discovering dry eye
Why Hydrogel Contact Lenses Stain
BY JASON J. NICHOLS, OD, MS, MPH
Surface staining is a prominent feature of dry eye disease, and staining severity increases with aqueous deficiency. Surface staining is also prominent in contact lens wear. Fluorescein will show breaks in the epithelial layer of the cornea, while both rose bengal and lissamine green stain conjunctival epithelia that's devoid of mucous protection.
Many hydrogel lens wearers exhibit some corneal fluorescein staining, and studies have shown that the cornea typically stains in the inferior region. Although not well studied, most hydrogel contact lens wearers also exhibit some conjunctival staining that usually follows the edge of the lens (Figure 1).
Figure 1. Nasal and temporal circumlimbal conjunctival staining associated with hydrogel lens
Searching for the Cause
How this type of circumlimbal staining develops isn't entirely clear; the process most likely involves many factors, and some potential explanations exist. Probably the most obvious mechanism is a mechanical desiccation of the conjunctival epithelium. In es-sence, the edge of the lens continually rubs on the conjunctival epithelium and results in the observed staining.
However, it's unknown how other factors influence this sort of mechanical interaction and why it occurs in some patients but not in others. Perhaps a reduction in tear exchange or tear thickness behind a hydrogel lens causes the staining phenomenon. Measuring both tear exchange and thickness are difficult, but studies are beginning to unravel some of these issues.
Hydrogel contact lenses that dehydrate to a certain magnitude on the eye may also promote the mechanical desiccation of the ocular surface. In general, high water content hydrogel contact lenses dehydrate to a greater extent on the eye than do low water content contact lenses; thus, dehydration and high water content contact lenses may play a role in the process.
Another contributing factor may be the conjunctival topography, as clinicians see conjunctival folds somewhat frequently in practice. Each of these factors may play a critical role in causing this sort of surface staining, which may be related to symptoms or comfort issues.
Treating Circumlimbal Staining
Although no real clinical trials have assessed treatments for this sort of desiccation, most clinicians would agree that these patients should use lubricant or rewetting drops during lens wear. This sort of lubrication will promote healing of the ocular surface, in addition to reducing any potential tear film disturbance and keeping the lens hydrated.
Evidence also indicates that an investigational agent (diqua-fosol tetrasodium -- INS365, Inspire Pharmaceutical's P2Y2 purinergic receptor agonist) promotes epithelial recovery from staining caused by dry eye disease through ocular surface rehydration. Such an agent may promote surface recovery in lens wearers.
If I believe lens dehydration may be a factor, I switch patients to a lower water content lens.
Aspiring to Perfection
Contact lens manufacturers should make it a long-term goal to develop contact lenses that resist dehydration and/or promote tear exchange beneath the lens. Not only should this reduce ocular surface desiccation, but it also should reduce inflammatory and infectious complications associated with contact lens wear.
Dr. Nichols is a senior research associate at The Ohio State University College of Optometry.