Clearing the Fog
Maria K. Walker OD MS
The advent of modern scleral contact lenses (ScCL) has dramatically changed the landscape of specialty contact lens care; practitioners are now able to successfully correct vision in individuals who would otherwise be limited by suboptimal contact lens correction. Many patients who would have been referred for a corneal transplant are now able to achieve tremendous comfort and vision with scleral lenses.
However, this relatively new modality lacks fundamental data on the acute and long-term complications. One of the common scleral lens complications is “mid-day fogging,” in which particulate matter accumulates in the tear chamber beneath the contact lens, creating fog-like blur in susceptible patients (this has been reported to occur in approximately 20-30% of scleral lens patients).1-3
The fluid forces associated with a scleral lens pressure system are likely responsible for the introduction of debris into the tear chamber, which causes mid-day fogging in susceptible individuals. The force is induced by the coup-contracoup movement of a scleral lens that occurs during a blink – as the eyelids close, they apply directional pressure on the lens toward the anterior surface of the eye (retro-pulsion), and as the eye reopens and the pressure is released, the lens rebounds outward and particulate matter is drawn from the peripheral confines of the lens chamber into suspension within the chamber fluid. Over the course of a few hours, the chamber fluid becomes saturated with debris, creating turbidity within the fluid.
Of course, we all know that these lenses have a net loss of tear reservoir fluid over time, termed “settling,” indicating that the retro-pulsing pressure is stronger than the recoil that occurs as the lid pressure is released. In addition to fluid forces, clinical observations of decreased fogging in individuals who use high-solute application solutions may suggest that there is also a diffusive gradient that encourages the movement of particles from the hypertonic tears into the hypotonic solution beneath a scleral lens.
It appears that this debris responsible for creating turbidity within the chamber fluid either comes through or directly from the perilimbal conjunctival tissue. This has been tested in the clinic at Pacific University by applying a scleral lens design on "foggers” that essentially plugs up the peripheral cornea and eliminates the presence of conjunctival tissue within the confines of the peripheral tear chamber. This virtually eliminated fogging in all of five subjects that were tested, indicating that the perilimbal conjunctival tissue is the route of entry for this foggy precipitate. Unfortunately, fitting scleral lenses that rest on the limbus is not a viable option, as it is important to avoid lens interaction with this vital tissue.
We can speculate as to why this phenomenon occurs, but the precise cause of fogging in susceptible wearers is still ultimately unknown. Because it does not occur in every scleral lens-wearing patient, there is clearly more exploration needed to fully understand the etiology. This understanding starts with determining what the fog is actually composed of.
Many practitioners initially hypothesized that the debris was composed of mucin secreted from conjunctival goblet cells. It was believe that the goblet cells were being “milked” into the tear chamber. However, protein analyses of several post-lens tear samples at Pacific University have detected virtually no mucin in either foggy or clear samples. Further, in five subjects tested for protein content (3 foggy, 2 clear samples), there are no differences in overall protein content between clear and foggy sample types (Figure 1).
Figure 1. The ten most abundant proteins detected in the post-lens tear reservoir of five subjects: 3 of which had turbid tear reservoirs and 2 of which had clear reservoirs. A total of 218 individual proteins were detected in the samples, and there were no significant differences between clear and turbid sample protein profiles.
Non-specific lipid staining with oil-red-O (Figure 2a) has led us to believe that a lipid substance may be primarily responsible for the post-lens clouding. Preliminary results of more specific lipid analyses have shown an increase in esterfied cholesterol (Chl-E) in 4 out of 5 of the foggy samples tested (Figure 2b-c). Considering the hydrophobic nature of Chl-E, it is logical that these molecules would form a precipitate, as they are unable to dissolve in the aqueous solution beneath the lens.
Figure 2a. Magnified (40x) images of clear and turbid post-lens tear reservoir samples stained withoil-red O. The clear samples show less staining than the turbid samples.
Figure 2b. Cholesterol detection in clear and turbid post-lens tear reservoir samples. Four out of five foggy samples showed increased levels of esterified cholesterol when compared to four clear tear reservoir samples.
Figure 2c. OCT images showing the opacification of tear film reservoirs in six of the nine subjects tested – with clear (A-C) and turbid (D-F) tear reservoirs. The values beneath each image show the concentration of esterified cholesterol in each reservoir.
Susceptibility to mid-day fogging has been observed in patients with greater than average lens vault1 and dry eye patients4 (who often have abnormal lipid profiles). To reduce fogging in these individuals, the design of a scleral lens can be manipulated to discourage the movement of particulate matter under the lens. Decreased apical clearance has been shown to reduce the fog in many patients by thinning the layer of fog in general and by decreasing limbal clearance, which narrows the channel for entering debris. Applying the lenses with a high-viscosity / high-solute solution (preservative-free artificial tears) can also be an effective method of managing mid-day fog. The mechanisms of these artificial tear solutions are likely related to both the resistance of the increased viscosity solution to the applied eyelid pressure and to the lipid solubility of the solution.
Even with these “tricks” for reducing fog, it is still prevalent among ScCL wearers, and the ultimate goal must be to eliminate the presence of the fog-inducing substance. Certainly, there is more work to be done in clarifying the mechanism of fogging and determining exactly what the turbid-creating substance is.
The author would like to thank Professor Patrick Caroline and Sheila Morrison at Pacific University College of Optometry for their assistance in obtaining the data presented here.
1. Caroline PJ, Andre M. Cloudy Vision With Sclerals. Contact Lens Spectrum. Vol 27; pg 56. June 2012
2. Miller WL. Scleral Contact Lens Fog. Contact Lens Spectrum. Vol. 28; pg 52. Sept 2013.
3. Rathi VM, Mandathara PS, Vaddavalli PK, Srikanth D, Sangwan VS. Fluid filled scleral contact lens in pediatric patients: Challenges and outcome. Contact Lens & Anterior Eye. 35(2012). 189-192.
4. McKinney A, Polizzi C, Miller W, van der Worp E, Bergmanson J, Leach N. Attributes of midday visual fogging in scleral lens wearers. Poster session presented at: Annual Conference of The American Academy of Optometry. 2013 October 23-26; Seattle, WA.
Maria K. Walker is currently an Assistant Clinical Professor at The University of Houston College of Optometry. She completed a Cornea & Contact Lens Residency at Pacific University College of Optometry in Forest Grove, Oregon, and received her Doctorate of Optometry and Master’s in Vision Science from The New England College of Optometry in 2013. Her main interests include contact lens optics, corneal physiology, and multifocal lens performance in presbyopia and myopia control.