OM t of c.iii - Vision Research Coordinating Center - Washington ...

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2/1/99 Chapter 2 Study Design page 2-7 The Executive Committee considered whether the presence of Vogt’s striae or a Fleischer’s ring or characteristic corneal scarring should be required in each eligible eye or in one eye. Keratoconus is bilateral 95% of the time (Krachmer et al., 1984). Because keratoconus can progress at different rates in each eye of a patient, Vogt’s striae or a Fleischer’s ring or corneal scarring may be present only in the more advanced eye. The probability that the corneal irregularity in an eye with a fellow eye showing Vogt’s striae or a Fleischer’s ring or corneal scarring does not represent keratoconus is very low. Also, such eyes typify the mild to moderate stages of the disease. Therefore, the eligibility criteria require either Vogt’s striae or a Fleischer’s ring or corneal scarring characteristic of keratoconus in at least one eye. 2.4.3d Contact Lenses Are Not an Entry Criterion Because contact lenses are an integral part of the standard care in keratoconus, it is reasonable to enter current wearers of contact lenses as well as non-wearers. 2.4.3e Other Criteria An entry criterion that was considered but rejected was evidence of an apical touch fluorescein pattern with a rigid contact lens whose base curve was equal to the flatter keratometric reading. Although this is an accepted clinical technique for early diagnosis of keratoconus, the reliability and validity of this procedure has not been evaluated. It would be prohibitively time-consuming to perform such an evaluation on all potentially eligible subjects. Another entry criterion that was suggested but rejected was the requirement of progressive change in the manifest refraction with time. This potential entry criterion would unnecessarily delay entry into the CLEK Study until some arbitrary increase in myopia and/or astigmatism was documented. Reliable manifest refractions are difficult to perform in keratoconus, so actual progression in the degree of myopia or astigmatism would be difficult to discern from the expected variability in subjective refraction. 2.4.3f Representativeness of the CLEK Study Sample The specific aim of the CLEK Study is to describe the course of patients with the diagnosis of keratoconus. No claim is made that patients identified with the diagnosis are representative of the undiagnosed keratoconus population or that the patients enrolled in the CLEK Study are representative of all diagnosed keratoconus patients. However, patients are typically first diagnosed with keratoconus by optometrists and typically receive eye care follow-up from optometrists prior to referral to ophthalmologists. Because the patients participating in the CLEK Study are recruited primarily from optometric clinics who receive referrals from community optometrists and ophthalmologists, the CLEK Study sample is “community” based. The alternative would have been to enroll patients from hundreds of smaller optometric and ophthalmologic practices which would be more costly and more time-consuming.
2/1/99 Chapter 2 Study Design page 2-8 2.5 CLEK Study Measures 2.5.1 Visual Acuity Visual acuity is an important measure for the following reasons: (1) It is widely acknowledged as a good clinical indicator of disease severity in keratoconus. (2) It relates directly to the keratoconus patient’s satisfaction with his or her mode of visual correction and frustration with the disease. (3) The clinically accepted criteria for penetrating keratoplasty in the 10% to 20% of keratoconus patients requiring surgery is insufficient visual acuity and/or intolerance to contact lenses that prevents the performance of the patient’s everyday tasks. (4) Measurement methods for visual acuity are straightforward and reliable. Visual acuity is measured in three ways: (1) High and low contrast Bailey-Lovie visual acuity with habitual correction, each eye separately and both eyes together; (2) High and low contrast Bailey-Lovie (Bailey and Lovie, 1976) visual acuity with best correction (rigid contact lenses (including Softperm and piggyback systems) with optimal over-refraction for rigid contact lens wearers or steep “K” contact lens (from the CLEK Study Trial Lens Set) with over-refraction for non-rigid contact lens wearers), each eye separately; and (3) High contrast Bailey-Lovie visual acuity with manifest refraction, each eye separately. The Bailey-Lovie chart is very similar to the visual acuity chart used in the Early Treatment of Diabetic Retinopathy Study (Ferris et al., 1982). The high and low contrast Bailey-Lovie charts are reversible wall charts. The chart is mounted at a distance of 4 meters from the patient. (If the largest letters cannot be read at 4m, the chart can be moved to 1 m.) The required chart luminance is 70 to 110 cd/m 2 . The same method for prompting the patient as used in the ETDRS (Chapter 6 of this Operations Manual) is employed to assure standardization of testing conditions, and a total number score (letters correct) is recorded. The CLEK Study visual acuity stopping rule requires that the patient reads letters across an entire row on the Bailey-Lovie chart. When 3 or more letters in a given row are read incorrectly, the visual acuity testing stops at the end of that row. Best corrected visual acuity provides a measure of the eye’s optimal level of performance and provides information comparing the current standard of care in optical management with the best achievable correction, including how much vision is lost by the inadequacy of current modes of correction to manage the corneal distortion and scarring exhibited in keratoconus. Habitual visual acuity provides an estimate of the vision that the patient experiences in the real world and may provide the best association with the visual quality of life estimates. This information also enables us to
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