Vascular Response

Kathryn Dumbleton - BSc (Hons), Uni of Wales1984 MCOptom 1985, MSc Uni of Waterloo 1988

Senior Researcher
Centre for Contact Lens Research (CCLR)
University of Waterloo, Ontario, Canada

The eye's vascular response to external stimuli has been an area of concern to eye care practitioners since contact lenses were first introduced. Mechanical pressure and the physiological effects of hypoxia result in the filling of the surface limbal arcades which is termed limbal hyperemia. Limbal hyperemia is temporary, reversible and does not cause permanent tissue damage1,2. It is however an undesirable clinical presentation for contact lens wearers.
Chronic hypoxia and injury or inflammation can result in the pathological process of neovascularization, or growth of new blood vessels3. In contact lens wearers corneal neovascularization occurs as an asymptomatic circumlimbal or regional encroachment into the cornea from the limbus of 1mm or less. In severe cases, the new vessels increase the risk of stromal opacification and hemorrhage which may reduce corneal clarity. Increasing the oxygen permeability of contact lenses can lessen these stresses and reduce the eye's vascular response.

A number of studies have been conducted to investigate the vascular response to the new generation high oxygen permeability (Dk) silicone hydrogel lens materials. Papas et al4 have shown that in neophytes (non lens wearers) limbal hyperemia occurs to a lesser degree in eyes wearing silicone hydrogel lenses than in eyes wearing conventional low Dk materials (Figure 1). Differences were reported during periods of open eye wear and were most marked following periods of closed eye wear. The degree of limbal redness following short term silicone hydrogel wear was in fact found to be similar to that recorded for no lens wear. An inverse relationship between the limbal vascular response and oxygen permeability during short-term wear of contact lenses has also been reported5.

Figure 1: Limbal hyperemia with conventional hydrogel lens wear (left) compared with silicone hydrogel high Dk lens wear (right). Courtesy of Eric Papas.

In addition to a reduction in vascular response, limbal hyperemia has also been shown to recover to baseline levels more rapidly after overnight eye closure with these materials when compared with lenses of lower Dk6. Within three hours of eye opening, the limbal hyperemia in eyes wearing silicone hydrogel materials had returned to baseline levels. Eyes wearing lower Dk lenses still showed significantly more limbal hyperemia three hours after eye opening than was recorded at baseline.
In a further study conducted at the CCLR7, Dumbleton et al compared the longer term vascular response to silicone hydrogel and conventional low Dk wear. Sixty-two subjects who were adapted to daily wear of hydrogel lenses were randomized to extended wear of either a high Dk lens (lotrafilcon A - Focus Night & Day) for up to 30 nights in a row or to a low Dk lens (etafilcon A) for up to 6 nights in a row. After nine (9) months of extended wear, subjects who wore the low Dk lenses presented with an increase in limbal hyperemia which was greatest for participants who initially presented with lower levels of baseline hyperemia. The level of hyperemia in the low Dk wearing group increased most over the first month of extended wear. Nine (9) months extended wear of the high Dk lenses resulted in no change in vascular response compared to their daily wear baseline level (Figure 2). This is consistent with the previous reports for short-term wear.

Figure 2: Mean ( SEM) change in limbal hyperemia over time for high Dk silicone hydrogel (n=39) and low Dk (n=23) lens wearers.

Moderate neovascularization also developed after 2-3 months of extended wear of low Dk hydrogels, while wearing the high Dk silicone hydrogel lenses on an extended wear basis for up to 30 nights caused no neovascularization (Figure 3). Neovascularization increased to a greater extent in subjects wearing the Low Dk lenses who initially presented with lower levels of neovascularization, similar to the pattern observed with limbal hyperemia. Subjects with higher levels of neovasculrization prior to study entry showed a significant decrease following extended wear with the high Dk lenses.

Figure 3: Mean ( SEM) change in neovascularization over time for high Dk silicone hydrogel (n=39) and low Dk (n=23) lens wearers.

In a further study conducted at the CCLRU / CRCERT9, the eyes of patients wearing silicone hydrogel lenses on a continuous wear basis were compared with age and gender matched non-lens wearing eyes in a masked, prospective study design. No differences in bulbar redness, limbal redness or vascular appearance, were found between the two groups and it was not possible, based on these clinical appearances, to distinguish between the lens wearers and the non-lens wearers.

As a result of the reduced limbal hyperemia, many patients notice that their eyes are indeed whiter as a result of silicone hydrogel lens wear. The level of acceptable contact lens induced hyperemia will undoubtedly be reduced for both daily and continuous lens wearers and many patients may be refitted into these materials simply to diminish the common hyperemic response associated with contact lens wear in general.
While neovascularization will never completely resolve, there have been many clinical reports of the emptying of vessels previously encroaching into the corneal tissue, termed ghost vessels, following removal of hypoxic stress. Similar claims are also now being made for patients refitted into high Dk silicone hydrogel lens materials. This is particularly encouraging for patients with marked noevascularization whose previous only option was rigid gas permeable lenses or spectacles.
Although neovascularization due to chronic hypoxia is not considered a contraindication to continuous wear with silicone hydrogel lenses, localized areas of vascularization should be treated with greater suspicion. These may be indicative of a previous inflammatory response, and it has been reported that patients with a history of inflammation are more likely to experience subsequent inflammation. These patients may be better suited to a flexible wearing schedule and should be monitored very closely.

The vascular response to silicone hydrogel lens wear is significantly less than for conventional hydrogel lens materials on either a daily or continuous wear basis. Eye care practitioners can safely fit their patients with these materials and not expect to see the previously accepted levels of induced limbal hyperemia and neovascularization. These lens materials may also be fitted as a problem solving lens to patients demonstrating prior vascular responses.

References

1. McMonnies CW, Chapman-Davies A, Holden BA. The vascular response to contact lens wear. Am J Optom Physiol Opt 1982;59:795-9.

2. Holden BA, Sweeney DF, Swarbrick H, Vannas A, Nillsson KT, Efron N The vascular response to long-term extended contact lens wear. Clin Exp Optom 1986;69:112-19.

3. Shah SS, Yeung KK, Weissman BA. Contact lens related deep stromal vascularization. ICLC 1998;25:128-136.

4. Papas EB, Vajdic CM, Austen R, Holden BA. High-oxygen-transmissiblity soft contact lenses do not induce limbal hyperaemia. Curr Eye Res 1997;6:942-8.

5. Papas, E. On the relationship between soft contact lens oxygen transmissibility and induced limbal hyperaemia. Exp Eye Res 1998;67(2):125-131.

6. du Toit R, Simpson TL, Fonn D, Chalmers RL. Recovery from hyperemia after overnight wear of low and high transmissibility hydrogel lenses. Curr Eye Res 2001;22(1):68-73.

7. Dumbleton KA, Chalmers RL, Richter DB, Fonn D. Vascular response to extended wear of hydrogel lenses with high and low oxygen permeability. Optom Vis Sci, 2001; 78(3): 147-151.

8. Covey M, Sweeney DF, Terry R, Sankaridurg PR, Holden BA. Hypoxic effects on the anterior eye of high-Dk soft contact lens wearers are negligible. Optom Vis Sci, 2001; 78(2): 95-99.