Nancy MacDougall -

Centre for Contact Lens Research, University of WaterlooWaterloo

Nancy MacDougall is currently a Research Associate at the Centre for Contact Lens Research at the University of Waterloo in Ontario, Canada, where she is responsible for conducting clinical research in the areas of contact lenses and refractive surgery. She graduated with honours in Optometry from the University of Waterloo and is currently working towards her MSc Degree in Vision Science on a part-time basis.

Manuscript Review

On the Relationship Between Soft Contact Lens Oxygen Transmissibility and Induced Limbal Hyperaemia

Eric Papas, Exp Eye Res 1998 Aug;67(2):125-131 By Nancy MacDougall

Anatomically there is an extensive network of blood vessels supplying the eye. The arterial supply of the bulbar conjunctiva consists of both anterior and posterior systems. The anterior (ciliary/limbal) system includes the anterior ciliary arteries, which originate from the muscular branches of the ophthalmic artery. Approximately 4mm from the corneo-scleral junction these arteries bifurcate forming the major circle of the iris. (1) At the bifurcation other branches continue as the anterior conjunctival arteries. These anastamose with each other forming an arcade, eventually giving rise to the ciliary/limbal plexus. Chronic forms of injury, such as hypoxia with certain types of contact lens wear, can cause these vessels to become injected giving rise to what's clinically known as limbal hyperaemia. (2-6)

Limbal hyperaemia is not only a cosmetic problem for a contact lens patient, but can potentially be more serious as evidence suggests that limbal hyperaemia is a precursor to corneal neovascularisation. (3, 7, 8) Although other mechanisms may exist, (6, 9) evidence has been mounting that the mechanism through which hydrogel lenses induce limbal hyperaemia is localised hypoxia in the region of the limbus, rather than the mechanical presence of the lens itself. (4, 5) A reduction, or elimination, of induced limbal hyperaemia would increase safety and cosmetic appeal for soft contact lens wearers. As a result, there have been many studies investigating the relationship between corneal hypoxia and limbal hyperaemia with contact lens use.

To date, it has been shown that hydrogel contact lenses are a stronger stimulus to hyperaemia than both rigid gas permeable contact lenses (3) and lenses with high oxygen permeability (high-Dk), including the new silicone-hydrogel lenses. In addition, a negligible difference has been shown between subjects wearing high-Dk lenses compared to non-contact lens wearers, with respect to hypoxic effects on the anterior eye. (10) It has been reported that the rate of recovery from hyperaemia following overnight eye closure is faster for subjects wearing high-Dk lenses compared to low-Dk lenses. (11) Furthermore, a recent study also reported a lower vascular response for subjects wearing high-Dk lenses over 9 months of extended wear, when compared with those who wore lenses of lower-Dk for the same period. (12)

The paper published by Eric Papas in the August 1998 issue of Experimental Eye Research investigates the relationship between lens oxygen transmissibility (Dk/t) and induced limbal hyperaemia and attempts to determine the nature of this association. Using information from several different studies where hydrogel lenses with different Dk values were worn for 8 hours, limbal hyperaemia was quantified by observing limbal redness in each of 4 quadrants using a modified subjective grading scale system (13, 14). Dk/t was calculated by measuring Dk with a coulometric method (15) and lens thickness using a digital gauge. Thickness was determined both centrally and peripherally to provide Dk/t estimates both at the lens centre and at a radius of 6mm. The change in limbal redness between baseline and the 8-hr point was taken as a measure of induced hyperaemia. Results confirmed that the level of limbal hyperaemia induced by a hydrogel lens is directly correlated with the Dk/t of that lens. A strong degree of association was found between the change in limbal redness and peripheral Dk/t.

As a result of this study and many other publications, (5, 9-12,16) there is strong evidence to support the hypothesis that high Dk/t lenses will minimise limbal hyperaemia. Previously, all that was known is that soft contact lenses with Dk/t in the region of 95x10-9 (cm s-1)(mlO2 ml-1xmmHg)(14) considerably minimised induced hyperaemia. Interestingly, this study further proposes that a minimum peripheral Dk/t of 55 units would be required for negligible interference with ocular physiology and a maximum value of around 280 units would be necessary for no additional effect on the limbal vasculature.

In summary, although the exact mechanism for limbal hyperaemia remains unclear, there is undoubtedly a relationship between limbal hyperaemia and the local availability of oxygen. Clinically, it would also be beneficial to know if there exists a defined level of hyperaemia that predisposes the cornea to neovascularisation. Results with high-Dk lenses have been extremely positive, showing significantly reduced levels of limbal hyperaemia with their use. (4, 5, 10, 16) In light of this, and early findings with long-term extended wear using high-Dk lenses, (12) contact lens-induced corneal neovascularisation will hopefully become a thing of the past with the use of these novel materials and limbal hyperaemia will become less of a clinical concern.


References

1. Greenburg DA. Clinical significance of ciliary/limbal injection. J Am Optom Assoc. 1980;51(11): 997-1001.
2. Young G, Coleman S. Poorly fitting soft lenses affect ocular integrity CLAO 2001;27(2):68-74.
3. McMonnies C, Chapman-Davies A, Holden B. The vascular response to contact lens wear. Am J Optom Physiol Optics. 1982;59:795-799.
4. Papas E, Fleming C, Austen R, Holden B. High Dk soft contact lenses reduce the limbal vasculature response. Optom Vis Sci. 1994:71:14.
5. Papas E, Vajdic CM, Austen R, Holden B. High-oxygen-transmissibility soft contact lenses do not induce limbal hyperaemia. Curr Eye Res. 1997;16:942-948.
6. McMonnies C, Chapman-Davies A. Assessment of conjunctival hyperaemia in contact lens wearers. Part II. Am J Optom Physiol Optics. 1987;64:251-255.
7. Young G: Soft lens fitting re-assessed. Cont Lens Spectrum. 1992;7(12):56-61.
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10. 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.
11. du Toit R, Simpson TL, Fonn D, Chalmers RL. Recovery from hyperaemia after overnight wear of low and high transmissibility hydrogel lenses. Curr Eye Res. 2001;22(1):68-73.
12. Dumbleton KA, Chalmers RL, Richter DB, Fonn D. Vascular response to extended wear of hydrogel lenses with high and low oxygen permeability. Optom and Vis Sci. 2001;78(3):147-151.
13. CCLRU: CCLRU grading scales. in Contact Lenses, A. Phillips and L. Speedwell, Editors. Oxford, Butterworth-Heinemann,1997, pp 863 - 867.
14. Bailey I, Bullimore M, Raasch T, et al.: Clinical grading and the effects of scaling. Invest Ophthalmol Vis Sci. 1991;32(2): 422-432.
15. Winterton L, White J, Su K: Coulometric method for measuring oxygen flux and Dk of contact lenses and lens materials. ICLC. 1987;14(11):441-451.
16. MacDonald K, Fonn D, Richter D, Robboy M. Comparison of the physiological response to extended wear of an experimental high Dk soft lens versus a 38% hema lens. Invest Ophthalmol Vis Sci. 1995;36:S310.