Aortic intramural hematoma (IMH) is an atypical form of aortic dissection due to a contained hemorrhage into the aortic wall usually from the vasa vasorum without an intimal tear. It forms part of the acute aortic syndrome spectrum along with penetrating atherosclerotic ulcer (PAU) and classical aortic dissection.
Typically aortic IMHs are seen in elderly hypertensive patients. The same condition may also develop as a result of blunt chest trauma with aortic wall injury or a PAU 1,2.
The clinical features of IMH are those of the acute aortic syndromes, namely chest pain radiating to the back and hypertension.
This condition is thought to begin with spontaneous rupture of the vasa vasorum, the blood vessels that penetrate the outer half of the aortic media from the adventitia and arborize within the media to supply the aortic wall 2. Other theories describing the pathogenesis include thrombosis of a dissection lumen, microscopic intimal tears, progression from a PAU and traumatic medial injury 13.
The hematoma propagates along the medial layer of the aorta. Consequently, intramural hematoma weakens the aorta and may progress either to outward rupture of the aortic wall or to inward disruption of the intima, the latter leading to a communicating aortic dissection 2.
Acute IMH may coexist with other forms of AAS.
Patients with aortic IMH and PAUs have an increased risk of concomitant abdominal aortic aneurysm 13.
There is a greater predilection to involve the descending aorta 13.
Similar to aortic dissections, aortic IMHs are classified according to the Stanford classification 4:
- type A: involves the ascending aorta, with or without descending aortic involvement
- type B: confined to the descending aorta, distal to the origin of the left subclavian artery
The DeBakey classification can also be used 5.
Acute intramural hematomas appear as focal, crescentic, high-attenuating (60-70 HU) regions of eccentrically thickened aortic wall on non-contrast CT (high-attenuation crescent sign). Narrow window width is essential for identifying subtle lesions 6. Intimal calcification may be displaced inwards, best appreciated on the non-contrast phase. Thickening is greater that the normal aortic wall thickness.
The lesions exhibit low attenuation in relation to the aortic lumen on post-contrast CT and can be far more subtle, hence a non-contrast phase before CTA is often done in an acute aortic syndrome protocol. Unlike aortic dissection, no intimal flap is present on the CTA.
On follow-up imaging, contrast can occasionally be seen within the intramural hematoma. It is important to distinguish between two entities that have different prognostic significance 11.
- intramural blood pool: small region of contrast accumulation within the hematoma with invisible or small (< 2 mm) communication with true lumen, often with a peripheral connection with an intercostal or lumbar artery 11. Often regress with no adverse prognostic implications 11.
- ulcer-like projection: new intimal disruption (not present on the original study) with a wide neck. These usually progressively enlarge and herald a poor prognosis 11.
The direction of atheromatous calcification displacement can help differentiate IMH from mural thrombus:
- calcification displaced towards the aortic lumen in IMHs
- calcification displaced away from the aortic lumen in mural thrombus
A CTA radiology report of IMHs should include 13:
- proximal and distal extent of the hematomas
- thickness of the hematoma
- maximal diameter of the affected aortic lumen
- presence and thickness of associated PAU
An intramural hematoma (IMH) may be readily visualized with transesophageal echocardiography, which offers superior visualization of the aorta than is usually available via transthoracic examinations. Defining features include 10:
- crescentic thickening of the aortic wall
- normal aortic wall thickness < 3 mm
- wall thickness must exceed 7 mm to diagnose IMH
- wall demonstrates mixed echogenicity
- predominantly echodense with scattered internal echolucencies
- no internal flow detectable
- lack of an intimal (dissection) flap
- the luminal surface in IMH tends to be smooth and continuous
MRI may also detect the abnormality but conventional angiography will not.
Treatment and prognosis
If an intramural hematoma involves the ascending aorta (Stanford A), surgical treatment is offered to prevent rupture and progression to classic aortic dissection. Medical management of type A IMHs leads to mortality of 40% 13.
Conservative management is indicated for an intramural hematoma of the descending aorta (Stanford B).
- 77% of intramural hematomas regress at 3 years
- survival of >90% at 5 years 7
RIsk factors for progression of IMH and worse prognosis include 13:
- presence of ulcer-like projections (ULPs)
- IMH thickness greater than 10 mm
- associated aortic aneurysm
- increase thickness of the IMH at follow up CTA
Untreated, an IMH can be life-threatening as it can lead to:
The main differential diagnoses are:
- thrombosed false lumen in classic aortic dissection: typically spirals longitudinally around the aorta whereas an intramural hematoma usually maintains a constant circumferential relationship with the aortic wall
- aortitis: typically shows concentric uniform thickening of the aortic wall with or without peri-aortic inflammatory stranding, whereas an intramural hematoma is often eccentric in configuration
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