Tony Chahine, OD is an optometrist with a private practice in La Cañada, a suburb outside Los Angeles, where he specializes in difficult and specialty contact lens fittings.MTC, a 46-year-old Caucasian female, had been a long-time wearer of spherical rigid gas permeable (RGP) contact lenses. Two years prior, her lenses were adjusted for monovision (OD distance, OS near):
| Entering visual acuity (monovision): | ||
| OD: | 20/20 at distance | J5 at 40 cm |
| OS: | 20/70 at distance | J2 at 40 cm |
| OU: | 20/20 at distance | J2 at 40 cm |
| Sphero-cylindrical over-refraction: | ||
| OD: | Plano DS | 20/20 |
| OS: | –1.25 DS | 20/20 |
As a research scientist on the NASA mission that sent two rovers to the planet Mars, MTC’s increasing workload during the mission’s success augmented her presbyopic symptoms. To perform her near and computer work (10 – 12 hours daily, both laptop and desktop), and still see well in the distance, MTC needed more help. She rejected enhancing her monovision on the principle of not sacrificing her distance vision any longer. This case report illustrates her refitting process.
| Manifest refraction: | ||
| OD: | –6.50 DS | 20/20 |
| OS: | –5.50 –0.50 X 045 | 20/20 |
| +1.50 add | 20/20 @ 40 cm | |
| Topography (Humphrey-Zeiss Atlas topographer): | ||
| OD: | 43.50 at 180 | 43.75 at 090 |
| OS: | 44.00 at 030 | 45.00 at 120 |
Following a discussion on the merits of monovision, translating bifocal and multifocal rigid gas permeable (RGP) contact lenses, MTC preferred the multifocal design.
I considered MTC to be an early presbyope with a slightly reduced tear film (her tear break-up time was 8 seconds in each eye). She had an 11.5 mm corneal diameter but only a 9.5 mm palpebral fissure. Her lower lid margin was about 1.0 mm above the lower limbus while her upper lid margin position varied according to her state of relaxation (between 1.0 and 2.0 mm below the superior limbus). The pupil size was 3.0 mm in each eye under mesopic conditions as measured with a ruler in the exam room and confirmed by the corneal topographer.
In preparation for her new lenses, MTC agreed to modify the placement of her work and home office computers and to change her habits by taking more frequent visual breaks and using artificial tears.
Aspheric multifocal RGP lens design:
When ordering aspheric multifocal RGP lenses, I have found it advantageous to control, or at least be aware of, as many parameters as possible. With proprietary designs, this is often challenging.
For MTC, I began with a custom design that I could modify to her demanding VDT (visual display terminal) needs, and ordered the first set of lenses based on an empirical fit. I chose an eccentricity value of 0.9 so that there would be adequate plus power as the base curve flattened toward the periphery.
As with other aspheric multifocals, MTC’s lenses had to be fit steep. To determine the base curve, I first determined the toricity (0.25D OD and1.00D OS). Using these values and Mandell’s recommendations as a guide, a spherical lens would be fit along the flat-K OD (43.50D) and 0.25D steeper than flat-K OS (44.25D). Because MTC was a younger presbyope, I wanted her aspheric lenses to be fit 2.50D steep. For older presbyopes, or those who need a stronger reading add, I normally fit 3.00D steeper.
In determining the back-vertex power of the central region (i.e. the distance power), the manifest refraction was adjusted for the vertex distance, -2.50D was added to account for the steep fit, and in an effort to avoid over-correction in the distance an extra +0.50 was added. In the event of subjective distance blur when the lenses are dispensed, this under-correction can be easily reversed by adding minus power with an in-office lens modification.
Hydro-2 material was chosen because it has a low wetting angle. Whereas higher Dk and index of refraction materials are available, they can come at the price of poorer wetting and hence reduced lens comfort. In my opinion, MTC’s low risk of corneal hypoxia with RGP lenses seemed much less worrisome than her potential for age and VDT-related dry eyes.
By favoring good wetting over superior Dk, the hope was to avoid a future situation where the RGP bifocals fit well and provide excellent vision, but are uncomfortable to wear.
With the above factored, the following was ordered:
| Lens: | OD aspheric multifocal | OS aspheric multifocal |
| Base curve: | 7.34 (46.00) | 7.22 (46.75) |
| Back vertex power: | -7.75 | -7.25 |
| Overall lens diameter: | 9.6 | 9.6 |
| Optic zone: | 8.8 | 8.8 |
| Peripheral curve radius/width: | 14.0 / 0.4 | 14.0 / 0.4 |
| Lens material: | Hydro-2 | Hydro-2 |
| Lens color: | Soft blue | Soft blue |
Contact lens progress check 1
After one week of multifocal lens wear, MTC felt that the lenses were comfortable but she complained that every aspect of her vision was sacrificed slightly. She felt her nighttime driving vision was reduced, and she could not see the slide presentations at meetings if the room lights were dimmed. She could see her 17” desktop VDT but had difficulties with her smaller laptop screen. She was very satisfied with her reading ability with printed material, provided that she used a reading lamp (which she did not mind doing). She also noticed that her spectacle-corrected vision was worse immediately after contact lens removal than in the morning before contact lens insertion.
| Visual acuity: | ||
| OD: | 20/25-2 at distance | J4 at 40 cm |
| OS: | 20/25-2 at distance | J4 at 40 cm |
| OU: | 20/20-3 at distance | J3 at 40 cm |
| Sphero-cylindrical over-refraction: | ||
| OD: | –0.50 DS | 20/20-1 |
| OS: | –0.50 DS | 20/20-1 |
| Contact lens fit: |
| Both lenses maintained a lid attachment fit with a slight superior decentration. There remained a 3-4 mm zone of NaFl pooling centrally, a 2-3 mm band of mid-peripheral touch and a good peripheral pooling system. Even though MTC could see 20/20-3 OU with her current lenses, I felt that she was more annoyed by the distance blur than she was pleased by her good VDT and near vision. I proceeded with an in-office modification (using a cone shaped sponge and polishing compound) to add–0.50 D to the central 4 mm of each lens. |
Contact lens progress check 2
One week later, MTC was much more satisfied with her distance vision in the daytime and noticed a slight improvement at nighttime. She was still not pleased with her vision when viewing slide presentations at meetings, since the slides usually contained a lot of information, much of which she was unable to see clearly. Regarding her VDT vision, she stated that she could manage, but only after fussing with lights and adjusting her working distance. Finally, she felt that her near vision was slightly worse following the contact lens modification and resigned herself to using a reading lamp, which helped tremendously.
| Visual acuity: | ||
| OD: | 20/20-2 at distance | J4 at 40 cm |
| OS: | 20/20-2 at distance | J4 at 40 cm |
| OU: | 20/20 at distance | J3 at 40 cm |
| Sphero-cylindrical over-refraction: | ||
| OD: | Plano DS | 20/20-2 |
| OS: | Plano DS | 20/20-2 |
The aspheric RGP multifocal lenses fit well and the vision was reasonable (from an objective point of view), but MTC seemed more resigned to the limitations than I had liked. Her quiet disappointment was really displeasure with simultaneous vision and the subtle, omnipresent blur. Because of the temporary spectacle blur induced by the steep fitting, aspheric RGP lenses, MTC avoided wearing her glasses in the evening and resorted to wearing her contact lenses until bedtime. She found her spectacle vision to be clear only in the morning, before inserting her contact lenses, and thus could only wear them then.
In short, MTC was a perfectionist, and so I felt I needed to try a different bifocal design if I were to come reasonably close to her high expectations. I explained that her visual requirements exceed the limitations of the pupillo-centric aspheric design, and that I might provide her with crisper vision if I employed an alternating vision translating bifocal. Because of her excessive computer use, however, I planned to incorporate a mild modified monovision approach to her refit. I chose the Tangent Streak Bifocal and followed their fitting guide.
OD:
- the guide suggests fitting 1.00D flatter than steep K since the toricity is less than 1.00D.
- The recommended base curve is 42.75 (7.89) if the horizontal diameter is 9.5, truncated to 9.1 vertically. Since I desired a smaller lens for MTC’s smaller palpebral aperture (9.2 / 8.8), they suggested steepening the base curve by 0.25D.
- The back vertex power reflected the vertex distance and recommended base curve adjustments, and the add was chosen to be +1.50D.
- The recommended segment height is 3.9 mm if the limbus meets the lower eyelid margin. Since MTC’s lower lid was about 1.0 mm above the lower limbus, I reduced the segment height of 3.8 mm.
- The lens material would remain Hydro-2 since comfort and dryness could be a bigger issue with this thicker lens design. Finally, the prism ballast was kept standard at 3.0∆ base down.
OS:
- The guide suggested fitting 1.00D flatter than steep K since the toricity was 1.00D.
- The recommended base curve is 44.00 (7.67) for a 9.5 / 9.1 diameter lens, but had to be steepened by 0.25D since I was ordering a smaller lens (9.2 / 8.8).
- Like the OD, the back vertex power reflected the vertex distance and recommended base curve changes, but with a minor adjustment. Since I desired a modified monovision, +0.50 would be added over the distance zone for VDT distances. The near add would then be reduced to +1.00D so that the left and right effective adds would be the same.
- The segment height and lens material would be like the OD and the prism ballast was kept standard at 3.0∆.
The following lenses were ordered:
| Lens: | OD Tangent Streak bifocal | OS Tangent Streak bifocal |
| Base curve: | 7.85 (43.00) | 7.63 (44.25) |
| Back vertex powers: | -5.25 with +1.50 add | -4.75 with +1.00 add |
| Horizontal and vertical lens diameter: | 9.2 truncated to 8.8 | 9.2 truncated to 8.8 |
| Segment height: | 3.8 mm | 3.8 mm |
| Center thickness: | Not specified | Not specified |
| Prism ballast: | 3.0∆ | 3.0∆ |
| Peripheral curve radius/ width: | Standard | Standard |
| Lens material: | Hydro-2 | Hydro-2 |
| Lens color: | Soft Blue | Soft Blue |
Contact lens dispense:
With a big smile, MTC said that she liked these contact lenses more than the previous design. Overall, she felt that her vision was much better.
| Visual acuity: | ||
| OD: | 20/20 at distance | J2 at 40 cm |
| OS: | 20/25 at distance | J2 at 40 cm |
| OU: | 20/20 at distance | J2 at 40 cm |
| Sphero-cylindrical over-refraction: | ||
| OD: | Plano DS | 20/20-2 |
| OS: | -0.50 DS | 20/20-2 |
| Contact lens fit: |
| Both lenses were slow to drop and rotated nasally by an estimated 30°. They were well centered and the seg line was just below the pupil margin. Following the application of NaFl, both lenses demonstrated central pooling, mid-peripheral touch and an adequate peripheral pooling system. The Tangent Streak fitting guide explains that lenses that are slow to drop and rotate nasally by more than 15° (among other things) are fit too steep. MTC’s lens behavior and NaFl pattern confirmed exactly this. |
A second pair was ordered with base curves flatter by 0.50D OD and 1.00D OS.
One-week progress check
At the one-week progress check, MTC said that she “loved” her contacts and vision.
| Visual acuity: | ||
| OD: | 20/20 at distance | J2 at 40 cm |
| OS: | 20/25- at distance | J2 at 40 cm |
| OU: | 20/20 at distance | J2 at 40 cm |
| Sphero-cylindrical over-refraction: | ||
| OD: | Plano DS | 20/20-2 |
| OS: | -0.50 DS | 20/20-2 |
| Contact lens fit: |
| They were very comfortable and she could see much better in bright and dim illuminations than with any other lenses. She had no difficulties seeing the slide presentations in dimly lit auditoriums. Furthermore, she expressed no difficulties in viewing her laptop or desktop screens and did not need extra lighting to read. Both lenses positioned interpalpebrally and translated well. NaFl revealed light touch under the distance portion of the lens, and slight pooling under the reading area, both of which are desirable. There was good mid-peripheral touch and a good peripheral pooling system. Following each blink, the lens rode up with the upper eyelid and then quickly dropped back into place. The truncated edge rested on the lower lid, there was no rotation and the segment line was positioned at the lower margin of the pupil. |
Discussion, summary and conclusion
This case of a bifocal fitting illustrates the value of having multiple lens designs at one’s disposal. Binocularly, MTC ended up with 20/20 vision in the distance and could read printed material easily. From an ergonomic standpoint, she consciously kept her desktop monitor at a comfortable position so that she could view it clearly through the top portion of the left lens, without tilting her neck. For the lap top computer, she indicated that it was not a problem with either the intermediate or near zone if she was conscious where she placed it. Finally, and much to her pleasure, she was able to alternate quickly between her contact lenses and spectacles because the RGP base curves were spherical.
