Scleral lenses typically won't cause a localised oedema effect, instead rather a diffuse reaction affecting the entire central cornea.
An oedema level of 5% is considered acceptable during daily wear of contact lenses, and overnight oedema levels as high as 8-10% are considered acceptable in extended wear if oedema subsides within 2-3 hours upon awakening (Efron and Holden, 1986a).
Beyond 5% oedema, vertical greyish-white lines called striae begin to appear in the posterior stroma. At 10-12% swelling, folds also appear, and contact lens wear is usually modified to increase oxygen levels. Oedema beyond 15% is considered pathological; at about 20%, the cornea begins to cloud and visual acuity drops. Within a few days, other complications such as infiltrates and epithelial microcysts occur. The induction of blood vessel growth within the cornea may also happen if this level of swelling persists.
Microcysts are the most distinctive and readily detectable indicator of contact lens-induced hypoxia. They should not be confused with cyst-like inclusions that occur in conditions such as Meesmann's dystrophy, bullous keratopathy, and Cogan's microcystic dystrophy, or with mucin balls, vacuoles, microcystic oedema, and infiltrates. There could be a good chance that you could be dealing with bullous keratopathy due to endothelial dysfunction.
Images of Epithelial microcysts:
With corneal Oedema, patients also frequently report Sattler's Veil where they notice coloured halos around lights. Sattler's Veil is due to a circular diffraction pattern formed by the basal epithelial cells and the extracellular spaces. (Lambert, 1981)
Upon removal of the cause, acute corneal oedema usually resolves itself within a matter of hours. Still, chronic oedema such as experienced by some extended wear patients after months of wear may take up to 7 days to disappear (Efron and Holden, 1986a).
Two studies investigated the occurrence of corneal hypoxia induced by scleral lens using a theoretical model (Michaud, 2012; Jaynes 2015). Both confirm that scleral lens wear may cause corneal oedema if the lens thickness exceeds 250 um and if the clearance under its surface is higher than 200 um.
Clinical studies have since been published confirming that scleral lens wear can be associated with an induced corneal swelling varying from 1 to 4% in normal corneas (Compañ 2014, Ng 2015, Miller 2015), in keratoconus (Soeters 2015) and especially in cases where the endothelial cell layer is compromised (Lau 2015, Riff 2015).
These studies are based on pachymetry. However, it is not still clear if hypoxia is the only factor involved in corneal swelling during scleral lens wear. More interestingly, an in vivo study, also confirmed a reduction in the oxygen diffusion to the cornea if tear fluid layer thickness increases (Giasson 2015). In this study, authors used the EOP approach and evaluated, directly on the cornea with probes, the oxygen consumption after exposure to several gases (reference curve) and then after wear of scleral lenses fitted with 200 and 400 microns of clearance. Their results are highly significant in that increased clearance reduces oxygen diffusion to the cornea by 30%.
Considering an optimal Equivalent Oxygen Percentage (EOP) level of 9.9% to avoid corneal hypoxia (Benjamin 1988), scleral lenses (18.0 mm in diameter, with an average lens thickness of 310 microns) fitted with 200 um of clearance lead to an EOP value of 9.0%, while the same lens with 400-micron clearance decreased EOP to 6.2%. The results show that scleral lenses are inducing chronic corneal oedema when the lens thickness is more than 250 um, and when the post lens tear thickness exceeds 200 um. (Figure 75) Vincent (2016) suggested that this level of hypoxia may be considered benign, comparable to physiological oedema seen after sleeping. However, this comparison is questioned as physiological oedema typically resolves within an hour of waking. In contrast, induced oedema persists for all lens wearing hours coupled with the restoration time when the cornea is exposed to air after lens removal.
It should be noted that hypoxia is not clinically visible before reaching 8-10% level. Also, oedema does not affect the limbal area, clearance over the limbus being limited under 100 um. Consequently, neovascularization is not triggered. Notwithstanding this absence of visible clinical signs, studies showed that oedema occurs with unknown long-term impacts.
From sclera-corneal swelling studies (Smith 2004), it also became clear the endothelial count plays a significant role in how much the corneal will swell under reduced oxygen condition. According to an article by Dr Christine W. Sindt, scleral lenses are contraindicated in grafts with endothelial cell counts of less than 800 mm2 since oedema may develop.
More studies are needed to research the impact of chronic corneal oedema, especially on compromised tissues. In the meantime, it is recommended to fit scleral lenses with the lowest post lens tear thickness possible and to manufacture scleral lenses as thin as possible using the highest DK material available.
Benjamin, WJ and RM Hill, Human cornea: individual responses to hypoxic environments. Graefes Arch Clin Exp Ophthalmol, 1988. 226(1): p. 45-8
Compañ V. Oliveira C. Aguilella-Arzo M. Molla S. et al. Oxygen diffusion and edema with modern scleral rigid gas permeable contact lenses. Investig Ophthalmol Vis Sci, 55 (2014), pp. 6421–6429
Holden BA, Mertz GW. Critical oxygen levels to avoid corneal edema for daily and extended wear contact lenses.
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Jaynes JM. Edrington TB. Weissman BA. Predicting scleral GP lens entrapped tear layer oxygen tensions. Contact Lens Ant Eye, 38 (2015), pp. 44–47.
Lambert SR, Klyce SD. The origins of Sattler's veil. Am J Ophthalmol. 1981 Jan;91(1):51-6.
Lau J. Reeder R. Localized corneal graft rejection from scleral lens wear with excessive limbal clearance. Poster presented at the GSLS meeting, Las Vegas, 2015. http://www.pentavisionevents.com/ckfinder/userfiles/files/Lau%20- %20Scleral%20Contact%20Lens%20Complications%20with%20Excessive%20Limbal%20Cleara nce.pdf
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(6):266-71. doi: 10.1016/j.clae.2012.07.004. Epub 2012 Aug 9.
Miller W.L Vance K. Johnson L. Bergmanson J.P. Scleral contact lens effect on central and peripheral corneal thickness. Investigative Ophthalmology & Visual Science June 2015, Vol.56, 6103. http://iovs.arvojournals.org/article.aspx?articleid=2336208&resultClick=1
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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(4):294-7
Vincent SJ, Alonso-Caneiro D, Collins MJ, Beanland A, Lam L, Lim CC, Loke A, Nguyen N. Hypoxic Corneal Changes following Eight Hours of Scleral Contact Lens Wear. Optom Vis Sci. 2016 Mar;93(3):293-9. doi: 10.1097/OPX.0000000000000803.