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

2/1/99 Chapter 1 Background page 1-10
Previous large-scale studies of keratoconus have focused on describing the
disease’s incidence and prevalence, (Kennedy et al., 1986) attempting to distinguish
disease etiologies (Macsai et al., 1990; Swann and Waldron, 1986), or on trends in the
clinical management of keratoconus (Smiddy et al., 1988; Lass et al., 1990; Fowler et al.,
1988 ; Belin et al., 1988). Few have characterized the course of the disease and its
associated factors in large samples of keratoconus patients (Eggink et al., 1988). All
previous studies have relied on retrospective evaluations of keratoconus patients’
records.
The studies outlined in Table 1-2 below represent the largest samples of
keratoconus patients assembled to date, but they suffer from the following limitations:
• These studies have not reported on the prevalence of the biomicroscopic slit lamp
signs associated with keratoconus, including scarring, in anything but small
samples (Kennedy et al., 1986; Swann and Waldron, 1986).
• None of the biomicroscopic signs of keratoconus has been evaluated prospectively
in a sample of keratoconus patients.
• None of the previous studies has characterized the interactions of corneal
curvature, biomicroscopic findings, and vision in keratoconus.
• Although non-acuity visual function has been described in small samples of
keratoconus patients (Carney, 1982a; Carney, 1982b; Mannis et al., 1986; Zadnik
et al., 1987; Zadnik et al., 1984), none of the large-scale studies in Table 1-2 have
characterized vision and visual function beyond retrospective Snellen
measurements as recorded by the examining clinician.
• No surveys of visual quality of life in keratoconus have been performed, although
clinical, anecdotal reports of keratoconus patients’ dissatisfaction with their
vision are common.
• No data exist comparing keratoconus patients’ visual symptoms with their
clinically measured visual performance.
• Attempts to stage and/or classify keratoconus have been based on the severity
of corneal distortion (Mandell, 1988), on the corneal thickness profile (Mandell
and Polse, 1969), and on cone “type” or shape (Perry, et al., 1980). None of these
systems use criteria that are measurable in a standardized fashion.