Diagnostic Images, Treatment Decisions

Case 1:

Middle-aged Man With Chest Pain

F

or 3 days, a 42-year-old man has had episodic dullchest pain. The anterior precordial and retrosternalpain intensifies with inspiration and movement. He has nohistory of recent viral infection, hypertension, coronaryartery disease, cardiac surgery, diabetes mellitus, or hyperlipidemia.There is no family history of cardiovasculardisease.The patient denies risk factors for HIV infection. Hetakes no medications or herbal remedies and does not useillicit drugs.In the emergency department, the patient is alert butuncomfortable. Blood pressure is 160/102 mm Hg in allextremities; heart rate, 103 beats per minute, regular andequal bilaterally. There is a faint early diastolic murmurin the aortic region; no gallop, pericardial rub, or knock isaudible. The lungs are clear; abdominal findings are unremarkable.The cranial nerves are intact; no neurologicdeficits are found.Serial measurement of cardiac isoenzymes revealsno evidence of myocardial injury. The patient's ECG andchest film are shown here.

What abnormalities are evident on these studies--and to what diagnosis do they point?

A.

Acute coronary syndrome

B.

Acute pericarditis

C.

Aortic dissection

D.

Ventricular aneurysm

E.

Aneurysm of the thoracic aorta

Case 1:

WHAT'S WRONG:

The chest film reveals that theaorta is enlarged, particularly for aperson of this age; other radiographicfindings are normal. The ECGshows diffuse ST-segment elevation.The absence of ECG evidence of anacute coronary syndrome and theenlarged aorta demonstrated on thechest film signaled the need for furtherworkup.

Diagnostic steps.

A transthoracicechocardiogram showed aorticroot dilatation but no aortic regurgitation,despite the patient'ssoft diastolic murmur. Systolic anddiastolic left ventricular functionwas normal.A CT scan of the chest revealeda proximal dissection of the

thoracic aorta with extension to thedescending aorta,

C

(

Figure

). Becausesymptoms had been presentfor less than 2 weeks, the dissectionwas considered acute. In about two thirds of affectedpatients, dissections are acute.

¹

Therapy.

The patient was given propranolol andnitroprusside in preparation for surgery. Subsequently,the aortic dissection was surgically repaired; the postoperativecourse was uneventful.

Histologic findings.

Histopathologic examinationof specimens obtained during surgery revealed cysticmedial necrosis, which is found in 30% to 40% of patientswith aortic dissection.2 This disease is characterized byfocal fragmentation of elastic fibers, loss of smooth musclecells of the media, and deposition of varying amountsof acid mucopolysaccharides. Typically, the disease involvesthe ascending aorta, predisposing the vessel todisruption. Cystic medial necrosis is seen frequently inpatients with Marfan syndrome and, to a lesser extent,in elderly persons; however, this patient had no identifiablerisk factors for this vascular disease.

AORTIC DISSECTION:AN OVERVIEW

Spontaneous thoracic aortic dissection must betreated immediately to prevent severe morbidity anddeath. Approximately 28% of untreated patients die within24 hours of the event, 50% within 48 hours, 70% within 1week, and 90% within 3 months.

^3,4

The annual incidence is thought to be between 5 and30 aortic dissections in every 1 million persons. Aortic dissectionoccurs 3 times more often in men than in women;the risk for African Americans is twice that for whites. Dissectionsof the proximal aorta typically occur in youngeradults (mean age, 49 years) and are often associated withdegenerative connective tissue diseases, such as Marfansyndrome; cystic medial necrosis, as in this patient; andEhlers-Danlos syndrome.

^5,6

Distal dissections occur in olderpersons (mean age, 60 years) and are typically associatedwith hypertension and atherosclerosis. Other predisposingfactors are listed in the

Table

.

⁷

CLASSIFICATION OFDISSECTIONS

Dissection--a dynamic process that can occur anywherein the aorta--usually arises in the ascending aorta.The greatest shear in the vascular system is produced byblood expulsed from the heart onto the right lateral wallof the aorta.

⁸

After an initial proximal tear, propagation ofblood through a false lumen can extend along the greatercurve of the aortic arch and down along the descendingaorta. Retrograde extension of the false lumen toward theaortic valve is also possible.

⁹

Aortic dissections that occurbeyond the ascending aorta typically originate immediatelydistal to the left subclavian artery in the vicinity of theligamentum arteriosum.Aortic dissections have been classified anatomicallyby 2 schemes. The Stanford classification modifies the3-pronged DeBakey system. Because neither of theseschemes can describe all dissections, a simplified schemehas recently been suggested (

Box

).

¹⁰

HISTORY ANDEXAMINATION

Symptoms.

Classically, the patient describes theabrupt onset of a tearing, excruciating pain in the anteriorchest, in the interscapular region, or in both areas. Thesite of the pain may be a clue to the location of the dissection;ascending aortic dissection is often associated withanterior pain, whereas distal dissection is more likely toproduce back pain. As the dissection propagates, the painmay migrate to the neck, arm, epigastrium, or leg. Painlessaortic dissection is uncommon.

¹⁰

About 12% of patients with proximal aortic dissectionexperience syncope. Ischemic paresis and monoparesis,peripheral nerve impairment, transient ischemic attacks,and spinal cord lesions also can occur.

¹¹

Physical findings.

These may be normal; however,assessment of the neck vessels, all peripheral pulses, bloodpressure in all extremities, and heart and lung sounds iswarranted. A thorough investigation of the cardiovascularsystem may provide clues to the correct diagnosis.

⁷

Subtlepericardial or pleural friction rubs, or signs of a pleural effusionusually on the left may be detected. A complete neurologicexamination is indicated.Aortic regurgitation occurs in as many as half of patientswith proximal aortic dissection, and a diastolicmurmur has been reported in 25% of patients.

^12,13

Severeacute aortic regurgitation is the second most commoncause of death in patients with aortic dissection. Thesepatients typically present with cardiac decompensationand shock.

NARROWING THEDIFFERENTIAL

The manifestations of aortic dissection are proteanand subtle. The disorder can mimic other cardiovasculardiseases, such as pericarditis, acute coronary syndrome,ventricular aneurysm, thoracic aortic aneurysm, andacute aortic regurgitation. The diagnosis may not bestraightforward; aortic dissection has been missed in 38%of patients on initial evaluation.

¹

Acute pericarditis

usually presents with chest painthat is exacerbated by inspiration, cough, and recumbency.ECG changes in uncomplicated acute pericarditis resemblethose in aortic dissection; the characteristic patternconsists of widespread ST-segment changes, absenceof reciprocal ST-segment depression, and depressed PRsegment in all leads except aVR and occasionally V

₁

.

¹⁴

Although electrocardiography is rarely helpful in diagnosingaortic dissection, the absence of ECG evidence ofacute myocardial ischemia or infarction may help rule outan

acute coronary syndrome.

However, 1% of patients withaortic dissection experience concomitant coronary arteryocclusion that may precipitate acute myocardial infarctionand left ventricular failure.

¹¹

Other ECG abnormalities thatoccur with aortic dissection include left ventricular hypertrophy,left axis deviation, heart blocks, and dysrhythmias.

¹¹

Ventricular aneurysm

develops in 5% to 10% of all patientswith anterior myocardial infarction.

¹⁵

The persistentST-segment elevation in an ECG area of infarction cansuggest aneurysm formation but may be mimicked by anaortic dissection. The diagnosis of a ventricular aneurysmis best made by echocardiography.Dissection of the thoracic aorta and

thoracic aorticaneurysm

are both life-threatening conditions that are frequentlyregarded as similar problems but, in fact, differsignificantly. Whereas aortic dissection involves an intimalrupture that leads to separation of the intima from the adventitiaand propagation of blood through the vesselmedia, a thoracic aneurysm is a localized dilatation of the aorta that involves all layers of the vessel and arises in aweakened and stretched arterial wall. Although aortic dissectionsare often erroneously referred to as "dissectinganeurysms," concomitant vascular dilatation and dissectionare rare.

¹⁰

DIAGNOSTIC STUDIES

Roentgenography.

In patients with aortic dissection,the chest film may show a widened mediastinum, awidened aortic knob, a difference in the diameter of theascending and descending aorta, and blurring of the aorticmargin secondary to local extravasation of blood.Other associated findings are unilateral or bilateral pleuraleffusion, left ventricular hypertrophy, displacement of theleft main bronchus or esophagus, and evidence of pericardialeffusion. The calcium sign--a 1-cm or more separationof intimal calcification from the outer border of theaortic knob--is highly suggestive of dissection but is onlyoccasionally present.

¹⁶

These findings may be subtle;therefore, the diagnostic value of a chest film in suspectedaortic dissection is limited.

Other imaging techniques.

Modalities that can beused to definitively diagnose aortic dissection are:

• CT. This highly sensitive and specific noninvasive techniqueallows for rapid diagnosis of aortic dissections.Thus, CT has emerged as the most commonly used diagnostictest for aortic dissection.11
• Transesophageal echocardiography (TEE). An accuratestudy, TEE may be performed quickly at the bedside withminimal risk. In addition, TEE can identify other cardiovasculardiseases.
• MRI. This technique is also highly sensitive and specific10;however, its availability is limited. Also, it is difficult tomonitor patients during imaging; moreover, this technologycannot be used in patients with implanted metallicdevices.
• Angiography. Once the gold standard in this setting, angiographyhas largely been supplanted by noninvasivediagnostic techniques. It is now rarely used as an initialstudy.

THERAPY

Medical therapy.

Emergent medical therapy is indicatedfor all patients with aortic dissection, except forthose with hypotension. The long-term goals of medicaltreatment are to stabilize the dissection, prevent rupture,reduce the risk of complications, and accelerate healing.

¹⁰

The immediate objective is to reduce the rate of rise ofpressure in the aorta and to decrease mean arterial bloodpressure. This is best accomplished by decreasing thevelocity of ventricular contraction. Thus, a β-blocker is theinitial drug of choice.Intravenous propranolol is given unless contraindicated.The heart rate is a measure of the drug's effectiveness.Nitroprusside is added if propranolol alone does not sufficientlylower mean arterial blood pressure. The β-blocker isadministered before the vasodilator to prevent propagationof the dissection from increased left ventricular contractionforce and aortic pulse caused by a reflex catecholamine releasesecondary to vasodilation. Trimethaphan, a ganglionicblocker as well as a direct vasodilator, is an effective alternativeto the β-blocker-nitroprusside combination; however,significant side effects may develop.Hypotensive patients require fluid resuscitation andemergent surgery. Often an intra-aortic balloon pumpis placed as a "bridge" therapy during the preparation forsurgery. Patients who present with hypotension causedby an aortic dissection have a poor prognosis.

Surgery.

Urgent surgical repair of proximal aorticdissections is indicated, unless comorbid conditions preventinvasive interventions. The origin of the dissection isexcised and replaced during surgery.The best surgical technique for the treatment of aorticdissection has yet to be determined. In approximatelyhalf of surgically treated patients, a segment of the aortawith the dissection persists.Patients with uncomplicated acute distal aortic dissectionscan be managed medically. About 75% of thesepatients survive whether they are treated medically or surgically.

¹⁷

However, urgent surgery needs to be performedin patients with distal dissections who have complications.Indications for surgery in these patients include rapid expansionof the false lumen, blood leakage, impending rupture,impaired blood flow to an organ or limb, and uncontrollablepain.

¹⁸

Endovascular stents.

Placement of an endovascularstent may offer potential benefits for patients with aorticdissection; however, this procedure requires further studybefore the indications, efficacy, and safety of the procedurecan be defined. Endovascular stent placement remains investigational.Endovascular stents are currently recommendedas a palliative measure for symptomatic patientswith distal aortic dissections or as a bridge to surgery forunstable patients with proximal aortic dissections.

¹⁰

PROGNOSIS

Despite improved diagnostic and therapeutic modalities,the in-hospital mortality among patients with either aproximal or distal aortic dissection has not decreased inthe past 3 decades.

^19,20

However, patients with proximalaortic dissections who undergo successful surgical repairhave a 10-year survival of 40%18; if untreated, only 5% ofpatients are alive 1 year after the event. The data on thelong-term prognosis of patients with distal aortic dissectionswho were treated medically versus those who weretreated surgically are limited. Death from aortic dissectionmost commonly occurs as a consequence of the complicationsof rupture of the aorta, aortic valve disruption, cardiactamponade, or coronary artery occlusion.

²¹

References:

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