Home About us Editorial board Search Ahead of print Current issue Archives Submit article Instructions Subscribe Contacts Login 
Home Print this page Email this page
Users Online:194


 
 Table of Contents  
ORIGINAL ARTICLE
Year : 2021  |  Volume : 9  |  Issue : 1  |  Page : 66-71

Born-free congenitally absent pericardium


Department of Cardiovascular and Thoracic Surgery, King George's Medical University, Lucknow, Uttar Pradesh, India

Date of Submission29-Sep-2020
Date of Decision30-Sep-2020
Date of Acceptance28-Dec-2020
Date of Web Publication30-Mar-2021

Correspondence Address:
Prof. Sushil Kumar Singh
Department of Cardiovascular and Thoracic Surgery, King George's Medical University, Shahmina Road, Lucknow - 226 003, Uttar Pradesh
India
Login to access the Email id

Source of Support: None, Conflict of Interest: None


DOI: 10.4103/heartindia.heartindia_44_20

Rights and Permissions
  Abstract 


Objectives: Congenital absence of pericardium (CAP) is a very rare condition, with an incidence of <1 in 10,000 and occurs in isolation or associated with various congenital cardiac or systemic disorders. We describe the two cases of incidentally diagnosed left-sided congenitally absent pericardium and evaluate the presentation, diagnosis, and management with recent work done in this area. Atrial septal defects have been associated with the absence of pericardium similar to one of our case. We also found this entity associated with aortic coarctation which has not previously been described.
Materials and Methods: Using our database, we retrospectively went through the details of patients operated in our center between January 2015 and December 2019 to identify the cases with CAP.
Results: Out of 2193 cardiac surgeries, 478 patients got operated for congenital heart disease out of which 2 were found with pericardial defects intraoperatively. No surgical intervention for the pericardial defects was done. Both patients did well after surgery for their primary disease.
Conclusions: Congenital defects of the pericardium are very rare. Although there are characteristic radiology findings, yet without a suspicion, these can be easily missed and are discovered during surgery. The associations with many cardiac conditions have been seen; however, association with aortic coarctation is extremely rare as we could not find any such association in literature.

Keywords: Absent pericardium, cardiac surgery, congenital heart disease


How to cite this article:
Tewarson V, Kumar S, Hakim MZ, Kumar S, Singh SK. Born-free congenitally absent pericardium. Heart India 2021;9:66-71

How to cite this URL:
Tewarson V, Kumar S, Hakim MZ, Kumar S, Singh SK. Born-free congenitally absent pericardium. Heart India [serial online] 2021 [cited 2021 Aug 1];9:66-71. Available from: https://www.heartindia.net/text.asp?2021/9/1/66/312485




  Introduction Top


Congenital absence of pericardium (CAP) is very rare and mostly asymptomatic. It is found incidentally during imaging studies, during surgery, or at postmodern examination.[1] We describe the two cases of incidentally diagnosed left-sided CAP and evaluate them with recent work done in this area. The signs and symptoms of the primary cardiac disorder masqueraded preoperative diagnosis of the pericardial defect during the evaluation for surgery for their primary cardiac disease, and hence, the pericardial defects were encountered intraoperatively. Out of the two cases, one was associated with aortic coarctation (CoA) which has not been previously reported.


  Materials and Methods Top


Using our database, we retrospectively went through the details of patients operated in our centre between January 2015 and December 2019 during which a total of 2193 patients were operated. Among these, there were 478 patients who underwent surgery for congenital heart disease, and these two cases came from this congenital heart disease group. The details of the patients are given in [Table 1]. As these were routine cardiac surgery cases, the involvement of the Institutional Review Board was not performed. Full informed consent was obtained from these patients before surgery and also after their surgery for usage of their data and images of their unique intraoperative surgical findings for the publication.
Table 1: Patient demography, surgery, and postoperative course

Click here to view


Patients

Case 1

A 25-year-old male was being evaluated for hypertension in the cardiology department of our institution. He denied having any other symptoms apart from on and off headaches attributable to his hypertension and very occasional lower limb pains on prolonged exertion. He was found to have elevated blood pressures (BP) in both upper extremities (right upper limb BP – 198/96 and left upper limb BP – 194/90 millimeters of mercury [mm Hg]) and a marked reduction in BP in the lower extremities (BP in the left leg 108/72 mm Hg and BP in the right leg 98/66 mm Hg) with a radio-femoral delay. He had a grade 4/6 systolic murmur posteriorly over the thoracic spine. Chest X-ray (CXR) did not demonstrate characteristic “figure of-3 sign” nor any prominent rib notching [Figure 1]. Transthoracic echocardiography (TTE) revealed left ventricular hypertrophy with tapering of proximal descending aorta with a peak systolic gradient of 56 mm Hg. Computed tomography aortogram (CTA) confirmed coarctation of the aorta, immediately distal to the origin of left subclavian artery [Figure 1]. The patient was started on calcium channel blockers and referred for surgical management. Preoperative blood workup was normal, and the patient was negative for any other comorbidities.
Figure 1: (a and b) Computed tomography aortogram of Case 1 demonstrating aortic coarctation distal to origin of left subclavian artery (yellow arrow), (c) Scannogram demonstrating the levoposition of cardiac silhouette (d) “Snoopy sign” evident on the same

Click here to view


[Figure 2] narrates the operative procedure and findings. The patient was planned for an extraanatomical bypass with a vascular graft for the CoA from the ascending to the descending thoracic aorta. After median sternotomy, the total absence of the pericardium on the left side was evident as shown in the picture. As good retraction of the heart for exposure for the distal graft anastomosis was necessary, we made the use of partial cardiopulmonary bypass (CPB). For the same reason of providing good exposure, we also used an Octopus® stabilizing device (Medtronic, Inc., Minneapolis, MN, USA) to enhance our field of work by retracting and keeping the heart in a stable position during CoA surgery as can be seen in the image. We used a size 20 Dacron tubular graft (Terumo® Aortic Medical Devices Inchinnan, Renfrewshire, Scotland) for the extra anatomical aorta to aorta bypass. This Dacron tube graft was fashioned for the appropriate length and was anastomosed to the descending thoracic aorta in the end-to-side fashion with the use of a side-biting clamp to prevent distal ischaemia. Proximal end of the graft was anastomosed to the ascending aorta in end-to-side fashion with the use of a side biting clamp. We did not repair the pericardium. Postoperative recovery was uneventful. The blood investigations were normal. The patient is now doing fine in the follow-up.
Figure 2: Intra-operative photograph of Case 1. (a) Exposure of descending thoracic aorta better enabled by Octopus® (Medtronic, Inc., Minneapolis, MN, USA) stabilizing device to retract the cardiac tissue. (b) Completed extra-anatomical bypass for aortic coarctation. Large defect of absent left pericardium evident

Click here to view


Case 2

A 19-year-old male presented to the cardiology department with complaints of palpitations, dyspnea, and dizziness for 5 years. Examination revealed a rumbling mid-systolic murmur at the left lower sternal border with a wide fixed split in second heart sound. TTE showed a 2.4 cm ostium secundum atrial septal defect (OS ASD) with deficient rim margins and with a left-to-right shunt (Qp:Qs = 2.8:1) with mild tricuspid regurgitation (TR) and normal biventricular function. The patient was planned for surgical repair of the OS ASD in our department. Other preoperative parameters were normal. At the time of surgery subsequent to midline sternotomy, it was noted that the left-sided pericardium was deficient and the heart was tilted into the left pleural cavity. We routinely utilize fresh autologous pericardium for OS ASD closure at our center. However, due to the absence of pericardium on the left side, the ASD was closed with a Dacron® patch in the standard fashion [Figure 3]. The pericardial defect was left as such. After meticulous de-airing, the patient was smoothly weaned off CPB. He has been doing fine in the follow-up.
Figure 3: (a) Intraoperative photograph of case 2 demonstrating the ostium secundum atrial septal defect (vent placed through into LA) upon right atriotomy (b) After completion of atrial septal defect repair, right atriotomy closed. Absent left pericardium evident

Click here to view



  Results Top


Out of the 2193 patients operated during the period of 5 years, the majority of the cases were of acquired heart disease. Congenital absence of the left pericardium in our two cases (in CoA and OS ASD) was seen only in association with the congenital heart disease group comprising 478 out of the total number of patients. Thus, pure pericardial defects as a stand-alone entity were not seen. The surgical and postoperative parameters are depicted in [Table 1]. While the first case was done on partial bypass which was required for a better exposure of the distal anastomosis site, the second case of OS ASD had a 20 min cross clamp time. Total bypass time was 50 min and 40 min in the first and second cases, respectively, and the total blood loss similarly was 250 ml and 300 ml. Both patients were extubated within 6 h and none required inotropic support. The postoperative intensive care unit stay was smooth, recovery for both was good, and the total hospital stay was 5 days each for both patients. They did well during follow-up with no new cardiovascular complaints [Table 1].


  Discussion Top


Prevalence and associations

Pericardial defects are extremely rare, their true occurrence usually underestimated (being mostly asymptomatic) and occur more commonly in males.[2] The etiology of CAP is thought to be the failure of pleuropericardial membrane fusion or if the heart enlarges before fusion of the pleuropericardial membrane.[2],[3] Another potential cause can be a tear induced by traction contributing to a defect in the pericardium.[3],[4] Finally, if the cardinal veins (duct of Cuvier) prematurely atrophy, a defective pericardium may form as a consequence of reduced blood supply.[3],[5]

Depending upon their extent and location, they may be either complete (bilateral, right, or left) or partial (right or left). Their prevalence in the decreasing order is – complete left sided (70%), complete right sided (17%), complete bilateral (9%), partial right or left sided (3–4%).[1],[6] In addition to the congenital anomalies mentioned earlier, to which CAP is mostly associated (namely ASD, tricuspid atresia, bronchogenic cyst, pectus excavatum, and VATER syndrome), it has also been found in a rare concomitant defect of vascular ring created by right-sided aortic arch with aberrant origin of the left subclavian artery and Kommerell's diverticulum. It has also been noted in Pallister–Killian syndrome and Marfan's syndrome with bicuspid aortic valve.[2],[7],[8]

Correlating this further, another study done earlier showed that approximately half of the patients with pericardial agenesis of some kind have an associated congenital heart defect which in addition to above diseases also included, sinus venosus defects with partial anomalous pulmonary venous drainage, tetralogy of Fallot, patent ductus arteriosus, and mitral valve disease.[9] In terms of mitral valve disease, our center performed 764 mitral valve cases during the same time period, and no pericardium abnormality was seen.

While associations with CoA could not be cited by us in literature, CAP very interestingly is associated with ASD sharing features of similarity and that is why many reports mention initial workups for ASD surgery only to be found having CAP later, for example, by Garnier et al.[1]

Clinical features

Kim et al. while demonstrating a way to distinguish ASD from CAP echocardiographically also mentioned features shared by both ASD and CAP like right bundle branch block on electrocardiogram, a systolic ejection murmur and wide splitting of S2 on auscultation. There is also the presence of right ventricular (RV) enlargement and TR.[10] The second of our cases had a well-documented presence of a 2.4 cm OS ASD with deficient rim margins unsuitable for device closure and therefore need of surgical ASD closure. The subtle shared symptomology between these two entities made OS ASD mask any sort of suspicion of a diagnosis of CAP preoperatively.

Although the majority of the cases are clinically silent,[11] we in our deal with this entity would agree here that, given its rare occurrence, many clinicians and imaging specialists will have little experience with CAP and may fail to recognize it on thoracic or cardiac studies for workup.[12],[13]

Again, while a majority of the silent cases are found with the complete absence of pericardium, it is the partial absence which is more symptomatic. Trepopnea is a characteristic complaint and is described as shortness of breath when a patient lies down toward one side, mostly the left.[14] Chest pain can also occur due to the kinking or compression of a coronary artery by the sharp rim of the pericardial defect and cases of sudden death have also been cited owing to this fact.[15] None of our cases presented with trepopnea or chest pain. On the other hand, our first case with CoA would qualify as one of the silent cases as far as symptoms for complete CAP are concerned. These, as mentioned above are mostly asymptomatic or at best have vague or nonspecific symptoms. Their diagnosis like ours is mostly incidental during surgery or on thoracic imaging performed for other conditions such as CTA which was done for CoA but missed the pericardial finding.[16]

Both our cases had murmurs justifiable to the respective etiology of cardiac disease; however, in CAP, many case reports have noted a systolic ejection murmur at the left side of the sternum which is considered to be due to the turbulence setup by various mechanical deformities at the base of an unusually mobile heart thus making distinguishing diagnosis further difficult.[17]

Radiological diagnosis

Ellis in 1959 first characterised the findings of the absence of pericardium on plain chest radiographs.[18] The first thing to mention here is that despite all of the cardiac rotations and displacements seen in this rare disorder which will be discussed shortly; the plain CXR remarkably does not cause any deviation of the trachea which is a midline structure in the CXR in these conditions.[2] In patients with a complete left CAP, CXR reveals the often mentioned characteristic “Snoopy Sign” which happens due to the posterior and leftward shift of the heart and is characterized by the straightening and elongation of the left border of cardiac silhouette and by the loss of its right border, covered by the spine,[19] and radiolucent bands of lung tissue between the aorta and the pulmonary artery and between the left hemidiaphragm and the base of the heart.[20],[21] In noncomplicated partial CAP, CXR is generally normal. Left-sided partial defects can occasionally present with a bulge of the left upper heart border which may suggest the herniation of the left atrial appendage through the defect.[22]

After surgery for CoA and OS ASD, respectively, we re-evaluated our patients' CXRs and these above-mentioned findings were noted from a new perspective. In particular, our second case of OS ASD had a leftward cardiac shadow and flattening and elongation of the left heart border. There was also lucency between the aorta and left pulmonary artery (correlating well to our intraoperative findings of a bridge or wedge of lung tissue lying between the two great arteries). The trachea was relatively midline. These findings were again suggestive of the “Snoopy sign” characteristic of left pericardial absence. The right border of the thoracic spine was also exposed [Figure 4].
Figure 4: (a) Chest X-ray demonstrating levoposition of cardiac shadow, wedge between aorta and pulmonary artery (arrow 1), elongated left heart border (arrow 2) lucency between the left ventricle and the diaphragm (arrow 3) and exposed right border of thoracic spine (arrow heads). Inset image shows the superimposition of our re-evaluation of the same X-ray showing the characteristic features again. (b) Intra-operative photograph showing the absence of left pericardium with the left lung covering the left atrium, root of pulmonary artery, and part of the ventricle and interposing between the aorta and pulmonary artery (arrow)

Click here to view


Despite the classical radiological findings mentioned above, the diagnosis becomes difficult due to a low suspicion for this exceedingly rare disease. Moreover, many of the findings are nonspecific and fit a variety of other disease presentations as seen above. Due to this problem, advanced imaging techniques are necessary to make the diagnosis while excluding other possibilities in the symptomatic patient with CAP.[8]

Both dedicated cardiac magnetic resonance imaging (CMR) and cardiac computed tomography (CT) are the advanced modes of imaging for pericardial defects with a more or less similar diagnostic accuracy. Clinical settings, cost factor, radiological expertise, and availability determine the usage of one over the other. While CMR provides a more comprehensive evaluation, which includes direct pericardial visualization and assessment of cardiac function without exposure to ionizing radiation, CT is quicker and gives submillimeter cuts in the good resolution for the visualization of the pericardium. The CTA of our first case during evaluation and three-dimensional reconstruction for CoA surgery workup did not reveal the pericardial absence on the left side [Figure 1]. This was a dedicated aortic CTA. It is noteworthy to mention here that although a cardiac CT scan can generally identify the pericardium, the most common sites of pericardial defects – the left sided ones, happen to correlate with those locations which are not ideally visualized on cardiac CT[5],[23] and therefore like our first case, can easily be missed.

According to Shah and Kronzon,[2] on conventional echocardiography, standard views to image for these defects are not always applicable and the traditional left parasternal view may show more of the RV area due to cardiac displacement, and therefore, the patient may falsely be diagnosed to be having RV dilatation. The dilemma here is that a significant left-to-right shunt causing RV dilatation is also an accepted indication of ASD closure.[24]

Treatment

For patients with complete bilateral or complete left-sided absence of the pericardium, as in our two cases, there is no indication for surgery to close the pericardial defect.[2] Whether or not surgery for the primarily diagnosed entity like CoA or OS ASD should be accompanied by concomitant pericardial closure when pericardial agenesis is detected is too early to say given the few number of cases but we went by the above convention and the pericardium was left as such. There were no more cardiovascular complaints after surgery further indicating that these patients' symptoms were due to CoA and OS ASD, respectively, (and had these two patients not been having CoA and OS ASD, their pericardial defects would probably have gone unnoticed). In addition, patients with a complete left absence of pericardium as a stand-alone finding have fared similar to normal participants in terms of the prognosis.[2] As far as moderate size defects or partial defects are concerned, prophylactic closure or repair is often advised as there may be a risk of strangulation due to herniation and the defect can be closed using a patch. Many times, simple pericardiectomy and division of adhesions solve the purpose.[12]

The very rare nature of CAP has precluded the laying down of well-established protocols of surgical management of this entity. This has made recommendations at best to be largely based on observational studies or case reports.[25] While most authors agree upon the previously mentioned rationale, there are some features which are helpful in identifying high risk patients and thus the need of surgery as described by Lopez amd Asher[11] – (a) a left ventricular myocardial crease or hinge point on cardiac imaging, (b) coronary compromise on conventional angiography or CT, (c) inducible ischaemia on stress perfusion studies, and (d) herniation. Surgery for large CAPs like in our cases does not provide any benefit.[11]


  Conclusions Top


Congenital defects of the pericardium are rare, and as in our cases, they mostly occur on the left side. Apart from the primary disease itself, when they occur in isolation; the signs and symptoms are vague. Surgery should be tailored individually and as shown in our patients, when warranted, the pericardial defects can be left alone. Again, although there are characteristic CXR, CCT, and CMR findings, yet without a suspicion, these can be easily missed and are discovered during surgery. Despite this CAP may still go unrecognized not because of the lack of sensitivity of criteria, but more likely due to the lack of suspicion for such a rare condition. Associations with many cardiac conditions have been seen, however, association with aortic coarctation is extremely rare as we could not find any such association in literature.

Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.

Ethical approval

As these were routine cardiac surgery cases, the involvement of the Institutional Review Board was not performed. Full informed consent was obtained from these patients before surgery and also after their surgery for usage of their data .

Authors' contributions

Vivek Tewarson: Performed the literature review and prepared the manuscript. Sarvesh Kumar: Helped with review of literature, preparation and editing of the manuscript.Mohammad Zeeshan Hakim: Helped with preparation of the manuscript and collection of data and literature.Shobhit Kumar: Helped with collection of data and literature. Sushil Kumar Singh: Conceived and designed this work, supervised the preparation of the manuscript and edited the manuscript prior to submission for publication. All authors have seen the final manuscript and approve it for publication.



 
  References Top

1.
Garnier F, Eicher JC, Philip JL, Lalande A, Bieber H, Voute MF, et al. Congenital complete absence of the left pericardium: A rare cause of chest pain or pseudo-right heart overload. Clin Cardiol 2010;33:E52-7.  Back to cited text no. 1
    
2.
Shah AB, Kronzon I. Congenital defects of the pericardium: A review. Eur Heart J Cardiovasc Imaging 2015;16:821-7.  Back to cited text no. 2
    
3.
Koo CW, Newburg A. Congenital absence of the right pericardium: Embryology and imaging. J Clin Imaging Sci 2015;5:12.  Back to cited text no. 3
[PUBMED]  [Full text]  
4.
Kaneko Y, Okabe H, Nagata N. Complete left pericardial defect with dual passage of the phrenic nerve: A challenge to the widely accepted embryogenic theory. Pediatr Cardiol 1998;19:414-7.  Back to cited text no. 4
    
5.
Bogaert J, Francone M. Pericardial disease: Value of CT and MR imaging. Radiology 2013;267:340-56.  Back to cited text no. 5
    
6.
Klein AL, Abbara S, Agler DA, Appleton CP, Asher CR, Hoit B, et al. American Society of Echocardiography clinical recommendations for multimodality cardiovascular imaging of patients with pericardial disease: Endorsed by the Society for Cardiovascular Magnetic Resonance and Society of Cardiovascular Computed Tomography. J Am Soc Echocardiogr 2013;26:965-1012.e15.  Back to cited text no. 6
    
7.
Sani ZA, Savand-Roomi Z, Vojdanparast M, Sarafan S, Seifi A, Nezafati P. Congenital partial absence of the pericardium presenting with a rare concurrent abnormality of vascular ring diagnosed by cardiac magnetic resonance imaging. Adv Biomed Res 2016;5:203.  Back to cited text no. 7
  [Full text]  
8.
Parmar YJ, Shah AB, Poon M, Kronzon I. Congenital abnormalities of the pericardium. Cardiol Clin 2017;35:601-14.  Back to cited text no. 8
    
9.
Bremerich J, Reddy GP, Higgins CB. Magnetic resonance image of cardiac structure. In: Pohost GM, O'Rourke RA, Berman D, Shah PM, editors. Imaging in Cardiovascular Disease. Philadelphia: Lippincott Williams & Wilkins; 2000. p. 409, 756.  Back to cited text no. 9
    
10.
Kim MJ, Kim HK, Jung JH, Yoon YE, Kim HL, Park JB, et al. Echocardiographic diagnosis of total or left congenital pericardial absence with positional change. Heart 2017;103:1203-9.  Back to cited text no. 10
    
11.
Lopez D, Asher CR. Congenital Absence of the Pericardium. Prog Cardiovasc Dis 2017;59:398-406.  Back to cited text no. 11
    
12.
Van Son JA, Danielson GK, Schaff HV, Mullany CJ, Julsrud PR, Breen JF. Congenital partial and complete absence of the pericardium. Mayo Clin Proc 1993;68:743-7.  Back to cited text no. 12
    
13.
Gersbach P, Kaufmann U, Althaus U. Congenital absence of the left pericardium with extreme intrathoracic displacement of the heart. Case report and review of the literature. Eur J Cardiothorac Surg 1996;10:473-6.  Back to cited text no. 13
    
14.
Wilson SR, Kronzon I, Machnicki SC, Ruiz CE. A constrained heart: A case of sudden onset unrelenting chest pain. Circulation 2014;130:1625-31.  Back to cited text no. 14
    
15.
Yared K, Baggish AL, Picard MH, Hoffmann U, Hung J. Multimodality imaging of pericardial diseases. JACC Cardiovasc Imaging 2010;3:650-60.  Back to cited text no. 15
    
16.
Bernardinello V, Cipriani A, Perazzolo Marra M, Motta R, Barchitta A. Congenital pericardial agenesis in asymptomatic individuals: Tips for the diagnosis. Circ Cardiovasc Imaging 2020;13:e010169.  Back to cited text no. 16
    
17.
Nasser WK. Congenital diseases of the pericardium. Cardiovasc Clin 1976;7:271-86.  Back to cited text no. 17
    
18.
Ellis K, Leeds NE, Himmelstein A. Congenital deficiencies in the parietal pericardium: A review with 2 new cases including successful diagnosis by plain roentgenography. Am J Roentgenol Radium Ther Nucl Med 1959;82:125-37.  Back to cited text no. 18
    
19.
Panagiotopoulos N, Barnard J, Wells F. The snoopy sign. Eur J Cardiothorac Surg 2014;45:586.  Back to cited text no. 19
    
20.
LeWinter MM, Samer K. Pericardial diseases. In: Zipes DP, Libby P, Bonow RO, Braunwald E, editors. Braunwald's Heart Disease: A Textbook of Cardiovascular Medicine 2. 7th ed. Philadelphia: Elsevier; 2005. p. 1757-8.  Back to cited text no. 20
    
21.
Gatzoulis MA, Munk MD, Merchant N, Van Arsdell GS, McCrindle BW, Webb GD. Isolated congenital absence of the pericardium: Clinical presentation, diagnosis, and management. Ann Thorac Surg 2000;69:1209-15.  Back to cited text no. 21
    
22.
Robin E, Ganguly SN, Fowler MS. Strangulation of the left atrial appendage through a congenital partial pericardial defect. Chest 1975;67:354-5.  Back to cited text no. 22
    
23.
Baim RS, MacDonald IL, Wise DJ, Lenkei SC. Computed tomography of absent left pericardium. Radiology 1980;135:127-8.  Back to cited text no. 23
    
24.
Baumgartner H, Bonhoeffer P, De Groot NM, de Haan F, Deanfield JE, Galie N, et al. ESC Guidelines for the management of grown-up congenital heart disease (new version 2010). Eur Heart J 2010;31:2915-57.  Back to cited text no. 24
    
25.
Ignaszewski M, Baturin B, Waldman B, Peters P. Cardiac eclipse: Congenital absence of the pericardium manifesting as atypical chest pain. CASE (Phila) 2020;4:59-62.  Back to cited text no. 25
    


    Figures

  [Figure 1], [Figure 2], [Figure 3], [Figure 4]
 
 
    Tables

  [Table 1]



 

Top
 
 
  Search
 
Similar in PUBMED
   Search Pubmed for
   Search in Google Scholar for
 Related articles
Access Statistics
Email Alert *
Add to My List *
* Registration required (free)

 
  In this article
Abstract
Introduction
Materials and Me...
Results
Discussion
Conclusions
References
Article Figures
Article Tables

 Article Access Statistics
    Viewed284    
    Printed0    
    Emailed0    
    PDF Downloaded30    
    Comments [Add]    

Recommend this journal


[TAG2]
[TAG3]
[TAG4]