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No evidence exists that refractive surgery causes chronic dry eye. In many cases, patients seeking refractive surgery already may have dry eye symptoms. It is the surgeon's responsibility to identify and treat dry eye prior to surgery, ensuring that the patient has a healthy ocular surface.
This patient population encompasses former contact-lens wearers who now are seeking refractive surgery because they have become contact-lens intolerant because of dry eye, he noted.
"It is extremely important to diagnose and treat dry eye effectively before LASIK," said Dr. McCulley, professor and chairman, Department of Ophthalmology, University of Texas Southwestern Medical Center, Dallas. "We need a healthy ocular surface, and if we don't [have one], we are asking for big troubles."
Individuals with healthy ocular surfaces should be prepared to experience dry eye symptoms for up to 12 months after LASIK. The reasons for this dry eye state:
"If the ocular surface is normal preoperatively in tear production, the rare patient will be tipped over to a dry eye state with the LASIK procedure," Dr. McCulley said. "Patients who have very marginal tear production that may or may not be obvious preoperatively are very apt to have the decrease in tear production associated with LASIK result in a postoperatively dry eye state.
"If these patients are identified either before or shortly after developing ocular surface disease, [dry eye] typically can be well controlled and effectively treated until their tear production recovers 6 to 12 months after surgery."
Often, patients don't notice the dryness initially because LASIK has denervated the ocular surface and the result is decreased corneal sensitivity. "This can result in a slower blink rate," Dr. McCulley said. "There is also an adverse effect on the neural loop so that a decrease in the neural stimulation of the lacrimal and accessory lacrimal glands-and possibly also the meibomian glands-results in decreased tear component production."
The combined ablation depth and flap thickness are risk factors for dry eye after LASIK because the deeper that nerves are cut from the corneal surface, the longer it takes for them to regenerate.
The debate continues about whether flap hinge position and flap hinge width contribute to the severity of dry eye after LASIK, Dr. McCulley noted. In his clinical experience, he said, he has not noticed an increased risk of dry eyes in patients receiving superior hinges compared with those receiving nasal hinges. Some researchers, however, have noted the advantages of the nasal hinge being less loss of corneal sensitivity or a quicker recovery of corneal sensitivity than in eyes receiving superior hinges.
"The length of the hinge [of the flap] would also theoretically have an impact, i.e., the longer the hinge the fewer nerves cut near the limbus," Dr. McCulley said. "I personally have not experienced adverse clinical events with varying hinge lengths."
Counsel patient carefully
All patients need to know that, after LASIK, they will experience decreased tear production. Those with dry eye symptoms before LASIK should expect worse symptoms after the procedure. Decreased tear production and dry eye symptoms are likely to persist for 6 to 12 months postoperatively, so it is important to identify and treat patients with dry eye preoperatively.
"If I identify a patient who has dry eyes preoperatively, I only perform LASIK if I can get the ocular surface completely healthy, with a reasonable amount of therapy preoperatively," Dr. McCulley said. "If I cannot get the ocular surface completely healthy preoperatively, I do not do LASIK."