discovering dry eye
Creating Dry Eye With Contact Lenses
BY CAROLYN BEGLEY, OD, MS, FAAO
As clinicians, we spend a lot of time and energy treating patients with dry eye syndrome. But do we ever create the condition in previously asymptomatic patients? A model of dry eye in humans would make it easier to understand what causes the condition, but we are constrained by ethical considerations. However, upon further deliberation it is obvious that we do create mild cases of dry eye in humans on a daily basis we fit our patients with contact lenses.
Contact lens wear is known to cause an increase in dry eye symptoms. Many patients who show no apparent dry eye symptoms without lenses become symptomatic with lenses. Recently, the Dry Eye Investigation (DREI) collected 1,054 dry eye questionnaires from patients entering five optometry schools in the United States and Canada and one private practice. Contact lens wearers reported more frequent ocular discomfort, dryness, visual disturbances, soreness, irritation and foreign body sensation than non-lens wearers. Approximately 50 percent of contact lens wearers reported moderate to intense eye dryness in the evening, compared to only 22 percent of non-wearers. The contact lens wearers also experienced more frequent and intense eye dryness. In addition, 172 former contact lens wearers reported that the top two reasons why they stopped wearing lenses were eye dryness and discomfort later in the day.
Sensory Feedback and Dry Eye
How does contact lens wear increase dry eye symptoms? The usual explanation is that the lens surface is not as wettable as the ocular surface, causing disruption of the tears. However, soft lenses also cover the cornea and shield it from the outside world, affecting sensory feedback of information from the ocular surface. This feedback may help determine tear levels at the surface.
In a recent study of punctal plugs, Drs. Alan Tomlinson, BOptom, PhD; Jennifer Craig, BOptom, PhD; and Gerald Lowther, OD, PhD, occluded the puncta of normal volunteers with non-dissolvable plugs and measured the resulting tear turnover rates over one week. The subjects teared excessively during the first few days. However, by the end of one week, tear turnover rates had adjusted to the new condition and returned to baseline levels. This implies that feedback of information from the ocular surface contributes to the control of tear levels. If that is the case, then soft contact lenses may interfere with that feedback by blanketing the cornea and limbal conjunctiva.
Contact Lens "Security Blanket"
I use peeling onions to provide an example of how much a soft contact lens can shield the cornea from sensation. I wear only one contact lens, so it is easy to determine the degree of protection. When I peel onions, the eye with the contact lens is completely comfortable, while the other eye burns furiously. Clearly, the soft contact lens protects my ocular surface from this noxious sensation.
How would that information contribute to what we know about dry eye? Sensory feedback from the surface may control tear levels and also may affect the blink rate, both of which could contribute to dry eye symptoms. Refractive surgery patients often suffer from dry eye immediately after the surgery, sometimes for six months or more. The proposed cause is severed surface nerves which affect sensory feedback from the corneal surface. Ocular surface conditions immediately following refractive surgery may be comparable to wearing a soft contact lens. Both conditions interfere or block normal sensory feedback from the surface of the eye.
Dr. Begley is an associate professor at the Indiana University School of Optometry and is also a member of the Graduate Faculty.