Porcelain Aorta

Etiology

Porcelain aorta is associated with several disease processes, including chronic kidney disease, mediastinal radiation, chronic systemic inflammatory diseases such as Takayasu arteritis, systemic lupus erythematosus, and rheumatoid arthritis. Multiple risk factors have been deemed responsible in PA in literature reviews. The factors commonly involved in both types are a familial factor, aging, diabetes mellitus, radiation, uremia, and Takayasu arteritis. The atherosclerotic type is associated with additional factors such as hypertension, hyperlipidemia, smoking, alcoholism, sedentary lifestyle, obesity, systemic lupus erythematosus, and rheumatoid arthritis.[1]

Epidemiology

Porcelain aorta is a relatively rare entity in the general population, but recently it has been increasingly recognized in patients 60 years and older, particularly those with coronary artery disease, chronic renal disease, and calcific aortic stenosis (AS). The reported incidence rate ranges from 1% to 20% based on different studies. Leyh et al. found that 23 out of 1861 (1.2%) patients who underwent coronary artery bypass grafting (CABG) had porcelain aorta.[4] PA was also detected in 15.1% (54 of 358) of patients enrolled in an inoperable cohort of PARTNER (placement of aortic transcatheter valves) trial.[5] PA was present in 18% (61 of 339) patients undergoing transcatheter aortic valve replacement (TAVR).[6] The FRANCE-2 TAVI registry and German TAVI registry published porcelain aorta incidence rates of 5% and 11%.[7][8] Faggiano et al. also evaluated 240 patients for AS and noted that 7.5% (18/240) had porcelain aorta.[9]

Several studies have reported discordant results for gender differences in the prevalence of porcelain aorta. The evaluation of PA among the heterogenic population has revealed some sex distribution discrepancies, showing a male predominance in some[10][11] and a female predominance in others.[12][13][14] Moreover, a series of patients with PA who underwent transaortic valve replacement (TAVR) for severe aortic stenosis demonstrated a female predominance (52.8% to 75.4%).[6][8][15] On the other hand, there is a male predominance in porcelain aorta prevalence among patients who underwent coronary revascularization surgery.[16]

Pathophysiology

The exact cause of porcelain aorta is still elusive, but two pathophysiological processes have been described in the pathogenesis of this condition. These mechanisms allowed us to divide porcelain aorta into the following two entities:

Atherosclerotic Porcelain Aorta

It is characterized by calcification of tunica intima of the aortic wall due to atherosclerotic plaques development. These atheromatous lesions are induced by an inflammatory endothelial injury that activates the endothelial cells to express adhesion molecules that attract inflammatory cells, mainly monocytes and T lymphocytes, into the subendothelial intima. These monocytes undergo a specific differentiation into macrophages, which phagocytose the modified lipoproteins producing foam macrophages. The activated macrophages secrete cytokines to recruit more inflammatory cells and growth factors that induce the proliferation of inflammatory cells and vascular smooth muscle cells (VSMCs) and endothelial cells. In turn, these factors stimulate the migration of VSMCs from the tunica media into the tunica intima.

The intimal VSMCs proliferate and ingest lipoproteins forming lipid-laden VSMCs. The layers of foam macrophages and lipid-laden VSMCs are collectively known as fatty streaks, which are recognized as an early sign of stable atherosclerosis. The advanced, stable atherosclerosis is characterized by further enlargement of fatty streaks and secretion of extracellular matrix proteins by VSMCs, forming a fibrous cap covering the lesion. This matrix is followed by apoptosis of foam macrophages, leading to necrosis of VSMCs and degradation of extracellular matrix proteins, thus producing a necrotic core associated with the advanced unstable lesion. The destabilization of atheromatic plaques increases the liability of rupture, resulting in platelet accumulation at the site of rupture and the formation of atherothrombosis that occludes the artery.[1]

Nonatherosclerotic Porcelain Aorta

In this type, calcification occurs in the tunica media of the aortic wall in the absence of atherosclerosis. It is induced by vascular inflammation, radiation, and uremia via triggering a metaplastic transformation of VSMCs into osteoblasts leading to the production of bone-associated proteins such as alkaline phosphatase, bone sialoprotein, bone GLa protein, and bone morphogenic protein 2. In cases of vascular inflammation and aging, apoptotic vesicles arising from dead VSMCs, and elastin degradation mediated by matrix metalloprotease will act as a nidus for medial calcification by sending a paracrine osteogenic signal that will provoke the aortic calcium deposition as well as the collagen deposition and loss of the elastic fibers causing an arterial wall stiffness.[1][17]

History and Physical

Pertinent aspects of history and physical examination in patients with porcelain aorta are related to underlying medical conditions and risk factors.

Evaluation

Porcelain aorta is an asymptomatic condition that usually appears as an incidental finding in patients evaluated for cardiovascular or pulmonary diseases.[1] It is commonly identified on chest radiographs or computerized tomographic scans done for other indications.

Different diagnostic tools have been suggested to diagnose porcelain aorta and evaluate its extent and exact location before any cardiac intervention. The most accurate modality is a multislice computed tomography (MSCT). It is used to accurately diagnose porcelain aorta and give valuable information about the exact level and site of aortic calcification, thereby distinguishing PA (circumferential calcification) from a less extensive aortic calcification. Furthermore, a 3-dimensional (3D), volume-rendered reconstructions of MSCT images can be done to allow 3D mapping of the aortic wall, thus providing us with useful information about the three-dimensional distribution of the calcification.[1][2][18][19][20]

Other modalities include:

• A chest X-ray may occasionally reveal a calcification in the thoracic aorta, but it is not accurate in defining PA.
• Fluoroscopy during coronary angiography is somehow sensitive in detecting an aortic calcification that suggests PA; however, it lacks the precision in assessing the distribution and localization of this calcification.

On the other hand, PA is recognizable during the cardiac surgery by performing an epiaortic echocardiographic scanning of the aorta in conjunction with manual palpation after sternotomy and exposure of the aorta. Such testing confirms the presence of PA, as well as its extent and exact location.[1][2][18][19][20]

Pre-operative identification of porcelain aorta is important because the procedural plan often changes if PA is discovered. When porcelain aorta is suspected based on incidental findings on other imaging modalities, pre-procedural computed tomography can evaluate PA.

Treatment / Management

During any cardiac procedure, the need for advanced surgical techniques in patients with concomitant porcelain aorta (PA) is mandatory to avoid manipulating the heavily calcified aorta, thus preventing unfavorable consequences from the calcified aorta leading to stroke and systemic embolism.

Porcelain Aorta During Coronary Bypass Surgery

Patients with porcelain aorta demonstrate an increased morbidity and mortality rate during CABG operation because of the elevated risk of embolic stroke from the atheromatous heavily calcified aorta. Such complication usually takes place during manipulation of the diseased aorta in three different maneuvers in CABG:

1. Cannulation of the aorta
2. Cross-clamping
3. Partial clamping for the construction of the proximal anastomosis[4] 

Several amendments have been described to avoid cannulation and clamping of the porcelain aorta, including:

• A no-touch technique or aortic off-pump CABG [1][21][22]
• CABG performed during deep hypothermic circulatory arrest [1][4][16][23]
• Femoral or axillary artery cannulation for cardiopulmonary bypass [1][4][16][24]
• Placement of proximal saphenous vein grafts onto the internal mammary artery, onto the innominate artery, onto the axillary artery, or the carotid artery[1][4][16][25][26]
• Single clamp technique[1][4][16][27]
• Intraluminal balloon catheter as a replacement for external clamps[1][4][16][28]
• Ascending aorta endarterectomy[1][4][16][29][30]
• Patch aortoplasty[1][4][16][31]
• Graft replacement of the ascending aorta[1][4][16]
(B2)

The most common and effective recent modality is "no-touch technique" or aortic off-pump CABG in which cannulation and clamping of the ascending aorta are averted. This technique is performed on the beating heart without using the heart-lung machine and, so-called, off-pump, or beating heart CABG. It is achieved by an arterial grafting using bilateral internal mammary grafts in addition to a radial artery or reverse saphenous venous grafts, forming a T, or Y shaped graft that will be connected to the innominate artery or common carotid artery that will act as the inflow source, thereby bypassing the blockage area without touching the diseased aorta.[1][18][21][16][22] Lev-Ran et al.[16] performed a retrospective analysis and comparison of CABG results with femoral artery cannulation in fifteen patients and off-pump CABG in 41 porcelain aorta patients. He found that the off-pump CABG group has only 1 case of perioperative mortality (2.4%) and no perioperative stroke or transient ischemic attack cases. While in CABG with femoral cannulation, there was one case of perioperative mortality (6.6%) and three cases of perioperative strokes or transient ischemic attacks (20%). These results reveal that the off-pump technique has a beneficial impact on reducing perioperative mortality and stroke risk compared to the conventional CABG procedure. However, this technique's major drawback in their study was the failure to achieve complete revascularization in 24.3% of patients compared to the lower rate in the normal CABG group. (B2)

If there is a small non-calcified area confirmed by epiaortic ultrasound, a safe proximal anastomosis construction can be done using a proximal seal system [1][25][21][16] to allow the surgeon to perform an anastomosis between the graft and the aorta without the need to do an aortic clamping. Hilker and his colleagues[32] published a study that reveals a stroke rate of 0.48% of 412 patients in which 542 had proximal anastomoses using a Heartstring device during CABG was performed. Dohmen G et al.[21] reported 25 successful anastomoses in 17 patients undergoing CABG. It was demonstrated that eight patients had porcelain aorta. Two of them developed neurological deficits after the procedure.(B2)

The other beneficial method is performing CABG during deep hypothermic circulatory arrest. In this practice, myocardial revascularization is performed using moderate hypothermia with a core temperature of 28-32°C for up to 40 minutes without harming the patient. In this case, sufficient time is given for the surgeon to perform safe proximal anastomoses without placing clamps on the aorta, minimizing the rate of cerebral complications. Salenger et al.[23] studied the outcome of this approach in 71 patients and found that one patient (1.4%) developed a mild stroke that resolved afterward, [26] analyzed six patients not suitable for clamping due to the severely calcified aorta. A deep hypothermic circulatory arrest for proximal anastomosis was made on all patients with complete recovery free of neurological problems.

Ascending aorta is the usual cannulation site in cardiac surgery.[18] The existence of a porcelain aorta precludes the conventional arterial cannulation.[18] Therefore, a suitable cannulation site should be chosen for safe surgery. Several places have been suggested, and the classic approach is performing arterial perfusion via the common femoral artery (CFA) because it can be quickly done but carries the risk of embolization from the atherosclerotic thoracic and abdominal aorta due to retrograde perfusion. If the CFA is atherosclerotic and cannot be used for cannulation, the alternative location will be through the axillary artery.[25] Perfusion through the axillary artery has the following advantages:(B2)

1. It carries a lower risk of cerebral embolization.
2. The rate of atherosclerosis is less than that of the ascending aorta or the femoral artery.
3. The risk of severe distal ischemic-reperfusion injury or embolization after cannulation is reduced due to the presence of abundant collaterals.

However, axillary artery cannulation can be accompanied by several local complications, including axillary artery dissection, thrombosis, and brachial plexus injury.[25]

Svensson et al.[24] conducted arterial perfusion via the axillary side graft in 299 patients and the femoral artery in 375 patients. They observed that the stroke occurred in 4% (12/299) of the axillary side graft group and 6.7% (25/375 patients) of the femoral artery group. The risk of hospital mortality was higher with femoral perfusion (11%, 42/375) than axillary side graft perfusion (7.0%, 21/299). 

Cannulation via the brachiocephalic artery could also be a feasible option in patients with porcelain aorta. Banbury et al. endorsed that brachiocephalic arterial cannulation is a good alternative in cases whose aorta cannot be manipulated. This option can avoid making a second incision in axillary artery cannulation or retrograde perfusion issues in femoral artery cannulation.[25]

Porcelain Aorta During Mitral Valve Surgery 

Hypothermia and a fibrillating heart can avoid aortic cross-clamping in mitral valve surgeries.[1][25][27] Loulmet et al. reported outstanding hypothermia results and a fibrillating heart method during mitral valve surgeries in patients with PA.[28](B2)

Porcelain Aorta and Aortic Stenosis 

Strategies like deep hypothermic circulatory arrest, the apico-aortic conduit technique have been proposed in the scholarly literature to avoid a cross-clamping of the aorta during surgical aortic valve surgery in patients with aortic stenosis and porcelain aorta.[1][25][21] Unfortunately, these technically challenging methods require significant surgical experience and don't completely avoid manipulating the porcelain aorta.[1](B2)

Another new promising alternative is transcatheter aortic valve replacement (TAVR). It is the least invasive procedure to replace the aortic valve in high surgical risk patients with severe aortic stenosis. Patients having porcelain aorta will benefit from this approach by replacing the aortic valve without an aortic cross-clamping. In a study of around 60 patients with severe AS and PA, TAVR was successful in 98.4% of the patients with PA. The stroke and thirty-day mortality rates were 1.6% (1/61 patients) and 11.5% (7/61 patients), respectively, showing no differences from patients without PA. Pascual et al. stated that 449 patients underwent TAVR, of which 36 patients (8%) had a porcelain ascending aorta. The procedure was successful in 94.4%. The stroke and thirty-day mortality rates were 2.8% (1/36 patients) and 5.6% (2/36 patients), respectively, similar to patients without PA.

Differential Diagnosis

Porcelain aorta is a radiographic diagnosis with few or no differential diagnosis. Often, minor forms of thoracic aortic calcification are found in older patients. PA is the most severe form of thoracic aortic calcification.[33]

Prognosis

Porcelain aorta is typically an incidental finding associated with diffuse atherosclerotic cardiovascular disease, chronic kidney disease, or prior chest radiation therapy. General prognosis in patients with PA is usually related to the underlying medical conditions.

Complications

Porcelain aorta is usually an incidental finding in patients being evaluated for cardiovascular or pulmonary diseases.[1] Porcelain aorta is frequently associated with valvular and coronary calcification, thus increasing the risk of stenotic valvular disorders and ischemic heart diseases due to coronary atherosclerosis in these patients. Furthermore, severe calcification of the aorta and resultant luminal narrowing increases the resistance to the blood flow ejected from the left ventricle, resulting in the development of cardiac hypertrophy, congestive heart failure, and arrhythmias over time. A few studies have also linked an elevated risk of long-term mortality and adverse cardiovascular events and stroke in patients with aortic calcification.[1] 

Cardiac surgery is more complicated in patients with porcelain aorta, and their management can be challenging due to the increased risk of perioperative atheroembolism and aortic dissection.[25]Patients with a PA are at high risk for embolic stroke due to manipulating aortic atheroma during surgery.[33]