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The successful clinical introduction of various non-invasive imaging modalities has contributed to the establishment of ‘acute aortic syndrome’, a relatively new clinical syndrome incorporating several disease entities with similar clinical features (at the time of clinical presentation). Aortic dissection (AD) with intimal flap and two aortic channels (true and false lumens) is the most important disease entity in acute aortic syndrome. Clinical characteristics of AD have been established, with standardized treatment strategies depending on the affected site of the aorta. For the past several decades, variant forms of classic AD, including aortic intramural hematoma (IMH) and incomplete dissection, have been increasingly diagnosed in routine clinical practice worldwide, and imaging findings characteristic of these variant forms have been established for their differential diagnosis. As imaging findings, rather than clinical features, are critical for the differential diagnosis of acute aortic syndrome, careful interpretation of imaging results is necessary for accurate diagnosis and better clinical decision-making, thus improving patient outcomes. IMH is probably the most important variant form of classic AD and has been the source of intense controversy regarding its pathogenesis and optimal treatment options. Clinical reports regarding outcomes of patients with IMH from different centers have shown conflicting results and risk stratification based on imaging findings remains a challenging clinical issue.

Intimal tears can have a very wide spectrum. For example, a stellate or linear intimal tear can involve the exposure of the underlying aortic media or adventitial layers but without the progression and separation of the medial layers, resulting in extensive undermining of the intimal layers. This incomplete dissection, characterized by an intimal tear without an intimal flap or hematoma, was reported to be present in 9 of 181 (5%) consecutive patients with ascending or aortic arch repairs. Currently available CT or TEE cannot identify this type of localized intimal tear, with the only imaging characteristic reported to be a localized eccentric bulging or aneurysm. This feature is too subtle to diagnose accurately using conventional imaging modalities, suggesting that surgical exploration be performed in patients with aortic pain and aortic root dilatation who show unexplained aortic regurgitation or pericardial effusion. Further investigations are necessary to evaluate the clinical significance and outcomes of patients with incomplete dissection.

Non-invasive imaging modalities such as CT and TEE have contributed to a better understanding of acute aortic syndrome, resulting in inclusion of IMH and atypical dissection as new disease entities. Differences in imaging findings may be due to differences in the hemodynamic status of the false lumen, resulting from the wide spectrum of intimal tears, from a localized or ‘micro-intimal tear’ to a large tear with flow communication resulting in a ‘double channel aorta’. Further investigation is required to determine whether the application of different treatment options following risk stratification based on imaging results is rational and can improve the clinical outcomes of patients with this syndrome.

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