discovering dry eye
Do Lens Wearers Ever Truly Adapt?
BY JASON J. NICHOLS, OD, MS, MPH
Most lens wearers have relatively few problems with dryness and discomfort during the first few hours of lens wear, but do experience symptoms during the latter part of the day. Lens application seems to be a significant insult to the normal homeostasis of the ocular surface, so it seems that symptoms should occur during adaptation or lens settling. Studies are searching for an answer to this puzzling matter.
Exploring Lens Adaptation
Harris and Mandell (1969) proposed the osmotic theory of contact lens adaptation. Their re search showed that a cornea of an unadapted lens wearer increased in thickness upon placing a rigid contact lens on the contralateral cornea. Because they predicted increased tear flow to occur in both eyes (rather than just the eye wearing the lens), they suggested that this reduced the normal tonicity of the tear film, resulting in an increase in corneal thickness. They also suggested that as the nerves adapt to the lens during the first few weeks of corneal wear, tear secretion will return to normal levels upon lens application.
In a follow-up study, Polse and Mandell (1971) suggested a second mechanism for contact lens adaptation (the oxygen theory). They proposed that contact lens wear results in an increase in blinking (for about one hour), followed by a period of blink suppression. During blink suppression, reduced tear exchange might cause a reduced oxygen supply to the cornea, with resultant corneal swelling. However, they suggested that tear tonicity remains the primary factor involved in lens adaptation.
Settling in Adapted Wearers
The normal, daily process of lens settling on the ocular surface for veteran lens wearers remains unclear. Clinicians predict that upon hydrogel lens application, a thick tear film may surround the lens. Reductions to normal pre- and post-lens tear thickness may follow this increased tear thickness over the first hour of wear. However, researchers know little about tear thickness immediately following lens application and how contact lens solutions may impact these layers.
Little and Bruce (1994) qualitatively examined the post-lens tear film (POLTF) thickness during the first six hours of lens wear in a group of normal subjects. They showed that the POLTF was thick immediately after lens application. However, during the next 30 minutes, the POLTF showed significant thinning with reduced lens movement. The POLTF returned to thicker values for the rest of the lens-wearing period. This finding seems to agree with what the osmotic theory predicts for an adapted lens wearer (neural desensitization results in normal tear secretion and tonicity for the adapted lens wearer).
Chauhan and Radke (2002) also studied how lens settling affects tear film thickness using theoretical models. They predict that the POLTF doesn't reach a steady state, but continues to thin toward the corneal surface, especially in the periphery. They suggest that this may cause superior epithelial arcuate lesions (SEALs) and adherent lenses.
These studies provide preliminary evidence that the tear film changes during the first hour, and potentially over the course of the day, for all contact lens wearers.
The Search Continues...
The tear film of symptomatic patients may not recover after the first hour of lens wear, as some of these studies suggest. Also, it seems that researchers have largely ignored the pre-lens tear film in all of this. The key to patient dryness symptoms, and lens dehydration, may lie within this important layer.
Dr. Nichols is a senior research associate at The Ohio State University College of Optometry.