Astigmatism is a refractive error in which light passing through the eyes focuses in different meridians instead of converging at a single point on the retina.1 It is the most common type of refractive error that affects 40% of the adult population and 15% of children.2,3
Some of the common symptoms resulting from this refractive error include blurry vision, strained eyes, sensitivity to light, difficulty seeing at night, and squinting.2
Astigmatism can be regular or irregular and primarily arises from corneal, lenticular, and to a lesser degree, retinal causes. It has been reported that roughly 50% of the population has 0.75D or more of astigmatism,4 however, a 2023 international contact lens prescribing survey noted that toric soft lens fitting accounted for 28% of the fits.5
Understanding the types of astigmatism
Regular astigmatism
The principal meridians of astigmatism, i.e., the steepest and flattest meridians, are perpendicular to each other.
- With-the-rule (WTR) astigmatism: The vertical meridian is steeper than the horizontal meridian.1-3
- Against-The-Rule (ATR) Astigmatism: The horizontal meridian is steeper than the vertical.1-3
- Oblique Astigmatism: The two principal meridians are orthogonal, however, it is in an oblique pattern.1-3
Irregular astigmatism
Irregular astigmatism occurs when the principal meridians are not orthogonal to each other and the power changes non-uniformly across the meridians. This type of astigmatism is difficult to correct with traditional spectacles and depending on the severity, it may be difficult to correct with traditional soft contact lenses. Irregular astigmatism can impact both visual acuity and contrast sensitivity of patients.1,2
Some causes of irregular astigmatism include:6
- Corneal Ectasia: Keratoconus, pellucid marginal degeneration, keratoglobus, and Terrien marginal degeneration
- Other Degenerations: Salzmann nodular degeneration, pterygium
- Corneal Damage: Traumatic corneal pathologies and scarring
- Ocular Surgery: Penetrating keratoplasty, post-laser assisted in-situ keratomileusis (LASIK) ectasia, endothelial keratoplasty
- Ocular surface disease: Dry eyes
5 steps to fitting contact lenses for astigmatism
1. Perform a comprehensive eye exam
Refraction
The first step in managing astigmatism is a thorough refraction to determine the magnitude and axis of astigmatism. When fitting contact lenses, we all know that for moderate to high astigmatic patients, correcting the cylindrical prescription is crucial to improving patients’ visual outcomes.
However, when patients have low astigmatism, some may question if the astigmatism should be corrected with a toric contact lens or if a spherical equivalent powered contact lens would be sufficient.
There have been studies that have demonstrated that 70% of patients with low astigmatism (0.75DC) preferred and visually benefited from toric contact lens correction.7 Toric lenses helped increase visual acuity by 3 to 5.5 letters in patients with low astigmatism in comparison to soft spherical lenses.7,8
2. Choose the right soft contact lens
Soft toric contact lenses are a great option for the correction of regular astigmatism. Today, there is a wide range of available parameters that can allow for patients to be fitted with soft torics successfully with good visual outcomes. With the availability of extended parameter ranges of astigmatism, there is tremendous potential for successful fits in this modality.
Ideal candidates for soft toric lenses include:11
- Patients with regular astigmatism that ranges anywhere between 0.75D and higher
- Patients with lenticular astigmatism, i.e., cases where patients have spherical keratometric values but present with refractive astigmatism secondary to the lens. They will benefit more from a soft toric lens in comparison to a corneal gas permeable lens
Patient preferences
Having a discussion with patients about frequency of use, previous lens modality, and ocular concerns such as environmental allergies can also help guide decision-making for lens replacement modality.
Patients with an active lifestyle or in sports may benefit from daily disposable lenses which offer greater hygiene and comfort. Similarly, presbyopic patients may benefit from toric multifocal lenses to help address both distance and near vision needs with contact lenses. As we know, currently toric multifocal lenses are limited to monthly replacement modality.
Contact lens parameters
Today, we have a wide selection of soft toric contact lenses available to us, however, not all fit the same. Emphasis should be placed in understanding how we can utilize different brands and their parameter ranges to manipulate the fit to optimize contact lens fitting and reduce lens changes.
As the prescription becomes stronger, the options for daily and biweekly modalities become limited. With astigmatic prescriptions 3.00D or higher, monthly contact lenses are the only viable soft contact lens option to trial with the patient. In such cases, replacement schedule, daily disinfection, and solution compatibility should be discussed at length with the patient.
Contact lens replacement modalities
Several factors, such as refractive error, replacement modality, lifestyle, and patient preferences, must be considered when determining the optimal lens replacement modality.
Understanding contact lens designs
Different manufacturers use different techniques to achieve rotational stability. The most common designs include prism ballast and dynamic stabilization/double thin zone.
- Prism-Ballast Design: This traditional lens design consists of a thin superior portion and a thicker base down prism in the inferior portion of the lens. This results in vertical prism being introduced in the optic zone of the lens.14
- Normally, if the vertical prism is equal in both eyes, it does not result in any visual concern. However, when patients have a toric lens in one eye and a spherical or multifocal in the other, vertical disparity can result causing symptoms of tension in the forehead, fatigue, double vision, and vertigo.14-16
- Peri-Ballast Design: This design aims to overcome the challenge of vertical prism by positioning the prism in the periphery of the lens. This avoids issues of visual disturbances resulting from prism and offers enhanced comfort for patients.14
Contact lens designs by brands
The majority of brands utilize an iteration of prism-ballast or peri-ballast designs, with the exception of Johnson & Johnson, which employs the double-thin-zone-like design highlighted below.
- Precision Balance 8|4: This design, as seen in Alcon contact lenses, implements a wide optic zone and thickened lens portions at 8 o’clock and 4 o’clock locations to help with stabilization.14
- Optimized Toric Lens Geometry: CooperVision utilizes this design comprised of wide ballast areas adjacent to the optical zone and uniform horizontal thickness that provides stability by minimizing rotation by blinking.14
- OpticAlign and Peri-Ballast Design: This design incorporates OpticAlign in its ultra series, with ballasting in the lower hemisphere of the lens. Whereas the Bio-True series of lenses uses a peri-ballast design to stabilize the lens.14
- Blink Stabilized Design: This design, utilized by Johnson & Johnson, uses four active zones of increased thickness in the mid-periphery of the lens and dual thin zones superiorly and inferiorly.
- It allows the lens to be rotated into place with blinking and it stays stable in place, even with changes in head position. Since this design does not rely on prism ballasting, there is no vertical prism in the optic zone.14
Table 1 lists the vertical prism in soft contact lenses.17
| Soft Toric Contact Lens | Stabilization Design | Mean Vertical Prism △ |
|---|---|---|
| ACUVUE OASYS for Astigamtism | Accelerated Stabilization Design | 0.01 |
| Air Optix for Astigmatism | Modified peri-ballast design | 0.52 |
| Biofinity Toric | Peri-ballast design | 0.77 |
| Clariti Toric | Prism-ballast design | 0.85 |
| PureVision 2 for Astigmatism | Modified peri-ballast design | 0.96 |
Table 1: Courtesy of Sulley et al.
All the designs work well and provide good rotational stability for most patients. Despite the presence of vertical prism, it has been shown that prism induced by prism-ballasted designs is tolerated well and does not interfere with the binocular system.18 However, caution is advised when working with patients with vertical phoria decompensation or vertical vergence issues.18
Tip: During the course of toric lens fitting, if you encounter issues with poor stability with one stabilisation design, try switching the patient to a different lens design.
3. Ensure proper fitting of the soft toric contact lens
Fitting soft toric contact lenses
After choosing a lens, we need to adjust the contact lens prescription for the vertex distance and find a trial lens to assess fit. Following application of the lens, have the wait for a few minutes to allow the lens to settle prior to assessing the lens fit.
After vertex conversion of the powers, if the exact power for the toric lens is not available, start with lower astigmatism power and then increase power if visual outcomes are affected. For example, after vertex conversion if the astigmatism power needed is -1.50 and it is not available, consider trialing -1.25 astigmatism and assessing visual outcomes. If the visual outcome is affected, then trial a stronger power, such as -1.75.
When evaluating a lens fit, it is important to ensure that the alignment of the toric markers is appropriate. As the lens rotates 10°, about ⅓ of the cylinder power remains uncorrected.19,20 This amount of uncorrected astigmatism increases with the rotation.19,20
Troubleshooting soft toric contact lenses
If you are experiencing challenges with the poor performance of a soft toric lens, the following steps can be adopted to improve visual outcomes.
Axis misalignment
Axis misalignment can result in reduced acuity and poor visual performance. In such cases, it is important to ensure the stability of the lens prior to making any lens changes. In cases of stable axis misalignment, use the LARS rule to modify the cylindrical axis.
For example, if the lens is rotated 10° to the left, add 10 to the refractive axis. With the new lens, the lens will still be rotated 10° to the left, however, it changes the optical orientation to correct vision.8,21
In cases where a patient has gone through multiple brands of toric lenses yet still complains of fluctuating vision, consider selecting a lens brand with a larger diameter and steeper base curve of the lens (i.e., increased sagittal depth of the lens) to achieve stability on the eye.8,21
Incorrect sphere or cylinder power
If a patient isn’t happy with their vision, it is always advisable to perform a sphero-cylinder over-refraction. If the spherical equivalent is plano, then the toric contact lens power is most likely correct.
However, in cases where the spherical equivalent is not zero, pay close attention to the axis of overrefraction:21
- If the axis of over-refraction matches the axis of the toric lens, it suggests that the patient needs increased astigmatism in the toric lens.
- If the axis of the over-refraction is 90° away from the axis of the toric lens, it suggests that the toric lens is overcorrected for astigmatism.
- If the axis of the over-refraction is at an oblique angle, it suggests axis misalignment of the lens.
4. Educate the patient
Topics to discuss with astigmatic patients after fitting a contact lens include:
- Adaptation Period: Discuss the potential adaptation period and vision fluctuations with blinking.
- Regular Follow-Ups: Schedule follow-up appointments to monitor lens fit, vision, and overall eye health.
5. Monitor patients on an ongoing basis
Regularly monitor the patient's vision and comfort. Adjustments may be necessary, especially during the initial adaptation period.
Follow-up appointments are crucial in catching such concerns early on to prevent contact lens drop-out and optimize outcomes for the patient.
Conclusions
Today, we have a wide range of toric lenses that we can offer to our patients to improve their visual outcomes.
Prior to initiating contact lens fitting, it is important to understand patients’ visual goals, lifestyle, and ocular health. All these factors are crucial in tailoring an appropriate lens fitting, ensuring optimal visual goals and overall ocular health outcomes.
Risk of infections, hypoxia, and neovascularization are always a concern when fitting a contact lens, however, with the appropriate selection of lens, we can reduce the risk of such complications and ensure safety.
