Parachute tricuspid valve: a systematic review

Background A parachute tricuspid valve is a very rare congenital cardiac anomaly. Its morphological features and clinical implications have not been sufficiently described so far. The purpose of the present systematic review is to disclose the morphological and clinical characteristics of parachute tricuspid valve, and to discuss its diagnostic methods, treatments and patients’ outcomes. Main body The Preferred Reporting Items for Systematic Reviews and Meta-analyses (PRISMA) statement guidelines were followed in this systematic review. Publications were systematically searched in the PubMed, Highwire Press, and the Cochrane Library databases. By comprehensive retrieval of the pertinent literature published between 1979 and 2019, 13 reports were collected with 14 patients recruited into this study. Their ages ranged from neonate to 52 years old with a median age of 23 years. Tricuspid valve regurgitation of a less-than-severe degree was seen in 6 (60%) patients, tricuspid valve stenosis was present in 3 (30%) patients and normally functioning tricuspid valve was noted in 1 (10%) patient. All patients had a single papillary muscle in the right ventricle. The chordae tendineae could be normal in length and thickness, or elongated, or shortened and thickened. Forty percent of the patients were asymptomatic or with only mild symptoms and did not need a surgical or interventional therapy, and 6 (60%) patients were indicated for a surgical/interventional treatment due to their severe presenting symptoms, associated congenital heart defects, and the resultant severe right ventricular inflow obstruction and (or) tricuspid stenosis. Patients’ outcomes varied depending on the substantial status of the patients with a survival rate of 70% and mortality rate of 30%. Conclusion A few patients with a parachute tricuspid valve are asymptomatic or only with mild symptoms and a surgical or interventional treatment is not required. The surgical/interventional indications for parachute tricuspid valve patients are their severe presenting symptoms, associated congenital heart defects, and the resultant severe right ventricular inflow obstruction and (or) tricuspid stenosis. The survival rate of this patient setting is satisfactory.


Introduction
An atrioventricular valve anomaly caused by a single papillary muscle with unifocal attachment of the chordae tendineae was firstly described by Swan et al. [1] in 1949. This anomaly was termed as "parachute" by Schiebler et al. [2] in 1961. Schiebler et al. [2] and El Sayed et al. [3] observed a parachute anomaly affecting the right-sided atrioventricular valve in patients with corrected transposition of the great arteries. The right atrioventricular valve in such patients is actually the morphological mitral valve, whereas the left-sided atrioventricular valve is the tricuspid valve. Thus it should be categorized as parachute mitral valve. Parachute tricuspid valve (PTV) is a very rare congenital lesion. Due to its rarity, the anatomical features and clinical implications of PTVs remain uncertain. The purpose of this article is to describe the anatomical and clinical features of PTV, and discuss the management, surgical indications and outcomes of the patients.

Materials and methods
The Preferred Reporting Items for Systematic Reviews and Meta-analyses (PRISMA) statement guidelines were followed in this systematic review. Publications were systematically searched in the PubMed, Highwire Press and the Cochrane Library databases. The period of study was from 1979 to 2019. The MeSH terms and keywords used to identify articles included "parachute tricuspid valve", "supravalvular tricuspid ring", "single papillary muscle", "congenital tricuspid stenosis", "tricuspid valve repair", and "tricuspid valve replacement". The screening of the bibliographic references helped in completing the literature retrieval. Twenty-six articles were found related to the topic and keywords in the literature search; and 13 articles, which met the inclusion criteria during preliminary assessment, were included in this review. The inclusion criteria were publications of any type (clinical research, case series, or case report) on PTV patients at all ages including children (age < 18 years old) and adults (age ≥ 18 years old) with substantial patient information for statistical analysis. The exclusion criteria were: parachute mitral valve (n = 7), parachute-like tricuspid valve (n = 2), parachute-like mitral valve (n = 1), parachutelike atrioventricular valve (n = 1), accessory tricuspid valve (n = 1), and lack of substantial patient information (n = 1). A flow chart of literature retrieval was shown in Fig. 1.
The data independently extracted from each study were the study population, demographics, morphology and function of the tricuspid valve, associated congenital heart defects, surgical indications, management and patients' outcomes.
The measurement data were expressed in mean ± standard deviation. The categorical variables were expressed by frequency and percentage, and were compared by Fisher's exact test. P < 0.05 was considered statistically significant. An IBM SPSS Statistics version 22.0 was used to perform the statistical analyses.
A cardiac murmur was audible in 11 patients. Actually a systolic murmur was audible in all 11 patients. In 10 patients, the murmur was audible along the sternal border, and in 1 patient, it was heard in the left intercostal space [8]. In one of the patients, an additional diastolic murmur was heard at the apex [4]. A fixed split of the second heart sound was heard in 3 (27.3%) patients [5,13,14].
The treatments were not described for 4 patients. Of the remaining 10 patients, a surgical/interventional treatment was required in 6 (60%) patients: tricuspid valve operation in 2 (33.3%) patients (annular ring resection plus tricuspid valve replacement [9], and chord transition [10]), and surgical/interventional repair of other associated intracardiac anomalies in 5 (83.3%) patients (the patient receiving chord transition had simultaneous atrial and ventricular septal defect closures [10]) including device closure of atrial septal defect [5,11], surgical closure of atrial septal defect [8], chord transition (to support the free edge of the neoseptal leaflet) plus atrial and ventricular septal defect closures [10] and straddling mitral valve repair plus ventricular septal defect closure [4]. In addition, 4 (40%) patients were on a follow-up only due to mild clinical symptoms. There was no significant difference in the prevalence between surgical/interventional and non-surgical/interventional patients (χ 2 = 0.8, P = 0.656).

Discussion
In 1979, Milo et al. [4] firstly reported a case of PTV associated with a straddling mitral valve and doubled outlet right ventricle in a baby. He was performed an operation at the age of 6 years 8 months but died soon after the operation. PTV is a very rare congenital cardiac anomaly. It is considered to have been underestimated, as the anatomical features are often sheltered by the associated congenital anomalies [12]. Its prevalence appears to be significantly lower than that of parachute mitral valve [4].
The anatomical features of the atrioventricular valve caused by two asymmetric papillary muscles are termed as a parachute-like valve [17]. The true type of PTVs should be distinguished from the parachute-like valve. In one report, a case of true PTV was mistaken as a parachute-like one [5]. This case was enrolled into this study as a case of true PTV.
Patients with a PTV can be asymptomatic if the valve function is normal or only with mild valve dysfunction [16]. However, in some patients, PTVs may cause tricuspid stenosis and (or) regurgitation. Patients with severe tricuspid stenosis may present with congestive heart failure [10].
A PTV can be visualized in two-and three-dimensional transthoracic and transesophageal echocardiography. A transgastric view is preferred to reveal the features of a PTV, including a solitary papillary muscle, pear-like right atrium and chordal redundancy. Multidetector computed tomography can also reveal the feature of a PTV with a single papillary muscle as well as its attachment site [8].
It has been reported in the literature that PTV was associated with right heart obstructions, such as tetralogy of Fallot [6] and double outlet right ventricle [4]. Atrial and (or) ventricular septal defects were often present in PTV patients. Parachute deformity of both mitral and tricuspid valves were reported in 2 patients [11,14]. Mohan et al. [14] summarized 7 cases of PTV patients from the literature, and found the ratio of complex and solitory PVT was 4:3 (1.3:1). Tricuspid stenosis was less significant in PTV patients [5]. Transthoracic/ transesophageal echocardiography was a usual diagnostic technique confirming the morphological features of PTV including single papillary muscle, doming shaped chords and pear-shaped right atrium [7]. Three-dimensional transesophageal echocardiography and computed tomography angiography also confirmed the features of PTVs [7]. Patients with a PTV often underwent a surgical treatment of the associated cardiac defects, and an operation for the tricuspid valve per se was usually not demanding unless there was severe tricuspid stenosis. Gupta et al. [10] reported their complex tricuspid valvuloplasty including single papillary muscle mobilization, chord transfer, anteroseptal commissure creation and regional annuloplasty for their PTV patient with a unicuspid tricuspid valve. Moreover, in PTV patients, transcatheter atrial septal defect closure could reduce the severity of tricuspid regurgitation as a result of reduction of tricuspid annular dimension and right ventricular volume overload.
The present study revealed that the morphological changes of the tricuspid valve of PTV patients could be functionally normal (10%), stenosed (30%), or regurgitant (60%). In line with what was reported in the literature, due to the small number of tricuspid stenosis cases and the mildly regurgitant tricuspid valve, the demanding of tricuspid valve operation was less likely in this patient setting. The tricuspid chords in patients with a PTV varied morphologically significantly from normal to shortened or elongated. In the present study, PTV associated with other cardiac anomalies was seen in 85.7% of the patients, while isolated PTV accounted for 14.3% with a ratio of 6:1, significantly exceeding the value reported in the literature. Surgical treatment was performed in 60% patients with a tricuspid valve operation in one-third and repair of associated cardiac defects in two-thirds of the patients. In 40% patients, only follow-up is advised due to no or mild symptoms. The prognoses of the patients were good with a survival rate of 70%.
As it was evidenced by the present study, when PTV patients became symptomatic, associated with additional congenital heart defects, or right ventricular inflow obstruction and (or) severe tricuspid stenosis, they were indicated for a surgical/interventional treatment [16]. Patients' outcomes vary from case to case depending on the substantial status of the patients [8]. The present article proved that 60% of patients warranted a surgical/ interventional treatment, and the mortality rate was 30%.
The study materials were based on some case reports, and this situation might bring about possible publication biases at an outcome level, and which was considered a major drawback of the study. In spite of the limitations, the value of the work still exists.

Conclusion
PTV is a very rare congenital cardiac anomaly. Minority of patients are asymptomatic or only with mild symptoms and do not need a surgical/interventional therapy. The surgical/interventional indications for PTV patients are their severe presenting symptoms, associated congenital heart defects, and the resultant severe right ventricular inflow obstruction and (or) tricuspid stenosis. Patients' outcomes vary depending on the substantial status of the patients.