Article

PRESCRIBING FOR ASTIGMATISM

WHEN IRREGULAR ASTIGMATISM BECOMES REGULAR

Patients who have corneal irregularities, including corneal ectasias and transplants, often experience poor vision in standard spectacle or contact lens correction and require GP contact lenses. GPs are able to neutralize the corneal irregularity and provide optimal vision. When fitting both corneal and scleral GP lenses with spherical optics, it is not uncommon to find residual uncorrected astigmatism when performing a sphero-cylindrical over-refraction (SCOR). Practitioners are then tasked with determining whether the residual cylinder should be corrected and how it should be incorporated. This article will discuss methods to correct residual astigmatism and the possible difficulties that may be encountered.

Five Methods to Correct Residual Astigmatism

  1. Corneal Front-Surface (F1) Toric F1 torics are traditionally prescribed for patients who have spherical or minimally toric corneas and refractive cylinder. Since the advent of widely available and reproducible soft toric contact lens designs, front-toric GP lenses are now infrequently used. Utilizing this design is a possibility for those who have corneal irregularities—but lens instability becomes an important factor to consider, as unstable lenses will induce unwanted cylinder correction. Methods of stabilizing the corneal GP lens include prism ballast and lens truncation. Increasing the amount of prism may be beneficial in stabilizing the lens but may lead to lens awareness and inferior lens position.
  2. Corneal Bitoric Bitoric corneal GPs are often utilized when there is regular corneal toricity of at least 2.00D to 2.50D. The back-surface toricity allows the lens to fit more uniformly to the corneal contour, while the front-surface optics allow for additional astigmatic optical correction. In patients who have corneal ectasias such as keratoconus, it is almost guaranteed that there will be at least 2.00D of toricity present. However, the corneal astigmatism in these patients is irregular; thus, back-surface toricity will not necessarily help contour the central fit. In addition, if a cylinder power effect (CPE) design is prescribed, any lens rotation will induce unwanted cylinder correction. However, back-surface-toric designs may be beneficial to provide more uniform edge lift, especially in cases in which there is excessive vertical edge lift and minimal horizontal edge lift.
  3. Piggyback Lens System A soft lens piggyback system is often utilized to help improve patient comfort or to protect the cornea from mechanical irritation. When incorporating a soft contact lens under a corneal GP, approximately 25% of the soft lens power remains. Because standard toric soft lens powers are readily available up to –2.75D, using a piggyback system could correct approximately 0.75D of residual cylinder.
  4. Over-Specs Spectacles are the easiest method to correct over-refraction astigmatism, as there are no concerns regarding toric GP lens cylinder misalignment or rotational instability. It is also a good option for those who require presbyopic correction. Additionally, it provides full-time ocular protection.
  5. Scleral Lenses Most labs offer the option to incorporate front-surface toric optics on their scleral lens designs. The cylinder correction is stabilized either with prism, with slab-off methods, or with toric peripheral curves (haptics) to lock into scleral toricity, if present. When refining the prescription based on the SCOR, it is imperative to note rotation of the lens to accurately incorporate the SCOR.

Give the Best Vision Possible

In many cases, incorporating cylinder correction into GP designs greatly improves a patient’s best-corrected vision. It is important to optimize the lens fit before incorporating cylinder correction, as decentered or flexing lenses may be the cause of cylinder in the over-refraction. CLS