The first case of aortic dissection was reported by Frank Nicholls in an autopsy

report of King George II of Great Britain who died suddenly while straining in a toilet [23] on 25 October, 1760 and found a dissection in the aortic arch which ruptured into the pericardium [24] [25] . J. P. Maunoir, a French physician introduced the term “aortic dissection” in 1802. Thomas Bevill Peacock, a London cardiologist reported two series as 19 cases in 1843 and 80 cases in 1863 in the literature [26] . In 1924, Boyd reviewed 4000 autopsy reports of thoracic aortic aneurysms, finding 1197 cases of rupture with a 4% occurrence of aortopulmonary fistula [27] . In 1970, Bory and associates first reported an aortopulmonary fistula in a young woman [28] . In 1972, McCarthy and associates described an acute aortic dissection spreading through the ductus arteriosus to the pulmonary artery [29] . In 1995, Spier and Associates [30] described a unique case of rupture into both right ventricle and pulmonary artery with good outcome. In 1998, Atay and associates [31] reported a case of aortopulmonary fistula in a 73-year-old man. In 1999, a fistula was found between the false lumen and main pulmonary artery in a man underwent surgery for type A aortic dissection [32] .

Aorta plays an important role in modulating left ventricular performance and arterial function throughout the entire cardiovascular system [33] . An increase in distending pressure during systole induces an increase in aortic dimension, which is directly related to the elastic properties of aorta. Aorta expands at a rate of 0.9 mm in men and 0.7 mm in women for each decade of life and a diameter larger than 6 cm are definitely more prone to rupture than smaller ones [34] . The ascending part of the aorta bears the largest pressure during the cardiac cycle [35] and its exposure to elevated pressures causes a tear in the intimal lining at the sites of greatest wall stress and results in dissection which propagates either anterograde or retrograde directions along the arterial wall. This entrance tear is the initiating event and blood enters into the media and splits the aortic lumen to form a double-channel aorta with a true and false lumen. The intima and the inner part of the media form the intimomedial flap which separates the false lumen from the true lumen and is composed mainly of aortic media delaminated from the aortic wall [36] . When blood flows into the false lumen, it may cause secondary tears in intima, blood can reenter the true lumen and creating additional communication between the true and false lumens in the distal aorta. The true lumen is usually small with high-velocity flow whereas the false lumen is larger with slower velocity and turtbulent flow.

In autopsy series, as many as 13% of aortic dissections do not have an identifiable intimal tear [37] . In a minority of cases, the initiating event is the medial hemorrhage due to rupture of vasa vasorum. The intramural hematoma weakens the aortic wall and may progress either to outward rupture of the aortic wall in 35% of cases or to inward disruption of the intima, leading to communicating dissection in 3% to 5% of cases [38] . In penetrating aortic ulcer, an atheromatous plaque burrowing deeply through the intima into the media [39] , the media is exposed to pulsatile arterial flow, which causes hemorrhage into the walls that then leads to intramural hematoma [40] and adventitial erosion may cause rupture in 42% of cases [41] . These nonflap lesions are more common in the descending aorta especially in elderly hypertensive individuals [42] . Spontaneous resolution of an aortic intramural hematoma also has been reported [43] . The noncommunicating dissection is characterized by no entry tear and no flow in the false lumen, the false lumen is more often filled with thrombus and the differentiation from intramural hematoma may be difficult [44] . In the Japanese Gui- delines for thoracic aortic dissection 2011, it is defined the non-communicating dissection as no communication between the false lumen and true lumen [45] .

Von Kodolitsch et al. [46] identified the independent predictors of aortic dissection in a study of 200 patients. Aortic dissection is also classified according to its duration, defined as length of time from symptom onset to medical evaluation as acute (<2 weeks), subacute (>2 weeks and chronic (>6 weeks after the onset of pain)). The characteristic symptom of tearing pain in the chest was recognized in 1855 when a case was diagnosed in life. Pain is the most common symptom of acute aortic dissection in 96% of cases and those without pain are found to have chronic dissection [47] . The pain could be sharp (64%), tearing or ripping type (51%) or knife-like and its abruptness is the most specific characteristic feature. Pain is severe, unbearable at its inception and forces the patient to writhe in agony, fall to the ground or pace restlessly in an attempt to gain relief and has a tendency to migrate along the path of dissection in 17% of cases. The pain was located in anterior chest in >90% of cases of ascending aortic dissection, in interscapular area (36%) and back (53%) in descending thoracic aortic dissection, in abdomen and lower extremities (30%) when the dissection extending distally into the abdominal aorta . The initial pain of aortic dissection may be followed by a pain-free interval lasting from hours to days and then pain returns, is an ominous sign of impending rupture [48] . About 4% of acute dissections may be painless especially in diabetes and older individuals, making a delay in diagnosis [49] . Pulse deficits were reported in <20% of patients with aortic dissection (50% in proximal and 15% in distal dissections). It includes a significant difference in pulse volume (pulse differentials) and blood pressure (BP differentials) in two upper extremities due to partial compression of one or both subclavian arteries or a sudden loss of pulse with abrupt onset of chest pain raise a high suspicion of aortic dissection. The difference of >20 mmHg of systolic blood pressure may be found in 10% to 20% of unaffected individuals [50] . The presence of pulse differential is the most specific physical sign of aortic dissection in 38% of cases, may be transient, secondary to decompression of the false lumen by distal reentry into the true lumen or secondary to the movement of the intimal flap away from the occluded orifice. When a false lumen communicates with the true lumen, the duplication of pulse due to difference in flow rates in the true and false channels occurs and it is a rare physical finding of aortic dissection [51] .

Myocardial infarction occurs in 1% - 2% of cases of aortic dissection secondary to the compression of coronary artery by the expansion of false lumen or the extension of dissection process. Involvement of right coronary artery is more common than the left one and, occasionally dissection and myocardial infarction occur concomitantly [52] .

Aortic regurgitation accompanies in 18% to 50% of cases of proximal aortic dissection [53] . The mechanisms include dilatation of aortic root, annular dilatation, tearing of annulus or valve cusps, rupture of the annular support of the leaflets as shown in Figure 8, cusp prolapse due to pressure from asymmetric false lumen and physical interference of valve closure by intimal flap (intimal flap prolapse). In Marfan’s syndrome, the aortic dilatation is confined to sinuses, effacement of sinotubular junction (failure to narrow at sinotubular junction) results in malcoaptation of leaflets and subsequent aortic regurgitation [54] .

The risk of fatal rupture in patients with untreated proximal aortic (the aortic root and ascending portion) dissection is around 90%, and 70% of ruptures occur into pericardium, pleural cavity and mediastinum [55] leading to cardiac tamponade, left sided pleural effusion, hypotension and shock. Rare cases have been described in which the dissection eroded into the pulmonary artery and producing aorto-pulmonary fistula as in Figure 7 [56] . It is important to rule out the presence of pseudohypotension due to obstruction of an aortic arch branch in aortoarteritis.

Raised concentrations of smooth muscle myosin heavy chain (a protein that released from damaged aortic medial smooth muscle) at higher levels (51 ± 52.3 µg/L) within 3 hours of symptoms) than in acute myocardial infarction (2.1 ± 1.6 µg/L) occurs in aortic dissection [57] and it is elevated (>2.5 µg/L) from the normal level (0.9 ± 0.4 µg/L) in healthy individuals within 6 hours of the onset of dissection. It may become a useful initial step in triaging the patients with suspected aortic dissection provided that the patients present within 6 hours and

preferably within 3 hours. Its level is higher in proximal dissection than the distal and levels >10 µg/L shows 100% specificity for aortic dissection within 30 mi- nutes of the assay. A blood D-dimer level <500 ng/ml may rule out aortic dissection [58] , but the AHA (American Heart Association) is not advising this test as evidence is still tentative [59] .

Radiologically, mediastinal widening (bulge to the right with ascending dissection and to the left with descending dissection), localized hump on the aortic arch with widening of aortic knob distal to left subclavian artery and the separation of intimal calcification from the outer aortic soft tissue border by 10 mm, a finding on chest X-ray is called as “calcium sign” are the characteristic features of aortic dissection. About 12% to 20% of aortic dissections had normal chest X-rays as shown in Figure 3.

It is important to prevent misdiagnosis of thoracic aortic dissection. About 20% of patients with type A dissection had ECG evidence of acute ischemia or acute myocardial infarction [60] and erroneous administration of thrombolytic therapy may be unavoidable and hazardous. A compilation of reminders, symptoms, and risk factors called as Ritter rules (named for the actor John Ritter who died due to thoracic aortic dissection in September 11, 2003 which was misdiagnosed and treated as acute myocardial infarction) was formulated to predict the aortic dissection [61] . Aortopulmonary fistula should be suspected in a patient who had aneurysmal dilatation of aortic root and exhibits signs of heart failure.

Two-dimensional echocardiography is currently considered as the first non- invasive technique when aortic dissection is clinically suspected in an emergency room [62] and it has a sensitivity of 78% to 100% to diagnose type A dissection [63] . A dissection may extends weeks to months after its onset [64] and the tear is defined as a disruption of the flap continuity as shown in Figure 4 with fluttering of the ruptured intimal borders [65] . The visualization of an intimal flap that separates the true and false lumens within the aorta is pathognomonic of aortic dissection [66] as shown in Figure 4 and Figure 5. The two-dimensional echocardiography appears to be superior [67] in the recognition of an intimal flap and its utility is limited in visualization of a small circumscript dissection segment in the distal part of ascending aorta and anterior part of the arch, known as “blind spot” due to interposition of trachea and left main stem bronchus between the esophagus and the aortic arch. Two echocardiographic criteria for the diagnosis of aortic dissection are:

a) Dilatation of one segment of the aorta.

b) An abnormal linear intraluminal echo corresponding to the intimal flap [68] [69] .

There are 3 types of intimal flap were identified as shown in Table 5.

In some cases, a linear echo criss-crossing or dividing the aorta perpendicularly is highly diagnostic of an intimal flap as shown in short axis image in Figure 5. If a thrombus is detected, the intimal flap is immobile and an aortic wall thickness >15 mm suggests dissection with thrombus in the false lumen. The intimal tear is usually >5 mm, located at the level of ascending aorta in 60% to 65% of cases and this segment is more prone to rupture. The oscillating intimo-intimal flap always indicate an acute dissection without thrombosis of false lumen [70] and a parallel wall motion with absence of oscillating flap indicates a chronic dissection [71] . The intimal flap, an anatomical marker of identifying the false lumen may appear as “cobweb” or “beak” appearance as shown in Figure 1 and Figure 9 [72] . In acute dissection, the false lumen is more often larger and it is smaller in chronic dissections. The intimal flap is usually thick, may be calcified, less mobile and fibrosed in chronic dissection as shown in Figure 4 and Figure 9. In rare instances, fistulous formation between the aorta and pulmonary artery may complicate dissection due to rupture as shown in Figure 6, Figure 7, Figure 10, and Figure 11. A disturbed diastolic flow signal was detected in the left ventricular outflow tract as aortic regurgitation as in Figure 1, Figure 8 and Figure 12 due to tear of annular support of leaflets at the site of rupture and this leak between aorta and pulmonary artery creating an acquired aortopulmonary fistula in this patient as shown in Figure 6 and Figure 7.

A dilated pear-shaped aortic root as in Figure 1 and Figure 4 with regional wall motion abnormalities, particularly the inferior hypokinesis as shown in Figure 13 due to right coronary occlusion is suggestive of acute type A aortic dissection with a dissection flap and small false lumen in the supero-medial aspect of the aortic root as in Figure 5 in the short axis image in an already existing chronic disease as in Figure 4, leading to contractile dysfunction (EF 40%) as shown in Figure 13. The aortic arch and descending aorta appeared normal as shown in Figure 14.

Therapy for aortic dissection is directed to halt the progress of dissection process since lethal complications may arise from the the intimal tear and its subsequent course may result in vascular compromise or aortic rupture [74] .

Since the arterial dP/dt reflects the force of LV ejection and it is the major stress acting on the aortic wall that contributes to both genesis and subsequent propagation of dissection, its diminution and reduction of systolic blood pressure are the two primary goals of pharmacological therapy. Acute dissection generally pre- sents as a hypertensive emergency and the prime consideration of medical management is to maintain the mean arterial pressure around 60 to 75 mmHg and the systolic blood pressure to be reduced to values between 100 and 120 mmHg with an initial decrease should be at 20% [75] . Historically, Wheat et al. [76] first introduced the aggressive drug therapy for aortic dissection by originally using reserpine and guanethidine. A combination of beta blocker and a vasodilator (sodium nitroprusside) remains as a standard medical therapy recently. To reduce the dP/dt, an intravenous beta blocker, preferably propranolol 1 mg every 3 to 5 minutes (0.15 mg/kg) until a satisfactory beta blockade (heart rate 60 to 80 beats/minute) up to a total dose of 2 to 6 mg and then at 4 to 6 hours interval to maintain the adequate heat rate. In patients with labile arterial pressure or obstructive pulmonary disease, an ultra-short-acting beta blocker esmolol 500 mcg/kg IV bolus, followed by continuous infusion at a rate of 50 mcg/kg/minute and titrated up to 200 mcg/kg/minute is useful especially if surgery is to be planned. Labetalol, an alpha and beta adrenergic receptor antagonist is an alternative drug to lower both dP/dt and arterial pressure. It is given in an initial dose 20 mg IV over 2 minutes period, followed by 40 to 80 mg every 10 to 15 minutes until the heart rate and blood pressure have been controlled and then a maintenance dose of 2 mg/minute.

The most appropriate drug to provide pain relief is an opiate, the IV morphine sulphate. It reduces the force of cardiac contraction and the rate of rise of aortic pressure (dP/dt), retards propagation of dissection; delays rupture and also prevent exacerbation of tachycardia and hypertension. AHA (American Heart Association) guidelines recommend beta blocker as a first-line treatment. Vasodilators should be given if systolic blood pressure >120 mmHg after beta blocker therapy and pain control. Appropriate rate control medications are mandatory before the initiation of vasodilator therapy as they induce reflex tachycardia and worsen the stress on the arterial wall. Calcium channel blockers and angiotensin converting enzyme (ACE) inhibitors are the acceptable alternatives in patients with contraindications to beta blockers. In refractory hypertension (persistent hypertension on the maximum dose of three different classes of antihypertensive drugs), an involvement of renal arteries with release of large amount of renin should be considered. In this setting, the most effective agent is IV ACE inhibitor, the enalaprilat given in an initial dose of 0.625 mg every 4 to 6 hours to control the blood pressure. Sublingual nifedipine is also successful to treat refractory hypertension associated with aortic dissection [77] . The efficacy of trimethaphan, a ganglionic blocker and a direct vasodilator is less predictable and it may cause tachyphylaxis, severe hypotension, urinary retention, and ileus. Hydralazine and minoxidil should be avoided since they produce a hyperdynamic response that would increase the dP/dt.

The potential problems encountered during medical therapy could be extension of dissection, aortic rupture and organ malperfusion resulting in hypotension and shock. In such cases attempts must be done to stabilize the patient with medical therapy which includes volume expansion with crystalloids and vasopressors (norepinephrine and phenylephrine) to titrate the mean arterial pressure at 70 mmHg. The extreme aggressive attempts at doing so may result in further propagation of false lumen [78] with adverse outcome, the aortic insufficiency and pericardial tamponade since both conditions are preload-dependent [79] . Therefore, treatment of shock associated with aortic dissection should be managed by rapid surgical intervention.

If coronary insufficiency occurs, nitrates may be used, but therapy with thrombolytic agents and aspirin should be avoided.

The principal aim of cardiac surgery in type A aortic dissection is to obliterate the entry tear in the ascending aorta, thereby preventing flow into the false lumen, encouraging thrombosis of the false lumen and healing of the aortic dissection [80] . Definitive surgical therapy was first developed by Michael E. DeBakey, Denton Cooley, and Oscar Creech, the cardiac surgeons associated with the Bay- lor college of medicine, Houston, Texas in 1954 [81] . DeBakey developed aortic dissection himself at the age of 97 in 2005 and underwent surgery in 2006 [82] . The fenestration procedure is the first surgical approach to aortic dissection in which the dissected aorta was incised and a distal communication was created between the true and false channels thereby decompress the false lumen. This procedure is used in selected cases of dissection involving the descending aorta to relieve the renal or mesenteric or limb ischemia [83] .

When the proximal aorta is fragile or badly torn or entire root is aneurysmatic, Bentall procedure, a radical approach is used mostly, in which a composite prosthetic graft (aortic valve plus ascending aorta tubular graft, i.e., a prosthetic aortic valve sewn on to the end of a Dacron tube graft) facilitates replacement of both ascending aorta and aortic valve together and coronary arteries are reimplanted as buttons of aortic tissue into the graft wall [84] [85] . Bypass grafting of coronary arteries using saphenous vein segments is to be done when the coronary ostia were torn by the dissection.

The first successful repair of aortopulmonary fistula was reported by Giacobine and Cooley in 1960 [86] . Closure of the fistula results in immediate hemodynamic and symptomatic relief.

Endovascular stent grafts are not indicated for repair of the ascending aorta or aortic arch and it is a standard treatment in most cases of distal aortic dissection. The fenestration/stent treatment could be used in unstable proximal dissection as a palliative treatment in high surgical risk patients. There are two endovascular techniques used to manage the acute vascular complications secondary to aortic dissection as follows.

a) Percutaneous balloon fenestration of the intimal flap which involves crossing an intact intimal flap with a wire, passing a balloon-tipped catheter over the wire, and then expanding the balloon to tear a hole in the intimal flap which allows the blood to flow from the false lumen into the true lumen, thereby decom- press the distended false lumen.

b) Percutaneous stenting of the affected arterial branch whose flow has been compromised by the dissection process, in which a stent is deployed within the lumen supplying the branch vessel to hold the lumen, opens by displacing the intimal flap towards and overcoming the pressure from the other lumen by a balloon fenestration procedure.

Percutaneous aortic graft stenting are currently available and shows better results than surgical graft stenting [87] . Aortic stenting is primarily used to supplement fenestration as well as to reduce the dissection flap. In high grade compression of true lumen supplying to major aortic branch, stenting may play a role to enlarge the compressed lumen and to increase the distal flow. An average stent grafts are 3.5 cm (2.4 - 4.5 cm) in diameter and 10.5 cm (preferably <15 cm) long. After deployment of a stent, a balloon incorporated in the system, allows further apposition of the stent struts to the wall of the true lumen and to occlude the intimal tear rather than wrapping of the intimal flaps to the wall of the false lumen. Attempting to eliminate tears and reentries starting at the beginning of the intervention may improve the prognosis. Use of candy-plug technique with an excluder aortic extender for distal occlusion of a large false lumen aneurysms in chronic aortic dissection and an Amplatzer vascular plug II as a sealing button to obliterate the false lumen have been reported recently [88] , [89] . “Tear-oriented surgery” for acute Type A dissection and open stent-graft- ing (OSG-Frozen®, Japan, Life line, Tokyo) or frozen elephant trunk (FET) [90] involves substituting “distal anastomosis to the descending aortic suture by fixation with the stent-graft” in aortic arch surgery was launched in Japanese patients in 2014 [91] .

The IRAD (International Registry of Acute Aortic Dissection) study reported mor- tality rate of 27% for type A aortic dissection in 464 patients undergone surgical therapy [92] and in experienced centers, the long-term survival rate was >90%. The Japanese association of thoracic surgery reported overall hospital mortality was 9.1% in 2013 in a study of 4444 patients with acute type A aortic dissection who underwent surgical correction.

The accepted critical aortic root diameter of 5 - 5.5 cm at any age even in infancy, the prophylactic replacement of aortic root should be performed at a diameter of 5.5 cm and this general agreement is not applicable to other segments of the aorta. All patients should receive lifelong antihypertensive medications including beta blockers, the blood pressure should not exceed 130 mmHg systolic for both children and adults [93] and to be titrated to <120/80 mmHg.