Aortic intramural hematoma

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 (PAU) and classical aortic dissection.

Epidemiology

Typically, aortic IMHsintramural 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 PAUpenetrating atherosclerotic ulcer ^1,2. 

Clinical presentation

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

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 PAUpenetrating atherosclerotic ulcer and traumatic medial injury ¹³.

The haematoma propagates along the medial layer of the aorta. Consequently, intramural hematomahaematoma 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 IMHintramural haematoma may coexist with other forms of AASacute aortic syndrome.

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

Location

There is a greater predilection to involve the descending aorta ¹³.

Classification

Similar to aortic dissections, aortic IMHsintramural 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). 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 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 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 hematomahaematoma. It is important to distinguish between two entities that have different prognostic significance ¹¹. 

1. intramural blood pool: small region of contrast accumulation within the hematomahaematoma 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. 
2. 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 IMHintramural haematoma from mural thrombus:

• calcification displaced towards the aortic lumen in IMHsintramural haematomas
• calcification displaced away from the aortic lumen in mural thrombus

A CTA radiology report of IMHsintramural haematoma should include ¹³:

• proximal and distal extent of the haematomashaematoma
• thickness of the haematoma
• maximal diameter of the affected aortic lumen
• presence and thickness of associated PAUpenetrating atherosclerotic ulcer

Echocardiography

An intramural hematoma (IMH)haematoma 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 IMHintramural haematoma
  • wall demonstrates mixed echogenicity
    • predominantly echodense with scattered internal echolucencies
• no internal flow detectable
  • colorcolour flow Doppler interrogation important to differentiate from aortic dissection
    • the true lumen of a 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 IMHintramural 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. Medical management of type A IMHsintramural haematomas 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 IMHintramural haematoma and worse prognosis include ¹³:

• presence of ulcer-like projections (ULPs)
• IMH thickness greater than 10 mm
• associated aortic aneurysm
• increase thickness of the IMHintramural haematoma at follow up CTA

Untreated, an IMHintramural 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