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Keratoconus

Information compiled by Candice Halstead

Keratoconus (from Greek: kerato- horn, cornea; and konos cone), is a degenerative disorder of the eye in which structural changes within the cornea or front surface of the eye cause it to thin and change to a more conical or cone-like  shape than its normal gradual curve. Keratoconus has been known to be called “bulging-eyes.”

People suffering from keratoconus may complain about the following symptons:

• distorted vision with multiple images
• streaking of their vision
• Sensitivity to light (also known as photophobia)
• difficulty with driving at night 
• Eye strain 
• Itching of the eyes associated with no pain

Keratoconus is usually diagnosed in the teenage years when vision trouble affects the learner at school but can also be missed if in it’s early stages and rather be diagnosed as a simple astigmatism.

For people with astigmatism, objects both near and far appear distorted. This occurs because some of the light rays are focussed and others are not. Astigmatism is a very common refractive error found by optometrists and or ophthalmologists.

Keratoconus is typically diagnosed in the patient’s adolescent years as mentioned above and attains its most severe state in the twenties and thirties. If both eyes are afflicted, the deterioration in vision can affect the patient’s ability to drive a car or read normal print. In most cases, corrective lenses are effective enough to allow the patient to continue to drive legally and likewise function normally. Further progression of the disease may require surgery including corneal transplantation. However, despite the disease’s unpredictable course, keratoconus can often be successfully managed with little or no impairment to the patient’s quality of life.

Keratoconus is the most common dystrophy of the cornea, affecting around one person in a thousand. It seems to occur in populations throughout the world, although it occurs more frequently in certain ethnic groups. The exact cause of keratoconus is uncertain, a genetic link seems likely, as the incidence rate is greater if a family member has been diagnosed. The progression of keratoconus is rapid in patients having Down syndrome.

Diagnosis
 
A schematic digram showing change in cornea

Prior to any physical examination, the diagnosis of keratoconus frequently begins with an ophthalmologist’s or optometrist’s assessment of the patient’s medical history, particularly the chief complaint and other visual symptoms, the presence of any history of ocular disease or injury which might affect vision, and the presence of any family history of ocular disease. An eye chart, such as a standard Snellen chart of progressively smaller letters, is then used to determine the patient’s visual acuity. The eye examination may proceed to measurement of the localised curvature of the cornea with a manual keratometer, with detection of irregular astigmatism suggesting a possibility of keratoconus. A further indication can be provided by retinoscopy, in which a light beam is focused on the patient’s retina and the reflection, or reflex, observed as the examiner tilts the light source back and forth.

Treatment
Contact lenses
 
Rigid gas permeable lens for keratoconus

In early stages of keratoconus, glasses or soft contact lenses can suffice to correct for the mild astigmatism. As the condition progresses, these may no longer provide the patient with a satisfactory degree of visual acuity, and most clinical practitioners will move to managing the condition with rigid contact lenses, known as rigid gas-permeables, or RGPs. RGP lenses provide a good level of visual correction, but do not arrest progression of the condition.

In keratoconic patients, rigid contact lenses improve vision by means of tear fluid filling the gap between the irregular corneal surface and the smooth regular inner surface of the lens, thereby creating the effect of a smoother cornea. Many specialized types of contact lenses have been developed for keratoconus, and affected people may seek out both doctors specialized in conditions of the cornea, and contact lens fitters who have experience managing patients with keratoconus. The irregular cone presents a challenge and the fitter will endeavour to produce a lens with the optimal contact, stability and steepness.

Traditionally, contact lenses for keratoconus have been the ‘hard’ or rigid gas-permeable variety, although manufacturers have also produced specialized ’soft’ or hydrophilic lenses and, most recently, silicone hydrogel lenses. A soft lens has a tendency to conform to the conical shape of the cornea, thus diminishing its effect. To counter this, hybrid lenses have been developed which are hard in the centre and encompassed by a soft skirt. Soft or hybrid lenses do not however prove effective for every patient.

Some patients also find good vision correction and comfort with a “piggyback” lens combination, in which gas permeable rigid lenses are worn over soft lenses, both providing a degree of vision correction. One form of piggyback lens makes use of a soft lens with a countersunk central area to accept the rigid lens. Fitting a piggyback lens combination requires experience on the part of the lens fitter, and tolerance on the part of the keratoconic patient.

Scleral lenses are sometimes prescribed for cases of advanced or very irregular keratoconus; these lenses cover a greater proportion of the surface of the eye and hence can offer improved stability. The larger size of the lenses may make them unappealing or uncomfortable to some, however their easier handling can find favour with patients with reduced dexterity, such as the elderly.

Surgical options

• Corneal transplant
• Corneal ring segment inserts
• Radial keratotomy

Reference:
1) www.bausch.co.za
2) www.wikipedia.co.za
3) Kanski JJ 3rd edition McMillan & Partners