High Astigmatism May Present Unique Myopia Control Challenges

High astigmatism creates a unique set of myopia control challenges for optometry — a specialty that continues to experience an evolving myopia management paradigm. Myopia prevalence has increased markedly in recent years, and fewer clinicians view the disorder as a simple refractive error. Many have begun adding myopia management strategies to their clinical practice and using various treatments, including atropine, contact lenses, and others to slow myopia progression in their pediatric patients. While correcting refractive error continues to remain a priority, it is important for clinicians to communicate the risks associated with high myopia to their patients and caretakers and to emphasize the importance of slowing its progression. 

Astigmatic Considerations

As more optometrists add myopia management therapies to their clinical practice, they face more challenging and complex cases — one of which includes concomitant astigmatism. Approximately 28% of children in the US have astigmatism, with a higher prevalence noted among children of Hispanic and Asian descent.¹ The prevalence of astigmatism in individuals with myopia is almost double compared with patients with hyperopia.² With increasing myopia, there is often an accompanying increase in astigmatism, and with-the-rule astigmatism is predominant among individuals with moderate to high myopia.^3,4

Astigmatism type and magnitude may change as a child grows. A higher degree of corneal astigmatism is often present in early childhood, with against-the-rule astigmatism being more common. As a child approaches their school-age years, the cornea flattens, reducing the amount of corneal astigmatism and making with-the-rule astigmatism more common.⁵ Nevertheless, a higher amount of astigmatism is likely to persist into adulthood, presenting contact lens fitting challenges, particularly with respect to orthokeratology and soft multifocal lenses.

Current Myopia Management Therapies

Current therapies for myopia management include atropine, orthokeratology, and peripheral defocus soft contact lenses. While all of these therapies are viable options for children with spherical myopia, children with high astigmatism have fewer options. 

Low-dose atropine, instilled on a nightly basis, may be one such option. Unlike other therapies, this treatment method has no refractive limitations since it does not provide visual correction. While most children may be candidates for this therapy, they also require visual correction with spectacles or contact lenses. 

Orthokeratology contact lenses are approved by the US Food and Drug Administration (FDA) for myopia correction of 6.00 diopters (D) or less and astigmatism correction up to 1.75 D. As technology improves, many orthokeratology lens manufacturers are offering more customizable parameters to maximize potential efficacy for slowing myopia progression and axial elongation. 

Toric periphery orthokeratology lenses are now available through most manufacturers. These designs include reverse curves or peripheral and alignment curves that can be made toric. Adding toricity to these lenses does not correct astigmatism; rather, it improves lens centration. A properly centered lens will ensure flattening occurs in the proper place and provide better visual outcomes. However, it is not uncommon for individuals with high astigmatism to have residual astigmatism after toric orthokeratology lens treatment. Software programs may allow clinicians to customize an orthokeratology lens, enabling them to design a lens with a toric base curve. 

Optometrists who attempt to control myopia in patients with high astigmatism may be met with challenges when implementing strategies involving soft contact lenses. While there are several different types of peripheral defocus soft contact lenses, few of them offer toric multifocal options. There are 2 commercially-available, monthly replacement soft toric multifocal lenses with an aspheric center distance design that can correct up to 5.75 D of astigmatism. Quarterly replacement custom soft toric multifocal lens options are available, but they  are made of a hydrogel material with lower oxygen transmissibility. No daily disposable peripheral defocus contact lenses that offer astigmatic correction are currently available in the US.

Efficacy of Astigmatic Myopia Management Therapies 

Research has failed to substantiate the efficacy of myopia management therapies for patients with higher amounts of astigmatism. According to a clinical summary of myopia control trials by the International Myopia Institute, the astigmatic cutoff for participants was frequently set between 1.00 and 1.50 D.⁶ The investigation further suggests that an ideal myopia control study would follow a refractive limit of 1.00 D or less of astigmatism for inclusion criteria. 

Only a handful of studies have investigated the efficacy of toric orthokeratology lenses for slowing myopia progression. However, the limited available research suggests that these lenses can slow axial elongation, and may outperform spectacle lenses and spherical periphery orthokeratology lenses.^7,8,9,10 No known studies have explored the use of soft toric multifocals for slowing myopia progression and axial elongation in children. Based on the limited research, clinicians can only assume that toric orthokeratology and soft toric multifocal lenses provide the same myopia management efficacy as the proven non toric versions.

However, one prospective, randomized, crossover study did assess vision among 30 adult participants without presbyopia who were treated with toric orthokeratology and soft toric multifocal lenses. Researchers randomly assigned study participants with moderate to high astigmatism to 10-day treatment with 1 of the 2 lenses before undergoing a washout period and crossing over to the other treatment. Although the study was conducted using adult participants, higher-order aberration and peripheral defocus measurements estimate the potential of both lenses to slow myopia progression. A comparison of the 2 lens types revealed that toric orthokeratology induced a greater amount of higher-order aberrations and more peripheral myopic defocus compared with soft toric multifocal lenses.^11,12 

The researchers also evaluated patient-reported outcomes and high and low contrast visual acuity and found that acuity was reduced with toric orthokeratology compared with soft toric multifocal lenses.¹³ However, participants preferred the toric orthokeratology lenses for vision and overall use.¹³ 

Comparative investigations such as these show that myopia management strategies for individuals with moderate to high astigmatism may improve visual acuity, but with known caveats, which include induced higher-order aberrations and reduced low contrast acuity. They may also confirm toric orthokeratology as the better therapy for these patients if greater higher-order aberrations and more peripheral myopic defocus are the drivers for slowing myopia progression.

Managing Children with Astigmatism

When managing children with moderate to high astigmatism, the amount of astigmatism, its orientation (with-the-rule vs against-the-rule), and corneal location (apical vs limbus-to-limbus) must be considered. Atropine or soft toric multifocal lens treatment may mitigate some of the limitations these considerations present, but poor oxygen transmissibility and infrequent lens replacement are limitations themselves for soft toric multifocal lens wear. 

For pediatric patients with corneal astigmatism exceeding 1.75 D, optometrists may want to proceed cautiously and set reasonable visual expectations for the patient and parent. Individuals who have with-the-rule astigmatism and apical astigmatism may experience better outcomes compared with patients with against-the-rule and limbus-to-limbus astigmatism, respectively.

Previously, clinicians assumed that measuring corneal elevation at an 8 mm chord length was necessary to determine how much toricity the reverse curve of an orthokeratology lens needed. Since the reverse curve lands on the cornea at approximately 8 mm, a difference in elevation would reflect the amount of toricity needed. However, most corneal astigmatism extends from limbus to limbus, so measuring central corneal astigmatism may be sufficient. Studies have shown a strong relationship between central corneal astigmatism and corneal elevation difference measured at an 8 mm chord length.^14,15 Following the manufacturer’s nomogram or fitting guide may be the best strategy for determining reverse curve toricity, as many have adopted the use of central corneal astigmatism. 

With a prevalence of approximately 28% among children in the US, clinicians can expect to encounter pediatric patients with astigmatism in their practice.¹ And while fewer therapies are available for these patients, treatment options for slowing myopia progression do exist and can be implemented to improve patient outcomes.