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Excimer Laser Phototherapeutic Keratectomy for Corneal Dystrophies

US Sensory Disorders Review, 2006:30-2 DOI:
Received: January 13, 2011 Accepted: January 13, 2011

While there are seemingly weekly advances in the genetic classification of corneal dystrophies, clinicians generally differentiate them by their anatomic depth. From a practical standpoint, the level of the pathology in the cornea directly impacts both the patient’s symptoms and their treatment options. Anterior disease is more likely to cause painful erosions and corneal epithelial irregularity. Mid-corneal disease tends to cause decreased vision more than pain. Posterior corneal disease tends not to cause symptoms until corneal edema develops.The more anterior the pathology, the more amenable it is to superficial keratectomy with a blade, or more recently, with the excimer laser.1

Anterior corneal dystrophies, such as anterior basement membrane dystrophy (ABMD) and the corneal dystrophies of Bowman’s membrane (classically Reis-Bücklers’ dystrophy), often cause disruption of the most superficial layer of the cornea, which can result in recurrent erosions. Recurrent erosions are painful breaks in the corneal epithelium, which can last from minutes to days, depending ontheir size. One of the worst aspects of recurrent erosions is that they are unpredictable.They may not occur for weeks to months, but then develop on the morning of an important event such as a business meeting or family wedding. Not only can erosions incapacitate patients due to pain and decreased vision, but they also greatly increase the risk of corneal infection. Depending on the degree of corneal irregularity and opacity, anterior dystrophies can also cause decreased vision even without recurrent erosion.

Corneal dystrophies involving the stroma—the ‘body’ of the cornea—such as granular, lattice, macular, and Schnyder’s crystalline dystrophy, tend to cause fewer painful erosions, but can certainly affect the vision depending on the degree of opacity. Multiple separate stromal opacities cause fewer disturbances to the vision than confluent opacities. Posterior corneal dystrophies, for example endothelial dystrophy, Fuchs’ dystrophy and posterior polymorphous dystrophy, affect the endothelium—the back layer of the cornea.

The endothelial cells pump fluid out of the cornea to keep it clear. When the endothelial cells are severely damaged, corneal edema develops, at which point the vision is affected and eventually the eye can become painful.

The argon-fluoride excimer laser uses 193nm wavelength ultraviolet light to remove corneal tissue. It can remove approximately 1/2,000th of the corneal thickness per laser pulse and the spot size can be adjusted from less than 1mm to almost the entire corneal diameter. The number of pulses delivered determines the depth of the ablation. The excimer laser removes tissue by breaking molecular bonds with minimal damage to adjacent or deep corneal tissue. It is US Food and Drug Administration (FDA)-approved to treat nearsightedness, farsightedness, and astigmatism by reshaping the corneal surface through tissue removal. Tissue can be removed from the superficial corneal stroma (surface ablation) or the mid-stroma (laser in situ keratomileusis—LASIK) after a partial thickness corneal flap is created and moved to the side. The excimer laser is also FDA-approved to remove anterior corneal pathology and smooth the corneal surface to improve visual function and painful symptoms in a procedure termed ‘phototherapeutic keratectomy’ (PTK). Excimer laser PTK is not useful for posterior corneal dystrophies. While the number of PTK procedures is tiny compared with the number of surface ablation and LASIK procedures, PTK can significantly improve the vision, pain and discomfort and consequently quality of life for many patients with superficial and anterior stromal dystrophies.2

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