Multifocal GP lenses can provide crisp vision for presbyopes who have corneal astigmatism. The toric lacrimal lens created by a GP lens corrects all of the corneal astigmatism, and then it’s just a matter of optimizing the multifocal optics to provide good vision at distance and near.
However, many patients are unable to adapt to GP lenses. Also, some patients have internal astigmatism that the lacrimal lens does not correct. And, even though internal astigmatism can be corrected with front-surface toric optics, the prism required to stabilize lens rotation can ruin proper lens position and visual performance. So, for many presbyopic astigmatic patients, contact lenses may not initially seem like a good option. But for motivated patients, soft toric multifocals can be a great solution.
Most soft toric multifocal lenses are available only as custom designs. One monthly replacement lens is available, but it is still a custom order. With the need for precise correction of the astigmatism plus the sensitive optical properties of simultaneous multifocal designs, it is an advantage to have as many design options as possible.
Center-Near or -Distance?
Some soft toric multifocals, including the aforementioned monthly, are available in both center-near and center-distance designs. Generally, center-distance designs provide better distance acuity. But, due to pupil size changes with accommodation, most soft multifocal designs are center-near. Sometimes, the best option is to fit one of each to enhance distance vision in one eye and near vision in the other.
Do Base Curve and Diameter Matter?
Disposable multifocal designs are generally available in only one base curve/diameter combination, which works fine for the majority of patients. Still, base curve and diameter choices can influence lens position, movement, comfort, and corneal draping; a problem with any of these factors can sabotage the fitting process. This is especially true for those patients who are at the edge of the bell curve for eye shape and size. Measuring corneal curvature and horizontal visible iris diameter (HVID) are important in determining the best starting point for these parameters and will make the fitting process more efficient.
What Add Power?
Choose the add power to provide the best balance between distance and near vision. The add power has to be strong enough to provide good near vision. Note: the higher the add power, the more distance vision is compromised.
Sometimes it helps to give your patient unequal add powers to enhance distance vision in one eye and near vision in the other. Having a full range of available add powers allows for flexibility in optimizing vision.
Adjusting Zone Sizes?
The relationship between a patient’s pupil size and the diameter of the various power zones can affect vision significantly. Careful pupil measurement prior to fitting is very important in selecting the initial zone diameters. Then, based on the patient’s visual symptoms, the zone sizes can be increased or decreased accordingly. As with the add power, zone sizes can be different between eyes to enhance distance and/or near vision.
One final critical parameter is the cylinder. Without accurate cylinder power and axis, vision cannot be optimal at any distance. Careful evaluation of lens rotation and sphero-cylindrical over-refraction help to determine the best overall power. Smooth draping can be evaluated by performing keratometry or corneal topography over the lens. Adjusting base curve and power to provide smooth draping is important. Increasing prism or other stabilizing mechanism also can help improve rotational accuracy to ensure that the cylinder correction is consistent.
These factors may make soft toric multifocal contact lens fitting sound challenging, but a little experience can turn it into a rewarding option for your astigmatic presbyopes. CLS