Aortic Arch Aneurysm

Etiology

Atherosclerosis or plaque build-up is the predominant etiology for an isolated aneurysm of the aortic arch as well as for those associated with descending aorta. The aneurysms found in relation to ascending thoracic aorta often result from cystic medial degeneration. The causes include Marfan syndrome, Loeys–Dietz syndrome, Ehlers– Danlos syndrome, Turner syndrome, familial TAA syndrome, and Behcet disease.[5] Deceleration injuries have been seen to cause dilation of the segment just after the aortic arch.[6][1]

Both infectious as well as non-infectious inflammatory conditions of the aorta, or aortitis can also result in TAA. These include syphilis, giant cell arteritis, and Takayasu arteritis.[6]

Along with the risk factors for atherosclerosis such as smoking, hypertension, and hypercholesterolemia, factors that increase aortic wall stress, including pheochromocytoma, cocaine use, coarctation, weight lifting, also increase the likelihood for development of TAA.[7]

Epidemiology

The true incidence and prevalence of aortic aneurysms are difficult to determine as most of them are often asymptomatic, with a large number of cases being diagnosed incidentally.[8] Thoracic aortic aneurysms (TAAs) have an estimated incidence of around 10 cases per 100 000 person-years. Aneurysms involving the aortic arch constitute a minor proportion of TAAs, accounting for about 10% of the total cases of thoracic aorta aneurysms.[9][3]

Males are two to four times more commonly affected than females, with the majority of patients in the sixth and seventh decade of life.[7] With the improvement in screening and advancement in imaging techniques, it is difficult to decipher whether the apparent increase in the number of patients with aortic arch aneurysms is a result of increased detection or due to a rise in the incidence of the disease.[10]

Pathophysiology

The literature on the pathophysiology of aortic aneurysm is continuously evolving and reflects an interplay of protein degeneration, thrombosis, hemodynamic stress, and inflammatory cytokines contributed by genetic triggers and developmental risk factors.

Major theories on the development of aneurysm have surfaced from in-vitro studies that emphasize the role of the breakdown of extracellular matrix proteins by proteolytic enzymes such as elastase, collagenase, plasmin, and matrix metalloproteinases (MMPs).[11] Endothelium, smooth muscle cells, and inflammatory cells infiltrating the media and adventitia act as the source for these proteases.[12] The resultant medial degeneration leads to the weakening of the aortic wall, which in turn results in aortic dilatation and aneurysm formation.

As most of the cases of an isolated aortic arch aneurysm are seen in relation to underlying atherosclerosis, lab findings suggest that inflammatory cells present within the atheroma accelerate the breakdown of matrix protein by the release of the inflammatory mediators.[10] Also, the narrowing of the arterial lumen by an atherosclerotic thrombus triggers a compensatory process in the media in response to the shear stress on the wall. This matrix remodeling promotes the expansion of the vessel in an attempt to normalize diameter and hemodynamic forces. Along with the mechanical weakening, atherosclerosis may also induce aneurysm formation by degenerative ischemic changes, through the obstruction of the vasa vasorum.

Independent risk factors for both atherosclerosis and aneurysm, including hypertension and smoking, exert a direct effect on the vasculature, adding to the multifactorial pathophysiology of aneurysm formation.

Histopathology

Histopathology of aortic aneurysm described as cystic medial degeneration is characterized by disintegration and loss of elastic fibers with an increase in the deposition of proteoglycans. Loss of smooth muscle cells in tunica media is typically seen.[3][1] Increased penetration of vasa vasorum into the medial layer was reported.[13] As atherosclerosis has been found commonly in relation to the aortic arch aneurysm, histological data also depicts the evidence of the presence of atheroma or fibro-fatty plaque in the lesion. However, these changes have been seen superimposed on the degenerative medical disease.[14]

History and Physical

The majority of patients with an aortic arch aneurysm are asymptomatic or directly presents with life-threatening complications. Aneurysms are typically discovered incidentally on imaging ordered for other indications. Vague chest discomfort, neck, and jaw pain may occur with aneurysms involving the arch. A large-sized aneurysm can impinge upon the adjacent anatomical structures and exert a local mass effect including hoarseness, from left recurrent laryngeal nerve stretching; stridor, from tracheal or bronchial compression; cough, dyspnea, and recurrent pneumonitis, from lung compression; dysphagia, from oesophageal compression; and plethora and edema, from the compression of the superior vena cava.[2]

The most serious consequence of TAA is aortic dissection or aortic rupture, which present as tearing, chest pain, and hypotension, respectively.[6]

Other less frequent manifestations include systemic embolization as a result of atheroembolism and gastrointestinal hemorrhage secondary to the formation of an aortoesophageal fistula.

Evaluation

As described earlier, thoracic aortic aneurysms are commonly suspected on chest radiographic films when done for an unrelated indication and are characterized by widening of the mediastinal silhouette, abnormalities of aortic contour, or tracheal deviation.[15] However, chest X-rays are not sensitive to exclude the presence of aortic aneurysms and are inadequate to differentiate between an aneurysm and conditions like mediastinal mass, torturous aorta, or aortic dissection with similar radiographic features and require definitive aortic imaging.

Computed tomography angiography (CTA) or magnetic resonance angiography (MRA) are imaging modalities of choice for accurate detection and measurement of TAA. It is preferred to delineate aortic anatomy, size, branch artery involvement, and to rule out other differentials.[2] These can be of disadvantage to patients with decreased kidney function as CTA requires the administration of intravenous contrast dye, and MRA usually requires the use of gadolinium.[16]

Echocardiography can also be used to visualize the aorta and its major branches. The suprasternal view is best for viewing the aortic arch, whereas orthogonal and longitudinal scanning plans are beneficial when the involvement of the innominate artery, left subclavian artery or left common carotid artery is also suspected.[16] Coronary angiography and echocardiography are also performed as part of usual preoperative investigations done in order to determine the need for a concomitant cardiac procedure. Also, as maintaining brain perfusion is critical while carrying out surgeries involving the aortic arch, carotid duplex scanning is also routinely instituted in order to access for carotid stenosis.[7]

If TAA is detected, it is recommended to image the abdominal aorta to screen for an abdominal aortic aneurysm.

Treatment / Management

Medical Management

The primary step in the management of aortic arch aneurysm is to administer medical therapy directed towards the control of risk factors for atherosclerosis, to slow the rate of expansion, and to lower the likelihood of development of the complications, including dissection or rupture.[17][18](B3)

2010 AHA guidelines on medical treatment of patients with thoracic aortic disease recommend that stringent control of hypertension, the optimization of lipid profile, smoking cessation, and other atherosclerosis risk-reduction measures should be instituted for patients with small aneurysms as well as for patients who are not considered to be surgical candidates (Class 1 recommendation, level of evidence: C).[3](B3)

Other important strategies of conservative treatment include patient education regarding the signs and symptoms indicating the development of complications, serial imaging of aneurysm to evaluate for expansion; screening for aneurysms at other locations including non-contagious aortic segments, and counseling for those suspected of having an associated genetic disorder.

Guidelines on surveillance imaging suggest that for isolated aortic arch aneurysms less than 4.0 cm in diameter, it is reasonable to re-image using CT or MRI, at 12-month intervals, to detect enlargement of the aneurysm (class of recommendation: 2a, level of evidence: C). Whereas for patients with isolated aortic arch aneurysms of 4.0 cm or greater in diameter, it is reasonable to re-image using computed tomographic imaging or magnetic resonance imaging, at 6-month intervals. (Class of recommendation: 2a, level of evidence: C).[3](B3)

Surgical Management

Surgical intervention for the aneurysms of aortic arch raises a particular concern due to higher rates of mortality, need for creating a bloodless field with circulatory arrest as well as challenges in maintaining perfusion to the head, neck, and upper extremities.[19] The risk of a surgical procedure is weighted against hazards of aortic rupture while formulating a decision on when and if to operate.

Criteria for the Candidacy of Operative Management

The 2014 ESC guidelines on the diagnosis and treatment of aortic diseases recommend that surgery should be considered in patients who have an isolated arch aneurysm with a maximal diameter of more than 5.5 cm (class of recommendation: 2b, level of evidence: C). Aortic arch repair may be considered in patients with aortic arch aneurysm who are already going to have surgery of an adjacent aneurysm located in the ascending or descending aorta (class of recommendation: 2b, level of evidence: C).[16] The 2010 ACCF/AHA/AATS/ACR/ASA/SCA/SCAI/SIR/STS/SVM guidelines also reveal similar recommendation that for patients with low operative risk having isolated degenerative or atherosclerotic aneurysm of the aortic arch, operative treatment is reasonable for asymptomatic patients when the diameter of the arch exceeds 5.5 cm (class of recommendation: 2a, level of evidence: B). In a nutshell, surgical treatment is often recommended for all symptomatic patients, patients with aneurysm size more than 5.5 cm, and any patient of TAA with a growth rate exceeding 0.5 cm per year.[11](A1)

Advancements in open surgical techniques, safer anesthetic practices, and improvised methods of maintaining cerebral perfusion have revamped the most widely used conventional open repair procedure. The newer endovascular and hybrid modalities have also started to emerge. These newer technologies have made surgical treatment possible even for a larger spectrum of patients, including those at high risk and with multiple comorbidities. Each of these techniques has its own merits and demerits, and the choice is tailored considering peri-operative risks, comorbidities, and anatomy of the lesion.

According to the 2010 ACCF/AHA/AATS/ACR/ASA/ SCA/SCAI/SIR/STS/SVM guidelines for the diagnosis and management of patients with thoracic aortic disease, class 2a recommendations for open surgical repair of asymptomatic patients with aortic arch aneurysms are:[2][19](B3)

• For thoracic aortic aneurysms also involving the proximal aortic arch, partial replacement of the arch together with the repair of the ascending aorta using right subclavian/axillary artery inflow and hypothermic circulatory arrest is reasonable. (level of evidence: B)
• Replacement of the entire arch is reasonable for acute dissections when the arch is aneurysmal, or there is excessive aortic arch destruction and leakage. (level of evidence: B)
• Replacement of the entire aortic arch is reasonable for aneurysms of the whole arch, for chronic dissection when the arch is dilated, and for distal arch aneurysms that also involve the proximal descending thoracic aorta, usually with the elephant trunk procedure. (level of evidence: B)

Open arch replacement usually employs the use of a classical two-staged elephant trunk (ET) or a frozen elephant trunk (FET) procedure wherein the first stage is an open repair of the ascending aorta and aortic arch along with the placement of the trunk followed by a second stage completion surgery which involves the intervention of the descending aorta via either open or the endovascular approach using thoracic endovascular stent graft (TEVAR). Deployment of TEVAR is done in a retrograde fashion in the setting of ET, whereas ante-grade deployment through a reconstructed transverse arch is achieved in a FET procedure. Island patch reconstruction or branch graft is often used to anastomose supra-aortic branches, and brain protection during the surgery is provided by a deep hypothermic circulatory arrest (HCA) or with newer strategies including antegrade and retrograde cerebral perfusion.[20]

Hybrid procedure renders a less invasive intervention and also often obviates the need for hypothermic circulatory arrest (HCA) and cardiopulmonary bypass (CPB) hence making the surgical treatment of aneurysms acceptable to a larger subset of a patient population with comorbid conditions who otherwise would not tolerate the technical demands of open repair under HCA. As described above, TEVAR can be integrated with conventional open repair as a safe alternative to open thoracotomy to complete distal aneurysm repair or is combined with open debranching of cerebral vessels.[21] 

Supra-aortic arch debranching involves bypass to all three arch branches from the ascending aorta followed by TEVAR, including the arch.[22] This can be achieved by sequential anastomosis of affected arch vessels to the dacron graft. In some patients with favorable anatomy, the hybrid procedure is carried out by construction of the extra-anatomic bypass, either carotid-carotid or carotid subclavian, to allow an endograft to cover the origin of the carotid artery. To accommodate TEVAR, the landing zone is created in the non-aneurysmal segment of the aorta having appropriate dimensions.[11] (B3)

With increasing competence in endovascular procedures, total endovascular repair surgeries of the aortic arch with the use of advanced generations of branched or fenestrated endografts or with chimney technique have been tried at some centers with technical success and low-perioperative morbidity and mortality; however, randomized multicenter trials and data on long term effectiveness and durability are still lacking.[22][23][24][25]

Differential Diagnosis

Other aortic arch conditions causing dilation of the vessel segment, including pseudo-aneurysm, intramural hematoma, and aortic dissection, can manifest with similar symptoms to those of aortic arch aneurysm. Pseudoaneurysms usually develop secondary to deceleration injury or torsional trauma from accidents and falls. The chest pain from aortic dissection is usually very severe and described as tearing and sharp in nature.[2]

These aortic lesions, along with some other conditions including mediastinal mass, torturous aorta, and senile ectasia, mimic the plain radiographic findings of the thoracic aneurysm, including widened mediastinal silhouette and altered aortic profile and hence necessitates the need for confirmatory diagnostic imaging studies like angiography to rule out these disorders.

Pertinent Studies and Ongoing Trials

The study reviewed a number of randomized clinical trials (RCTs) of losartan in Marfan syndrome - a monogenic disorder that predisposes to the thoracic aortic disease.[26]

Prognosis

The prognosis of the aortic arch aneurysm largely depends on the size of the lesion as the aneurysm size and rate of expansion are found to be the most significant predictors of the rupture. Prognosis is usually good if timely intervention is instituted before the rupture, which can have mortality as high as 80%, as observed in some studies.[19][27][28]

Complications

Aortic arch aneurysms can lead to life-threatening cardiovascular and neurological complications, including aortic rupture most often into the left chest or pericardium, presenting as severe chest pain and hypotension or shock; aortic dissection, and atheroembolism causing ischemic strokes. Some rare complications of the formation of aorto-esophageal or aorto-bronchial fistula have also been documented.[29]

Embolization during the placement of the graft leading to stroke or spinal ischemia from obstruction of spinal arteries, endoleaks, and hematoma formation are also some of the well known and concerning peri-operative complications associated with the aortic repair.[9]