A 22-year-old track-and-field athlete, recently diagnosed with mixed hyperopia and astigmatism, wanted to wear contact lenses. There were no ocular or systemic contraindications after careful examination of the case.

How many of you would recommend fitting this patient with a monthly replacement hydrogel lens? Probably none. The reason why is simple: concern that such a lens represents a risk of negative effects to eye health over the long term, especially because of the potential corneal hypoxia that would result from regular (and sometimes extended) wear associated with this type of lens.

This answer also implies that, as practitioners, we want to avoid hypoxia whenever the means exist to do so. This indicates that we most likely have zero tolerance for hypoxia-induced stress when considering long-term effects.

Considering the dust and wind conditions to which this athlete is exposed, you decide to proceed with scleral lenses. Are you still aiming for zero hypoxic stress over time? Logically, your answer would have to remain “yes.”

Hypoxia and Lens Wear

It is known that the vast majority of scleral lenses worn generate a chronic corneal edema varying between 1% and 3%. At this level, it is impossible to detect at the slit lamp; and yet, this does not mean that it is nonexistent. It may be considered benign, because this level is inferior to physiological edema, but this comparison is fallacious. Physiological edema does not last for more than one hour after corneal exposure to the atmosphere, whereas chronic corneal edema induced by scleral lens wear lasts for all wearing hours. The cornea can never restore itself. And we don’t know what the long-term impact of such chronic hypoxic stress will be, as stated in a recent study (Dhallu et al, 2020)that provides some insights that may change the way scleral lenses are prescribed today.

First, the authors point out that hypoxic stress occurs quickly after lens application and lasts throughout the wearing period, qualifying it as constant and cumulative. The consequences on a normal cornea remain uncertain. To determine which elements could be varied and what influences the amount of oxygen reaching the cornea, they fitted lenses made of materials of various permeability, adapted with a range of fluid reservoir thicknesses. All lenses were manufactured with the same thickness, eliminating the “t” from the Dk/t factor.

Their results confirmed that all scleral lenses are associated with corneal swelling over time, while no lens or silicone hydrogel lens wear led to corneal thinning compared to baseline. There was no statistical difference among materials ranging in Dk values from 125 to 180, despite the fact that higher permeability was determined to be more comfortable. This is the same finding with soft lenses, while for those in silicone hydrogel, more oxygenating lenses were preferred (Dillehay, 2007). Acknowledging that Dk values greater than 125 do not influence corneal swelling, it would seem that it is the fluid reservoir thickness that mostly drives the occurrence of hypoxia-induced stress with scleral lens wear.

Finally, the authors conclude that hypoxic stress should be minimized whenever possible by modifying factors such as the lens design, including material (minimum 125 Dk) and fitting (lowest reservoir thickness possible).

Optimizing Scleral Lenses

Clinically, only five GP materials meet the minimum 125-Dk criterion. Other factors should also be considered: optical quality, modulus, ease of manufacture, and wettability. In the scleral lens world, problems of surface deposits are common (Walker et al, 2016); thus, a lower wetting angle is preferred to minimize deposits (Barnett, 2018).

In conclusion, when it comes to determining the optimal parameters of a scleral lens, many factors are in play. As always, it is a matter of finding the right balance. CLS

For references, please visit www.clspectrum.com/references and click on document #305.