FREQUENCY OF POST-LASIK DRY EYE
Dry eye following LASIK is perhaps the procedure's most common complication. Virtually all patients experience dry eye after LASIK, at least transiently.1 Hovanesian et al2 reported that 50% of patients had symptoms related to dry eye 6 months following LASIK. According to a different study, 15% of patients experienced moderate dry eye that lasted at least 3 months, and 5% of patients experienced severe dry eye that lasted at least 6 months after LASIK.3 However, in general, only patients who had preoperative dry eye or who were marginally compensated with borderline dry eye before surgery experienced dry eye symptoms postoperatively. Most patients with post-LASIK dry eye suffered a disruption of the tear film and ocular surface and will experience fluctuating visual acuity between blinks at different times of the day. It is fortunate that, in our experience, the majority of patients' symptoms are resolved in 2 to 4 weeks following surgery.
Several possible causes of LASIK-associated dry eye exist. The high level of pressure induced by the suction ring during LASIK may damage the conjunctival goblet cells and compromise the mucin layer of the tear film.4 Significant alterations to the corneal curvature that occur after LASIK alter tear wetting as the lids move over the modified ocular surface. Medicamentosum, which is caused by epithelial toxic antibiotics, NSAIDs, and preservatives may also induce transient dry eye symptoms. Because intact corneal sensation drives tear production, corneal denervation associated with the LASIK procedure is the most significant cause of post-LASIK dry eye. During LASIK, the microkeratome severs the corneal nerve trunks and photoablation disrupts the anterior stromal nerves. Both processes damage corneal innervation. The reduction in corneal neuronal feedback to the brain stem reduces the brain stem's innervation of the lacrimal glands and diminishes tear production. The nerves regenerate postoperatively, and corneal sensation returns in approximately 3 to 6 months.5 The regained corneal sensation may help explain the transient dry eye and the return of tear function with time.
One of the most important ways to prevent symptomatic postoperative dry eye is to screen patients carefully. Many patients with dry eye are actually preselected candidates for refractive surgery because their pre-existing dry eye causes them to be uncomfortable wearing contact lenses or renders them contact lens intolerant. Any mention of contact lens intolerance during the course of the patient history suggests the possibility of underlying dry eye. Conversely, long-term contact lens wear, especially of hard lenses, should also be noted during the history-taking because long-term contact lens wear decreases corneal sensation and could contribute to decreased tear production.
The dry eye history is perhaps the most important part of the dry eye work-up. Any eye irritation, including sandy-gritty irritation, dryness, burning, and foreign body sensation, is suggestive of dry eye. The examiner should carefully inspect for signs of meibomianitis on the lids of patients who report eye irritation upon awakening. They should pay close attention to the status of the meibomian gland's orifices, the width of the palpebral fissure, and the volume of the tear film in patients who report that their symptoms worsen as the day progresses. Stenosis and closure of the meibomian glands, a large palpebral fissure width, and decreased tear production increase the tear film's osmolarity and cause dry eye. Tear testing such as tear break-up time, an examination for tear debris in the inferior cul-de-sac, and Schirmer testing with anesthesia are all appropriate. Most importantly, we suggest performing supravital staining of the conjunctiva with Lissamine Green (Accutome Inc., Malvern, PA) or rose bengal and fluorescein staining of the cornea to look for the classic staining pattern of the dry eye (Figure 1).
The efficacy and safety of LASIK are not necessarily affected by pre-existing dry eye,6 but it is a risk factor for symptomatic post-LASIK dry eye with measurably lower tear function and supravital staining of the ocular surface.6 Preoperative conjunctival staining represents a risk factor for dry eye postoperatively, and corneal staining is considered to be a relative contraindication for surgery until the ocular surface has stabilized.4 Women and any patients with reduced preoperative tear film stability are at risk for chronic dry eye after LASIK.4 In patients who have dry eye symptoms preoperatively, it is important to maximize the health of their ocular surface prior to surgery because it is easier to successfully treat these patients pre- versus postoperatively. Patients with symptoms of dry eye but no signs of corneal or conjunctival staining are generally excellent candidates for LASIK. Those who have dry eye symptoms with mild conjunctival staining should be treated with artificial nonpreserved tears to stabilize their ocular surface before surgery.7,8
Patients with corneal staining should also undergo treatment with artificial nonpreserved tears, lubricating ointment at night, topical cyclosporin A (Restasis; Allergan, Inc., Irvine, CA), and, if necessary, punctal occlusion. For individuals who have meibomian gland disease, we add 100mg of oral doxycycline twice a day for 2 weeks and then once a day for an additional month.
EFFECT OF LASIK ON CORNEAL SENSATION
The cornea is one of the most densely innervated and highly sensitive tissues in the body. Corneal sensation is vital for maintaining corneal epithelial integrity and tear film function, and it is provided by the long ciliary nerves of the ophthalmic division of the fifth (trigeminal) cranial nerve. The long ciliary nerve trunk travels through the suprachoroidal space, where it branches several times before entering the cornea at the limbus. The large nerves enter the limbus predominantly at the 9- and 3-o'clock positions (Figure 2). These nerves then bifurcate and move toward the 12- and 6-o'clock positions. After a second bifurcation, they again run toward the 9- and 3-o'clock positions. The fact that the long ciliary nerves enter the eye at the aforementioned locations explains why corneal sensation is significantly greater at the temporal and nasal limbi than inferiorly.
A self-controlled, masked clinical study demonstrated that patients with superior-hinge flaps (Figure 3) had a significantly greater loss of sensation and significantly increased corneal and conjunctival Lissamine Green staining at all time intervals compared with patients who had nasal-hinge flaps (Figure 4).3 The study also revealed a trend toward lower Schirmer scores and shorter tear break-up times in patients with a superior-hinge flap compared to those with nasal flaps.3
INTRA- AND POSTOPERATIVE MANAGEMENT
The careful intraoperative management of LASIK can decrease the risk of dry eye by preserving the corneal epithelium and preventing corneal abrasions. We suggest minimizing the use of topical anesthetics by giving the first dose when a patient enters the laser suite and the second dose immediately before surgery. To reduce the risk of a corneal abrasion, we suggest lubricating the ocular surface with proparacaine, which has a glycerin base, prior to the microkeratome's pass. After repositioning the flap, we place a small amount of carboxymethylcellulose 1% on the corneal surface to prevent surface dessication. We apply the nonpreserved NSAID, ketorolac, a fluoroquinolone antibiotic, and prednisolone acetate 1% intraoperatively before removing the lid speculum. We instruct patients to close their eyes for 15 minutes before we examine the flap. We then ask them to keep their eyes closed for 4 hours to promote epithelial healing.
To help epithelial healing and reduce the incidence of post-LASIK dry eye, patients are given a structured schedule for postoperative artificial tear use. On the first postoperative day, patients use a more viscous tear, carboxymethylcellulose 1%, every 2 hours. For the remainder of the first postoperative week, they use transiently preserved tears. If a patient has symptoms of dry eye following LASIK, then we insert inferior punctal plugs in an attempt to stabilize the ocular surface. If this treatment is not successful, then we add oral doxycycline to the postoperative regimen. We have found that many patients with meibomian gland disease also benefit from Refresh Endura lipid emulsion (Allergan, Inc.) twice a day and nutritional supplements containing omega-3 essential fatty acids. Finally, patients who have recalcitrant, long-term dry eye complications, have benefited significantly from tCSA 0.05% twice daily for 6 months.
LASIK causes significant changes to the ocular surface that may be transient or permanent. The most frequent nonkeratome-related complication of LASIK surgery is dry eye. Its incidence can be decreased by identifying patients at risk for dry eye, maximizing their tear film stability preoperatively, and minimizing their dry eye through intra- and postoperative therapeutic interventions. Refractive surgeons should take the appropriate steps at each stage of the surgical process to optimize the refractive outcome, minimize postoperative complications, and maximize the stability of the patient's ocular surface.
Renée Solomon, MD, states that she holds no financial interest in any product or company mentioned herein. She may be reached at email@example.com.
Eric D. Donnenfeld, MD, is a consultant for Allergan, Inc., Alcon Laboratories, Inc., and Bausch & Lomb, and he performs research for Alcon Laboratories, Inc., Allergan, Inc., and Santen, Inc. He states that he holds no financial interest in any products mentioned herein. Dr. Donnenfeld may be reached at (516) 766-2519; firstname.lastname@example.org.
Y. Ralph Chu, MD, is Medical Director, Chu Vision Institute in Edina, Minnesota. He states that he holds no financial interest in any product or company mentioned herein. Dr. Chu may be reached at (952) 835-1235; email@example.com.
Nina Goyal, MD, is a resident in ophthalmology at the Rush University Medical Center in Chicago. She states that she holds no financial interest in any product or company mentioned herein. Dr. Goyal may be reached at (312) 942-5315; firstname.lastname@example.org.
Wei Jiang, MD, is a resident in ophthalmology at the Jules Stein Eye Institute in Los Angeles. She states that she holds no financial interest in any product or company mentioned herein. Dr. Jiang may be reached at (310) 825-5000; email@example.com.
Baseer Khan, MD, is a senior resident in ophthalmology in the Department of Ophthalmology at the University of Toronto. He states that he holds no financial interest in any product or company mentioned herein. Dr. Khan may be reached at (415) 258-8211; firstname.lastname@example.org.
Patty Lin, MD, MBA, is a resident in ophthalmology at the Jules Stein Eye Institute in Los Angeles. She states that she holds no financial interest in any company or product mentioned herein. Dr. Lin may be reached at (310) 825-5000; email@example.com.
Gregory J. McCormick, MD, is an ophthalmology resident at the University of Rochester Eye Institute in New York. He states that he holds no financial interest in any company or product mentioned herein. Dr. McCormick, may be reached at (585) 256-2569; firstname.lastname@example.org.
Jay S. Pepose, MD, PhD, is Professor of Clinical Ophthalmology & Visual Sciences, Washington University School of Medicine, St. Louis. He states that he holds no financial interest in any product or company mentioned herein. Dr. Pepose may be reached at (636) 728-0111; email@example.com.
Paul Sanghera, MD, is a resident in ophthalmology in the Department of Ophthalmology and Vision Sciences at the University of Toronto. He states that he holds no financial interest in any product or company mentioned herein. Dr. Sanghera may be reached at (416) 666-7115; firstname.lastname@example.org.
Tracy Swartz, OD, MS, states that she holds no financial interest in any product or company mentioned herein. She may be reached at (615) 321-8881;
Ming Wang, MD, PhD, states that he holds no financial interest in any product or company mentioned herein. He may be reached at (615) 321-8881;