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Silicone hydrogels alleviate many risks associated with extended wear
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Silicone hydrogels are a relatively new advance for the contact lens practitioner. Because of the difficulties in the past with devastating diseases such as microbial keratitis, many practitioners have considered extended wear a high risk option. Many of us are in the “wait and see” mode when it comes to silicone hydrogels. But it seems that they are a leap forward in alleviating the risk of microbial keratitis.
The key aspect of using silicone hydrogels is the high oxygen transmissibility. This has been achieved by incorporating the silicone monomer with the hydrophilic vinyl carbamate group and a cross-linker. The results are Dks over 100 (110 barrers for balafilcon, 140 barrers for lotrafilcon). When evaluating clinical performance, silicone hydrogels either outperform or are comparable to disposable soft contact lenses (Sweeney D. Silicone Hydrogels: The Rebirth of Continuous Wear Contact Lenses. Boston: Butterworth-Heinemann, 2000).
Big improvement over mid-water
When considering corneal swelling, formation of striae and folds, and epithelial microcysts, silicone hydrogels offer much improvement over mid-water disposable soft lenses. Corneal swelling seen with silicone hydrogels is comparable to no lens wear. Striae and folds are infrequent. Epithelial microcysts are significantly less frequent. The risk of microbial keratitis has also been greatly reduced with silicone hydrogels; there are no reports to date of microbial keratitis with silicone hydrogel extended wear.
With regard to hyperemia and vascularization, occurrences of bulbar and limbal hyperemia are similar to disposable soft lenses at baseline. Limbal hyperemia does show a reduction over a 12-month period. Limbal vascularization is also reported to be less.
See the photos below for an example of limbal vascularization. The patient had been wearing a mid-water content lens on a daily-wear basis. Two vessels can be seen at the 7 o’clock position penetrating the cornea.
The patient was re-fit into extended wear with a silicone hydrogel lens. The neovascularization resolved after 3 weeks. Even on an extended-wear basis, the oxygen transmissibility of silicone hydrogels was great enough to overcome daily wear neovascularization in this case.
Silicone hydrogels and dryness
Studies reveal some interesting findings regarding the relationships of silicone hydrogels to dryness (Fonn D, Pritchard N, Dumbleton K. Factors affecting the success of silicone hydrogels. Clinical trial at CCLR, University of Waterloo. In: D. Sweeney. Silicone hydrogels. Boston: Butterworth-Heinemann, 2000). Dryness can take many forms: general dryness, end-of-day dryness and beginning-of-day dryness.
Beginning-of-day dryness is commonly seen with all types of extended wear and can usually be relieved with a wetting drop. For general dryness, silicone hydrogels are comparable to disposable soft lenses. However, silicone hydrogels show greater comfort with reduced end-of-day dryness. The reason end-of-day dryness is reduced may be due to the lack of hypoxia.
Several years ago, Epstein and Freedman identified hypoxia as a cause of dryness (Epstein AB Freedman JM. The two sides of the dry eye syndrome. Contact Lens Forum. 1985;10(8):33-34). With the extremely high Dk of silicone hydrogels, hypoxia has become inconsequential. This may be the reason these lenses have reduced end-of-day dryness.
Bulbar, indentation staining
Two types of conjunctival staining are characteristically seen with silicone hydrogel lenses: bulbar and indentation. When bulbar staining appears, the natural inclination is to assume the patient has dry eyes. However, as covered earlier, end-of-day dryness is relieved with silicone hydrogel. In my experience, patients who wear silicone hydrogels and have bulbar staining are usually asymptomatic.
A paradox is formed when one sees clinical signs of dryness without symptoms. It seems the rigidity of the high Dk lenses are the probable cause. A typical mid-water content lens, such as etafilcon A (Acuvue, Vistakon), has a modulus of 0.26 mN/m2. Silicone hydrogels have a modulus of 1.1 mN/m2, approximately four times the rigidity of disposable soft lenses. The higher rigidity of the lenses may be altering the lid wiping. Tear coverage of the bulbar conjunctiva is changed, and staining results.
This is not unlike the situation with rigid gas-permeable (RGP) lenses. For those of us who fit many RGPs in our practice, bulbar staining (not corneal) is regarded as a normal occurrence. RGPs have much higher rigidity than disposable soft lenses.
Surface interaction
The surface interaction between the silicone hydrogel and the mucin layer may be yet another contributing factor. We know that mucus balls form as a result of surface interaction. This interaction may alter the mucin layer under the lens, as well as over the entire ocular surface. An altered mucin layer can appear as bulbar staining.
A dryness contradiction results with respect to bulbar staining. The staining strongly suggests dryness, but the symptoms suggest otherwise. The eye appears dry, but is actually not.
Indentation stain is another characteristic conjunctival stain. Although indentation stain is seen with disposable lenses, it seems to be a more frequent occurrence with silicone hydrogels. Higher lens rigidity is again the probable cause.
SEAL
Finally, superior epithelial arcuate staining or lesion (SEAL) can be seen with the new silicone hydrogels. Ronald Watanabe, OD, FAAO, of New England College of Optometry, tells the story of a patient with a K reading in the right eye of 41.75/42.50 @90; refraction: –4.00 –0.50 x 180 20/20. The patient was wearing 1-day Acuvue 9.0/14.2/–3.75 and was refitted to PureVision (balafilcon A, Bausch & Lomb) 8.6/14.0/–3.75.
The patient’s wearing schedule was 7 days/6 nights extended wear. On the third day of the second week, the patient awoke with a foreign body sensation. The sensation worsened with lens removal. Visual acuity remained good.
The patient chose to discontinue wear, and symptoms reduced after 1 day. The patient then decided to return to another night of extended wear. The next day, the foreign body sensation increased to pain and worsened with lens removal. SEAL was seen. The patient was treated with lubricants and discontinued wear for 1 week. The patient was then returned to Acuvue 1-day.
The higher rigidity of the silicone hydrogels are a causal factor in this case of SEAL.
Although SEAL has been seen in the past with extended wear, it occurs at higher rates with silicone hydrogels. The lid shearing forces create pressure on the superior cornea, resulting in SEAL. The higher rigidity silicone hydrogel transmits more of these forces.
Despite the complications, silicone hydrogels are welcomed options. In some cases, silicone hydrogels in the extended-wear mode are healthier than daily wear.
For Your Information:
- Milton M. Hom, OD, FAAO, practices in Azusa, Clif., and is a member of the Primary Care Optometry News Editorial Board. He can be reached at 1131 E. Alosta, Azusa, CA 91702; (626) 963-7100; fax: (626)335-1402; e-mail: eye-mage@jps.net. Dr. Hom has no direct financial interest in the products mentioned in this article, nor is he a paid consultant for any of the companies mentioned.