Aortic Dissection

Aortic dissection is an acute, life-threatening emergency characterized by separation of the medial layer of the aortic wall, creating a false lumen that courses between the intima and adventitia. This catastrophic vascular event represents one of the most lethal acute cardiovascular emergencies, with mortality exceeding 1% per hour in the first 48 hours if untreated. The incidence is approximately 3–4 per 100,000 person-years, with peak occurrence in the sixth to eighth decades of life; men are affected 2–3 times more frequently than women. Hypertension is present in 60–90% of cases, making it the dominant modifiable risk factor. For USMLE Step 2 CK, aortic dissection represents a critical "don't miss" diagnosis requiring rapid recognition, appropriate imaging, and urgent medical or surgical intervention based on anatomical classification.

The pathophysiology of aortic dissection begins with disruption of the medial layer (tunica media), which bears the mechanical stress of systolic blood pressure oscillations. The media comprises concentric layers of smooth muscle cells and elastic fibers held together by elastic laminae. A tear in the intima permits blood under systemic pressure to penetrate into the media, separating it from the outer adventitia. The initial site of intimal rupture is typically in the ascending aorta (just distal to the aortic valve) for Type A dissections, or in the descending thoracic aorta (distal to the left subclavian artery) for Type B dissections. This location predilection reflects zones of highest shear stress (dP/dt), where the rate of pressure change is greatest.

Uncontrolled hypertension is the fundamental driving force in the majority of dissections. Systemic hypertension increases both the absolute transmural pressure gradient across the aortic wall and—critically—the shear stress (dP/dt, the rate of change of pressure). Shear stress creates lateral forces perpendicular to blood flow that mechanically stress the intimal-medial interface. In hypertensive patients, chronic exposure to elevated shear stress causes medial smooth muscle cell apoptosis, deposition of proteoglycans, and progressive degradation of elastic laminae architecture. When acute hypertensive surges occur (particularly during emotional stress or exertion), the accelerated dP/dt overwhelms the structural integrity of a weakened medial layer, precipitating intimal rupture. Mathematical modeling demonstrates that dP/dt is a more predictive variable than absolute blood pressure (BP) alone in determining dissection risk.

Histopathologically, aortic dissections are characterized by cystic medial necrosis (also called medial cystic degeneration)—a pathological finding describing areas of smooth muscle cell loss, elastic fiber fragmentation, and accumulation of mucopolysaccharide material. This process occurs both as a consequence of chronic hypertension and as a primary structural abnormality in genetic aortopathies (Marfan syndrome, Ehlers-Danlos syndrome). In Marfan syndrome, mutations in the fibrillin-1 gene (FBN1) result in defective fibrillin microfibrils, which are crucial for both elastic fiber assembly and transforming growth factor-beta (TGF-β) sequestration. Reduced fibrillin leads to excessive TGF-β signaling, which paradoxically promotes smooth muscle apoptosis and collagen deposition while simultaneously degrading elastic fibers. The result is a medial layer that is structurally weak and prone to dissection at lower pressures than in the general population. Histological studies show that the medial layer in dissected aortas contains approximately 30–50% fewer elastic laminae than normal aorta.

Once the intimal tear is established, blood flow from the true lumen enters the false lumen, which then propagates both proximally and distally. The extent of dissection depends on the balance between intimal tear size, blood pressure, aortic compliance, and the geometry of the dissection pathway. Dynamic obstruction occurs when the dissection flap—the strip of intima and inner media separating true and false lumens—buckles into the true lumen during systole, narrowing or obliterating true lumen perfusion. Alternatively, the false lumen may become thrombosed, creating a static obstruction. As dissection propagates distally, it can selectively compromise branch vessel ostia (coronary arteries, brachiocephalic arteries, mesenteric arteries, renal arteries) by either direct involvement of the ostia within the dissection or by compression of the ostium as the dissection flap moves. Progressive dissection also weakens the aortic wall by removing elastic support, increasing the risk of aortic rupture through the adventitia into the pericardium (causing tamponade), pleural space, or retroperitoneum.

Recent evidence indicates that aortic dissection is not purely a mechanical phenomenon but involves active molecular pathology. Inflammatory activation occurs in the aortic wall, with increased expression of matrix metalloproteinases (MMPs), particularly MMP-2 and MMP-9. These proteases degrade the extracellular matrix (collagen and elastin), accelerating weakening of the media. Additionally, NF-κB-mediated inflammatory signaling is upregulated, leading to increased production of pro-inflammatory cytokines (IL-6, IL-8, TNF-α). This inflammatory state explains why patients with connective tissue disorders and those with chronic inflammatory conditions (e.g., giant cell arteritis, Takayasu arteritis, systemic lupus erythematosus) are at higher dissection risk. Histologic examination of dissected aortas reveals increased density of macrophages and T lymphocytes in the media, supporting an active inflammatory process.

Systemic hypertension is present in 60–90% of aortic dissection cases, making it the overwhelming leading risk factor. Chronic hypertension drives medial degeneration through sustained elevation of shear stress and pressure-induced smooth muscle apoptosis. Critically, it is not merely the absolute BP level but the rate of change of pressure (dP/dt) during systolic ejection that precipitates dissection. This explains why acute hypertensive crises—whether from cocaine use, amphetamine intoxication, abrupt antihypertensive medication discontinuation, or severe emotional stress—carry particularly high dissection risk. The combination of baseline hypertension plus an acute hypertensive surge creates the highest-risk scenario. Paradoxically, some patients present with relatively normal or even low BP at the time of dissection, either because dissection has compromised aortic perfusion or because the initial BP elevation has already resolved.

The genetic aortopathies dramatically increase dissection risk and do so at relatively young ages (mean 40–50 years). Marfan syndrome (FBN1 mutations) confers a lifetime dissection risk of 1–2% without aortic root replacement and approximately 20–fold increased dissection incidence compared to the general population. Patients with Marfan syndrome frequently present with Type A dissection even without significant hypertension; this reflects the primary structural weakness of the media. Ehlers-Danlos syndrome (EDS), particularly the vascular type (EDS Type IV, caused by COL3A1 mutations), carries an extremely high dissection risk (up to 50% by age 50 years) due to defective Type III collagen, which is abundant in the aortic media and adventitia. Other connective tissue disorders at risk include Loeys-Dietz syndrome (TGFBR1/TGFBR2 mutations), familial thoracic aortic aneurysm and dissection (FTAAD) associated with ACTA2, MYH11, and MYLK mutations, and Turner syndrome (associated with bicuspid aortic valve, aortic stenosis, and inherent media weakness). These genetic conditions should be suspected in any patient presenting with dissection before age 50 or without significant hypertension history.

Pre-existing thoracic aortic aneurysm (TAA) significantly increases dissection risk. An enlarged aortic root (diameter >5.5 cm) has substantially increased wall tension and shear stress according to the Laplace relation (wall stress = pressure × radius / wall thickness). Additionally, the medial histology in aneurysmal aortas shows cystic medial necrosis and elastic fiber loss, predisposing to intimal rupture. Patients with known TAA require intensive BP control and β-blockade to reduce dP/dt, and many require prophylactic surgical repair at defined size thresholds (typically 5.0–5.5 cm depending on risk factors) to prevent dissection.

Cocaine and amphetamines (methamphetamine, MDMA) cause acute dissection through two mechanisms: (1) sudden, severe elevation of blood pressure and dP/dt from catecholamine release and direct sympathomimetic effects, and (2) direct toxic effects on the aortic media causing smooth muscle necrosis and elastic fiber degradation. Dissections related to stimulant use tend to occur in younger patients (average age 30–40 years) and may present without pre-existing hypertension. The timing of dissection relative to drug use is often within hours of administration, and cocaine-associated dissections carry higher in-hospital mortality rates. Notably, cocaine dissections frequently involve the ascending aorta (Type A), suggesting that the acute sympathomimetic surge preferentially stresses the proximal aorta.

Pregnancy, particularly the third trimester and immediate postpartum period, carries a 5–10 fold increased dissection risk compared to non-pregnant women of similar age. The pathophysiology involves physiological changes inherent to pregnancy: increased cardiac output (leading to increased dP/dt), volume expansion, hormonal effects on aortic compliance (relaxin and estrogen modulate elastic fiber properties), and hemodynamic stress. The peripartum dissections are often Type A and carry particularly poor outcomes if they occur during labor and delivery. Patients with pre-existing hypertension, preeclampsia, or underlying aortopathy face even higher risk. This dissection risk is a major reason that patients with Marfan syndrome or other connective tissue disorders require careful counseling regarding pregnancy.

Bicuspid aortic valve is present in 1–2% of the general population but occurs in 5–10% of patients with aortic dissection. The abnormal valve geometry may alter hemodynamics, creating asymmetric flow patterns and increased shear stress on the ascending aorta. Additionally, bicuspid valve is frequently associated with aortic root dilatation and underlying medial abnormalities. Patients with bicuspid aortic valve require periodic imaging surveillance for aortic root expansion and should maintain stringent BP control.

Chronic inflammatory conditions damage the aortic wall and predispose to dissection. Giant cell arteritis (temporal arteritis) predominantly affects the aorta and its major branches; aortic dissection and rupture occur in approximately 5–15% of GCA patients, particularly those with delayed diagnosis or inadequate immunosuppressive therapy. Takayasu arteritis is a chronic inflammatory disease affecting the aorta and major branches, with dissection occurring in 1–10% of cases depending on disease stage. Behçet disease can cause aortitis with secondary dissection. Systemic lupus erythematosus (SLE) and rheumatoid arthritis (RA) have been associated with increased dissection risk, likely through inflammation-mediated medial degeneration. Ankylosing spondylitis rarely causes aortic involvement but when present, dissection risk is elevated.

Patients with hypertrophic cardiomyopathy, particularly those with MYBPC3 and MYH7 mutations, have modest increased dissection risk. The proposed mechanism involves abnormal dP/dt from the hypercontractile left ventricle and potential underlying myofibrillar disorganization extending to the aortic wall.

Advanced age and male sex are non-modifiable risk factors. Cigarette smoking increases dissection risk through direct toxic effects on the aortic media and acceleration of atherosclerotic disease. Chronic kidney disease with uncontrolled hypertension increases risk, as does diabetes mellitus (though the association is less strong than hypertension). Prior aortic valve replacement and cardiac surgery (particularly those involving aortic manipulation or cross-clamping) increase the risk of late dissection, as do indwelling aortic catheters or intra-aortic balloon pumps (IABP).

The prototypical presentation of aortic dissection is sudden onset of severe, maximal-intensity chest pain at the moment of dissection. The pain is classically described as "tearing," "ripping," or "stabbing" in character and is distinguished from acute coronary syndrome (ACS) by its abrupt initiation at full severity (rather than gradual crescendo) and its characteristic descriptors. The pathophysiological basis is the acute separation of medial layers and stretching of the aortic wall, which activates nociceptors in the adventitia and surrounding tissues. The pain typically has a migration pattern: in Type A dissections, pain often originates in the anterior chest and radiates to the back, between the scapulae, or to the interscapular region. In Type B dissections, pain more commonly involves the lower back, flanks, or abdomen. This migration reflects the propagation of the dissection through the aorta. Importantly, 10–15% of dissection patients experience painless dissection, particularly those with acute aortic regurgitation, neurological deficits, or those in shock; these patients represent particularly challenging diagnostic scenarios. Painless presentations may be more common in patients with pre-existing aortic disease, those on analgesics, or those with diabetic neuropathy.

The blood pressure presentation at initial evaluation varies widely and is often misunderstood. Hypertension (SBP >160 mmHg) is present in 60–70% of acute Type A dissections and is the typical picture. However, normotension or hypotension occurs in 25–30% of cases and carries worse prognosis. Hypotension in dissection indicates (1) aortic rupture with hemorrhage, (2) acute severe aortic regurgitation with acute heart failure and reduced cardiac output, (3) pericardial tamponade from rupture into the pericardium, or (4) dissection extending to branch vessels causing ischemia of vital organs (brachiocephalic, celiac, or renal artery involvement). The presence of hypotension in a patient with apparent ACS should raise suspicion for dissection rather than simple MI. The pulse deficit phenomenon—discrepancy in systolic BP or pulse quality between upper extremities—occurs in 15–30% of Type A dissections and reflects compression or obstruction of brachiocephalic or left subclavian artery by the dissection flap. A systolic BP differential >20 mmHg between the two arms is suggestive of dissection.

Aortic dissection can present with acute stroke (5–8% of dissections) when the dissection involves the ascending aorta and extends into the innominate or left carotid artery ostia, compromising cerebral blood flow. The resulting neurological deficits (hemiparesis, aphasia, ataxia) may superficially resemble primary stroke, but the presence of severe chest or back pain, hemodynamic abnormality, and aortic imaging findings clarify the diagnosis. Spinal cord ischemia results from dissection extending into the descending thoracic aorta, compromising the artery of Adamkiewicz (typically at T9–L2 level), which supplies the lower thoracic and lumbar spinal cord. This presents as acute paraplegia or paraparesis, often with sensory level and bowel/bladder dysfunction, and carries particularly poor outcomes. These neurological presentations underscore the importance of imaging the entire aorta and considering