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

2/1/99 Chapter 1 Background page 1-9
age, high and low contrast best corrected and habitual visual acuity, whether the patient
wears spectacles or contact lenses, the type of contact lens worn, and the first definite
apical clearance contact lens base curve.
It is expected that the results of these four sets of evaluations, and the
relationship between those results will lead to a better understanding of the clinical
course of keratoconus and will provide the framework for new approaches to the
management of keratoconus.
1.3 Diagnosis of Keratoconus
Keratoconus is a progressive disease characterized by steepening and distortion
of the cornea, thinning of the apical cornea, scarring, and treatment-related sequelae,
such as abrasions from contact lenses and surgical complications. Although various
geometries of keratoconus have been described (Caroline et al., 1978; Perry et al., 1980)
it is not usually possible to identify these types in the early stages of the disease, except
keratoglobus, which is actually a separate disease (Krachmer et al., 1984). The diagnosis
of early keratoconus depends primarily on assessment of the corneal topography
(Krachmer et al., 1984). An irregular, scissoring motion can be detected by viewing the
retinoscopic reflex (Swann and Waldron, 1986). Inferior corneal steepening (Edmund,
1987b; Zabala and Archila, 1988) and irregular mires are observed with the keratometer.
Devices which more accurately depict corneal topography, such as a hand-held Placido
disc, photokeratoscope, or videokeratography system show inferior corneal steepening
as well as irregular astigmatism. Documented increases in keratometric curvature of the
cornea over time are valuable in diagnosing early keratoconus (Krachmer et al., 1984).
Lack of agreement between corneal toricity and refractive astigmatism is also a sign of
early keratoconus, especially when accompanied by documented irregular astigmatism.
There are several characteristic biomicroscopic signs which increase as the
disease progresses (Krachmer et al., 1984). These include an inferiorly displaced,
thinned protrusion of the cornea, visually evident corneal thinning over the apex,
Vogt’s (1919) striae at the level of Descemet’s membrane, superficial scars at the level of
Bowman’s membrane, and Fleischer’s ring (of iron) at the base of the cone, either full or
partial. Vogt’s striae and Fleischer’s ring are considered pathognomonic for
keratoconus. The CLEK Study protocol requires a patient to have at least one of these
two classic biomicroscopic findings or corneal scarring in at least one eye in order to be
eligible for the study. This stringent eligibility criterion confirms the diagnosis of
keratoconus and excludes alternative conditions such as irregular corneal surface
without these signs (Rabinowitz et al., 1990), pellucid marginal degeneration, and
keratoglobus.
1.4 Course of Keratoconus