Aortic intramural haematoma

Aortic intramural haematoma (IMH) is an atypical form of aortic dissection due to a contained haemorrhage 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 and classical aortic dissection.

Epidemiology

Typically, aortic intramural haematomas 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 penetrating atherosclerotic ulcer ^1,2. 

Associations

Acute intramural haematoma may coexist with other forms of acute aortic syndrome (e.g. aortic dissection) and/or haemopericardium ¹⁴.

Patients with aortic intramural haematoma and penetrating atherosclerotic ulcers have an increased risk of concomitant abdominal aortic aneurysm ¹³.

Clinical presentation

The clinical features of intramural haematoma are those of the acute aortic syndromes, namely chest pain radiating to the back and hypertension ¹⁴.

Pathology

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 penetrating atherosclerotic ulcer and traumatic medial injury ¹³.

The haematoma propagates along the medial layer of the aorta. Consequently, intramural haematoma 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.

Location

Aortic intramural haematoma has a greater predilection to involve the descending aorta ¹³.

Classification

Similar to aortic dissections, aortic intramural haematomas 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. 

Radiographic features

CT

Acute intramural haematomas appear as focal, crescentic, high-attenuating (60-70 HU) regions of eccentrically thickened aortic wall on non-contrast CT (high-attenuation crescent sign). A narrow window width is essential for identifying subtle lesions ⁶. Intimal calcification may be displaced inwards, best appreciated in the non-contrast phase. Thickening is greater than 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 the first series performed 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 haematoma. It is important to distinguish between aortic dissection and acute intramural haematoma as they have different prognostic significance ¹¹. 

• intramural blood pool

  • ​small region of contrast accumulation within the haematoma with invisible or small (<2 mm) communication with the 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 in 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 intramural haematoma from mural thrombus:

• calcification displaced towards the aortic lumen in intramural haematomas

• calcification displaced away from the aortic lumen in mural thrombus

A CTA radiology report of an intramural haematoma should include ¹³:

• proximal and distal extent of the haematoma

• the thickness of the haematoma

• maximal diameter of the affected aortic lumen

• presence and thickness of associated penetrating atherosclerotic ulcer

Echocardiography

An intramural haematoma may be readily visualised with transoesophageal echocardiography, which offers superior visualisation 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 intramural haematoma

  • wall demonstrates mixed echogenicity

    • predominantly echodense with scattered internal echolucencies

• no internal flow detectable

  • colour flow Doppler interrogation is important to differentiate from aortic dissection

    • the true lumen of dissection will demonstrate systolic flow

    • variable flow patterns may be present in a false lumen, which tends to expand in size during diastole

• lack of an intimal (dissection) flap

  • the luminal surface in intramural haematoma tends to be smooth and continuous

Other modalities

MRI may also detect the abnormality but conventional angiography will not.

Treatment and prognosis

If an intramural haematoma involves the ascending aorta (Stanford A), surgical treatment is offered to prevent rupture and progression to classic aortic dissection, which occurs in ~30% (range 20-45%) of patients ¹⁴. Medical management of type A intramural haematoma leads to mortality of 40% ¹³.

Conservative management is indicated for an intramural haematoma of the descending aorta (Stanford B).

• 77% of intramural haematomas regress at 3 years

• survival of >90% at 5 years ⁷

Risk factors for progression of intramural haematoma and worse prognosis include ¹³:

• presence of ulcer-like projections (ULPs) ¹⁵

• IMH thickness greater than 10 mm

• associated aortic aneurysm

• increase in the thickness of the intramural haematoma at follow-up CTA

Untreated, an intramural haematoma can be life-threatening as it can lead to:

• aortic rupture

• aortic dissection

• aortic aneurysm

Differential diagnosis

The main differential diagnoses are:

• thrombosed false lumen in classic aortic dissection: typically spirals longitudinally around the aorta whereas an intramural haematoma 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 haematoma is often eccentric in configuration