| AAA – Imaging Strategies
CLINICAL CONSIDERATIONS– AAA presents with a pulsatile mass, back pain, or prominent femoral and or popliteal arteries and the question would be what imaging study would be the most prudent to make the diagnosis, how the aneurysmm should be followed, and when the aneurysm should be referred for surgery. IMAGING STRATEGIES For the initial study to diagnose the choices are between an USscan, CT scan and an MRI, There is general consensus that US is the study of choice in this scenario since it is inexpensive, safe, with high sensitivity (90-97%) , specificity and accuracy. (Sprouse) The key issue is to accurately size the aneurym requiring the aorta to be imaged along its short axis in the transverse plane, and along its long axis. WHAT STUDY? The study of choice is an US examination of the retroperitoneum with attention to the maximum diameter of the aorta. Follow up examinations are directed based on the size of the aneurysm (see protocol below) and the size considered ripe for surgery is usually 5.5cms.(American Family Physician). WHY? To its advantage, it is accurate, safe, and inexpensive, does not involve radiation. On the other hand it is operator dependant and patient dependant. Factors that affect the quality and accuracy of the study include the size of the patient and the presence of bowel gas. SCREENING? Since the clinical examination for screening is so poor with a sensitivity of 20-90% , and for US 90-97% , and with the knowledge that there is a natural tendency for the aorta to enlarge with age,it may be prudent to screen a select group of males over 60years with hypertension, claudication, family history, or other vascular disease. BACKGROUND TO THE DISEASE Causes and Predisposing Factors The factors that cause and predispose to aneurysmal disease include the atherosclerotic process, with associated inflammatory and degenerative changes of the aortic wall. The degenerative changes in the matrix of the aortic wall occur with age, proteolysis, and inflammation induced by the atherosclerotic process, Circulating proteolytic activity results in an increase in the elastase in the aortic wall and a subsequent breakdown of the elastin. In COPD there is increased elastase activity as well resullting in increased elastolysis. AAA is more commonly seen in males over 60 years, with a M:F ratio of 5:1. The incidence in Western cultures has tripled over the last 3 decades, due to the larger numbers of people in the elderly population. Improved diagnostic techniques have also been a major factor in the apparent increased incidence. In the USA, AAA is 13th in the leading causes of death. Only about 50% percent of patients survive a ruptured abdominal aorta. Other factors that contribute to the development of aneurysmal disease of the abdominal aorta include smoking, hypertension and familial factors. 90 percent of patients with aneurysms have a smoking history and 60% have hypertension – (Pokrovskii ) Asociated diseases include coronary artery disease and peripheral vascular disease. 90% of patients with AAA have CAD (Pokrovskii ) Statistics 5 percent of men older than 65 years of age will have an occult aneurysm (3 to 6 cm in diameter). 15,000 deaths per year are directly attributable to an abdominal aortic aneurysm, making it the 13th leading cause of death in the United States. AAA account for 95% of all aneurysms. M:F 4:1 peak incidence men occurs in men >80 and women >90 The incidence in patients <60 years is less than 1% Associated Diseases Common iliac artery aneurysms 20%(Armon) Popliteal aneurysms 8-10% (Ebaugh,MacSweeney) Common femoral artery aneurysms association COPD Pathogensis Law of Laplace Tension = radius X pressure/wall thickness RESULTS: In most instances (90%), AAA is infrarenal and the aneurysm extends into the iliac arteries in about 2/3 of the patients. Rupture occurs in 25% of patients. Factors that predispose to aneurysmal rupture include large aneurysm size, presence of hypertension and chronic obstructive pulmonary disease (COPD). Complications: Size vs risk of rupture 4cms <5%/year 5cms 5 – 10% per year 7cms 15-25% per year 8cms 25-40% per year >9cms 75% per year Dx 75% of patients with an AAA are asymptomatic. Symptoms include abdominal pain or tenderness, back pain. Aneurysms become palpable when they are larger than 4 cm in diameter. The aneurysm is commonly identified on plain film and is easier seen on a lateral abdominal film, since 90% have mural calcification. However AAA are evaluated by their transverse dimension which cannot be measured on a lateral examination. Surgery is indicated for aorta 5.5 cm or larger Aneurysmal rupture usually occurs when the aneurysm enlarges to greater than 6 cms. The rupture rate at 5 years for an aneurysm 7 cm or more in diameter exceeds 75%. For aneurysms 6 cm in diameter, the rate is about 35%, and for those 5 to 5.9 cm in diameter, it is about 25%. At this time, data are insufficient to accurately estimate the risk of rupture in aneurysms less than 5 cm (2 in.) in diameter. RX Indications for surgical intervention include an aneurysm greater than 5cms, back pain related to the aneurysm or rapid growth of the aneurysm (enlarging 0.5 cm in 6 months). Both conventional surgery as well as endovascular stent-graft placement can be used in the treatment. Gold standard is surgery with graft patency at 5 years of 90% Mortality for elective surgery is 2%, for the AAA that is symptomatic it is 20% and for the ruptured AAA it is 50%. Graft material dacron or ePTFE Indications for endovascular stent graft High risk for surgery Infrarenal neck 1cms-2.8cms Other considerations relate to the landing sites which include iliac tortuosity and size. Diagnosis The aorta is considered aneurysmal, when the transverse diameter is greater than 3cms or when a true cross sectional diameter at any level is greater than 1.5 times that of the smallest diameter. Surgery is considered when the size reaches 5.5cms although in some centers 5.0cms is the cut off. Rapid expansion is an indication for surgery and is considered when the diameter increases by more than 0.5 cm over 6 months. New back pain in a patient with a known aneurysm is a concerning symptom and requires meticulous CT evaluation to exclude a leak best studid by non contrast CT scan. The information provided by the study should include the maximum dimension of the aneurysm perpindicular to the longitudinal axis of the aorta. The maximum measurement is determined either in the A-P plane or the transverse plane. The length of the aneurysm has no bearing on the surgery, although its involvement with the iliacs and renal arteries does. The most common site of aneurysmal disease is the infrarenal aorta, which is more easily evaluated than the suprarenal aorta by US. The distal aorta above the bifurcation may also be difficult to visualise. However the purpose of the US is to measure the size of the aorta and for this purpose the low cost/low risk study is adequate. When the patient is preop, then a CT is required to provide the clarity of these relationships. US does not provide the clarity needed by the surgeon for operative planning. There is a standard error of measurement of AAA of 4-5mm. In the sizing and subsequent follow up of patients a difference therefore of 5mm may be a falsley high or low and thus decisions on size changes must take this error factor into account. (Lederle) . A change of greater than 7mm is on the other hand considered to be significant. When a major decision needs to be made based on size it is worth discussing the study and studies with the raiologist to assess accuracy of reported measurements. When measurements are considered accurate then an increase in size by 2mm per year is considered a “slow growing” aneurysm while a “fast grower” enlarges by 5mm per year. Surgery is indicated when the aneurysm reaches greater than 5cms, and usually when it is in the 5.5cms range. If the patient in fact has a fast growing AAA currently measuring 3.5cms, it would reach target surgical size in 3-4 years, taking into consideration that rate of increase in size is yet greater as the aneurysms get larger. Thus if the patient is imaged in 18 months to two years one would be able to safely determine rate of growth and also determine size. A 3.5cms fast grower would be between 5 and 5.5cms in two years INDICATIONS FOR SURGERY Elective Surgical Repair Indications Aortic aneurysm diameter >=5.5 cm AAA diameter 4-5 cm and enlarging 0.5 cm in 6 months WHEN? Protocol for imaging follow up Ultrasound for aorta < 5 cm (by ultrasound) Aortic dimension 2.5 -3.4cms repeat ultrasound in 5 years Aortic dimension 3.5-3.9cms repeat ultrasound in 2 years Aortic dimension 4- 5 cms repeat ultrasound in q6 months Aortic dimension 5-5.4 cm repeat ultrasound in q3 months and do a planning CTangio with contrast Aortic dimension >5.5cms elective surgery Aortic dimension >8cms admit the patient HOW TO ORDER? When ordering an ultrasound for AAA it is important to reference the symptom or the sign as the clinical indication and not the diagnosis. ie Patient with “pulsatile mass” is acceptable The appropriate study to request is an ultrasound of the aorta and NOT an abdominal ultrasound. A request for an abdominal ultrasound will provide you with a full study of the abdomen which includes the liver pancreas gallbladder and spleen – whereas an US of the aorta will give you the aorta and the kidneys. The evaluation of the abdominal structures is not necessary in this situation and would only generate an extra charge and unwanted information. PATIENT INFORMATION The patient should be NPO for 4 hours to limit the amount of overlying gas in the abdomen. The exam takes about 30 minutes to perform, with the patient lying on his/her back, ans well as on either side. The test is painless but may require some deep pushing of the probe on the abdomen. The technologist and radiologist do not discuss the results with the patient. CLINICAL RED FLAGS New back pain is an important symptom and must be taken seriously. It would be extremely unusual for a 4.5cms aneurysm to rupture and the usual cause for back pain, even in the patient with an aneurym is discogenic disease. If the pain raises clinical concern then the study of choice is a CT scan of the abdomen and pelvis without IV contrast to evaluate for leak. Why the pelvis? Finding a leaking iliac aneurysm is a possible clinical scenario and blood or fluid in the cul de sac would be a radiological sign of an important clinical event in the abdominal cavity. References Nevitt MP, Ballard DJ, Hallett JW Jr. Scott RA, Kim LG, Ashton HA; Multi-centre Aneurysm Screening Study Group. |