Discussion

Malposition of the branch pulmonary arteries is a condition that is underdiagnosed and underreported. As per available literature, we found that the classic type was about two times more common than the lesser type, which is similar to the result reported by Hui Liu. ^{Reference Liu, Juan, Wang, Xie and Chen6} The patients with isolated crossed pulmonary arteries are usually asymptomatic. They did not require special intervention unless if there is stenosis of pulmonary arteries branches or CHDs requiring repair. Malposition of the branch pulmonary arteries does not cause mechanical airway obstruction by itself. If it occurs, it is related to associated cardiovascular anomalies like pulmonary arterial sling. ^{Reference Chen, Hsieh, Chiu, Chen and Wu5,Reference Liu, Juan, Wang, Xie and Chen6,Reference Babaoglu and Do11,Reference Hernandez, Anderson, Hoggard, Louis and Shepard12} Crossed pulmonary arteries must be distinguished from pulmonary artery sling and anomalous origin of one pulmonary artery ^{Reference Novelli, Malkani and Cerrone3,Reference Chen, Hsieh, Chiu, Chen and Wu5,Reference Zhang, Gao, Huang, Hu, Yang and Ma10} and also from the abnormal course of the left pulmonary artery in the setting of a horseshoe lung. ^{Reference Chaturvedi, Mikailian and Freedom8}

Echocardiography is the primary method used to demonstrate crossed pulmonary arteries in most of the reported cases, as it was a safe and convenient economical image technique. ^{Reference Zimmerman, Berdusis and Wright7,Reference Zhang, Gao, Huang, Hu, Yang and Ma10,Reference Babaoglu and Do11} Modern imaging methods, such as cardiac magnetic resonance angiography and CT, help to diagnose this disease precisely and may aid in correct cardiac repair planning. ^{Reference Cuturilo, Drakulic and Krstic13} Many of the authors believe that echocardiography is enough to diagnose crossed pulmonary arteries in most patients, and the cardiac CT is indicated in special cases, for example, poor echo window suspected associated aortic arch anomalies or need to have more information about the course and location of the pulmonary artery branches and related anatomic structures, especially with complex cardiac anomalies. ^{Reference Babaoglu and Do11} On the other hand, Zhang et al and Jin Chen supported the use of CT in the diagnosis of crossed pulmonary arteries if there is any doubt by echocardiography as they thought that many cases are missing. ^{Reference Zhang, Gao, Huang, Hu, Yang and Ma10,Reference Chen and Feng14}

The disease is found to have a male predilection as per the reports available in literature. ^{Reference Recto, Parness, Gelb and Lopez4,Reference Liu, Juan, Wang, Xie and Chen6,Reference Zimmerman, Berdusis and Wright7,Reference Raza, Khandwala, Qayyum and Saeed9–Reference Babaoglu and Do11,Reference El Batti, Ben Abdallah, Julia, Alsac and Vouhé15} A good number of cases were reported mainly from China. ^{Reference Chen, Hsieh, Chiu, Chen and Wu5,Reference Liu, Juan, Wang, Xie and Chen6,Reference Zhang, Gao, Huang, Hu, Yang and Ma10,Reference Cheng, Zhang and Meng16–Reference Altun, Babaoğlu, Oğuz and Dönmez19} Genetic syndrome was detected in 45% cases reported in literature; the commonest reported syndrome was DiGeorge syndrome. Other chromosome abnormalities include trisomy 18, partial monosomy Xq, partial trisomy 1q, Holt–Oram, Noonan, Turner, Castello, and Frankter Haar syndrome. ^{Reference Hernandez, Anderson, Hoggard, Louis and Shepard12,Reference Wolf, Casta and Nichols20}

As per the available literature, the classical crossing ^{Reference Becker, Becker and Edwards2,Reference Recto, Parness, Gelb and Lopez4–Reference Chaturvedi, Mikailian and Freedom8,Reference Babaoglu and Do11–Reference Chen and Feng14,Reference Cheng, Zhang and Meng16,Reference Fu, Yang, Zhang and Feng17,Reference Agarwal, Arulselvam, Rashid, Amer and Kalis21–Reference Shunmugasundaram, Bijulal and Titus31} is found in majority of cases reported in literature as compared to reverse crossing ^{Reference Raza, Khandwala, Qayyum and Saeed9} among typical variety. The typical variety is classified into crossed pulmonary arteries and reverse crossed pulmonary arteries. The crossed pulmonary arteries is associated with left pulmonary artery origin superior to the right of the right pulmonary artery, and the two branches crisscross one another along with their courses to their respective lungs, whereas the reverse crossed pulmonary arteries is described as right pulmonary artery origin superior to the left of the left pulmonary artery, and the two branches crisscross one another along with their courses to their respective lungs. Among the lesser type, common variant left pulmonary artery superior to the right pulmonary artery is seen more commonly. ^{Reference Becker, Becker and Edwards2,Reference Liu, Juan, Wang, Xie and Chen6,Reference Zhang, Gao, Huang, Hu, Yang and Ma10,Reference Cuturilo, Drakulic and Krstic13} The lesser form or atypical variety is further described as common variant and rare variant. The common variant has left pulmonary artery openings directly above the right pulmonary artery, and the two branches do not crisscross one another whereas in the rare variant right pulmonary artery openings directly above the left pulmonary artery, and the two branches do not crisscross one another.

Malposition of the branch pulmonary arteries is not always associated with other cardiovascular anomalies, ^{Reference Recto, Parness, Gelb and Lopez4–Reference Liu, Juan, Wang, Xie and Chen6,Reference Babaoglu and Do11} as we found a total of 15 cases with isolated crossed pulmonary arteries, but however associated CHD is very common as it had been reported in 86.8% of cases and 30% were complex cases, and this result is comparable to the 20% reported by Babaoglu et al. and 40% reported by Hui Liu. ^{Reference Liu, Juan, Wang, Xie and Chen6,Reference Babaoglu and Do11}

Since Becker E. et al. in 1970 reported the association of crossed pulmonary arteries with truncus arteriosus, many authors postulated the link of crossed pulmonary arteries with conotruncal anomaly. ^{Reference Recto, Parness, Gelb and Lopez4,Reference Cuturilo, Drakulic and Krstic13} However, according to the available data, we found ventricular septal defect is the most common cardiac defect associated with crossed pulmonary arteries representing 38% (43 cases) of all patients with malposition of the branch pulmonary arteries (ventricular septal defect accompanied with complex CHD not taken in the account), and this result is near to that of Babaoglu et al., ^{Reference Babaoglu and Do11} which show ventricular septal defect represent 55% (11/22) of all cases, while in Hui Liu et al. ^{Reference Liu, Juan, Wang, Xie and Chen6} study, ventricular septal defect represents 80% (12/15) and in Jing Zhang ^{Reference Zhang, Gao, Huang, Hu, Yang and Ma10} study represents 13.7% (4/29). Isolated crossed pulmonary arteries generally is asymptomatic. However, they will express symptoms secondary to the associated heart abnormalities. ^{Reference Babaoglu and Do11} Many of the previous reports showed that malposition of the branch pulmonary arteries does not cause mechanical airway obstruction, ^{Reference Chen, Hsieh, Chiu, Chen and Wu5,Reference Babaoglu and Do11} while Hui Liu’s study showed that 33% of his patients were manifested with airway compression secondary to external compression. Three of them received successful surgery to relieve the external airway compression. ^{Reference Liu, Juan, Wang, Xie and Chen6} Only seven cases had found to be reported in our survey, and this may be because most of the authors did not mention this in their reports, or most of the cases were diagnosed by echocardiography. In the literature coexisting anomalies of the aortic arch, such as interrupted aortic arch, right-sided aortic arch, aberrant left subclavian artery present in 71% of cases which have reported by and Recto MR and 80% reported by Hui Liu. ^{Reference Recto, Parness, Gelb and Lopez4,Reference Liu, Juan, Wang, Xie and Chen6,Reference Cuturilo, Drakulic and Krstic13} In our review, aortic arch anomalies were detected only in 42% of malposition of the branch pulmonary arteries patients, which is close to the results of Babaoglu et al. ^{Reference Babaoglu and Do11} with 35% (7/20) and the difference could be attributed to the use of echocardiography in most of the cases rather than cardiac CT angiography. The prognosis and surgical outcome depend on the associated cardiac anomalies, extra-cardiac malformations, and genetic defects. ^{Reference Babaoglu and Do11} As per literature available, 53 patients (46% of the total) underwent successful surgery for their associated cardiovascular anomalies. In four patients who underwent pulmonary artery banding, all of them developed severe right pulmonary artery stenosis secondary to band impingement. ^{Reference Recto, Parness, Gelb and Lopez4,Reference Hernandez, Anderson, Hoggard, Louis and Shepard12} That is why complete early repair is preferential in this setting, rather than palliative procedures. ^{Reference Hauck, Monge, Popescu, Forbess and Backer22} Therefore, a preoperative diagnosis of crossed pulmonary arteries and recognition of this variant may influence management decisions. ^{Reference Piluiko, Poynter and Nemeh23–Reference Shunmugasundaram, Bijulal and Titus31}

There are some limitations in this study. Firstly, this is a retrospective review based on available literature. Hence, there may be certain lacunae in the information. Secondly, we believed that this condition is underdiagnosed and underreported, so describing its association precisely with various CHDs could be deceptive until further studies are performed. Thirdly, there is a variation in the methods employed for diagnosis in various studies which may give variable results.