The ASP score was established according to IA size, location and shape and was developed to identify RIAs in SAH patients harboring MIAs [6]. In this study, we applied the ASP score in 134 SAH patients with MIAs and found that the sensitivity, specificity, false omission rate, diagnostic error rate, and diagnostic accuracy were 87.3%, 89.1%, 12.7%, 10.9%, and 88.3%, respectively.
Traditionally, size has been considered an important factor in IA rupture, and a large IA is considered more prone to rupture than a small IA. Some studies have reported that size is a significant predictive factor for IA rupture [12,13]. Although Björkman et al. [14] indicated that IA size was associated with IA rupture, the RIA was not of the largest size in 13% of their study cohort, and they found that irregular shape may identify the RIA better than size in patients presenting with SAH and MIAs. In addition, Backes et al. [2] reported that RIA was not the largest IA in 29% of patients with MIAs. In this study, 18.7% (25/134) of the patients had an unruptured IA with the largest diameter, and 15 of them did not have the largest ASP score.
Irregular shape was thought to be associated with IA rupture [12,13], possibly because the irregular shape increases the local hemodynamic stress [15]. Backes et al. [2] reported that irregular shape is associated with IA rupture independent of IA size and location and independent of patient characteristics. Björkman et al. [14] showed that shape and size had the best diagnostic value for identifying RIAs in patients presenting with SAH and MIAs, but shape may be better than size. However, Orning et al. [4] reported that it is unreliable to use morphological features of IA in determining rupture sites in nondefinitive ASH patterns. Another study also showed that morphological and hemodynamic parameters seem to have no or only low effect on the prediction of RIA in patients with MIAs [16]. The present results showed that 39 (29.1%) RIAs had regular shapes, and 36 (23.1%) unruptured IAs had irregular shapes.
IAs located in the AcoA, PcomA, or PC are considered to have a high risk of rupture [17-19]. However, we did not find that location significantly increased the rupture risk in our previous studies [20-22]. Similar to our study, 2 previous studies also confirmed that location was not related to IA rupture [2,14]. In this study, although IAs located in the PcomA ruptured more often than IAs in other locations, the diagnostic accuracy was only 62.4%.
The aim in developing the ASP scoring system was to identify RIAs in SAH patients with MIAs, and the prediction score had high accuracy in a small prospective sample [6]. In this study, the ASP score had high sensitivity and specificity, but 17 UIAs were misdiagnosed as RIAs. On the other hand, the area under the curve of the ASP score was lower than that of maximum diameter, indicating that the performance of the ASP score was not satisfactory. One of the reasons is that IA size and shape may change after rupture. Another reason is the inherent flaws of the ASP scoring system: sometimes the location and shape of IAs may lead to a decrease in the ASP score. The coefficients need to be optimized to further improve the rate of recognition of RIAs. In addition, morphological characteristics such as location of bifurcation, small-diameter of the parent artery, and location of the AcomA with A1 dominance are risk factors for IA rupture [22,23]. Some studies reported that an aspect ratio ≥1.3 or the size ratio were the best factor for identifying RIAs [2,24].
Limitations
The present study had a limitations. First, the shape or size of the RIAs might have changed due to the rupture, and the results may be biased. Second, this study considered only MIAs with SAH, and the results may not be applicable to patients with a single IA or unruptured MIAs. Third, as we used CTA data in this study, conus arteriosus could have been misdiagnosed as an IA, causing a patient with a single real IA to be identified as one with “MIAs”, although this situation is unlikely. Last, this study only validated the accuracy of the ASP scoring system and did not compare it with other scoring systems. A multicenter study with a large sample size is needed in the future.