BTAI is more common than BAAI [5, 6] and had thus been more widely studied [9,10,11,12,13]. BTAI has even been considered equivalent to BAI in a broad sense [9]. In 2009, Demetriades et al. [12] reported that the overall mortality of BTAI was only 12.4%, while Fox et al. [9] reported that the mortality in BTAI patients undergoing thoracic endovascular aortic repair was as low as 9% in a large sample meta-analysis in 2015. The reason for the relatively low mortality may be due to the accumulation of substantial clinical experience. In contrast, BAAI has a significantly higher mortality. In the present study, we found that the reported lower hospital mortality still reached 28.0% [30]. Because the mortality is significantly higher and the anatomical features and treatment modalities are quite different, BAAI is considered a completely different disease from BTAI. In contrast to BTAI, there are only a few studies on BAAI, and most of them are case reports. Because there were no previous meta-analyses focusing on BAAI alone, the present study was performed to evaluate the prognosis of BAAI patients more accurately, thus enabling BAAI patients to receive more timely and accurate treatments.

In addition to the five included studies, four studies containing potentially adequate patients attracted our attention. Of them, two studies [31, 32] did not report the number of BAAI patients, one study [33] included 20 patients without reported mortality, and one study [15] reported a 92% mortality, including prehospital death. Unfortunately, we have not been able to obtain the relevant data of these studies.

Five studies [1, 5, 34,35,36] were excluded due to complete duplication of data. However, three [4, 28, 30] of the included studies may have some degree of duplication because the centres may have been duplicated across these multicentre studies. Considering that the data duplication was not serious according to the comparisons of their inclusion criteria (year, age, etc.), these three studies were included in the present study, which may have increased the error of the present study to some extent.

In addition, the five included studies were all from the USA. Except for the excluded article that exclusively included juveniles and had a sample size of only 16 [27], 2 of the remaining 4 exclusively included adults [4, 30]. These factors may reduce the fitting effect of the results of the present study for populations overall.

In the process of extracting data, we determined all the variables with data that could be displayed or analysed. In addition to the primary outcome measure (i.e. OHM), the HMO and HMNO of BAAI in the studies were also calculated [4] or directly extracted, and they were considered the secondary outcome measures. Due to the lack of other available data on BAAI patients in these articles, however, it was difficult to perform other quantitative analyses, such as comparison of mortalities from various treatment modalities and screening of predictors for death.

Because this study was a single-arm meta-analysis involving no control groups, we considered all included studies for quality assessment as simple observational studies without intervention. The result that no low-quality studies were found was satisfactory.

Due to a higher statistical performance and applicability to samples with a very low value [37], we performed the F-T double arcsine transformation of data before the meta-analysis, which reduced the risk of blind exclusion of the above described studies. Unfortunately, the study reporting an OHM of 0% was still excluded due to high heterogeneity (I2 = 78.8%, P = 0.001 for Q test). Of note, this was a single-centre study with a small sample size that only enrolled paediatric individuals < 18 years of age.

We were unable to determine the impact of the excluded study on each of the three variables, namely year of cases, gender, and injury severity score [38]. To explore the reasons for the extremely low mortality in this study, we investigated the factors or variables that may affect the risk of death in BAAI patients. Unfortunately, after an extensive search, it appears that only one study [30] has performed this work, suggesting that in adult BAAI patients, increased age is a risk factor for death. However, it is not known whether this conclusion applies to paediatric patients. Due to the limited data, we were unable to evaluate the differences in other factors affecting mortality between the excluded study and the other four studies [30].

It is worth noting that the study [30] that explored predictors of death in BAAI did not include the location and grade of aortic lesions, which are two important variables. Shalhub et al. [1] reported that the AA is divided into three zones [zone I, above the superior mesenteric artery (SMA); zone II, from the SMA to renal artery; and zone III, infrarenal] to guide the operation for BAAI, which is a widely recognized method [27,28,29, 39, 40]. Although this study was excluded from the present meta-analysis of OHM because its data were completely duplicated with one [28] of the included studies, it reported mortality rates of BAAI by aortic lesion location, i.e. 60% in zone I, 100% in zone II, and 15% in zone III. In combination with the opinions of Shalhub et al. [1], we consider that the reason for the significantly lower mortality in zone III is that compared to those in the other two zones, the aortic injuries in this zone are less complicated with the injuries of other organs or aortic branches, and they are easier to expose by open surgery or to repair endovascularly. Of note, all the locations of aortic lesions in patients in the excluded study [27] were zone III, whereas the other two available studies reported the proportion of this lesion location to be 66.4% [28] and 68.8% [29], respectively. Although the pooled analysis was not completed due to high heterogeneity to obtain a specific value, we found that zone III injuries accounted for the majority, providing some confidence to clinicians and patients. Moreover, the grade of BAAI lesion severity should be defined. At present, there is no consensus in this regard specifically for BAAI. The assessment methods of all previous studies were the same as those of BTAI, and it remains unknown whether they apply to BAAI. Azizzadeh et al. [41] classified the aortic lesions of BTAI patients into four grades as follows: internal tear, intramural haematoma, pseudoaneurysm, and rupture. Rabin et al. [42] utilized a different classification standard as follows: internal tear or intramural haematoma, small pseudoaneurysm (< 50% of the aortic circulation), large pseudoaneurysm (> 50% of the aortic circulation), and rupture or transection. The pooled analysis yielded an aortic rupture rate of 12.2%, thus indicating the mortality risk of BAAI. Due to the paucity of studies and data available, we were unable to determine the relationships between the location and severity of aortic lesions and the risk of mortality in BAAI patients by quantitative calculations. However, we believe that patients who have aortic lesions that are more easily repaired or of lesser severity will have a lower risk of mortality.

In the excluded study [27], the mechanisms of injury for all patients were all seat belt-related motor vehicle accidents. Although the rate of seat belt-related mechanisms could not be extracted directly from the three included studies [4, 28, 30] that reported relevant data due to nonuniformity in classification standards and missing data, the rate is expected to greatly differ from 100%. It is unknown whether this difference is one of the reasons for the large difference in OHM. The ambiguity in the classification standard for the mechanisms of injury in BAAI represents insufficient knowledge among investigators in this regard. In 1962, BAAI and simultaneous lumbar spine fracture were initially described as “seat belt syndrome” [1]. BAAIs with seat belt injury mechanisms are not uncommon in case reports [43,44,45,46]. Additional studies are required to understand whether there are essential differences among the injury mechanisms of seat belts, those of motor vehicle accidents without seat belts, and even those of nonmotor vehicle-induced trauma to cause different death risks from BAAI.

In summary, a lower mean age, high proportions of seat belt-related injury mechanisms and injuries in zone III may contribute to the unusually low mortality (as low as zero) in the excluded study [27]. However, if the sample size increases, the OHM may be valuable.

Of the four included studies, two studies included only adults (≥ 16 years old [4] and ≥ 18 years old [30]) without explanations. In addition, one study [29] had a sample size of only 16 cases, which was small compared to the other studies, and the differences between the data available for extraction were not significant. Therefore, the differences in age and the small sample size may explain the heterogeneity among the studies.

After the heterogeneity among the included studies was demonstrated to be low (I2 = 47.6%, P = 0.126 for Q test), we adopted the fixed effects model to determine a true pooled ES [47]. The OHM of BAAI was found to be 28.8% (95% CI 26.5–31.1%). This rate of nearly 30% of BAAI varied from the 10% rate of BTAI, confirming our hypothesis that they are two different diseases.

This result was consistent with that obtained by the random effects model (30.1%, 95% CI 26.3–33.9%), and no ES was found to be outside the previous 95% CI after omitting studies one by one, which demonstrated that the final model had good stability. The P value obtained by Egger’s test was 0.339, which was far higher than 0.05, suggesting that the model had a low level of publication bias.

In addition, through statistical analyses by the same method, we also obtained an HMO of 13.5% and an HMNO of 28.4%. Although the included studies were slightly different, the significantly lower hospital mortality (13.5%) suggests that the operation has great benefits for BAAI patients compared to simple observation. However, it is necessary to hierarchically consider which treatment modality is more beneficial for each BAAI patient. Because BAAI is a fatal disease and that operation is a good treatment choice, it is important to define which patients are at higher risk of death and need prompt operation. Thus, the predictors of hospital death in BAAI patients need to be identified, which will allow more rational treatment.

To the best of our knowledge, the present study is the first meta-analysis on the mortality of BAAI. This study provided insight into the mortality of BAAI, a rare but fatal disease, through an extensive search and scientific analysis. However, this study had several limitations. First, there may be a small degree of duplication in the samples of the included multicentre studies, which may increase the statistical error. Second, the number of studies included in the analysis was small, which may reduce the statistical power.