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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.