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Contact Lens Spectrum

Article

THE SCLERAL LENS VAULT

PRISMATIC SCLERAL LENS TIPS

By: KAREN LEE, OD
Contact Lens Spectrum November 1, 2022 Vol 37, Issue November 2022 Page(s): 45

In the past, contact lenses provided minimal relief for patients who had symptomatic horizontal binocular vision ailments. Base-down prism in small diameter corneal GP lenses or soft contact lenses, where the thick prism base interacts with gravitational and lid forces, can stabilize lens rotation (Hanks, 1983).

A small amount of prism is often used, as this ballasting methoddoes increase lens thickness and may lead to patient discomfort.Theoretically, two-thirds or more of this prism is translated into the optical axis and may help alleviate mild vertical binocular vision imbalances (Lindsay and Crock, 2005); patient tolerability, however, is limited by peripheral lens thickness (Lam et al, 2022).

Scleral lenses semi-seal to the ocular surface, trapping a layer of fluid between the lens and cornea, and are designed to align with the patient’s sclera (van der Worp, 2015). Rotationally stable scleral lenses do not always require a prism ballast, especially when quadrant-specific landing zones or free-form SLs are customized for optimal fit.

Manufacturers recommend a sagittal height difference of 100 microns or more between meridians before attempting a toric or quadrant-specific scleral zone, as anything less may result in rotational instability. These lock-and-key fits allow for the incorporation of optical prism in any direction (Vincent and Fadel, 2019), and successful cases correcting horizontal or vertical diplopias have been published (DeNaeyer, 2022; Frogozo, 2016) (Figure 1).

Figure 1. A prismatic scleral lens designed with corneoscleral topography displaying a thickened area of prism in the optic zone.
Figure 1. A prismatic scleral lens designed with corneoscleral topography displaying a thickened area of prism in the optic zone.

Here are clinical pearls to consider when fitting scleral lens patients who have binocular misalignments:

  1. Verify laboratory manufacturing capabilities and utilize a design that offers optical prism correction. At the time of this writing, optical prism could not be combined with front-surface toric lens power correction.
  2. Corneoscleral profilometry or corneal molding can greatly expedite the fitting process and is strongly recommended.
  3. Optimize lens fit prior to incorporating prism, however, be prepared to tackle prism-induced lens rotation.
  4. Currently, scleral lenses are limited to 4 prism diopters (PD) in any direction per lens and can correct a combined total of 8PD between both eyes.
  5. Necessary PD may differ when transferring correction from spectacles to contact lenses. Trial frame and use the lowest amount of prism that results in binocular fusion.
  6. Monitor periodically for binocular adaptation and corneal decompensation. A thickened scleral lens may cause corneal hypoxia with long periods of wear.

Case Report

A 35-year-old white female patient who wanted to wear contact lenses for sports reported for a scleral lens fitting. She was seeing well through her current spectacles prescription: OD +0.25 –1.00 x 065 3PD base out, 20/20 and OS plano –1.25 x 125 3PD base out, 20/20.

Free-form scleral lenses, manufactured with the help of corneoscleral topography, with three base-out prism split equally between the two eyes, resulted in binocular fusion and acceptable visual acuity of OD 20/25, OS 20/20. Excessive lens movement along the vertical meridian resulted in air bubble formation and lens awareness (Figure 2). Landing zone customizations focusing on improving scleral lens alignment resulted in greater lens stabilization and comfort.

Figure 2. Air bubbles noted in the tear reservoir after five hours of scleral lens wear secondary to lens movement with blink.
Figure 2. Air bubbles noted in the tear reservoir after five hours of scleral lens wear secondary to lens movement with blink.

Prismatic scleral lenses are a viable treatment option for diplopic patients who wish to be free of overlay glasses. However, both patients and practitioners must understand that the fitting process can be lengthy, and success is not guaranteed. CLS

REFERENCES

  1. Hanks AJ. The watermelon seed principle. Cont lens Forum. 1983;8(9):31-35.
  2. Sulley A, Hawke R, Lorenz KO, Toubouti Y, Olivares G. Resultant vertical prism in toric soft contact lenses. Cont Lens Anterior Eye. 2015 Aug;38:253-257.
  3. Lindsay RG, Crock GW. A bitoric rigid contact lens with base down prism to eliminate binocular vertical diplopia. Clin Exp Optom. 2005 Jan;88:55-57.
  4. Lam H, Zabrowski C, Lee M. Correction of vertical diplopia with soft contact lenses. Eye (Lond). 2022 Mar;36:639-640.
  5. van der Worp E. A Guide to Scleral Lens Fitting (2 ed). Forest Grove, OR: Pacific University; 2015. Available at commons.pacificu.edu/mono/10 . Accessed Sep. 21, 2022.
  6. Vincent SJ, Fadel D. Optical considerations for scleral contact lenses: A review. Cont Lens Anterior Eye. 2019 Dec;42:598-613.
  7. DeNaeyer G. Utilizing Prismatic Scleral Lenses for the Correction of Diplopia. Poster presented at the 2022 Global Specialty Lens Symposium, las Vegas. January 2022. Available at https://ss-usa.s3.amazonaws.com/c/308476694/media/348061ef10fe398fd82881573427447/CaseReport%20-%20Prism.pdf . Accessed Sep. 21, 2022.
  8. Frogozo M. Treatment of Horizontal Diplopia with Prism Correction in Scleral Gas Permeable Prosthetic Device. Poster presented at the 2016 Global Specialty Lens Symposium, Las Vegas. January 2016. Available at sclerallens.com/PDFs/Frogozo.pdf . Accessed Sep. 21, 2022.

Dr. Lee is a clinical assistant professor at the University of Houston College of Optometry. She is also the past president of the Scleral Lens Education Society. She has received travel funding and lecture or authorship honoraria from Alcon and Bausch & Lomb.

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