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December 2020

Corneal Edema with Scleral Lens Wear – Is Any Amount Safe?

By Melissa Barnett, OD
According to theoretical and clinical studies, scleral lenses induce corneal edema with variable lens thicknesses and corneal clearances.1-8 There is significant debate about corneal oxygenation during scleral lens wear based on potential hypoxia as a result of the scleral lens itself and a fluid reservoir.9 Some practitioners deem this level of induced edema benign because it is comparable to physiological corneal edema upon awakening. It has been recommended to limit overnight corneal swelling to 4%, which is similar to corneal swelling seen without lens wear.10 But unlike physiological edema, with scleral lenses, the induced edema lasts for the entire duration of wear,11 and the effect of chronic exposure is unknown.
 
With advances in scleral lenses manufactured with hyper-oxygen-permeable rigid lens materials (Dk ≥ 100), numerous short-term studies have demonstrated that contemporary scleral lenses induce approximately 2% corneal edema in healthy eyes1-3,12-14 or keratoconic eyes15-18 and, on average, up to 4% in post-graft corneas.16,19

In eyes that have a compromised corneal endothelial layer, corneal edema may appear rapidly. However, hypoxia is not always clinically visible. Corneal edema is apparent when hypoxia reaches 5% to 8%; clinical signs correlate with the amount of edema.20 Corneal edema is predominantly stromal in nature and is increased with greater fluid reservoir thickness.21

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Microcystic epithelial edema and vascularization may occur with chronic hypoxia due to insufficient midperipheral or limbal vault.22,23 Epithelial bullae have been reported in compromised corneas, even with sufficient lens clearance.24 When edema reaches 15%, a loss of corneal transparency ensues.25
 
Because scleral lenses are a sealed system with minimal tear exchange after settling, it is important to consider fluid reservoir thickness and available oxygen to the cornea.21 Various theoretical models have estimated more edema and decreased oxygen delivery with a greater central corneal vault.20-24 It also has been proposed that this value is overestimated for both open- and closed-eye environments in young, healthy corneas.9,26,27
 
Although additional research needs to be performed, it appears that modern scleral lenses induce less than physiological hypoxic swelling in healthy corneas during open-eye wear and are relatively safe to wear.5 Scleral lens wear in a closed-eye situation appears unclear at this time except in special circumstances.
Reference(s):
  1. Vincent SJ, Alonso-Caneiro D, Collins MJ. The time course and nature of corneal oedema during sealed miniscleral contact lens wear. Cont Lens Anterior Eye. 2019 Feb;42:49-54.
  2. Vincent SJ, Alonso-Caneiro D, Collins MJ, et al. Hypoxic corneal changes following eight hours of scleral contact lens wear. Optom Vis Sci. 2016 Mar;93:293-299.
  3. Vincent SJ, Alonso-Caneiro D, Collins MJ. Corneal changes following short-term miniscleral contact lens wear. Cont Lens Anterior Eye. 2014 Dec;37:461-468.
  4. Michaud L, van der Worp E, Brazeau D, Warde R, Giasson CJ. Predicting estimates of oxygen transmissibility for scleral lenses. Cont Lens Anterior Eye. 2012 Dec;35:266-271.
  5. Kim YH, Tan B, Lin MC, Radke CJ. Central Corneal Edema with Scleral-Lens Wear. Curr Eye Res. 2018 Nov;43:1305-1315.
  6. Jaynes JM, Edrington TB, Weissman BA. Predicting scleral GP lens entrapped tear layer oxygen tensions. Cont Lens Anterior Eye. 2015 Feb;38:44-47.
  7. Weissman BA, Ye P. Calculated tear oxygen tension under contact lenses offering resistance in series: Piggyback and scleral lenses. Cont Lens Anterior Eye. 2006 Dec;29:231-237.
  8. Compañ V, Aguilella-Arzo M, Edrington TB, Weissman BA. Modeling Corneal Oxygen with Scleral Gas Permeable Lens Wear. Optom Vis Sci. 2016 Nov;93:1339-1348.
  9. Fisher D, Collins MJ, Vincent SJ. Fluid reservoir thickness and corneal oedema during closed eye scleral lens wear. Cont Lens Anterior Eye. 2020 Aug 19:S1367-0484(20)30154-5. [Online ahead of print]
  10. Holden BA, Mertz GW. Critical oxygen levels to avoid corneal edema for daily and extended wear contact lenses. Invest Ophthalmol Vis Sci. 1984 Oct;25:1161-1167.
  11. Michaud L. Scleral Lenses for the Regular / Normal / Non-Diseased Cornea in Barnett M, Johns LK. Contemporary Scleral Lenses: Theory and Application. Bentham Science 2017;4:170-171.
  12. Michaud L, Samaha D, Giasson CJ. Intra-ocular pressure variation associated with the wear of scleral lenses of different diameters. Cont Lens Anterior Eye. 2019 Feb;42:104-110.
  13. Tan B, Zhou Y, Yuen TL, Lin K, Michaud L, Lin MC. Effects of Scleral-lens Tear Clearance on Corneal Edema and Post-lens Tear Dynamics: A Pilot Study. Optom Vis Sci. 2018 Jun;95:481-490.
  14. Tan B, Tse V, Kim YH, Lin K, Zhou Y, Lin MC. Effects of scleral-lens oxygen transmissibility on corneal thickness: A pilot study. Cont Lens Anterior Eye. 2019 Aug;42:366-372.
  15. Esen F, Toker E. Influence of Apical Clearance on Mini-Scleral Lens Settling, Clinical Performance, and Corneal Thickness Changes. Eye Contact Lens. 2017 Jul;43:230-235.
  16. Macedo-de-Araújo RJ, Amorim-de-Sousa A, van der Worp E, González-Méijome JM. Clinical Findings and Ocular Symptoms Over 1 Year in a Sample of Scleral Lens Wearers. Eye Contact Lens. 2020 Nov;46:e40-e55.
  17. Soeters N, Visser ES, Imhof SM, Tahzib NG. Scleral lens influence on corneal curvature and pachymetry in keratoconus patients. Cont Lens Anterior Eye. 2015 Aug;38:294-297.
  18. de Luis Eguileor B, Acera A, Santamaría Carro A, Feijoo Lera R, Escudero Argaluza J, Etxebarria Ecenarro J. Changes in the corneal thickness and limbus after 1 year of scleral contact lens use. Eye (Lond). 2020 Sep;34:1654-1661.
  19. Kumar M, Shetty R, Khamar P, Vincent SJ. Scleral Lens-Induced Corneal Edema after Penetrating Keratoplasty. Optom Vis Sci. 2020 Sep;97:697-702.
  20. Efron N., ed. Stromal edema. Contact Lens Complications, Second edition. Butterworth-Heinemann, Edinburgh (2004), pp. 132-140.
  21. Fisher D, Collins MJ, Vincent SJ. Fluid Reservoir Thickness and Corneal Edema during Open-eye Scleral Lens Wear. Optom Vis Sci. 2020 Sep;97:683-689.
  22. Nixon AD, Barr JT, VanNasdale DA. Corneal epithelial bullae after short-term wear of small diameter scleral lenses. Cont Lens Anterior Eye. 2017 Apr;40:116-126.
  23. Guillon NC, Godfrey A, Hammond DS. Corneal oedema in a unilateral corneal graft patient induced by high Dk mini-scleral contact lens. Cont Lens Anterior Eye. 2018 Oct;41:458-462.
  24. Isozaki VL, Chiu GB. Transient corneal epithelial bullae associated with large diameter scleral lens wear: A case series. Cont Lens Anterior Eye. 2018 Oct;41:463-468.
  25. Efron N. Contact lens-induced changes in the anterior eye as observed in vivo with the confocal microscope. Prog Retin Eye Res. 2007 Jul;26:398-436.
  26. Fisher D, Collins MJ, Vincent SJ. Fluid reservoir thickness and corneal oedema during closed eye scleral lens wear: Experimental and theoretical outcomes. Cont Lens Anterior Eye. 2020 Oct 24:S1367-0484(20)30175-2. [Online ahead of print]
  27. Fisher D, Collins MJ, Vincent SJ. Fluid Reservoir Thickness and Corneal Edema during Open-eye Scleral Lens Wear. Optom Vis Sci. 2020 Sep;97:683-689.
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