LEARNING METHOD AND MEDIUM

This educational activity consists of a written article and 20 study questions. The participant should, in order, read the Activity Description listed at the beginning of this activity, read the material, answer all questions in the post test, and then complete the Activity Evaluation/Credit Request form. To receive credit for this activity, please follow the instructions provided below in the section titled To Obtain CE Credit. This educational activity should take a maximum of two hours to complete.

CONTENT SOURCE

This continuing education (CE) activity captures key statistics and insights from contributing faculty.

ACTIVITY DESCRIPTION

The goal of this article is to explore clinical challenges and/or potential challenges as they relate to scleral lenses, including intraocular pressure, corneal oxygenation, clearing the limbus, and conjunctival prolapse.

TARGET AUDIENCE

This educational activity is intended for optometrists, contact lens specialists, and other eyecare professionals.

ACCREDITATION DESIGNATION STATEMENT

This course is COPE-approved for two hours of CE credit.COPE Course ID: 79130-CL

DISCLOSURES

Stephen Vincent, OD, PhD, has received research grants from Azura Ophthalmics. Jan PG Bergmanson, OD, PhD, has received educational grants from Alcon, CooperVision, and Basuch + Lomb.

DISCLOSURE ATTESTATION

The contributing faculty member has attested to the following:

1. That the relationships/affiliations noted will not bias or otherwise influence their involvement in this activity;
2. That practice recommendations given relevant to the companies with whom they have relationships/affiliations will be supported by the best available evidence or, absent evidence, will be consistent with generally accepted medical practice;
3. That all reasonable clinical alternatives will be discussed when making practice recommendations.

TO OBTAIN CE CREDIT

To obtain COPE CE credit and instant certificate processing for this activity, read the material in its entirety and consult referenced sources as necessary. Please take the post test and evaluation by following the link below and clicking on the CE Information tab. 

Current Controversies in Scleral Lens

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NO-FEE CONTINUING EDUCATION

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DISCLAIMER

The views and opinions expressed in this educational activity are those of the faculty and do not necessarily represent the views of Contact Lens Spectrum. This activity is copyrighted to PentaVision LLC ©2022. All rights reserved.

This activity is supported by unrestricted educational grants from ABB Optical, Acculens, Art Optical, Boston Sight, and Contamac.

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RELEASE DATE: AUGUST 1, 2022EXPIRATION DATE: JUNE 21, 2025

Despite technological advances in ocular imaging and innovations in scleral lens (SL) designs, coatings, and materials that have expanded our clinical armamentarium, there are still numerous contentious topics being discussed.

NOTHING NEW UNDER THE SUN

Recently, McMonies proposed that a SL that lands firmly on the ocular surface near the limbus may compress vessels carrying aqueous humor sufficiently to raise the intraocular pressure (IOP).¹ However, Pearson pointed out that this concern is as old as this device itself, almost one-and-a-half centuries old.² This idea was revived in the 1950s by Huggert, who reported that wearing SLs elevated IOP and more so in people who were already afflicted with glaucoma.^3,4 His IOP measurements were obtained after removal of the glass SLs.

With the arrival of modern scleral designs and new materials, several studies have attempted to determine the true effect of SLs on aqueous humor dynamics.^5-10 The reported effects of SLs on IOP have been diverse; some reported minimal to moderate elevation while others reported no change or even a reduction in IOP.

An inherent dilemma with studying IOP in SL wearers is that IOP cannot be measured during lens wear unless the measurement is taken over the sclera. The problem is that tonometers are calibrated for a corneal application. If the SL is removed prior, the tonometer may be applied to the cornea, but then we do not have an accurate measurement from a lens-wearing eye.

Studying the anatomy and physiology of aqueous humor dynamics can teach practitioners about the likelihood of SL wear as a risk factor in developing glaucoma. Therefore, the next logical questions are: What does the current literature tell us about aqueous drainage? How many ways can aqueous escape from the eye?

At a recent anterior segment meeting in Texas, eye-care practitioners were asked how many routes aqueous may follow, and 68% of attendees did not know.¹¹ This is understandable because, in recent years, a greater understanding of aqueous dynamics has revealed alternative passages that a drop of aqueous may take. There are six different ways for aqueous to leave the eye^12-14 (Figure 1): 1) conventional outflow: aqueous passes from Schlemm’s canal (Figure 2) to the venous plexi in the conjunctiva, sclera, and ciliary body; 2) uveoscleral outflow; 3) transscleral outflow; and 4) uveolymphatic outflow.

Only one outflow route, via the conjunctival venous plexus, is plausibly affected while wearing a SL. The pathways to the sclera and ciliary body are located deep in the dense, fibrous outer coat and would, for this reason, be unlikely to be indented by a SL and to impede the aqueous drainage and to elevate IOP.

The other routes are away from areas that the SL touches. If one pathway is blocked or partially blocked, it is logical that aqueous will seek to flow into one of the other five routes available. For example, the proportion of aqueous escaping through the uveoscleral pathway has not been determined because it varies according to IOP and venous pressure (i.e., the system has flexibility).

Future research on the influence of SL wear on IOP will be welcome and important. Until we know more, we will offer our SL patients routine glaucoma screening, but there is no real need to go beyond that.

THE VAULT

It is generally agreed that a SL should vault the cornea and limbus. The controversy is how big of a jump this should be. That is, what is the corneal diameter and width of the limbus? It seems like a simple question to answer, but the lack of exact and agreed-upon reference points has made this a thorny and unresolved issue.

A literature search on this topic yielded few scientifically based attempts to measure this region and a surprising disinterest among contact lens-focused texts in quantifying corneal and limbal dimensions.^15,16 Structural dimension is an anatomical issue, and visiting anatomy texts for this information revealed that many use the same dimensions to describe the cornea (11.7mm x 10.6mm).

This peculiar and unanimous agreement arose because no one measured this dimension, but rather each text relied on the same source for this data. The origin of this data is Duke-Elder and Wybar’s 1961 publication,¹⁷ but they did not make these measurements; rather, the measurements were taken from seven studies up to 132 years old! These old numbers are not far off, but surely we can do better today. Let’s treat these numbers as the best approximation that we have today.

Now we need to add the limbal width to the corneal diameter to determine the overall diameter (OAD) required for a SL prescription. If one finds the approximation of the corneal diameter unsatisfactory, then the limbal dimensions are worse because no one has systematically measured this region with reasonable accuracy.

Anatomical texts estimate that limbal widths vary between 1mm and 2mm.¹⁵ Clinical measurements of visible corneal diameter are subjective and approximate.

The limbus is a transitionary region blending the cornea with the sclera; therefore, without using specified reference points, the measurements are only approximations. Good reference points for such measurements exist in histology but are more difficult to isolate clinically. Although the anatomical literature provides a uniform measurement for quantifying limbus width, the width varies depending on whether it is measured anteriorly near the ocular surface or posteriorly along the anterior chamber.¹⁴

If we use the smaller estimated limbal width (1.0mm) to calculate the OAD, we need to clear a zone of at least 14.7mm along the horizontal meridian. For a SL with a 15.5mm OAD, this leaves only 0.4mm for the landing zone (LZ) on each side. Such an abrupt landing will not be comfortable. If the LZ width is increased (without altering the OAD), theoretically we will not clear the limbus, nor will we if the limbus is wider than 1.0mm. In contrast, an 18mm OAD will offer a 1.65mm LZ width on each side, or four times wider than a 15.5mm lens.

Based on these calculations, one might assume that most SLs prescribed would have larger diameters. However, based on a practitioner survey, Harthan and colleagues reported that only 17% of SLs had an OAD of > 17mm and, similarly, in 2021 Contamac estimated that less than 20% of their orders were for blanks larger than 17mm.¹⁸

Why, then, is a sizable portion of the market choosing a smaller lens? Are larger diameters still intimidating for patients and practitioners? Is it still an oxygen question, even if the extra plastic rests on the conjunctiva, which does not have the oxygen requirements of the cornea? A larger, well-fitted lens will be more comfortable simply because it will land on a less sensitive region of the ocular surface; the further from the cornea and limbus a lens lands, the more comfortable the lens will be.

A further bonus with larger SLs for practitioners concerned about the IOP in SL wearers is that the weight of a larger lens is distributed over a greater surface area and is further from the limbus where some of the vessels carrying aqueous emerge. This has the potential to minimize any negative SL wear effect on IOP.

Research using new technology will be most welcome in establishing normative data on the structure on which we are trying to place a contact lens. New instrument designs using advances in technology and previously untried approaches could potentially find their way into the clinical setting, allowing accurate measurements on the eye being prescribed a SL.

OXYGEN DELIVERY

A topic of some debate over the past decade is whether modern SLs provide sufficient oxygen to the cornea. And what is the best clinical approach to optimize its delivery and minimize edema? Despite the lack of tear exchange in modern “sealed” (non-fenestrated) lenses and the increased fluid reservoir thickness and lens thickness compared to corneal rigid lenses, most studies of young, healthy eyes report that short-term daily wear of SLs induced edema in the range of ~1% to 3%.¹⁹

This is broadly consistent with Fatt’s hypothesis that if sufficient tear mixing occurred within the fluid reservoir of a sealed SL, there would be sufficient oxygen supply to almost meet the metabolic requirements of the cornea for daily wear.¹⁹ However, considering oxygenation alone may be an oversimplification of the metabolic processes.²⁰ This is an area of ongoing interest, as most of the people fitted with SLs are keratoconus or post-graft patients who have a greater likelihood of altered endothelial cell function and propensity for corneal edema.²¹

Fitting Philosophies While some practitioners strive for maximum lens material oxygen permeability and minimum reservoir and lens thickness to minimize corneal hypoxia, is this always necessary? Recent laboratory-based studies of young, healthy eyes have provided further insights into the effect of manipulating these variables on corneal edema (Figure 3). Dhallu and colleagues²² demonstrated that increasing the lens oxygen permeability beyond a Dk of 100 provided no further reduction in corneal edema, and Fisher and coworkers²³ highlighted that minimizing the fluid reservoir thickness (Dk of ~80) provided a greater reduction (~two times greater) in corneal edema than similar modifications for lens thickness values (Dk of 141).

In clinical practice, there are always patients who prove to be exceptions to the rule or who have ocular responses that vary from the expected physiological outcome. In SL practice, are certain individuals’ corneas exceptions to the rules? Perhaps children who have robust endothelial cell function or aphakes who have a reduced corneal metabolic demand fall into this category.

Evidence from clinical practice highlights that some corneas can indeed tolerate fluid reservoir and lens thickness values up to 1,000µm with no overt long-term complications. For example, Phillis and coworkers utilized a high-Dk SL intentionally fitted with a generous central reservoir thickness of 900µm (later increased to 1,100µm) to improve lid position in a 7-year-old who had superior ptosis, facial paralysis, and corneal irregularity.²⁴ Despite this fluid reservoir thickness, no clinical signs of corneal hypoxia were observed over eight years of follow-up.

Murphy and colleagues also observed no signs of corneal hypoxia over six months of lens wear by a 6-year-old aphake who required a SL to vault elevated scar tissue and to optimize vision.²⁵ In this case, the required back-vertex power was +24D with a center lens thickness of 970µm (Dk of 141, central fluid reservoir thickness 250µm).

Holier-Than-Thou The use of fenestrations in high-oxygen-permeable SLs to minimize hypoxia is also part of the corneal oxygen debate. While there is a history of fenestrating glass, polymethyl methacrylate (PMMA), and low-Dk SLs dating back to the work of Dallos and Bier in the 1940s, there is limited research into the actual corneal edema reduction achieved by fenestrating a SL.

Numerous studies have reported on the effect of fenestrating soft and rigid corneal lenses with mixed results.²⁶ Similarly, Ko and colleagues examined tear replacement within the fluid reservoir of habitual SL wearers: four eyes fitted with a single 1.0mm to 1.5mm limbal fenestration, and eight eyes fitted with a channeled haptic.²⁷ The authors observed that the extent of tear mixing within the fluid reservoir was extremely variable between subjects and concluded that fenestrations and channels were most likely of limited value in terms of oxygen replenishment.

Clinical reports have demonstrated that a single fenestration in a high-Dk SL can reduce epithelial edema in an aging corneal graft that has a low endothelial cell count and can relieve lens suction or cling, which can aid lens removal.²⁸ Fisher and colleagues recently examined the effect of incorporating a single limbal 0.3mm fenestration into a high-Dk (141 Dk) sealed SL design in a cohort of healthy patients who had normal corneas.²⁹ A modest reduction in edema was observed (a 19% reduction relative to a non-fenestrated lens), which was substantially less than the reductions obtained through reducing the fluid reservoir or lens thickness. Future research in eyes that have keratoconus and following penetrating keratoplasty is still required to determine the efficacy of SL fenestrations and channels, including their optimal size and location.

CONJUNCTIVAL PROLAPSE

The bulbar conjunctival tissue is movable, malleable, and compressible. Even with a scleral LZ that is optimally aligned, some conjunctival compression can typically be seen following lens removal using optical coherence tomography (OCT) or a biomicroscope. Haptic compression of the conjunctiva and its superficial vasculature have been observed for many years in clinical practice and was used to determine which regions of the LZ required modifications to improve the fit. A more recent conjunctival phenomenon that appears to be unique to modern SLs is conjunctival prolapse.

Conjunctival prolapse (aka conjunctival “hooding,” “chalasis,” “inlapse,” “billowing,” “entrapment,” and “tenting”) refers to the anterior projection of conjunctival tissue beneath the periphery of a SL, sandwiched between the limbus and the innermost aspect of the LZ (Figure 4).³⁰ The conjunctival tissue may be displaced over the limbus and peripheral cornea, but typically returns to its normal position following lens removal.

More extensive conjunctival prolapses can become a cosmetic issue. The current controversies surrounding conjunctival prolapse include its etiology, potential long-term sequelae, and optimal management strategy.

Etiology and Prevalence While the exact cause of conjunctival prolapse is unknown, it is hypothesized to occur because of the negative pressure generated within a sealed SL system and is often observed in association with excessive limbal clearance due to a region of relative corneal depression.³⁰ Other contributing fitting factors may be the extent of conjunctival tissue compression beneath the scleral LZ or the force of lens application. Prolapse may be more likely to occur in patients whose eyes have altered anatomy, such as older patients who have less adherent and more redundant conjunctival tissue (conjunctivochalasis), following surgery (e.g., pterygium removal or correction of strabismus), or tissue inflammation.

There is limited data about its prevalence in SL wearers. In a retrospective chart review, Severinsky and colleagues reported prolapse in only 1 of 36 (~3%) post-graft eyes fitted with 18.5mm to 19.0mm diameter non-fenestrated SLs followed for five years.³¹ In a repeated measures laboratory-based experiment, Fisher and coworkers observed 22 instances of conjunctival prolapse across nasal and temporal locations for 30 different lens fits for a 16.5mm diameter non-fenestrated lens design.³² However, the authors defined prolapse as any increase in total tissue thickness from the anterior conjunctiva to the posterior sclera beneath the lens, measured with OCT. Similarly, Courey and colleagues reported a 20% incidence of conjunctival prolapse in a short-term laboratory study using an 18mm-diameter non-fenestrated lens design.³³

Long-Term Sequelae Based on survey data, the number of clock hours of conjunctival prolapse considered acceptable by practitioners varies significantly depending on their SL fitting experience; more seasoned clinicians may consider prolapse a relatively benign cosmetic issue, particularly if the entrapped tissue recedes immediately after lens removal.³⁴ Although prolapse does not appear to significantly affect short-term lens performance, there is the potential for interference with tear exchange or mixing, and prolonged periods of conjunctival adherence to the limbus can result in localized vascularization, conjunctivalization (fibrovascular pannus), and impaired stem cell function.

Management Numerous strategies have been proposed to eliminate conjunctival prolapse associated with modern SLs. The most common recommendation is to alter the fitting relationship to decrease the fluid reservoir thickness at the limbus and create a more uniform or symmetric post-lens tear layer. However, Fisher and colleagues reported no association between the magnitude of conjunctival prolapse and limbal fluid reservoir thickness asymmetry in their short-term study.³²

Other approaches include altering the lens diameter, improving LZ alignment with the underlying conjunctiva (by using a toric, quadrant-specific, or customized lens periphery), or adding fenestrations or channels to reduce suction forces (although loose conjunctival tissue can plug a fenestration). In cases of redundant lax conjunctival tissue prior to SL application, consideration of surgical excision is reasonable. A SL piggybacked on a high-Dk soft lens may also be an option to prevent prolapse, but it may add corneal hypoxic stress. Patient reeducation may be required if forceful lens application is a contributing factor.

IN SUMMARY

This article looked at existing or potential clinical challenges related to scleral lenses. SL fitting issues related to IOP, corneal oxygenation, clearing the limbus, and conjunctival prolapse remain unresolved. These are not necessarily divisive issues, but they require further evidence to inform optimal management approaches. CLS

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