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

2/1/99 Chapter 2 Study Design page 2-9
assess the adequacy of current standards of optical management in the CLEK Study
patient sample. Because keratoconus patients often request back-up eyeglasses, the
information from the manifest refraction supplies information on the visual acuity that
can be achieved with this mode of correction. The CLEK Study will also determine the
stability of the resultant prescription and will provide the ophthalmic community with
guidelines for prescribing spectacles for keratoconus patients.
2.5.2 Corneal Curvature
Corneal curvature and changes in corneal curvature (central keratometry) are
monitored for disease progression. Although videokeratography has been repeatedly
recommended to the CLEK Executive Committee for characterizing disease severity
and progression, we selected keratometry over corneal topography for the following
reasons.
It has been demonstrated that current videokeratoscopy systems can measure
spherical surfaces with at least ±0.37 D accuracy (Hannush, et al., 1989; Koch, et al.,
1992). The normal corneal surface can be measured with repeatability of ±0.37 D to
±0.50 D (Zadnik, et al., 1992; Williams, et al., 1991). However, the instrumentation lacks
normative standards and reproducibility standards for the measurement of abnormal
corneas (National Advisory Eye Council, 1994; Antalis, et al., 1993), such as keratoconic
corneas. Mandell and Shie (1993) have documented a videokeratographic value of 57.79
D for a keratoconus patient’s decentered corneal apex in primary gaze. Upon changing
the patient’s fixation to center the keratoconic corneal apex, the value was 76.70 D. This
example illustrates the limitations of the currently available software with regard to
evaluating irregular corneal surfaces (Mandell and Shie, 1993).
Nonetheless, videokeratography will be a required measure for all CLEK
Participating Clinics. CLEK Clinics will collect videokeratography data with Tomey’s
Topographic Modeling System if they have one, or with whatever videokeratography
system they do have if not a TMS, for future analysis and possible ancillary studies.
Data from Clinics with TMS devices are sent to the Chairman’s Office for storage and
future analysis. Data from Clinics with non-TMS devices are stored at the Clinics.
2.5.3 Corneal Scarring
Corneal scarring is a direct measure of corneal compromise. Scarring will be
observed clinically, documented photographically, and evaluated in a masked fashion
by the CLEK Photography Reading Center.
Corneal scarring is measured in two ways: (1) by an on-site Clinician and (2) by
independent, masked reading of corneal photographs.
Apical corneal scarring is directly related to many of the parameters that
determine contact lens success in keratoconus. Scarring of the corneal apex contributes