Transcript
Hindawi Publishing Corporation
Journal of Ophthalmology
Volume 2014, Article ID 402193, 6 pages
http://dx.doi.org/10.1155/2014/402193
Research Article
Impaired Corneal Biomechanical Properties and
the Prevalence of Keratoconus in Mitral Valve Prolapse
Emine Kalkan Akcay,1 Murat Akcay,2 Betul Seher Uysal,1
Pinar Kosekahya,1 Abdullah Nabi Aslan,2 Mehtap Caglayan,1
Cemal Koseoglu,2 Fatma Yulek,1 and Nurullah Cagil1
1
2
Department of Ophthalmology, Ankara Ataturk Training and Research Hospital, Yildirim Beyazit University, 06810 Ankara, Turkey
Department of Cardiology, Ankara Ataturk Training and Research Hospital, Yildirim Beyazit University, 06810 Ankara, Turkey
Correspondence should be addressed to Emine Kalkan Akcay;
[email protected]
Received 16 February 2014; Revised 26 March 2014; Accepted 31 March 2014; Published 17 April 2014
Academic Editor: Antonio Queiros
Copyright © 2014 Emine Kalkan Akcay et al. This is an open access article distributed under the Creative Commons Attribution
License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly
cited.
Objective. To investigate the biomechanical characteristics of the cornea in patients with mitral valve prolapse (MVP) and the
prevalence of keratoconus (KC) in MVP. Materials and Methods. Fifty-two patients with MVP, 39 patients with KC, and 45 control
individuals were recruited in this study. All the participants underwent ophthalmologic examination, corneal analysis with the Sirius
system (CSO), and the corneal biomechanical evaluation with Reichert ocular response analyzer (ORA). Results. KC was found in
six eyes of four patients (5.7%) and suspect KC in eight eyes of five patients (7.7%) in the MVP group. KC was found in one eye of
one patient (1.1%) in the control group (𝑃 = 0.035). A significant difference occurred in the mean CH and CRF between the MVP
and control groups (𝑃 = 0.006 and 𝑃 = 0.009, resp.). All corneal biomechanical and topographical parameters except IOPcc were
significantly different between the KC-MVP groups (𝑃 < 0.05). Conclusions. KC prevalence is higher than control individuals in
MVP patients and the biomechanical properties of the cornea are altered in patients with MVP. These findings should be considered
when the MVP patients are evaluated before refractive surgery.
1. Introduction
Mitral valve prolapse (MVP) is a primary connective tissue
abnormality of leaflets, the chordae tendineae, and the annulus of the mitral valves [1]. The prevalence of MVP in the
general population ranges from 0.6% to10% and can change
according to the diagnostic methods used, the diagnostic criteria, and the population assessed [2]. MVP can be associated
with ophthalmological diseases such as keratoconus (KC),
chronic progressive external ophthalmoplegia, and retinal
artery embolism [3, 4].
KC is a progressive, noninflammatory, idiopathic corneal
ectasia characterized by changes in corneal collagen structure
and organization [5, 6]. The prevalence of KC in the general
population is 50–230 per 100,000 (approximately 1/2000)
[7]. It is most commonly an isolated condition, despite the
multiple singular reports of coexistence with other disorders
[8]. Commonly recognized associations include MVP, Down
syndrome, Leber’s congenital amaurosis, and various connective tissue disorders [8].
The corneal stroma is the main structure that provides
corneal refraction, mechanical properties, and corneal transparency. The anterior stromal part plays especially important
role in corneal stability and shape [9]. Aging, corneal pathologies, corneal surgery (e.g., LASIK), and systemic diseases may
affect corneal biomechanical characteristics by affecting this
stromal structure rich in collagen connective tissue [10–14].
Given the cornea’s rich collagen connective tissue, corneal
biomechanics may be affected by collagen connective tissue
diseases.
Both KC and MVP are noninflammatory conditions, the
etiology of which has not yet been clearly identified. In
most cases of KC, thinning and ectasia of the central cornea
with subsequent progressive reduction of vision occurs [15].
2
Furthermore, both diseases are associated with systemic
collagen diseases such as pseudoxanthoma elasticum, Marfan syndrome, Ehlers-Danlos syndrome, and osteogenesis
imperfecta [16–18].
Corneal biomechanical properties can be assessed in vivo
with an ocular response analyzer (ORA, Reichert Ophthalmic
Instruments, Buffalo, NY, USA). An ORA provides measurements of Goldmann-related intraocular pressure (IOPg),
corneal-compensated intraocular pressure (IOPcc), corneal
hysteresis (CH), and the corneal resistance factor (CRF).
CH is related to the viscoelastic property of the cornea.
CRF is a parameter that shows the general resistance of the
cornea, while IOPcc is a parameter measured according to
CH value. The assessment of corneal biomechanics can be
used to prevent the misinterpretation of intraocular pressure
(IOP) measurements, the preoperative evaluation of patients
for refractive surgery, and the separation of healthy and
abnormal corneas [12–14, 19, 20].
Despite various studies of MVP prevalence in patients
with KC only two studies of KC prevalence in patients with
MVP exist [4, 21]. Furthermore, no research on corneal
biomechanical characteristics in patients with MVP is available.
This study therefore aimed to compare the biomechanical
characteristics of the cornea in MVP patients, as well as to
investigate the prevalence of KC in MVP.
2. Materials and Methods
This prospective, cross-sectional, comparative study was performed at the Ophthalmology and Cardiology Departments
at Yildirim Beyazit University Ankara Ataturk Training and
Research Hospital. The study followed the tenets of the
Declaration of Helsinki and was approved by the Local
Ethics Committee. All participants received oral and written
information about the study and, prior to participating, each
provided informed, written consent.
Participants were divided into three groups: the MVP
group, the KC group, and the control group. The MVP group
included 104 eyes of 52 patients, the KC group included 78
eyes of 39 patients, and the control group included 90 eyes of
45 participants.
Transthoracic echocardiography was performed by
using a Vingmed System V echocardiography system (GE
Vingmed, Horten, Norway) and a 2.5–3.5 MHz transducer
in the Cardiology Department. Two investigators performed
independent echocardiographic evaluation, and in the case
of any discrepancy, consensus was reached between them.
Diagnosis of primary MVP was based on the condition that
one or both mitral leaflets prolapsed into the left atrium by
passing the level of the mitral annulus by at least 2 mm along
the parasternal long axis during the systolic phase [22, 23].
Each participant received a complete ophthalmic evaluation that included the assessment of visual acuity, slit-lamp
biomicroscopic examination, and fundoscopy. All participants also received a corneal topographical evaluation with a
Scheimpflug camera combined with Placido corneal topography (Sirius, version 1.2, CSO, Firenze, Italy), which has
Journal of Ophthalmology
been found to be repeatable and reliable [24]. Measurements
of simulated keratometry 1 (Sim-K1), simulated keratometry
2 (Sim-K2), apical corneal thickness (ACT), and central
corneal thickness (CCT) were taken by an experienced
examiner in the Refractive Surgery and KC Unit according to
Sirius manufacturer’s guidelines. Three measurements were
made per eye, and the measurement with the best alignment
and fixation was selected for data analysis.
Three ORA measurements were taken in all participants
by an experienced clinician. Three high-quality (symmetric,
well-defined inward, and outward applanation spike height)
measurements were obtained for each eye. In the right eye of
each participant, four parameters were obtained: CH, CRF,
IOPg, and IOPcc.
All 52 patients of the MVP group had pure MVP (i.e.,
myxomatous degeneration) and no history of rheumatic
heart disease, systemic collagen disease, or any other ocular
disease.
Topography findings and clinical examination was used
to classify the eye as normal, suspect KC, or KC [25]. The eyes
with normal corneal thickness (>505 𝜇m), with no sign of
ectasia in anterior and posterior corneal curvature were classified as normal. Suspect KC is defined as abnormal localized
steepening of the cornea or an asymmetric bow-tie pattern
on corneal topographic examination, a normal-appearing
cornea on biomicroscopy, and at least 1 of the following signs:
steep keratometric curvature greater than 47.0 D, oblique
cylinder greater than 1.50 D, an inferior-superior dioptric
asymmetry difference in 1.4 to 1.9 D gradient, central corneal
thickness less than 505 𝜇m, and an elevation of the posterior
corneal surface [26, 27]. KC was diagnosed as having KC by
the topographic pattern (asymmetric bow-tie pattern with or
without skewed axes) and at least one KC sign in clinical
examination (stromal thinning, conical protrusion of the
cornea at the apex, Fleischer ring, Vogt striae, and anterior
stromal scar) [28]. Control group included patients who
were referred to Ophthalmology Department for refractive
surgery.
Exclusion criteria were any history of corneal or intraocular surgery, history of glaucoma, unreliable corneal topography (e.g., corneal scar, history of previous keratitis, and
corneal inflammation), or poor cooperation for reliable
examination and heart disease other than MVP. Contact
lenses had to be removed at least 3 weeks prior to examination
in the MVP and control groups. In the KC group, all first
examinations of patients were included in the study.
2.1. Statistical Data Analysis. Continuous variables were evaluated as mean ± SD in statistical analysis and were compared
with Student’s 𝑡-test. Nominal data were analyzed by Pearson’s
chi-square test, and the correlation between parameters
was interpreted with Pearson’s correlation coefficient. Any
difference was considered to be statistically significant when
the 𝑃 value was less than 0.05. Analysis was conducted by
using SPSS version 17 (Statistical Package for Social Sciences
Inc., Chicago, IL, USA).
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3
Table 1: Demographic characteristics of the patients and the control Group.
Variables
Age
Mean ± SD
Range
Gender
Female
Male
a
MVP group
(𝑛: 52)
KC group
(𝑛: 39)
Control group
(𝑛: 45)
36.2 ± 12.6
19–65
24.8 ± 5.9
19–40
32.1 ± 6.6
21–47
𝑃 value
0.053a
0.001b
0.600c
31 (59.6%)
21 (40.4%)
17 (43.5%)
22 (56.5%)
25 (55.6%)
20 (44.4%)
Student’s 𝑡-test, MVP-control group, b Student’s 𝑡- test, MVP-KC group, c Pearson’s Chi-Square test, MVP: mitral valve prolapse, KC: keratoconus.
Table 2: Corneal biomechanical properties of the patients and the control groups.
Variables
MVP group
(𝑛: 52)
KC group
(𝑛: 39)
Control group
(𝑛: 45)
𝑃 valuea
𝑃 valueb
𝑃 valuec
CH, mmHg
CRF, mmHg
IOPg, mmHg
IOPcc, mmHg
9.46 ± 1.57
9.51 ± 1.55
14.49 ± 3.59
15.44 ± 3.08
7.48 ± 1.35
6.48 ± 1.78
10.38 ± 2.78
14.25 ± 2.35
10.33 ± 1.47
10.42 ± 1.79
15.75 ± 3.59
15.97 ± 2.88
0.006
0.009
0.067
0.38
0.001
0.001
0.001
0.048
0.001
0.001
0.001
0.001
a
Student’s 𝑡-test, MVP-control group, b Student’s 𝑡-test, MVP-KC group, c Student’s 𝑡-test, KC-control group, MVP: mitral valve prolapse, KC: keratoconus, CH:
corneal hysteresis, CRF: corneal resistance factor, IOPg: Goldmann-related intraocular pressure, IOPcc: cornea compensated intraocular pressure.
3. Results
4. Discussion
Table 1 shows the baseline characteristics of the MVP, KC,
and control groups. Regarding age and gender, no significant
differences occurred between the MVP and control groups
(𝑃 = 0.053 and 𝑃 = 0.6, resp.). In the KC group, the mean
age was significantly less than that of the MVP and control
groups (𝑃 < 0.001 and 𝑃 < 0.001, resp.).
A significant difference occurred in the mean CH and
CRF between the MVP and control groups (𝑃 = 0.006 and
𝑃 = 0.009, resp.) (Table 2). In the MVP group, mean IOPg,
IOPcc, ACT, and CCT measurements were less than those
of the control group, though they did not differ significantly
(𝑃 = 0.067, 𝑃 = 0.38, 𝑃 = 0.39, and 𝑃 = 0.26, resp.)
(Tables 2 and 3). Moreover, mean Sim-K1 and Sim-K2 did
not differ significantly between the MVP and control groups
(𝑃 = 0.61 and 𝑃 = 0.52, resp.). All corneal biomechanical and
topographical parameters except IOPcc were significantly
different between the KC-MVP groups and the KC-control
groups (𝑃 < 0.05) (Tables 2 and 3). The cut-off value for
corneal thickness was 505 𝜇m, as per studies by Fontes et al.
[12, 27]. In our study, CCT fell below the cut-off value in 11 of
the 52 patients in the MVP group and one of the 45 control
group participants in the psychometric analysis (𝑃 > 0.05).
In correlation analysis, we found that in the MVP group,
the CH and CRF values were moderately correlated with CCT
(𝑟 = 0.535, 𝑃 = 0.001, and 𝑟 = 0.643, 𝑃 = 0.001, resp.)
(Figures 1 and 2). The ACT was also moderately correlated
with the CH and CRF values (𝑟 = 0.462, 𝑃 = 0.001, and 𝑟 =
0.574, 𝑃 = 0.001, resp.).
Furthermore, KC was found in six eyes of four patients
(5.7%) and suspect KC in eight eyes of five patients (7.7%) in
the MVP group. KC was found in one eye of one patient (1.1%)
and suspect KC not found in the control group (𝑃 = 0.035).
It is well known that human heart valves, like the cornea, are
composed of collagen types I and V with a small proportion
of type III [29, 30]. Frequently associated with myxomatous
degeneration (i.e., the pathological weakening of the connective tissue), MVP is the most common cause of mitral
regurgitation and affects 2–22% of the general population
[31]. It has been shown that myxomatous degeneration affecting the cornea shares some features with KC; the damage
is present particularly in the anterior part of the stroma,
Bowman’s membrane is disrupted, and stromal keratocytes
are transformed into cells with myofibroblastic differentiation
[32]. Since the alteration of collagen subtypes occurs in
both KC and MVP, it is possible that a single event during
embryogenesis affects both structures, for both the corneal
stroma and the atrioventricular valves form during the sixth
to seventh week of fetal life [9, 16]. Dudakova and Jirsova
hypothesized that very similar changes in the extracellular
matrix, particularly at the level of collagen metabolism,
including lysyl oxidase (LOX) impairment in mitral leaflets,
may reflect an association between KC and mitral valve
prolapse. As such, these findings may also indicate a very
similar origin of KC and MVP [33].
Numerous studies have confirmed a statistically significant higher occurrence of MVP in KC patients, with
a reported prevalence of 23–66% compared to the 7–13%
prevalence of MVP in the normal population [15, 34, 35].
Though many studies have reported the prevalence of
MVP in KC patients, to our knowledge only two studies have
reported the prevalence of KC in MVP patients. Lichter et al.
studied 72 eyes of 36 MVP patients and 50 eyes of 25 control
patients and diagnosed KC in eight eyes of 36 MVP patients
(11.1%) and in one (2%) eye of a (4%) patient in the control
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Table 3: Corneal topographical properties of the patients and the control groups.
Variables
MVP group
(𝑛: 52)
KC group
(𝑛: 39)
Control group
(𝑛: 45)
𝑃 valuea
𝑃 valueb
𝑃 valuec
Sim-K1, D
Sim-K2, D
ACT, 𝜇m
CCT, 𝜇m
42.92 ± 1.42
43.97 ± 1.53
548.4 ± 31.0
539.3 ± 33.4
47.69 ± 3.34
52.22 ± 3.86
441.8 ± 48.8
443.2 ± 42.6
42.77 ± 1.31
44.16 ± 1.33
554.0 ± 31.9
546.69 ± 30.1
0.61
0.52
0.39
0.26
0.001
0.001
0.001
0.001
0.001
0.001
0.001
0.001
a
Student’s 𝑡-test, MVP-control group, b Student’s 𝑡-test, MVP-KC group, c Student’s 𝑡-test, KC-control group, MVP: mitral valve prolapse, KC: keratoconus, SimK1: simulated keratometry 1, Sim-K2: simulated keratometry 2, ACT: apical corneal thickness, CCT: central corneal thickness.
14.00
14.00
r = 0.535
r = 0.643
12.00
CRF
CH
12.00
10.00
10.00
8.00
8.00
6.00
475
500
525
550
575
600
CCT
475
500
525
550
575
600
CCT
Figure 1: Correlation graphic between CH and CCT. CH: corneal
hysteresis, CCT: central corneal thickness.
Figure 2: Correlation graphic between CRF and CCT. CRF: corneal
resistance factor, CCT: central corneal thickness.
group [4]. To do so, they used a videokeratography system
(Topographic Modeling System, TMS-1, TOMY Computed
Anatomy, NY, USA), which is the most sensitive method used
in KC diagnosis and also allows early diagnosis. By contrast,
Javadi et al. generally used conventional topography in their
study but videokeratography (Orbscan, Bausch & Lomb,
Rochester, NY, USA) in two patients suspected of having KC
yet did not report any KC in 392 MVP patients [21]. In our
study, all participants analyzed with a Scheimpflug camera
combined with Placido corneal topography, and similar to
Lichter et al.’s study, KC and suspect KC prevalence increased
(13.4%) in MVP patients compared to those in the control
group (1.1%).
Previous studies have found that corneal biomechanical
characteristics measured by ORA were affected by numerous
corneal diseases, such as KC, Fuchs’ endothelial dystrophy, and corneal edema, as well as some systemic factors,
including diabetes and menstruation [36, 37]. However, this
is the first study conducted to compare the biomechanical
properties of the cornea in patients with MVP or KC and a
control group.
Our results revealed that, in the MVP group, CH and CRF
values were significantly less than those in the control group.
These results indicate that corneal viscous and elastic properties between normal and MVP patients differ. To distinguish
normal from abnormal corneas before corneal surgery, it is
important to assess this biomechanical difference.
Other studies have shown that, in keratoconic eyes, CH
and CRF values were less than those in normal eyes [10,
19, 36]. Our results were similar to these; in our study’s
KC group, values of CH, CRF, IOPcc, and IOPg were less
than those in the MVP and control groups. These results
indicate that keratoconic eyes are more elastic and less rigid
than normal eyes, which emphasizes that the biomechanical
characteristics of keratoconic eyes differ from those of normal
eyes.
Also in this study, IOPcc measurements did not significantly correlate with CCT, which reveals that IOPcc values
provided by ORA seem unaffected by corneal thickness.
Similarly, though IOP measurements significantly correlated
with CRF, IOPcc values were uninfluenced by CRF, which
suggests that IOPcc measurements were also uninfluenced by
corneal characteristics.
It is known that keratoconic corneas are thinner, more
fragile, and more prone to deformation than normal corneas
[27]. We want to underscore that the eyes of MVP patients
Journal of Ophthalmology
are similar in some aspects to those of KC patients. Due to
structural similarity, corneal thickness in both MVP and KC
patients can be less compared to that of normal patients.
In our study, CCT was less in 1/5 of MVP patients than
in normal individuals (<505 𝜇m). In MVP patients, corneal
rigidity and resistance decreased as the cornea thinned [36].
At the same time, ocular resistance was less in MVP patients’
eyes due to the combined effect of corneal thickness, ocular
rigidity, and characteristics of corneal viscoelasticity. As such,
for MVP patients—particularly those diagnosed at an early
age—ophthalmologic examination and CCT follow-up are
clinically crucial.
Corneal biomechanical characteristics are clinically significant in selecting patients for corneal refractive surgery
[19, 38]. Recent reports have suggested that patients with early
KC or suspect KC comprise 2–5% of patients who present for
refractive surgery for myopia [39, 40].
While evaluating eye problems in MVP patients, especially when planning refractive surgery, corneal biomechanical characteristics and the association of KC with MVP
should be considered.
Conflict of Interests
The authors report no conflict of interests. The authors alone
are responsible for the content and writing of the paper.
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