Juniper Publishers-Journal of Ophthalmology
Abstract
Background
This case report aims to evaluate
safety, efficacy and feasibility of anterior corneal surface imaging by a
novel multi-color Light-Emitting-Diode (LED) tear film-reflection
topographer in comparison to several standard corneal imaging modalities
in patient with severe ocular and corneal changes due to old
acanthamoeba keratitis.
Case Description
An18-year old female patient,
infected approximately two years ago with acanthamoeba in her right (OD)
eye, was subjected to a multitude of anterior-segment and ocular
imaging diagnostic devices. The imaging modalities included Placido
topography, Scheimpflug tomography, Anterior-Segment Optical Coherence
Tomography (AS-OCT), ocular stray-light measurement quantifier, and the
Cassini, a novel multi-spot, multi-color LED reflection topographer.
Clinical Relevance
The ease of use and comparable
results offered by the Cassini, in comparison to established Placido
topography and Scheimpflug tomography, as well as the increased
sensitivity that the novel topographer may offer new clinical diagnostic
significance. Scheimpflug-imaging derived pachymetry has limitations in
cases of partially opaque corneas.
Accurate
This case report, based on a novel
corneal topography imaging in comparison to a variety of corneal imaging
modalities, verifies the clinical applicability of the new technology
in the imaging of disturbed and highly asymmetric cornea.
Keywords:
LED Cassini; multi-color LED
topography; Acanthamoeba keratitis; Point-source topography; Pentacam
HR; Placido topography; Scheimpflug topometry; Differential topography;
Irregular corneal astigmatism; Stray light measurements; C-Quant;
Anterior-Segment Optical Coherence Tomography
Introduction
The limitations of Placido-based topography have been
identified in the past, particularly pertaining to imaging of radial,
rather than only contour topographic changes [1]. Topography systems
with color-coded LED reflection forward ray tracing have been proposed
[2] as an alternative to Placido ring imaging [3]. The VU topographer
(Vrije Universiteit Medical Center, Amsterdam, The Netherlands) [2]
introduced a different approach, with a color-coded squares in a
chess-pattern array projected on the cornea instead of the Placido rings
[4]. The algorithm for the surface reconstruction employed data from
the pattern crossing points to eliminate source-image mismatch, enabling
one-to-one match. In principle, this color-coded topographer was found
to be more efficient in reconstructing the non-rotationally symmetric
anterior corneal surface [5].
The Cassini (i-Optics, The Hague, The Netherlands)
topography system is a novel topographer employing multi spot (up to
700), multicolor (red, yellow and green) Light Emitting Diode (LED) tear
film-reflection-imaging, following the steps of the VU topographer.
The difference is instead of a limited number of color-coded squares,
there are hundreds of LED spots on radial and contour arrangement imaged
on the cornea. Image processing algorithm locates feature points in
the reflection image and accounts for smearing and deformation in
irregular corneas. The system has been recently introduced [6] and has
received FDA approval for clinical use in corneal topography.
Due to the novelty, clinical validations of this
novel topography system and its clinical implications have not yet been
extensively investigated and reported. We have recently reported [7]
clinical results of the Cassini in a case of forme fruste keratoconus
(FFKC).
The scope of this manuscript is to examine the
clinical feasibility of this newly-introduced corneal topographer in a
case of a patient with old acanthamoeba infection. Very little has been
documented in the last ten years in cases involving acanthamoeba
keratitis – infected eyes, from the point-of-view by contemporary
anterior segment imaging.
Case Report
Case Report
We present the case of an 18-year old female subject,
diagnosed with acanthamoeba keratitis [8] on her right (OD) eye,
affected at approximately two years ago. Informed consent was obtained
from the subject at the time of the first clinical visit. This study
adhered to the tenets of the Declaration of Helsinki and was approved by
the Ethics Committee of our Institution.
The non-affected left eye (OS) had best-corrected
distance visual acuity (CDVA) 20/20 and manifest refraction -7.00 S-0.25
C × 180°. The affected OD eye had CDVA 20/50 when wearing the manifest
refraction of -3.50 S -2.00 C × 18°.
The Cassini (running on software version 1.2, updated
September 2013) was employed to provide anterior cornea imaging. The
system produces anterior elevation, tangential, axial curvature, and
refractive power three-dimensional maps covering approximately a 7.5-mm
diameter area. The report provides keratometry data (steep and flat K,
simulated astigmatism), and computerized topographic and keratoconus
indices. In addition, aberrations report is provided, according to the
Zernike nomenclature. Four additional ocular imaging modalities were
employed in this work. Placido imaging was provided by the Wave Light
Allegro Topolyzer (Alcon Surgical, Ft Worth, and TX). The Topolyzer is a
wide-cone corneal topographic Placido system with 22 concentric rings
for the detection of up to 22,000 elevation points. Scheimpflug
topometry was provided by the Wave Light Oculyzer II (Alcon Surgical, Ft
Worth, TX), a Pentacam HR (high-resolution) camera providing corneal
pachymetry and tomography imaging [9] was also included in this study.
The Fourier-domain Anterior-Segment Optical Coherence
Tomography (AS-OCT) system RTVue-100 (Optovue Inc., Fremont, CA),
running on analysis and report software version A6 (9,0,27) was employed
in the study to provide cornea cross-section, and corneal and
epithelial thickness 3-dimensional mapping [10]. Finally, the C-Quant
(Oculus Optikgeräte GmbH, Wetzlar, Germany), a system that measures
stray-light scattering in the eye [11] was employed to provide
quantitative data of scatter in relation to the identified corneal
opacity.
Figure 1 presents OCT high-resolution meridian scan
of the affected eye, corneal and epithelial pachymetry 3-dimensional
maps. We observed the deep stromal scar, appearing as opacity
associated with the residual stromal scarring, located in the near
inferior-nasal area. Local corneal thinnest point and epithelial
hyperplasia were also indicated in the same affected area. Central
corneal thickness and minimum corneal thickness, as measured by the OCT
were 381 and 318 μm respectively. A thinner epithelium (56 μm) over the
thinnest, most ectatic region of the cornea also noted; overall,
however, the epithelium was thick, in the vicinity of 70 – 74 μm,
associated with large topographic variability (standard deviation of
pachymetry) computed from the Scheimpflug system. The imaging
successfully depicts the corneal opacity. The thinnest point also
located inferior-nasally; however we note that the Scheimpflug
imaging-derived central and minimum corneal thickness were 438 μm and
167 μm, a distinct disparity when compared to the OCT measurements (381
μm and 318 μm, as noted above). As it can be noticed in Figure 2a, the
posterior surface was poorly identified at the lesion, a possible
consequence of the corneal opacity. This is a clinical example of a
cornea with considerable opacity that alters the ‘normal’ densitometry
interpolation of the Scheimpflug algorithm and can possibly be
associated with the reported cornea thickness results by the device.
Figure 3 presents stray-light measurement report by
the C-Quant. The measurement, based on the compensation method, [12]
reported a stray-light logarithm value of 1.43±0.05. When comparing
this result to the average value of 0.80±0.05, obtained from a large
number of healthy subjects of the same age (reference light curve in the
upper graph), [13] it is clear that the measurement suggests that the
affected eye has a substantially ‘abnormal’ ocular scatter.
Figure 4 presents Placido ring imaging raw data and
the corresponding refractive map. We observed the highly distorted ring
pattern along the vertical direction. The refractive map indicated
very large variation of the cornea refraction, ranging from a localized
33.5 D inferiorly, to more than 48 D superiorly, and a distinct
‘astigmatic’ – like pattern in the mid-superior cornea. Figure 5A
presents Cassini imaging raw data imaging on the affected eye. Anterior
surface axial curvature and refractive map (C) computerized with the
above data are reported in Figures 5B and 5C, respectively.
Discussion
Acanthamoeba keratitis is a rare disease in which
amoebae invade the cornea [14-16]. The disease [17] as well as it
sequalae is a common cause of corneal blindness in the United States,
often a result of improper disinfection of contact lens use [17-19]. If
it is not diagnosed early and not treated aggressively, infection by
trophozoites [20] may result in extensive ocular damage, and enucleating
may be required [21]. Even when cured, the disease may result in
permanent irregular corneal surface and extensive central opacities,[22]
thus rendering accurate imaging of the cornea is challenging.
Subsequent corneal scarring may have severe impact on the anterior
surface regularity, tear film regularity, as well as stromal density, as
indicated by the OCT imaging, renders Placido and Scheimpflug imaging
in this case challenging, as expected.
In this work we examined the clinical feasibility of
the Cassini, the newly-introduced topographer in a case of Acanthamoeba
keratitis in comparison to the established methods of clinical imaging,
namely Placido topography, Scheimpflug topometry, and AS-OCT.
Thinnest cornea pachymetry, measured with Scheimpflug
topometry (167μm) was in large disparity with the OCT-measured value
(318μm). We believe this is a disadvantage of the Scheimpflug imaging
principle, which assumes clear cornea interpolation of the posterior
surface.
When correlating the novel Cassini to the established
Placido topographer, it is clear from this case that the increased
sensitivity, which includes both radial and contour differences, based
on differential spot-imaging, enabled proper imaging of this highly
irregular cornea. Particularly considering the extent of the
abnormality presented in this case, and the fact that significant
distortion was also present at the corneal center, the Placido device
was not successful in imaging this case. The Scheimpflug topometry, on
the other hand, was successful in imaging the anterior surface, but not
the posterior.
Discussion
The facility and comparable results offered by the
novel Cassini topographer, in comparison to established Placido and
Scheimpflug imaging, as well as the possibility of increased
predictability that may be offered by the novel corneal imaging system
in highly irregular corneas, may hold promise for wider clinical
applications, such as screening of highly asymmetric and corneas with
highly coinciding opacities.
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