Aorta is the largest artery in the body. It originates from the Left Ventricle. It carries the oxygenated (pure) blood to various parts of body through its numerous branches. At the origin, the flow of blood from the left ventricle to aorta is controlled by Aortic Valve.
Normal aorta is 2.5-3.0cm in diameter at the origin. The diameter gradually comes down as it gives away branches. Before bifurcating in the abdomen it is about 1.5-2.0cm.
First branches of Aorta are the Coronary arteries. After giving off these branches aorta climbs up to the upper part of chest and turns downward to form an arch. Thus there are three distinct parts of aorta at this area.
- Ascending Aorta – the part which goes up
- Arch of Aorta – the arching part which also gives off branches that supply the brain (carotid arteries) and the two upper limbs (Subclavian arteries)
- Descending Aorta – the part which comes down in the chest after the arch. This part gives of intercostal arteries. Intercostal arteries have a major role in the blood supply of spinal cord.
The descending aorta travels down and goes across diaphragm to enter abdomen (Abdominal Aorta). Abdominal aorta supplies the intestine and all the organs in the abdomen.
The branches of Abdominal aorta are
- Coeliac Artery – supplies liver, spleen, gallbladder and pancreas
- Superior Mesenteric Artery – Supplies the intestines
- Inferior Mesenteric Artery – Supplies the intestines
- Renal Arteries – Normally one for either side and supplies kidney
Aorta has very specific branches at specific locations that supply specific parts of the body. Any disease that affect the aorta in a particular location will affect the corresponding organs. The symptoms of the same disease at different locations can thus be different.
After giving off these branches aorta divides into two large branches called Common Iliac Arteries(CIA) one for either side. CIA's in turn divide into Internal and External Iliac Arteries. Internal Iliacs supply the pelvic organs. External Iliacs go on to become the femoral arteries that supply legs.
DISEASES OF AORTA:
The important diseases that affect the Aorta are
- Aortic Aneurysm
- Aortic Dissection
Aneurysm is the ballooning of the arterial wall due to some weakness in it. It can develop in any artery in the body. When it affects the aorta, it is known as aortic aneurysm.
How it happens?
Aortic wall has 3 layers – Intima (inner layer), media (middle layer) and the adventitia (outer layer). Some diseases affect the media and cause weakness in the layer. The internal pressure of blood acts constantly on the aortic wall. When it acts on the weakened section of the wall, it balloons out. Gradually an aneurysm forms. The commonest disease that weakens aortic wall is Atherosclerosis.
Some of the connective tissue disorders are genetic and runs in families. (eg. Marfan’s syndrome and Ehler-Danlos syndrome) These conditions cause weakness in the aortic wall. The affected persons have the risk of developing aneurysms at a young age.
Age related degeneration can also lead to weakness in the arterial walls and cause aneurysm.
Hypertension is an aggravating risk factor which hastens the enlargement of the aneurysm.
As discussed in earlier section, symptoms of aortic aneurysm vary depending on the site of the disease. Stretch on the aortic wall can cause pain usually of dull and aching variety. Due to the mass effect, it can exert pressure on the neighboring structures. At the ascending aorta dyspnea and chest pain can occur. At the arch and proximal descending aorta patients can have difficulty in swallowing (dysphagia), difficulty in breathing or change in voice due to compression on the nerve (recurrent laryngeal nerve).
In the abdomen symptoms can be confusing – from mild abdominal pain, colicky pain, burning micturition and low backache to jaundice and bleeding. Some patients notice prominent pulsating swelling in the abdomen.
When aneurysm is large and threatening to rupture, pain may be more severe. It may present with abdominal or back pain.
Rupture is associated with excruciating back or abdominal pain. Deterioration can be rapid due to blood loss into peritoneum or chest cavity.
Aneurysm close to intestines can erode into them and rupture (Aorto-enteric fistula). This is a very serious complication and treatment is difficult with poor prognosis in spite of timely surgery.
An Ultrasound of abdomen or an Echo may detect the aneurysm for the first time. The precise evaluation is done by a CT-Aortogram. This contrast CT would provide accurate information on the size, location, extent and branch involvement of the aneurysm.
In the initial stages the treatment is conservative and revolves around risk factor control. Hypertension should be treated aggressively. Traditionally treatment is recommended once the aneurysm grows beyond 5 cm in diameter. At this point the statistical chance of rupture starts to increase rapidly.
Surgery is the traditional form of treatment. Depending on the site the surgical options defer.
- Aortic root – Aortic root replacement
- Ascending aorta - Interposition graft
- Arch of aorta – Arch repair or replacement
- Descending or Abdominal aorta – repair with interposition graft with/without branch re-implantation
New onset pain in a known case of aneurysm is a sinister symptom and should be taken seriously.
Endo-Vascular Repair of aortic aneurysm using a stent is a viable option in many situations. It is suitable for straight segments with no branch involvement. There should be clear disease free areas on either side of the disease for the stent to be seated well (landing zones).
Hybrid is a method of combining surgery with stenting. This can reduce the morbidity of the treatment and widen the scope of treatment as the two methods complement each other.
Aortic dissection is a condition which involves a tear in the inner layer of aorta. Aortic wall as already described in other section, has three layers. When the tear occurs in the intima, blood flowing inside the aorta under pressure creeps into the layers of aortic wall and dissects it. One of the major contributing factors is uncontrolled hypertension.
The extent of the split can vary and this decides the severity and symptoms of Aortic dissection.
Aortic dissection can be classified based on
- Location and extent of dissection
- Duration of symptoms
Different segments of aorta can be affected by the dissection.
Type A Dissection:
When the ascending aorta is affected irrespective of the extent
Type B Dissection:
When Ascending Aorta is not affected. Here usually dissection is limited to the Descending aorta.
Types according to onset and duration:
Acute Aortic dissection:
Sudden and recent onset is described as acute. Usually less than 4 weeks in duration
Chronic Aortic Dissection:
Duration more than 4 weeks
PAIN: Aortic dissection is usually associated with severe pain. Site of pain may vary depending on the location of dissection. For example, ascending and arch dissection will have severe chest pain. Descending thoracic aortic dissection may have chest and upper back pain. Abdominal aortic dissection presents with severe lower back pain and abdominal pain.
OTHER SYMPTOMS: Depending on the aortic branches affected by the dissection some symptoms may appear. When the abdominal branches are involved pain may be predominant. Dissection can affect the lower limb vessels and that can cause severe leg pain. If coronary arteries are involved, the symptoms may be severe and patient may have angina or even heart attack (MI). Involvement of Carotid arteries can rarely cause strokes.
Clinical suspicion is very crucial in clinching the diagnosis, as the situation can mimic many conditions. The definite diagnosis is made by an ECHO and CT aortogram.
Echo can detect the dissection flap and the extent can be seen. Aortic valve can be assessed. CT aortogram can give precise location, extent, branch involvement and any rupture of pseudo lumen. Due to the accuracy and detail, CT aortogram is the GOLD STANDARD in aortic dissection.
Sometimes, a patient presenting to Emergency room with type A aortic dissection may be too unstable to undergo CT scan. In such dire emergencies, immediate surgery may be the only solution. A quick diagnosis by Trans-thoracic Echo is done pre-operatively and a Trans-esophageal Echo is done during surgery.
Treatment broadly depends on the type of dissection. As a rule of thumb, Acute type A dissection warrants an emergency surgery. This is because it can potentially affect the coronaries and can be fatal. The mortality approaches 60% with out surgical intervention.
On the contrary, type B dissections are largely managed conservatively. Surgery is generally reserved for complications like visceral, renal or limb artery ischemia or aneurysm of pseudolumen.
Type A Aortic Dissection:
As soon as the diagnosis is made, planning of treatment should be started. The extent of dissection, aneurysm formation, presence of rupture and the branch involvement usually dictates the type of surgery.
The commonest scenario is a dissection starting at the ascending aorta just above the level of Right Coronary Artery and extending to variable distance of aorta. The tear usually involves the arch partly or completely. It may even involve one or more of the major arch vessels. But as long as the perfusion to these vessels are intact and there is no aneurysm formation of the pseudolumen, surgical intervention of arch is not indicated. The treatment then would be an Aortic root replacement. The valve may be replaced or preserved depending on the presence or absence of valve or annulus involvement.
When the arch of aorta is badly damaged or the psuedolumen is aneurysmal in this area, the arch should be addressed during surgery. Depending on the extent of involvement, part (Hemiarch) or entire (Total Arch) arch may be replaced.
Type B Aortic Dissection:
Traditionally, type B dissections (Dissections beyond the left subclavian artery) are treated conservatively unless one of the following situations is present.
- Aneurysm of pseudolumen.
- Tear or leak of the psudolumen
- Viscral branch or limb branch involvement causing decrease in perfusion of an organ.
However, persisting pain not responding to conservative treatment is a relative indication for intervention now a days. Various methods of interventional procedures are attempted to control the primary tear in the intima.
One common interventional method is to stent across the proximal intimal tear (figure)
This prevents the blood from entering the pseudo-lumen and eventually blood in the pseudo-lumen gets thrombosed.
Whenever decision is made to treat the type B Dissection conservatively, a close follow up is mandatory. CT Aortogram/MR Aortogram should be performed at 3 months and one year.
Some Special Cases
Aorta can be injured in variety of accidents or trauma. A piercing or penetrating injury by a sharp object can directly injure the aorta in the thorax or abdomen. These injuries are often instantly fatal due to severe bleeding.
More commonly aorta is injured in blunt trauma especially in high velocity deceleration accidents. Typically the person is thrown from a speeding bike and comes crashing. The relatively fixed part of aorta (close to the position of ligamentum arteriosum) takes the brunt of impact. So commonest site of injury in these injuries is opposite the left subclavian artery. Aorta may be sustain partial tear of intima and/or media or a complete tear. The complete tear is a devastating injury accounting for heavy mortality- 20% of victims die before reaching medical facility. Untreated another 10% die every hour following injury. Needless to say, this is a true surgical emergency.
Sometimes the extensive traumatic dissection can cause malperfusion of the visceral vessels or intercostals. Bleeding into the chest can severe enough to cause hypotension and shock.
Blunt chest trauma or decelerating injury is an indication for CT scan of chest with contrast. Apart from identifying the bony or soft tissue injuries, it can clearly demonstrate vascular trauma including aortic tears. Fig(x) shows the tear in the aorta (arrow).
Once the CT scan is done the extent and severity of aortic injury becomes clear. Depending on the type of injury the treatment varies. A small hematoma on the aortic wall may be left alone. Even a small tear in the aortic intima (inner layer of aorta) may be treated conservatively with caution and rigorous follow up. However, more serious injuries need definitive treatment.
A full thickness tear in the aortic wall is a potentially fatal injury. If the bleeding continues. the victim may develop hypovolemic (low blood volume in the circulation) shock. Large volume of blood can get collected in the thoracic cavity. Often after the initial bleed, the blood clot in the vicinity of injury prevents further bleeding. In these circumstances proper treatment can save the person fig(1).
There are two types of definitive treatment. Open Surgery and Endovascular Stenting. In open surgery, the injured part of aorta is replaced with artificial tubes or prosthetic grafts made of PTFE, Dacron or Polyester fig(2).
In Endovascular procedures, a folded tube with a self-expanding stent in inserted into the aorta from the femoral artery (Groin) and expanded inside the injured portion to seal the area and prevent bleeding.
Sometimes a HYBRID technique is employed. ie., a combination of surgery and Endovascular procedure (fig 3)
It is important to keep a close watch on these patients and get a CT aortogram at 3 and 12 months