The major concern in fitting polymethyl methacrylate (PMMA) scleral contact lenses (ScCLs) are complications associated with hypoxia. The introduction of advanced rigid gas-permeable (RGP) materials significantly reduced complications related to corneal hypoxia using ScCL, increasing their popularity among practitioners.1–5 However, new issues and complications have appeared, and some of these issues and complications require study of their etiology and new approaches for their management.
The introduction of new diagnostic instruments to detect the ocular surface has allowed professionals to better understand the contour of the ocular surface, leading to the formulation of new scleral lens designs and techniques, such as those with a toric back surface periphery, quadrant-specific design, one or more reverse zones, and toric front surface. These designs and techniques are meant to manage issues related to the fitting relationship with the sclera. However, several issues still occur, and importantly, not all clinicians have advanced instruments available.
Reports of adverse reactions related to ScCL wear are quite rare.6 Therefore, in this article, the non–adverse events will be denoted as issues and only adverse events will be considered to be complications.
Issues caused by nonoptimal ScCL fitting do not cause persistent alterations to the ocular surface. However, they affect patient satisfaction and lead them to discontinue ScCL use. Some issues may be challenging, adding to frustration, time, and costs for both practitioner and patient, especially if they occur in combination. Management of the process for modifying the lens fitting relationship is crucial because most of the time, the decision to use ScCL represents a life-changing event for patients, and problems with these may result in eye surgery and/or depression or psychological problems and emotional instability. Management strategies vary depending on the type of issue, often requiring different changes (Tables 1 and 2).
The purpose of this article is to examine, in detail, the causes of all issues and complications that ScCL fitters may encounter during the fitting of ScCL, referring to clinical experience and literature reports, and to provide guidelines, sometimes to both practitioners and patients, to resolve such issues. However, it should be clarified that some fitting guidelines may vary from practitioner to practitioner or even from condition to condition.
The literature reviewed was from PubMed on the July 18, 2017, using different combinations of keywords including “Scleral contact lenses; complications; issues; hypoxia; infection; inflammation; microbial keratitis”; the search retrieved 47 articles that included studies and case reports.
ISSUES (NON–ADVERSE EVENTS)
Excessive Central Clearance
The liquid reservoir created between the posterior surface of the lens and the anterior corneal surface is also referred as “lens clearance” or “vault.” Its thickness is associated with the lens sagittal height exceeding the sagittal height of the eye at a specific chord that is being fit, and it is expressed in microns (μm). Usually, the amount of central clearance required depends on the ocular surface conditions, and it is minor in normal eyes and higher in irregular corneas and eyes with diseases. It may range from 200 to 300 μm before lens settling and 50 to 200 μm after lens settling (Fig. 1).7
Excessive clearance after lens settling may influence oxygen delivery to the cornea,8–11 reduce visual acuity, lead to visual disturbance because of the fishbowl effect, and, in the presence of a small landing zone, air bubble formation.12 A study has shown that clearance exceeding 500 μm may cause midday fogging (MDF).13 Diminishing lens sagittal height is required. The sagittal height of an ScCL is reduced by flattening the transition zone, the zone between the optical zone, the landing zone, or the base curve.
Corneal bearing when using an ScCL does not occur frequently because of the lens vaulting over the cornea. When ScCL bears on the cornea, lens adhesion,12 microcysts,14,15 corneal molding, corneal staining, and epithelial bullae may occur.16,17
Scleral contact lens bearing on the apex of the cornea indicates that the lens sagittal height is not sufficient and needs to be increased. The lens sagittal height may be increased by steepening the transition zone or the base curve. If the lens is fitted too flat, steepening the peripheral curves and the corneal curves may be necessary (Fig. 2).
Peripheral bearing may also indicate a small ScCL for the specific eye. Increasing the ScCL diameter by enlarging the corneal and limbal diameter will alleviate this problem.
When fitting a prolate ScCL design on an oblate cornea (e.g., after refractive surgery for myopia or penetrating keratoplasty), the lens will have an optimal central clearance but bear peripherally. Increasing the corneal clearance will allow an adequate corneal peripheral clearance but an excessive central clearance. Switching to an ScCL with an oblate design may be the ideal option.
Corneal staining is evaluated after ScCL removal with fluorescein. It may be induced by mechanical, hypoxic, and toxic factors,12 and it may cause discomfort, foreign body sensation, and occasional eye dryness. It appears in different patterns, either localized or diffused over the cornea.
Delimited corneal staining during scleral lens wear could be because of mechanical causes, such as lens bearing on the cornea, improper ScCL handling, or the presence of an air bubble.
If corneal bearing shows in the center or in mid periphery, that is, in cases with keratoconus, then staining will be localized in the corneal apex, and increasing the lens sagittal height will be necessary.
In presence of decentered ectasia, corneal staining may appear in the mid-periphery. In this case, increasing the corneal, peripheral, and limbal clearance, and steepening the transition zone, is necessary.
In the case of fenestrated lenses and reduced tear reservoir, the holes may cause corneal abrasions. Increasing clearance will solve this condition.
The presence of air bubbles may cause a localized area of dryness in the cornea, stained typically with a circular pattern when the lens is removed (Fig. 3). Air bubbles may be the result of improper lens handling, such as insufficient filling of the ScCL with saline or not opening the eyelid properly during lens insertion; the lid will decenter the lens on the plunger or fingers. The occurrence of air bubbles may also be the result of fitting a spherical ScCL on a significantly toric or asymmetric sclera or fitting a fenestrated ScCL. Switching to an ScCL with a toric back periphery may be beneficial. In some cases, air bubbles may persist. Using a more viscous lens filling solution may be indicated.
A more diffuse punctate erosion pattern is typically caused by hypoxia, toxic reaction to solutions, and the fluid reservoir containing contaminants, such as debris. These can be addressed, respectively, by increasing the oxygen transmissibility, reviewing solutions, using nonpreserved solutions, verifying compliance, and inspecting the presence of debris.
Air bubble formation is one of the significant issues related to ScCL fitting. Small air bubbles may disappear as the lens settles, but even if they remain, they may be acceptable if they are floating. A large and static bubble, or one located centrally, especially over the pupil, may interfere with vision, cause discomfort, induce corneal dellen formation, and corneal desiccation in the area where it occurs, leading to lens dropout.
The formation of air bubbles may be caused by improper lens handling during its insertion, nonoptimal lens fitting, and fenestrated ScCL.
Excessive corneal clearance associated with a small or misaligned landing zone may also be responsible for bubble formation. Decreasing the corneal clearance or increasing the landing zone width may be necessary. Lens lifting in the periphery in at least one meridian indicates a toric sclera or an asymmetric regular or irregular sclera. Bubbles will enter in the area where the lens is lifted off. In the case of toric or asymmetric regular sclera, steepening the ScCL periphery in the specific meridian or quadrant to avoid the creation of an entry path for air bubbles is an optimal option. Flattening the inner curves of the lens scleral zone to move the bearing area closer to the limbus where the sclera is more likely spherical or simply choosing a smaller lens may be good alternatives as well.
In the case of an irregular sclera, such as scar tissue, symblepharon, pterygium, making a notch, or a localized areas of increased elevation into the ScCL periphery may be beneficial. If the elevation is adjacent to the limbus, then a larger diameter with a slight compression on the pinguecula may also be indicated. However, switching to a molded/impression ScCL design may be the optimal alternative.
The hole of a fenestrated ScCL that permits the passage of oxygen may also represent an entrance to air bubbles. Thus, choosing a nonfenestrated ScCL may be indicated.
Finally, a more viscous solution in combination with saline to fill the ScCL may be used. The higher viscosity in the solution will also impede the entrance of debris. The more air bubbles that are present, the more viscous solution should be added in combination with saline to fill the ScCL.
Midday fogging refers to the presence of debris in the post–lens fluid reservoir. This phenomenon appears only with ScCL wear; it is multifactorial and occurs in 33% of wearers, especially in those with a predisposition to dry eye.18 It has been described by patients experiencing debris incidence as a decrease of at least two lines of visual acuity after 4 hours of lens wear.19 Another study reported a 25% dropout rate because of visual dissatisfaction, potentially associated with MDF.20 Diffused corneal punctuate staining as a result of toxicity may also appear. However, it is possible to differentiate MDF from corneal swelling symptoms. In case of corneal edema, a rainbow-like pattern around lights, haloes, or eye redness may be observed, and removing the lens will not relieve symptoms.
There are three types of debris that may occur: Mucus debris appears as small, white, and fluffy particles in the reservoir. The second type of debris is a fogging associated with atopic disease that appears as diluted milk fogging. The third type is lipid particles showing as oil droplet debris.21 Atopic diseases, nonoptimal fitting relationship, and the release of cells from the cornea may be factors contributing to MDF.
A nonoptimal fitting relationship, such as excessive vault, a tight fit, mechanical stress of an excessively flat lens-landing zone, and edge liftoff may contribute to MDF. Accumulation of debris in the reservoir has been described in patients fit with excessive clearance.22 A study showed an association between fogging and high average post–lens tear layer thickness (more than 500 μm) and tight peripheral edge fit.13 Lowering the central clearance will thin out the debris layer, resulting in better visual acuity.
Additionally, decreasing the limbal clearance will decrease the negative pressure, which is responsible for the debris influx behind the lens, and will narrow the channels of debris migration.23
The friction of a flat lens on the conjunctiva may increase mucin production, which, with fluid forces, may enter into the fluid reservoir. Steepening the landing zone, assessing lens edge, or switching to a toric back surface periphery will alleviate this problem.24
To minimize MDF, further recommendations may be given. Patients may wash their eyes with eyebath in the morning before lens application. Additionally, they may apply, remove, and reapply ScCL to clear eye of the morning secretions. Taking breaks to remove the lenses, cleaning, and refilling them with fresh, nonpreserved solution may increase MDF. An effective alternative may be holding preservative-free saline to the lens edge and squeezing the solution under the lens. The use of a more viscous, preservative-free solution may be beneficial. The amount of a viscous solution may be varied depending on the symptoms.
Stem cells, essential for corneal epithelium regeneration and proliferation, are located in the limbus. Thus, limbal bearing may trigger the failure of the epithelium, provoking several complications, such as limbal staining, limbal stem cell deficiency,25 limbal edema, epithelial bullae,17 and keratitis.26 A subepithelial fibrosis, in the area of the ScCL bearing at the graft/host edge, has been reported in a post–penetrating keratoplasty (PKP) patient.27 Additionally, limbal bearing may cause discomfort, leading to reduced patient satisfaction and lens dropout. For maximum comfort, the ScCL should land gently and as far away as possible from the corneal nerve endings.28
Scleral contact lens fitting should be evaluated in the five gaze directions (straight, superiorly, inferiorly, nasally, and temporally). When the lens decenters inferiorly, a superior limbal bearing may appear; thus, the corneal surface in the area of superior bearing should be examined at each follow-up visit. Referring to clinical experience, it has been reported that 20% of circumferential limbal bearing may be tolerated.23 Otherwise, it may be necessary to increase the lens diameter and the sagittal height in the limbal zone. A limbal bearing that occurs in one meridian indicates an oval shape of the limbus, because of the horizontal visible iris diameter (HVID) majorly being larger than the vertical visible iris diameter. This requires switching to an ScCL with an oval trend, which has a larger vault diameter in the meridian, where the limbus is larger, and a toric limbal zone.29
Conjunctival prolapse appears when the conjunctival tissue near the limbal region migrates under the ScCL as a result of the negative pressure beneath the lens. It occurs mostly in elderly patients, in those with inflammatory conditions, atopy, limbal stem cell deficiency, pellucid-marginal degeneration, in those who have undergone several ocular surgeries, such as strabismus and retinal surgery, and in those experiencing dermatochalasis.30
Conjunctival prolapse may be considered benign if after lens removal, the conjunctival tissue retrocedes; however, the long-term effects are still unknown. Tissue that remains adherent to the cornea may be removed with a clean finger or cotton swab. If this manipulation fails, then the conjunctival prolapse may represent a concern because it covers the stem cells and may provoke neovascularization. Excessive tissue adherence may be removed surgically; however, it may reemerge after fitting ScCLs.12
Modifying the ScCL fitting relationship may minimize conjunctival prolapse and prevent tissue adherence. Conjunctival prolapse appears mostly in the limbal zone, in the less elevated, temporal, inferior area, where a larger clearance is evident, which triggers the negative pressure that pulls in the loose conjunctival tissue. Quite rarely, it may occur in the optical zone. Reducing the limbal clearance may be suggested to alleviate conjunctival prolapse. Smaller lenses may also be indicated, although in patients having an oval limbus, the lens will bear on the cornea in the horizontal meridian, resulting in stem cell failure. Indeed, fitting a spherical ScCL in a patient having a prominent oval-shaped limbus may be responsible for this phenomenon because the lens will start landing close to the limbus, in the meridian where the visible iris diameter (VID) is larger (generally the horizontal meridian) and far from the limbus in the meridian where VID is smaller (generally the vertical one). This fitting relationship will create a larger clearance in the vertical meridian, generating a negative pressure causing the conjunctival prolapse.29 Additionally, a significant difference in height between two meridians in the limbal zone may generate excessive clearance. In this case, an ScCL with an oval trend shape and a toric limbal zone may be indicated.
Conjunctival blanching is a whitening effect of the bulbar conjunctiva that results from localized pressure of the ScCL landing zone on the conjunctival vessels, restricting the blood flow. Because the lens on the conjunctival vessels causes the compression, rebound hyperemia at the location of the compression will appear when the lens is removed and typically will not result in conjunctival staining. To manage the conjunctival blanching correctly, it is important to observe its localization, which may be circumferential or sectorial and may occur in the inner or outer area of the landing zone.
Circumferential blanching is caused by a nonoptimal landing zone, which may be too steep or too flat. All peripheral curves, that is, corneal peripheral, limbal, and scleral curves, should be flattened. Flattening only the scleral curves may lead to a tighter fit because this creates a hinge effect, resulting in compression as the outer edge is lifted.31 In the case of a tight fit, decreasing the lens sagittal height, flattening the base curve, or flattening the lens peripheral zones may minimize compression. On the other hand, increasing the landing zone width and increasing the lens total diameter (TD) will allow a better distribution of the lens pressure on the conjunctiva, alleviating vessel compression.
Sectorial blanching refers to lens compression in one or more quadrants and is the result of a misalignment of the ScCL on a toric or asymmetric sclera or an irregular conjunctiva. When fitting a spherical ScCL on a significantly toric sclera, the lens will touch the sclera on the flat meridian and will lift in the steep one. If the bearing is excessive, then compression will occur. In sclera having a with-the-rule toricity pattern, the sectorial blanching will appear in the horizontal meridian because this is the flatter one, causing mechanical stress in the underlying area. It is important to evaluate the lens in the opposite meridian for an excessive liftoff, which will lead to lens awareness and influx of debris and air bubbles. The use of a toric back surface periphery may be an option as well (Fig. 4).
The sclera has been reported to exhibit the steepest curvature in the temporal curvature compared with the nasal.32 Thus, blanching may ensue in a precise quadrant, which is more likely to be nasal, where the sclera is flatter. A quadrant-specific lens design will allow a better fitting relationship. Referring to a study reporting that the scleral asymmetry is greater toward the extraocular muscles, decreasing the ScCL TD and avoiding the interaction with a more toric or asymmetric sclera may be valuable remedies.33
The presence of a pinguecula may cause blanching in a specific area because the pinguecula is a conjunctival elevation and is vascularized. Reducing the lens diameter to prevent the interaction with such an elevation may be a remedy to the situation. A notch or a localized areas of increased elevation into the ScCL periphery may also be helpful. A smaller ScCL that avoids the interaction with the irregularity or a large diameter with a slight compression on the pinguecula may also be indicated. Switching to a molded/impression ScCL design may also be beneficial.
Conjunctival blanching may occur in the inner or the outer area of the landing zone. Compression in the inner area indicates a landing zone that is too flat. The presence of air bubbles or frothing under the lens edge will demonstrate the same effect, requiring a steepening in that specific area (Fig. 5). Blanching in the outer area of the landing zone with excessive clearance in the inner region indicates a peripheral landing zone that is too steep and will need to be flattened.
It is necessary to evaluate the lens on the eye in different gaze positions because decentered lenses can produce a different pattern of blanching in comparison with the static straight eye gaze position.
Conjunctival impingement occurs when the lens edge is too steep and pinches on the conjunctiva, resulting in conjunctival staining after lens removal. A tight-fitting lens may obstruct blood flow through the vessels, resulting in impingement and blanching, both occurring under the edge area. Symptoms may depend on the severity of the lens indentation into the conjunctiva. Additionally, patients with impingement and negative pressure buildup may not feel any symptoms during ScCL wear, but they will complain of discomfort after lens removal and will be incapable of wearing the lens the next day.34 It is necessary to manage this condition because long-term impingement may result in conjunctival hypertrophy.
Impingement may be circumferential or sectorial and may appear in the inner edge, adjacent to the landing zone, or in the outer edge. A full ring of impingement is the result of an edge profile steeper than the sclera, which needs to be flattened. However, lens edge modification may not be available in all lens designs. Therefore, other parameters may be varied to remedy an edge that is too steep. Thus, relief in conjunctival impingement may be obtained by decreasing the lens sagittal height or flattening the base curve, the transition zone, or the landing zone.
The sectorial impingement pattern indicates a significantly toric, asymmetric sclera, necessitating a smaller lens, toric back surface periphery, or quadrant-specific lens design.
Additionally, conjunctival irregularities, such as pinguecula, symblepharon, and scars, may cause impingement and conjunctival staining in a specific area. Even a touch with the lens edge may cause conjunctival inflammation. In these cases, a notch, a localized peripheral areas of increased elevation, a quadrant-specific design, or a larger lens with minor compression may be indicated.
Finally, an inner edge impingement may be the result of a sharp junction between the landing zone and the edge, which needs to be smoothed.
Conjunctival redness related to ScCL use may have different origins, including conjunctival vessel compression, tightly fitting lens, lens adhesion, irritation, metabolic influence, toxic reaction, and chemical influences from using nonadequate solutions. The condition may cause occasional dryness.
Conjunctival vessel rebound after lens removal indicates vessel compression by the landing zone, tight fitting, and lens adhesion (Fig. 6). As such, it will be necessary to modify the lens parameters as described in the sections Conjunctival Blanching and Lens Adhesion.
The touch of the lens edge with conjunctival irregularities, such as pinguecula, symblepharon, scars, and conjunctival elevations, may cause their inflammation, resulting in sectorial conjunctival redness. Reducing the ScCL TD and preventing the interaction with conjunctival irregularities may be a good option (Fig. 7). A notch or a localized peripheral areas of increased elevation into the lens, quadrant-specific lens design, or a larger ScCL with a slight compression may also be indicated. Choosing a molded/impression ScCL design may be an optimal alternative.
Metabolic issues related to redness may be caused by hypoxia, hypercapnia, and the accumulation of lactic acid. Switching to an ScCL with higher gas transmissibility (Dk/t) and decreasing the central clearance may alleviate conjunctival redness. A tight fitting lens and lens adhesion cause the accumulation of lactic acid in the peripheral cornea. Increasing ScCL diameter or flattening the ScCL fitting will alleviate a tight-fitting lens. Additionally, increasing the central clearance or flattening the landing zone will reduce lens adhesion.
Conjunctival staining is more common than corneal staining because the ScCL bears on the conjunctival tissue. Conjunctival staining may be the result of a lens edge that is too steep, which will require flattening, and a mechanical friction of the landing zone on the conjunctiva in the horizontal meridian, which is flatter. Flattening the landing zone in the flatter meridian and steepening the landing zone in the steepest meridian allows for the adequate alignment with the conjunctiva.
Conjunctival staining may also occur at the 3- and the 9-o'clock areas that are not covered by the ScCL. Patients with ocular surface diseases may complain of discomfort in these exposed scleral areas. A larger diameter, providing a wider protection of the ocular surface, may alleviate this condition.
Conjunctival imprint is a mechanical effect of ScCL on the conjunctiva that is observed at lens removal. It may appear even in the absence of conjunctival staining. Although it may be considered a benign condition because it disappears a few hours after removing the ScCL, the long-term effects on conjunctival tissue are still unknown.
If the conjunctival imprint is circumferential, then the cause may be circumferential conjunctival blanching, a tightly fitting lens, lens adhesion, or the landing zone and lens edge being too steep. Managing these causes may be indicated.
Its appearance in specific areas indicates a toric or an asymmetric sclera. Switching to an ScCL with a toric back surface periphery or a quadrant-specific design may alleviate the impression.
Excessive Lens Edge Lifting
The fitting relationship of the lens edge may be evaluated with the slit lamp, rotating the slit beam at 90°, and using the white light. An edge lifting will appear as a dark band or shadow under the lens edge. Lens edge lifting may be assessed using a slit lamp with parallelepiped or direct focal illumination with white light. The use of fluorescein will show an excessive pooling of stained liquid under the lens edge. Although it is possible to evaluate the lens edge fitting relationship using ocular coherent tomography, this may result in artifact because the instrument is capable of calculating a single index. This assessment is referred to as the 50/50 ScCL edge rate, where 50% of the edge apex sinks softly into to conjunctiva and 50% of the edge apex is out the conjunctiva. A higher sinking rate will cause lens impingement. If more than half of the edge apex is lifting off, then the lens will cause discomfort.35
The lens edge in the area where it lifts off should be steepened (Fig. 8). Increasing the lens sagittal height and steepening the transition zone, the base curve, and the landing zone may remedy a flattened edge.
Lens adhesion is a quite rare condition that may cause discomfort, reduced wearing time, and ocular health alteration.12 A complete or nearly complete adherence may minimize tear exchange and produce toxic metabolic waste.27 Severe adverse reactions may ensue such as edema, inflammation, and infection. Lens removal may provoke severe complications, such as trauma or rupturing of the globe at the corneal incision or the host/donor interface sites because of biomechanical weakening of the stroma after the surgery.23
In patients with keratitis sicca, oblate corneas such in post–radial keratotomy, laser-assisted in situ keratomileusis, and post-PKP/lamellar keratoplasty, lens adhesion may occur, where a suction force during blinking ensues or excessive pressure is exerted during lens removal.23 Taking breaks during the day will help to avoid ocular damages.24,27 Lubricating drops during ScCL wear or choosing fenestrated ScCL may also be helpful.
Low corneal clearance and a steep landing zone may provoke lens adhesion.12 In the case of excessive lens pressure on the conjunctiva, switching to a toric periphery or a larger landing zone to create a better distribution of lens pressure may be an optimal option.
Lens flexure may also provoke lens adhesion, which can be remedied by switching to a toric lens periphery. Similarly, a thicker lens or thicker junctions may be indicated.
Lens adhesion may be the result of a lens sinking in a swollen conjunctiva. If this is the case, then increasing the limbal clearance may solve conjunctival swelling.
If lens adhesion leads to problems in its removal, then applying pressure on the ocular surface close to the lens edge may help to break the seal of the lens.12
Scleral Lens Instability
In most cases, the ScCL decenters inferior temporally, inducing a thinning of the fluid reservoir in the superior nasal quadrant.36 Lens decentration dislocates the lens optics, causing prismatic effects and aberrations, and makes the management of nasal and temporal limbal clearance differences more challenging, thus provoking discomfort.12 It may be caused by too little apical clearance, excessive central clearance, scleral asymmetry, scleral toricity, large vault diameter, and lens mass.
Lens instability may be the result of a lens “rocking” on the central cornea, inducing discomfort and decentration caused by a small apical clearance that needs to be increased.
An excessive central clearance may also induce lens decentration; the higher the lens vault, the greater the lid pressure on the lens, resulting in inferior decentration.37 In this case, reducing the central vaulting may lead to more lens stability.
A large vault diameter leads to an excessive limbal clearance, causing lens instability. Reducing the lens-vaulting diameter by referring to the HVID and calculating roughly a limbal zone width of 1.00 mm per part38 will lead to a better lens centration. On the other hand, an oval limbus shape will create a high limbal clearance in the vertical meridian. An ScCL with an oval trend and a toric limbal zone will remedy this issue.
Fitting a spherical ScCL on a significantly toric sclera will not allow an optimal lens pressure distribution on the sclera, leading to lens instability and discomfort. In this case, the lens will bear only on the flattest meridian, rocking on the steepest one. A toric back surface periphery will lead to a better lens landing on the ocular surface and better stability. It has been reported that switching to the toric back periphery increased comfort, wearing time, and overall patients' satisfaction.38–40
Fitting a spherical lens on a significantly asymmetric sclera will bear on the area, which is more elevated (nasally) and pushed in the opposite direction, in the area less elevated (temporally). A quadrant-specific ScCL design may reduce lens decentration.
Gravitational forces play an essential role in the lens mass of the ScCL decentering inferiorly. Reducing the lens mass, decreasing the TD, or making the lens thinner will mitigate the effect of gravity.
Lens flexure on-eye may occur with ScCL made with high oxygen permeability (Dk) materials, and it may be detected using corneal topography or keratometry. Making a thicker lens or thicker junctions may render the lens more resistant to flexure. Lens distortion may be triggered by lens misalignment with the sclera, causing unwanted astigmatism. Choosing a back surface toricity or quadrant-specific design may alleviate lens distortion, increasing visual acuity. Switching to a smaller lens with less interference from scleral asymmetry and a higher rigidity may be beneficial as well.
In some cases, astigmatism from lens flexure may remain, and it will be necessary to add a front toric surface. This requires stabilization by prism ballasting, dual thin zones, or double slab-off off prism. When the sclera exhibits a clinically significant amount of toricity, a toric back periphery may be sufficient to provide a stable lens and thus a stable vision.
COMPLICATIONS (ADVERSE EVENTS)
Bullae may appear oval shaped, clustered, larger than 40 μm in diameter, with indistinct margins.41 Their etiology is not well understood; it has been shown that mechanical stress in areas of persistent unintended contact lens interaction with the ocular surface results in epithelial erosions and epithelial bullae.17 Symptoms are not well defined; patients may complain of only mild to moderate dryness. However, symptoms are consistent in frequency and magnitude with the wearing of other modalities of contact lenses. Epithelial bullae are associated with negative staining. Clinical signs, such as redness, may also be observed.
The prognosis of recovery from epithelial bullae is generally good; any inclusion of the epithelium will be removed within a few days. In severe cases, bullae may break through the epithelium surface and progress into epithelial defects.16
Switching to an ScCL with larger diameter and increasing the clearance in the corneal and limbal areas is essential.
The incidence of a significant number of microcysts may be alarming, as it is a characteristic of epithelial metabolic distress.16 Yet with even a high amount of microcysts, patients may report little discomfort.
It has been stated that the presence of microcysts is an evidence of chronic metabolic stress.42 During ScCL wear, microcysts may occur because of inadequate oxygen supply to the cornea. This is addressed by increasing the ScCL Dk/t and decreasing the thickness of the fluid reservoir beneath the lens.
Additionally, because mechanical lens trauma can induce microcysts,14,15 an ScCL that exerts pressure on the cornea may provoke microcysts. Thus, the proper management of ScCL interaction with the cornea, as suggested earlier, is needed.
The advent of high Dk materials has not provided a definitive solution to complications associated with corneal hypoxia. Most of the time, ScCLs are fitted in patients with compromised corneas that need more oxygen, especially in those having a compromised endothelium layer, such as in post–corneal graft patients.
The transmissibility of oxygen through the ScCL may be lower compared with other modalities because it depends on the lens system composing the ScCL itself and the fluid reservoir behind the lens. Tear permeability has a Dk value of approximately 80 × 10−11 Fatt Dk units, which is relatively low compared with some high Dk materials, which may range from 100 × 10−11 Fatt Dk units to 163 × 10−11 Fatt Dk units.43,44 Additionally, when fitting ScCLs, the tear exchange at lens settling is limited to 0.2% per minute.45 However, no studies have shown clinically significant levels of hypoxia during daily wear of ScCL, even in cases with oxygen permeability below Holden–Mertz criteria.
Even if clinical corneal edema has not been revealed, subclinical levels of swelling may occur. Various researches examining the corneal response to modern ScCLs reported low corneal swelling, less than 4%.46–50 The amount of corneal edema remained within the physiological range even when fitting lenses with high Dk materials having a thickness of up to 120 μm51 and with clearance thickness up to 350 μm.9 These outcomes diverge from theoretical models referring to oxygen delivery using ScCL.8–11 A theoretical model predicted that an ScCL made with materials having more than 100 × 10−11 Fatt Dk units, a thickness of less than 220 to 260 μm, and a clearance height less than 150 μm prevents corneal swelling related to hypoxia.8 Another report, divided into two parts, clinical and theoretical, stated that an ScCL made with materials having more than 125 × 10−11 Fatt Dk units and a thickness of 200 μm should be fit, provided there is a clearance height of less than 150 μm to prevent corneal swelling.9 Further theoretical models showed similar results, that is, ScCLs made with materials with high Fatt Dk units and fit with minimal central clearance reduce hypoxia.10,11
Consequently, even if the ideal vault is yet to be determined, it may be recommended to fit an ScCL with minimal clearance height to prevent corneal swelling. Special considerations should be addressed for patients with a compromised endothelial layer because it plays a crucial role in delivering enough oxygen to the cornea. A low endothelial cell density of less than 800 cells per square millimeter may be one of the few contraindications of ScCL wear. Endothelial cell density lower than 1,000 cells per square millimeter should be managed with intense attention.12,52
Neovascularization has been a large concern when fitting PMMA ScCL. If this condition occurs in the ScCL with high Dk RGP materials, it is more likely because of mechanical stress in the limbal area rather than hypoxia. Increasing the lens sagittal height in this zone is crucial. Additionally, lens adhesion may cause neovascularization.12 Increasing central clearance or flattening ScCL landing zone will alleviate lens adhesion.
A tight fitting ScCL may also be implicated in this complication. A tightly fitted lens may indent the conjunctiva and restrict the blood flow, causing lactic acid to accumulate in the peripheral cornea, triggering neovascularization.16
Additionally, neovascularization has been observed when conjunctival prolapse adhered to the cornea for extended periods.25
Chemosis is the swelling of the conjunctiva and may occur during ScCL use because of low limbal clearance or from trauma by a sharp or damaged lens or fingernail. Conjunctival swelling may cause lens adhesion because the lens sinks into the altered conjunctival tissue.12 Increasing the limbal clearance is fundamentally important.
Giant Papillary Conjunctivitis
As with soft and corneal RGP lenses, the deposits on the anterior surface of the lens may cause a giant papillary conjunctivitis (GPC), which does not appear to be more prevalent in ScCL wearers compared with those with corneal RGP and soft lenses.12
Its etiology has yet to be defined. It is most likely provoked by a mechanical irritation12,16 or an immunologically mediated process frequently associated with atopy16,53 or from a toxic or allergic factor of the tarsal conjunctiva because of solutions or denatured protein on the contact lens surface or an edge lifted off.12 These factors can lead to an inflammation cascade, initiating GPC. Thus, special attention should be addressed to ScCL care.
A proper alignment of the edge with the sclera is also vital to prevent GPC and discomfort. An excessively lifted lens edge may provoke mechanical irritation on the tarsal conjunctiva triggering an inflammation reaction. Managing the edge relationship with the ocular surface is necessary (see section Excessive Lens Edge Lifting).
Gas-permeable materials and ScCL techniques are continuously evolving to minimize complications and optimize their fitting relationship. On the other hand, international associations provide continuous professional education to spread knowledge of ScCL indications, their intrinsic advantages, and troubleshooting. However, some practitioners remain intimidated by preconceived notions, the fitting process, and, especially, complications and their management. As a consequence, practitioners rarely, if not at all, prescribe ScCLs. Describing in detail all of the issues and complications that can ensue from fitting ScCLs will allow a better understanding of their origin and thus their resolution. Expansion of the use of these lenses depends on the increase of knowledge of ScCL fitting and management, and patients will benefit from this knowledge expansion. Indeed, this is the objective of this article.
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