Aortic Dissection: Aetiology, Anatomy and Pathophysiology

• Aortic dissection involves a tear in the aortic wall's intimal layer, causing blood flow between the intima and media, creating a false lumen.
• Acute aortic dissection (AAD) is the most common aortic emergency, with 3-4 cases per 100,000 annually in the UK and a high mortality rate.

Aetiology

Anatomy

• The aortic wall has three layers: intima, media, and adventitia.
• The initial tear commonly occurs in the ascending aorta.
• Aortic dissection involves blood flow between the intima and media, creating a false lumen.

Pathophysiology

• The true lumen is the normal arterial lumen, and the false lumen is the blood-filled channel in the media.
• The true lumen often decreases in size due to the false lumen compressing it.
• Aortic dissection can spread along the aorta, either anterograde or retrograde.
• Dissection propagation can cause branch occlusion and ischaemia in the affected arterial territory, referred to as aortic dissection with end-organ malperfusion.
• Proximal dissections can affect the aortic valve root, leading to cardiac tamponade, acute aortic regurgitation, and/or aortic rupture.

Risk factors

• Male sex
• Age 50-70 years
• Hypertension is the most common risk factor
• Connective tissue disorders like Marfan’s syndrome or Ehlers-Danlos syndrome
• Abrupt, severe increase in blood pressure related to stress, pain, substance use, or heavy lifting
• Atherosclerotic disease
• Pre-existing aortic aneurysm
• Bicuspid aortic valve or coarctation of the aorta
• Iatrogenic causes include aortic instrumentation or surgery, such as stenting or catheter insertion

Clinical Features

History

• Aortic dissection diagnosis is challenging due to vague clinical presentation and varied symptoms.
• AAD typically presents as sudden, severe chest or inter-scapular back pain, described as sharp, ripping, or tearing.
• The pain may subside spontaneously.
• 10% of AAD cases are painless and found incidentally during imaging for other conditions.
• Less typical symptoms include abdominal or flank pain and end-organ malperfusion symptoms.
• AAD can also manifest as cardiovascular collapse due to aortic rupture.

Atypical presentations

• Neurological deficits such as syncope, seizure, limb paraesthesia, or paraplegia.
• Limb pain and/or pallor due to acute limb ischaemia.
• Flank pain and/or reduced urine output due to renal artery involvement.
• Abdominal pain due to compromised gut perfusion (e.g., mesenteric ischaemia).

Clinical examination

• A full cardiovascular and abdominal examination is needed if AAD is suspected.
• Measure blood pressure in both upper limbs.
• Typical clinical findings include:
• Pulse deficit or asymmetric blood pressure readings are associated with type A dissections, but present in only 20% of cases.
• Hypertension is common and often extreme.
• Hypotension may be spurious or indicate cardiac tamponade, aortic regurgitation, hypovolaemia (from aortic rupture), or neurogenic shock.
• Tachycardia
• Diastolic murmur from aortic regurgitation.
• Pulsus paradoxus, muffled heart sounds, or distended neck veins suggest cardiac tamponade.
• Decreased breath sounds indicate haemothorax.
• Pulse deficit is a difference of >20 mmHg in blood pressure between limbs or a weaker/absent pulse compared to the contralateral side.

Differential Diagnoses

• Acute coronary syndrome: crushing chest pain, cardiac ischaemia signs on ECG, raised troponin.
• Cardiac tamponade without dissection
• Pericarditis
• Spontaneous pneumothorax
• Pulmonary embolism
• Oesophageal rupture
• Musculoskeletal pain

Investigations

Bedside Investigations

• ECG: to assess for myocardial ischaemia, which is common in AAD and may indicate coronary involvement.

Laboratory Investigations

• FBC, U&Es, LFTs, and coagulation screen
• Arterial blood gas (including lactate): elevated lactate might indicate potential tissue ischaemia
• Group and save and crossmatch (if concerns over bleeding)
• Troponin: may be elevated if dissection causes myocardial ischaemia
• D-dimer: a negative D-dimer indicates that dissection is very unlikely, but a positive result is not sufficient to diagnose aortic dissection.

Imaging Investigations

• Imaging is essential to confirm aortic dissection.
• Urgent CT angiogram (CTA) of the whole aorta is the initial investigation.

Chest X-ray findings

• Widened mediastinum (>8cm) is a classic finding, but only present in approximately 60% of cases
• Double or irregular aortic contour occurs in 50% of cases
• Inward displacement of atherosclerotic calcification
• Pleural effusion or haemothorax indicates dissection rupture
• Chest X-ray is normal in around 10 – 15% of patients with AAD

Cross-sectional Imaging

• CT angiogram (CTA) whole aorta confirms diagnosis, classifies dissection, assesses distal complications, and assists surgical planning.
• CTA has nearly 100% sensitivity and specificity for acute aortic dissection.
• CTA is not 100% sensitive in detecting end-organ malperfusion, especially if vessel occlusion is dynamic.
• History, clinical examination, and serial blood markers are important for assessing end-organ malperfusion
• Voltage-gated CT offers superior resolution.

CT Angiogram Findings

• Double lumen (true and false lumens) confirms AAD diagnosis.
• Identify the entry tear (where the dissection begins).
• Assess for aortic dilatation (aneurysmal change).
• Evidence of end-organ malperfusion (e.g., non-enhancing kidney).
• Features of acute rupture (including extravasation of contrast or haemothorax).
• Magnetic resonance angiography (MRA) also has excellent sensitivity and specificity.
• Use in emergencies is limited by availability and monitoring difficulties.

Echocardiography

• Transthoracic echocardiography (TTE) can be used, but transoesophageal echocardiography (TOE) is more sensitive and specific.
• TOE assesses ascending aorta involvement, pericardial effusion, and aortic regurgitation.
• TOE is more invasive than TTE and requires specialist expertise, limiting its availability.

Diagnosis

• Aortic dissections are classified using the Stanford classification.

Stanford classification

• Type A: involves the ascending aorta (with or without the arch and descending aorta), accounting for 60-70% of cases.
• Type B (TBAD): involves only the descending aorta (distal to the left subclavian artery) and/or abdominal aorta, accounting for 30-40% of cases.

DeBakey classification

• Type 1: intimal tear originates in the ascending aorta and involves the ascending aorta and aortic arch and variable amounts of the descending aorta.
• Type 2: dissection is confined to the ascending aorta.
• Type 3: intimal tear sited in the descending aorta, distal to the left subclavian artery
• Further classified as Type IIIa (affected region is confined above the diaphragm) or Type IIIb (affected region extends below the level of the diaphragm)

Management

Initial Management

• ABCDE assessment for all AAD cases.
• Priorities include high-flow oxygen, IV access, continuous observations, and invasive monitoring.
• Seek senior support from anaesthetics/critical care, cardiothoracic or vascular surgery, and interventional radiology.
• Discuss the patient urgently with the on-call vascular or cardiothoracic team and arrange urgent transfer to an appropriate centre.
• Provide adequate analgesia with strong IV opiate analgesia to decrease sympathetic tone and facilitate blood pressure control.

Blood pressure control

• Actively manage blood pressure to rapidly lower systolic BP, pulse pressure, and pulse rate.
• Target heart rate is 60-80 bpm and target systolic BP is 100-120 mmHg.
• Intravenous beta-blocker infusion (such as labetalol) is the first-line agent.
• Second-line agents are IV calcium channel blockers (such as nicardipine).
• IV nitrate infusion or vasodilators (such as sodium nitroprusside) are used in cases of refractory hypertension.
• An arterial line must be placed to facilitate control of blood pressure with IV agents.

Surgical Management

• Surgical management depends on the classification of AAD.
• Type A dissections require open surgery to prevent aortic rupture and generally carry a worse prognosis than Type B dissections.
• Type B dissections (TBAD) are usually managed medically, with endovascular intervention indicated for complicated dissections.

Type A Management

• Type A aortic dissections have high mortality if managed medically due to the risk of aortic rupture.
• Discuss cases urgently with a cardiac or vascular surgeon for consideration of urgent surgical repair.
• Surgical management involves removal of the ascending aorta with or without the aortic arch and replacement with a synthetic graft.
• If the dissection has damaged the aortic valve, this will also require repair or replacement.
• Branches of the aortic arch involved in the dissection may require reimplantation into the graft.

Type B Management

• TBAD is traditionally described as acute (≤ 14 days) or chronic (> 14 days).
• More recent classifications include the following groups: hyperacute (3 months).
• This has relevance when deciding upon surgical management.

Complicated Type B Dissections

• Defined by aortic rupture, impending rupture, rapidly expanding aortic diameter, malperfusion, ongoing pain, and refractory hypertension.
• Intervention is almost exclusively with endovascular stent graft placement, (thoracic endovascular aortic repair (TEVAR)).
• Aim of endovascular management is to stent and occlude the proximal entry tear, promoting false lumen thrombosis and aortic remodelling.