CONTACT LENSES AND ASTIGMATISM

BY PAUL WHITE, O.D.
AUG. 1997

About 40 percent of spectacle-corrected patients have 0.75D or more of cylindrical correction. Spherical soft contact lenses do not correct this astigmatism and may degrade visual acuity and comfort. Despite this, fewer than 15 percent of soft contact lens patients wear toric designs to correct their astigmatism. Although not all soft lens patients who have a spectacle cylinder of 0.75D or more require toric soft lenses, about 25 percent of them would benefit.

Contemporary toric soft lenses have an extensive parameter availability, good design and much-improved manufacturing accuracy and reproducibility. Rigid gas permeable contact lenses are also a viable option for many astigmats.

TORIC SOFT LENS DESIGN

Combating Rotation

Adherence of the eyelid to a contact lens with blinking is the primary cause of lens rotation. The amplitude and speed of rotation are functions of the lens fit, material and design, and their ability to resist the forces of blink action, eyelid tonus, palpebral aperture width, eyelid positioning and global location within the orbit.

Toric soft lenses incorporate design characteristics to reduce rotation and to provide appropriate meridional orientation of the cylinder axis. The average diameter of a spherical soft lens is 13.5mm to 14.0mm. The diameter of a toric soft lens is about 1.0mm larger, increasing adherence between the lens and the eye, which slows rotation.

Prism, thin zones (slab-off), truncation and periballast are the primary methods for lens stabilization (Fig. 1), although minus lenticular periballast and truncation are usually inadequate, and truncation often causes discomfort. Contemporary toric soft lenses use combinations of prism, thin zones and lenticulation to produce a desired front surface angulation and peripheral lens thinness. Prism creates a weight differential between the thinner apex and the thicker base and displaces the center of gravity downward. Although this contributes to lens orientation, more important is the steeper, front surface angulation produced by prism. Thinner, steeper peripheral and midperipheral front surface areas reduce the eyelid's ability to grasp and rotate a lens. This is the "watermelon seed" principle. Because prism increases overall lens thickness, the minimum amount that stabilizes the lens is used, usually about 1.5 prism diopters.

A single thin zone may be incorporated toward the base of a prismatic lens. This reduces the thickness in this area, increases comfort and creates a partial central thicker wedge. Eccentric (off-center) plus carrier lenticulation on a prismatic lens does this better than an inferior thin zone. A front surface, eccentric lenticular is wider toward the prism's base than its apex, which thins and steepens the peripheral and midperipheral inferior portions more than the already thin superior apex areas. The prism is then relatively restricted to the nonlenticularized central 60 percent of the lens, creating a wedge. Thus, mass is reduced, superior and inferior thicknesses are similar, stability and comfort are improved, and limbal-scleral draping is better with less compression of the underlying tissues. Many toric soft lens designs incorporate prism and eccentric lenticulation (Fig. 2).

Role of Cylinder in Rotation

The cylinder itself creates a thickness and front surface angulation difference between the principal meridians, and the direction of the cylinder helps predict direction of rotation. WTR minus cylinders increase vertical lens thickness and angulation; ATR minus cylinders increase horizontal lens thickness and angulation; oblique minus cylinders produce asymmetric thickness and angulation changes. Obviously, higher cylinder powers result in greater changes. When superior peripheral and midperipheral lens thickness and angulation are similar on both sides of the lens at the areas of upper lid contact during blinking, there is little or no rotation. Thicker meridians of ATR minus cylinders are relatively parallel to the movement of the upper eyelid, and the lens often has no rotation or a slight nasal rotation. Thicker meridians of WTR minus cylinders are relatively perpendicular to the upper eyelid's movement, and lens rotation may be nasal or temporal. Thicker meridians of oblique minus cylinders are neither parallel nor perpendicular to the upper eyelid's movement. Lenses with minus cylinder axis about 45 degrees have greater thickness about 135 degrees, and the upper eyelid during blinking will first contact the thicker edge at about 10 o'clock and impart a nasal lens rotation (OD). With minus cylinder axis about 135 degrees, eyelid contact at about 2 o'clock creates a temporal rotation (OD). In essence, the greater force is on the side with the greater thickness, and rotation is downward on that side. Although some patients with oblique astigmatism may be successful with toric soft lenses, there is often the greatest rotation, and the prognosis for patients who need cylinder axis 90 � 20 degrees or 180 � 20 degrees is better, especially with higher cylinders. As with spectacle correction, improperly corrected oblique astigmatism is less acceptable to patients than WTR or ATR.

Most toric soft lenses are manufactured with back surface toricity. This improves stabilization, especially with higher corneal toricity, because the back of the lens conforms to the toric cornea. The toric curves of many back toric lenses are confined to the central optic zone. This reduces differential edge thickness and blink-related lens rotation.

PATIENT SELECTION

Determine the patient history, ocular measurements, tissue health, corneal curvature and refraction. Refer spectacle refraction in both principal meridians to the corneal plane, then calculate the amount of uncorrected astigmatism for a spherical soft or RGP lens.

Residual Astigmatism

Not only do spherical soft lenses have little or no lacrimal lens to neutralize corneal astigmatism, but they also flex with a general conformity to the cornea and transmit its cylinder. So-called masking of corneal astigmatism by spherical soft lenses is usually 15 percent or less of keratometric astigmatism (AK). Spherical lenses do not neutralize any of the internal astigmatism (AI) from the interfaces behind the precorneal fluid and the air. Total astigmatism (AT) is indicated by the spectacle plane refraction referred to the corneal plane. With spherical soft lenses, assume that corneal plane AT will be uncorrected. This calculated residual astigmatism (CRA) correlates to measured residual astigmatism (MRA), which is determined by overrefracting a spherical soft lens in situ. In some patients there will be differences between CRA and MRA, so determine MRA. With nonflexing spherical base curve RGP lenses, the lacrimal lens neutralizes AK. To determine CRA, use the simple formula AI = AT - AK, in which AK is expressed as the needed corrective minus cylinder. Again, CRA and MRA may differ. MRA may help you decide whether a soft or an RGP lens is preferable.

MRA Dictates Lens Choice

When the MRA is less than 0.75D, spherical soft lenses are usually indicated. The 0.25D to 0.50D of astigmatism is left uncorrected or a spherical equivalent may be prescribed. To correct MRA of 0.75D or 1.00D, you may use a spherical equivalent and/or spectacles over the contact lenses with the appropriate cylinder for tasks that require better acuity, such as prolonged reading or night driving. These procedures are often more acceptable to patients who don't have very demanding vision needs -- especially those whose spherical prescription is 3.00D or more or those with little or no astigmatism in one eye.

Low eccentricity, front surface aspheric soft lenses may be an option for some patients who would have 0.50D to 1.00D of MRA. They don't correct astigmatism, but they may reduce spherical aberration, which may reduce the size of the retinal blur circle and improve resolution and acuity. Many patients with 0.75D to 2.00D of MRA are better fitted with toric soft lenses. Although patients with MRA greater than 2.00D may be successfully fitted with toric soft lenses, a spherical or bitoric RGP lens should also be considered. Patients with 1.25D of MRA are often purposefully undercorrected by 0.50D of cylinder with toric soft lenses.

Ideal Toric Lens Candidate

When evaluating a patient for toric soft lenses, look for a 9mm or larger vertical palpebral aperture, normal lid closure, average-to-loose lid tension, lower lid approximately tangent to the inferior limbus, absence of raised conjunctival areas and satisfactory precorneal fluid. Patients with narrow palpebral apertures, harsh lid closure patterns and tight lids often produce too much lid action on a lens to maintain satisfactory cylinder axis position. When the lower lid is high on the cornea, sharply angled from horizontal, or moves very obliquely with blinking, axis mislocation and instability often result. Pingueculae or other raised ocular surface areas often cause lens malposition or become irritated by the larger toric soft lenses. Insufficient precorneal fluid may cause more lens dehydration and cylinder axis shift.

Patients who are very sensitive to slight prescription changes or who have extreme visual demands are less likely to accept the vision quality of toric soft lenses. Patients with very low spherical refractive errors, especially combined with moderate or high cylindrical refractive errors, are also less likely to be successful. Don't prescribe toric soft lenses for patients with corneal curvature or regularity changes induced by previous contact lens wear until these alterations have recovered and stabilized.

FITTING EVALUATION

Physical fit of a toric soft lens must fulfill the same criteria as for a spherical soft lens:

1. The lens should center and provide full corneal and limbal coverage. Decentration of the reference mark(s) appears to change the axis angle (Figs. 3 & 4).
2. Vertical movement with blinking should be about 1mm to 2mm.
3. The relationship between the back surface of the lens and the front surface of the eye should be flat enough to provide apical and peripheral alignment.

Steep lenses may stabilize better, but may lock in an undesired meridian, return to position slowly after a blink or cause physiologic problems. Flat lenses may not stabilize consistently and may rotate excessively with blinking. A balance is desirable, and good physical fit is required before you can make a valid appraisal of cylindrical optical factors.

Determining the Physical Fit

There are four methods to determine the physical and optical fit. First is the empirical method. Without placing lenses on a patient's eyes, order the most logical base curve/lens diameter combination based upon keratometric findings with a spherical and cylindrical power based upon the patient's corneal plane refraction. Second, use a spherical soft lens with parameters comparable to a toric soft lens. Third, use a small diagnostic set of a toric soft lens, then overrefract and compensate for the cylinder axis. Fourth, use a large toric soft lens inventory, whereby the lenses placed on the patient's eyes will exactly or closely have all of the parameters, including cylinder power and axis. Although this provides the best evaluation for the ordered lenses, very few practitioners maintain such a large toric soft lens inventory. All of these methods may be satisfactory as a starting point as long as you realize that the ordered lenses are, in effect, diagnostic lenses. This is the first time that lenses are placed on the patient's eyes with all of the requisite parameters.

If the physical fit of the ordered lenses is acceptable and the patient has good comfort and a visual acuity within at least two lines of best-corrected acuity, dispense the lenses. Do not overreact at this time by ordering other lenses. Place the patient on a wearing schedule and see him in about a week for further evaluation. When the first lenses are not satisfactory, diagnose the causes, make adjustments and reorder.

Determining the Optical Fit

After the physical fit is satisfactory, evaluate the optical fit. Cylinder axis should locate properly with primary fixation and no blink action. It should not rotate significantly from this position with change of fixation or blinking and it should recover quickly to its position after blinking. Cylinder axis mislocation, rotation and slow recovery produce greater effects on vision with strong cylinder powers.

To determine axis location, use the reference marks. On a prismatic lens, a single reference mark in the middle of the base should locate at 6 o'clock (Fig. 5); when there are three marks, the central one is in the middle of the base and should align at 6 o'clock. These marks do not represent the cylinder axis. Additional marks help determine the amount of axis mislocation. As each hour of a clock is separated by 30 degrees, this can be used as a gross approximation of any axis mislocation. For a more refined evaluation, place a protractor-type reticule on the eyepiece of a slit lamp, or place an ophthalmic trial lens with an axis line drawn across it in a trial frame on the patient, align it with the reference dot on the contact lens and read the axis designation from the trial frame (Fig. 6).

Allow the lens to settle for at least 15 minutes before evaluating axis location. Have the patient blink several times with primary fixation to assess rotation and recovery. You can also evaluate axis position recovery by moving the lens off axis manually, then allowing the patient to blink normally until the lens reorients. The fewer blinks required, the better the recovery rate. To help determine the appropriate axis location, rotate the lens gently and slowly until the patient reports best vision. Identify this position and then compensate as follows.

If a reference mark mislocates to your right as you look at the patient, subtract the angular amount of mislocation from the prescription. If it mislocates to your left, add the amount to the axis needed by the patient. This is the LARS rule: left add, right subtract (Figs. 7 & 8). For example, if a patient needs axis 90, and the reference mark mislocates 15 degrees to the right, order an axis of 75. In general, do not compensate for mislocation of greater than 20 degrees, but try other lens brands or parameters instead. Remember that when you place the new lens on the patient's eye, the base reference mark will position as on the original lens because the axis has been compensated in relation to this.

Overrefraction

When you overrefract, pay special attention to the possible occurrence of obliquely crossed cylinders over toric soft lenses. Assuming that the cylindrical power is the same on the eye as in air, obliquely crossed cylinders can be resolved easily. Place ophthalmic trial lenses of both the soft lens and the overrefractive spheres and cylinders on axis in an ophthalmic trial frame, then read the resultant cylinder power and axis in the lensometer. You may use standard ophthalmic optics formulae for obliquely crossed cylinders or the available computer programs.

The spherical component of the overrefraction may differ from that predicted. Just as the differential flexing of high plus spherical soft lenses often produces a need for additional plus in overrefraction, the thickness of the prism may create a similar differential flexing and need for 0.25D to 0.50D of less minus or more plus. Many practitioners believe that toric soft lens cylinder power is the same in air and on the eye. This is true for correctly labeled lenses that do not flex on the eye, but flexure in situ may alter the power. Flexure-induced changes are unpredictable and usually can be determined by overrefraction of a toric soft lens in situ. Differential meridional flexing may produce a need for an overrefraction cylinder other than that predicted. Although physically thin compared to spectacle lenses, contact lenses are optically thick. The differential thickness from the apex to the base of a prism can alter the effective power. Some patients may need a toric soft lens for one eye only. Because a prism in a contact lens deviates light similarly to a spectacle lens, a vertical imbalance may be induced, but many patients adapt to this.

Optical fit for close vision is difficult to evaluate, but it is of great clinical importance. Some contact lens wearers may have satisfactory distance vision, but unsatisfactory near vision. This may be due to any of the reasons that apply to spherical soft lens wearers, or there may be a different cylinder axis location and lens flexure with near fixation due to ocular cyclotorsion, palpebral aperture narrowing and increased lid pressure on the lens.

THE PROGRESS VISIT

Progress evaluations should include history, physical fit, vision and physiologic response. Obviously, anything that could cause a problem for a spherical soft lens wearer applies to toric soft lenses. The major additional cause of vision problems with toric soft lens wearers is improper cylinder rest axis location. Next, in order, are: excessive blink-initiated axis rotation; slow axis recovery after blinking; incorrectly ordered sphere or cylinder; lens flexure; and mislabeled lenses.

Mislabeled lenses can be determined by lensometry verification, which is less accurate with soft lenses than with RGPs. Prepare a toric soft lens for verification by lightly blotting its surfaces with a lint-free cloth to remove surface fluid. Then geometrically center the concave surface of the lens on the lensometer stop with the prism base or lens marking at the 90-degree position. Plastic mounts placed over the eyepiece reduce the aperture to about 3mm, eliminating peripheral aberrations and allowing a more accurate determination of power and axis. Special devices for verifying toric soft lens optical factors are available.

The first lenses ordered may be the final lenses, especially with lower cylinder powers whose axes are 90 and 180 � 20 degrees, but it's more likely that additional lenses may be needed. All toric soft lens manufacturers have warranty policies.

PLANNED REPLACEMENT

Just as with spherical soft lenses, planned replacement of toric soft lenses minimizes the tissue reactions produced by coated lenses. Because toric soft lenses are thicker, however, their oxygen transmission (Dk/L) is less than that of spherical soft lenses. Discourage extended wear because it produces hypoxia and corneal edema during sleep, and it increases the probability of corneal microcysts, infiltrates, ulcers, and acute red eye syndrome (ARE).

RIGID LENSES

Some astigmats are better served by RGPs, either spherical, front surface toric, aspheric or bitoric.

Prism and Flexure

A nonflexing, spherical RGP lens neutralizes AK and leaves a residual AI, and MRA is easy to determine by overrefraction. About a third of patients will have an AI of 0.75D or greater. For patients with 1.00D or less of AI, use the same optical procedures as for spherical soft lenses. On-the-eye flexure of an originally spherical RGP lens may alter the in situ residual spherical or cylindrical power. Factors that increase the probability of undesired flexure are superior positioning, tight upper eyelids, greater corneal toricity, steeper base curves or thinner lenses. Flexure may be constant or intermittent, and it may create regular or irregular surfaces.

When AI with a spherical RGP lens is 0.75D or more and AK is less than two diopters, a spherical base curve, front surface toric lens may be indicated. To neutralize lens rotation, 1.50 � 0.50 prism diopters base down is incorporated. Within this range, less prism is needed with spherical lens powers of -4.00D or greater, while greater prism is needed for lenses with lower minus or with plus spherical lens power. Apex thickness should be about 0.10mm to avoid making the lens thicker.

The inherent increased thickness and weight from the prism usually precludes a superior-positioning fit, so the lens should be positioned intrapalpebrally. This usually involves lens diameters of 8.5mm to 9.0mm and a base curve that is 0.10mm to 0.15mm steeper than required to achieve central alignment. This steeper fit offsets the prism forces that often direct the lens to an inferior location.

With an inferior aperture position, the upper eyelid blink action required to clean the anterior lens surface, remove debris from between the lens and the cornea, and wet the lateral cornea adjacent to the lens is reduced. This increases the probability and severity of 3 and 9 o'clock staining, which is further exacerbated by the thickness of the lens and the resultant increased lid gap.

Although truncation is sometimes used with prism ballast, front surface toric RGP lenses, this type lens is often uncomfortable and a circular lens is preferable. After the lens has stabilized, evaluate cylinder axis location, rotation and recovery as you would for toric soft lenses. Then use the LARS procedure and make appropriate compensations.

Asphericity and Toricity

In less than 10 percent of patients, corneal toricity is 2.00D or more. Corneas are aspheric, ellipsoidal and have an eccentricity value of about 0.5. When a spherical base curve doesn't match the toricity and asphericity of the cornea, undesired lens position, movement, rocking, discomfort, topographic changes and spectacle blur may result. Low aspheric bases may be used to address corneal asphericity, improving lens position, movement and comfort, even though the base curve's noncoincidence with corneal toricity is still present. Eccentricities of 0.2 to 0.4 may be used. Greater eccentricities may improve the relationship but are more difficult to fit, and the induced progressive power changes attendant to aspheric curves can create vision problems, especially when the lens is not centered well.

When a toric base curve is used to provide a better lens-to-cornea relationship, there is a residual induced cylinder when the lens is on the eye. This is created by the indicial difference between the approximate 1.33 index of the precorneal fluid and the higher index of the RGP material, which is about 1.47. This residual induced cylinder is about 30 percent of the toric surface's cylindrical power in air, and it is a minus cylinder whose axis is along the flatter base curve principal meridian.

There are two methods to neutralize residual astigmatism with a toric base curve. For a spherical power effect (SPE) design, the manufacturer computes the required front surface plus cylinder to neutralize only the induced back surface cylinder. For a cylindrical power effect (CPE), the front surface cylinder neutralizes both the induced back surface cylinder and the AI. SPE is used more often when AI is less than 0.75D, and CPE when it is greater than this. Although this sounds complicated, the computations are simple and laboratory consultants can help.

Bitoric RGP lenses usually position intrapalpebrally, and lens diameters are often 8.5mm to 9.0mm. Most practitioners do not have inventory or diagnostic sets of bitoric RGPs. The base curve radii of the initially ordered lenses is done with compensation to the principal meridians' keratometric readings. Although there are many methods for doing this, their common denominator is to fit flatter-than-K for each meridian, especially the steeper one. A practical starting point is to fit the flatter meridian 0.25D (0.05mm.) and the steeper meridian 0.50D to 0.75D (0.10mm to 0.15mm) flatter than the corresponding keratometric readings. Lacrimal lens optical compensations are made in the usual way.

The SoftPerm contact lens has an 8.0mm RGP center (Dk 14) surrounded by a soft material skirt (Dk 25) extending to 14.3mm diameter. This hybrid lens provides the central optics of an RGP and the stability and comfort of a soft lens. With some patients, it may be used instead of a bitoric RGP corneal lens.

Other hybrid approaches are to piggyback a small (8.0mm), thin RGP lens over a soft lens or to use a soft lens which has a central recessed bed for insertion of a 7.5mm to 9.5mm RGP lens. Hybrid lenses are sometimes used for keratoconus or after penetrating keratoplasty, but only when corneal RGPs or other lens designs have been unsuccessful on these very atypical, asymmetric and highly astigmatic topographies. One problem with hybrid combinations is increased hypoxia. RGP scleral lenses have been used successfully for unusual corneal topography, Stevens-Johnson Syndrome and ocular cicatricial pemphigoid. CLS