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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 curve. Keratoconus affects around one person in a thousand. The exact cause of keratoconus is uncertain, but has been associated with detrimental enzyme activity within the cornea. Environmental and genetic factors are considered possible causes, but the findings are still yet inconclusive.
The cornea is the clear front part of the eye, and functions much like a window that you look through, except that it is a living structure. It is composed of many thin stacked layers of protein (collagen) and supporting cells (keratocytes) that keep the protein fibers strong. In the condition known as keratoconus, the cornea becomes gradually thinner in its center. Then, the normal fluid pressure in the eye causes this area to bulge forward (like a thin area on tire).
On the outside of the eye, this bulging causes the cornea's normal round curvature to become more irregular. Patients with keratoconus gradually develop distorted vision. Because the cornea's bulging has an irregular shape, glasses and conventional contact lenses cannot provide clear vision.
In early stages of keratoconus, spectacles 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. As the name implies, these lenses have a rigid shape, allowing them to create a new, smooth front surface for the eye. They can provide acceptable vision, but for some people the contacts can become very uncomfortable and can easily fall out. This type of lens is also considerably more costly than soft contact lenses.
In the past, when rigid contact lenses failed, the only other option for treating keratoconus was corneal transplantation. This procedure requires a recovery period of six to 18 months before stable vision can be achieved. After recovery, many patients still require rigid contact lenses to see well.
For keratoconus among patients who can not tolerate contact lenses, a recent surgical alternative to corneal transplant is the insertion of intrastromal corneal ring segments, called Intacs. Intacs are tiny (0.25 mm thickness) acrylic rings that are placed between the layers of the cornea. These rings were first designed to treat nearsightedness, and since then have been the subject of considerable research on keratoconus. Several studies have shown they can help provide a more normal shape to a patient whose cornea is weakened by keratoconus.
Like a corneal transplant, Intacs rings actually add structural integrity to the cornea, which is lacking in keratoconus. This treatment can be performed as an outpatient and allows a much quicker recovery than corneal transplantation. 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.
A small incision is made in the periphery of the cornea to a depth of 80% of the corneal thickness using a precisely calibrated instrument with a blade made of diamond.
Through this incision, the surgeon creates channels (tunnels) through the corneal tissue for the rings to be placed.
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. When in place, these rings alter the cornea's shape, creating a more normal surface contour.
Both in the U.S. and abroad, studies on the effects of Intacs on keratoconus have shown that the cornea may take on a more regular, round shape after the rings are implanted. Results depend on the individual patient's corneal shape, but roughly half of patients who have the rings placed have an increase in uncorrected vision of about five lines on a standard Snellen eye chart. This is the equivalent of vision improving from 20/200 (legal blindness) to 20/50 (acceptable for driving in most states). Corrected vision increased by about three eye chart lines in most patients.
A thorough eye exam by a corneal specialist is necessary to determine if an individual is a candidate for Intacs for keratoconus. For information on Intacs Corneal Implants at Harvard Eye Associates, please contact us.
As always, Harvard Eye is first and foremost concerned about what is best for each individual patient. Call us today for your personalized consultation!
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