Stanford type B dissection

Images Earl Butler, text Genevieve Carbonatto

A 55 year old lady presents with acute numbness to her right leg and “dizziness” lasting a few hours. No back pain. No chest pain. There are no other neurological symptoms.  BP 175/85 HR 72/min, RR 16/min saturations 96% on room air. A brain CT COW is organised and is normal. An aorta point of care abdominal ultrasound is performed.

Proximal aorta

The proximal aorta measures 2.66 cm

Mid aorta in the transverse plane


Colour Doppler shows 2 channels with turbulence in the true lumen.

Mid aorta in the longitudinal plane

Colour Doppler shows 2 channels. The flow is bidirectional on either side of the flap

The dissection is seen to descend into the right common iliac artery

Discussion

Ultrasound in aortic dissection

  • An aortic flap may not always be visible with ultrasound. It will depend on it’s orientation to the ultrasound beam. If a dissection is suspected it needs to be examined from several angles in an effort to show the flap (1)
  • Spectral and colour Doppler will show the presence and the character of the flow in both the true and false lumens. Even if the flap is not visible, the different flows in the 2 channels will be apparent on Doppler
  • Reversed flow may be seen in the non dominant channel due to compression in systole
  • If one channel is thrombosed it may be more confusing
  • Doppler can be used to assess blood flow in the major branches supplying the bowel, the liver, kidneys and lower limbs which may be supplied by the true or the false lumen

Both type A and type B aortic dissections are treated medically with

  • Analgesia
  • Control of hypertension and heart rate with B blockers. B blockers also protect the myocardium against ischaemia.
  • Aim to get BP between 100 and 120 mmHg
  • Aim to get HR to 60/min
  • Alternative to B blockers are vasodilators such as Ca channel blockers ( nicardipine or diltiazem) or nitroglycerine

Type B dissections require surgery or thoracic endovascular aortic repair (TEVAR) if there is

  • rupture of the descending aorta
  • uncontrolled pain
  • malperfusion of aortic branches or lower extremities

 

  • The aim of surgery in patients with type B dissection is to resect the primary entry tear and to replace the dissected descending aorta, which increases blood flow to the true lumen and improves organ ischaemia. The aim of TEVAR is to close the primary entry tear and thus redirect blood flow to the true lumen. It also has the advantage of stabilising the dissected aorta to prevent late complications by inducing aortic remodeling. Thrombosis of the false lumen may result in shrinkage and prevent aneurysmal degeneration.
  • The entry site for spontaneous isolated abdominal aortic dissections most commonly occur between the renal arteries and the inferior mesentery artery
  • Repair of the descending aorta is associated with a higher incidence of paraplegia than repair of other types of dissections because of interruption of segmental blood supply to the spinal cord.
  • The operative mortality rate is approximately 5%

Differentiation between the true and false lumen is important especially for the placement of an endoluminal graft. If there is no clear continuation of one lumen with the artery then it can be difficult.  The following features may distinguish one lumen from the other.

True lumen

  • often compressed by the false lumen and the smaller of the two
  • outer wall calcification (helpful in acute dissections)
  • origin of celiac trunk, SMA and right renal artery usually from true lumen

False lumen

  • Often larger lumen size due to higher false luminal pressures
  • At risk for rupture due to reduced elastic recoil and dilation
  • Beak sign
  • Cobweb sign (as slender linear areas of low attenuation specific to the false lumen due to residual ribbons of media that have incompletely sheared away during the dissection process)
  • Often of lower contrast density due to delayed opacification
  • Maybe thrombosed and seen as mural low density only (more common in chronic dissections)
  • Origin of left renal artery usually from false lumen
  • Surrounds true lumen in Stanford type A
  • Chronic dissection flaps are often thicker and straighter than those seen in acute dissections

References

  1. Clinical Ultrasound : Paul Allen
  2. Management of acute aortic dissection and thoracic aortic rupture Toshihiro Fukui  Journal of Intensive Care20186:1
  3.  Up to date : aortic dissection
  4. Gen Thorac Cardiovasc Surg. 2019 Feb;67(2):203-207. doi: 10.1007/s11748-018-1030-y. Epub 2018 Nov 19.
    Acute medical management of aortic dissection
  5. Medscape :Acute aortic dissection
  6. Radiopaedia : Aortic dissection

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