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Original Research |
1 Department of Radiology, Nagasaki University School of Medicine, 1-7-1
Sakamoto, Nagasaki 852-8501, Japan.
2 Department of Radiology, Omura Municipal Hospital, Omura 856-0817,
Japan.
3 Department of Radiology, National Nagasaki Medical Center, Omura 856-8562,
Japan.
Received February 18, 2005;
accepted after revision May 25, 2005.
Address correspondence to E. Sueyoshi.
Abstract
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MATERIALS AND METHODS. Fifty-four patients were entered into this study, and regular follow-up CT studies (mean ± SD, 46.9 ± 27.2 months; range, 5-136 months) were performed. The affected aortas and iliac arteries were divided into five segments. A total of 127 segments were evaluated (aortic arch, n = 47; descending thoracic aorta, n = 51; suprarenal abdominal aorta, n = 24; infrarenal abdominal aorta, n = 3; and iliac artery, n = 2). The growth rate of each segment was obtained on CT. The factors influencing increase in the diameter and growth rate—age, sex, diabetes mellitus, atherosclerotic disease, history of smoking 20 years, chronic renal failure, blood pressure, initial diameter of the lumen, the presence of blood flow in the false lumen—were evaluated by univariate analysis and a multivariate logistic regression model.
RESULTS. Twenty (37.0%) of 54 patients had one or more segments that increased in size during the follow-up period. Of a total of 127 segments, 35 (27.6%) increased in size, and for all, the mean growth rate was -0.5 ± 2.9 mm/year. This negative growth rate represents shrinkage. The mean growth rates within the first year and after 1 year from onset were -2.2 ± 5.7 and 0.4 ± 3.2 mm/year, respectively, and a significant difference was observed between them (p < 0.0001). An initial diameter of 40 mm or greater and the presence of blood flow in the false lumen were significant risk factors for an increase in the diameter, as confirmed by univariate and multivariate analyses.
CONCLUSION. In patients with type B intramural hematoma, the affected aortas did not show a high incidence of enlargement during the follow-up period, but the affected aortas tended to increase in size after 1 year from onset. An initial diameter of 40 mm or greater and the presence of blood flow in the false lumen were important risk factors for enlargement during the follow-up period.
Keywords: aorta • cardiovascular disease • CT • emergency radiology
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In contrast, patients who have type B intramural hematoma without complications should be treated with hypotensive drugs during the acute phase. Surgical treatment should be selected if the aortic diameter becomes enlarged or a new complication arises during the chronic phase. Most institutions may select a similar therapeutic strategy for patients with type B intramural hematoma [13-18]. The affected aorta can progress to double-barreled aortic dissection, aortic aneurysm, or pseudoaneurysm [11, 12, 17-19]. According to recent reports, the most frequent long-term evolution of intramural hematoma is to aortic aneurysm or pseudoaneurysm, which is the most frequent cause of death [19]. Recently, several groups of researchers have reported the growth rate of aortic aneurysms measured with CT [20-24]. However, there have been few reports about the growth rate of intramural hematoma, and the natural history of the affected aorta of patients with type B intramural hematoma is not clearly understood [25]. The purpose of this study was to evaluate the growth rate of aortic diameter affected by acute type B intramural hematoma using repeated CT examinations and the factors influencing its enlargement during the chronic phase.
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The diagnosis of type B intramural hematoma was established on CT using the following criteria: first, a crescent-shaped or circumferential area along the wall of the aorta having higher attenuation than blood on unenhanced CT; second, no contrast enhancement within the area on enhanced CT; third, no intimal flap; and fourth, no intimal tear or penetrating atherosclerotic ulcer [1, 7, 26]. In addition, the initial diagnosis of intramural hematoma was reconfirmed for all patients who had undergone TEE, MRI, or both. Forty-seven (87.0%) of 54 patients had an intramural hematoma that involved the aortic arch. In five patients, the intramural hematoma also involved the right innominate artery, left common carotid artery, or left subclavian artery or a combination of these arteries.
The mean CT follow-up period was 46.9 ± 27.2 months with a range of 5-136 months. Regular follow-up studies were performed every week during the first month, and one to three times a year thereafter. In seven patients, the follow-up period was less than 1 year. For patients with a new episode (recurrence of chest pain) suggesting complications during the follow-up period, additional studies were performed. The results of the CT follow-up period after surgery were excluded from this study. All patients with hypertension were managed with ß-blockers.
During the follow-up period, seven (13.0%) of the 54 patients had surgery 6-69 months after onset because aneurysmal dilatation occurred in five, aortic rupture in one, and a change to a type A double-barreled aortic dissection in one. Seven of the remaining 47 patients who did not undergo surgery died 5-82 months after onset due to malignancy in three, aortic rupture in one, cerebral infarction in two, and renal failure in one. Three patients were lost to follow-up from 43 to 62 months after onset. The remaining 37 patients have been followed up continuously.
CT Examination
For 54 patients, a total of 354 studies were performed using conventional
(n = 111) or helical (n = 243) CT scanners. The initial and
follow-up CT studies were performed with unenhanced and enhanced CT in all
patients. Enhanced CT was performed using a bolus injection of 100 mL of
nonionic contrast material. CT was performed on a 9800, HighSpeed Advantage,
LightSpeed Qx/i scanner (all, GE Healthcare), or a Somatom Plus 4 scanner
(Siemens Medical Solutions) generating axial images with contiguous 5-mm-thick
sections from the top of the aortic arch to the abdominal aorta. The area of
coverage of helical CT was the same as with the conventional acquisitions. In
the helical CT scanner, a power injector was used, and scanning began at 20-30
sec and 120-150 sec (two phases) after the start of injection of contrast
material.
Image Analysis
CT images were evaluated by two experienced cardiovascular radiologists
(> 10 years' experience). The affected aortas were divided into five
segments: aortic arch, descending thoracic aorta, suprarenal abdominal aorta,
infrarenal abdominal aorta, and iliac artery. Seven of 54 patients had aortic
dissection limited to one segment, the descending thoracic aorta. The
remaining 47 patients had two or more affected segments. A total of 127
segments were evaluated: aortic arch, n = 47; descending thoracic
aorta, n = 51; suprarenal abdominal aorta, n = 24;
infrarenal abdominal aorta, n = 3; and iliac artery, n = 2.
The remaining 143 segments were not affected by intramural hematoma. The
aortic arch was defined as the segment between the brachiocephalic artery and
the ligamentum arteriosus. The descending aorta was defined as the segment
between the ligamentum arteriosus and the aortic hiatus of the diaphragm.
All 127 segments had no blood flow in the false lumen on the initial CT. The presence or absence of blood flow in the false lumen, including ulcerlike projection, and the aortic diameter of each segment were evaluated during the follow-up period. The absence of blood flow in the false lumen was defined as no contrast enhancement in the false lumen on enhanced CT. An ulcerlike projection (ulcerlike lesion) was defined as a localized blood-filled pouch protruding into the thrombosed lumen of the aorta. It showed the same degree of contrast enhancement as the aortic lumen on enhanced CT scans [27]. Final decisions regarding the findings were reached by consensus. To compare the growth rate in the thoracic aorta with that in the abdominal aorta, we chose the higher value if intramural hematoma involved two segments of the thoracic or abdominal aorta. The growth rates in the thoracic and abdominal aorta were independently calculated if intramural hematoma involved both segments of the thoracic and abdominal aorta.
The initial and final CT measurements were used to calculate changes in aortic size at the same level in each segment. The largest short axial diameter of the outer contour of the affected segment of aorta was measured [20] (Fig. 1A). In the aortic arch, the largest diameter perpendicular to the curvature was measured [20] (Fig. 1B). The diameters were measured using direct-reading calipers from hard-copy images and were corrected for the appropriate scale.
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Statistical Analysis
All values are expressed as mean ± SD. Statistical analysis was
performed on clinical and morphologic variables, with the chi-square or
Fisher's exact test used for categoric variables and the paired Student's
t test and Mann-Whitney U test for continuous variables. If
the expected number of cells was less than five, Fisher's exact test was used
for categoric variables. Variables with statistical significance, set at
p < 0.05 (two-sided), were included in a multivariate logistic
regression model. Estimates of risk (odds ratios) were calculated on the basis
of coefficients from the logistic models. In all tests, a p value of
< 0.05 was considered to be statistically significant. Data analysis was
performed using Stat-View J-5.0 software (Abacus Concepts) for Windows
(Microsoft).
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In 21 segments (16.5%), the false lumens had blood flow (ulcerlike projection, n = 13; double-barreled aortic dissection, n = 8) from 1 day to 2 weeks after onset (aortic arch, n = 4; descending thoracic aorta, n = 11; suprarenal abdominal aorta, n = 3; infrarenal abdominal aorta, n = 1; and iliac artery, n = 2). However, two segments with ulcerlike projection were completely thrombosed again during the follow-up period (descending thoracic aorta, n = 2). In the remaining 106 segments (83.5%), the false lumens had been completely thrombosed until the final CT (aortic arch, n = 43; descending thoracic aorta, n = 40; suprarenal abdominal aorta, n = 21; infrarenal abdominal aorta, n = 2; and iliac artery, n = 0). Of the 19 segments with blood flow in the false lumen, 14 (73.7%) increased in size. In contrast, among 108 segments without blood flow in the false lumen, 21 (19.4%) increased in size during the follow-up period.
The mean growth rates within the first year (n = 127) and after 1 year (n = 120) from the onset were -2.2 ± 5.7 and 0.4 ± 3.2 mm/year, respectively, and there was a significant difference between them (p < 0.0001). The growth rate after 1 year was faster than that within the first year from onset.
Of 54 patients, 35 (64.8%) had a segment with the largest diameter in the aortic arch, 16 (29.6%) in the descending aorta, and three (5.6%) in the suprarenal abdominal aorta. The mean largest diameter on the final CT was 42.6 ± 13.1 mm. On the other hand, 24 patients (44.4%) had a segment with the fastest growth rate in the aortic arch, 24 (44.4%) in the descending aorta, and six (11.2%) in the suprarenal abdominal aorta. The mean fastest growth rate was 0.3 ± 2.3 mm/year. In 37 (68.5%) of 54 patients, the same segment had the largest diameter and the fastest growth rate. In 17 patients (31.5%), the segments with the largest diameter did not show the fastest growth rate.
We divided all 127 segments into two groups with an increase or no increase in the diameter. Table 2 shows that patient characteristics (risk factors) such as age of 60 years, male sex, diabetes mellitus, atherosclerotic disease (including atherosclerotic aneurysm, ischemic heart disease, and cerebrovascular disease), history of smoking 20 years, chronic renal failure, and blood pressure of 140 mm Hg during the follow-up period were not significant risk factors for an increase in the diameter in univariate or multivariate analysis. The presence of blood flow in the false lumen during the follow-up period and an initial diameter of 40 mm were the significant risk factors for an increase in the diameter as determined by the univariate and multivariate analyses.
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Table 3 shows the mean growth rates of the segments in two groups divided by sex or by the presence or absence of characteristics such as age of 60 years, diabetes mellitus, atherosclerotic disease, history of smoking 20 years, presence of blood flow in the false lumen, an initial diameter of 40 mm, chronic renal failure, and blood pressure of 140 mm Hg during the follow-up period. An initial diameter of 40 mm and presence of blood flow in the false lumen were significantly different between the two groups; their p values were 0.0037 and 0.0020, respectively. The segments with an initial diameter of 40 mm and presence of blood flow in the false lumen had a significantly higher mean growth rate.
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Table 4 compares lesions of the thoracic aorta and those of the abdominal aorta regarding the mean initial diameter, final diameter, and growth rate. To compare the growth rate in the thoracic aorta with that in the abdominal aorta, we chose the higher value if intramural hematoma involved two segments of the thoracic or abdominal aorta. In 49 of 125 segments between the aortic arch and infrarenal abdominal aorta, 49 lesions consisted of two segments of the thoracic or abdominal aorta. Therefore, 76 lesions were evaluated. Of the 76 lesions, 26 (34.2%) increased in size during the follow-up period, but there was no significant difference in the growth rate between the two groups. Of the 52 lesions in the thoracic aorta, 46 lesions involved two segments, consisting of the aortic arch and descending aorta. In 22 of these 46 lesions, distal (descending aorta) segments grew at a faster rate than did proximal (arch) segments. Of the 24 lesions in the abdominal aorta, three lesions involved two segments (suprarenal and infrarenal abdominal aorta). In all three lesions, proximal (suprarenal abdominal aorta) segments grew at a faster rate than did distal (infrarenal abdominal aorta) segments.
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In the present study, 20 (37.0%) of 54 patients had one or more segments increase in size, and 35 (27.6%) of the 127 segments increased in size during the follow-up period. Ninety-two segments (72.4%) remained unchanged or decreased in size. Aortas affected by type B intramural hematoma had a relatively low incidence of enlargement during the follow-up period.
In this study, the mean growth rate of the 127 segments was -0.5 ± 2.9 mm/year. According to previous studies, the growth rate of type B aortic dissection is positive, and the growth rate of thoracic and abdominal aortic aneurysms ranges between 1.3 and 5.7 mm/year [20, 21, 28-30]. Accordingly, there was a large difference between these previous results and the mean growth rate of intramural hematoma. In many cases of intramural hematoma, the thrombosed false lumen itself shrinks during the follow-up period, which might be a factor contributing to the observed negative growth rate. The growth rate after 1 year (0.4 ± 3.2 mm/year) was faster than that of the first year (-2.2 ± 5.7 mm/year) from onset. These results suggest that affected aortas tend to decrease in size within 1 year from onset because the thrombosed false lumen itself shrinks within 1 year from the onset in many cases. On the other hand, affected aortas tend to increase in size after 1 year from onset because both structural weakness of the aortic wall caused by intramural hematoma and continuous mechanical stress from blood flow may cause progressive enlargement of the aorta. Therefore, long-term follow-up studies are needed to examine patients with type B intramural hematoma.
This study shows that the segment with the largest diameter does not always show the fastest growth rate. In addition, the proximal portion of the aorta does not always show the fastest growth rate. Recognition of these results is useful for following up patients with type B intramural hematoma.
Blood flow in the false lumen (progression to double-barreled aortic dissection or ulcerlike projection) was a significant risk factor for an increase in the diameter, as confirmed by univariate and multivariate analyses (Table 2). Blood flow in the false lumen was also associated with a significantly higher mean growth rate (Table 3). Segments with blood flow in the false lumen frequently become a double-barrel chamber, resulting in enlargement of the affected aorta. In addition, structural weakness of the aortic wall and mechanical stress resulting from blood flow cause progressive enlargement. Previous studies have revealed that blood flow in the false lumen is a significant risk factor for aortic enlargement of chronic type B aortic dissection [31]. Our results also showed that the presence of blood flow in the false lumen is a significant risk factor for aortic enlargement.
An initial diameter of 40 mm was a significant risk factor for increase in the diameter (Table 2), and segments with an initial diameter of 40 mm had a significantly higher mean growth rate (Table 3). Previous studies have revealed that aortic diameter is a predictor of rupture and increase in the diameter of aortic aneurysm, aortic dissection, and intramural hematoma [7, 22, 23, 31, 32]. In one study, researchers reported that an initial diameter of 40 mm was a significant risk factor for progression of an affected aorta (expansion, progression to aortic dissection, increased false lumen, or rupture) in patients with type B intramural hematoma. The findings of our study also suggest that aortic diameter is a predictor of the increase in the diameter of type B intramural hematoma, and this can be explained by Laplace's law [20, 22]: It states that the perpendicular stress on a cylinder is directly proportional to the pressure exerted by the fluid contents and its radius and is inversely proportional to the wall thickness. This means that the larger the diameter, the faster the growth rate at a constant pressure.
In this study, almost all patients with hypertension were well managed on ß-blockers. It may be that patients whose blood pressure is well managed have less expansion of their aortic diameter. In a recent report, researchers suggested that ß-blocker therapy may improve the long-term prognosis of patients with intramural hematoma [11]. Therefore, ß-blocker therapy might have prevented expansion of aortic diameter in this study.
In a previous report, researchers found that aortic dissections and aortic aneurysms in the thoracic aorta grow at a faster rate than those in the abdominal aorta [20, 28]. However, we found no significant difference in the growth rate of affected aorta between the thoracic aorta and the abdominal aorta (Table 4). The reason for this result is unclear, and further studies may be needed.
One limitation of this study was the difficulty in obtaining accurate measurements of the aortic diameter from axial CT images. Although previous studies used a measurement method similar to ours [20, 21], multiplanar reconstructed CT images or other techniques such as MRI and TEE may be needed to obtain more accurate sizing of the aorta. Another limitation was that the diagnosis of intramural hematoma was established only by imaging methods. On imaging, intramural hematoma must be diagnosed by means of the findings of aortic intramural hematoma formation without intimal disruption or the presence of a penetrating ulcer. We might have overlooked undetectable intimal disruption given that some reports have suggested that small intimal disruption may be overlooked even during surgery and autopsy [16, 27, 33]. However, such patients may have a prognosis similar to that of patients with intramural hematoma because the aortic pathology and flow dynamics are similar to each other. Other limitations were that the sample size was small and the follow-up periods of the patients varied. Additional studies addressing other factors and involving larger numbers of patients with a longer follow-up period are therefore needed.
In conclusion, in patients with type B intramural hematoma, affected aortas do not show a high incidence of enlargement during the follow-up period. However, they tend to increase in size after 1 year from onset. Therefore, long-term follow-up is needed for patients with type B intramural hematoma. An initial diameter of 40 mm and blood flow in the false lumen are important risk factors for enlargement. More careful follow-up studies are needed for patients with these characteristics. In addition, we should recognize that the largest diameter or the proximal portion of the aorta does not always show the fastest growth rate.
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