There are two proposed mechanisms of causation- intimal tear with blood dissecting through diseased media and medial hemorrhage due to rupture of vasa vasorum rupturing through the intima.

 

Classification

The two common sites of intimal tear are near the aortic valve and at the ligamentum arteriosum.

Stanford classification is into types A and B. Type A involves the ascending aorta and type B does not involve the ascending aorta. They are also called proximal and distal respectively.

DeBakey classification is into types I, II and III. Type II originates in the ascending aorta and remains there. Type I originates in the ascending aorta and extends at least till the aortic arch. Type III originates in the descending aorta and may propagate proximally or distally.

The time cutoff for acute and chronic dissections is 2 weeks. Majority are acute when diagnosed.

 

Pathology

The causes are

1.      Cystic medial degeneration-

a.       Marfan (5% of all cases)

b.      Ehlers-Danlos

c.       Bicuspid aortic valve (5% of all cases) (risk of dissection is independent of severity of aortic stenosis)

2.      Hypertension

3.      Old age

4.      Pregnancy (usually in third trimester)

5.      Coarctation of aorta

6.      Turner syndrome

7.      Cocaine

8.      Giant cell arteritis

9.      Trauma

10.  Iatrogenic- catheterization, IABP, dilation of coarctation, post cardiac sugery, especially after aortic valve replacement

 

Usually occurs in 6th or 7th decade. Men are affected twice more commonly.

 

Symptoms

All acute dissections have pain. Anterior chest pain indicates ascending aortic involvement while posterior chest pain indicates descending aortic involvement. Neck pain indicates ascending aortic involvement. Abdominal and lower limb pains indicate descending aortic involvement. The pain is severe and has maximum intensity at the onset itself unlike the crescendo pain in acute myocardial involvement.

 

Signs

  1. Aortic branch vessel involvement (due to blood in false lumen pushing flap into true lumen or due to flap overlying ostium) (may be transient)
    1. Coronary- usually right coronary ostium is involved, causing IWMI
    2. Neck vessels- hemiplegia, syncope, coma
    3. Subclavian- pulse asymmetry, pseudohypotension
    4. Spinal arteries- paraplegia
    5. Mesenteric ischemia
    6. Renal arteries- renal failure, hypertension
    7. Lower limb ischemia
  2. Acute aortic regurgitation- due to sinotubular junction enlargement, leaflet detachment or flap prolapse. Causes heart failure.
  3. Pleural effusion- left sided small pleural effusion due to inflammation around aorta
  4. Aortic rupture- into pleural space, trachea, heart etc

 

Investigations

 

Chest X-ray is abnormal in 90%.

1.      Widening of the cardiac silhouette is the commonest sign.

2.      Calcium- separation of aortic knuckle calcium from outer margin by more than 1 cm.

3.      Left sided pleural effusion

D-dimer is more than 400 ng/ml in almost all patients.

Aortography diagnoses 90% cases, but misses the diagnosis if there is false lumen thrombosis or if there is equal opacification of true and false lumens. Also, it will miss intramural hematomas. It is invasive and takes lot of valuable time. Its advantages are good visualization of branches, coronary angiogram and detection of aortic regurgitation. It is not used nowadays for the diagnosis, but used to be the gold standard earlier.

Contrast CT is the preferred initial diagnostic test now. It detects almost all cases. Dissection is detected by demonstration of intimal flap or differential rate of contrast opacification. It shows branch vessels well.

MRI also detects almost all cases, but is not used commonly due to three reasons. It takes lot of time. It does not allow patient monitoring with cables. It needs gadolinium to see braches.

Echocardiogram detects dissection by demonstrating the flap in more than one view with independent motion. Different color flows are seen in the true and in the false lumens. TTE does not detect one-third cases. TEE detects almost all cases and is used by many as the initial diagnostic test. It has limited ability to see the distal ascending aorta and the proximal arch due to intervening air passages.

Preoperative coronary angiogram is not indicated unless there is a specific reason to suspect coronary artery disease.

 

Management

In acute dissection, without treatment, a quarter dies in the first day, another quarter in the first week and another quarter in the first month. Only 10% survive for one year.

For acute proximal dissection, surgery is needed. For acute distal dissection and for all chronic dissections, medical management is advocated unless any one of the above indications for surgery is present-

1.       Vital organ compromise

2.       Uncontrolled pain

3.       Rupture or impending rupture (saccular aneurysm, rapid expansion)

4.       Marfan

5.       Aortic regurgitation

 

Medical management

 

1.       Control of pain. IV morphine is preferred.

2.       Reduction of heart rate, BP and dP/dt are needed. Heart rate is to be reduced to 60 to 80 pm. Keep systolic BP at 100 to 120 and mean BP at 60 to 75 mm Hg.

If there is no hypertension, a beta blocker alone is used. If there is hypertension, usually, it needs to be combined with sodium nitroprusside is used. The beta blocker used is esmolol if immediate surgery is planned or propranolol if immediate surgery is not planned. (Metoprolol, atenolol or labetolol can also be used, but these have long half life. Fenoldopam is used instead of sodium nitroprusside if there is renal insufficiency.) This combination is used because beta blocker reduces heart rate while sodium nitroprusside reduces BP. Beta blocker reduces dP/dt while sodium nitroprusside increases dP/dt. (Note that unlike propranolol, esmolol can reduce BP.)

If beta blockade is contraindicated (eg bronchospasm), diltiazem or verapamil can be used. These can decrease heart rate, BP and dP/dt.

Doses-

·         IV propranolol is given at a dose of 1 mg every 5 minutes till a HR in the range of 60 to 80 pm is achieved. Then 4 hourly dose or infusion is given.

·         Esmolol is especially useful if surgery is planned as its effect stops in half an hour on stopping the infusion. Dose is 500 mcg/kg bolus followed by 50 mcg/kg/min infusion. Unlike propranolol, esmolol can reduce BP also.

·         Sodium nitroprusside, titrated up from 20 mcg/min, is useful for BP control. It increases dP/dt and so a beta blocker should always be coadministered.

·         Labetolol is an alpha + beta blocker and so can decrease heart rate, BP and dP/dt. 20 mg bolus followed by 40 mg boluses are given every 10 minutes till heart rate and BP are controlled. Then 2 mg/min infusion is started.

·         Fenoldopam is an alternative to sodium nitroprusside if there is renal insufficiency. It is a peripheral dopamine-1 receptor agonist which produces vasodilation and BP reduction. Dose is 0.1 mcg/kg/min.

3.       If severe hypertension occurs due to renal artery occlusion producing renin release, IV enalaprilat titrated up from 0.625 mg q6h is effective.

4.       Cardiac tamponade is likely to occur in acute proximal dissection. If there is severe hypotension or pulseless electrical activity, pericardiocentesis of minimum essential quantity is to be done to increase BP to lowest acceptable level as further aspiration will worsen the patient due to recurrent bleeding into the pericardial space. Otherwise, proceed to urgent surgery without pericardiocentesis.

 

Surgery

Resect the damaged part of the aorta and put a graft. Usually the part with the intimal tear is also included in the resection. If the intimal tear is in the arch, but the dissection is elsewhere, the arch is usually left alone as arch replacement increases chance of complications.

Sometimes, only the most damaged part is resected and grafted. In the less affected dissected part, suture or glue is used to seal off the false lumen.

Aortic regurgitation– If possible, aortic valve repair is done by the resuspension technique. If repair is not possible, valve is replaced.

Endovascular techniques

a.       A wire is used to puncture the flap. A balloon is passed over this wire and dilated to produce an outlet for blood from the false lumen, thus relieving true lumen compression.

b.      An aortic branch, whose ostium has been occluded by dissection, can be stented.

c.       The aortic dissection itself can be stented. This is mainly used for distal dissection. Initial studies have shown lower morbidity and mortality. The INSTEAD study is presently studying this.

 

In acute distal dissection being treated medically, there may occur a severe hypertensive crisis lasting for one week due to sympathetic stimulation by aortic wall inflammation.

Follow up– During follow up, BP should be controlled, to below 130 mm Hg systolic, with beta blockers, non DHP calcium channel blockers, ACEIs or ARBs, avoiding stand alone DHP calcium channel blockers and hydralazine as these increased dP/dt. There is a high chance of dissection and aneurysm formation and so repeat CT examinations are needed.