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The utility of multiple imaging modalities to diagnose acute aortic dissection

Case Reports

Katrina F. Hurley, MD;* James Ducharme, MD†

*Emergency Medicine Resident, Department of Emergency Medicine, Dalhousie University, Halifax, NS, †Clinical Professor of Medicine, Division of Emergency Medicine, McMaster University, Hamilton, Ont.

CJEM 2008;10(1):75-80

Abstract

A 21-year-old man with Marfan syndrome and known aortic root aneurysm presented to our emergency department with symptoms suggestive of acute aortic dissection. The patient was hemodynamically stable and bilateral upper extremity blood pressures were similar. There was no mediastinal widening on portable chest radiograph. Both contrast CT and retrograde angiography of the aorta failed to identify dissection. Subsequent transesophageal echocardiography demonstrated a Stanford classification type A dissection. This case demonstrates the utility of multiple imaging modalities for identifying aortic dissection in high-risk patients.

Résumé

Un jeune homme de 21 ans atteint du syndrome de Marfan avec présence connue d'un anévrisme de la racine aortique s'est présenté à l'urgence avec des symptômes évoquant une dissection aiguë de l'aorte. Le patient était stable sur le plan hémodynamique et les pressions artérielles bilatérales des membres supérieurs étaient similaires. Les résultats de la radiographie pulmonaire réalisée sur un appareil portatif n'indiquaient pas d'élargissement du médiastin. Ni la tomodensitométrie avec agent de contraste ni l'angiographie rétrograde de l'aorte n'ont permis de déceler la dissection. Une échocardiographie transœsophagienne subséquente a montré qu'il s'agissait d'une dissection de type A selon la classification de Stanford. Ce cas met en évidence l'utilité d'avoir recours à de multiples techniques d'imagerie médicale pour établir un diagnostic de dissection aortique chez les patients à haut risque.

Introduction

Aortic dissection is reported to be one of the "most undiagnosed serious conditions."1 In one study, nearly 30% of cases were not diagnosed until the post-mortem examination.2 Given a mortality rate of 1% per hour in patients with untreated thoracic aortic dissection, diagnosis must be prompt.3,4 Unfortunately, the diagnostic imaging modalities available have limited sensitivity for identifying aortic dissection. Patients with connective tissue disorders, such as Marfan syndrome account for more than 5% of all patients with aortic dissection1,4 and many of the cases involving younger patients.5 We describe a case of acute aortic dissection in a young adult with Marfan syndrome for whom contrast CT and retrograde angiography of the aorta were nondiagnostic, while transesophageal echocardiography (TEE) demonstrated a Stanford classification type A aortic dissection.

Case report

A 21-year-old man presented to our emergency department (ED) after developing severe chest pain during a game of ball hockey. He described his pain as severe retrosternal chest heaviness (graded 8-9 out of 10). His review of systems included shortness of breath, but he denied nausea, vomiting or palpitations. Past medical history was significant for Marfan syndrome diagnosed at age 5 years and related problems, including an aortic root aneurysm, mitral valve prolapse and bilateral lens dislocations. He had a documented prior episode of ventricular tachycardia and was being treated with sotalol 80 mg daily and atenolol 50 mg twice daily.

Initial examination revealed a regular pulse of 72 beats/ minute (bpm), a respiratory rate of 20 breaths/minute, and blood pressures of 128/88 mm Hg in the right arm and 118/78 mm Hg in the left arm. Breath sounds were normal bilaterally and cardiac auscultation revealed a normal S1, a distant S2 and a grade III/VI pansystolic murmur that was loudest at the apex and radiated to the left axilla. The patient showed physical features consistent with Marfan syndrome, including a marked pectus excavatum. The remainder of the examination was unremarkable.

His electrocardiogram showed a ventricular rate of 68 bpm, downsloping ST-segments in the inferior leads, and evidence of left ventricular hypertrophy and left atrial enlargement. A portable chest radiograph revealed a left heart shift, clear lung fields and scoliosis but no mediastinal widening (Fig. 1). Initial laboratory investigations included a hemoglobin level of 139 g/L, a platelet count of 124 × 109/L and an international normalized ration (INR) of 1.35.

A presumptive diagnosis of acute aortic dissection was made. Non-helical CT with contrast and retrograde aortography failed to demonstrate aortic dissection (Fig. 2). Because clinical suspicion remained high, TEE was undertaken with procedural sedation and analgesia in the ED. This revealed an intimal flap in the ascending aorta proximal to the right coronary artery ostium, extending to the mid-ascending aorta (Fig. 3). Trivial aortic insufficiency and severe mitral insufficiency were also noted. All diagnostic tests were performed within 6 hours. The patient remained hemodynamically stable throughout his ED stay.

Figure 1

Fig. 1. Portable chest radiograph showing a left heart shift, clear lung fields and scoliosis.

Figure 2

Fig. 2. Retrograde angiography of the aorta showing a grossly dilated aortic root. No dissecting lesion was visible.

The patient was admitted to the intensive care unit and underwent repair with the Bentall procedure.6 Postoperatively, he developed anterior compartment syndrome in the right leg, necessitating fasciotomy. He recovered well and was discharged home in stable condition.

Figure 3

Fig. 3. Transesophageal echocardiography demonstrating dilated aortic root and dissection flap (arrow).

Discussion

Aortic dissection occurs when an intimal tear develops, allowing blood to penetrate the aortic wall, dissect longitudinally through the media and form a false lumen.1 Conditions associated with medial degeneration, such as connective tissue disorders (e.g., Marfan syndrome) and hypertension, increase the risk for dissection. There are 4 main imaging modalities used to diagnose aortic dissection: retrograde angiography; ultrasound, including transesophageal TEE; CT scanning; and magnetic resonance imaging (MRI).

Retrograde angiography is the historical criterion standard for diagnosing aortic dissection. However, retrograde angiography is invasive and may extend the dissection (Table 1). The patient is also exposed to contrast media and radiation. Retrograde angiography may appear inappropriately normal if the false tract has thrombosed. Recent studies suggest that less invasive studies such as TEE, CT scanning and MRI provide excellent diagnostic accuracy and mitigate some of the risks inherent to retrograde angiography (Table 2).7-19 Unfortunately, the methodology of the research studies supporting the use of these alternative imaging modalities may make their results inappropriate for populations with a relatively low prevalence of aortic dissection.

Growing familiarity with ultrasound technology and the use of multi-planar (M-mode) echocardiography rather than 2-dimensional (biplanar) scans have led to higher diagnostic sensitivity and specificity — approaching 100% — for aortic dissection.7,10,13,14,16,17,19,20 In particular, TEE is reliable for localizing intimal tears and has the added potential benefit of assessing valve function and flow in false lumens.17,21,22 In addition, TEE may be performed at the bedside in the ED, yielding advantages for hemodynamically unstable patients (Table 2). However, TEE requires esophageal intubation, which may increase systolic blood pressure in awake or inadequately sedated patients, increasing the risk of extension of the dissection or aortic rupture.23

CT is now the most frequently ordered diagnostic imaging modality for the initial evaluation of patients with suspected aortic dissection.24 Multi-detector row CT is the most rapid diagnostic test for aortic dissection, with data acquisition accomplished in less than 30 seconds.18 Reported sensitivities and specificities range from 79% to 100% for CT, but early studies should be interpreted with caution as this technology is evolving rapidly.7,11,12,15,16,18 Combining colour Doppler transthoracic echocardiography (TTE) with CT increases diagnostic accuracy. However, TTE may be limited in patients with chest deformities such as those with Marfan syndrome and pectus excavatum.16

In experienced hands, MRI is highly sensitive and specific for aortic dissection.8,9,11,15,16 However, MRIs are not universally available and image acquisition time is up to 30 minutes.18 This relatively prolonged time away from the ED places hemodynamically tenuous patients at risk for delayed aggressive stabilization should they decompensate during the study. Decreasing the duration of MRI studies would, at least partially, mitigate this concern. Two small studies have described faster MRI techniques that do not require either electrocardiographic gating or breath-holding.25,26 If these techniques, which can be performed in less than a minute, prove accurate, MRI may become the imaging modality of choice for evaluating patients for aortic dissection. Of note, MRI is an observer-dependant technology with reported sensitivities ranging from 52% to 100% depending on the experience of the radiologist.21,27

A recent systematic review compared the diagnostic accuracy of TEE, CT scanning and MRI. The authors concluded that these studies are equally reliable for diagnosing or ruling out thoracic aortic dissection.28 The 95% confidence intervals for sensitivity were 95%-99% for TEE, 96%-100% for CT scanning and 95%-99% for MRI. One interpretation of these findings is that up to 5% of thoracic aortic dissections can be missed when only a single imaging modality is used to diagnose aortic dissection.28 The 95% confidence intervals for specificity were 92%-97% for TEE, 87%-99% for CT scanning and 95%-100% for MRI. The authors found significant heterogeneity in the study populations. Unfortunately, pooling the results from heterogeneous populations may yield invalid results.29 In addition, the authors of this systematic review do not discuss the impact of one negative test result on the test characteristics of subsequent tests. Further, the studies included in this systematic review did not adequately take into account pretest probability.

Two studies did take into account sequential diagnostic imaging modalities and pretest probability. A decision analysis by Sarasin and colleagues concluded that, in patients with low pretest probability (< 15%), a single negative TEE, CT, MRI or aortic angiogram is sufficient to rule out dissection, while in patients with higher pretest probability, additional imaging is required.30 Unfortunately, this model has not been validated and is limited by the quality of published data and the ability of clinicians to determine pretest probability. An analysis by Barbant and colleagues also discusses the importance of pretest probability and the application of Bayes' theorem.31 These authors reported that when the prevalence of disease was varied from intermediate (10%) to high (50%), the negative predictive value of CT decreased from 98% to 85%, reflecting the need for a second diagnostic test in high-risk populations.31 In our very high-risk patient, 3 imaging modalities were needed to convincingly diagnose aortic dissection.

Table 1. Comparison of the advantages and disadvantages of TEE, CT, MRI and retrograde angiography of the aorta in diagnosis of acute aortic dissection
Advantages and disadvantages TEE CT MRI Angiography
Advantages        
Performed at bedside x      
No radiation x   x  
Expeditious x x    
Assesses entire aorta and branches   x x x
Potentially identifies alternative diagnoses   x x  
Specific advantages Evaluation of cardiac function and flow in true/false lumens Most readily available    
Disadvantages        
Invasive       x
Requires experienced operator/interpreter x   x  
Expensive     x x
Requires contrast   x   x
Specific risks Aspiration; exacerbation of hypertension   Contraindicated in patients with some types of aneurysm clips or claustrophobia Retrograde extension of dissection; cholesterol embolization syndrome; falsely negative when false lumen thrombosed
TEE = transesophageal echocardiography; MRI = magnetic resonance imaging.
Table 2. Studies assessing the characteristics of diagnostic imaging in aortic dissection
Study Period Prevalence, % Criterion standard n No. of confirmed cases* Sensitivity, % Specificity, %
Erbel et al7 1983-87 50 OR findings, autopsy or agreement on 2 of 3 imaging studies 164 47† TEE 99
CT 83
Angio 88		 TEE 98
CT 100
Angio 94
Nienaber et al8 1988-91 58 OR findings, autopsy or angiography 53 31 TEE 100
MRI 100		 TEE 68
MRI 100
Nienaber et al9   74 OR findings, autopsy or angiography 35 21 TEE 100
MRI 100		 TEE 78
MRI 100
Chirillo et al10 1990-93 57 OR findings, autopsy or 6 mo of follow-up 70   TEE 98
Angio 88		 TEE 97
Angio 97
Sommer et al11   67 OR findings, autopsy, angiography or at least 3 mo of follow-up 49 28 TEE 100
CT 100
MRI 100		 TEE 94
CT 100
MRI 94
Small et al12 1990-95 32 OR findings, autopsy, other imaging or follow-up 81 17 CT 96 CT 96
Keren et al13 1991-94 43 OR findings, autopsy or other imaging 112 60 TEE 95 TEE 98
Evangelista et al14   49 OR findings, autopsy, CT/MRI 132 29 TEE 97 TEE 100
von Kodolitsch et al15   64 OR findings, autopsy or angiography 120 105 TEE 100
CT 83
MRI 100		 TEE 88
CT 90
MRI 96
Losi et al19   65 OR findings or angiography 46 45 TEE 97 TEE 100
Kodolitsch et al16 1984-94 45 OR findings, autopsy or angiography 86 59 TEE 100
CT 79
MRI 100		 TEE 96
CT 87
MRI 96
Pepi et al17 1995-98 62 OR findings, autopsy or angiography 86 59 TEE 100 TEE 100
Hayter et al18‡ 2002-03 18 OR findings, autopsy, DC Dx, follow-up or other imaging 373 37 CT 99 CT 100
OR = operating room; TEE = transesophageal echocardiography; angio = angiography; MRI = magnetic resonance imaging; DC Dx = discharge diagnosis.
*No. of anatomically confirmed cases of aortic dissection (e.g., OR findings or autopsy).
†Registry study.
‡Study included disorders other than aortic dissection.

Conclusion

Our case demonstrates that for high-risk patients multiple imaging modalities may be needed to adequately diagnose aortic dissection. Given the associated mortality, reliance on a single imaging modality to rule out aortic dissection in high-risk patients is unwise.

References

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Competing Interests: 
None declared.
Correspondence to: 

Dr. Katrina Hurley, Emergency Department, QEII, Halifax Infirmary — Rm 3021, 1796 Summer St., Halifax NS B3H 3A7; kfhurley@dal.ca

Received: Mar. 1, 2007; revisions received: July 12, 2007; accepted: Aug. 3, 2007
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