Download Is Mitral Valve Prolapse Due to Cardiac

Is Mitral Valve Prolapse Due to Cardiac
Entrapment in the Chest Cavity?*
A CT View
Paolo Raggi, MD; Tracy Q. Callister, MD, FCCP; Nicholas J. Lippolis, MD; and
Donald J. Russo, MD
Background: Mitral valve prolapse (MVP) is the most frequently diagnosed valvular disease, but
its pathophysiology remains elusive. Its complete absence in 1,734 neonatal echocardiographic
studies suggests that this may be an acquired rather than a congenital disease. We observed
several patients with distorted cardiac and valvular anatomies on electron beam CT (EBCT)
images of the chest who reported symptoms reminiscent of MVP. In these patients, the heart is
compressed between the spine and the anterior chest wall and it appears trapped in a chest cavity
that is too small for its size.
Methods: We performed EBCT in 66 patients with echocardiographically proven MVP and no
clinical pectus excavatum (group A; 80% were women; mean age, 48 ⴞ 12 years) and in 96 control
patients without MVP by echocardiography (group B; 72% were women; mean age, 49 ⴞ 10
years). EBCT alone was also performed on 200 patients who had reported atypical chest
discomfort and palpitations to their physicians (group C) and on 200 asymptomatic patients
(group D). The EBCT measurements included the following: anteroposterior chest diameter
(APD); the angle formed by the confluence of the mitral valve ring with the interatrial septum
(ANGLE); and the contact area between the posterior surface of the anterior chest wall and the
myocardium (CA). Entrapment was considered present if the individual patient’s measurements
varied by more than two SDs compared to measurements made in control subjects (group B).
Results: EBCT images demonstrated cardiac entrapment in 82% of group A patients and in 4.2%
of group B patients (p < 0.001). ANGLE and CA were significantly larger in MVP patients than
in group B patients (114 ⴞ 9° vs 91 ⴞ 5° and 6,230 ⴞ 2,020 mm2 vs 476 ⴞ 1,009 mm2, respectively; p < 0.001 for both comparisons), while APD was significantly smaller (91 ⴞ 16 mm vs
128 ⴞ 17 mm, respectively; p < 0.001). The prevalence of entrapment was significantly greater in
group C patients than in group D patients (22% vs 6.5%; p < 0.001).
Conclusions: MVP may be an acquired condition caused by a growth disproportion between the
heart and the chest cavity, with distortion of the mitral valve annulus and subsequent leaflet
prolapse. A narrow APD, a wide ANGLE, and a large CA characterize this condition. Similar
findings are found in a sizable proportion of patients with atypical chest pain symptoms and
palpitations.
(CHEST 2000; 117:636 – 642)
Key words: electron beam computed tomography; mitral valve prolapse; thoracic radiography; thorax
Abbreviations: ANGLE ⫽ angle formed by the confluence of the mitral valve ring with the interatrial septum;
APD ⫽ anteroposterior chest diameter; CA ⫽ contact area between the posterior surface of the anterior chest wall and
the myocardium; EBCT ⫽ electron beam CT; MVP ⫽ mitral valve prolapse; NS ⫽ not significant
valve prolapse (MVP) is currently the most
M itral
commonly diagnosed form of valvular heart
disease in the western countries,1–3 and yet its
*From the EBT Research Foundation, Nashville, TN.
Presented at the XIXth World Congress on Diseases of the Chest
and the 64th Chest Annual Scientific Assembly, Toronto, Ontario, Canada, November 8 –12, 1998.
Manuscript received April 1, 1999; revision accepted August 13,
1999.
Correspondence to: Paolo Raggi, MD, Director, EBT Research
Foundation, 64 Valleybrook Dr, Hendersonville, TN 37075;
e-mail: [email protected]
etiology and pathophysiology remain unknown.
Some investigators have suggested that this condition is inherited as an autosomal dominant trait.4,5
However, the reported complete absence of MVP in
1,734 newborn echocardiographic studies seems to
refute this opinion.6 Several other pathophysiologic
explanations have been proposed, such as undetected rheumatic fever in childhood,7,8 myxomatous
degeneration of valve leaflets and chordae tendinae,9
and valvular-ventricular disproportion.10 –13 MVP has
also been described in association with Marfan’s
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Clinical Investigations
syndrome, straight back syndrome, scoliosis, and
pectus excavatum.14 –21 These associations indicate
that chest deformities may play an important role in
the pathophysiology of this condition or, alternatively, that these conditions are in some way developmentally related. We observed several patients
with unusual anatomic cardiac features on electron
beam CT (EBCT) images of the chest who reported
symptoms reminiscent of the MVP syndrome. In the
EBCT images, the heart appears trapped in the
chest cavity and the mitral valve annulus undergoes
a morphologic distortion that could predispose to the
development of MVP. Therefore, we undertook this
EBCT and echocardiographic comparative study to
assess the relationship between echocardiographically proven MVP and cardiac entrapment on EBCT.
Subsequently, we assessed the prevalence of cardiac
entrapment in a subset of patients with chest discomfort and palpitations but no evidence of MVP
and in a matched group of control subjects with no
symptoms. This was done to verify the frequency
with which the cardiothoracic morphologic abnormalities we describe in this study are found in the
population at large and in a group of patients
lamenting symptoms similar to those of MVP patients.
in a supine position on a radiologic cradle during two 30- to 40-s
breath-holding periods at end-inspiration. In these conditions,
the respiratory motion artifacts and the area of contact between
the myocardium and the anterior chest wall are reduced to a
minimum. Images were obtained with 100-ms scan time and
3-mm single-slice thickness, with a total of 36 slices starting at the
level of the carina and proceeding to the level of the diaphragm.
Tomographic imaging was electrocardiographically triggered to
80% of the R-R interval. Off-line analysis of chest images was
performed on a NetraMD workstation (ScImage; Los Altos, CA)
with fly-through, calipers, and three-dimensional reconstruction
capabilities. Interpretation of the EBCT images was performed
by two expert investigators blinded to the results of the echocardiographic studies performed in patients in groups A and B. The
following measurements were performed on all EBCT images
(Fig 1, top, A and bottom, B): (1) The anteroposterior chest
diameter (APD), as measured between the posterior surface of
the anterior chest wall and the anterior border of a vertebral
body. The measurement was made at the level of the narrowest
Materials and Methods
Patients
The study was approved by the Internal Review Board of our
institution, and all patients signed an informed consent form
prior to participating in the study. For the first part of the study,
we enrolled 66 patients with echocardiographically proven MVP
(group A) and 96 control patients (group B) with no evidence of
MVP on echocardiography. None of these patients showed
evidence of pectus excavatum on physical examination, and they
all underwent echocardiography followed by EBCT imaging. Of
the 66 patients in group A, 53 were women (80%; mean ⫾ SD
age, 48 ⫾ 12 years). Of the 96 patients in group B, 69 were
women (72%; mean ⫾ SD age, 49 ⫾ 10 years). For the second
part of the study, EBCT scanning was performed on 200 patients
who had reported atypical chest discomfort and palpitations to
their physicians (group C; 50% were women; mean ⫾ SD age,
50 ⫾ 11 years) and on 200 asymptomatic patients (group D, 50%
were women; mean ⫾ SD age, 52 ⫾ 12 years).
Imaging Protocols
Electron Beam CT: Electron beam CT is a radiologic imaging
technique that allows the acquisition of accurate images of the
heart and coronary arteries at very high speed to perform their
computerized tomographic reconstruction.22–24 The fast imaging
speed is necessary to prevent blurring of the images due to the
continuous motion of the heart. The high quality of the images
obtained allows for a detailed analysis of the cardiothoracic
structures included in the field of view. All study patients
underwent EBCT imaging (Imatron C-100 scanner; Imatron; San
Francisco, CA). Scanning was performed with the patients lying
Figure 1. Top, A: An axial CT view of the chest in a patient
without MVP. Note the spherical shape of the heart and the
absence of contact between the myocardium and the anterior
chest wall. Bottom, B: An axial CT view of the chest in a patient
with echocardiographically proven MVP. Note the narrow APD,
the obtuse ANGLE, and the large CA. The chest configuration
appears similar to a horizontally placed “figure eight.”
AP ⫽ APD.
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637
thoracic point still including all four cardiac chambers. (2) The
angle formed by the confluence of the mitral valve ring with the
interatrial septum (ANGLE; Fig 2, top, A and bottom, B). The
mitral valve ring plane lies along the atrioventricular groove, and
it can be identified by drawing a line joining the proximal
segments of the left circumflex and right coronary artery. The
interatrial septum plane can then be easily identified by scrolling
through multiple cranial-to-caudal axial images of the heart. (3)
The contact area between the posterior surface of the anterior
chest wall and the myocardium (CA). To calculate this area, we
multiplied the maximum contact lengths between chest wall and
myocardium measured in the axial and sagittal views of the chest.
The EBCT measurements made in the 96 control patients
enrolled in group B were used as reference measurements. For
the patients in groups A, C, and D, cardiac entrapment was
considered present if their EBCT measurements differed by
more than two SDs compared to those made in group B patients.
We developed these computed tomographic criteria from a
cohort of asymptomatic control subjects with echocardiographically proven absence of MVP (group B patients), since there are
no pertinent published criteria in the current radiologic literature.
In two patients, 125 mL of IV iodine contrast were injected at
a rate of 5 mL/s to perform a noninvasive contrast cineangiogram
for visualization of the cardiac chambers and valvular structures
(Fig 3). In these patients, images were acquired using the
multislice scanning mode and a slice thickness of 8 mm.22
Echocardiography: Echocardiographic measurements were
made using commercially available phased-array echocardiographic systems (CFM750; VingMed Sound A/S; Horten, Norway and Sonos 2000; Hewlett-Packard; Palo Alto, CA) equipped
with 2.5- and 3.5-MHz transducers. Comprehensive two-dimensional and M-mode scanning, as well as Doppler interrogation
and color flow analysis were performed in the parasternal
long-axis and apical 2, 4 and 5 chamber views. All studies were
recorded on one-half inch VHS-format videotapes and reviewed
off-line by an expert investigator independent of the EBCT
investigator and unaware of the results of the EBCT imaging
procedures. MVP was diagnosed by the following standard
echocardiographic criteria: (A) two-dimensional mode, the posterior-superior displacement of one or both leaflets above the
mitral valve annulus in the parasternal long-axis view; and (B)
M-mode, the systolic posterior displacement of a mitral leaflet of
ⱖ 3 mm.
Statistical Analysis
The two-tailed t test was used to compare continuous variables
between patient groups. The ␹2 and test-on-proportions were
utilized for comparison of categorical variables. All variables were
expressed as mean ⫾ SD. A p value ⬍ 0.05 was considered
statistically significant.
Figure 2. An electron beam CT image showing the reference
markers used for the identification of the mitral valve plane and
the interatrial septum. Top, A: the image is shown without the
lines identifying the AV groove and the septum. Bottom, B: the
same image is now shown after the addition of the lines used to
define the ANGLE.
Figure 3. An electron beam CT systolic angiographic frame
demonstrating prolapse of the posterolateral leaflet of the mitral
valve inside the left atrium (arrow) in a patient with cardiac
entrapment. LA ⫽ left atrium; LV ⫽ left ventricle; RA ⫽ right
atrium; RV ⫽ right ventricle.
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Clinical Investigations
Results
Group A vs Group B Subjects
The clinical characteristics and EBCT measurements for the patients with echocardiographically
proven MVP and for the respective age-matched
control subjects are presented in Table 1. As shown,
the mean ANGLE and CA were significantly greater,
while the mean APD was significantly smaller in
patients with echocardiographically proven MVP
than in group B patients (114 ⫾ 9° vs 91 ⫾ 5° and
6,230 ⫾ 2,020 mm2 vs 476 ⫾ 1,009 mm2, and
91 ⫾ 16 mm vs 128 ⫾ 17 mm, respectively;
p ⬍ 0.001 for all comparisons). All group A patients
presented measurements of ANGLE and CA above
the mean of the normal control subjects, and all had
APD measurements below the mean for the control
subjects. Cardiac entrapment, as defined by a difference of more than two SDs from expected values,
was present in 82% of group A patients and in 4.2%
of group B patients (p ⬍ 0.001).
No significant differences in APD, ANGLE, and
CA measurements were found between male patients and female patients in group A, although there
was a tendency for female patients to show a smaller
APD than male patients (89 ⫾ 16 mm vs 98 ⫾ 16
mm; p ⫽ not significant [NS]). This tendency is
known to occur in the general population as well.25
Indeed, the APD was significantly smaller in female
patients than in male patients in group B (122 ⫾ 12
vs 144 ⫾ 17; p ⬍ 0.001). The APD measurements
made on EBCT images for group A and B patients
compared statistically well with measurements made
on plain chest roentgenograms in patients with
straight back syndrome and normal control subjects
by Twigg et al25 (p ⫽ NS for comparison of means).
Figure 1 shows a comparison of a tomographic
chest image of a normal subject (top, A) with that of
a patient with echocardiographically proven MVP
(bottom, B). The deformed cardiac anatomy in the
MVP patient is clearly demonstrated with a wide
ANGLE, a narrow APD, and a large CA. Figure 3
shows an EBCT angiographic frame from a patient
with echocardiographically proven MVP. The posterolateral leaflet of the mitral valve is showing
prolapsing inside the left atrium during systole (black
arrow). Figure 4 is an M-mode echocardiographic
image obtained in the parasternal long-axis view in a
patient with MVP and cardiac entrapment on EBCT.
In this image, systolic posterior displacement of the
posterior mitral valve leaflet is clearly visible (arrow).
In patients with echocardiographically proven
MVP, we identified two types of chest deformities
causing cardiac entrapment. In the first type, the
chest is shaped in the semblance of a horizontally
placed “figure eight” (Fig 1, bottom, B); in the
second type, the thoracic cage is shaped as a reversed
letter “C” (Fig 5). In the “figure eight” shape type of
deformity, the sternum is sunken into the rib cage
and the thoracic spine shows an accentuated lordotic
curvature. In the second type of deformity, the
accentuated lordotic curvature of the spine is the
only noticeable skeletal abnormality. The actual
pathophysiologic significance of these different phenotypes is unclear at this time.
Group C vs Group D Subjects
The prevalence of cardiac entrapment on EBCT
images was statistically greater in symptomatic patients (group C) than in asymptomatic patients
(group D; 28% vs 6.5%; p ⬍ 0.001). The prevalence
of cardiac entrapment did not differ between male
patients and female patients in group D (4% vs 9%;
p ⫽ NS). On the contrary, in group C there was a
statistically greater proportion of women than men
with entrapment on EBCT images of the chest (41%
vs 17%; p ⫽ 0.002).
Discussion
Our study shows that there is a close correlation
between MVP and a structural chest anomaly not
Table 1—Clinical Characteristics and Chest CT Measurements*
Variables
Group A†
(n ⫽ 66)
Group B‡
(n ⫽ 96)
p Value
Age, yr
Women, No. (%)
Prevalence of CE, %
APD, mm
ANGLE, degree
CA, mm2
48 ⫾ 12
53 (80)
82
91 ⫾ 16
114 ⫾ 9
6,230 ⫾ 2,020
49 ⫾ 10
69 (72)
4.2
128 ⫾ 17
91 ⫾ 5
476 ⫾ 1,009
NS
⬍ 0.001
⬍ 0.001
⬍ 0.001
⬍ 0.001
⬍ 0.001
*Data are presented as mean ⫾ SD, unless otherwise indicated; CE ⫽ cardiac entrapment.
†Patients with echocardiographically proven MVP.
‡Control patients without MVP.
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639
Figure 4. M-mode systolic frame obtained in a patient with
entrapment on EBCT showing classical displacement of the
posterior mitral leaflet (arrow).
readily discernible by visual inspection, but easily
detected with high-quality chest CT imaging. In this
condition, the heart appears trapped in a chest cavity
too small for its size, and its anatomic architecture is
seemingly altered in an attempt to accommodate
these insufficient dimensions. Our findings, along
with the complete absence of MVP in 1,734 consecutive neonates reported by Nascimento et al,6 argue
in favor of an acquired etiology of this frequently
diagnosed valvular condition.
Other authors have previously reported the
association of pectus excavatum and the straight
back syndrome with MVP and physiologic mitral
regurgitation,18 –21,26 and have suggested that flattening of the thorax during growth may cause
morphologic abnormalities of the left atrium and
Figure 5. The configuration of the chest in this patient with
cardiac entrapment and MVP by echocardiography resembles a
reversed letter “C.” Note the morphologic difference compared
to the patient in Figure 1, bottom, B.
ventricle. The association of MVP with the straight
back syndrome has been reported to be due to an
autosomal dominant inheritance,27 and Chen et
al19 proposed that these associated conditions
might be features of a more generalized disorder
with incomplete penetrance. However, no prior
study clearly demonstrated the presence of a
distorted cardiac anatomy in association with skeletal abnormalities, as we have shown in this chest
CT analysis.
Murakami et al28 studied the mechanisms responsible for mitral valve malcoaptation in patients with
the straight back syndrome. They concluded that
anteroposterior flattening of the left ventricle predisposes to asynchronous motion of the papillary muscles at end-systole, causing leaflet malcoaptation and
mitral regurgitation. Lee et al29 maintained that the
cause of MVP is excessive papillary muscle traction
with superior displacement of the mitral leaflets due
to the presence of a valvular-ventricular disproportion, the mitral annulus being dilated compared to
the size of the left ventricle. In this condition,
continuing papillary muscles and chordal stretching
eventually causes valvular prolapse. However, in
others authors’ views, the prolapse should be considered primary and the chordal stretching secondary.10,13,30,31
Because of the presence of myxoid degenerative
changes and redundancy of the mitral leaflets sometimes found in this condition, MVP has been considered a forme fruste of Marfan’s syndrome,14,32,33
although this opinion is disputed by some investigators.17
Palpitations are almost ubiquitous in patients
with MVP,2,34,35 and atypical chest discomfort is
reported very frequently.2,36,37 Some of the proposed dysrhythmic mechanisms include traction of
the papillary muscles, preexcitation, focal cardiomyopathic changes, increased QRS complex dispersion, and long QT syndrome.38 Though lethal
arrhythmias have been associated with the MVP
syndrome, the arrhythmias in MVP are usually
benign. Interestingly, in our study, cardiac entrapment was not only seen in the large majority of
patients with echocardiographically proven MVP,
but also in a sizeable proportion of non-MVP
patients reporting chest discomfort and palpitations, particularly women.
Finally, presyncope and orthostatic hypotension
are often reported by patients with MVP and are
thought by some investigators to be pathophysiologically linked to the small size of the left ventricular
cavity and the subsequent underfilling during times
of hyperdynamic function.2,11
We believe that our findings offer an opportunity to develop a unifying theory for the morpho-
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Clinical Investigations
genesis of MVP, its symptoms and hemodynamic
findings. If an inherited disorder were truly the
cause of this condition, MVP might be the consequence of a congenital predisposition to cardiothoracic growth disproportion. Compression of the
left ventricle in a small space could effectively
cause valvular-ventricular disproportion due to
distortion and enlargement of the mitral valve
annulus. The probable subsequent stretching of
the papillary muscles and of the prolapsing leaflets
could provoke palpitations and chest discomfort as
proposed by Slovut and Lurie.37 On the other
hand, prolonged traction and friction of the prolapsing leaflets and chordae may conceivably cause
myxoid-like degeneration of these structures.
Although the present study offers only observational data and no cause-and-effect link can be
established between the morphologic findings described on EBCT and MVP, it suggests that a
developmental mechanism may be responsible for
this frequently diagnosed valvular abnormality.
Our study also offers an apparent explanation for
the higher prevalence of MVP in women. In fact,
women on the average tend to have smaller APDs
than men, and a myocardial-chest disproportion
may occur more easily under these conditions.
Furthermore, a smaller chest cavity may predispose women to the more frequent development of
atypical chest pain symptoms than men, as was
seen in our group of patients with nonanginal
chest discomfort and palpitations in the absence of
true MVP.
The morphologic anomalies of the heart and rib
cage seen on a high-quality chest CT as described in
this study should alert the interpreting physician to
the possible presence of underlying MVP. Moreover,
this abnormality can help the specialist using chest
CT imaging to explain some of the nonanginal
symptoms reported by patients without overt coronary artery disease.
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Clinical Investigations