Close to the Edge: Oxygen at the Lens Periphery
BY ERIC PAPAS, PHD, MCOPTOM, DIPCL
In the days before silicone hydrogel materials became available, contact lens practitioners spent a great deal of time worrying about oxygen transmissibility (Dk/t). These concerns seem to have faded into the background in recent years, but should we forget about the issue altogether? In a silicone hydrogel world where daily wear is the dominant modality, and — apart from blinking — there is no eye closure during lens wear, are there still risks?
Measuring Dk/t: Location Matters
Manufacturers specify the Dk/t of their contact lens materials based on measurements taken at the center of a representative lens, usually a lens with a nominal back vertex power of −3.00D. For any prescribed lens, however, the thickness profile can vary significantly from the center to the periphery, depending on the design and the back vertex power. Even for a −3.00D spherical lens, the thickness can be 2 or 3 times greater at some positions outside the center.
Because the Dk/t of a lens is governed by its thickness, we need to pay closest attention to the regions of the ocular surface covered by the thickest portions of the lens. For most modern lenses with diameters of 13.5 mm to 14.5 mm, the peripheral cornea and the limbus are likely to be the main sites under threat.
Corneal Swelling Response to Hypoxia
Perhaps the most commonly studied response to ocular hypoxia is corneal swelling. Swelling is less pronounced at the periphery of the cornea than at the center because anatomical factors progressively limit the extent to which swelling can occur as we approach the limbus. For this reason, there is little value in measuring the far periphery of the cornea, but some studies have examined slightly less extreme locations.
One such investigation showed that certain features of a hydrogel lens, such as thickened toric stabilization zones, can limit oxygen flow and cause regional and localized swelling under daily wear conditions (Tyagi et al, 2010). Another study, which may have more general application, found that after 3 hours of open eye wear, spherical lenses with lower oxygen transmissibility can produce significant peripheral swelling (Morgan et al, 2010). These researchers analyzed their data to estimate the minimum Dk/t needed to avoid peripheral corneal swelling in daily wear. They calculated a value of 33 × 10-9 (cm s-1)(mLO2 mL-1 mm Hg) across the entire lens.
Because most practitioners do not routinely use pachometry in daily practice, corneal swelling is not useful as a clinical indicator of the oxygen supply to the corneolimbal region. The most easily accessible alternative marker is the behavior of the limbal vasculature.
Hyperemia as an Alternate Marker
Blood vessels throughout the body respond to the level of oxygen in their local environment, and those at the limbus are no exception. In the short term, reduced oxygen causes hyperemia, which is perceived as ocular redness or limbal flush. Although these changes are reversible, prolonged periods of hyperemia can lead to vascularization of the peripheral cornea, which is a more serious condition with the potential for permanent consequences.
One of the earliest observations made with what were then experimental silicone hydrogel lenses was that eyes remained whiter-looking (Papas et al, 1997). Until then, we believed that mechanical irritation from the lens edge caused the redness habitually associated with soft lenses. Later experiments using goggles filled with different gas mixtures demonstrated that oxygen is an important factor mediating these responses (Papas, 2003). In addition, a number of studies have confirmed the value of silicone hydrogels in avoiding limbal redness (Du Toit et al, 2001; Dumbleton et al, 2001; Maldonado-Codina et al, 2004; Brennan et al, 2006; Dumbleton et al, 2006).
From a clinical standpoint, the amount of redness produced by a contact lens is inversely related to the oxygen transmissibility at its periphery (Papas, 1998); however, the minimum transmissibility required to prevent hyperemia from occurring remains a subject of active debate. Thus far, the only work that has directly addressed this question suggested a Dk/t in the lens periphery of about 55 × 10-9 (cm s-1)(mLO2 mL-1 mm Hg) as a reasonable minimum target for the open eye (Papas, 1998). This research was conducted more than 15 years ago at a time when few silicone hydrogel lenses were available, so it is surprising that there has been little activity to update this value.
Silicone Hydrogel and the Redness Response
Of some relevance is a study that compared the limbal redness responses of two silicone hydrogel lenses with that of a conventional hydrogel lens during daily wear (Maldonado-Codina et al, 2004). Although the silicone hydrogel lenses were made from materials of dissimilar oxygen permeability, the results showed that, on average, they caused minimal amounts of redness that were similar in magnitude to the hyperemia observed in the non-lens-wearing control subjects. Eyes wearing the conventional hydrogel lenses were significantly redder than those wearing either of the two silicone hydrogel lenses.
Although the researchers in the 2004 study did not try to determine a transmissibility threshold for limbal redness, their observations were subsequently interpreted as evidence that the amount of oxygen transmitted by the silicone hydrogel lens with the lower Dk/t was sufficient to prevent a vascular response in the open eye (Morgan et al, 2010). The peripheral transmissibility of this lens was 36 x 10-9 (cm s-1) (mLO2 mL1 mm Hg), a value that potentially lowers the transmissibility requirement somewhat and is similar to the value I mentioned earlier for peripheral corneal swelling.
Consider These Qualifying Issues
Before accepting this conclusion regarding minimum oxygen transmissibility, however, we should consider two qualifying issues. First, this estimate of oxygen transmissibility (Morgan et al, 2010) was made some years after the original clinical observations (Maldonado-Codina et al, 2004) by measuring a lens with a power of −3.00D. Lenses in the study ranged from −1.00D to −4.00D, so the single value would not have equally represented the experience of all participants.
Second, the peripheral thickness measurement for the reference lens in the 2004 study was made at a diameter of 8 mm. Although this was appropriate in the context of the study’s primary aim, which was to examine corneal swelling, it does not correspond to the area of the lens that would influence vascular changes at the limbus. Thus, more data must be accumulated to satisfactorily resolve the uncertainty surrounding these numbers.
Regardless of the precise values, it seems clear the open eye is not immune to the effects of hypoxia that may occur from a contact lens that has poor oxygen-transmitting properties. Fortunately, clinicians can avoid this problem by choosing carefully from the array of contemporary lenses that are available. CLS
For references, please visit www.clspectrum.com/references.asp and click on document SE2013.
Associate Professor Papas is executive director of Research & Development, Brien Holden Vision Institute and Vision Cooperative Research Centre, and senior visiting fellow, School of Optometry & Vision Science, University of New South Wales, Sydney, Australia. The Brien Holden Vision Institute and Vision Cooperative Research Centre have received research funds from B+L, AMO, and Allergan and have proprietary interest in products from Alcon, CooperVision, and Carl Zeiss. You can reach him at email@example.com.