GP insights

Corneal Thinning With GP Wear

BY ANN LAURENZI-JONES, OD, FAAO

When fitting contact lenses, we are acutely aware of the complications that we can induce such as mechanical trauma, dry eye, increased infection rates, and overall ocular irritation. We are also well aware of lens-induced corneal hypoxic events and the cascade of ocular consequences that can occur related to hypoxia. Induced hypoxia can cause morphologic, metabolic, and structural changes that can be observed throughout the corneal epithelium, stroma, and endothelium (Liesegang, 2002). The clinical results may include edema, pannus, neovascularization, pleomorphism, and polymorphism. Many of these ocular events require immediate intervention in the form of fit changes, material changes, or abortion of lens wear. High- and hyper-Dk lens materials (soft and GP) have greatly lowered the risk of these complications.

Lens-Induced Corneal Thinning

One long-term contact lens complication that may not be immediately observed is corneal thinning associated with GP (and soft) contact lens wear. Research shows that contact lens wear can cause corneal thickening in the first three hours to three months of lens wear but that longer-term wear is associated with corneal thinning (Yeniad et al, 2003).

Corneal thinning has been observed in daily wear of GP lenses that had Dk values accepted for supplying appropriate amounts of oxygen for corneal metabolism. One study reported the mean reduction in central corneal thickness to be 24 microns more in GP wearers than in a control population of non-contact lens wearers (Braun and Anderson Penno, 2003). Another study found statistically significant thinning in GP patients wearing lenses of various Dk values after one month of wear that remained when examined at 12 months (Yeniad et al, 2003).

Also different from edema is that corneal thinning is not transient. A study of long-term GP wearers reported that corneal thickness was an average of 37 microns thinner when compared to eyes that had not worn contact lenses, and there was no significant change in this reduction after discontinuation of rigid lens wears for six months (Myrowitz et al, 2002). There also seems to be a negative correlation between central corneal thickness and length of time wearing contact lenses (Lui and Pflugfelder, 2003). This study also concluded that central corneal thickness was lower in GP wearers versus non-contact lens wearers. Another study highlighted thinning differences in diagnosing true keratoconus versus GP contact lens wear-induced corneal thinning (Pflugfelder et al, 2002).

None of these studies have determined the direct cause of corneal thinning with contact lens wear, but possible explanations include contact lens-related chronic edema of the corneal stroma and biochemical changes in corneal stromal composition resulting from an accumulation of lactic acid secondary to hypoxia; keratocyte apoptosis from either chronic hypoxia or cell trauma with release of interleukin 1; chronic exposure to hyperosmotic tears resulting from increased tear osmolarity in lens wearers; and mechanical epithelial thinning in GP wear resulting in either epithelial cell loss or redistribution.

Clinical Implications

Knowing that there is an association between lens wear and corneal thinning may help us in our practices when considering referrals for refractive surgery procedures that further thin the cornea or in treating a glaucoma patient who has a history of GP wear or who needs to be fit in GP lenses.

The causes of lens-associated corneal thinning may not be mutually exclusive from one another, which makes it even more important for us to perform our professional duties rigorously: we must provide our patients with the best lens fit possible in high-Dk materials to allow for the best longterm ocular heath. CLS

For references, please visit www.clspectrum.com/references.asp and click on document #172.