Orthokeratological Reduction of Astigmatism

BY ANTHONY VINCENT POTTS, O.D., M.S., F.I.O.S., F.A.A.O
JUNE 1997

Through this programmed application of orthokeratology, the high astigmat can be effectively brought to sphericalization using standard rigid gas permeable contact lenses.

The scope of orthokeratology now extends far beyond the mid-level myope with a steep cornea -- long considered the ideal candidate. Orthokeratologists have discovered that highly myopic patients (over four diopters) are generally very grateful for any amount of reduction in myopia. Now the astigmat can also benefit from corneal reshaping.

Achieving sphericalization of an astigmatic eye is slightly more difficult than the general orthokeratological reduction of myopia. Performing ortho-k without leaving telltale signs -- the negative shape factor, negative eccentricity values or distortion -- usually requires four or five sets of contact lenses and can be accomplished without the use of reverse geometry lenses which may leave artifacts deleterious to pilots and visual acuity-critical occupations.

There are two methods that we can use in the two-step process of reducing myopia and with-the-rule astigmatism: computerized corneal topography or a mathematical formula combined with diagnostic fitting.

TOPOGRAPHICAL ANALYSIS

By using corneal topography and spherical rigid gas pemeable contact lenses, we can successfully reduce myopia combined with with-the-rule astigmatism as illustrated by the topographical map in Figure 1. Fit the lenses so they appear to seal at the 3 and 9 o'clock position at the junctures of the secondary curve and the optic zone. Clearance should be slight at the 6 and 12 o'clock position. This will provide a horizontal fulcrum, allowing tear exchange vertically; the result will be a decrease or flattening of the vertical meridian and an increase or steepening of the horizontal meridian. As the reduction of the astigmatism begins, blend the optic zone in 0.1mm steps until the astigmatism is one diopter or less.

FIG. 1: TOPOGRAPHICAL MAPPING OF A WITH-THE-RULE ASTIGMAT.

 

At this point, change the base curve to achieve an alignment fitting pattern, reducing the optic zone by 0.1mm per 0.12D of corneal flattening. Change the lenses with every half-diopter of corneal flattening, starting with a new optic zone as explained in the mathematical formula which follows.

MATHEMATICAL FORMULA AND DIAGNOSTIC FITTING TECHNIQUE

If you are using keratometry instead of topography, and if the with-the-rule astigmatism is from 1.50D to 3.50D, proceed as follows.

Fit the first set of lenses steeper than flattest K. Take the difference between the steepest and flattest K reading in diopters and divide by three. Add this number to the flattest K reading to determine the base curve for the first set of contact lenses. Figure 2 illustrates a typical fluorescein pattern for a first set of lenses.

FIG. 2: TYPICAL FLUORESCEIN PATTERN OF FIRST SET OF LENSES.

 

First Lens for Reduction of With-the-Rule Astigmatism:

BC = flattest K + (steepest K - flattest K )/3

Diameter = BC in mm + 1.3mm

Secondary Curve (SC) = BC in mm + 1.1mm

Secondary Curve Width (SCW) = 0.3mm

Peripheral Curve (PC) = BC in mm + 1.7mm

Peripheral Curve Width (PCW) = 0.3mm

Optic Zone = BC in mm

Blend = light

Center Thickness (CT) = Power and diameter dependent (never go below 0.16mm to provide stability and prevent flexure)

Edge = 12.50mm edge lift

Note: Corneal astigmatism greater than 3.50D will need an aspheric contact lens design.

If a patient has never worn contact lenses, it's important to start the wearing schedule slowly due to the slight tightness of the lenses. To prevent edema, start at two hours with a two-hour increase per day. If the astigmatism worsens, increase the diameter by 0.2mm, keeping all other dimensions per the formula.

See the patient one week after dispensing the first lenses to monitor the molding process. As the flattest K meridian becomes steeper and the steepest K meridian becomes flatter, flatten the optic zone by 0.1mm per 0.12D change. Modifying too soon can negate the sphericalization process. If there is a 0.12D change in the first week, wait two weeks before modifying to ensure stabilization of this change.

Always make sure that the lens is not sealing off in both meridians at the same time. The lens may appear to be sealing off in the horizontal meridian, but it should show tear exchange in the vertical meridian in a with-the-rule astigmat. By the third to fourth week of wear, you can reduce or blend the optic zone by 0.1mm to 0.15mm per 0.12D change of corneal astigmatism. More frequent visits plus modification or blending may be necessary earlier if corneal changes are occurring sooner and in greater amounts than anticipated.

See the patient four weeks after the initial modification and reduce the optic zone by the same formula until the corneal astigmatism is reduced to one diopter or less. Then switch to an on-K or parallel fitting technique, starting again with a large optic zone and contact lens dimensions per the original formula. Schedule monthly follow-up visits, and reduce the optic zone by 0.1mm per 0.12D of corneal change. If there is no corneal change, then check the center thickness. Remember, in orthokeratology, thin is not better! Checking all dimensions before dispensing is very important for consistent results.

You will be changing lenses with every half-diopter of corneal flattening. At the half-diopter change position, you would have modified the lens about four times and reduced the optic zone by about 0.4mm to 0.5mm. At this point, the patient will start noticing that the prescription is too strong, and you will have run out of optic zone. Thus, the next lenses will be a half-diopter less minus power and a half-diopter flatter than the previous lenses, starting again with an optic zone per formula.

FINISHING THE THERAPY

The endpoint is usually when the cornea no longer changes with three months of optic zone modification. (The center thickness can be increased to see if this will elicit any further change. Or you could flatten the base curve radius by 0.5mm and increase the diameter.) When the central topographical reading is the same as the peripheral topographical reading, the cornea has reached sphericalization.

RETAINER WEAR

To stabilize the corneal curvature, patients should wear the last set of contact lenses full-time for three months. If the unaided acuity is 20/20, or if the patient wants to reduce his or her wearing time and return to another refractive modality (soft lenses or eyeglasses), then reduce the wearing time one hour per week to a total of four hours.

For example, a patient who is wearing the lenses 14 hours a day should reduce the wearing schedule to 13 hours a day for one week, 12 hours a day for one week, and so on, stopping at 10 hours per day. If no regression is apparent at the tenth hour, reduce the wearing time another four hours, one hour per week as above, stopping at the six-hour level. Again, check for regression. Regression usually occurs between six and eight hours for high myopes and between four and six hours for low myopes. When you note slight regression, add two hours to the wearing time, and check the eye in one month. This should reverse the regression and stabilize the results.

MAINTAINING RESULTS

The orthokeratological results are consistent as long as the patient wears the retainer lenses for the prescribed time. Retainer lens wearing time can be task-dependent. A patient who does more near-point work will need more wearing time. Also, the higher the refractive error, the more wearing time is needed. Freeman reports that "... for patients who start with three diopters or less of refractive error and are able to attain 20/30 or better acuity during the program, I find that wear of four to eight hours per day is attainable 70 percent of the time. For patients with greater than three diopters of (refractive) error and who attain 20/30 or better acuity, I find retainer wear of four to eight hours per day is only attainable 40 percent of the time."

I usually advise students and anyone under the age of 21 to wear the retainer lenses eight to 10 hours until school or growth is completed. A low plus spectacle prescription worn over the contact lenses can be beneficial to the academically oriented patient.

ASTIGMATIC REDUCTION CASE

R.C., a 25-year-old man with astigmatism corrected with spectacles came to us for an orthokeratological work-up. Family history was negative for diabetes, glaucoma, detached retina or other ocular disease. All examination findings were normal. Presenting refraction was: OD -0.75 -2.75 x 142, 20/20; and OS -1.00 -2.25 x 036, 20/20. Unaided visual acuity was 20/200 OD and 20/200 OS. Beginning keratometry readings were: 45.87 @ 065/43.37 @ 155 OD; 46.50 @ 115/43.75 @ 025 OS.

We ordered the initial pair of lenses as follows:

OD / OS

BC 44.12 / 44.62

PC 9.3 / 9.3

PCW 0.3 / 0.3

SC 8.7 / 8.7

SCW 0.3 / 0.3

Power -1.50 / -2.00

Diameter 8.9 / 8.9

OZ 7.6 / 7.6

CT 0.18 / 0.18

Blend Light / Light

At R.C.'s first progress visit, we noted a 1.50D reduction of corneal curvature in the vertical meridian with no change in the horizontal meridian of the right eye. His left eye showed a 1.50D reduction of corneal curvature in the vertical meridian and 0.25D increase in corneal curvature in the horizontal meridian. In one week, the first set of lenses had reduced the corneal astigmatism by 1.50D in the right eye and 1.75D in the left eye. To allow the tissue movement or molding process to equilibrate with the present lens specifications, we did not modify the lenses.

The second progress appointment (three weeks after the initial dispensing) showed a 2.50D reduction in the vertical meridian and a 0.12D increase in the horizontal curvature of the right eye. The left eye showed 2.12D of curvature reduction in the vertical meridian and a 0.37D increase in horizontal curvature. Unaided visual acuity improved to 20/40 OD and 20/60(-1) OS. The manifest refraction was OD -1.00 -0.50 x 130, 20/20 and OS
-1.25 -0.75 x 047, 20/25+. By this visit, the corneal astigmatism had decreased 2.37D in the right eye and 2.50D in the left. We noted that the lenses were starting to seal off, so we reduced the optic zone by 0.2mm OU to bring them into a parallel fitting pattern. This is contrary to reverse geometry lenses where reducing the optic zone would tighten the lens. With reverse geometry lenses, the practitioner must order a second set of this type of lens to prevent sealing, thus becoming dependent on the laboratory.

R.C.'s third progress examination revealed a 2.37D reduction in the vertical meridian and a 0.25D reduction in the horizontal meridian of the right eye. The left eye showed a 2.25D reduction in the vertical meridian and a 0.37D increase in the horizontal meridian.

We decided to blend the optic zone by 0.2mm again. The first lenses no longer elicited corneal change, so we ordered the second set of lenses as follows:

OD / OS

BC 43.50 / 44.00

PC 9.5 / 9.5

PCW 0.3 / 0.3

SC. 8.9 / 8.9

SCW 0.3 / 0.3

Power -0.87 / -1.37

Diameter 8.9 / 8.9

OZ 7.6 / 7.6

CT 0.20 / 0.20

Blend Light / Light

We dispensed the second set of lenses using the same fitting formula as we did with the first set, but starting again with a large, non-modified optic zone. We adjusted power accordingly.

The fifth progress visit showed that after having reduced the astigmatism by 2.37D in the right eye and 2.00D in the left, the second set of lenses was remolding the horizontal meridian, bringing about an improvement in the manifest refraction and unaided visual acuity.

The sixth, seventh and eighth progress visits, scheduled at one-month intervals, showed the astigmatic reduction had stabilized while myopia reduction continued. We blended the optic zone by 0.1mm at each visit to elicit further myopic reduction.

By the ninth visit, the reduction of the myopic component had stabilized in the left eye and reversed slightly in the right after several reductions of the optic zone. Due to the sphericalization of the K reading in the right eye and a need for a more parallel fit in both eyes, we dispensed a third pair of contact lenses. At this point, the astigmatic reduction had stabilized and the overall refraction was being reduced. Unaided visual acuity had greatly improved from the starting point.

The third set of lenses (see below) continued to reduce both meridians in the right eye and equalized the K readings in the left eye. We blended the optic zone by 0.1mm.

OD / OS

BC 43.00 / 43.50

PC 9.5 / 9.5

PCW 0.3 / 0.3

SC 8.9 / 8.9

SCW 0.3 / 0.3

Power -0.37 / -0.87

Diameter 8.9 / 8.9

OZ 7.6 / 7.6

CT 0.21 / 0.21

Blend Light / Light

Corneal change for both eyes was stable for several months. Unaided visual acuity was 20/20 OD, 20/20-2 OS, 20/20 OU. We ordered the last pair of lenses (see below) to maintain parallel fit and to stabilize results.

We instructed R.C. to wear this last set of contact lenses full-time for four to six months and then gradually reduce wearing time one hour per week. We scheduled progress visits every month to monitor stabilization and to adjust retainer lens wearing schedule.

OD / OS

BC 42.50 / 43.00

PC 9.5 / 9.5

PCW 0.3 / 0.3

SC 8.9 / 8.9

SCW 0.3 / 0.3

Power +0.12 / -0.37

Diameter 8.9 / 8.9

OZ 7.6 / 7.6

CT 0.21 / 0.21

Blend Light / Light

CLS

The methods, opinions and views expressed in this article do not represent the official position of the U.S. Navy or the Department of Defense.

References are available upon request to the editors. To receive references via fax, call (800) 239-4684 and request document #25.

Dr. Potts, former chairman and administrative director of the International Orthokeratology Society, is on active duty with the U.S. Navy. He is a consultant on contact lenses to the specialty advisor for Navy optometry in Great Lakes, Ill.