Keratoconus (KC) is an eye condition where the cornea progressively becomes thinner causing a cone-like bulge to develop that distorts the vision.
The cornea is the clear outer layer of your eye covering the iris, pupil, and anterior chamber. The cornea, combined with the anterior chamber and lens, refracts light and directs the light to the back of your eye to the retina.
Providing about two-thirds of the eye’s total focusing power, the cornea, along with the white of your eye also helps protect your eye and maintain moisture levels.
Most refractive errors, nearsightedness, farsightedness, and astigmatism, are caused by a cornea with a less-than-optimal curvature and symmetry. This is why the shape of the cornea is so important and why keratoconus, with its cone-like bulge, distorts vision.
How does Keratoconus affect eyesight?
The cornea needs to have the correct shape to transmit and direct incoming light precisely. As the regularly dome-shaped cornea, increasingly becomes thinner, a cone-shaped bulge pushes up at the center of the cornea in the area of most thinning.
The irregular cone-shape alters the ability of the cornea to focus light accurately on the retina. This causes nearsightedness and irregular astigmatism to develop. Other problems with distorted and blurred vision and light sensitivity also may develop.
Vision changes will differ from one individual to another, with the impairment ranging from mild vision loss to acute vision loss. If the vision loss is too severe, there can be a decrease in the ability to see clearly, even with corrective lenses.
Typically, Keratoconus doesn’t result in total blindness. But the vision can degrade to where it greatly impacts the quality of life of those with this disease.
While keratoconus is a progressive disease, the shape of the patient’s cornea may change quickly or the shape may change over several years. It is difficult to predict the progress for, at any time, the changes may halt or go on for decades.
One or both eyes can be affected. Often though, both eyes are affected after a time, developing first in one eye and later in the other eye, although not always to the same degree.
Because keratoconic patients experience these variable changes in their vision, their eyeglass prescriptions tend to be different every time they visit their eye doctor.
Who’s at risk for Keratoconus?
Keratoconus can start in childhood, in the teenage years or early 20s, and in people up to age 30. It is uncommon, but it may occur in those 40 or older. However, it usually starts in the teenage years. Onset is normally during puberty with 75% of cases being diagnosed before the age of 25 years.
The condition continues to progress until the third decade of life, after which progression often slows down. The severity of its progression is partly due to how early it appears. Although evidence on the influence of sex in onset and progression is inconsistent, there appears to be a higher tendency for keratoconus to affect males.
Your optometrist can usually diagnose keratoconus through a comprehensive eye exam using a “slit-lamp.” This instrument focuses a thin sheet of light into the eye. A low-powered microscope is then used to look for definitive signs of keratoconus such as corneal thinning and scarring.
The exam may also include a pachymetry test. A simple, painless test that measures the thickness over the whole cornea looking for areas of thinning.
More advanced tests can be done using computerized corneal mapping. Corneal tomography and corneal topography are photographic tests that create a detailed map of the shape and thickness of your cornea. Corneal mapping can detect signs of keratoconus before it can be seen with a slit-lamp examination.
Keratoconus Treatment: What Can Be Done About it?
Patients who suffer from Keratoconus have a diminished quality of life. The treatments and the methods used to correct the refractive errors of patients can impact the disease and improve their quality of life.
Rehabilitating keratoconic corneas is accomplished by addressing three concerns:
- Halting the forward bulging and thinning of the cornea
- Improving the corneal shape
- Minimizing residual refractive error
Several effective treatments can be used as the disease progresses.
In early cases of keratoconus, spectacles can be used to enhance visual acuity. However, in later stages of the disease irregular astigmatism, (an imperfectly shaped cornea that distorts vision,) can develop. This cannot be corrected with spectacles, so the next step would be correction with special contact lenses.
Contact lenses are a common and successful treatment option for early to moderate cases of keratoconus. The lenses must be carefully fitted, and multiple checkups and lens changes may be needed to maintain good vision.
Devices called Intacs, and intracorneal rings can be used to improve the fit of the contact lens. Further on as the cornea progressively thins and changes shape, a rigid gas permeable (RGP) contact lens can continue correcting the vision.
Corneal crosslinking is a treatment that first appeared as an option in the late 1990s and has been under investigation as a way to halt the progression of keratoconus.
The treatment method uses a combination of ultraviolet light and a “chromophore” (vitamin B2, riboflavin), to stiffen the cornea.
Your cornea is held in place by tiny fibers of protein called collagen, a substance that generally helps hold your whole body together. A decrease in protective antioxidants in the cornea weakens the collagen fibers and they cannot hold the shape. So, the cornea bulges out a becomes more cone-shaped.
With corneal crosslinking, the photochemical reaction creates a chemical bond between collagen fibrils, making them stronger. Often with a single application, the progressive thinning of keratoconus can be arrested.
If there is extreme thinning, scarring, or contact lens intolerance, a corneal transplant, (keratoplasty) with healthy donor tissue may be necessary. Only 15-20% of people with keratoconus would eventually require a corneal transplant.
Recent advancements in keratoplasty techniques have evolved which help to improve the outcomes. Using computerized corneal tomography and lasers during surgery are the most important.