GP Lens Case Grand Rounds Troubleshooting Guide – 35

Moderate Keratoconus: Bruce Anderson, OD, FAAO

Background

A 36-year-old male patient presented on for a contact lens fitting for keratoconus. He was currently wearing soft toric contact lenses but complained that he was not obtaining adequate vision with his current lenses. He had a history of keratoconus and had not previously been fitted with gas permeable (GP) lenses. He was interested in pursuing new lenses at this time. His vision with his current contact lenses was 20/30- in the right eye and 20/40- in the left eye. However, he complained of significant ghosting and shadowing with the current contact lenses.

Test Procedures, Fitting/Refitting, Design & Ordering

Slit Lamp Evaluation:

Evaluation of his eye without his contact lenses revealed slight central corneal thinning in both eyes. There were several Vogt’s Striae slightly inferior to the central cornea noted in both eyes as well. There is no significant scarring and the central cornea was clear.

Diagnostic Data at Fitting Visit:

Manifest Refraction:

OD: -3.25 -4.00 x 075 20/30+
OS: -3.00 -6.00 x 092 20/40-

Keratometry (Auto):

OD: 45.00 @ 145; 46.50 @ 055
OS: 46.00 @ 140; 55.00 @ 050

Corneal Topography:

Corneal topography revealed very significant inferior cones as typically noted with keratoconus. See the topographies on the last page. I proceeded with the contact lens fitting using the following diagnostic lenses.

Diagnostic Lenses:

OD: Base Curve Radius (BCR): 47.00D
Power: -3.00D
Overall/Optical Zone Diameter (OAD/OZD): 8.6/7.2mm
Secondary Curve Radius/width (SCR/W): 9.00/.5mm
Peripheral Curve Radius/width (PCR/W): 12.20/.2mm

OS: Base Curve Radius (BCR): 52.00D
Power: -8.00D
Overall/Optical Zone Diameter (OAD/OZD): 8.6/6.0mm
Secondary Curve Radius/width (SCR/W): 7.09/.3mm
Intermediate Curve Radius/width (ICR/W): 8.29/.3mm
Intermediate Curve Radius/width (ICR/W): 10.09/.3mm
Peripheral Curve Radius/width (PCR/W): 12.09/.4mm

Over-Refraction:

OD: -3.00DS 20/25
OS: -4.75DS 20/30-

The right contact lens was a standard lens design, and the left lens was a Maguire keratoconic design, both used for diagnostic evaluation with the keratoconus fitting.

Slit Lamp Evaluation:

Slit lamp evaluation of the contact lenses revealed that the right contact lens was fitting slightly flat centrally. The peripheral curve of the contact lens was fitting slightly tight. The diameter of the contact lens was such that it was interpalpebral, not lid attachment. Evaluation of the fit of the left contact lens revealed that the lens was also fitting flat centrally. The peripheral edge of the contact lens was lifting slightly tight and the lens was not fitting under the upper lid. Both trial lenses positioned slightly inferior and were centering over the cone. The initial contact lenses were ordered:

Contact Lenses Ordered:

Diagnostic Lenses:
OD: Base Curve Radius (BCR): 47.50D
Power: -6.50D
Overall/Optical Zone Diameter (OAD/OZD): 9.0/7.4mm
Secondary Curve Radius/width (SCR/W): 9.00/.5mm
Peripheral Curve Radius/width (PCR/W): 12.50/.3mm

OS: Base Curve Radius (BCR): 52.25D
Power: -12.75D
Overall/Optical Zone Diameter (OAD/OZD): 9.0/6.2mm
Secondary Curve Radius/width (SCR/W): 7.10/.3mm
Intermediate Curve Radius/width (ICR/W): 8.35/.3mm
Intermediate Curve Radius/width (ICR/W): 10.20/.3mm
Peripheral Curve Radius/width (PCR/W): 12.25/.4mm

Follow-Up Care/Final Outcome

At the dispensing visit the visual acuity in the right eye was found to be 20/25; the left eye was 20/30. The over refraction in the right eye was -0.25DS with no improvement in the vision. The left eye was +0.50DS which resulted in somewhat variable vision.

Slit lamp evaluation of the lenses revealed that both lenses were positioning slightly inferior. Both lenses exhibited an alignment fit centrally. The edges of both contact lenses exhibited adequate clearance. The lenses moved approximately 1 to 1 ½ mm with the blink. He was asked to return in two weeks for follow-up visit.

At the follow-up visit, he was only wearing the lenses approximately two to three hours each day. Visual acuity in the right eye was 20/25+, and the left eye was 20/20-. Over- refraction of the right eye was -0.25DS with no improvement in vision. The over-refraction in the left eye was +0.25DS resulting in 20/20- visual acuity. Both contact lenses were decentering slightly inferior (over the cone). There was adequate movement. There was an alignment lens-to-cornea fitting relationship centrally with slightly excessive clearance peripherally. Another follow-up visit was scheduled at one month.

At the one month evaluation, he stated that he was able to wear the lenses most of the day. The visual acuity in the right eye was 20/20- and in the left eye was 20/20- as well. There was a plano over refraction in each eye resulting in the same visual acuity. Both contact lenses exhibited an alignment fitting relationship centrally. The lenses were again decentering slightly inferior with slightly excessive clearance peripherally. Upon blinking, the lid would move smoothly over the top edge of the contact lens.

Discussion/Alternative Management Options

In fitting this patient, who had moderate keratoconus, with the left eye being more affected than the right eye, different lens designs can be considered. The right eye which had mild keratoconus was fit with a more traditional GP lens design with good success. The initial diagnostic lens placed on the right eye was a smaller diameter; however, I enlarged the final diameter to encourage more corneal coverage and try to achieve lid attachment.

The fluorescein pattern needs to be evaluated with each lens dispensed. I prefer a light touch to slight pooling centrally so as not to irritate the central cornea over the cone. The peripheral edges need to have slight edge clearance to encourage proper movement and prevent mechanical staining via edge sealoff. The diameter was increased from the initial diagnostic lenses to encourage this more appropriate fit. The base curve radii were determined from the fluorescein pattern as evaluated from the initial trial lenses. These initial lenses were based on the keratometry readings, as well as corneal topography readings.

The diagnostic lenses that were selected were biased toward the steeper corneal curvature. When selecting an initial diagnostic lens for keratoconus (Average keratometry value: 45.00 to 50.00D), I will use the steeper keratometry reading (or simulated “K” reading from topography) as the selector for the trial lens base curve. This usually works well as a starting point, because in most cases, the keratometry readings tend to be slightly flatter than the steepest region of the cornea due to the typical decentration of the cone.

The peripheral curves were determined from the fluorescein pattern of the diagnostic lenses. Knowing the diagnostic lens peripheral curve values, these were modified accordingly depending upon the appearance of the edge clearance as observed with fluorescein. It is very important to emphasize the need for evaluating the fluorescein patterns of a diagnostic lens on the eye in determining the final contact lens design.

Depending upon the centration of the lens, different lens diameters may be used. As a rule of thumb, the smaller the cone and the less progressed the keratoconus, the more traditional the fitting philosophy that could be used. A larger diameter lens may be designed in this case. A larger diameter lens will fit more centrally and, hopefully, under the upper lid. This would create a more comfortable design.

As the cone becomes more progressed, a smaller design is needed. When a larger diameter is used on an advanced cone, it tends to result in excessive edge clearance inferiorly. The contact lens powers were determined from the over-refraction with adjustment for vertex distance and any changes in base curve radius that were made from the initial diagnostic lens.

It is important to follow these patients over time. Once the contact lens has been worn for a period of time, the contact lenses tend to shape and re-mold the eye and the fit of the contact lens will shift and change. Frequently, the cone will be slightly compressed from the wearing of the rigid lens on the eye and once changes have occurred in the shape of the eye, then the fitting relationship may no longer be ideal. Then adjustments need to be made in the contact lens design over time.

Therefore, it is important to evaluate the fit of the lenses when they are dispensed and provide adequate intermediate and long term follow-up visits to ensure that the adjustments that need to be made can be performed to ensure the most comfortable design and fit for any particular patient is obtained.

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