Astigmatism: A Brief Review 

Astigmatism is a common refractive error that is correctable through the use of eyeglasses, contact lenses, or surgery. If we think of the normal cornea as being essentially spherical in shape, when light enters the eye, it is refracted or “bent” evenly in all directions, resulting in a sharp point of focus on the retina. However, if astigmatism is present, the cornea is shaped more like a football or the back of a spoon. So when light enters the eye, it is refracted more in one direction than the other, resulting in blurred vision in at least one of the two principal meridians. Astigmatism can be hereditary and is often present at birth. It can also result from pressure from the eyelids on the cornea or incorrect posture. Untreated, it can cause headaches, fatigue, eyestrain, and blurred vision at all distances. Astigmatism can almost always be corrected with properly prescribed eyeglasses or contact lenses. In cases with only a small amount of astigmatism, corrective lenses may not be needed at all, as long as other conditions such as myopia or hyperopia are not present.

Correcting Astigmatism 

Corrective spectacles or contact lenses containing cylinder are used for the correction of astigmatism. These lenses have greater refractive power in one direction than the other. Another method for correcting astigmatism is by changing the shape of the cornea through refractive or laser eye surgery. While there is more than one type of refractive surgery, specific treatments are recommended on an individual basis. Refractive surgeries require healthy eyes that are free from retinal problems, corneal scars, and any eye disease.

 Regular and Irregular Astigmatism 

Astigmatism may be divided into two broad categories: regular and irregular. Irregular astigmatism is caused by a diseased or damaged cornea, resulting in light scattering in the crystalline lens. Irregular astigmatism cannot be corrected by standard spectacle lenses. However, rigid gas permeable (RGP) contact lenses can help treat irregular astigmatism due to the tear lens, which fills in the irregularities and re-creates a smooth optical surface. There will be more on the tear lens later in this article. Regular astigmatism, which can be caused by either the cornea or crystalline lens, can be corrected by a toric (cylindrical) lens. A toric surface resembles the surface of a football, where there are two curves, one steeper than the other. This optical shape results in regular astigmatism.

 With-the-Rule and Against-the-Rule Astigmatism 

In with-the-rule corneal astigmatism, the cornea is steeper in the vertical meridian and flatter in the horizontal. Imagine holding a football with the tips of the ball held horizontally. The flatter curve is in the horizontal, and the steeper curve is in the vertical. That would represent with-the-rule astigmatism. If the football were held vertically, with the steeper curve in the horizontal and the flatter curve in the vertical, that would represent against-the-rule astigmatism. 

In with-the-rule astigmatism, the eye sees vertical lines sharper than horizontal lines. In against-the-rule astigmatism, the horizontal meridians are sharper. The axis is always recorded as an angle in degrees between 0 and 180 degrees in a counter-clockwise direction. 0 and 180 lie on a horizontal line at the level of the center of the pupil, and as seen by an observer, 0 lies on the right of both eyes.

Choosing the Right Contact Lens for Your Astigmatism

Selecting the appropriate contact lens for astigmatism involves careful consideration of various factors. Today, there are numerous excellent options available for correcting astigmatism with contact lenses, dispelling the misconception that individuals with this condition cannot wear them. 

Factors to Consider

When choosing contact lenses for astigmatism, it’s crucial to consult an eye care professional who specializes in fitting these lenses. They will conduct a thorough contact lens exam to determine the best type of lens for your specific needs. The severity of astigmatism plays a significant role in lens selection. For instance, prescriptions with very low astigmatism can often be corrected with soft spherical contact lenses, while moderate or higher amounts of astigmatism require more specialized lenses.

Soft Toric vs. RGP Lenses

Soft toric lenses and rigid gas permeable (RGP) lenses are two primary options for correcting astigmatism:

1. Soft Toric Lenses:
   • Designed with a toric shape to neutralize astigmatism
   • Comfortable and easy to adapt to
   • Available in various materials and replacement schedules
   • Slightly more expensive than spherical soft lenses
2. RGP Lenses:
   • Can correct moderate levels of astigmatism with a spherical design
   • Provide significant benefits to visual acuity, even for severe astigmatism
   • Require less frequent replacement than soft lenses
   • May be less comfortable initially and require more maintenance

Soft Toric Contact Lenses for the Correction of Astigmatism 

Toric soft lenses have different curves in different meridians to correct astigmatism. Special weighting and/or thin zones in toric lenses keeps the proper lens meridian in front of the corresponding meridian of the eye and prevents the lens from rotating during wear. Because they have a more sophisticated design, toric soft lenses are more expensive to manufacture and cost more than regular soft lenses. Spherical soft contact lenses have the same curve in all meridians and cannot correct astigmatism. 

Rigid Gas Permeable (RGP) Contact Lenses 

As their name implies, rigid gas permeable (RGP) lenses are hard lenses. They maintain their shape on the eye and don’t conform to the unequal contours of an astigmatic eye like soft lenses do.

When an RGP contact lens is worn on an eye that has mild to moderate astigmatism, the space between the (spherical) back surface of the contact lens and the (astigmatic) front surface of the cornea is filled with tears. This layer of tears is shaped in such a way that it becomes, in effect, a lens that fully corrects the astigmatism of the eye. This “tear lens” behind the RGP contact lens is unaffected by lens rotation. 

Therefore, the only lens power required in the contact lens is the power needed to correct any myopia or hyperopia that exists along with the astigmatism. In cases of moderate to severe astigmatism, a standard rigid gas permeable contact lens may be uncomfortable or unstable on the eye because the shape of the cornea is so irregular that the contact lens doesn’t center properly on the eye and moves (or “rocks”) too much during blinks. In these cases, a toric RGP contact lens may be used.

 A toric RGP lens has a spoon-shaped back surface that more closely matches the front surface of an astigmatic eye for a more stable and comfortable fit. This closer fitting relationship also keeps the lens from rotating on the eye, allowing different powers (curves) to be ground on different meridians of a toric RGP lens. The variable powers on the lens surface correct the astigmatism without having to rely solely on the power of the “tear lens” between the cornea and the back surface of the lens. 

The front surface of a toric RGP lens may be spherical or toric, depending on the required prescription. (If both the front and back surfaces are toric, the lens is called a bitoric lens.) Because toric rigid gas permeable lenses have a more sophisticated design, they are more expensive to manufacture and therefore cost more than regular RGP contact lenses. 

Toric contact lenses may be indicated when residual astigmatism results in vision which is compromised. Residual astigmatism may result from a toric posterior corneal surface, a toric crystalline lens, or a partially dislocated crystalline lens. Toric lenses may also be required when moderate to high corneal astigmatism results in either mechanical irritation or centration problems with spherical hard lenses. 

Astigmatic contact lenses may be classified into the following categories and these are applicable to both hard and soft lenses.

1. A) Anterior toric, with spherical back surface
   B) Posterior toric base curves
   C.) Posterior toric only
   2. Bitoric 

Anterior toric with spherical back surfaces: These are available in both hard and soft lenses. They are typically fit when there is a significant amount, generally more than 0.75 D, of uncorrected residual astigmatism. 

The correcting cylinder is put on the front surface of the lens while the diameter and posterior curves remain the same. 

Posterior base curve toric lens: These are available in both hard and soft lens materials, although they are used predominately with hard lenses. They are indicated when a spherical lens does not provide a stable fit and when there is at least 1.50 diopters of corneal astigmatism. They can also be indicated when irritation is experienced due to inadequate clearance of a spherical lens on a corneal cylinder. 

A bitoric lens design is needed when the back toric surface results in enough residual astigmatism so that a front surface correcting cylinder is needed. At least 1.50 diopters of corneal astigmatism should be present. 

When to Consider Specialty Lenses

In some cases, specialty contact lenses may be necessary:

1. Hybrid Lenses: These combine a rigid gas permeable center with a soft lens skirt, offering both comfort and customizable optics. They’re ideal for patients with corneal astigmatism or those who have difficulty achieving optimal vision with soft lenses [2].
2. Scleral Lenses: These larger lenses can be beneficial for patients with severe astigmatism or irregular corneas.[2]
3. Custom-made Lenses: For individuals with high levels of astigmatism or unique eye shapes, custom-made lenses may be the best option.

Ultimately, the choice between different types of contact lenses for astigmatism depends on factors such as the degree of astigmatism, lifestyle needs, and personal comfort preferences. Regular follow-ups with an eye care professional are essential to ensure the chosen lenses continue to provide clear vision and maintain eye health.

Stabilization Techniques for Toric Contact Lenses 

Many of the techniques used for stabilizing toric contact lenses are also used for the stabilization of segment style multifocal lenses. In either case, for obvious reasons, lens rotation on the eye needs to be minimized.

Prism Ballast: This is one of the most common stabilizing techniques. A prism of between 1.00 to 1.50 D is ground base down into the lens. However, greater amounts of prism may be needed for patients with particularly tight lids, flat corneas, or oblique axis astigmatism. The lens will tend to rotate so that the base of the prism is oriented inferiorly. The added thickness of the lens along the prism base can reduce oxygen permeability through that portion of the lens, resulting in possible hypoxic disturbances in the inferior zone of the cornea. 

Truncation: When a lens is truncated, a portion of it is sectioned off. It is usually 0.50 to 1.5 mm on the lower edge of the lens. The amount sectioned off will depend on the size of the lens; larger lenses require greater amounts than smaller lenses. Occasionally, the upper edge is sectioned off as well resulting in a double truncated lens. The truncation will serve to stabilize a lens when the lower flat edge comes to lie adjacent to the lower eyelid margin. Truncation is often combined with prism ballast. When a lens is truncated its diameter is effectively reduced which results in a looser fit. To compensate for this the base curves of truncated lenses are generally made somewhat steeper. 

Double Slab-off lenses: This technique creates a lens which is thicker along its central body which lies along the palpebral fissure and thinner along the inferior and superior edges which come to lie under the upper and lower lids. This technique is comfortable since there is no lid impact along the inferior surface. However it does not offer as much lens stability as the truncated or posterior toric techniques. Double slab-off lenses are often combined with a prism ballast to help prevent rotation. 

Posterior Toric Lenses: A back toric surface can be used as a lens stabilizing technique. When the shape of the posterior contact lens surface closely parallels that of the cornea lens rotation can be minimized. 

Aspheric Lens Surface: An aspheric surface can aid in lens-axis stabilization by adding drag to the motion of the lens. It is generally used in combination with truncation or prism ballast since it is only minimally effective by itself

Toric Reference Markings 

Because it is necessary to inhibit lens rotation, toric lenses will typically come with reference markings so the fitter can determine how the lens is oriented on the cornea. If one were to superimpose the face of a clock over the cornea, these markings, which may be circles or lines, are located at the six-o-clock or three and nine-o-clock meridians. Each hour on the clock represents 30º. So if a lens becomes oriented at five-o-clock, that means it has rotated 30º to the right, while seven-o-clock is 30º to the left. The expression LARS stands for left add, right subtract. If the lens is rotated to the fitter’s left, the appropriate number of degrees is added to the prescribed axis. If it is rotated to the right, the appropriate number of degrees is subtracted. 

Lens rotation can be measured using a slit lamp equipped with a protractor. The use of trial lenses is especially important when fitting toric contact lenses. 

How the Tear Lens Can Correct for Significant Amounts of Corneal Astigmatism 

Let’s first consider a spherical lens on a spherical cornea. 

Figure A below illustrates a rigid contact lens fit “on K” where the base curve of the lens parallels the curvature of the cornea. In this case, the power of the tear lens is plano. When there is corneal astigmatism, a lens fit “on K” parallels the flatter of the two corneal meridians. 

Figure B shows a rigid contact lens that is fit steeper than K. Note that the shape of the resultant tear lens is thicker in the center and thinner on the edges which produces a lacrimal lens with plus power.

The power of the lacrimal or liquid lens must be considered when determining the correct power the contact lens. 

In figure C, the rigid contact lens is fit flatter than K resulting in a tear lens which is thinner in the center and thicker on the edges. The lacrimal lens in this case will contain minus power.

Now let’s do a few examples in calculating lens power when it is fit on K, steeper than K, and flatter than K.

Example 1: 

Spectacle Rx: -3.25 K: 43.50 Vertex distance = 12 mm Using the data above, what would be the power of a rigid the contact lens which is fit on K? 

When a lens is fit “on K” it indicates that its base curve is designed to parallel the flattest corneal meridian. In this case, we are presented with a “spherical” cornea. Therefore, a lens fit on K would contain a base curve of 43.50, which is equal to the curvature of the flattest (in this case only) corneal meridian.

As indicated in figure A in the previous illustration, when the back curve of the contact lens parallels the front curve of the cornea, the power of the tear lens produced is plano or zero. The Rx in this example tells us that a spectacle lens with a power of -3.25 D is needed to correct this person’s myopia. Since the power at the spectacle plane is less than -4.00, it is not necessary to compensate for vertex distance. The total power of the contact lens plus the power of the tear lens needs to equal -3.25. Since the tear lens is plano, the power of the contact lens would be -3.25. 

Example 2:

Using the data above, what is the power of a rigid contact lens fit 0.50 D steeper than K? In this example, a lens fit 0.50 D steeper than K would contain a base curve of 44.00 diopters. Since it is steeper than K, the shape of the tear lens would resemble the one in figure B on the previous illustration, thicker in the center and thinner on the edges thereby creating a liquid lens of plus power. How much plus power? If it is 0.50 D steeper than K, the power of the tear lens is approximately equal to +0.50 D. As previously stated, the total power needed at the corneal plane is -3.25 D, which will be the combined power of the contact lens plus the tear lens. If the tear lens contains a power of +0.50, then the contact lens must contain a power of -3.75. So, -3.75 (contact lens power) combined with +0.50 (tear lens power) results in the desired -3.25 diopters of power at the corneal plane. 

Example 3:

Using the data in example one above what is the power of a rigid contact lens which is fit 0.25 D flatter than K? 

When the lens is fit flatter than K, the shape of the tear lens will resemble figure C in the previous illustration, thinner in the center and thicker on the edges, thus creating a liquid lens containing minus power. How much minus power? When the contact lens is fit 0.25 diopter flatter than K, the power of the tear lens is equal to approximately -0.25 diopter. Since the combined power of both the contact lens and tear lens needs to be -3.25 D, the contact lens would require a power of -3.00. So -3.00 (contact lens power) combined with -0.25 (tear lens power) results in the needed -3.25 diopters at the corneal plane. 

Daily vs. Monthly Replacement Schedules for Astigmatic Lenses

When choosing contact lenses for astigmatism, one crucial decision is selecting between daily and monthly replacement schedules. Each option has its own set of advantages and considerations, which can significantly impact the wearer’s experience and eye health.

Pros and Cons of Daily Lenses

Daily disposable contact lenses offer several benefits for individuals with astigmatism. These lenses provide exceptional convenience, as wearers simply insert a fresh pair each morning and discard them at night.[1] This eliminates the need for cleaning and storage, making them ideal same for those with busy lifestyles or limited experience with contact lenses.[1]

Daily lenses also promote better eye health by reducing the accumulation of deposits on the lens surface.[2] This can be particularly beneficial for individuals with sensitive eyes or those prone to allergies. Additionally, daily lenses are available in various prescriptions, including options for astigmatism and multifocal correction.

However, daily disposables do have some drawbacks. They tend to have a higher initial cost compared to monthly lenses, although this may be offset by the elimination of expenses for cleaning solutions and storage containers.[1] Daily lenses are also thinner, which can make them more susceptible to tearing and require careful handling [1].

Benefits of Monthly Lenses

Monthly replacement contact lenses offer their own set of advantages for astigmatism correction. These lenses are often made from silicone hydrogel materials, which allow for increased oxygen permeability, promoting better eye health and comfort. The more durable construction of monthly lenses can make them suitable for individuals who require higher prescriptions or have more severe astigmatism.

Monthly lenses may be a more cost-effective option for full-time wearers, as fewer lenses are purchased throughout the year. They also generate less waste, which can be an important consideration for environmentally conscious individuals.

Cost Comparison

The cost difference between daily and monthly contact lenses for astigmatism can be significant. Daily disposables typically have a higher upfront cost due to the need for more lenses. However, when considering the total cost of ownership, it’s important to factor in the expenses associated with monthly lenses, such as cleaning solutions and storage cases.

For occasional wearers, daily disposables might be more economical, as they eliminate the need for maintenance products and reduce the risk of lens wastage. Conversely, full-time wearers may find monthly lenses more cost-effective in the long run, despite the additional care requirements.

Advancements in Contact Lens Technology for Astigmatism

New Materials and Designs

Recent advancements in contact lens technology have revolutionized the options available for individuals with astigmatism. Toric contact lenses, designed specifically to address astigmatism, now offer comfort levels comparable to spherical lenses.[1] These lenses feature unique designs that interact with the eyelids to maintain stability during blinking, ensuring consistent visual performance.

One notable innovation is the development of silicone hydrogel materials for toric lenses. For instance, the Biofinity Toric lens utilizes kalifilcon A, which combines 55% moisture content with high breathability (107 Dk/t) and a low modulus for enhanced comfort.[4] This material allows for extended wear times while maintaining eye health.

Digital Stabilization Techniques

Modern toric lenses employ various stabilization methods to prevent rotation and maintain proper alignment with the eye’s astigmatic axis. These techniques include:

1. Prism ballast: A common approach that adds 1.00 to 1.50 D of prism base down into the lens.[1]
2. Truncation: Sectioning off a portion of the lens, usually 0.50 to 1.5 mm on the lower edge.[1]
3. Double slab-off: Creating a thicker central body with thinner edges for comfort and stability.[1]
4. Posterior toric surface: Shaping the back surface to closely match the corneal contour.[1]

Studies have shown that optimized prism-ballast designs, such as those used in the Biofinity Toric, perform exceptionally well. These lenses demonstrate minimal rotation from the vertical position and rapid reorientation after blinking.[2]

Future Innovations

The contact lens industry continues to push boundaries in astigmatism correction. Hybrid lenses, combining rigid gas-permeable centers with soft hydrogel or silicone hydrogel edges, offer a promising solution for those seeking both comfort and visual clarity. These lenses are particularly effective for correcting irregular astigmatism.

Another exciting development is the introduction of daily disposable toric lenses with advanced moisture retention properties. For example, Bausch + Lomb’s INFUSE for Astigmatism lenses incorporate a blend of osmoprotectants, electrolytes, and moisturizers to maintain comfort for up to 16 hours. These lenses also feature ProBalance technology and OpticAlign design, which help reduce spherical aberration and maintain clear vision in various lighting conditions. ]

As research continues, we can expect to see further improvements in lens materials, stabilization techniques, and customization options, making contact lenses an increasingly viable and comfortable solution for individuals with astigmatism.

OTI Take Away Points to Give You an Edge as an Optician

The advancements in contact lens technology have ushered in a new era for individuals with astigmatism. From innovative materials and designs to digital stabilization techniques, these developments have a significant impact on comfort, visual clarity, and overall eye health. The introduction of silicone hydrogel materials, hybrid lenses, and daily disposables with enhanced moisture retention properties offers a wide range of options to suit different needs and preferences.

As the field continues to evolve, the future looks bright for astigmatism correction through contact lenses. These improvements not only enhance the quality of life for wearers but also open up new possibilities for those who previously struggled with traditional lenses. To stay ahead in this rapidly changing landscape, eye care professionals and patients alike should keep themselves informed about the latest developments. 

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FAQs

What is the most effective type of contact lens for correcting astigmatism?
Toric contact lenses are generally the best option for those with astigmatism. They are specially designed with different refractive powers to correct the irregular curvature associated with both corneal and lenticular astigmatism.

Which contact lenses are known for their comfort when dealing with astigmatism?
For those with dry eyes and astigmatism, Acuvue Oasys for Astigmatism is highly recommended. Air Optix plus Hydraglyde for Astigmatism is noted for its comfort, CooperVision Biofinity Toric is recognized for its breathability, and Biotrue ONEday for Astigmatism is favored for its visual clarity.

What challenges do people face when using contact lenses for astigmatism?
Individuals with astigmatism face difficulties using traditional spherical soft contact lenses due to the irregular curvature of their corneas. These lenses can rotate with each blink, causing the vision to shift in and out of focus.

Is there a specific method to apply contact lenses for astigmatism?
While toric contact lenses can generally be applied in the same manner as other contacts, it is beneficial to pay attention to their orientation. This can be managed by using the markings on the lens as a guide and ensuring the weighted or thicker side of the lens is positioned at the bottom.

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