Aortic dissection occurs when the inner layer of the wall of the aorta tears and splits open (dissects).  This is more likely to occur where pressure on the artery wall from blood flow is high.  A common place is the ascending aorta, where the aorta originates from the heart’s left ventricle. When the aortic wall splits, the blood enters the artery wall under the intima (inner layer).  This makes the aorta split further.  The tear usually continues distally (away from the heart) down the descending aorta and into its major branches. When the cerebral arteries are involved this can lead to neurological symptoms. When the visceral vessels are involved this can lead to ischemia of the involved organs.
The presence of an intima flap is pathognomonic for a dissection.
When there is a proximal and a distal tear, blood flows on either side of the  intima layer. Which is the case in the abdominal aorta in this patient. Sometimes however there is antegrade flow in the true lumen and retrograde flow in the false lumen. Several examples of antegrade and retrograde flow in cases of a dissection of the aorta can be found on www.ultrasoundcases.info category 1.6.5 Aortic dissection

When there is no distal tear, blood pools behind the detached intima layer which leads to a reduction of the lumen and eventualy an occlusion.Which is the case in the common iliac artery in this patient. In this case the dissection continued to the left external iliac artery. In some cases the dissection continues ever more distally.
A case of iliac an femoral artery involvement with severe stenosis can be found on www.ultrasoundcases.info category 10.1.2 Peripheral arteries

The dissection weakens the wall of the aorta which can lead to an aneurysmal dilatation and rupture.
Patients with Marfan's syndrome have a high incidence of developing an aortic dissection and aneurysmal dilatation  at a very young age. In category 1.6.5 of the above mentioned website is a case of a 37 year old patient with an aortic and iliac dissection and aneurysmal dilatation.

Acute aortic dissection usually causes sudden chest pain.  This pain is often described as very severe and tearing; it’s associated with cold sweat.  Typically the pain moves as the dissection extends distally.  Other symptoms and signs depend on the arterial branches involved.

Aortic dissection is more common in males than in females.  The male-to-female ratio ranges from 2:1 to 5:1.  The peak age of occurrence of proximal dissection is between ages 50 and 55.  Distal dissection occurs most often between ages 60 and 70.  High blood pressure is the most common factor predisposing the aorta to dissection. Dissection can also be the result of a chest trauma
Several different classification systems have been used to describe aortic dissections. A common classification is the DeBakey classification system. It categorizes the dissection based on where the original intimal tear is located and the extent of the dissection (localized to either the ascending aorta or descending aorta, or involves both the ascending and descending aorta).

• Type I - Originates in the ascending aorta, propagates at least to the aortic arch and often beyond it distally.
• Type II – Originates in and is confined to the ascending aorta.
• Type III – Originates in descending aorta, rarely extends proximally.

In this patient the dissection originated in the ascending aorta and extended far distally so it is a DeBakey type 1

Because ultrasound doesn’t show the intrathoracic aorta, the full extend of an aortic dissection can only be examined with CT or MRI. However since the initial examination in many emergency departments is still ultrasound, it is good to be aware of the signs of a dissection. Also color doppler and spectral doppler can give information about the flow (direction and velocity) in the true and false lumen.