Digging Through the Literature
Use this review to catch up with the latest contact lens and nutrition news.
By Marjorie J. Rah, OD, PhD
If you are looking for a way to catch up on the latest contact lens literature, but do not have time to search through all of those journals, then the January 2003 supplement to Eye & Contact Lens: Science and Clinical Practice journal is a good place to start. This particular issue is a great compilation of articles written after the Continuous Wear Contact Lenses for the New Millennium: Challenges, Controversies, and New Opportunities World Summit Symposium that was held in San Diego in August 2002.
But just in case you do not have the time to track down even that issue, I will provide the highlights of its supplement, along with summaries of additional articles that I have found interesting over the past few months, in this article.
The FDA recently approved the following lenses for 30-day extended wear: Focus Night & Day (CIBA Vision), PureVision (Bausch & Lomb) and Menicon Z (Menicon). The introduction of these hyper oxygen transmissible lenses has brought forth several questions. Do we finally have lenses that will meet the criteria of oxygen transmissibility to avoid overnight corneal edema? If so, will they eliminate the risks associated with extended wear of hydrogel lenses? Will silicone hydrogel contact lenses replace conventional soft lens materials?
To determine the effects that extended wear soft lenses of varying Dk values have on the cornea, Ladage et al (2003) studied epithelial homeostasis. Although silicone hydrogel lenses suppressed the basal cell proliferation rate the least, the authors noted a reduced proliferation rate for all extended wear lens types. Similarly, short-term extended wear of all lens types suppressed the normal movement of basal cells toward the corneal surface and interfered with the normal apoptotic exfoliation of surface epithelial cells. The clinical significance of slower epithelial renewal rates warrants further study.
Cavanagh et al (2003) detected reduced levels of bacterial binding with silicone hydrogel lenses when compared to Vistakon's Acuvue lenses. The authors hypothesized that the reduction in Pseudomonas binding will result in a lower risk of microbial keratitis in both daily and extended wear of silicone hydrogel lenses.
However, because of the large number of patients required, it is difficult to accurately determine the incidence of microbial keratitis. McNally et al (2003) looked at risk factors for infiltrative events in silicone hydrogel lens wearers, defined as a corneal infiltrate grade 3 or greater or any corneal infiltrate with an overlying epithelial defect. Out of 658 patients, 33 (five percent) experienced an infiltrative event. Ten of the 33 patients (30 percent) also experienced a second infiltrative event. Significant risk factors include smoking, age, smoking and young age combined and a history of corneal scar, contact lens acute red eye or corneal infiltrates. These results show us that hyper oxygen transmissible lenses have not completely eradicated adverse reactions in soft lens wear. Social risks such as smoking and poor compliance will continue to plague contact lens wear.
The good news is that silicone hydrogel lens wear results in less limbal injection and corneal vascularization than with additional hydrogel lenses. Fewer microcysts and lower levels of corneal edema have also been noted (Sweeney 2003) in silicone hydrogels than in other lens materials. Protein deposition is less of a problem with silicone hydrogel lenses; however, lipid deposition may be greater in some patients (Jones et al 2003).
Reports on the mechanism, efficacy and safety of contact lens corneal reshaping continue to appear.
Measuring Efficacy Sridharan and Swarbrick (2003) recently reported on corneal changes following short-term lens wear. The researchers asked nine adult patients to wear reverse geometry lenses in one eye for 10, 30 and 60 minutes while keeping their eyes open, and for eight hours while their eyes remained closed during sleep. Each of these lens-wearing sessions took place at least one week apart to ensure complete recovery of the cornea between sessions. The investigators noted statistically significant reductions in apical corneal power compared to baseline values for all sessions (p<0.001). Keratometry readings confirmed this finding. The changes for both apical corneal power (0.61D ±0.35D after 10 minutes and 1.63D ± 0.46D after eight hours) and keratometry readings (horizontal: 0.35D ±0.38D after 10 minutes and 0.90D ±0.63D after eight hours) increased with increased length of lens wear. The study investigators also noted significant changes in treatment zone diameter and uncorrected logMAR visual acuity. Again, the changes increased with increased length of lens wear (UVA: 0.16D ±0.18D after 10 minutes and 0.68D ±0.25D after eight hours; TZD: 3.86mm ± 0.88mm after 10 minutes and 5.59mm ±0.83mm after eight hours). This study shows that significant changes in corneal curvature occur after as little as 10 minutes of reverse geometry lens wear.
Corneal Reshaping and Thinning In another current article, Alharbi and Swarbrick (2003) report fitting reverse geometry lenses on both eyes of 18 patients and conventional gas permeable lenses on one eye of 10 patients (the controls). The total diameter, material, center thickness and Dk were similar for the two lens types. The researchers performed optical pachymetry on all patients at baseline and after one, four, 10, 30, 60 and 90 days of overnight lens wear. The measurements were taken approximately nine hours following lens removal.
Over the entire study period (three months), the researchers noted no significant changes in central total, epithelial or stromal thickness. They did, however, notice significant thinning of the central epithelium in the reverse geometry lens-wearing patients from day one. This thinning did stabilize at 10 days. No significant changes in the central stromal thickness occurred over the study period in this group of patients. The reverse was true in the nasal and temporal midperipheral epithelium and stroma. The researchers did observe a significant increase in stromal thickness in these regions and the epithelial thickness remained relatively constant. The researchers noted no changes in the peripheral total, epithelial or stromal thickness in either group. So what do these data tell us? The changes in corneal topography that occur with overnight wear of reverse geometry lenses primarily result from central epithelial thinning and midperipheral stromal thickening of the cornea. What these data do not tell us is whether these changes are caused by redistribution of existing tissue, compression or remodeling of tissue or edematous changes.
Examining the Safety of Corneal Reshaping On a different note, the safety of this procedure has been questioned by reports of infectious keratitis in patients undergoing contact lens corneal reshaping. Although it is difficult to determine the incidence of cases attributable to this procedure, since 2001 five papers have reported cases of microbial keratitis. The reports span four countries, though none from the United States, and the patients range in age from 9 to 37. Four cases of Pseudomonas aeruginosa infection were detected, one case of Serratia Marcescens, one of Acanthamoeba and one case where no organism was detected following culture. Poor lens hygiene and compliance may have caused some of the cases.
Iron deposition in the cornea, a much more benign finding, may also occur in patients undergoing corneal reshaping. The appearance is similar to a Fleischer's Ring or to iron deposition detected following refractive surgery or penetrating keratoplasty. The finding may result from tear pooling in the region of the steep secondary curve of the lenses. It is considered to be non-sight threatening and requires no treatment.
Changes Following Cessation of Lens Wear
How long should a patient abstain from contact lens wear before refractive surgery? If a patient hasn't already asked you this question, then no doubt one will at some time or another. This question was even the basis for a study conducted by Wang et al (2002) in which they asked 165 consecutive contact lens-wearing patients to abstain from lens wear before refractive surgery. The purpose was to evaluate the time to resolution of contact lens-induced corneal warpage defined as a manifest refraction change less than or equal to ±0.50D, keratometry change less than or equal to ±0.50D and corneal topography pattern normalization.
Of the 165 patients, 88 wore soft lenses and 77 wore GP lenses and the mean duration of contact lens wear was 18.9 ± 9.9 years (range 2 to 40 years). Of these patients, 20 eyes of 11 patients had corneal warpage. However, the corneal warpage was not limited to the GP lens wearers. In fact, of the eyes with corneal warpage, 12 wore soft lenses and eight wore GPs.
Among the soft lens wearers, six wore extended wear lenses, four wore toric lenses and two wore disposable daily wear lenses. When asked to cease lens wear, it took an average of 7.8 ± 6.7 weeks (range one to 20 weeks) for corneal recovery with the longest recovery time occurring in the soft extended wear patients at a mean of 11.6 ± 8.5 weeks, followed by soft toric lenses at a mean of 5.5 ± 4.9 weeks, soft daily wear (2.5 ± 2.1 weeks) and GPs (8.8 ± 6.8 weeks). As you can see, all types of contact lenses can cause corneal warpage. The take-home message is that patients should discontinue contact lens wear of any type until corneal stabilization is evident before undergoing refractive surgery.
Lutein, Zeaxanthin and AMD
Now for the posterior portion of the eye. Gale et al (2003) reported their results using the Wisconsin Age-Related Maculopathy Grading System to evaluate the relation between plasma concentrations of lutein and zeaxanthin, xanthophylls, carotenoids and the presence of age-related macular degeneration (AMD). Of the 207 men and 173 women they examined, nearly 21 percent had signs of either early or late macular degeneration. Those who had macular degeneration had significantly lower concentrations of plasma zeaxanthin. The plasma concentrations of lutein in patients who had macular degeneration were lower than those who didn't have signs of the disease, but the difference did not reach statistical significance. Patients who exhibited signs of AMD also were more likely to have had coronary artery surgery and reported a history of consuming beer at least once each week.
To receive references via fax, call (800) 239-4684 and request document #97. (Have a fax number ready.)
Dr. Rah is an assistant professor at the New England College of Optometry, where she works primarily in the Cornea and Contact Lens Service in patient care, teaching and research.