Laboratory data are usually within normal limits in patients with acute dissection. The white blood cell count may be slightly elevated to 12,000 to 20,000/μL, most likely as a stress response. Electrocardiogram (ECG) interpretation may show left ventricular hypertrophy due to chronic hypertension, but other changes are rare. Acute ischemic changes should raise the concern of coronary artery involvement by the dissection in the patient with a typical history. Conversely, to avoid the dire consequences of misdiagnosis, any patient presenting to the ER with ECG changes suggesting myocardial ischemia (especially with evidence of right coronary artery involvement) should have their history considered carefully before immediately moving to anticoagulation, thrombolytics, and urgent cardiac catheterization to treat the more prevalent condition of atherosclerotic coronary artery disease.
Diagnostic imaging is the most important investigation for the diagnosis and classification of aortic dissections. Standard anteroposterior and upright lateral chest x-rays often reveal a widened mediastinum, although this may be absent in up to 40% of type A dissections (Fig. 30-4). Classically, the aorta bulges to the right with type A and to the left with type B dissections. Occasionally a double rim of calcification may be present in the distal aortic arch or a pleural effusion may be present (left more than right), due mainly to a serous sympathetic reaction rather than to frank blood from a rupture.
Chest x-ray illustrating widened mediastinum with blunting of the aortic knob.
Significant debate exists as to the best imaging modality presently available for aortic dissections. The ideal modality would be readily available, easy to interpret, represent no risk to the patient, and provide all the information the surgeon requires. These needs include assessment of the ascending aorta, identifying the site of the intimal tear, the full extent of the dissection, existence of branch vessel involvement, presence of a pericardial effusion, and presence and severity of aortic valve insufficiency.19 Several imaging techniques including aortography, computed tomographic (CT) scanning, magnetic resonance imaging (MRI), and echocardiography are highly accurate for the diagnosis and classification of dissections.20 Controversy exists as to which of these is the most accurate.19,20–24 In the past many authors believed that aortic angiography was the most definitive diagnostic method, with sensitivity and specificity of 88% to 90% and 90% to 95%, respectively.2,25 In addition to confirming the diagnosis by illustrating the true and false channels, the aortogram could pinpoint the site of the intimal tear (Fig. 30-5), establish the extent of the dissection, quantitate the severity of aortic insufficiency if present, and identify the presence and degree of aortic branch occlusions. Some authors, however, have expressed concerns over false-negative reports due to viewing the flap and both lumens en face, with the central beam missing a small localized dissection, and possibly missing the flap due to simultaneous opacification of both true and false lumens.12 The procedure is also invasive and requires a contrast load, so less invasive investigations are preferred.
A. Aortogram (lateral projection). Grossly dilated ascending aorta (open arrow) with visible intimal flap (solid arrow). Note normal descending aorta. B. Contrast-enhanced CT scan of the thorax of the same patient. Arrow identifies intimal flap. (Reprinted with permission from Kotler N, Steiner RM: Cardiac Imaging: New Technologies and Clinical Applications. Philadelphia, FA Davis, 1986.)
CT scanning is a noninvasive technique that yields excellent images of dissections, especially type B. Specific identification of true and false lumens with a flap is possible, as well as detection of pericardial effusions and accurate depiction of the extent of the dissection (Figs. 30-6 and 30-7). The new third-generation helical CT scanners yield excellent images both in two and three dimensions (Fig. 30-8). CT is very specific and accurate (∼100%) for the diagnosis of dissection,18,25 and 96% accurate for the presence of aortic branch involvement.19 CT scanning does not yield information regarding aortic insufficiency or left ventricular function, and requires both contrast administration and exposure to radiation. CT scanning is also inferior to aortography for identifying the site of the intimal tear. Recently, however, the site of the intimal tear has had debatable bearing on the type of dissection or surgical success. Since CT scanning lacks somewhat in sensitivity, if the CT scan is negative and the suspicion of aortic dissection remains high, other investigations should be done to rule out a dissection. Helical CT scanning is the author's recommended imaging technique for the long-term follow-up of the patient, given its low expense, high reproducibility, and minimal patient discomfort.
Contrast-enhanced CT scan of the thorax. Aneurysmal dilation of the aorta is clearly evident (open arrow). An intimal flap seen in the ascending aorta (long arrow) and continuing into the descending aorta (short arrow).
Contrast-enhanced CT scan of the thorax. A dilated aortic root and intimal tear are present. Contrast material fills the true lumen first (open arrow), while false-lumen filling is delayed (solid arrow).
A. Two-dimensional images of a spiral CT scan from a patient with an acute type A aortic dissection. (Open arrows point to saccular dilation of the false channel outside the true one.) B. Three-dimensional image reconstruction of the spiral CT scan above. Observe the false channel (solid arrows) surrounding the true one. (Reprinted with permission from Borst HG, Heinemann MK, Stone C: Surgical Treatment of Aortic Dissection. New York, Churchill-Livingstone, 1996, p 121.)
Echocardiography is likely the diagnostic modality of choice for acute aortic dissections. Both transthoracic and transesophageal two-dimensional echocardiography can accurately evaluate the most important issues of dissections; involvement of the ascending aorta, presence and severity of aortic insufficiency, the entry sites in the intimal flap, and presence of aortic arch dilation. Two lumens may be seen separated by a flap, or there may be central displacement of intimal calcification. The addition of color-coded Doppler allows better identification of the true and false lumens, even if the false lumen is thrombosed. The presence of a pericardial effusion and any left ventricular wall motion abnormalities also may be assessed accurately. Transesophageal echocardiography (TEE) was previously criticized for being unable to visualize the aortic arch, but current multiplanar TEE transducers have few blind areas. Its main limitation is inability to see the intra-abdominal aorta, but this can be assessed with simple ultrasound.23
Transthoracic echocardiography is less accurate than transesophageal echocardiography in the diagnosis of aortic dissections. It is also frequently technically limited due to chest wall abnormalities, chronic obstructive pulmonary disease, and obesity.
TEE21,23 is the most efficient and accurate diagnostic modality available (Fig. 30-9). Its sensitivity and specificity are over 98%, so this technique very accurately identifies the presence or absence of a dissection. Notably, it can be performed in less than 30 minutes in an emergency room or intensive care unit. The efficiency and mobility are very important in the management of critically ill patients, who are best cared for in a well-monitored ICU rather than in the radiology suite. Occasional complications (<1%) have included transient arrhythmias (atrioventricular block, bradyarrhythmias, or premature ventricular contractions), reports of rupture of the dissection during the insertion of the probe, and esophageal perforation. Generally the procedure is well tolerated with appropriate control of pain and hypertension.21,22,24
A. Transesophageal echocardiogram of type A dissection. Note clear depiction of true and false lumens as well as intimal flap originating in the ascending aorta (two arrows in bottom left image) and extending into the descending aorta. (Reprinted with permission from Erbel et al.21) B. Transesophageal echocardiogram of type B dissection. True and false lumens are illustrated with spiraling of the dissecting hematoma around the aorta. (Reprinted with permission from Erbel et al.21)
MRI is very accurate, sensitive, and specific in dissections, but has been used sparingly as the procedure requires 45 minutes and can be difficult due to limited access to a critically ill patient.19,23–25 It is, however, the only modality that provides all of the previously mentioned preoperative needs of the surgeon. It is noninvasive and does not require contrast material or ionizing radiation. Excellent contrast can be obtained between extraluminal structures, and it allows the visualization of vascular walls and both clotted and flowing blood. MR technology is advancing rapidly, with accuracy of 100% with phase-contrast cine MR angiography,1–9 and new MRI sequences (such as the breath-hold gradient-echo) have been able to reduce the procedure time to less than 5 minutes without compromising accuracy.26
The author presently recommends TEE as the initial investigation. If this is not immediately available, then a helical CT scan of the thorax is performed. If this is negative and there is still a strong suspicion of dissection, then MR angiography may be performed in a stable patient. Of course, this approach should be modified by availability and the local expertise with these imaging techniques at each institution.