Evaluating the Cornea

Studies have shown that 50% of patients complain of ocular irritation during the first few months following LASIK and that approximately 10% of patients have a clinically significant dry eye after undergoing the procedure.1,2 Moreover, based on the amount of referrals I receive, the number of cases of post-LASIK dry eye appears to be increasing. Assessing the health of a candidate's corneal surface before that person undergoes refractive surgery is essential to identifying patients with keratitis sicca, which may contribute to a poor surgical outcome. Fluorescein-dye staining offers the greatest sensitivity for detecting corneal epithelial disease.

PREOPERATIVE EVALUATION

Contact-Lens Intolerance
Many patients who cannot tolerate their contact lenses seek refractive surgery. It is important to recognize that the reason for their contact lens intolerance is often dry eye. Additionally, long-term lens wearers generally have subnormal corneal sensation, so they will be prone to dry eye postoperatively, if they do not already have this condition.

A lack of corneal sensation reduces a patient's blink rate, so his tear film evaporates more rapidly and any tears produced are not spread over the ocular surface. In addition, because tear production is largely reflexive, a reduction in corneal sensation will decrease the drive for the lacrimal gland to produce tears. LASIK worsens the already poor corneal sensation of many contact lens-intolerant patients by cutting the majority of their corneal nerves. For these reasons, refractive surgeons must have a high index of suspicion for dry eye in lens wearers and lens-intolerant patients, and they should warn these patients that their postoperative visual recovery may be more prolonged than normal. Medications

Be certain to inquire about the patient's use of medications. Oral medications such as antihistamines and antidepressants decrease tear production. I frequently instruct antihistamine-users to stop taking the medication for 1 week prior to and 1 week following refractive surgery.

Cosmetic Surgery
Many patients interested in LASIK have undergone cosmetic facial surgery. Be aware that individuals who have had blepharoplasties or a facelift frequently do not completely close their eyes. These patients exhibit either inferior corneal or exposure zone staining.

If the patient history includes cosmetic facial surgery, first observe the patient's rate of blinking. In some, it will be obvious that they are staring unblinkingly at you. Others may blink twice per minute, and their blink rate may be more difficult to perceive. Next, ask the patient to gently close his eyes and examine him with a penlight in order to ascertain whether the eyelids close completely.

If a patient does not close his eyes completely, he will require vigorous ocular lubrication following LASIK, or else the flap will not heal properly. The ocular dryness of several patients referred to me has prevented their epithelium from healing over the inferior edge of the LASIK flap, thereby leading to scarring and even melting of the flap and putting these individuals at risk for infection.

THE SUPERIORITY OF FLUORESCEIN
Although rose bengal is a good method for corneal staining when in liquid form, which is now available only from formulation pharmacies, it is more difficult to visualize when strips are used. In that form, both it and Lissamine Green (Accutome, Malvern, PA) simply do not deliver a sufficiently high concentration of dye for optimal staining. There is an ongoing debate about whether corne

al staining with fluorescein is superior to Schirmer testing as a screening method for dry eye. Overall, an analysis of the Schirmer test in hundreds of patients will demonstrate that it detects ocular surface disease, dry eye, and corneal erosions. Nevertheless, a low Schirmer test score in a given patient who has no symptoms of ocular surface disease is not a contraindication for LASIK. Similarly, many patients will have Schirmer test results of greater than 10 mm but exhibit punctate staining of the cornea. In these individuals, LASIK will likely exacerbate their ocular surface disease and limit their visual recovery.

METHODS
After instilling fluorescein into the patient's eyes, the first thing to do is a breakup time. Normally, tears break up in approximately 10 seconds. If they break up in less than 5 seconds, the patient's tear film is unstable. This instability will likely worsen after LASIK, and patients will complain of blurred and fluctuating vision with blinking.

Next, examine the cornea for staining. Viewing the eye through a yellow filter (Bausch & Lomb, Rochester, NY) will enhance the dye's sensitivity two- to threefold. It is important, however, to allow the dye 1 to 2 minutes to permeate the corneal epithelium. Evaluating the eye immediately after instilling fluorescein can make staining patterns difficult to perceive.

Corneal topography is another useful modality for examining the ocular surface. Some topographers include surface regularity indices. Dry eye produces an irregular corneal surface, so a highly irregular index is a warning sign that the patient may experience significant problems with dry eye following refractive surgery.

PATTERNS
A small amount of scattered staining, especially near the inferior limbus, is normal. More extensive staining of the inferior cornea generally occurs in patients with meibomian gland disease or lipid tear deficiency (Figure 1). Exposure zone staining across the central cornea usually indicates aqueous tear deficiency (Figure 2). Lissamine green is the best dye to stain the exposed conjunctiva of a patient with aqueous tear deficiency.

A patient with lipid tear deficiency normally benefits from lid hygiene and oral tetracycline antibiotics. A newly approved option is Restasis (Allergan, Inc., Irvine, CA), while the usefulness of nutritional supplements containing essential fatty acids is currently unproven. Patients with aqueous tear deficiency require vigorous lubrication and often punctal occlusion. It is important to treat the dry eye of both types of patients prior to performing any refractive surgery.

CONCLUSION
Dry eye is the most common problem following LASIK, and it is more likely to occur in patients who have the condition preoperatively. Identifying and treating dry eye prior to performing LASIK may improve surgical outcomes in these individuals. Surgeons should inform patients of this common but potentially sight-threatening complication.

Stephen C. Pflugfelder, MD, is Professor of Ophthalmology and Director of the Ocular Surface Center for the Cullen Eye Institute at Baylor College of Medicine in Houston. He is a consultant for Allergan, Inc. Dr. Pflugfelder may be reached at (713) 798-4732; steven@bcm.tmc.edu.
1. Hovanesian JA, Shah SS, Maloney RK. Symptoms of dry eye and recurrent erosion syndrome after refractive surgery. J Cataract Refract Surg. 2001;27:577-584.
2. Yu EY, Leung A, Rao S, Lam DS. Effect of laser in situ keratomileusis on tear stability. Ophthalmology. 2000;107:2131-2135.