Dry eye is one of the most common side effects that patients experience following LASIK. An estimated 85% of LASIK patients experience dry eye symptoms one week following surgery, and another 60% will have similar symptoms after one month. (1) For most patients, this is a temporary problem that resolves itself a short time after surgery.
For a small percentage of patients, however, the dryness symptoms can be very frustrating. Dry eye following LASIK can cause discomfort and blurred and fluctuating vision.
Another consideration: Wavefront laser ablations require aberrometry measurements to measure slight irregularities of the visual system. Disruptions in the tear film surface may potentially affect the aberrometer measurements, skewing the results of the procedure. (2)
Pre-existing keratitis sicca is not an absolute contraindication for a keratorefractive procedure. However, O.D.s should identify and treat dry eye patients prior to surgery.
Several theories have been presented to explain why patients develop dry eye following LASIK. Among them:
* The "neural feedback loop theory." This is the most commonly cited hypothesis. It suggests that the corneal nerve fibers are disrupted during creation of the LASIK flap and stromal ablation. This, in turn, interferes with the cornea-central nervous system-lacrimal gland regulatory loop. (3,4) Reduced corneal sensitivity causes a decreased blink rate and diminishes reflex tear production. The combination of these two factors increases the time the cornea is unprotected by the tear film.
Clinical experience has shown that LASIK-induced dry eye typically improves over time. Indeed, researchers have found that corneal sensation continues to improve up to six months after surgery. (3) The more severe cases of post-LASIK dry eye have even been termed "LASIK-induced neurotrophic keratopathy," or LINK.
* Goblet cell damage. A normal lipid layer is necessary to prevent evaporation of the tear film. (5,6) Likewise, a normal mucin layer is critical to a healthy ocular surface. However, prolonged microkeratome pressure can damage conjunctival goblet cells, disrupting the normal tear film composition. The resultant unstable mucin layer will decrease the tear film break-up time and increase dry eye symptoms. (6)
* Change in corneal curvature. While the change in curvature provides better vision, it also affects how the tear film overlays the cornea. The change in tear function is evident months after surgery. It presents as iron-stained epithelium.
* Osmolarity changes and exposure keratopathy. Studies have also shown an incomplete blink in patients following LASIK, which results in osmolarity changes and potential exposure keratopathy. (7)
* Type of ablation. Hyperopic ablations affect the tear film more than myopic ablations. (8) Specifically, the tear film has more trouble overlaying the steeper cornea following hyperopic surgery, thus causing an increase in dry eye symptoms.
Also, higher myopic prescriptions tend to cause greater dryness than lower myopic modifications. (9,10) This is because myopic ablations greater than -6.00D will produce a greater change in corneal curvature, possibly affecting more corneal nerve fibers.
Researchers have found that patients who undergo photorefractive keratectomy (PRK) tend to have less dryness problems than LASIK patients. (10)
In a prospective study, researchers compared...