Aortic disease (dissection, IMH, PAU)

Acute aortic syndrome (AAS) is a group of life-threatening aortic conditions sharing one presentation: abrupt, severe chest or back pain. It is frequently misdiagnosed — acute coronary syndrome is the most common wrong answer — and any of these lesions can progress to rupture, especially in the first week.

The entities are named separately below because you need the vocabulary. But the single most useful thing to carry to the workstation is that they are one process at different depths, not four unrelated diseases — see the continuum.

Key reference: Vilacosta I, San Román JA, di Bartolomeo R, et al. Acute Aortic Syndrome Revisited: JACC State-of-the-Art Review. J Am Coll Cardiol. 2021;78(21):2106–25. DOI: 10.1016/j.jacc.2021.09.022

1 The four constituent entities

The term was coined in 1998 and fully described in 2001. Note that it is four entities, not three — incomplete dissection is routinely left out and routinely missed.

On pathophysiology. The traditional split — CD arises from an intimal tear, IMH from vasa vasorum rupture — is now questioned. Intimal tears are found at surgery in supposed IMH often enough that a single shared mechanism is plausible; imaging simply fails to resolve them. The consensus view is that the debate should not distract from IMH’s essential feature: restricted flow within the aortic wall, which drives its morphology and its behavior over time.

Medial degeneration — elastic fiber fragmentation, thinning, mucoid extracellular matrix accumulation — is the final common pathway across causes (hypertension, Marfan, and others). CD additionally requires a trigger to create the entry site, presumed to be high blood pressure and flow producing wall shear stress.

2 One continuum, not four separate diseases

The four entities above are conventionally taught as distinct diagnoses arising from distinct processes. That framing is the main reason they are hard to tell apart. They are far better read as stages of increasing depth and severity along one process — which explains why they overlap on imaging, why one lesion converts into another, and why a scan is a snapshot of a moving target rather than a fixed label.

The atherosclerotic limb — the plaque gets deeper.

1. Irregular soft plaque / mural thrombus. By far the most common finding, and the starting point for everything downstream.
2. The plaque ulcerates → an ulcerated plaque or ulcer-like projection. A crater has formed, but it is still contained within the wall.
3. The ulceration deepens until it breaches the native wall → a penetrating aortic ulcer, which is itself a form of dissection.

How to call step 3 on CT. Ask whether the depth of the crater extends beyond where the aortic wall should sit. Wall calcification is the most useful landmark you have — intimal calcium marks the expected wall line, so if the ulcer is deeper than the calcium, it has penetrated. Contained ulceration stays shallower than the calcium.

The dissection limb — the wall splits and blood enters.

4. The wall tears, and blood collects within it as an IMH — which is exactly why it is visible on the noncontrast series.
5. From there the lesion goes one of two ways:
   • It stays stable and clots off — the hematoma remains contained, and may reabsorb.
   • It progresses — inflowing blood expands the plane, lifts a dissection flap, and a true dissection forms.
6. Once a dissection exists, the false lumen either remains patent or thromboses, decided by the differential flow and pressure between the true and false lumens.

Why this matters at the workstation. Reading for a stage rather than a label changes what you do:

• Overlapping appearances stop being a failure of the scan. A thrombosed dissection resembling an IMH is not a diagnostic contradiction — it is a lesion caught between two points on the same line.
• Depth becomes the question you actually ask — how far through the wall has this gone? — instead of trying to force the lesion into one of four boxes.
• Follow-up is expected, not incidental. These lesions move along the continuum in both directions; serial CT is how you learn which way this one is heading.

The consensus itself points the same way: the traditional split between CD and IMH is questioned precisely because a single mechanism plausibly gives rise to both, and imaging simply cannot always resolve the tear.

3 Classification

Non-A non-B is an unusual entity: the lesion is confined to the arch, or the arch is involved by retrograde propagation of a lesion originating distal to the left subclavian without reaching the ascending aorta. These patients have a complicated course. The SVS/STS more recently classifies as type B any dissection with an entry tear originating distal to the innominate ostium — a definition the consensus authors are not persuaded by, since it underweights ascending involvement.

Stanford type A predominates in CD and ID; type B predominates in IMH and PAU.

4 Epidemiology worth knowing

• Incidence of CD: 6–7.2 per 100,000 patient-years (Swedish and British population studies); 4.4 per 100,000 person-years in Olmsted County, where all acute aortic lesions together ran 7.7 per 100,000
• Sex: incidence is nearly 2-fold higher in men, and rises with age
• Age: mean age at diagnosis is lowest for CD (66–72 years) and highest for PAU; women present older than men
• Women with CD present later, in worse clinical status (coma, tamponade), and have higher mortality than men
• 30–50% of type A CD patients die at home or before reaching hospital — so hospital-based registries substantially underestimate true incidence, type A especially
• In-hospital mortality (largest US database, 2012–2016) is 26% and has not improved over time

5 Classic dissection

Blood enters the media through an intimal defect and splits the wall longitudinally, producing a dissection flap, two aortic channels, and an entry tear.

The most frequent histopathologic picture is medial layer degeneration. The dissection plane sits consistently at the level of the vasa vasorum network in the outer media — which is why the outer wall is thin and rupture-prone.

6 Intramural hematoma

Hemorrhage within the media, typically without an identifiable intimal tear or flap.

CT findings:

• Noncontrast CT: crescentic or eccentric hyperattenuation along the aortic wall — this is the diagnostic sequence, and skipping it causes false negatives
• CTA: crescentic wall thickening narrowing the lumen, with a smooth luminal interface
• Wall thickening is crescentic (noncircumferential) — the single most useful feature separating it from aortitis

Complications. IMH carries a higher rate of rupture (up to 35%) than classic dissection, because the hematoma sits closer to the adventitia — thinner outer wall, greater permeability, periaortic hemorrhage.

High-risk features predicting poor prognosis:

• Ascending aorta > 48 mm
• IMH thickness > 10 mm
• Ulcer-like projections — contrast outpouchings with a wide intimal opening (> 3 mm)

IMH can reabsorb spontaneously or convert to CD; it is a dynamic lesion requiring serial imaging.

7 Penetrating aortic ulcer

Ulceration of an atherosclerotic plaque penetrating the internal elastic lamina into the media. Calcified atherosclerosis is a prerequisite — wall calcification and inflammation are thought to hamper hemorrhage propagation, producing a characteristically focal lesion rather than a long dissection.

CT findings:

• Wide-mouth saccular contrast outpouching extending beyond the expected aortic contour
• Occurs in severe, extensive atheromatous disease, most often the descending thoracic aorta
• Typically remodels the aortic wall contour and is accompanied by localized intramedial hemorrhage

Use the calcium to judge depth. This is the practical test for separating a penetrating ulcer from a contained one. Intimal calcification marks where the wall line should be, so if the crater is deeper than the calcium, the ulcer has penetrated. A contained ulcer stays shallower. → See the continuum for where this sits in the progression.

Course. Most PAU patients are asymptomatic; a minority present with AAS. PAU may break through the adventitia to cause rupture, or form a pseudoaneurysm producing an aortic wall bulge. PAU only rarely serves as the entrance tear for a focal dissection.

8 Incomplete dissection — the commonly missed one

Also called acute limited intimal tear, subtle or discrete dissection, or limited dissection, and described as early as 1973. It is intimal and subjacent medial laceration without significant intramedial hematoma — Svensson type III.

• ~5% of all AAS patients, mostly ascending aorta
• The base of the lacerated area usually contains some medial tissue and adventitia (partial-thickness tear)
• Produces a localized, eccentric aortic wall bulge with subadventitial hematoma — and no false lumen
• A normal TTE does not rule it out. The flap can be subtle enough that CT is equivocal; TEE may be required

9 The 3-step diagnostic algorithm

The consensus proposes a deliberately simple pathway, because the practical problems are low prevalence, nonspecific presentation, and no specific biomarker.

Step 1 — Clinical suspicion. Calculate pretest probability from the three categories of the Aortic Dissection Detection Risk Score: predisposing conditions, pain characteristics, physical examination findings. One point per category with ≥ 1 risk marker. Score 0 = low risk; 1 = intermediate; ≥ 2 = high. This assessment is a Class I recommendation.

The score has real limits: a score of 0 does not exclude AAS with certainty, and specificity is poor at ≥ 1, producing many false positives and radiologic overtesting. Note also that hypertension — present in 86% of people who later develop CD — is not a high-risk predisposing condition in the score.

Step 2 — ECG, chest X-ray, troponin, D-dimer. This is the key stage.

The alerting triad: in a patient with aortic pain, normal ECG + normal troponin + elevated D-dimer is a clear warning pattern for AAS and makes acute coronary syndrome unlikely.

• D-dimer has high sensitivity, correlates with the extent of the aortic lesion, and is higher in CD than IMH. The higher the value, the greater the likelihood — particularly > 1,600 ng/mL (normal ≤ 500). Its negative predictive value is what makes it useful: combined with a low risk score, it helps rule out
• Raised D-dimer cannot distinguish AAS from pulmonary embolism — but should prompt urgent thoracic CT, which excludes both
• Troponin positivity does not rule out AAS. It may reflect coronary involvement by the flap, or ischemia from acute AR or hypotension. Elevated troponin does not remove the need for definitive imaging when suspicion is high
• A normal chest radiograph does not exclude AAS — it is quite frequent in these patients

Step 3 — CT of the entire aorta + focused TTE. CT confirms or excludes the diagnosis. Perform it in all patients with a risk score > 1 and raised D-dimers, particularly with normal troponin and no ECG changes.

10 CT technique

CT is the modality of choice in the emergency setting — wide availability, rapid acquisition.

TTE in the emergency room can help reach the diagnosis and assesses pericardial effusion, aortic regurgitation, and ventricular function — but its accuracy for AAS is low and a normal TTE does not rule out AAS. TEE has excellent diagnostic accuracy and is the alternative when CT is nondiagnostic or the patient is unstable. MR has no practical role as an initial test in AAS.

11 Avoiding misinterpretation — the mimics

Accurate reading depends on recognizing acutely thrombosed CD, mural thrombus, aortitis, ulcer-like projections, and intramural blood pools. The position of calcium is the most useful single discriminator.

Acutely thrombosed CD vs IMH. Both are noncommunicating — no contrast enters the wall. When the entry tear is small and the exit tear absent or sealed, false lumen thrombosis is rapid and the CT resembles IMH. Combining axial and sagittal planes to find a focal intimal contour alteration (the dissection tear) is what separates them. IMH has no apparent entrance tear, or a microscopic one. CT cannot always distinguish them.

Aortitis vs IMH. IMH is crescentic with a smooth luminal surface and a hyperattenuated wall contour on noncontrast. Aortitis is circumferential, with homogeneous wall enhancement on contrast-enhanced CT — though not invariably. PET/CT depicts the inflammatory process and settles ambiguous cases.

Mural thrombus vs IMH. Mural thrombus in a dilated aorta can look crescentic, but:

• Its luminal surface is irregular, not smooth
• It does not show high attenuation on noncontrast CT
• Calcification sits along the outer aortic border or within the thrombus — whereas IMH displaces intimal calcification inward

PAU vs ULP vs IBP. These three are not equivalent terms and are commonly conflated.

• ULPs appear as small saccular areas of enhancement with wide mouths protruding from the lumen into the wall. They arise in any segment and carry higher adverse aortic event rates — aneurysm formation, focal dissection, rupture. The consensus argues the term causes confusion and should be replaced by subtle intimal tear or focal intimal disruption
• IBPs communicate with the true lumen through the ostia of intercostal and lumbar arteries severed by the dissecting hematoma — which is why they are exclusively descending. They appear as rounded areas of enhancement, or focal color Doppler puddles on TEE. Finding an IBP within an IMH is not an ominous sign, but warrants close CT monitoring

In summary: shape, interface with the lumen, hyperattenuation on noncontrast CT, coexisting atherosclerosis, location of calcium, and the relationship of enhancing blood to the lumen are what separate these entities. Some cases remain genuinely difficult.

12 What to report

Findings that change clinical decisions:

• Involvement of the ascending aorta
• Site of the entrance tear
• Severe pericardial effusion / cardiac tamponade
• Significant aortic regurgitation, and its mechanism
• Signs of aortic rupture
• Signs of end-organ ischemia or malperfusion

13 Management essentials

Hyperacute phase (first 24 hours) — cardiovascular care unit, with three targets:

IV beta-blockers are used simultaneously with combined antihypertensive therapy.

Type A CD — emergency open surgical repair as a rule. Ascending aorta and hemiarch replacement in most cases; composite valve graft (Bentall-de Bono) or valve-sparing root operations when the root is involved or in connective tissue disease.

Type B CD — best medical therapy for uncomplicated disease. TEVAR is the established treatment of choice for complicated type B (ESC/ESVS). High-risk features predicting early adverse events:

• Total aortic diameter > 44 mm
• False lumen diameter > 22 mm
• Large proximal entrance tears
• Refractory pain, refractory hypertension

Centralization matters. Outcomes track surgeon and center volume, which is the argument for aorta centers, an “aorta code” (a streamlined emergent pathway activatable from any emergency room at all hours), and a multidisciplinary aorta team. Treating AAS at a high-volume center with low-volume surgeons is not optimal.