The fluid reservoir of a scleral lens allows it to correct astigmatism, mask front-surface corneal irregularity, and manage severe ocular surface disease. It has long been suspected that the semi-sealed fit of a scleral lens prevents significant tear exchange. Tear exchange is thought to be paramount for healthy contact lens use, as it allows for oxygen exchange and the flushing of metabolic waste. Recent research is starting to quantify exchange rates and the potential implications.
For perspective, previous studies have approximated tear exchange rates for corneal GPs at 10% to 20% per blink and for soft lenses at 1% to 2% per blink (Skidmore et al, 2019). For scleral lenses, presumed outflow takes place with settling, which averages 100 microns (Skidmore et al, 2019; Kauffman et al, 2014). In recent studies using fluorophotometry, Skidmore et al (2019) noted inflow rates of scleral lenses at < 1%, and Paugh et al (2018) saw elimination rates of reservoir fluid at < 1%. Observing outflow/inflow of fluorescein, Tse et al (2019) determined that only 13% of change in fluorescence intensity was due to exchange or mixing. Tan et al (2018) found that 33% of subjects had no reservoir mixing within the first five minutes and had no subjective discomfort or clinical complications. One important difference among the above-cited studies was that only Paugh et al (2018) used patients wearing either sphere or toric haptics, whereas the subjects in the other studies were wearing spherical-only haptic lens designs.
Midday fogging (MDF) occurs when particulates accumulate within the reservoir (Pucker and Laurent, 2018) (Figure 1). This affects 20% to 30% of scleral lens patients (Walker et al, 2016). In a recent Contact Lenses Today poll, MDF was cited by 44% of respondents as the most frequent scleral lens complication that they encounter in practice (Nichols, 2018).
Although MDF has not been shown to negatively affect the anterior ocular surface, it can result in loss of clarity and the need to refresh the lens during the course of daily wear (Carracedo et al, 2017).
Even though it is commonly thought that the influx of fluid into the reservoir contributes to MDF, results from the above-cited Skidmore et al (2019) and Tse et al (2019) studies show that tear inflow was not a factor of MDF. A recent study from Postnikoff et al (2019) demonstrated that increased scleral clearance was risk factor for MDF; for every 50 microns of clearance, there was 2.24 higher odds rate of MDF as a result of an increase in inflammatory cells. A novel filling solution, with an ionic composition and pH to match natural tears, may benefit some patients (Karres et al, 2019). Anecdotally, the off-label use of carboxymethylcellulose added to the bowl of a scleral lens reduces MDF for select patients.
Although most of the research arguably raises more questions than it answers, there are some conclusions that we can now clinically consider. First, scleral lens-wearing patients who do exhibit tear exchange have rates that are significantly less than those of corneal GP and soft contact lens wearers. It appears that inflow is not necessary for scleral lens success. MDF does not seem to be a result of inflow or mixing. Reservoir thickness seems to be the most significant factor for patients who experience MDF; reduced corneal clearance lessens this complication. CLS
For references, please visit www.clspectrum.com/references and click on document #282.