What is Type B Aortic Aneurysm (AAA)?

Type B Aortic Aneurysm (AAA)

Type B aortic dissection is defined as a dissection that does not involve the ascending aorta, according to the Stanford classification system. This classification is clinically practical as it guides management decisions between surgical and medical approaches 1.

Classification Systems

• Stanford Type B dissection involves the descending aorta but does not involve the ascending aorta, and is typically managed medically unless complications occur 1, 2
• In the DeBakey classification, Type B dissection corresponds to DeBakey Type III, which originates in the descending aorta and propagates distally 1, 2
• The Stanford classification is more clinically relevant as it directly impacts treatment decisions, while the DeBakey classification provides more detailed anatomical information 1

Pathophysiology

• Type B dissection involves disruption of the media layer of the aorta with bleeding within and along the wall of the aorta, creating a false lumen separated from the true lumen by an intimal flap 2
• The dissection process can lead to malperfusion of organs, rupture, or progressive aneurysmal dilatation of the affected segment 2
• Risk factors include hypertension, smoking, atherosclerosis, and connective tissue disorders such as Marfan syndrome 2

Clinical Presentation

• Many patients present with sudden, severe, tearing back or abdominal pain 2
• Some patients may be asymptomatic, with the dissection discovered incidentally on imaging 3
• Complications can include malperfusion syndromes affecting the kidneys, intestines, or lower extremities 2, 4
• Neurological manifestations occur in 22-30% of cases, with spinal ischemia and ischemic neuropathy being more common in Type B dissections 4

Diagnosis

• CT angiography is the gold standard for diagnosis, showing an intimal flap separating true and false lumens 2
• MRI is an excellent alternative, particularly for patients with contraindications to CT contrast 2
• Transesophageal echocardiography may be used in unstable patients or when CT/MRI is not available 2

Management

Uncomplicated Type B Dissection

• Medical management is the first-line treatment for uncomplicated Type B dissections 2
• Aggressive blood pressure control is essential, with target systolic blood pressure typically <120 mmHg 2
• Beta-blockers are the preferred antihypertensive agents 2
• Close surveillance with imaging is required to monitor for progression or complications 2

Complicated Type B Dissection

• Thoracic endovascular aortic repair (TEVAR) is the treatment of choice for complicated Type B dissections 2
• Complications warranting intervention include:
  • Persistent or recurrent pain 2
  • Uncontrolled hypertension despite full medication 2
  • Rapid aortic expansion 2
  • Malperfusion syndromes 2
  • Signs of rupture (hemothorax, increasing periaortic hematoma) 2
• Open surgical repair is considered when TEVAR is not feasible due to anatomical constraints 2

Long-term Follow-up

• Regular imaging surveillance is essential, typically with CT or MRI every 6-12 months initially, then annually if stable 2
• Lifelong blood pressure control is crucial to prevent progression 2
• Patients remain at increased risk for cardiovascular events and require comprehensive cardiovascular risk reduction 2

Special Considerations

• In patients with Marfan syndrome, Type B dissection occurs in about 10% of cases, sometimes without significant prior dilation of the descending aorta 2
• Type B dissection may occur after previous elective aortic root replacement, highlighting the importance of whole-aorta surveillance 2
• The mortality rate for Type B dissection is lower than Type A but remains significant, particularly when complications develop 4

Common Pitfalls

• Misdiagnosing the pain as musculoskeletal or gastrointestinal in origin, leading to delayed diagnosis 2
• Failing to recognize malperfusion syndromes, which may present subtly 2
• Inadequate blood pressure control leading to progression of dissection 2
• Overlooking the need for comprehensive aortic imaging, as patients with Type B dissection may develop aneurysms in other aortic segments 2