Research has shown that uncorrected astigmatism as low as 0.75D can lead to nearly a full line drop in acuity under mesopic conditions (Richdale et al, 2007). When multifocal lens candidates present with astigmatism of this degree or more, what’s the best course of action?
Compare Corneal to Spectacle Astigmatism
The first step is to compare the corneal toricity to the vertexed refractive astigmatism. If they are equal, many options can be considered, including corneal and scleral GP multifocals, hybrid multifocals, and toric soft multifocals. If they are not equal, a non-flexing rigid optical system will not properly correct the astigmatism, making a soft toric multifocal option attractive. Madrid-Costa et al (2012) found that soft toric multifocal lenses provided acuity within one or two letters of toric monovision in subjects who had 0.75D to 2.75D of astigmatism, while providing superior binocularity.
When fitting a soft toric multifocal contact lens, three different optical components must be managed: spherical, astigmatic, and presbyopic correction.
Correct Astigmatism Before Troubleshooting Presbyopia
After applying a soft toric multifocal contact lens to the eye, let it settle for at least 10 minutes prior to assessing vision. If vision is not satisfactory, before doing a spherical over-refraction with loose lenses as you would for a spherical soft multifocal, first look at the toric markings on the lenses. If they suggest that the astigmatic power in the lens is not aligned with the axis of the refractive astigmatism, stop and make appropriate adjustments before doing anything else.
If the axes do not align, check for lens rotational stability. Lenses that exhibit large degrees of rotation (30º or greater) are more likely to be unstable (Figure 1). If the lens axis shifts with blinking or with changes in gaze, modify the base curve, diameter, or lens type until you’ve achieved a stable axis position.
If the lens rotates, but does so consistently in the same direction and to the same degree, simply change the axis using the LARS principle (if the lens rotates Left, Add to the lens axis; if the lens rotates Right, Subtract from the lens axis). For example, if the spectacle axis for the right lens is 090 and the lens rotates 10º to the left, add 10º to the 090 degree axis, ordering axis 100 in the contact lens.
After Astigmatism, Probe Presbyopia
Once you are confident that the astigmatism is properly corrected, follow your usual problem-solving strategies for enhancing the performance of any simultaneous vision multifocal contact lens. If patients are able to comfortably read text on their smart phone and see well enough to drive confidently, don’t change anything! Release them for one to two weeks to experience vision in their habitual environment, and give their visual system time to adapt to multifocal optics.
Research shows that both distance and near visual acuity improves with multifocal contact lenses over at least a 15-day period (Fernandes et al, 2013). Conversely, acuity with monovision over the same time period either remains the same or worsens, so time is your friend with multifocals. CLS
For references, please visit www.clspectrum.com/references and click on document #266.