straight back syndrome has been reported to be a  pseudo - heart disease  that can mimic congenital abnormalities , especially atrial septal defect ( asd ) ( 1 )
. however , three - dimensional anatomical recognition of this rare disease entity appears to be insufficient . in this case report
, we demonstrate the detailed anatomical background of straight back syndrome with a series of images obtained from low - dose dual - source computed tomography ( dsct ) .
a 23-year - old asymptomatic woman was referred to our hospital for a detailed examination of a systolic ejection murmur with fixed splitting of the second heart sound auscultated at the third left sternal border .
electrocardiography showed right axis deviation and an incomplete right bundle branch block . according to these findings ,
the patient was suspected of having left - to - right shunt disease , including asd .
contrary to expectations , transthoracic echocardiography did not indicate asd , but instead only showed the presence of trivial mitral valve regurgitation due to anterior mitral valve prolapse .
because the four pulmonary veins could not be adequately visualized , she underwent low - dose electrocardiogram - gated contrast - enhanced cardiac computed tomography ( ct ) using commercially available third - generation dsct ( somatom force , siemens healthcare , forchheim , germany ) for further differential diagnosis , including partial anomalous pulmonary venous return , unroofed coronary sinus , subvalvular pulmonary stenosis , and double - chambered right ventricle . a low - dose protocol ( acquisition mode , high - pitch dual spiral scan ; tube voltage , 70 kvp ; rotation time , 250 ms ; effective radiation dose , around 1 msv ) was performed with a contrast agent volume of only 20 ml . the dsct findings ruled out the possibility of any congenital heart diseases , whereas the axial images clearly showed a markedly shortened anteroposterior diameter of the chest ( fig .
1 ) , which led us to a diagnosis of straight back syndrome . a vertically oriented  pancake  appearance ( a compressed heart that appears to be enlarged in frontal images ) of the heart ( 1 ) located in the so - called valentine position ( the heart positioned on its apex ) ( 2 ) ( fig .
2b ) , and compression of the entire right ventricular outflow tract ( rvot ) ( figure 3 , 4 ) were clearly demonstrated on the reconstructed images .
an accelerated flow in the compressed rvot was confirmed with subsequently re - examined transthoracic echocardiography .
compression of the rvot and the accelerated flow were compatible findings as the cause of her systolic ejection murmur with fixed splitting of the second heart sound .
axial image showing thoracic cage compression observed as a shortening of the anteroposterior diameter of the chest .
a vertically oriented  pancake  appearance of the heart ( a ) and a straight vertebral column ( b ) are shown .
ao : ascending aorta , icv : inferior caval vein , la : left atrium , laa : left atrial appendage , lv : left ventricle , lvot : left ventricular outflow tract , pa : pulmonary artery , raa : right atrial appendage , rv : right ventricle , rvot : right ventricular outflow tract , scv : superior caval vein , sv : sinus of valsalva compression of the right ventricular outflow tract : virtual dissection images .
compression of the entire right ventricular outflow tract ( rvot ) between the aortic root and sternum are shown in the multiplanar reconstruction image ( a ) and volume - rendered image viewed from the cranial ( b ) and right ( c ) directions .
the sectional plane of panel a is on the same level as that of panel b. the anteroposterior diameter ( red arrowheads ) of the rvot is 11.6 mm in systole . as : atrial septum ,
cs : coronary sinus , dao : descending aorta , l : left coronary aortic sinus , la : left atrium , laa : left atrial appendage , lad : left anterior descending artery , lcx : left circumflex artery , lipv : left inferior pulmonary vein , lmt : left main trunk , mb : moderator band , mva : mitral valvular attachment , n : non - adjacent aortic sinus , r : right coronary aortic sinus , ra : right atrium , rca : right coronary artery , ripv : right inferior pulmonary vein , spt : septoparietal trabecula , tva : tricuspid valvular attachment compression of the right ventricular outflow tract : dye - cast images .
the relationship between the sternum and right ventricular outflow tract is shown in the frontal image ( a ) .
viewed from the left posterior oblique 120 , the three - dimensional aspect of the entirely compressed right ventricular outflow tract ( yellow dotted circle ) between the sternum and aortic root is observed ( b ) .
the ascending aorta , aortic root , and left ventricle are removed from panel b ( c ) .
lv : left ventricle , ra : right atrium , raa : right atrial appendage , rv : right ventricle
straight back syndrome has been reported to be a  pseudo - heart disease  that can mimic congenital abnormalities , especially asd ( 1,3 ) .
it typically occurs in young , thin persons with a reduced anteroposterior diameter of the chest because of a straight thoracic vertebral column caused by the absence of normal thoracic kyphosis .
exaggerated splitting of the second heart sound and an incomplete right bundle branch block are common associated findings with asd ( 1,3 ) .
although the systolic ejection murmur , often heard at the left sternal border , originates due to rvot compression , this murmur lacks the typical rivero - carvallo sign .
rather , this murmur decreases on deep inspiration and increases on deep expiration ( 3 ) , because deep inspiration enlarges the thoracic cage and releases compression of the rvot .
reported that the average transverse - to - anteroposterior thoracic diameter ratio of patients with straight back syndrome is 3.80 , which is compatible with the present case ( figure 1 , 5 ) , whereas that of the normal group is 2.17 ( 4 ) .
the difference in the anteroposterior diameter of the rvot measured using dsct and echocardiography could be explained by the differences in the respiratory phase of image data acquisition ; the dsct scan was obtained on deep inspiration in systole , whereas the echocardiographic image was obtained on deep expiration and measured in systole .
figure 5 , 6 show a comparison of the present case with a control case of a 30-year - old woman
. comparison of axial images of the present case with those of the control case .
axial images at the level of the ninth thoracic vertebra showing a more flattened chest in the present case ( a ) than that in the control case ( b ) .
comparison of volume - rendered images of the present case with those of the control case .
volume - rendered thoracic images revealing a straight thoracic vertebra due to the absence of normal thoracic kyphosis ( a ) , narrowed thoracic cages ( a , b ) , a compressed heart ( b ) , and a vertical median recess ( red arrows ) at the back between the scapulae ( c ) . within the pericardial space , the posterior surface of the heart
therefore , compared with the posterior part , the anterior to apical part of the heart is relatively easy to rotate . a considerable proportion of patients with pectus excavatum present with right atrial and ventricular compression due to narrowing of the anteroposterior diameter of the chest ( 6 ) .
however , compression of the rvot with a systolic ejection murmur and accentuated splitting of the second heart sound appears to be less common than straight back syndrome . the frequently observed leftward cardiac displacement in pectus excavatum ( 7 )
although it remains a speculation , in pectus excavatum , the negatively bulged anterior chest wall may allow the heart to be displaced , often toward the left side ( 7 ) .
on the other hand , the relatively flat anterior chest wall in straight back syndrome may completely prevent the displacement of the heart , as observed in the present case .
regardless , in both anatomical situations , whether the rvot can escape from considerable compression is relevant . to the best of our knowledge ,
this is the first three - dimensional image of straight back syndrome reconstructed using low - dose dsct .
as demonstrated , low - dose dsct with minimal contrast volume could comprehensively visualize the detailed anatomical background of this rare disease entity .
the reconstructed images help identify the morphology of the heart and thorax in straight back syndrome and reinforce the importance of upper body inspection ( including the superior dorsal region ) , careful auscultation , and lateral chest radiography in young patients who present with clinical findings suggesting asd .
 a 23-year - old asymptomatic woman was referred to our hospital for further examination of a systolic ejection murmur with fixed splitting of the second heart sound auscultated at the third left sternal border . 
 initial echocardiography could not detect the cause . 
 subsequently performed low - dose computed tomography , however , ruled out the possibility of any congenital heart diseases , but revealed a markedly shortened anteroposterior diameter of the chest , which led us to a diagnosis of straight back syndrome . 
 a vertically oriented  pancake  appearance of the heart , straight vertebral column , and compression of the right ventricular outflow tract were clearly demonstrated on the reconstructed images . 
Introduction
Case Report
Discussion
The authors state that they have no Conflict of Interest (COI).