Thoracic aortic dissection

My DDU point form notes on dissection and other thoracic aortic syndromes…

The rest of my DDU notes are here.

Anatomy

aorta tapers

2.1 cm/m² in ascending aorta to 1.6 cm/m² at aortic arch

aging

↑ size ~1-2mm (?really) per year

media elastin ↓, collagen ↑

Pathogenesis – stresses

circumferential → dilation

circumferential stress affects media predominantly

Laplace: stress = Pr/2h → ↑ in dilated, thin walled aorta

dilating aorta more spherical

↑ circumferential stress, ↑ ↑ longitudinal stress

∴ most atraumatic aortic tears are transverse, from longitudinal stress

local factors focally ↑ stress

atheroma

intramural haematoma

vasa vasorum bleed into media

expands → can rupture into lumen (also external rupture)

dissection

intima and internal elastic lamina peel off

shear stress propagates dissection

Risk factors and mechanisms

atherosclerosis

especially with dilated aorta / symptoms

1 year survival with thoracic aortic aneurysm is < 60%

trauma

risk sites

highest at aortic isthmus – tethering by ligamentum arteriosum

R brachiocephalic

ascending aorta above sinuses

high risk mechanism

>~65 km / hr or fall from > 3 m

injuries

dissection

rupture of adventitia

→ risk of lethal haemorrhage

even if contained, high risk of rupture within 24 hrs

intramural haematoma

thrombus

mediastinal haematoma will note↑ distance from TOE probe to aorta

prognosis

significant aortic trauma → 30% mortality in 24 hrs without surgery

TOE safe and highly sensitive post MVA

however, CT better for transverse arch TOE blind spot

see flow on both sides of wall – can be quite focal (few cm)

detects other cardiac injuries

surgical

CPB sites, proximal anastomotic sites for CABG, post aortic / valve surgery

rarely, coronary angiography

hypertension

cocaine and methamphetamine

consider in young

giant cell arteritis, syphilis

pregnancy

congenital

coarctation

bicuspid aortic valve

9x ↑ risk cf normal valves, valve replacement doesn’t stop progression

electively replace ascending aorta when aortic diameter > 50 mm

connective tissue disease

Marfan

dilate → tensile failure → dissection

↑ risk of dissection with age, males, aortic root > 60 mm, rate of ↑, FHx dissection

put on β blockers and followed, elective surgery at ~ 45-50mm

Turner

>100 x risk of normals

Noonan, Ehlers-Danlos, osteogenesis imperfecta

familial

autosomal dominant

Aortic syndromes

dissection

important

type A dissection has a high mortality – 50% by 48 hours

frequently delayed / misdiagnosis

consider when chest pain and multiorgan disease

similar to MI

pain, Δ ECG, ↑ CK

~7% get coronary involvement

caution with thrombolytics, antiplatelets, coronary angio

usually transverse, ?initiated by intramural haematoma

65% within 3 cm of coronary ostia, 10% in arch, 10% in descending thoracic (rare in abdominal aorta)

classification

DeBakey

I: ascending + transverse arch, distal aorta

II: ascending only

III: descending only

Stanford

A: ascending aorta involved (I+II)

B: not involved (III)

 multiple distal exit tears common

can be chronic

d/dx aneurysm – hard to tell with thrombosed false lumen

 tests

CT usually faster

TOE role in instability + t/f to ICU / OR / serial assessment

 complications of dissection

aortic rupture

coronary / limb / cerebral / spinal / bowel / renal ischaemia

rupture into pericardium → tamponade

valve leaflet attachment disrupted → AR

intramural haematoma

vaso vasorum bleed into media

expands → can cause

aortic dilation

rupture

into lumen → dissection (33%)

externally (27%)

pleural / pericardial effusions

AR

can heal on medical Rx aggressive antiHT

high risk group

persistent pain

expanding haematoma / progressive thickening of aortic wall

>50mm aortic diameter

ascending aortic involvement

penetrating aortic ulcer

ulceration of atheroma into internal elastic lamina of aorta

commonest in descending aorta

can progress to dissection, intramural haematoma, rupture

Mx controversial

Imaging

if equivocal with one modality, use another

TTE

useful for AR, ventricular function, RWMA

may see flap in aortic root, arch, descending aorta but insensitive

CT

usual first choice, best for ruling out

no blind spot cf TOE

equal flow in both lumens can cause false –ve (rare)

MRI

best for ruling in, but logistically difficult

angiography

sensitivity and specificity limited by false lumen thrombosis, intramural haematoma and equal flow in true and false lumens

TOE

good views of intrathoracic aorta except blind spot

distal ascending aorta, superior transverse arch, origins of great vessels

due to airways

~98% specific and 99% sensitive

also informs re LV, RV function, AR including severity, mechanism → replacement vs repair, MR

can be done in ICU / OR etc

avoid contrast, X-ray, and hostile environments

can be associated with ↑ BP, arrhythmias, vagal AV block

Rx as required eg. have iv β blockers ± vasodilators for BP ready

record location of findings as distance from incisors

diagnostic features on TOE

dissection

flap

mobile, linear echo

motion towards false lumen in systole helpful

false lumen often bigger

flow on both sides: forward systolic flow in true lumen, variable + complex in false lumen

may be forward systolic flow near the entry tear, with diastolic reversal

often peak velocity later than true lumen

often swirling, with SEC / thrombus – d/dx contained rupture – ensure imaging depth adequate to see

thickened wall > 15 mm

thrombosed false lumen

beware of artefacts use multiplane to examine

near field artefacts

artefacts from atherosclerosis

shelf from abrupt aortic dilation relatively immobile with homogeneous flow on each side cf flap

common to find atherosclerosis / dilation in these patients

look for complications

rupture

coronary ostial involvement

intramural haematoma

100% sensitive, 91% specific

thickened aortic wall (>7 mm) with intramural echo free spaces, without colour flow in wall

↑ distance from TOE probe to inner aortic wall

focal distortion of aortic circularity

penetrating ulcer

crater like outpouching of aorta with jagged edges

associated with atheromatous plaque

Cover image: Modified from TAstent by James Heilman