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

2/1/99 Chapter 2 Study Design page 2-11
since large degrees of impairment are likely to be rare. (3) The instrument needs to
measure several dimensions of well-being. We considered using the CES-Depression
scale in addition to the SF-36; however, results using the CES-Depression scale that
were administered at the same time as the SF-36 in the CLEK pilot study showed
absolutely no trend for increased risk of depression in the total score for the CES-D or in
the mental health and emotional role functioning scale of the SF-36. Thus, the
measurement of emotional functioning does not justify a separate instrument like the
CES-D. (4) We desire a scale that could be self-administered without compromising
reliability. (5) We desire an instrument with strong psychometric properties including
good factor structure, high internal consistency reliability, high test-retest reliability,
and good external validity. For these reasons, the Medical Outcomes Short Form with
36 items (SF-36) was selected.
In addition to the SF-36 scale, a vision specific scale will be administered. The
Javitt Visual Function Questionnaire (Javitt et al., 1993) was selected because the format
of questions resembles the SF-36, which reduces response errors, and because it has
high test-retest reliability. The National Eye Institute Visual Function Questionnaire is
administered beginning in the first year of follow-up in the CLEK Study (Mangione et
al., 1998).
2.5.6 First Definite Apical Clearance Lens
At each CLEK Study Visit, the Clinician applies a series of CLEK Study trial
lenses, beginning with the lens with base curve equal to the steep keratometric reading,
until the first definite apical clearance fluorescein pattern is observed. This fluorescein
pattern is photographed, as is the lens that is 0.2 mm flatter than the first definite apical
clearance lens. This measurement provides an estimate of how flat or steep the patient’s
current lenses are and better defines the current standard of contact lens care in the
CLEK Study sample.
Thus, the CLEK Photography Reading Center provides an independent, masked,
standardized assessment of the habitual contact lens fit, the First Definite Apical
Clearance Lens, and the 0.2 mm flatter lens throughout the course of the study. In doing
so, the CLEK Photography Reading Center provides photodocumentation of the
standard of contact lens care of keratoconus patients in the CLEK Study sample and an
estimate of the degree of flatness (or steepness) of the habitual contact lens as compared
to the first definite apical clearance lens. Additionally, the First Definite Apical
Clearance Lens provides an estimate of the progression of the steepness of the apex of
the cornea during the Study.
2.5.7 Surgical Intervention
Corneal surgery is indicated in 10-20% of all cases of keratoconus (Kennedy et
al., 1986; Smiddy et al., 1988). The need for surgery has high clinical relevance, definable