Asymmetric Keratoconus Study: Novel Tomographic Parameters to enhance sensitivity to detect abnormalities in eyes with normal axial curvature maps from patients with obvious keratoconus in the fellow eye

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Asymmetric Keratoconus Study Novel Tomographic
Parameters to enhance sensitivity to detect
abnormalities in eyes with normal axial curvature
maps from patients with obvious keratoconus in
the fellow eye

• Marcella Q. Salomão, MD José Salgado-Borges, MD
  Eduardo Viteri, MD
• Canrobert Oliveira, MD Renato Ambrósio Jr.MD,
  PhD.
• No financial interest.

Rio de Janeiro - Brazil
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Purpose
San Francisco 2009

• To introduce new tomographic diagnostic indices,
  beyond surface topography, to detect very early
  signs of ectasia.
• To calculate the sensitivity of these new
  parameters.

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Justification
San Francisco 2009

• Keratoconus is, by definition, a bilateral
  disease however, a small percentage of cases
  present with a very asymmetric pattern. In some
  cases, the asymmetry can be so significant that
  the contra-lateral eye presents with normal
  anterior curvature maps.
• A bilateral presentation as in these normal
  eyes can occur and could even be the explanation
  for many cases of ectasia after LASIK with no
  identifiable pre-operative risk factors.
• Identifying asymptomatic cases with normal slit
  lamp exam, good spectacle corrected visual
  acuity, and normal corneal topography is
  critical not only for screening of refractive
  surgery candidates, but to allow an enhanced
  treatment choice.

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Patients and Methods
San Francisco 2009

• 53 eyes showing normal anterior curvature maps
  (based on standard Rabinowitz criteria) from
  patients with advanced keratoconus in the fellow
  eye were evaluated in this multicenter study.
• All patients had tomographic exams using the
  Oculus Pentacam.
• Patient demographics
• 42 patients ? 11 patients ?
• Average age 35.34 y (17 to 78)

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New tomographic parameters
San Francisco 2009

• Thickness Profiles1 Corneal Thickness Spatial
  Profile (CTSP) and Percentage Thickness Increase
  (PTI)
• Enhanced anterior and posterior elevation best
  fit sphere (BFS) 2
• 1- Ambrósio R Jr, Alonso RS, Luz A, Coca
  Velarde LG. Corneal-thickness spatial profile and
  corneal-volume distribution tomographic indices
  to detect keratoconus. J Cataract Refract Surg.
  2006 Nov32(11)1851-9.
• 2- Belin MW, Khachikian SS, Ambrósio R Jr,
  Salomão MQ. Keratoconus / Ectasia detection with
  the Oculus Pentacam Belin/Ambrosio enhanced
  ectasia display. Highlights of Ophthalmology.
  Volume 35- number 6.

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CTSP and PTI
San Francisco 2009
The Corneal Thickness Spatial Profile (CTSP)
represents the averages of thickness values of
the points on 22 imaginary circles centered on
the thinnest point with increased diameters at
0.4 mm steps. The Percentage of increase in
thickness (PTI) starting from the thinnest point
is also calculated.

• The

The calculated values are displayed in a
progression graph, starting on the thinnest
point, as the CTSP and PTI lines. Data from a 95
confidence interval of a normal population is
also displayed, so that the clinician can compare
the profile of each eye to a normal population.
Abnormally thin corneas (ectatic) usually show
abrupt and abnormal profiles, out of the 95 CI.
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Enhanced BFS
San Francisco 2009
The Standard BFS is basically an average of high
and low elevations of the cornea. It is
calculated considering the best reference sphere
for the whole corneal surface. Thus, in an
abnormal eye, data from an abnormal area might be
used in the BFS calculation.
The Enhanced BFS was calculated utilizing all the
valid data from within the 9.0 mm central cornea
with the exception of a 4mm area centered on the
thinnest point. This would better approximate the
normal cornea and accentuate the abnormal portion.
Elevation maps using this new reference shape
were calculated for both anterior and posterior
corneal surfaces.
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Methods
San Francisco 2009

• Elevation maps considering the standard BFS were
  subtracted from the elevation maps with the
  enhanced BFS and the highest difference between
  the maps in the central 4mm area was noted for
  anterior and posterior corneal surfaces.
• The differential map contains only 3 colors, each
  one corresponding to the amount of elevation
  change that occurs when moving between the
  standard and enhanced maps.
• Anterior - green is anything lt 6, yellow is
  between 6 - 12, red is gt 12
• Posterior green is anything lt 8, yellow is
  between 8 - 20, red is gt 20
• Yellow and red maps were considered abnormal.

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Results
San Francisco 2009
Approach KC eyes Contra-lateral eyes
Pachymmetric Profiles 53 (100) 46 (87)
Enhanced Anterior Elevation 46 (87) 8 (15)
Enhanced Posterior Elevation 53 (100) 47 (89)
All eyes but 1 (52/53 98) presented at least
one abnormal finding in the new parameters
described. The combination of Pachymetric
Profiles and Enhanced Elevation add to each other
in sensitivity and specificity. A new display
(enhanced ectasia detection),combining the two
indices, was developed.
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Enhanced Ectasia Detection Display
San Francisco 2009
On the left side of the display we can see
anterior and posterior elevation maps. The bottom
2 maps are difference maps showing the relative
change in elevation from the baseline elevation
map to the exclusion map. On the right side of
the display well see the thickness profiles. In
this clinical example, the enhanced display of
the left eye showed an abnormal finding in the
posterior elevation approach (yellow) along with
abnormal thickness profiles, despite a normal
anterior surface map.
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Enhanced Ectasia Detection Display
San Francisco 2009
In this clinical example, both thickness profiles
were normal in the right eye however, the
posterior elevation approach demonstrated an
abnormal finding (yellow).
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Conclusion
San Francisco 2009

• The combination of Pachymetric Profiles and
  Enhanced Elevation was capable of identifying
  abnormalities in the majority of eyes with normal
  anterior curvature maps of patients with very
  asymmetric keratoconus.
• The new parameters increase sensitivity and
  specificity for the screening of refractive
  surgery candidates.