Aneurysm or Dissection of the Aorta and Peripheral Arteries
The recommendations in this chapter concern definitions, the specification of disabling symptoms and signs, and a fuller set of predisposing conditions. Aneurysm as well as dissection should be specified because they can occur separately. The dilation of aneurysms can cause pain, and both aneurysms and dissections can reduce or cut off blood flow to vital organs, including the heart, lungs, brain, viscera, and limbs. According to clinical guidelines, limits on severe physical exertion and heavy lifting may be prescribed to prevent an aneurysm from rupturing or dissecting. Connective tissue disorders, including Marfan, Loeys-Dietz, Ehlers-Danlos, and Turner syndromes, may weaken the aortic and other vessel walls and need intervention at an earlier stage.
The aorta is the largest artery in the body and is divided into four parts: the aortic root (originating at the heart), the ascending aorta (rising to the head), the aortic arch, and the descending aorta (descending along the spine through the chest and upper abdomen before splitting into two branches, one into each leg). The peripheral arteries and other vessels extending from the aorta deliver freshly oxygenated blood throughout the entire body (Hiratzka et al., 2010; O’Gara, 2003). The aorta is composed of three layers: (1) the inner intimal lining, (2) the medial layer containing connective tissue interspersed with smooth muscle cells, and (3) the outer adventitial shell of connective tissue composed of collagen as well as
very small blood vessels (the vasovasorum) that provide nutrients to the vessel wall.
An aneurysm may occur when a blister or bulge develops in weak areas of the aorta or peripheral arteries that expand under constant pressure from flowing blood. When the bulge grows to 50 percent or greater than the normal size of the vessel, it is classified as an aneurysm (Hiratzka et al., 2010; O’Gara, 2003). Modest arterial dilation that is less than 150 percent of the normal vessel dimension is commonly defined as vascular “ectasia.” Aneurysms can occur in the aorta as well as peripheral vessels such as the popliteal artery (the vessel behind the knee) or small vessels in the brain (sometimes referred to as “berry aneurysms”). Untreated, most aneurysms may grow to sizes that pose a major risk of rupture. A ruptured aortic aneurysm is a life-threatening event, making early detection and medical management important to avoid death. Similarly, a ruptured small-vessel aneurysm in the brain can cause a fatal hemorrhagic stroke or result in long-term neurological deficits.
The histopathology of aortic aneurysm and dissection is characterized by degeneration of the muscle or medial layer of the aortic wall as well as disruption of elastic fibers and increased deposition of connective tissue components such as proteoglycans. The former use of the term cystic medial necrosis is now recognized as a misnomer that inaccurately describes the progressive loss of smooth muscle cells in the aortic aneurysm. In most cases of aneurysmal disease, the process of medial degeneration is associated with a superimposed atherosclerotic lesion.
Aortic dissection occurs with a “tear” or disruption within the degenerated medial layer of the aorta, involving bleeding within and along the aortic wall. Dissection may occur without the presence of an aneurysm (Hiratzka et al., 2010). Genetic disorders such as Marfan and Loeys-Dietz syndromes affect the composition and function of the aorta such that individuals are predisposed to aneurysm formation and dissection at early ages.
An aortic dissection should be distinguished from an “intramural hematoma,” which lacks evidence of blood flow in a false lumen channel within the medial layer or evidence of a blood channel connection to the vessel lumen through an intimal tear. Intramural hematomas typically occur in the descending aorta of elderly individuals and are postulated to arise from a rupture of the small vessels in the outer layer that feed the aortic wall (the vasovasorum).
EPIDEMIOLOGY, NATURAL HISTORY, AND TYPES OF AORTIC ANEURYSMS
Abdominal aortic aneurysms (AAAs), involving the infradiaphragmatic abdominal area, are the most common type of aneurysms. In addition,
aneurysms may occur in various peripheral arteries such as subclavian, popliteal, or cerebral vessels. Approximately 15,000 deaths occur every year in the United States due to AAAs. Risk factors include cigarette smoking, high blood pressure, family history of AAA in the first-degree male relative (e.g., father, brother), and signs of cholesterol buildup in other parts of the body. AAAs are more common in men than in women and typically occur after age 50 (O’Gara, 2003).
Aneurysms originating in the descending thoracic aorta (TA) occur in 5.9 to 10.4 per 100,000 person-years and rupture at a rate of 3.5 per 100,000 person-years. The incidence of TA aneurysms is estimated to be increasing and is now approximately 10.4 cases per 100,000 person-years. Most TA aneurysms are associated with atherosclerosis and medial layer degeneration. Risk factors for development of TA aneurysms include hypertension and smoking.
It is important to recognize that the size of the normal aorta increases with age and is affected by factors such as gender and body weight. Because the aorta gradually tapers along its course, it is critically important for measurements of aortic dimension to be localized precisely in defining abnormal vessel dimensions or determining rates of expansion. Studies have documented that the risk of death due to rupture and dissection of the aorta is directly associated with the size of the aorta. Accordingly, the guidelines for recommending prophylactic aortic repair are based on aortic dimensions in which the risks of complications are deemed to outweigh the risks of the reparative procedure (Conrad and Cambria, 2008; Davies et al., 2002; Elefteriades, 2002; Lobato and Puech-Leão, 1998). The rate of growth appears to increase as descending TA aneurysms become larger, further increasing the risk of rupture (Davies et al., 2002; Elefteriades, 2002; Lobato and Puech-Leão, 1998). The average rate of expansion of TA aneurysms is estimated to be 0.10 to 0.42 cm/year. This ongoing expansion of aneurysms serves as the basis for clinical practice guidelines that recommend annual noninvasive imaging tests to evaluate aortic dimension, such as a computed tomography (CT) or magnetic resonance imaging (MRI) scan.
Although the most common causes of aortic aneurysms are related to underlying atherosclerosis and medial degeneration, a variety of pathological processes can produce similar aneurysmal changes observed on an imaging study (e.g., CT scan). There are forms of congenital heart disease such as a bicuspid aortic valve that predispose patients to aortic dilation. Similarly, inflammatory conditions can predispose to aneurysm formation, such as giant cell arteritis, Takayasu arteritis, and Behçet disease. Under rare circumstances, aneurysms can be caused by infections due to viruses,
bacteria, fungi, spirochetes (i.e., syphilis), or tubercles bacilli (i.e., tuberculosis). Aneurysms due to infectious causes have been variously described as “mycotic aneurysms” or more recently as either an “infected aneurysm” or “infectious aortitis.”
Some forms of aortic disease are clustered in families and are due to genetic mutations that are either inherited from parents or occur de novo. Marfan syndrome is a genetic connective tissue disease that results from mutations in the fibrillin gene and is characterized by a high predisposition to aortic disease as well as cardinal clinical features affecting the eye (e.g., ectopia lentis, causing dislocation of the lens of the eye) and musculoskeletal system (e.g., joint laxity and dural ectasia). Nearly all patients with Marfan syndrome will eventually develop aortic disease and typically require ongoing annual surveillance of the status of their heart and aortic disease. In addition, patients with Marfan syndrome can exhibit other cardiovascular conditions, such as mitral regurgitation secondary to mitral valve prolapse or aortic regurgitation secondary to distortion of the valve cusps due to aortic dilation.
Loeys-Dietz syndrome results from mutations in the transforming growth factor receptor Type I or II genes and is characterized by the triad of arterial tortuousity and aneurysms, hypertelorism (wide-set eyes), and bifid uvula or cleft palate. These patients also have a spectrum of other features that overlap with the Marfan syndrome (e.g., joint laxity, dural ectasia) as well as features that are distinctly different from Marfan syndrome (e.g., presence of bifid uvula; translucent, easily bruised skin; or absence of ectopia lentis). Patients with Loeys-Dietz syndrome exhibit arterial tortuousity most commonly observed in the vessels of the head and neck, but this can also occur in other vessels. The vascular disease in these patients is particularly aggressive, with a mean age of death of 26 years. Children with prominent craniofacial features and severe systemic manifestations of Loeys-Dietz syndrome often exhibit more severe aortic disease. Accordingly, the American College of Cardiology/American Heart Association (ACC/AHA) guidelines recommend prophylactic repair once the aortic diameter exceeds the 99th percentile for age and the aortic valve annulus reaches 1.8 to 2.0 cm in children. Patients who undergo vascular repair remain at risk to develop additional vascular lesions at other sites. It is recommended that patients undergo annual surveillance of the vasculature by noninvasive imaging modalities (e.g., MRI) (Hiratzka et al., 2010).
In addition to Marfan and Loeys-Dietz syndromes, there are other heritable forms of aortic disease, such as Ehlers-Danlos and Turner syndromes, that remain to be further characterized as well-defined genetic diseases with vascular features.
DIAGNOSTIC CRITERIA AND METHODS
The ACC/AHA 2010 Guidelines for the Diagnosis and Management of Patients with Thoracic Aortic Disease recommend an aneurysm be identified by imaging techniques. The size of aortic aneurysm and the extent of aortic dissection can be visualized by several modalities, including helical CT, MRI, angiography, and echocardiogram/ultrasound. A previously diagnosed aneurysm or dissection should be reimaged at least every year or when symptoms develop to determine if enlargement has occurred (Hiratzka et al., 2010).
The treatment for aortic aneurysms may involve surgical or endovascular repair of the weakened arterial wall. Endovascular aneurysm repair (EVAR) now accounts for 56 percent of repairs in patients with AAA. Surgical repair of TA aneurysms is less successful than AAAs and can be associated with a significant risk of morbidity and mortality. Therefore, most guidelines weigh the risk of rupture and complications related to the size of the aortic dilation relative to the risks of the surgical repair. There appears to be increasing use of EVAR for AAA and some small TA aneurysms in the descending aorta.
Acute dissection involving the ascending thoracic aorta typically requires urgent surgical repair. Acute dissection involving the descending thoracic aorta can often be managed with medical treatment in the absence of significant compromise to vital organs.
Patients with TA aneurysms should have a long-term medical treatment regimen designed to achieve stringent control of cardiovascular risk factors such as hypertension, dyslipidemia, and tobacco exposure. Drugs that block the beta-adrenergic receptor are most clearly indicated for all patients with Marfan syndrome and aortic aneurysm to reduce the rate of aortic dilation and the risk of complications. Recent evidence supports the adjunctive use of drugs that block the angiotensin receptor as an additional means of reducing the rate of aortic dilation in patients with Marfan syndrome.
ACC/AHA guidelines currently recommend prophylactic surgical repair for asymptomatic patients with various forms of aortic disease, such as degenerative thoracic aneurysm, chronic aortic dissection, and mycotic aneurysm with an ascending aorta or aortic sinus diameter of 5.5 cm or greater. In certain subsets of high-risk patients with aortic disease, earlier intervention at smaller aortic dimensions (4 to 5 cm depending on the condition) is recommended. These patients include those with aortic disease and genetic predispositions (e.g., Marfan, Loeys-Dietz, Turner, and Ehlers-Danlos syndromes) or certain associated disorders (bicuspid aortic valve).
Patients who exhibit a rapid rate of increase in aortic dimensions (greater than 0.5 cm/year) are at increased risk of complications and should also be considered for earlier prophylactic repair at TA dimensions less than 5.5 cm (Hiratzka et al., 2010).
In addition to the acute, catastrophic symptoms that result from arterial rupture, aneurysms and aortic dissections can also produce a variety of chronic debilitating symptoms. For example, aortic dilation may compromise the normal function of the aortic valve and produce chronic aortic regurgitation that eventually results in heart failure. An arterial aneurysm can compress a peripheral nerve and induce neuropathic pain, numbness, or limb weakness, which could impair a patient’s functional capacity. An aneurysm could be the source of a blood clot that travels to the brain to cause a stroke or travels to the leg to cause critical limb ischemia. An aortic dissection may chronically disrupt the blood flow to vital organs such as the digestive tract or spinal cord and thereby result in pain syndromes or organ dysfunction that could compromise the capacity of a patient to engage in gainful employment.
Although surgical and endovascular repair of aortic disease is often successful enough to restore the individual to a good functional status, aortic surgery can be complicated by long-term sequelae such as spinal cord injury or kidney failure that could result in chronic disabling symptoms (Hiratzka et al., 2010).
Overall, most of the chronic debilitating symptoms related to aneurysms or aortic dissection are secondary to its effects on impairing blood flow to vital organs (i.e., tissue ischemia) or direct mechanical effects (i.e., compression) that compromise the function of vital organs. Accordingly, the disability listings related to aortic disease will most likely be met by reference to specific listings of the vital organ affected by the aortic disease.
In defining the criteria for abnormal aortic dilation, it must be recognized that the diameter of the aorta varies by several patient characteristics including age, sex, and weight (see Table 14-1). In addition to absolute size criteria, an abnormal aortic dilation may be defined by the relative relationship between the size of the ascending aorta and the descending aorta. Assessing these relative differences in dimensions may be particularly helpful in evaluating abnormal aortic dilation in children.
CONCLUSIONS AND RECOMMENDATIONS
The current listing for aneurysm (Listing 4.10, Aneurysm of aorta or major branches) is met if patients have an aneurysm of the aorta or major
TABLE 14-1 Normal Adult Thoracic Aortic Diameters
branches, due to any cause, demonstrated by medical imaging and uncontrolled by treatment (see Box 14-1). The most common and appropriate management of symptomatic cases involves surgical or endovascular treatment of the aneurysm or dissection. In high-risk patients with aortic disease and/or a genetic predisposition to aneurysm formation or aortic dissection (e.g., Marfan syndrome), the standard medical management may include substantial restrictions in physical activity in adults and children with asymptomatic aortic aneurysms or dissections. The prescribed treatment may include limitations on strenuous physical exertion or heavy lifting that may have implications on the capacity to perform certain activities of gainful employment.
Current Listing for Aneurysm of Aorta or Major Branches
4.10 Aneurysm of aorta or major branches, due to any cause (e.g., atherosclerosis, cystic medial necrosis, Marfan syndrome, trauma), demonstrated by appropriate medically acceptable imaging, with dissection not controlled by prescribed treatment (see 4.00H6).
SOURCE: SSA, 2008.
The committee concludes that the medical record should provide documentation of the aneurysm or dissection by imaging studies (e.g., CT, MRI) that are accompanied by evidence of impairment due to chronic debilitating symptoms resulting from the effects of the aneurysm or dissections on the heart, brain, peripheral nerves, or limbs. The Social Security Administration should reference other appropriate listings for the criteria for evaluating impairments on these related systems or organs.
RECOMMENDATION 14-1. Revise listing (4.10) to require the presence of chronic disabling symptoms due to the aneurysm or dissection. Disabling symptoms may be the result of the functional impairment to the heart, brain, peripheral nerves, or limbs due to the aneurysm or dissection. Claimants should be evaluated under the appropriate related cardiovascular listings or listings for other body systems if necessary.
The committee reviewed the current introductory text (i.e., the preamble) relevant to current listing 4.10 and recommended revisions to the text. These revisions will provide disability examiners with clear instructions for reviewing the medical records of patients with aneurysms or dissections and making appropriate evaluations of disability claims.
RECOMMENDATION 14-2. Revise the introductory text to the cardiovascular system to account for the following changes:
Include the term dissection in the primary description of the condition (i.e., aneurysm or dissection);
Develop the definitions for aneurysm and dissection to include: An aneurysm is a bulge in the aorta or a peripheral artery. A dissection of the aorta or its branches occurs when the inner lining of the artery is “torn” and begins to separate from the rest of the arterial wall. An aneurysm or dissection may compromise organ function and produce symptoms by the compression of other structures in the tissue or body compartment or induce ischemia by compromising the flow of blood to the heart, kidneys, brain, or other organs;
Revise the effects of aneurysm or dissection to include: An aneurysm or dissection can cause heart failure, renal (kidney) failure, or neurological complications. If an aneurysm or dissection is present, there must be one or more of these associated symptomatic conditions; the condition(s) are evaluated using the appropriate associated listings; and
Revise the diagnostic criteria of Marfan syndrome to include: There is no specific laboratory test to diagnose Marfan syndrome, although the mutation in the gene that causes it has been defined. The diagnosis is generally made by medical history, including family history and physical examination including an evaluation of the musculoskeletal features, a slit-lamp eye examination, and a heart test(s), such as an echocardiogram. In some cases, a genetic analysis may be useful, but such analyses may not provide any additional helpful information. Include a description of Loeys-Dietz syndrome as another example of a genetic disorder with increased risk of aortic aneurysm and/or dissection affecting both children and adults.
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SSA (Social Security Administration). 2008. Listing of impairments—Adult listings (Part A). Disability evaluation under Social Security (Blue Book). http://www.socialsecurity.gov/disability/professionals/bluebook/AdultListings.htm (accessed July 22, 2010).