Ruptured Abdominal Aortic Aneurysm

An aneurysm is a localized dilatation of an artery, with an increase in diameter of greater than 50% of the normal size. Abdominal aortic aneurysms (AAAs) are most commonly encountered in the infrarenal region. In general, AAAs occur when the maximal anteroposterior diameter is greater than 3 cm. As the population ages, the incidence of AAAs is increasing. Approximately 1.5 million Americans have AAAs, and 200,000 are diagnosed each year.¹ Most AAAs are asymptomatic and are detected during routine physical examinations or radiologic procedures for other problems. Elective repair is reserved for subjects with AAAs of at least 5 cm in diameter, and postoperative survival is approximately 95%. Symptomatic aneurysms may result in abdominal, flank, or back pain, distal embolization, thrombosis, or rupture. The latter complication is usually fatal if untreated; surgical repair carries a 30% to 65% survival rate. Success is highly dependent on rapid diagnosis and transport to the operating room.²

Ultrasonography is often used as the initial procedure for diagnosis of AAAs. It is also used for screening and serial measurement of AAA size. When aneurysms rupture, only 50% of patients present with the “classic” triad of abdominal or back pain, hypotension, and pulsatile abdominal mass. The clinical diagnosis may, therefore, pose a challenge, particularly in patients without a known history of AAA. An accurate imaging diagnosis of retroperitoneal hematoma in the presence of AAA enables prompt initiation of surgical treatment.

Although noncontrast computed tomography (CT) is also useful for this purpose, ultrasound is more expeditious in these potentially unstable patients. Ultrasound diagnosis of an abdominal aneurysm has a sensitivity of 98% and specificity of 95% in the setting of abdominal pain and hemodynamic instability.³ Although active extravasation is never brisk enough to demonstrate by Doppler ultrasound, the resultant hematoma provides evidence of rupture (Figure 19-1). Initially, intramural hemorrhage may be visible as an echogenic crescent within the aneurysmal wall. Following rupture, blood initially accumulates in the para-aortic space, extending toward the flanks via the pararenal space. Hemorrhage may track along the course of the iliac arteries to the extraperitoneal spaces of the pelvis (Figure 19-2). Anterior extension may transgress the posterior peritoneum, resulting in hemoperitoneum.

FIGURE 19-1 Ruptured aortic aneurysm. Transverse (A) and oblique (B) views of aortic aneurysm (cursors) with abundant mural thrombus and a small periaortic hematoma (arrow).

FIGURE 19-2 Retroperitoneal hematoma. A, Computed tomographic image shows a large abdominal aortic aneurysm with intramural (arrowhead) and retroperitoneal (arrow) hemorrhage. B, Ultrasound shows a large retroperitoneal hematoma in the left flank, infiltrating the psoas muscle (arrow) and distorting tissue planes.

When the patient presents with abdominal pain, the operator may be reluctant to use probe pressure to visualize the aorta for fear of provoking or exacerbating rupture. Although this is rarely a practical concern, the use of a coronal approach from the left flank avoids trapping the aneurysm between the transducer and the spine. In addition, this approach frequently provides improved visualization of the aorta by circumventing overlying bowel gas.

Nonetheless, conventional sonography has some limitations in imaging patients with a ruptured aneurysm because retroperitoneal hematoma is not always readily apparent, and the rupture itself is not directly visualized. In the appropriate clinical setting, contrast-enhanced sonography can be performed in less time than is required for contrast-enhanced CT. Findings associated with aneurysm rupture in these studies include delayed or protracted aortic lumen opacification, contrast leakage through luminal thrombus or around the aneurysm, and dependent contrast pooling around the aneurysm.⁴ There are no large studies evaluating the diagnostic accuracy of ultrasound in patients with suspected acute rupture of an AAA.

Aortic dissection may be encountered as an alternative diagnosis in patients with a clinical presentation suggesting aneurysm rupture. Although ultrasound is not a primary diagnostic modality, since it cannot reveal the full extent of dissection in the thorax, the characteristic intimal flap and altered blood flow pattern are readily visible on abdominal sonography and can lead to appropriate workup and treatment (Figure 19-3).⁵ In a recent study, the use of contrast-enhanced sonography increased the sensitivity for abdominal aortic dissection to 97%, from 68% for combined gray scale and duplex sonography.⁶ Additional details about aortic aneurysm and dissection are presented in Chapter 27.

FIGURE 19-3 Aortic dissection. Sagittal (A) image of the abdominal aorta shows an echogenic intimal flap (arrow). Transverse color view (B) clearly distinguished the patent true lumen (arrow) from the thrombosed false lumen (arrowhead).

Carotid Artery Stenosis

Symptomatic carotid artery stenosis is manifested by transient ischemic attacks (TIAs) or stroke (cerebrovascular accident, CVA). Neurologically unstable patients in whom there is no evidence of intracranial hemorrhage are potential candidates for emergency carotid endarterectomy. These cases include patients presenting with crescendo TIAs, stroke in evolution, fluctuating or fixed neurologic deficits caused by acute carotid artery thrombosis, and free-floating thrombus. In these cases, rapid sonographic evaluation of the carotid bifurcation can result in timely intervention to prevent stroke or death.

Sonographic detection and quantification of carotid stenosis in the emergency setting (i.e., progressive neurologic deficit) is similar to the nonacute setting and is described in Chapter 9. It is important to distinguish acute carotid thrombosis from stable (chronic) carotid occlusion. Whereas revascularization of a chronic occlusion is generally contraindicated, timely intervention may avoid or limit neurologic sequelae in cases of acute thrombosis.

Acute thrombosis may occur as a complication of endarterectomy or stenting, or it may represent acute progression of carotid stenosis. The thrombus is usually heterogeneously echogenic (Figure 19-4). The vessel lumen is of normal caliber or slightly expanded, as opposed to chronic occlusion, which may result in luminal narrowing or obliteration.⁷ Pulsations may be observed in the vessel wall, which retains its normal compliance. Swirling, sludgelike flow may be observed in the carotid bulb at the interface of the thrombosed and patent lumen. Spectral waveforms assume a high-impedance, hammer-like, or to-and-fro configuration proximal to the thrombus. Internalization of the external carotid artery waveform is uncommonly seen with acute thrombosis due to insufficient time to develop collateral pathways.