How is an aortic dissection diagnosed?
Because of the varying symptoms and signs of aortic dissection depending on the initial intimal tear and the extent of the dissection, the proper diagnosis is sometimes difficult to make. The key to diagnosing an aortic dissection is to confirm that it is in fact a dissection and not a heart attack, and which type it is (as the treatment options vary significantly).
The pain experienced by the patient is the first symptom of aortic dissection and is unique. The pain is usually described by the patient as "tearing, ripping, or stabbing." This is in contrast to the pain associated with heart attacks. The patient frequently has a reduced or absent pulse in the extremities. A murmur may be heard if the dissection is close to the heart. An enlarged aorta will usually appear in the chest x rays and ultrasound exams of most patients. The use of a blood dye in angiograms and/or CT scans (computed tomography scans) will aid in diagnosing and visualizing the dissection.
While taking a good history from the individual may be strongly suggestive of an aortic dissection, the diagnosis cannot always be made by history and physical signs alone. Often the diagnosis is made by visualization of the intimal flap on a diagnositic imaging test. Common tests used to diagnose an aortic dissection include a CAT scan of the chest with iodinated contrast material and a trans-esophageal echocardiogram. Other tests that may be used include an aortogram or magnetic resonance angiogram (MRA) of the aorta. Each of these test have varying pros and cons and they do not have equal sensitivities and specificities in the diagnosis of aortic dissection.
In general, the imaging technique chosen is based on the pre-test likelyhood of the diagnosis, availability of the testing modality, patient stability, and the sensitivity and specificity of the test.
Chest X-ray: Widening of the mediastinum on an x-ray of the chest has a high sensitivity (81-90%) in the setting of aortic dissection. However, it has low specificity, as many other conditions can cause a widening of the mediastinum on chest x-ray. The calcium sign is a finding on chest x-ray that suggests aortic dissection. It is the separation of the intimal calcification from the outer aortic soft tissue border by 1 cm. Pleural effusions may be seen on chest x-ray. They are more commonly seen in descending aortic dissections. If seen, they are typically in the left hemithorax. About 12% of individuals presenting with an aortic dissection have a "normal" chest x-ray.
EKG: There are no specific electrocardiographic findings associated with aortic dissection. About 1/3 of the time, the EKG will show signs of left ventricular hypertrophy, which is due to the long-standing hypertension seen in these individuals. Another 1/3 of the time the EKG would be considered "normal". If the EKG suggests cardiac ischemia in the setting of aortic dissection, involvement of the coronary arteries should be suspected.
Biochemical markers: While there are currently no blood tests that can accurately diagnose aortic dissection, research has been performed into the serial measurement of monoclonal antibodies to smooth muscle myosin heavy chains that appears to be both sensitive and specific for aortic dissection.1 The sensitivity of this test is about 90% and the specificity is 97% within the first 12 hours of the beginning of the dissection, and this assay can accurately differentiate myocardial infarction from aortic dissection. This test is not currently available for the diagnosis of aortic dissection in the clinical setting.
Transesophageal echocardiography: The transesophageal echocardiogram (TEE) is a relatively good test in the diagnosis of aortic dissection, with a sensitivity of up to 98% and a specificity of up to 97%. It is a relatively non-invasive test, requiring the individual to swallow the echocardiography probe. It is especially good in the evaluation of AI in the setting of ascending aortic dissection, and to determine whether the ostia (origins) of the coronary arteries are involved. While many institutions give sedation during transesophageal echocardiography for added patient-comfort, it can be performed in cooperative individuals without the use of sedation. Disadvantages of the TEE include the inability to visualize the distal ascending aorta (the beginning of the aortic arch), and the descending abdominal aorta that lies bellow the stomach. A TEE may be technically difficult to perform in individuals with esophageal strictures or varicies.
Aortogram: An aortogram involves placement of a catheter in the aorta and injection of contrast material while taking x-rays of the aorta. The diagnosis of aortic dissection can be made by visualization of the intimal flap and flow of contrast material in both the true lumen and the false lumen. The aortogram was previously considered the gold standard test for the diagnosis of aortic dissection, with a sensitivity of up to 88% and a specificity of about 94%. It is especially poor in the diagnosis of cases where the dissection is due to hemorrhage within the media without any initiating intimal tear. The advantage of the aortogram in the diagnosis of aortic dissection is that it can delineate the extent of involvement of the aorta and branch vessels and can diagnose aortic insufficiency. The disadvantages of the aortogram are that it is an invasive procedure and it requires the use of iodinated contrast material.
Spiral CT with contrast: The spiral CT scan with contrast is a fast non-invasive test that will give an accurate three-dimensional view of the aorta. It is performed by taking rapid-cut radiographs of the chest and combining them in the computer to create cross-sectional slices of the chest. In order to delineate the aorta to the accuracy necessary to make the proper diagnosis, an iodinated contrast material is injected into a peripheral vein at a properly timed moment so that it will enter the aorta at the time that the aorta is being imaged. It has a sensitivity of 96 - 100% and a specificity of 96 to 100%. Disadvantages include the need for iodinated contrast material and the inability to diagnose the site of the intimal tear.
MRI: Magnetic resonance imaging (MRI) is currently the gold standard test for the detection and assessment of aortic dissection, with a sensitivity of 98% and a specificity of 98%. An MRI examination of the aorta will produce a three-dimensional reconstruction of the aorta, allowing the physician to determine the location of the intimal tear, the involvement of branch vessels, and locate any secondary tears. It is a non-invasive test, does not require the use of iodinated contrast material, and can detect and quantitate the degree of aortic insufficiency.
The disadvantage of the MRI scan in the face of aortic dissection is that it has limited availability and is often located only in the larger hospitals, and the scan is relatively time consuming. Due to the high intensity of the magnetic waves used during MRI, an MRI scan is contraindicated in individuals with metalic implants. In addition, many individuals succumb to claustrophobia while in the MRI scanning tube. |
