Aortic Dissection

Definition and Pathophysiology

1. The sinotubular junction of the ascending aorta (50-65% of cases) - where hydraulic shear stress is highest
2. Just distal to the left subclavian artery (20-30%) at the junction of the arch and descending aorta

Epidemiology

• Incidence: 2.9-4.7 cases per 100,000 per year; 22% are undiagnosed prior to death
• Bimodal age distribution:
  • Younger patients with connective tissue disorders (Marfan syndrome, Ehlers-Danlos syndrome type IV)
  • Patients >50 years with chronic hypertension/atherosclerosis (the dominant group - >90% of cases)
• IRAD data: 65% are male, mean age 63; women present slightly older (mean 67)
• Hypertension coexists in 70% of patients
• Dissection is unusual in the presence of significant atherosclerosis, since medial fibrosis inhibits hematoma propagation

Risk Factors

Classification

Stanford System (most clinically used)

DeBakey System

Stanford A = DeBakey I + II. Stanford B = DeBakey III. The Stanford classification is more practical for management decisions since DeBakey I and II are treated identically.

Clinical Presentation

Symptoms

• Classic presentation: Abrupt, severe "tearing" or "ripping" chest pain, often the worst pain the patient has ever experienced, radiating to the interscapular region
• 60% have anterior chest pain (more common in Type A); abdominal/back pain more common in Type B
• ~40% of patients (particularly in Asian populations) may present without chest pain
• Syncope occurs in ~10% - more common with Type A (tamponade/severe AR)
• Neurologic symptoms (stroke, paraplegia) occur in ~20% of Type A - marks poorer prognosis

Physical Examination Findings

1. Category 1 - Predisposing condition: Marfan syndrome, family history of aortic disease, bicuspid aortic valve, recent aortic manipulation
2. Category 2 - Pain character: Abrupt onset, severe intensity, ripping/tearing quality
3. Category 3 - Exam findings: Pulse deficit, focal neuro deficit + chest/back/abdominal pain, new aortic insufficiency murmur, shock/hypotension

Acute Aortic Syndromes Spectrum

1. Aortic dissection - intimal tear with false lumen formation
2. Intramural hematoma (IMH) - rupture of vasa vasorum into the media; crescentic/circumferential aortic wall thickening on non-contrast CT; no flow in false lumen; ~10-20% of acute aortic syndromes; majority occur in descending aorta
3. Penetrating atherosclerotic ulcer (PAU) - plaque erosion through internal elastic lamina into media; mainly in mid/distal descending thoracic aorta; can progress to IMH, pseudoaneurysm, or rupture
4. Aortic rupture - direct full-thickness tear; highly fatal

Diagnosis

Imaging Modalities

Management

Immediate Medical Therapy (All Patients)

• IV beta-blocker (e.g., esmolol - 500 mcg/kg bolus, then 0.05-0.2 mcg/kg/min)
• Target HR <60 bpm
• Beta-blockers must be given BEFORE vasodilators to prevent reflex tachycardia

• Target SBP <110-120 mmHg (stricter than other hypertensive emergencies)
• Agents: sodium nitroprusside, nicardipine, or clevidipine - all rapidly titratable
• Labetalol (alpha + beta blocker): acceptable alternative, but long half-life (4-6h) limits titratability
• Diltiazem or verapamil: may be used but not ideal as monotherapy

Type A Dissection

• Emergency surgical repair is indicated in virtually all patients
• Mortality >50% within 48 hours if untreated
• Fatal complications without surgery: aortic rupture, cardiac tamponade, acute aortic regurgitation, acute MI
• In octogenarians with uncomplicated Type A: surgery associated with 63% 1-year survival vs. 45-62% hospital mortality with medical management alone
• Exception: complicated Type A (neurologic deficit, mesenteric ischemia, or CPR required) - medical management may be preferable

Type B Dissection

• Medical management preferred
• Maintain HR <60 bpm and SBP 100-120 mmHg with IV beta-blockers
• Both medical and emergency surgery have similar mortality rates in uncomplicated cases

• Surgical mortality >50%; endovascular repair (TEVAR) preferred
• IRAD registry shows increasing use of endovascular repair

1. Stent-graft placement - covers entry tear, promotes false lumen thrombosis, reduces risk of rupture (20-50% risk within 1-5 years without treatment)
2. Stent insertion - for branch vessel ischemia from true lumen collapse
3. Fenestration of the intimal flap - now less commonly used since stent grafts were introduced

• Early surgery recommended for Type B dissection in patients with Marfan syndrome

Long-Term Follow-Up

• Serial CTA or MRI to monitor false lumen for aneurysmal dilatation and distal extension
• Required in both conservatively managed patients and after endovascular repair

Prognosis

• Without treatment, Type A mortality: >1% per hour in the first 24-48 hours
• 1-, 5-, and 10-year actuarial survival for operative patients: 92%, 77%, 57%
• Cocaine-related dissection: implicated in 0.5-37% of cases; average interval from cocaine use to symptom onset is 12 hours (range 0-24 hours)
• Hypotension at presentation carries an ominous prognosis in both age groups

Key Differentiating Points from Other Causes of Chest Pain

• Aortic dissection pain is maximal at onset (unlike STEMI which builds gradually)
• Pulse deficits and BP differentials between extremities are not seen in ACS
• D-dimer is often elevated but is non-specific; a negative D-dimer with an ADD (Aortic Dissection Detection) Risk Score of 0 may help exclude dissection
• Thrombolytics are contraindicated if dissection is suspected
• CXR may show mediastinal widening (~60%) or abnormal aortic contour, but a normal CXR does not exclude dissection