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Diagnosis
Early stage.
Computerized corneal
topography (CCT) techniques
using curvature-based
analysis and newer forms of
elevation-based tomography
appear to be the most
sensitive methods for
detecting early keratoconus.
Prior to these, the most
sensitive methods included
1) apical decentration on
the keratometer using the
Soper topogometer, 2) subtle
changes in the retinoscopic
reflex that progress to
early scissoring and 3)
changes in the ability to
visualize the endothelial
specular reflection relative
to the epithelial reflection
during slit-lamp
biomicroscopy.
Elements of earlier
techniques- keratometry and
Placido disk–based
keratoscopy-have now been
integrated into
sophisticated, computerized
systems capable of
generating color-coded
curvature and elevation
maps. Even with these
technological advances, it
is increasingly recognized
that elevation data using
scanning slit or Scheimpflug
imaging provides data that
are diagnostically very
helpful when screening
patients with possible
corneal ectasia.
A number of diagnostic
algorithms can help diagnose
early keratoconus; however,
there remains no universally
agreed-upon diagnostic
criterion that, by itself,
conclusively diagnoses the
earliest cases. Typically,
changes in elevation
relative to a best-fit
sphere occur on the
posterior corneal surface
more rapidly than similar
changes are seen on the
anterior surface. Other
findings include apical
decentration, corneal
thinning, increasing
irregular corneal
astigmatism, and Rizzutti’s
sign, which is a sharply
focused conical reflection
obtained on the nasal cornea
after a penlight is shone on
the temporal side.
Intermediate stage.
Even in patients with
intermediate-stage disease,
corneal changes may still be
subtle and not readily
apparent by slit-lamp
biomicroscopy alone. The
retinoscopic streak usually
demonstrates scissoring, and
it is hard to neutralize the
streak reflection during
refraction. Computerized
corneal topography and
elevation-based
tomography-Orbscan IIz
(Bausch & Lomb) and Pentacam
(Oculus) are probably the
most widely used-typically
demonstrate findings that
are usually more obvious
with regard to both
diagnosis and monitoring
disease progression.
Advanced stage. In
more advanced cases,
keratoconus is readily
diagnosed by characteristic
slit-lamp findings:
-
Stromal thinning-usually
inferiorly.
-
Apical decentration and
conical protrusion.
-
A Fleischer ring-an iron
line within the deep
epithelium surrounding
the cone at the base.
-
Vogt lines-fine vertical
lines in the deep stroma
and Descemet’s membrane
that disappear
transiently with gentle
digital pressure.
In addition to these corneal
signs, external ocular
manifestations include:
-
Munson’s sign.
The lower lid protrudes
when a patient with
advanced corneal disease
looks downward.
-
Corneal hydrops.
Occasionally, patients
can progress to acute
corneal hydrops, a
stromal edema caused by
aqueous penetration
through breaks in
Descemet’s membrane. The
onset of corneal hydrops
is classically
associated with a sudden
decrease in both
uncorrected and
corrected vision as well
as redness, pain and
photophobia. The corneal
edema may persist
unpredictably for weeks
or months, with gradual
resolution as the
posterior break is
replaced with a
posterior collagenous
scar. Topical osmotic
agents such as
hypertonic saline may
accelerate the recovery
of vision; however, this
treatment is most
helpful to alleviate
edema within the
epithelium and anterior
stroma.
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