GP Lens Case Grand Rounds Troubleshooting Guide – 39

Advanced Keratoconus (hydrops): Bruce W. Anderson, OD

Background

A 42-year-old male presented on for a contact lens fitting for his left eye. He stated that he had advanced keratoconus which had progressed to hydrops approximately eight months previously. His eye had stabilized since that time. He did not have a corneal transplant, and he was hoping to wear a contact lens rather than proceed with surgery. His vision and corneal curvature had since changed dramatically. His visual acuity in his left eye, which was uncorrected, was count fingers at 2 feet.

Test Procedures, Fitting/Refitting, Design & Ordering

A manifest refraction was attempted; however, no improvement in visual acuity could be obtained with this refraction.

Slit Lamp Evaluation:

Slit lamp evaluation of his eye revealed a deep and clear anterior chamber. The conjunctiva was white; however, the cornea showed 3+ central scarring. There was also 3+ central thinning. There was a significant split in Descemet’s membrane, easily observed with the biomicroscope. There was a trace of corneal edema; however, overall, there was more scarring as opposed to edema reducing corneal transparency.

Corneal Topography:

I proceeded with a contact lens fitting using the following Maguire cone lens design.

Diagnostic Fitting:

OS:
Base Curve Radius (BCR): 59.00D
Power: -8.00D
Overall/Optical Zone Diameter (OAD/OZD): 8.6/6.0mm
Secondary Curve Radius/width (SCR/W): 6.32/.3mm
Intermediate Curve Radius/width (ICR/W): 7.52/.3mm
Intermediate Curve Radius/width (ICR/W): 9.32/.3mm
Peripheral Curve Radius/width (PCR/W): 11.32/.4mm

Over-Refraction:

OS: -8.50DS 20/80

The fit of this diagnostic contact lens, centrally, was exhibited excessive apical clearance. The peripheral edge of the contact lens was lifting slightly off the inferior cornea. The lens positioned over the central cornea but showed excessive movement. From evaluation of the fluorescein pattern and how the lens positioned on the eye, an initial contact lens was ordered.

Contact Lens Ordered:

OS:
Base Curve Radius (BCR): 57.00D
Back Vertex Power: -13.50D*
Overall/Optical Zone Diameter (OAD/OZD): 9.2/6.4mm
Secondary Curve Radius/width (SCR/W): 6.25/.3mm
Intermediate Curve Radius/width (ICR/W): 7.00/.3mm
Intermediate Curve Radius/width (ICR/W): 8.00/.3mm
Intermediate Curve Radius/width (ICR/W): 9.00/.3mm
Peripheral Curve Radius/width (PCR/W): 10.00/.2mm
Material: Boston XO
Plus Lenticular

*This power was obtained by adding the over-refraction to the diagnostic lens power and then factoring in the base curve radius change and vertexing the over-refraction to the corneal plane. The diagnostic lens is -8.00D with a -8.50D over-refraction. This equals -16.50D. The over-refraction would equal -7.50D at the corneal plane and an additional two diopters of plus power need to be added to compensate for the two diopter flatter base curve radius used in this case.

At the dispensing visit, the visual acuity with the new contact lens in the left eye was found to be 20/70. The vision was somewhat variable; however, the over-refraction was plano. The contact lens exhibited some apical clearance with slightly excessive peripheral clearance. The lens exhibited good centration and was moving approximately 1 to 1 ½ mm. upon the blink. The lens was dispensed and a follow-up visit was scheduled for one week.

Follow-Up Care/Final Outcome

First Follow-up Visit:

At the follow-up visit, this patient reported that the contact lens was staying on the eye; however, it was somewhat irritating to his eye after two to three hours of wearing time. Visual acuity with the contact lens was 20/100+.

Over-Refraction:

OS: +0.50DS with just a slight improvement of the vision to 20/80.

Slit Lamp Evaluation:

Slit lamp evaluation of the contact lens-to-cornea fitting relationship revealed that it was positioning fairly central. There was minimal movement of approximately 0.5mm with the blink. There was slight fluorescein pooling centrally; however, the superior edge of the contact lens was extremely tight with an absence of edge clearance.

With lens removal, it was found that in the superior cornea, relating to the region of peripheral edge sealoff, there was an arcuate staining abrasion extending from approximately 11:00 to 1:00 on the peripheral cornea. Because of the tight fit, a new contact lens was ordered:

Contact Lens Ordered:

OS:
Base Curve Radius (BCR): 57.00D
Back Vertex Power: -13.00D*
Overall/Optical Zone Diameter (OAD/OZD): 9.2/6.4mm
Secondary Curve Radius/width (SCR/W): 6.10/.2mm
Intermediate Curve Radius/width (ICR/W): 7.50/.3mm
Intermediate Curve Radius/width (ICR/W): 9.00/.3mm
Intermediate Curve Radius/width (ICR/W): 10.50/.3mm
Peripheral Curve Radius/width (PCR/W): 12.50/.3mm

Second Follow-Up Visit and Lens Dispensing:

This new contact lens was dispensed and the vision was determined to be 20/80.

Over-Refraction:
OS: +0.50DS 20/80+

Evaluation of the lens-to-cornea fitting relationship with fluorescein revealed that a slight apical clearance pattern was observed. Superiorly, slight edge clearance and no excessive bearing was present. The lens was dispensed and a follow-up visit was scheduled for two weeks later.

Third Follow-Up Visit:

At this third and final follow-up visit, he stated that he was able to wear the lens very comfortably. The visual acuity in the left eye was still found to be 20/80+. The over- refraction was plano with no improvement in vision.

Discussion/Alternative Management Options

The initial contact lens ordered for this patient was based on many different features relative to his corneal topography. Using the simulated “K” measurements which were found to be 50.93/62.43 @ 010.(as taken from topography) with a significantly steeper zone inferior, a base curve radius of 59.00D was selected from the diagnostic fitting set.

In selecting the diagnostic lens in this type of eye, I tend to bias the initial diagnostic lens closer to the steeper curvature rather than the flatter measurement, especially when there is such a large difference in the corneal curvature values. The diagnostic lens was placed on the eye, and evaluation using fluorescein was performed. Fluorescein evaluation should always be performed for each region of the contact lens from the center to the edge 360º around the contact lens.

Knowing the peripheral curve radii values for the initial diagnostic lens and noting that slightly excessive edge clearance was observed peripherally, steepening the curvatures of the peripheral radii in order to create a lower edge clearance relationship was performed. As his cornea was somewhat “bulbous” in appearance as opposed to normal keratoconus (which was created from the hydrops), a larger diameter contact lens was ordered. Because the cornea was somewhat protruding, steeper peripheral curve radii were initially ordered in an attempt to keep the contact lens stable on the eye.

However, once the contact lens was worn for a period of time, the cornea was molded and reshaped from this first contact lens and the peripheral clearance reduced over time. Therefore, the second contact lens was designed with flatter peripheral curve radii to decrease this fit-related problem.

This type of shift in the cornea is not unusual in any patient with keratoconus, especially those patients who have had hydrops. Therefore, close observation needs to be made during the fitting process and, more importantly, during the follow-up visit to ensure that if the cornea does change due to the wearing of the contact lens the appropriate changes can be made within the design of the lens to alleviate any areas of irritation or heavy bearing.

The base curve radius for the initial contact lens ordered was flattened significantly from the diagnostic lens. This is due to the steep fitting nature of the diagnostic lens that was initially evaluated on the eye. The overall diameter was decided to be enlarged from the 8.6 diameter of the initial diagnostic lens as this lens was fairly small and appeared to be “bumping” into the upper lid upon blinking. A larger diameter would allow a better fit of the lens edge under the upper lid.

There were multiple peripheral curves which were designed in an attempt to make a gradual transition from the steep base curve to a flatter peripheral curve. Knowing the peripheral curve radii of the diagnostic lens, and knowing that these peripheral curve radii resulted in excessive edge clearance (i.e., too flat), a steeper peripheral design was ordered for the initial lens to be dispensed.

After evaluation of the first dispensed lens after one week, it was observed that the peripheral curve-cornea relationship actually resulted in insufficient edge clearance (i.e., too steep). Therefore, this this lens had to be redesigned with flatter peripheral curve radii to allow more movement and adequate edge clearance to prevent the recurrent of the mechanically-induced corneal staining.

Typically, multiple peripheral curves with small widths that constantly change in curvature will allow a gradual transition from the steep central base curve radius to the flatter peripheral edge of the contact lens. This helps to prevent areas of abrupt change on the posterior surface of the lens and create a more comfortable design.

Also, it is important to note that the visual acuity of this patient was marginal at best. This patient was motivated for a contact lens as an alternative to a corneal transplant. He had other health problems that could affect his success with the surgery. I had encouraged him to deal with the other health problems first and then pursue the corneal transplant.

Even though this contact lens was fit successfully, and he was able to wear it successfully long term, the level of vision was marginally acceptable. However, when the two eyes are used together, a visual acuity of 20/70 to 20/80 in one eye when the other eye has significantly better vision may be acceptable to a patient not prepared or motivated to have a corneal transplant.

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