Prescribing for Astigmatism

Bitoric GP Troubleshooting Tips

By Pam Satjawatcharaphong, OD, and Tim Edrington, OD, MS

Often, bitoric lenses designed empirically fit patients well, and they are ecstatic about the clarity of vision through their new contact lenses. It is likely, however, that the first pair of ordered lenses will need to be adjusted to fine-tune vision and create the ideal fit. In this column, we review some common issues that arise while fitting bitoric GP lenses and how to address them.

Over-refraction (OR)

With bitoric GP fitting, it is best to perform a spherocylinder OR and modify the lens powers as shown in Table 1.

Fluorescein Pattern

When examining central and peripheral fitting relationships, consider the following:

1. Central touch: If noted, steepen the base curves of both meridians as indicated by the fluorescein pattern and increase minus power in both meridians accordingly.

2. Central clearance: If noted, flatten the base curves of both meridians as indicated by the fluorescein pattern and increase plus power in both meridians accordingly.

3. Residual tear lens toricity: If the fluorescein pattern shows excessive toricity (Figure 1), adjust the difference between the two base curves, and adjust the powers of both meridians using steeper add minus-flatter add plus (SAM-FAP).

4. Secondary curve toricity: It is not uncommon to observe areas of minimal and excessive peripheral clearance in the same lens (e.g., too thin at 3 o'clock and 9 o'clock and too thick at 6 o'clock and 12 o'clock), which may result in 3-9 staining. In these cases, instruct your lab to make the peripheral curve toric by the same amount as the base curve toricity.
These steps should help simplify your troubleshooting efforts if fitting problems arise with your initial bitoric lenses. CLS

TABLE 1 | Modifying Lens Power From OR Example 1: If your OR is spherical, simply add the power found to both meridians as follows: Note: If you find a low amount of cylinder (0.25DC to 0.50DC) in the OR, add the spherical equivalent to both meridians. Example 2: If you find a significant amount of cylinder (≥0.75DC), use optical crosses to determine the new lens powers as follows: Note: The resultant lens is a cylindrical power effect (CPE) bitoric, where the difference between base curves ≠ the difference between powers, and lens rotation may adversely affect vision.

Figure 1. Bitoric contact lens with residual with-the-rule toricity. Photo courtesy of Annie Chang, OD, and Dawn Lam, OD.

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Dr. Satjawatcharaphong is the cornea and contact lens resident at the Southern California College of Optometry (SCCO). Dr. Edrington is a professor at SCCO. He has worked as an advisor to B+L. You can reach him at tedrington@scco.edu.