Read the sidebars: How to Calculate the Final Power
Make a Preemptive Strike
The trouble we often encounter with soft toric contact lenses lies not so much in fitting the patient, but resolving her subjective complaints and/or clinical concerns afterward. We must assume the role of problem solver.
The most common problems associated with toric lenses are usually related to comfort and vision. Problems with visual acuity usually are constant or variable perceptions of vision. These occur due to improper lens positioning or errors in calculating the required correction. In some instances the solution is simple; in others it requires a bit of persistence on your part.
It’s best to problem solve or troubleshoot before the trouble even presents. Here are some of the problems you might encounter that could affect your astigmatic patient’s comfort and visual acuity in contact lenses.
Lens Awareness
Patients’ complaints about lens discomfort are primarily associated with increased lens mass and thickness profile. If the patient experiences lens awareness, you may need to refit her into a thinner lens. If discomfort due to lens deposits or defects are the root of the patient’s discomfort, inspect the lenses to verify their presence and fit a new lens if necessary.
Rotational Stability
After you insert the lenses and at follow-up visits, ask the patient if her vision remains stable or is variable throughout the day, or is constantly blurred.
Variable vision sometimes occurs when a lens is so flat and loose that the lid induces an unwanted lens rotation. These lid interactions may cause the lens to rotate nasally upon closure, particularly with nearpoint viewing, and temporally upon lid opening. Try steepening the base curve to correct this problem.
Constant blur may imply a constant misalignment of the lens axis. If the lens is misaligned, place it into its proper position manually, then monitor it for misalignment after the patient blinks. If it misaligns, the lens is flat. You need to fit a steeper base curve. If it stays in position, instruct the patient to manually align the lens after insertion.
If the patient experiences constant blur, the lens may be too tight. This, in effect, locks the lens out of proper position. Also monitor the lens for these clinical signs of steepness: conjunctival congestion and injection, difficult lens removal, or induced apical corneal distortions due to vaulting. If the lens appears to rotate without proper reorientation, you may need to flatten the base curve. This would allow for an increased freedom of movement. 1
To rule out basic rotational problems, simply observe a trial lens on the eye. To compensate for minor lens rotation (within 15 of an otherwise well-fitting lens), first determine the amount and direction of rotation. Here’s where the LARS method (left add, right subtract) comes in. If the trial lens rotates to your left, add the amount of rotation to the spectacle axis. If the lens rotates to your right, subtract it. I cannot overemphasize the importance of documenting the rotational characteristics of the lens when determining what axis to order.
The use of a slit lamp reticule or slit beam rotation gauge is very helpful to accomplish this. Also keep in mind that each clock hour of rotation represents 30 when estimating rotation. Realize, though, that LARS is an estimated compensation and may, in fact, induce a definite error. That’s why I rely on spherocylindrical over-refraction.
Spherocylindrical Over-refraction
Factors other than rotation can affect visual performance of contact lenses. Uncorrected refractive error—sphere, cylinder and/or axis—is one. First, check the order and verify that the lens parameters are accurate.
Also check for misalignment of the axis. It’s also possible that you have the correct lens powers and axis, but that the axis is still misaligned. In any case, perform a spherocylindrical over-refraction (SCOR), which will disclose many of the other contributing effects besides lens rotation. Change the lens accordingly.
Three possible results you might get from your SCOR:
- The axis equals the refractive axis. If this is the case, the power is incorrect. Recheck the manifest refraction and the lens, and adjust the power accordingly. If you don’t have the necessary powers in a soft design, you can get them in a custom RGP bitoric lens.
- The axis is 90 away from the refraction axis. This implies that the cylinder power is too high. Decrease the lens cylinder power.
- The axis is an oblique refractive axis. This result is more likely than the prior two. Choose between three methods to determine the final contact lens power.
Dryness and Deposits
Lens dehydration may cause both comfort and visual problems as the lens vaults over the cornea or locks out of position. Use a thicker/ prism-ballast lens, lower water content material.
Instruct the patient to use wetting drops tid to qid. These will add to his or her visual and physical comfort.
Also, surface dryness disturbs vision. As with lens dehydration, prescribe wetting drops tid to qid. It also may imply that the lens has a variable hydration level. There are obvious and simple remedies for both situations.
If increasing visual problems over time are the result of lens deposits, you may need to reinforce proper contact lens care. Also consider having the patient replace his or her lenses more frequently, and perhaps switch the patient to a different material.
To avoid or solve problems when fitting toric lenses, listen to the patient’s needs. Also, be creative and versatile with various toric lens designs.
The trouble many doctors have with torics arise because they do not address the observed fitting relationship of the lens on the eye, nor do they properly deal with SCOR. With the many new designs for frequent replacement and disposable torics, the fitting of these lenses has been greatly simplified, thus fostering a heightened success rate of fitting low to moderate levels of astigmatism.
Make a Preemptive Strike
The best time to begin troubleshooting soft torics is before you even fit the patient. This means taking a careful history to determine your patient’s needs and to identify any potential problems that could cause discomfort or poor visual quality. Also, you may need to be creative and consider many options rather than limit yourself to using one or two lens designs. To troubleshoot problems before they begin, you’ll need to answer these four questions:
- What are the patient’s everyday needs? This will determine whether you recommend a soft or rigid material, and whether you recommend daily, flexible or extended wear.
- Is the astigmatism corneal, lenticular or a combination? If it’s a combination, how much lenticular vs. corneal astigmatism is there? The answer will help determine which lens design would best suit that patient.
To determine what portion of the patient’s astigmatism is lenticular vs. corneal, simply subtract the corneal cylinder from the manifest refraction. In other words, a patient whose manifest refraction is –2.00 –3.00 x 180, and whose K readings are 44@90 x 46@180 (2.00D of corneal astigmatism), has 1.00D of residual (lenticular) astigmatism.
This patient would have limited success with a soft toric and would probably benefit more from a bitoric RGP. A general rule of thumb: The more potential residual astigmatism, the more particular your choice of lens design must be. - What does the topography look like? Does the patient have regular or irregular astigmatism? Should she get a front, back, eccentrically lenticulated, prism-ballast, peri-ballast, double thin zone, hydrogel or rigid lens design. For example, the figures on the previous page show one patient with pure astigmatism and another with keratoconus. While both patients have astigmatism, there is a very big difference.
- How sensitive is the patient to cylinder changes? Perform a spherocylindrical over-refraction to determine the patient’s sensitivity to full cylindrical correction. This can help you predict how sensitive the patient is to axis rotation and whether the patient will have difficulty with toric lenses.
- Place the full manifest refraction in a trial frame, isolate the patient’s best line of acuity, then slowly rotate the cylinder axis until the patient notes a change in visual quality (the “just noticeable difference” or JND). Note the amount of rotation, return to the original axis and repeat this test in the opposite direction.
- Return to the original refraction, and then reduce the cylinder in 0.25D steps until you note a JND in visual quality. Reduce the trial frame cylinder and repeat.2
To determine cylinder sensitivity:
Soft torics are not always the best option. If the patient is very sensitive to the cylinder power or misalignment of axis, consider an RGP instead. If the patient is not sensitive to cylinder power changes or axis misalignment, then soft torics may be the best first option. But at least you’ve taken a preemptive strike.—K.D.
How to Calculate the Final Power
Here are three methods you can use to determine the final power for the contact lens when there is a significant spherocylindrical over-refraction. Each assumes that the lens is oriented correctly on the eye.
- Empirical (balance) method. This three-step method compares the known lens correction on the eye to the SCOR. It’s similar to balancing a scale in that it only gives you an estimate, not an exact calculation of the final required power.
Suppose, for example, you have an Rx of –1.00 –2.50 x 090 and an over-refraction of +0.50 –0.75 x 070. Here’s how you would determine the new prescription:- Sphere. Add the sphere of the over-refraction to that of the original Rx, for a final sphere power of –0.50.
- Cylinder. Add the over-refraction cylinder to the prescription cylinder. Final cylinder is –3.25. Keep in mind that the weight of the –0.75D cylindrical overcorrection is 30% of the original Rx.
- Axis. First, calculate the difference between the two axes, which in this example would be 20. Adjust this difference by the same percentage as the cylinder (30%), and you end up with 6. Subtract this from the original axis of 90 for a final axis of 84.
- Trial lens method. This one is both simple and quick. Simply place the known contact lens power into a trial frame. Place a trial lens that equals the spherocylindrical over-refraction onto the same trial frame. Place the trial frame on the lensometer and read the resultant power.
- Oblique cross cylinder. This method requires a complicated calculation that accounts for the spherocylindrical power and its axis with respect to the contact lens power. It’s based upon vector analysis, a complicated trigonometric formula. 3,4
Your vendors should be able to supply you with these, either in a software program or as a preprogrammed calculator.—K.D.
Dr. Daniels is in private practice in Hopewell, N.J, and is an adjunct assistant clinical professor at Pennsylvania College of Optometry. He is also a National Eye Institute investigator for the Collaborative Longitudinal Evaluation of Keratoconus (CLEK) Study.
