– by Dr. Rosmary Sanchez OD
Keratoconus is the most common primary ectasia condition. It is a non-inflammatory degenerative condition characterized by localized thinning that leads to protrusion of the cornea. It is caused by changes in the structure and organization of the corneal collagen, which may cause myopia and irregular astigmatism. Its etiology is multifactorial; however, there is evidence that suggest that the expression of this condition has a genetic predisposition with environmental influences such as vigorous eye rubbing and UV damage.
Signs and Symptoms
Signs and symptoms may change according to the stage of progression. Early in the disease visual acuity may be normal and asymptomatic. The classic symptom is ghosting of images, in which the patient sees multiple images of one object. As the disease progresses, the patient may experience blurred vision and/or distortion, change in astigmatic correction, photophobia and glare, monocular diplopia, distortion and ghosting of images.
The diagnosis of keratoconus is done after a complete examination performed by an eye specialist. It is based on changes in prescription, poor spectacle or soft contact lenses correction vision, corneal thinning, and other characteristic clinical findings. If keratoconus is suspected, other ancillary tests are done to clinical findings observed such as tomography or topography. For a more definite diagnosis.
In early stages of keratoconus, spectacles and soft contact lenses can be used to correct for vision. Patients may still not me satisfied with corrected vision since they still experience the ghosting of images, poor night vision, and shadow effect die to the irregularity of the cornea.
Specialty lenses are used to fit any type of keratoconus patient depending on the severity of the condition.
Rigid gas permeable contact lenses (RGP lenses) are recommended as the condition progresses. RGP lenses resurface irregular corneas. They usually provide a dramatic improvement in quality of vision. In essence, the combined effect of the tears and the rigidity of the lens provide a regular and uniform smooth surface creating almost like an unaffected cornea.
Hybrid contact lenses combine the benefits of both soft and rigid contact lenses in one lens. It provides comfort by having a soft edge or skirt, and good optics by having a hard central zone that vaults over the cone.
Scleral and semi-scleral lenses are used to fit any type of keratoconus patient. They have a large diameter which vaults over the cornea, providing comfort and good vision. These lenses are custom made depending on the shape of the patient’s cornea.
Intracorneal segments (INTACS) are crescent shaped segments which are placed in the peripheral two thirds of corneal depth. It flattens central cornea by mechanically re-distributing the corneal shape. As a result, INTACS can make vision correction easier with glasses or contact lenses after surgery. They also have the advantage of being removed or changed once the prescription changes with time. INTACS may delay the need for other more invasive surgical procedures such as corneal transplants.
Corneal crosslinking (CXL) aims to increase the corneal rigidity and biochemical stability by the stiffening the number of anchors that bond the collagen. This procedure has not been FDA approved; however, clinical trials in the U.S began in 2008 to determine the effectiveness and safety of CXL.There are two different techniques used to accomplish CXL: epithelium-off and epithelium-on method. With epithelium off method, the outer layer of the cornea, epithelium, is removed. A type of vitamin B, riboflavin is applied into the cornea and it is activated with UVB light for an approximation of 30 min. The epithelium on method, the outer layer of the cornea is left intact; however, it requires longer time for the cornea to be exposed to riboflavin to penetrate into the cornea. The advantages of epithelium on technique includes less risk for infections, faster recovery time, and less discomfort.
There is active research, especially in the United States, to shorten the length of the procedure and to avoid the removal of the corneal epithelium. They include doing modifications of the vehicle of the riboflavin solution to increase epithelial permeability at a shorter time. Also, they have reported similar results obtained with either epithelium on or off techniques.
When the keratoconic patient no longer can tolerate the use of contact lenses, or they develop other clinical findings such as scarring or significant thinning impending perforation, the best option may be corneal transplant. Keratoplasty is corneal transplant in which diseased corneal tissue is excised from the host and replaced by healthy donor cornea. It can be partial or complete corneal transplant. With either technique, the patient end up needing glasses or contacts and reaching a much better comfortable vision correction. These surgical process is the most invasive and can take up to one year of recovery time.
Implantation of phakic intraocular lens can potentially improve vision without the need of corneal surgery. The requirements to do this procedure are showing stability of the condition as well as having significantly high myopia and astigmatism.
Topography-guided conductive keratoplasty regularizes the cornea with a limited ablation using Excimer laser, improving the quality of vision and the possibility of better visual correction. The procedures uses custom corneal topographies for guidance of treatment that is designed to normalize the anterior surface of the cornea only with a very small ablation treatment of less than 50 microns. The technology to accomplish this technique has not been approved in the United States; however, a phase III trial in under way.