An aneurysm is defined as a localized, pathological, blood-filled dilatation of a blood vessel caused by a disease or weakening of the vessel’s wall. These diseases include, but are not limited to, atherosclerosis, hypertension, syphilis, trauma, congenital malformations, and bacterial and fungal infections. Aneurysms fall into one of two general categories: True aneurysms, which involve all three layers of the artery (intima, media, and adventitia), and false aneurysms, which do not involve all of the layers.
Aneurysms most commonly occur in the abdominal aorta, thoracic aorta, and intracranial arteries, but can also affect the coronary, femoral, popliteal, splenic, carotid, and renal arteries. The shape of an aneurysm is either saccular (like a sack) or fusiform (like a spindle). Both are susceptible to growth, rupture, and dreadful consequences.
Abdominal aortic aneurysms (AAA) are defined as an aortic diameter of at least 1.5 times the diameter measured at the level of the renal arteries. In most people, this diameter works out to be 3 centimeters. Abdominal aortic aneurysms are the most common type of aneurysms and occur between the renal and inferior mesenteric arteries. (Anything located more superiorly is a thoracic aortic aneurysm.) They are true aneurysms and therefore involve all layers of the aorta. Atherosclerosis is considered the most common cause of AAA, although other etiologies include cystic medial necrosis and infections. Aneurysms are also associated with diseases such as Marfan and Ehlers-Danlos syndromes. In the United States, AAA kills 15,000 people per year and is the 10th most common cause of death for people older than 55 years.
Why and how abdominal aortic aneurysms develop is uncertain, although there are theories. The fact that AAA develops in many members of some families suggests a genetic basis, and chromosome 19 has been implicated. Another possibility involves enzymes called proteases that destroy important components of the arterial wall such as elastin and collagen. Finally, it is thought that inflammation in the arterial wall may play a role in the development of AAA as well.
Risk factors for the development of AAA include age greater than 60 (5 to 10 percent prevalence), male sex, Caucasian race, atherosclerosis, hypertension, and a family history of the disease. The strongest risk factor for developing AAA, however, is cigarette smoking. Cigarette smoking carries an odds ratio of 5 (i.e., a smoker is five times more likely to develop AAA than a nonsmoker) and a longer history of smoking confers a greater risk. It is estimated that smoking alone causes 75 percent of all AAA greater than 4 centimeters in diameter. Smoking also increases the rate at which an aneurysm grows.
Patients with AAA are frequently asymptomatic and the disease is discovered only incidentally via imaging studies obtained for other reasons. The two most important symptoms are abdominal or back pain as well as abdominal tenderness to palpation. Other findings on physical examination may include a pulsatile mass at or above the navel, which is approximately where the aorta divides and a common location for AAA, as well as an audible bruit (a harsh, shooting sound of blood flowing) when the abdomen is auscultated with a stethoscope. Patients with a ruptured aneurysm will be hypotensive due to exsanguination of blood into the abdominal cavity; the survival rate of patients with ruptured AAA is less than 50 percent. The diagnosis of AAA can be accomplished with physical examination and imaging studies. As mentioned, physical examination may reveal a pulsatile abdominal mass. Imaging studies include ultrasonography (preferred), computed tomography (CT), and magnetic resonance imaging (MRI).A small percentage of patients with AAA have an entity called inflammatory aneurysm. Additional findings of inflammatory aneurysm include weight loss and an elevated erythrocyte sedimentation rate (a type of blood measurement indicative of systemic inflammation).
The treatment of AAA is medical and/or surgical. Medical therapy involves quitting cigarette smoking, as well as reducing risk factors such as high blood pressure and high cholesterol (such as with beta-blocker and statin medications, respectively). Researchers are also looking into the role of antibiotic therapy, given the role that infection may play in AAA development.
Records show that surgery for AAA was attempted since the 100s c.e. in ancient Rome, but success did not occur until the 1920s. Surgical therapy involves replacement of the aneurysm with a prosthetic graft, although endovascular techniques are also being developed. The indications for surgery include aneurysms that are symptomatic and any aneurysm greater than 5.5 centimeters in diameter. Cases that are planned in advance are now associated with a low mortality rate of less than 5 percent, but emergent cases for aneurysm rupture have a mortality rate of greater than half. The chances for surgical success are improved at a hospital and with a surgeon who does many procedures repairing AAA.
Just like there are risk factors for aneurysm development, there are risk factors for aneurysm rupture. These factors include aneurysm diameter, rate of expansion, and gender. The annual rupture risk depends on the size of the aneurysm diameter: 0 percent for aneurysms less than four centimeters, 3 percent for aneurysms four to five centimeters, 10 percent for five to six centimeters, 15 percent for six to seven centimeters, 30 percent for seven to eight centimeters, and 40 percent for aneurysms greater than eight centimeters. In terms of aneurysmal expansion rate, the faster an aneurysm expands, the increased risk it has of rupturing. In addition, larger aneurysms tend to expand more rapidly. Last, although males are more likely to develop AAA, females have a four times higher risk of aneurysm rupture. Other factors that increase the risk of AAA rupture include smoking and hypertension.
Patients at risk of developing AAA should undergo screening examinations. Men between the age of 65 and 75 should be screened once with ultrasound if they ever smoked cigarettes. Men who are older than 60 years with a nuclear family history of AAA should also be screened. Other current recommendations are to monitor aneurysms three to four centimeters in diameter with ultrasound every few years, while aneurysms four to five centimeters in diameter should be monitored with ultrasound or CT every six to 12 months.
Thoracic aortic aneurysms (TAA) are less common than abdominal aortic aneurysms but are just as frightening. Even more frightening is the fact that one-fifth to one-fourth of patients with TAA have a coexisting AAA. TAA occur at an incidence of six per 100,000 patient-years and are more common in males in their 50s and 60s.
TAA are divided into four anatomic categories: ascending aorta, aortic arch, descending aorta, and thoracoabdominal. Two classification systems exist, namely the DeBakey and Stanford systems. Aneurysms that originate in the ascending aorta and involve the aortic arch (30 percent of TAA) are termed DeBakey Class I. DeBakey Class II aneurysms involve the ascending aorta only (20 percent). DeBakey Class III aneurysms originate in the descending aorta and comprise the other half of TAA. In the Stanford system, group A aneurysms have involvement of the ascending aorta, while group B aneurysms involve only the aortic arch and descending aorta.
Cystic medial degeneration (or necrosis) is a natural process whereby the middle layer of an artery degenerates and consequently weakens the blood vessel. Atherosclerosis is a major cause of cystic medial degeneration, as is protease-mediated breakdown of vascular proteins such as elastin and collagen. These processes are the pathophysiology behind vessel dilatation and TAA formation. The biggest risk factor for cystic medial degeneration and TAA is hypertension. Two genetic conditions also present patients with a higher risk of developing TAA, namely Marfan and Ehlers-Danlos syndromes. Other risk factors include a family history (which is often due to mutations in the transforming growth factor beta receptor 2 gene), a bicuspid aortic valve, and infectious or inflammatory disorders of the esophagus (such as giant cell arteritis, syphilitic aortitis, etc.).
Patients with TAA are usually asymptomatic. Symptoms, when they do occur, are usually due to compression of other structures and vessels by the aneurysm, or from emboli spewed off from the aneurysm causing ischemia of different organs. Such symptoms can include pain in the chest, back, flank, or abdomen. Furthermore, compression of the vagus or recurrent laryngeal nerves can cause vocal hoarseness; compression of the trachea, bronchi, or lungs can cause respiratory symptoms such as wheezing, coughing, or shortness of breath; compression of the phrenic nerve can cause paralysis of the diaphragm; and compression of the esophagus can cause problems with swallowing. Also, an ascending TAA can cause aortic regurgitation, which can itself cause heart failure. A ruptured TAA can cause pain, hypotension, shock, or even hematemesis (coughing up blood).
The most important factor related to the actual rupture of TAA is the aneurysm size. The five-year risk of rupture is 0 percent for aneurysms less than 40 millimeters in diameter, 15 percent for aneurysms 40 to 59 millimeters, and 30 percent if the aneurysm is 60 millimeters or larger. On average, ascending TAA are about 60 millimeters in diameter at time of rupture while descending TAA are about 70 mm. Thoracic aortic aneurysms grow anywhere between one to 10 millimeters per year, and the aneurysms that grow fastest are the ones that are already large or are located along the descending aorta.
The preferred methods of diagnosing TAA are CT and magnetic resonance angiography (MRA). Other options include chest X-ray, echocardiography, and contrast angiography. The medical treatment of asymptomatic TAA includes blood pressure control (with beta-blocker medications), surveillance, and serial imaging to monitor the growth of the aneurysm. Aneurysms require surgical intervention if the patient is symptomatic; the size of the aneurysm is greater than 50 millimeters or 60 millimeters for ascending and descending aortic aneurysms, respectively; the aneurysm grows faster than 10 millimeters in a year; or there is evidence of aortic dissection. Surgical repair involves substituting a graft for the diseased section of aorta.
Outcomes following surgery reveal the problematic nature of this disease. The one-month mortality rate is at least 10 percent (greater than 40 percent if the surgery is emergent), stroke rate is 20 percent for ascending aneurysms, and injury rates of the spine and/or kidneys are 15 percent for descending aneurysms. Patient factors predictive of poor outcome following surgery include symptomatic aneurysms, preoperative kidney disease, older age, and descending aneurysms.
Another option for aneurysm repair is endovascular stenting. This new technique is minimally invasive and has been shown to be associated with fewer perioperative complications when compared to surgery. However, long-term follow-up has not yet shown an advantage. Undoubtedly, this technology will continue to improve and one day present a viable option for patients.
Cerebral aneurysms are the primary cause of subarachnoid hemorrhage (SAH); therefore, the discussion of one is equivalent to a discussion of the other. Most cerebral aneurysms are of the saccular (berry) type, and the prevalence is estimated to be 5 percent; in other words, 300 million people worldwide have at least one brain aneurysm (approximately one-fourth of patients have two or more). Fortunately, most of these are small enough to be asymptomatic and remain unruptured; aneurysmal SAH occurs at a rate of “only” approximately 12 per 100,000.
The average age of cerebral aneurysm rupture is 51 years, and certain risk factors exist that predispose people to aneurysm formation. The most preventable risk factor is cigarette smoking, which increases the risk of aneurysm formation by 2.5 to 5 times compared to nonsmokers. This relationship is also dose dependent, so the more one smokes, the greater risk one has of developing a brain aneurysm. Other strong risk factors include a family history, which increases the risk by four times; hypertension, which increases the risk by 2.5 times; and alcohol, which increases the risk by a factor of 1.5 to 2. Genetics, the medication phenylpropanolamine, and estrogen deficiency are also thought to play a role in the development of aneurysms.
Aneurysms of the brain cause symptoms in two circumstances: if it is large enough or if it ruptures. The major symptom of a ruptured aneurysm is a headache, usually described as “the worst headache of my life.” Other symptoms include loss of consciousness, nausea and vomiting, seizures, and even low back pain. A finding on physical examination, aside from potential neurologic deficits, is retinal hemorrhages.
The most useful tool in diagnosing SAH is a head CT scan without contrast. If the head CT scan is normal, it may have missed the SAH, so a lumbar puncture may contribute to the diagnosis if there is an elevated opening pressure or red blood cell count in the cerebrospinal fluid (CSF). Other diagnostic tools include cerebral angiography and MRI.
Treating cerebral aneurysms can be performed with surgery or with endovascular techniques, but only one-third of patients will have favorable results. Surgical intervention is also known as aneurysm “clipping.” In this procedure, the patient receives a craniotomy which allows the neurosurgeon access to the aneurysm. Then, the neurosurgeon places a titanium clip at the base of the aneurysm. This occludes blood flow into the aneurysm and prevents or controls bleeding for unruptured or ruptured aneurysms, respectively. The endovascular treatment of brain aneurysms, also known as “coiling,” is a minimally invasive technique where the neurosurgeon or interventional radiologist enters the patient’s circulatory system through a peripheral vessel and thereby gains access to the cerebral vasculature. Using real-time radiologic imaging, the aneurysm is located and a thin, flexible, wiry material is injected into it until the aneurysm is filled with coils. This also occludes the aneurysm, preventing blood flow and consequent rupture.
The complications following SAH are many. The most important one is death. The fatality rate following SAH is 50 percent, with 10 percent of patients dying before reaching the hospital, another 15 percent dying within one day, and another 20 percent dying within one month. Other complications include increased intracranial pressure (ICP) (55 percent of patients), cerebral infarction (50 percent of survivors), vasospasm (34 percent of patients), cardiac dysfunction (20 percent of patients), hydrocephalus (15 percent of patients), seizures (7 percent of patients), rebleeding (7 percent of patients), and hyponatremia, all of which may lead to death.
Vasospasm, also known as delayed ischemic neurologic deficit, is the leading cause of morbidity and mortality following aneurysm rupture and affects one-third of patients with SAH. The pathogenesis of vasospasm is fascinating: When an aneurysm ruptures, blood comes into contact with the peri-adventitial space of the blood vessel, resulting in an upregulation of inflammatory cell mediators on the blood vessel wall as well as on circulating white blood cells. As a result, the white blood cells stick to the blood vessel wall and migrate through it, becoming entrapped in the subarachnoid space. This causes vasospasm in two ways. First, white blood cells are a source of endothelins, which are potent vasoconstrictors. Second, as the white blood cells die they release oxygen free radicals, which work to destroy the vasodilator nitric oxide (NO) that is normally released by the blood vessel wall.
Patients with vasospasm present with symptoms of altered mental status and/or focal neurologic deficit (i.e., weakness, hemiparesis) anytime between the third and twentieth day following aneurysm rupture, although most commonly it occurs between the sixth and eighth day. Transcranial doppler (TCD) studies can be used to detect vasospasm, although the tool is of low specificity and is particularly unhelpful in elderly patients, who have diminished vasomotor reactivity. The primary treatment and prevention of vasospasm involves hyperdynamic therapy (also known as triple-H therapy: hemodilution, hypertension, and hypervolemia).
As mentioned, cerebral infarction (i.e., strokes) occurs in half of SAH survivors; the most common cause is vasospasm. Ten percent of all strokes are due to SAH. Cardiac dysfunction is due to inadequate blood flow to the hypothalamus, which results in an increase in hormone release within the heart and consequent injury. These changes are evident on electrocardiography (EKG). Hyponatremia is caused by injury to the hypothalamus and the syndrome of inappropriate antidiuretic hormone (SIADH) secretion.
Screening for cerebral aneurysms is useful only in four select populations. First, patients with two or more first-degree relatives with aneurysms should receive screening. So should patients with autosomal dominant polycystic kidney disease who have a personal or family history of aneurysm rupture, symptoms, a high-risk occupation, and/or an upcoming surgical procedure. Third, patients with gluco-corticoid-remediable aldosteronism should receive screening as these patients have a higher incidence of aneurysms. Last, patients with connective-tissue diseases such as Ehlers-Danlos syndrome or pseudo-xanthoma elasticum should also receive screening. Screening involves the use of MRA or computed tomography angiography (CTA). Patients selected for screening will need annual imaging done for the first three years; if these studies are normal, screening can be decreased in frequency to once every five years.
Cholesterol; Ehlers-Danlos Syndrome; High Blood Pressure; Marfan Syndrome; Smoking.
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