The time course and nature of corneal oedema during sealed miniscleral contact lens wear
Introduction
The symptoms of scleral contact lens induced corneal oedema were first described by Muller following his experimentation with glass haptics in the 1880’s [1], however the physiological mechanism underlying Sattler’s Veil (epithelial oedema) remained elusive and was fiercely debated for the next half a century [2]. Early clinical attempts to delay the inevitable, but highly variable onset of corneal haze, included fitting slightly flat or loose in the haptic zone to encourage tear exchange at the expense of comfort, and altering the composition or pH of the post-lens fluid reservoir [3]. These approaches to minimising corneal oedema were unpredictable, and until “ventilated” (fenestrated) scleral lenses were introduced and much later oxygen permeable RGP sclerals [4], patients often removed their lenses for extended periods throughout the day to restore vision.
Although the oxygen permeability of rigid lens materials has improved substantially in recent years, there is still debate surrounding the corneal oxygen requirements for successful scleral contact lens wear; particularly since modern sealed scleral lenses are manufactured with a minimum centre thickness typically ≥ 300 μm to minimise lens flexure, exhibit limited to no tear exchange following lens settling, and the relatively thick post-lens tear layer itself may inhibit oxygen transmission to the cornea. However, despite these potential barriers to atmospheric oxygen, a number of short-term clinical studies have confirmed that modern highly oxygen permeable scleral contact lenses worn on a daily wear basis, typically induce ≤2% of total corneal oedema in young healthy eyes [[5], [6], [7], [8]]. These studies have quantified the change in the total corneal thickness using optical coherence tomography (OCT) or Scheimpflug imaging following a specified period of lens wear, typically after lens removal to facilitate imaging. Consequently, the time course (both the onset and recovery) and nature (epithelial and/or stromal) of scleral lens induced corneal oedema has not been investigated in detail.
Studies using soft contact lenses, eyelid closure, or gases of varying oxygen concentration suggest that the cornea swells rapidly under hypoxic conditions and stabilises after 2–4 h dependent upon the extent of the hypoxic stress [[9], [10], [11]]. However, the onset of corneal oedema during scleral lens wear may differ substantially to oedema associated with soft contact lenses or corneal RGP’s, given the lack of tear exchange in a sealed system and the dynamic thinning of the post-lens tear layer associated with scleral lens landing zone settling into the underlying conjunctiva and episclera [12]. Pullum et al. [13] examined the time course of corneal oedema in a single sealed PMMA scleral lens wearer and observed no swelling after 1 h of lens wear, followed by a steady increase to 8% after 3 h which then plateaued, suggesting that the post-lens tear layer may provide a reservoir of oxygen during the first hour after lens insertion.
Theoretical models of oxygen diffusion and scleral lenses have utilised a resistance in series approach [14] based on Fatt’s [15] earlier modelling of piggyback systems (soft and rigid contact lenses worn in combination), or have also taken into consideration the oxygen consumption of each corneal layer [16,17]. These models suggest that scleral lenses should be fitted with minimal central and limbal clearance (e.g. not exceeding 200 μm and 50–60 μm respectively [14]) to maximise the overall Dk/t of the scleral lens system [14] or the tear layer oxygen tension [17] to minimise potential anterior segment hypoxia. However, clinical case series of patients with both ocular surface disease and corneal ectasia suggest that modern highly oxygen permeable scleral contact lenses can be worn successfully and provide good vision and comfort with substantially greater levels of apical clearance (e.g. 600–1000 μm central vault) [18,19]. A small number of studies have systematically examined the influence of altering apical clearance upon scleral lens induced corneal oedema, typically in young participants with keratoconus [5] or healthy corneae [6,20]. These studies have utilised a repeated measures design to control for confounding variables such as lens design (e.g. thickness, oxygen permeability, peripheral seal off or tear exchange), wearing time, and individual variations in the corneal response, and suggest that modifying the apical clearance does not substantially alter the magnitude of corneal oedema following 3–8 h of lens wear. Esen and Toker [5] observed no significant difference in the extent of corneal swelling after 8 h of lens wear in keratoconic patients fitted with low (100–200 μm) medium (200–300 μm) or high (>300 μm) apical clearance (less than 4 μm or ∼0.9% difference in oedema between the various clearance groups), and observed a trend towards greater levels of oedema with lower levels of apical clearance also reported in other short-term studies [7,8]. Arlt [6] also found only 0.5% more oedema in young patients fitted with over 600 μm of central clearance (2.1% swelling) compared to the same miniscleral lens fitted with only 200 μm of central clearance (1.6% swelling). However in one study [20], a larger apical clearance (350 μm) was associated with significantly more corneal oedema (∼2% greater) than a lower clearance (150 μm). The methodology of this particular study differs somewhat from other studies that report no significant effect of altering corneal clearance [5,6] since a smaller diameter scleral lens was used (15.5 mm), the clearance values were estimated subjectively, and corneal thickness was measured using a manual ultrasound pachymeter rather than high resolution OCT.
In the current study the change in central corneal thickness (epithelial, stromal, and total corneal thickness) was measured using high resolution OCT imaging over an 8 h period of sealed miniscleral lens wear to examine the time course and nature of the induced corneal swelling and the influence of central corneal clearance. The change in total corneal thickness was also compared to previously published data to estimate oxygen delivery to the cornea during scleral lens wear.
Section snippets
Methods
The details of the study participants, contact lens fitting process, imaging procedures and experimental design have been described previously [21] and are summarised below. Fifteen young, healthy adults (mean age: 22 ± 1 years, 8 female and 7 male) with visual acuity of 0.00 logMAR or better in both eyes were recruited from the Queensland University of Technology (QUT). Participants underwent a screening examination to exclude those with any ocular or vision abnormalities, or contraindications
Measurement repeatability
The mean difference (95% limits of agreement) between the two analysed OCT images at the baseline measurement session was 0 μm (−1 to +2 μm) for the epithelium, −1 μm (−3 to +2 μm) for the stroma, and 0 μm (−2 to +2 μm) for the total cornea. This indicates excellent repeatability with the 95% limits of agreement for each corneal thickness measurement less than the axial resolution of the instrument, and is in close agreement with inter-observer repeatability data for total central corneal
Discussion
The mean peak total corneal oedema observed in our subjects after 90 min of sealed miniscleral lens wear (1.18 ± 0.20%) is consistent with a number of previous studies also using high resolution imaging techniques to quantify corneal thickness changes following 3–8 h of high Dk scleral lens wear in young participants (typically between 1 and 2% swelling) [[5], [6], [7], [8]]. Lafosse et al. [25], however, recently reported corneal oedema of 5.1% after 8 h of 18 mm diameter high Dk (100) scleral
Conclusion
Young healthy eyes with normal corneae fitted with a high Dk sealed miniscleral contact lens (ICD 16.5) displayed small but highly significant variations in epithelial, stromal, and total corneal thickness during lens wear. Corneal oedema was primarily stromal in nature, rapid in onset and peaked 60–90 min after lens insertion (<1.5% swelling) before stabilising. The total corneal swelling observed after an hour of sealed scleral lens wear provides a reasonable estimate of end of the day oedema
Conflicts of interest
The authors have no financial interest in any of the products mentioned in the manuscript.
Acknowledgements
The authors thank Alison Beanland, Linda Lam, Ching Chong Lim, Alyssa Loke, and Nhi Nguyen for assistance with data collection. This study was partially supported through an IHBI Vision and Eye Program Research Development Grant.
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