact lenses.

To provide better vision & make the fit more comfortable, Piggyback contact lensesare advocated in some cases ofkeratoconus. This method involves placing a soft contact lens, such as one made of  silicon hydrogel, over the eye and then fitting a GP lens over the soft lens. This approach increases wearer's comfort because the soft lens acts like a cushioning pad under the rigid GP lens. I personally have not found these lenses satisfactory since the cornea is deprived of oxygen. These lenses are not much in use.

ClearKone hybrid contact lenses (SynergEyes Inc., Carlsbad, Calif.)

These hybrid contact lenses combine a highly oxygen-permeable rigid center with a soft peripheral "skirt." The ClearKone version was designed specifically for keratoconus and vaults above the eye's cone shape for increased comfort. The hybrid contacts provide the crisp optics of a GP lens and wearing comfort that rivals that of soft contact lenses.

These lenses have to be custom designed & hence they are extremely costly & as yet not available in India

 

Rose K Lens

Rose-K lens introduced by New Zealand Optometrist, Dr. Paul Rose in 1989 & perfected it in year 2000. These lenses are latest innovation in the management of Keratoconus. Unlike traditional GP lenses, the complex geometry built into every Rose-K contact lens closely mimic the cone like shape of the cornea, for every stage of thecondition. This enhances the fitting comfort of the contact lens as well provide better visual acuity.

The Rose K lens's complex geometry has only become possible since computer controlled contact lens lathes were developed to cut sophisticated oxygen permeable polymers to the right shape. The practitioner has to maintain Rose-K trial set of 26 lenses to achieve successful fit.

The Rose K2

Paul Rose further improved the Rose-K lens which he labeled Rose K2 lens. This lens minimizes aberrations by applying very small changes to the curves on both the front and back of the lens in an attempt to bring the light passing through the lens within the pupil zone to a single point

By nature, the keratoconic cornea is asymmetric, where the inferior quadrant is frequently significantly steeper than the superior portion, causing the GP lens to lift at 6 o clock position. Rose K lens incorporating ACT are designed to accommodate this asymmetry (good edge fit at 3, 9 and 12 o clock but lift at 6 o clock). The inferior quadrant of the lens is steeper than the superior quadrants, providing a more accurate fit at 6 o clock, there by enhancing the comfort & stability of the lens & vision. A toric periphery of Rose K2 lens (TP) further enhances the fit & improves vision. These lenses are manufactured by David Tomas, U K and are now available in India. Recently, Menicon.co.Ltd has taken over the David Thomas Contact Lenses Ltd, UK and now Rose-K Lenses shall be manufactured & marketed by Menicon, a leading Japanese contact lens manufacturing company.

Apart from Keratoconus, Rose K lenses are also recommended for post keratoplasty astigmatism & post Lasik keratectasia.

 

Scleral and semi-scleral lenses ( Boston Scleral Lens Prosthetic Device: BSLPD)

These lenses are of only historical importance in the management of Keratoconus since the introduction of advanced management procedures including keratoplasty.

Intacs

A recent surgical alternative to corneal transplant is the insertion of intrastromal corneal ring segments. A small incision is made in the periphery of the cornea and two thin arcs of polymethyl methacrylate are slid between the layers of the stroma on either side of the pupil before the incision is closed. The segments push out against the curvature of the cornea, flattening the peak of the cone and returning it to a more natural shape. The procedure, carried out on an outpatient basis under local anaesthesia, offers the benefit of being reversible and even potentially exchangeable as it involves no removal of eye tissue. These inserts if required can be removed.

The principal intrastromal ring available is known by the trade name of Intacs. Internationally, Ferrara Rings are also available. Intacs are a patented technology and are placed outside the optical zone of 7.0 mm versus the smaller prismatic Ferrararings that are placed just inside the 5 mm optical zone. Intacs are the only corneal implants that have gone through the FDA Phase I, II and III clinical trials and were first approved by the Food and Drug Administration (FDA) in the United States in 1999 for myopia; this was extended to the treatment of keratoconus in July 2004.

Collagen Cross Linking.(C3-R)

This non-invasive procedure strengthens corneal tissue to halt bulging of the eye's surface in keratoconus.

While various methods are under investigation, one brand name associated with the procedure is ‘corneal cross linking or C3-R (Boxer Wachler Vision, Los Angeles)'. In the C3-R procedure, eye drops containing photosensitizer riboflavin phosphate (vitamin B2) are placed on the cornea and are then activated by ultraviolet light (365nm) for 30 minutes to strengthen links between the connective tissue ( collagen) fibers within the cornea. Such a cross linking of collagen fibers increases their physical strength by 300% and regresses the keratoconus by 15-20 percent over a period of one year.

In the United States, FDA clinical trials for corneal collagen cross linking began in early 2008.

This simple treatment with or without INTACS might reduce significantly the need for corneal transplants among keratoconus patients. Corneal cross linking also is being investigated as a way to treat or prevent keratoconus-like complications following LASIK

Penetrating Keratoplasty

Between 10% and 25% of cases of keratoconus will progress to a point where vision correction is no longer possible, thinning of the cornea becomes excessive, or scarring as a result of contact lens wear causes problems of its own, and a corneal transplantation or penetrating keratoplasty becomes essential. Keratoconus is the most common ground for conducting a penetrating keratoplasty, generally accounting for around a quarter of such procedures with extremely rewarding results. Since most of keratoconus corneas have no vascular invasion & hence chances of stromal rejection is greatly reduced.

PK is a well-established technique. However, PK breaches the structural and immunologic integrity of the eye, thereby, exposing the eye to formidable challenge of endothelial rejection.

Deep Lamellar Keratoplasty (DLK)

Deep lamellar keratoplasty (DLK) is an effective alternative to penetrating keratoplasty (PK) with similar best-corrected visual acuity (BVCA) and refractive results, while reducing the risk of endothelial rejection in the surgical treatment of keratoconus.

In recent years, DLK techniques have improved so that a full-thickness corneal stroma and epithelial button may be placed into a host bed containing little or no stromal tissue on top of Descemet's membrane (DM).DLK is more technically challenging than PK but eliminates the possibility of endothelial rejection, has minimal effect on the endothelial cell count, and may reduce the risk of late endothelial failure.

Summary

The advancements in the last decade has given great relief to keratoconus patients. However, the management of Keratoconus continues to be a nightmare for the ophthalmic practiotioner. Proper understanding of the condition by the optometrist & an Ophthalmologist may significantly help the patient. In a myopic patient, constant increase in astigmatism & subnormal improvement of vision must alarm the eye care person.

 

 

 

About the Author

PROF DR M. R. JAIN FAMS, FICS

Medical Director, M. R. J Institute & Jain Eye Hospital, Jaipur

Dr Jain is Ophthalmologist of the country with special interest in Glaucoma, Phaco & squint Surgery. He is also contact lens expert. He is Awarded Life Time Achievement Award by All India Ophthalmological Society for his contribution in the field of Glaucoma & drug Delivery to eye. He performed First IOL in 1976 in Jaipur. He has been Prof & Head at various Medical Colleges of Rajasthan including SMS Medical College Jaipur. ( INDIA)

FSA/Vision "Metron" Aero Shifter Interbike 2010