Determining the Oxygen Needs of Patients
SiHy EXTENDED WEAR LENSES
Determining the Oxygen Needs of Patients
Eyes vary in the amount of oxygen they require.
Dr. Eiden: We examine a host of clinical factors before we prescribe silicone hydrogels (SiHy) or any other contact lenses. Certainly, ocular surface disease and tear quality are important as well as corneal hypoxic responses over time with existing contact lens wear.
INDIVIDUAL OXYGEN NEEDS
Dr. Eiden: When we consider prescribing a contact lens design and a wearing modality, typically we think about how the patient may respond. However, significant individual variations in physiological responses may exist. What individual variability do you see in oxygen requirements and corneal edema response to overnight contact lens wear? Do you believe that maximizing oxygen transmission with SiHy lenses creates a safety net for patients in terms of potential complications associated with hypoxia?
Dr. Szczotka-Flynn: Most clinicians believe we've solved the oxygen issue for the majority of patients. High-Dk/t SiHy lenses give us a large safety net. For example, the lenses can help patients who have high-powered prescriptions and whose lenses are thicker than the average prescription.
But some people have higher oxygen requirements than what SiHy lenses can provide, and these patients are difficult to identify. In a recent study1 I completed, 205 patients slept in Night & Day lenses, which have the highest Dk/t of any soft lens on the market. Still, we discontinued two patients from the study for corneal swelling. That was shocking to me, because I don't experience this in my clinical practice. According to my study,1 corneal edema can exist even at the highest Dk/t levels during continuous wear if we look closely at enough patients.
Dr. Eiden: When you looked at those two patients with edema, did you have any thoughts about what separated them from the rest of the study participants?
Dr. Szczotka-Flynn: I thought the findings were very odd. Both patients had central microcystic epithelial edema. I tried to decrease the length of wear, but it didn't go away. The edema resolved when the patients discontinued lens wear and returned to spectacles.
Dr. Hom: Some studies have shown that in addition to patient-to-patient variability, there's variability within a single patient. Overnight swelling in one eye can create significant variability — from 3.3% to 11.3%, according to one study.2 This makes the concept of an oxygen safety net even more important.
TESTS FOR SUCCESS
Dr. Eiden: Some patients may not be getting sufficient oxygen transmission through their contact lenses. Their corneal oxygen demand may be greater, or their lens design or wearing habits may create greater oxygen transmission demands. Can we determine the oxygen requirements of patients to predict whether high-Dk/t SiHy lenses will succeed and benefit them? What clinical testing do you perform to determine this?
Dr. Davis: With neophytes, it's best to start them with daily wear lenses to gauge their corneal response. We can work up to continuous wear if that's what they desire, but first I need to see them back in 6 months to determine how they're doing.
Dr. Sorrenson: I determine if the patient has limbal injection or neovascularization. If I see any signs of neovascularization or limbal injection, I strongly recommend SiHy lenses. I ask the patient if he experiences end-of-day discomfort or end-of-day redness. If he complains about end-of-day redness or discomfort, I suggest a trial with SiHy lenses to determine if they'll improve symptoms.
Dr. Goldberg: Pachymetry is affordable and available, and it's a practical tool. I see several patients who have keratoconus, and pachymetry helps me find the thinnest portion of the cornea. However, at the same time, I can get a finding for corneal edema.
Dr. Eiden: Are there any other methods you use?
Dr. Szczotka-Flynn: Solomon did a study more than 10 years ago, using a corneal stress test to establish hypoxic needs during extended wear.3 His cutoff for acceptable corneal swelling was 5% after one night of wear, which determined if a patient was a good candidate for extended wear. I'm not familiar with any other provocative test before the start of lens wear, so I never promise continuous-wear patients they're going to be successful.
I look for evidence of limbal stem cell deficiency or limbal epithelial hypertrophy inside the limbus (Figure 1) to determine if contact lens patients might have been hypoxic responders in the past.
Figure 1. Signs of limbal epithelial hypertrophy may help determine if patients are good candidates for extended wear silicone hydrogel lenses.
The cause of soft contact lens-related stem cell deficiency is unknown. It's probably a mixture of mechanical and hypoxic problems. Limbal epithelial hyperplasia (LEH) often is seen in long-term wearers of low-Dk soft lenses.
In both of these situations, you have to watch the Dk/t in the area covering the limbus. One study4 reported that instead of looking at the maximum Dk/t value of a lens in the thinnest area, we should look at Dk/t in the thickest area of the lens, which should determine what the minimum oxygen transmission should be for that patient.
BROAD SAFETY NET
Dr. Eiden: As you've mentioned, maximizing the Dk/t value helps create the largest safety net for patients. Could you explain this in greater detail?
Dr. Davis: Each patient's biochemistry and physiological requirements vary. What's important to remember is that studies represent the average patient's corneal response to extended wear. Theoretically, this means hyper-Dk contact lenses with a minimum Dk/t of 125 satisfy the physiological needs of only 50% of patients, so many of our patients still exhibit a less-than-optimum corneal swelling response.5–8
When you understand that individual oxygen requirements vary, it makes good sense for practitioners to choose a lens with the maximum oxygen transmission — the largest safety net.
Dr. Eiden: In the next article, we'll separate truth from misconception as we review the safety data of extended wear SiHy contact lenses. CLS
1. Szczotka-Flynn L, Stokkermans T. The Longitudinal Analysis of Silicone Hydrogel (LASH) Contact Lens Study: 1st Interim Analysis. Optom Vis Sci. E-abstract. 070036;2007.
2. CIBA Vision data on file, 2005.
3. Solomon OD. Corneal stress test for extended wear. CLAO J. 1996;22:75–78.
4. Bruce A. Local oxygen transmissibility of disposable contact lenses. Con Lens Ant Eye. 2003;26:189–196.
5. Harvitt D, Bonanno JA. Re-evaluation of the oxygen diffusion model for predicting minimum contact lens Dk/t values needed to avoid corneal anoxia. Optom Vis Sci. 1999;76:712–719.
6. Sweeney DF. Silicone Hydrogels: The Rebirth of Continuous Wear Contact Lenses. Oxford: Butterworth Heinemann;2000:93.
7. Papas, E. On the relationship between soft contact lens oxygen transmissibility and induced limbal hyperemia. Exp Eye Res. 1998;67:125–131.
8. Ghormley NR. How much oxygen is enough for safe lens wear? Contact Lens Spectrum. March 2005.
S. Barry Eiden, OD, FAAO, (moderator) is president and medical director of North Suburban Vision Consultants, Ltd., a private group practice specializing in primary eye care, complex contact lens management, treatment of eye diseases and refractive surgery. He's cofounder and president of EyeVis Eye and Vision Research Institute. He's an assistant clinical professor at the University of Illinois at Chicago Medical Center in the department of ophthalmology, Cornea and Contact Lens Service, and an adjunct faculty member of the Illinois, Salus, and UMSL Colleges of Optometry. Dr. Eiden has served as a consultant to Alcon, CIBA Vision and Synergeyes. He has received research funding from Alcon, Coopervision, EyeVis, Hydrogel and Synergeyes. He has been a member of advisory panels for CIBA Vision, Coopervision, Special Eyes and Synergeyes.
Jennifer E. Davis, OD, practices in Waynesboro, Va., and has been in private practice for 8 years. She was selected as Virginia's Young Optometrist of the Year in 2007. She's an adjunct assistant clinical professor at Pacific University College of Optometry and a visiting assistant professor of ophthalmology at the University of Virginia. Dr. Davis is a member of the Alcon speaker's alliance and the CIBA Vision speaker's bureau.
Fred Goldberg, OD, FAAO, is the founder of McLean Eyecare Center, a group practice in McLean, Va. He's served as a clinical faculty member at the Pennsylvania College of Optometry. He's lectured on pediatric contact practice and written articles on practice record keeping. He's the immediate past president of the Virginia Optometric Association and is the 2009 Virginia Optometrist of the Year. Dr. Goldberg has consulted for CIBA Vision.
Milton M. Hom, OD, FAAO (DipCL), practices in Azuza, Calif. He's a Diplomate in Cornea and Contact Lenses and has written more than 150 articles in various publications. He serves on several editorial boards and has published more than 25 abstracts and peer-reviewed studies. He's the author of Mosby's Ocular Drug Consult and Manual of Contact Lens Prescribing and Fitting, Third Edition. Dr. Hom has received research funding from Alcon, Allergan, AMO, Bausch & Lomb, CIBA Vision and Inspire.
Laurie Sorrenson, OD, FAAO, is part of a group private practice in Austin, Texas, and is 4th year Practice Management Coursemaster at the University of Houston College of Optometry. She's spoken extensively on therapeutic management of LASIK, glaucoma, dry eyes, keratoconus, topography, contact lenses, anterior segment pathology and oral medications. Dr. Sorrenson has been an advisor to CIBA Vision and Crystal Practice Management.
Loretta Szczotka-Flynn, OD, MS, FAAO (DipCL), is associate professor at Case Western Reserve University, Department of Ophthalmology & Visual Sciences, and director of the Contact Lens Service at the University Hospitals Case Medical Center in Cleveland. She's a Diplomate in the Cornea and Contact Lens Section of the American Academy of Optometry and Diplomate award chair for the section. She has received research funding from Alcon, CIBA Vision, Coopervision and Vistakon.
Contact Lens Spectrum, Issue: January 2010