Should the Holden & Mertz Criterion of 87 x 10^-9 Units for Overnight Lens Wear be Revised?

Desmond Fonn Professor, School of Optometry, University of Waterloo Director, Centre for Contact Lens Research

Introduction

The importance of oxygen for safe, edema-free wear of contact lenses when worn on both a daily wear and an overnight basis is well established. Sufficient oxygen is required to maintain corneal integrity. Since the cornea is avascular it has to rely principally on the atmosphere for its oxygen supply. All contact lenses act as a potential barrier to oxygen reaching the cornea and the ability of a lens to transmit oxygen is a major factor in determining its clinical success. For more than three decades the widely accepted measure of oxygen provision to the cornea has been oxygen transmissibility (Dk/t). Dk/t is calculated from the oxygen permeability (Dk) of the material and the thickness (t) of the lens and the units for this parameter are (cm/sec)(mlO₂/ml x mmHg).

The amount of oxygen that a contact lens supplies to the cornea can be measured indirectly through epithelial oxygen consumption after removal of the lens, changes in stromal pH, and the corneal swelling response after wearing a lens. The corneal swelling response is recognized as one of the most reliable indirect measures of oxygen provision. In the early 1980’s Holden and Mertz [1] measured the corneal swelling response following lens wear with a number of conventional hydrogel contact lens materials, and one silicone lens material, and compared these values to those measured without lens wear [2]. From that research they established the minimum acceptable Dk/t required to prevent lens induced corneal swelling as 24 x 10^-9 units for daily wear and 87 x 10^-9 units for overnight or extended wear. These values were derived from regression curves fitted to the data for the swelling measured with each of the lenses used, for both open eye and overnight lens wear. In the open eye case, the Dk/t was simply that which corresponded to zero corneal swelling from the curve. The Dk/t for overnight lens wear was more complicated because in addition to the lens, eye closure induces corneal swelling. Holden & Mertz’s critical Dk/t for extended wear of 87 x 10^-9 was established on the basis that non-wearing eyes swelled approximately 4% overnight, which was derived from a previous study by Mertz [2].

Our experience with extended and daily wear of disposable contact lenses and silicone hydrogel lenses since this time has led us to reconsider the Holden and Mertz criteria.

How much swelling occurs overnight with no lens wear?

Mertz’s [2] original approximation of the level of overnight corneal swelling appears to be slightly high. Since Mertz’s estimate, overnight corneal swelling without lenses has been shown to be closer to 3% when results from a number of different studies are considered [3]. Extrapolation from the Holden - Mertz relationship using 3% results in a higher critical Dk/t to avoid additional lens induced overnight corneal swelling than originally proposed. It is also important to note that the oxygen transmissibilities that Holden and Mertz used were calculated from average thickness of the lenses and would therefore would be higher if centre thickness is used.

Overnight Swelling with Silicone Hydrogel lenses

More recent studies have compared the overnight swelling response induced by conventional hydrogel lenses with silicone hydrogel lenses. Fonn et al [4] found that the swelling response was significantly lower (2.7 ± 1.9%) with lotrafilcon A lenses (Dk/t of 175 x 10^-9) compared to low-Dk conventional hydrogel lenses (8.7 ± 2.8%). Although the swelling response induced by the lotrafilcon A lenses was very low (2.7%), it was still significantly higher than the non-lens wearing contralateral control eyes (1.4 ± 0.9%). This difference, although small, demonstrates that a high Dk silicone hydrogel lens with a Dk/t of 175 x 10^-9 produces slightly more swelling than non lens wearing closed eyes and implies that the Holden and Mertz criterion of 87 x 10^-9 underestimates the minimum Dk/t required for overnight lens wear.

Do these values work for all patients and all lenses?

In any discussion of the minimum requirements for Dk/t for overnight lens wear, one must keep in mind that these values are “averages” and that patients exhibit widely different corneal swelling responses [5 ,6] . In addition, many patients wear lenses that have significantly thicker lens profiles than -3.00D, for example hyperopes and those requiring toric lenses, and will still experience corneal swelling with high-Dk contact lens materials.

What other methods can be used to assess corneal oxygen requirements?

Other measures for assessing the critical Dk/t of contact lenses include limbal redness and corneal acidosis. Papas [7] showed that the mean peripheral Dk/t required to avoid a change in limbal redness is 125 x 10^-9 units. It must be remembered that the value was estimated from daily wear studies and therefore the value for overnight lens wear may be even greater.

Harvitt and Bonanno’s [8] model for the distribution of oxygen across the cornea to incorporate increased oxygen consumption occurring with acidosis found that 35 x 10^-9 units was required for daily wear and 125 x 10^-9 units for overnight wear, to prevent anoxia throughout the entire cornea. A number of the assumptions made in this model have however been recently challenged and alternative methods for calculating the oxygen requirements for both daily and overnight lens wear have been proposed [9]. It is important however to understand that these calculations are all based on models and not on the response to wearing contact lenses of differing oxygen transmissibilities which the original Holden and Mertz study and other studies investigating corneal swelling have employed.

In Conclusion

In the light of recent data and a better understanding of the corneal swelling response it now appears that the Holden and Mertz criterion for the minimum Dk/t for overnight contact lens wear should be revised to a higher value. While it may not be appropriate to suggest a single number, a value of at least 125 x 10^-9 is proposed.

References

1. Holden BA, Mertz GW. Critical oxygen levels to avoid corneal edema for daily and extended wear contact lenses. Invest Ophthalmol.Vis.Sci. 1984;25:1161-7.

2. Mertz GW. Overnight swelling of the living human cornea. J Am Optom Assoc 1980;51:211-4.

3. Fonn D, Sweeney D, Holden BA, Cavanagh D. Corneal oxygen deficiency. Eye Contact Lens 2005;31:23-7.

4. Fonn D, du T, Simpson TL, Vega JA, Situ P, Chalmers RL. Sympathetic swelling response of the control eye to soft lenses in the other eye. Investigative Ophthalmology and Visual Science 1999;40:3116-21.

5. Bruce A, Brennan NA. Warmer corneas swell more during closed eye contact lens wear. Optometry and Vision Science 1999;76:238.

6. Efron N. Intersubject variability in corneal swelling response to anoxia. Acta Ophthalmol (Copenh) 1986;64:302-5.

7. Papas, E. (1998). On the relationship between soft contact lens oxygen transmissibility and induced limbal hyperaemia. Exp Eye Res 67: 125-31.

8. Harvitt DM, Bonanno JA. Re-evaluation of the oxygen diffusion model for predicting minimum contact lens Dk/t values needed to avoid corneal anoxia. Optom.Vis.Sci. 1999;76:712-9.

9. Brennan NA. Corneal oxygenation during contact lens wear: comparison of diffusion and EOP-based models. Eye Contact Lens 2005;31:103-8.