By Lisa Ly
BOptom (Hons) GradCertOcTher
State president South Australia, Cornea & Contact Lens Society of Australia
Silicone hydrogels have revolutionised soft contact lenses, providing increased oxygen permeability and comfort. How do they perform in a conventional lens design?
A 55-year-old woman presented for her routine after-care to replace her 18-month-old conventional silicone hydrogel lenses. She was a daily wearer, using AO Sept and Ultrazyme protein remover. Her main concern was worse distance vision in her contact lenses compared to her glasses, causing a loss of confidence in night-time driving. Her near vision was acceptable.
Her last glasses prescription was: SVD R -12.50/-2.75x6 L -12.75/3.00x174.
Her last contact lens prescription was:
SH74 BC8.7 Diam 14 mm
R -11.00/-2.00x180 (6/12) ORx-1.00/-0.50x180 6/6-
L -11.25/-3.50x180 (6/24) ORx-0.75/-0.50x180 6/7.5+2
Near VA OU N5
Lens fitting was acceptable although compromised surface quality was noted. The woman was trial framed in monovision but immediately reported feeling ‘unbalanced’. After discussing her options for presbyopic correction while improving her distance vision at night, she chose to have her full distance correction with readers over the top.
Replacement lenses were ordered to:
SH74 BC8.7 Diam 14 mm R -12.00/-2.50x180 L -12.00/-4.00x180.
After trialling her new lenses, the woman felt ‘sick’ as she could not see near as well as she had previously. She also reported fluctuating vision with blinks, and less comfort when compared to her older lenses. After a lengthy discussion, she preferred better near vision with compromised distance, and over-spectacles for night driving.
Replacement lenses were ordered in her original prescription. After trialling these, the woman surprisingly remarked that her distance vision was great, but her near vision was ‘hopeless’. Her VA was R 6/6 L6/7.5-2, OU N6.3@40 cm ORx R+0.75 6/7.5- L +0.50 6/7.5-2. Over-refraction to achieve her original entrance acuity was R +2.00 6/12 L +3.25 6/24.
At this point the clinical findings did not reflect the old lenses she had presented with, so the old lenses were sent to the laboratory for verification. They measured R -9.00/-1.25x180 L -9.25-1.75x180 with an expanded diameter of 15.5 mm. Apparently the lenses had swelled, which can be caused by multipurpose solution or protein remover.
The laboratory reported that the old lenses had swollen, resulting in a hyperopic shift of 2.00 Dsph in each eye, and 0.75 Dcyl and 1.75 Dcyl in the right and left lenses, respectively, as well as a 1.5 mm increase in the lens diameter. This was attributed to the Ultrazyme protein remover the patient had been using regularly.
It was also advised that the SH74 material is harder to cut and less stable during the hydration process, compared to hydrogel materials, producing variable results and poor reproducibility. With future years in mind, a refitting with RGP lenses was recommended but the patient could not be convinced to try them.
Replacement lenses were ordered to: SH74 BC8.7 Diam 15 mm R -9.00/-2.00x180 L -9.00/-2.00x180. The diameter was increased to mimic the increased comfort in the swollen lenses.
On collection, VA was R 6/15- L 6/15- OU 6/12 with ORx R -1.50/-0.50x180 6/6- L -1.50/-0.50x180 6/6. Both lenses were comfortable and fitted well, but a replacement right lens was ordered to account for mild lens rotation (rotated clockwise by five degrees).
On collection of the new right lens (-9.00/-2.00x5) VA was unexpectedly significantly improved to 6/7.5-2, with +1.00 pushing VA to 6/15. She was able to see N3 at 25 cm comfortably.
Despite the inaccurately reproduced right lens, the patient was pleased with the unexpected compromised monovision and she was given over-spectacles for night driving. The happy camper was recommended to continue with AO Sept but to cease protein remover use, and to adhere to the recommended 12-month replacement schedule.
This woman was historically adverse to RGP lenses, due to their reputation for being uncomfortable, and was therefore refitted with the SH74 material to increase the oxygen permeability of her existing conventional hydrogel custom lenses.
With the increased oxygen permeability of a silicone hydrogel come increased deposits, which are not so noticeable in a monthly or daily replacement lens but are definitely noticeable in an annual replacement modality.
Protein remover is necessary for this reason but can cause the lenses to swell or discolour. As multipurpose solution can also cause lens swelling, peroxide solution is the manufacturer’s recommended disinfection modality. A strict 12-month replacement schedule is recommended to maintain maximum lens quality and integrity.