Type B Aortic Dissection: Management in a Young Man With Neurologic Sequelae

A 39-year-old man who had never seen a physician noted the sudden onset of central chest pressure while working on his roof. The pain was severe, radiated to his midback, and was accompanied by shortness of breath. Simultaneously, he developed bilateral leg weakness. In the emergency department, the patient's blood pressure was 169/81 mm Hg, with a heart rate of 76 bpm. Cardiac examination revealed a regular rate and rhythm without gallops, murmurs, or ectopy. Peripheral pulses were present, and there were no abdominal masses or bruits. He was unable to lift either lower extremity, but had normal strength in both upper extremities. The patient did not have long thin extremities or loose joints. Laboratory examination revealed a blood urea nitrogen of 16 mg/dL (normal <19 mg/dL), a creatinine of 0.9 mg/dL (normal 0.7–1.3 mg/dL), a hemoglobin of 15.4 mg/dL (normal 14–18 g/dL), and a troponin I of 0.01 mg/mL (normal <0.04 mg/mL). Urinalysis showed no cells and was negative for protein. The electrocardiogram revealed minimal nonspecific ST change. The mediastinal contour on the admitting chest radiograph was normal. Computed tomography (CT) scanning in the emergency department showed an intimal flap in the proximal descending aorta just distal to the takeoff of the left subclavian artery with a false lumen extending to both common iliac arteries, consistent with a type B aortic dissection (Figure 1). The patient was given a 10-mg bolus of IV labetalol followed 10 minutes later by a 20-mg bolus and then a 4-mg/min continuous infusion, achieving systolic pressures of 106–112 mm Hg and heart rates of 64–72 bpm. His chest pain resolved after about 12 hours, and he was able to lift each leg weakly off the bed. Oral medications were initiated and, on the third hospital day, he had a blood pressure of 112/68 mm Hg and a heart rate of 64 bpm off all IV medication and receiving hydrochlorothiazide (HCTZ) 25 mg, atenolol 50 mg b.i.d., lisinopril 40 mg, and nifedipine sustained-release 30 mg daily. Removal of his Foley catheter led to urinary retention and reinstallation of the catheter. On the evening of the third day, he experienced more substernal chest pressure with interscapular radiation; the systolic pressure rose to 162 mm Hg. A chest radiograph revealed a new small left pleural effusion (Figure 2). On IV labetalol, the pain resolved and a CT scan showed no change in the proximal extent of the aortic dissection. A large aortic dissection flap just distal to the left subclavian artery was clearly visualized on transesophageal echocardiography (TEE). There was no extension of the tear to the ascending aorta, and the aortic valve had normal trileaflet structure. Magnetic resonance angiography (MRA) also did not show more proximal extension of the original aortic tear (Figure 3). CT image showing findings of type B aortic dissection. Note intimal flap in descending aorta with some enhancement of the false lumen. The curvilinear focus in the ascending aorta is artifactual. PA=pulmonary artery; AA=ascending aorta; TL=true lumen; FL=false lumen Chest radiograph on hospital day 3 shows development of a new left pleural effusion. Magnetic resonance angiography of the aortic arch and descending aorta showing intimal flap in the proximal to mid-descending aorta (type B dissection). Intercostal arteries show flow. Over the next 2 days, the patient was again tapered off of the labetalol drip and had no recurrence of chest discomfort. He was discharged on hospital day 8 on HCTZ 25 mg, atenolol 75 mg b.i.d., lisinopril 40 mg, and terazosin 5 mg daily with a blood pressure of 108/58 mm Hg and a heart rate of 62 bpm. At the time of a 3-week post-hospitalization office visit, the patient was in a wheelchair and unable to ambulate because of continuing bilateral leg weakness and required bladder catheterizations 4–6 times daily for urinary retention. Nine months later, the patient was ambulatory without assistance. Acute aortic dissection is an uncommon medical emergency that demands early diagnosis and classification to expedite triage to medical vs. surgical management. This case had a rather unusual presentation because of its onset in a 39-year-old. However, the association of acute paraparesis with acute chest pain limited the differential diagnosis. The severity of chest discomfort suggested a differential diagnosis of acute coronary syndrome, pulmonary embolism, and acute pericarditis in addition to aortic dissection. The differential diagnosis of sudden, acute paraparesis includes throm-boembolism of a spinal artery, shock, epidural or subdural hematoma at the spinal level, tumor bleeding, rupture of an arteriovenous malformation, multiple myeloma, and transverse myelitis, in addition to aortic dissection with spinal artery involvement.1 The match-up of these two sets of symptoms made acute aortic dissection a reasonable initial working diagnosis. The three major noninvasive imaging options for making a diagnosis of aortic dissection are contrast-enhanced x-ray CT, magnetic resonance imaging (MRI), and TEE. A prospective comparison of these techniques in 110 patients showed slight preference for an MRI (one false negative, 98.3% sensitivity), compared with TEE (one false negative, 97.7% sensitivity), and CT (three false negatives, 93.8% sensitivity), relative to routine aortography in which two false negatives were reported.2 Specificity comparisons also favored MRI, with one false positive on MRI, four false-positive CT results, and six false positives on TEE (Table I). At the level of the thoracic aorta where detection of the proximal extent of the dissection is the key management indicator, MRI and TEE had comparable sensitivity. More false-positive TEE results were attributed to plaque formation or echo reverberations in an ectatic vessel.2 Since this study was published in January 1993, newer CT scanners have mostly replaced earlier models and probably offer sensitivity similar to MRI and TEE. Therefore, in most hospitals, CT is the first imaging modality due to availability and speed of testing. This can be followed by a confirmatory MRI if the patient is stable and there is any uncertainty with regard to either the diagnosis or the most proximal location of the tear. Significant renal insufficiency would suggest either MRI or TEE, and hemodynamic instability might indicate a TEE. Transthoracic echocardiography (TTE) can also make a diagnosis of aortic dissection but has suboptimal sensitivity and specificity (Table I). Detection of the most proximal portion of the tear in acute aortic dissection is the key management indicator and is based on an anatomic landmark. Dissection occurring proximal to the take-off of the left subclavian artery is classified as Stanford type A and distal to the left subclavian as type B. There is greater risk associated with type A dissections since more proximal extension can bleed into the pericardium, causing fatal tamponade. In addition, greater shear pressures where the aorta is at its largest diameter (according to LaPlace's law relating internal pressure to vessel diameter) are more likely to cause free rupture of the vessel. Acuity of the dissection is determined by duration: up until 14 days is defined as “acute,” and past 14 days is classified as “chronic,” because of patient stabilization and the tailing off of morbidity and mortality with time.3 A diagnosis of acute Stanford type B aortic dissection was established for this patient on the basis of a CT, but a follow-up TEE and MRI were necessary to confirm the most proximal portion of the tear when the patient had recurrence of pain and elevated blood pressures following transition to oral antihypertensive therapy. Fortunately, there was no evidence that he had converted to a type A dissection. Outcome data from the International Registry of Aortic Dissection (IRAD)3 suggests that mortality from the type B dissections treated medically vs. surgically is 10.7% compared with 31.4%, and the mortality from type A dissections treated medically compared with surgically is 58.0% vs. 26.0%. These outcome data support the recommendation to triage the acute type A dissections to urgent surgical repair and the acute type Bs to medical management. IRAD, which has collected prospective data from 18 large contributing centers since January 1996, has been a rich source of clinical data. Some demographic, symptomatic, and radiologic presentation data from IRAD are presented in Table II. When a follow-up chest radiograph in this case showed the interval development of a new left pleural effusion, there was concern for vessel leakage and conversion to a type A dissection. The IRAD data report that pleural effusion is an uncommon finding in acute aortic dissection, mostly nonspecific, and as likely to be seen in type A dissection (17.3%) as in type B dissection (21.8%) (Table II). This case showed a completely normal chest radiograph on presentation; the lack of mediastinal widening is seen in approximately 40% of cases (Table II). IRAD has separately analyzed a subgroup of 68 younger patients (younger than 40) with acute aortic dissection (7% of the total 951 in the registry).4 Findings included a high prevalence of unique clinical risk factors such as Marfan's syndrome, bicuspid aortic valve, and larger aortic dimensions. It can be seen in Table III that Marfan's syndrome occurred in 50% of patients younger than 40 and was more common than hypertension, which occurred in 34%. Previously undetected hypertension was believed to be the major risk factor for this 39-year-old black patient. Comparing age younger than 40 with age older than 40, mortality rates were the same despite a higher rate of cardiovascular comorbidities in the older group; this phenomenon is not well explained. There was a higher rate of limb ischemia in the younger age range, 18% vs. 10%, respectively (p=0.06), but there was no difference in neurological sequelae (both age groups, 10%).4 Acute paraparesis in association with aortic dissection may be transient or permanent. Painless transient paralysis of the lower extremities has been reported as a sole manifestation of acute thoracoabdominal aortic dissection.5 Nine months following the present case presentation, the patient had become ambulatory. Medical management of acute aortic dissection demands rapid blood pressure lowering with agents that also reduce shear pressure and pulsatile load, dp/dt. Target systolic pressure is <120 mm Hg or even <100 mm Hg if tolerated.6 Primary vasodilators such as nitroprusside should be used only in conjunction with IV β blockade or α/β blockers such as labetalol to reduce the increased shear pressure that might otherwise occur with a pure vasodilator despite blood pressure reduction. Alternative IV β-blocking agents include esmolol and metoprolol.7 IV fenoldopam, a selective peripheral dopamine-receptor agonist is an alternative vasodilator to nitroprusside, which does not accumulate or have toxic metabolites in patients with renal insufficiency.8 Hydralazine and minoxidil are generally avoided because, as pure vasodilators, they are more likely to increase pulsatile aortic force.9Table IV describes the recommended antihypertensive agents and dose ranges for aortic dissection. Transitioning from acute IV to oral antihypertensive therapy is accomplished gradually. The patient first needs to demonstrate complete stability and lack of symptoms for at least 24 hours to allow transition to oral drugs. This case demonstrated a recurrence of symptoms 24 hours following conversion to oral therapy. This exacerbation necessitated urgent imaging reevaluation and a return to IV labetalol. He then tolerated a second transition attempt to oral drugs.