Astigmatism Contact Lenses - Hydrogel contact lenses for astigmatism

Soft contact lenses (CLs) were once difficult to fit for astigmatic eyes because every toric CL was unique and fit differently with every lens. However, technology has dramatically improved, allowing for consistent fit and a myriad of CLs. A list of CLs is available in Tyler's Quarterly.

Diagram of a contact lens.
Diagram of a contact lens.

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Diagram of a contact lens.

Diagram of a contact lens.

Soft toric CLs can be fit empirically from the patient's subjective refraction. For prescriptions greater than +4.00 D, the vertex power is used to select the initial lens. Markings on the CLs enable axis alignment for astigmatic orientation. Markings differ and vary among manufacturers. For example, the marking may be a single dot at the bottom of the lens or 3 slashes (1 at the bottom of the lens, 1 at the 3-o'clock position, and 1 at the 9-o'clock position).

For CLs that do not align themselves properly on the eye, the CLs can be aligned in 1 of 2 ways, as follows:

1. One method is left add, right subtract (LARS). If the lens is rotated to the left by an estimated 10°, add 10° to the axis of the new lens. Likewise, if the lens is rotated to the right (from the practitioner's point of view), subtract the estimated degree of rotation from the original axis.
2. Overrefraction-correction toric calculators are available online free of charge. By knowing the patient's original subjective refraction, the current CL parameters, and the overrefraction, the calculators determine the new CL parameters for the practitioner.

Rigid gas-permeable contact lenses for astigmatism

For corneas with low toricity, spherical, rigid gas-permeable (RGP) lenses can often mask the astigmatism. If the spherical RGP lens fits well on the cornea but astigmatism persists, an anterior toric RGP lens is appropriate because it has a spherical base curve with a toric front surface to correct the astigmatism. Prism is often added to the lens to maintain orientation of the lens.

For corneas with large amounts of toricity not correctable with spherical RGP lenses, back toric RGP lenses are used. The back of the lens has 2 curves, 90° apart, to saddle the toric cornea. The front of the RGP lens has a spherical central zone. Back toric RGP lenses can also be used on irregular corneas, such as those observed in Terrien marginal degeneration or pellucid marginal degeneration, to improve centration of the RGP lens.

When residual astigmatism persists over a back toric RGP lens, a bitoric RGP lens can be used. The back surface of this lens is similar to that of a back toric RGP lens, and it has 2 curves to stabilize the RGP lens on the toric cornea. This lens also has 2 curves on the front to correct for the residual astigmatism.

The Mandell-Moore bitoric fitting guide can be used to determine the appropriate base curves and CL powers. It relies on accurate and refined refractions and keratometric readings. Trial fittings in the office with diagnostic RGP lenses often result in the most accurate orders for RGP lenses.
Presbyopic Contact Lenses

CL options for patients with presbyopia include distance CLs with glasses for near reading, CLs for near vision with spectacles for distance vision, monovision CLs, RGP bifocal CLs, soft bifocal CLs, and modified monovision CLs.1

Because patients with presbyopia are middle aged or older, they may have reduced tear quantity and quality, reduced eyelid tonicity, and small pupils. These characteristics can be relative contraindications for CL wear; however, with proper prescreening, appropriate lens selection, and patient education regarding expectations, most patients with presbyopia can be successfully fit with CLs.

Ideal patients are highly motivated to wear CLs, they understand the limitations in correcting presbyopia with CLs (ie, vision will not be perfect for all distances), they do not have an exceptionally small or large pupil, and they are willing to make several office visits to find the best fit.

Distance CLs with the use of glasses for near reading provide the patient with the best binocularity at all distances. The disadvantages include the need for the patient to put on glasses for reading.

Near-vision CLs with distance-vision spectacles can provide a patient with excellent binocularity. Disadvantages are similar to that mentioned above, with a need to put on glasses for distance viewing. In addition, the eyes may become dry when the patient reads for a prolonged time, especially with CL wear.

Monovision involves fitting 1 eye for distance vision and 1 eye for near vision. Any CL modality (eg, soft CLs, RGP lenses) can be used. Reported success rates vary, but the best candidates for monovision correction with soft CLs are patients with minimal astigmatism. For large amounts of astigmatism, RGP or soft toric CLs should be considered. Patients should be educated that monovision can affect binocularity and cause intermittent blurriness because of the inability to suppress the out-of-focus eye. For patients who drive, the prescription of distance-correcting spectacles to provide binocularity can be important for medicolegal purposes. For high levels of presbyopia, explaining to patients that monovision may not provide an adequate intermediate working distance is also recommended.

Modified monovision involves either the combination of a bifocal CL on 1 eye and a single vision CL on the other eye or the use of 2 bifocal CLs with 1 CL correcting for distance and intermediate vision and the other CL correcting for intermediate and near vision.

When bifocal CLs fit appropriately, they provide stereopsis greater than that achieved with monovision because both eyes are viewing together and not independently. Bifocal lenses may not be as clear as single-vision lenses, and their use often requires additional patience on the part of the patient and the physician to achieve success. Optical designs for bifocal soft CLs include the following:

* Concentric center near: The near prescription is in the center of the lens, and the distance prescription is in the outer annulus.
* Concentric center distance: The distance prescription is in the center of the lens, and the near prescription is in the outer annulus.
* Aspheric: This type of lens has a gradual progression of minus correction from its center to the midperiphery.
* Multiple zones: Multiple annuli alternate between distance prescriptions and near prescriptions.
* Diffractive: Phase plates throughout the lens refract light from all distances to the retina.

The key to successful bifocal CL fitting is matching the correct CL design to the patient's needs. Aspheric lenses are progressive, with a gradual transition from distance through intermediate to near vision; therefore, they are best suited for patients with a need for clarity at an intermediate working distance correction (eg, for computer use).

Pupil size plays an important role in fitting bifocal CLs. If the pupil is too small, the patient may not appreciate the midperipheral optics. If the pupil is too large, it can prevent the eye from using the central optics. Diffractive designs have phase plates that focus light from both distance and near sources, eliminating the need for an optimal pupil size. However, the optics of this design may cause ghost images and limit its usefulness.

RGP bifocal CLs typically provide optics better than those of soft bifocal lenses because they may correct for low amounts of astigmatism and provide a crisp refraction. Alternating (translating) RGP lenses are segmented and use prism ballasting to stabilize the RGP lens. RGP bifocal lenses are available in add powers higher than those of aspheric designs but do not provide intermediate-distance correction. RGP aspheric (soft or rigid) lenses require excellent CL centration. Back surface aspheric multifocals are often designed for superior lid attachment.
Bandage Contact Lenses

Soft CLs can be used to reduce discomfort from corneal epithelial defects, to protect the cornea from drying or mechanical trauma, and to promote healing of corneal wounds. Indications for bandage CLs include the management of recurrent corneal erosion, superficial punctate keratitis, epithelial defects, corneal dystrophies, and wound dehiscence after surgery.

Silicone CLs are optimal bandage CLs because of their increased oxygen permeability. Silicone lenses are associated with a lowered rate of corneal complications, especially in an extended-wear modality.2,3

Bandage CLs should fit snuggly on the cornea with slight movement (approximately 0.25 mm) with each blink. The appropriate base curve can be determined by using the keratometric readings of the contralateral eye. A lens that is too tight blanches the conjunctival vessels and cannot be easily removed. A bandage CL that fits too flat decenters easily and lags with each blink. The bandage CL should be large enough in diameter to cover the affected area on the cornea. Oversized lenses can be prescribed to bandage filtering blebs.

Unpreserved artificial tears instilled on the eye can help loosen the bandage CL when it is removed from the eye. All bandage CLs should be evaluated after 1 day and at 1 week and then as often as needed to monitor for infection.
Keratoconus

Keratoconus is a noninflammatory, asymmetrical, bilateral, progressive thinning of the cornea that distorts the patient's vision. Patients typically have increased, irregular astigmatism and inferior and nasal displacement of the corneal cone (see Media file 2).

Topographic map shows corneal steepening inferior...
Topographic map shows corneal steepening inferiorly. This is typically seen in patients with keratoconus.

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Topographic map shows corneal steepening inferior...

Topographic map shows corneal steepening inferiorly. This is typically seen in patients with keratoconus.

In the early stages of keratoconus, spectacles or soft CLs can help improve the patient's visual acuity. In the moderate-to-late stages of keratoconus, only RGP lenses can mask the underlying corneal distortion and provide the crispest possible vision. Piggyback CLs (an RGP lens on top of a soft CL) are used when patients can no longer tolerate wearing RGP lenses. Silicone lenses are recommended as the soft CL carrier to improve oxygen flow to the cornea.4

Use of diagnostic RGP lenses in the office provides the most accurate fit and prescription for the patient, minimizing multiple follow-up visits. Some laboratories can help design a lens for the practitioner on the basis of corneal topographic results. Many keratoconic RGP lens designs (eg, Soper, McGuire, Rose K) are available for selection depending on the steepness, size, and location of the cone on the cornea. The ultimate goal is to fit the RGP lens with a 3-point touch (trace apical touch over the cone and in the periphery) to distribute the weight of the RGP lens evenly across the cornea.

Some practitioners have advocated the use of scleral lenses or RGP lenses surrounded by a soft skirt (eg, Saturn lenses; CIBA Vision, Duluth, GA) for keratoconic eyes.5 The theory is that the above-mentioned lenses improve the patient's comfort. Disadvantages of the RGP lenses surrounded by a soft skirt include corneal neovascularization, difficulties cleaning the lenses at the junction of the rigid and soft portion of the CL, and CL tearing at the RGP CL–soft CL junction.
Contact Lenses After Trauma
Corneal trauma and corneal surgery frequently result in irregular astigmatism. In some cases, the cornea may remain elevated along the area where sutures were used to repair a laceration. In other cases (ie, chemical burn), a flattened area of corneal scarring may be present. Each case must be individually evaluated for tear quality and deformity of the corneal surface (irregular central vs peripheral cornea). Other factors that should be considered include aphakia (anisometropia), eyelid condition (damage), and iridodialysis requiring a prosthetic iris CL.

The corneal should be completely healed before an RGP lens is fit. In general, exposed sutures must be removed to reduce the risk of infection and to increase CL comfort. In cases involving penetrating keratoplasty (PKP), a thorough evaluation for graft rejection should always be performed (see Media file 4). Signs of rejection include keratic precipitates, an epithelial rejection line, subepithelial infiltrate, corneal edema, an anterior chamber reaction, and corneal neovascularization. CLs made of high-Dk material (material with high oxygen permeability) are recommended, especially for compromised corneas. In cases of severe astigmatism, selective suture removal or relaxing incisions should be considered to ease the fitting process.