AJR:203, July 2014 29

Imaging Findings and Clinical Features of Abdominal Vascular Compression Syndromes

Jeffrey Kah Keng Fong1

Angeline Choo Choo PohAndrew Gee Seng Tan

Ranu Taneja

Fong JKK, Poh ACC, Tan AGS, Taneja R

1 All authors: Department of Radiology, Changi General Hospital, 2 Simei St 3, 529889, Singapore. Address correspondence to J. K. K. Fong (jeffrey.fong@mohh.com.sg).

Gastrointest ina l Imaging • Review

This article is available for credit.

AJR 2014; 203:29–36

0361–803X/14/2031–29

© American Roentgen Ray Society

Keywords: abdomen, compression, CT, syndrome, vascular

DOI:10.2214/AJR.13.11598

Received July 22, 2013; accepted after revision September 26, 2013.

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compressed between the aorta and vertebral body, which is termed “posterior nutcracker syndrome” [2].

Theories about the cause of nutcracker syndrome include posterior renal ptosis, an abnormally high course of the LRV, and an abnormal SMA branching from the aorta [3].

Compression of the LRV predisposes to venous hypertension and the formation of periureteric varices [1].

Demographics and Clinical FeaturesThe exact prevalence of nutcracker syn-

drome is unknown, likely because of the variable presenting features [3]. However, it is estimated to be relatively more common in females and usually presents in the 3rd or 4th decade of life [4].

The severity of nutcracker syndrome is variable, and affected individuals may be completely asymptomatic or, in the most extreme cases, experience severe pelvic congestion [5].” However, the most com-mon presenting symptom is micro- or mac-roscopic hematuria. Hematuria has been attributed to hemorrhage into the calyce-al fornices from the thin-walled varices, which develop secondary to renal venous hypertension [6]. Other symptoms or com-plications that may occur include left flank

The world of medicine is full of eponymous conditions, an im-mortal catalog to the legacies of past pioneers. In this article,

three eponymous conditions are presented along with a more generically named disease under the common theme of “abdominal vas-cular compression syndromes.”

The underlying pathophysiology that ac-counts for these clinical syndromes, patient demographics and clinical presentation, and typical imaging findings for each condition are described. A thorough understanding of these disease processes will enable the accu-rate diagnosis of disease in the small subset of patients who require treatment and the ex-clusion of individuals with anatomic findings associated with these syndromes but who are asymptomatic and do not require treatment.

Nutcracker SyndromeBackground

The term “nutcracker syndrome” is at-tributed to de Schepper [1] and describes compression of the left renal vein (LRV) between the aorta and superior mesenter-ic artery (SMA). This condition is also re-ferred to as “anterior nutcracker syndrome.” Less commonly, a circumaortic (up to 17%) or completely retroaortic (3%) LRV may be

OBJECTIVE. This article describes the typical imaging findings and clinical features that are associated with four abdominal vascular compression syndromes. We explain the underlying pathophysiology that results in these clinical syndromes so that the patient subset who will benefit from treatment can be identified.

CONCLUSION. The abdominal vascular compression syndromes discussed here are un-common and are potentially easily missed on a cursory review of radiologic examinations, par-ticularly in a nonspecific and vague clinical setting. Hence, knowledge of the typical imaging findings and associated clinical symptoms is essential so that the they can be carefully sought and excluded. However, because these findings may also exist in healthy individuals as anatom-ic variants, it is important to correlate radiologic findings with clinical symptoms to identify the subset of patients who will benefit from treatment.

Fong et al. Abdominal Vascular Compression Syndromes

Gastrointestinal ImagingReview

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pain, pelvic discomfort, varicocele, or ovar-ian vein syndrome [7].

Radiologic FindingsOn CT, the beak sign (Fig. 1A) is an abrupt

narrowing of the LRV between the aorta and SMA, with proximal dilatation of the LRV. This sign has a sensitivity of 91.7% and spec-

ificity of 88.9% in the diagnosis of nutcrack-er syndrome. Also, a ratio of the LRV diam-eters at the hilar and aortomesenteric regions of more than 4.9 (i.e., a dilated proximal LRV; Fig. 1B) has a sensitivity of 66.7% and specific-ity of 100% for this condition. An aortomes-enteric angle (between the superior mesenteric artery [SMA] and aorta; Fig. 1C) of less than

41° is 100% sensitive and 55.6% specific [8] for nutcracker syndrome; a normal aortomesen-teric angle measures approximately 90° [4]. A beak angle measurement may also be obtained but is cumbersome to perform. It is obtained by drawing two lines along the anterior and pos-terior walls of the LRV where it passes under-neath the SMA to the point of narrowing of the

A B

C

Fig. 1—Contrast-enhanced CT images of 69-year-old woman who presented with history of repeated hospital admissions for undiagnosed abdominal pain. A, CT image shows beak sign (arrow), which is narrowing of left renal vein (LRV) between aorta and superior mesenteric artery (SMA) (arrowhead). B, Ratio of more than 4.9 of anteroposterior diameter of LRV at aortomesenteric (left arrowhead and short line) and hilar (right arrowhead and long line) regions is considered 100% specific for nutcracker syndrome. C, Oblique sagittal reformation image with SMA and aorta in same plane shows aortomesenteric angle (lines) of 35°; value of less than 41° is 100% sensitive for diagnosing nutcracker syndrome. D, Beak angle obtained by intersection of line from points A and C, which is drawn from anterior wall of LRV where it passes deep to SMA to point of LRV narrowing, and intersection of line from points B and D, which is drawn from posterior renal wall to point of narrowing. In this case, beak angle is 35°. Value of more than 32° is diagnostic of nutcracker syndrome.E, Oblique coronal image shows dilated tortuous collateral vein (arrowheads) between LRV (arrow) and inferior vena cava.

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AJR:203, July 2014 31

Abdominal Vascular Compression Syndromes

LRV lumen (Fig. 1D). A measured beak angle of more than 32° has a diagnostic sensitivity of 83.3% and specificity of 88.9% [8].

Classic retrograde venography with a renocaval pressure gradient measurement of more than 3 mm Hg is considered the refer-ence standard for the diagnosis of nutcracker syndrome but has the inherent drawback of being invasive [4, 9].

On ultrasound, measurements of both an-teroposterior diameter and peak velocity of the LRV at the renal hilum and the aortomesenter-ic junction should be obtained. A ratio of more than 5 for both is suggestive of nutcracker syn-drome [10]. Dilated tributaries to the LRV (left gonadal, left adrenal, and lumbar veins) may be seen if the internal valves are incompetent (Fig. 1E). Alternatively, collateral formation, in which the LRV is decompressed and nondis-tended, may be a feature. Color Doppler ultra-sound to look for flow away from the LRV has a diagnostic sensitivity of 78% and specificity of 100% for nutcracker syndrome [9].

ManagementBecause 44% of asymptomatic patients

were found to have mild narrowing of the LRV without the beak sign [8], it is important to correlate radiologic findings with the pa-tient’s clinical condition to determine wheth-er further management is needed. There is no consensus about which symptoms are suffi-ciently severe to warrant definitive manage-ment, but severe pain, gross hematuria, renal insufficiency, and 2 years of failed conserva-

tive management have been suggested [3]. Various surgical approaches, ranging from a straightforward LRV bypass to the more rad-ical option of nephrectomy, have been de-scribed [3]. Recently, endovascular stenting has been used as a less invasive alternative with good outcomes, but it does have inherent drawbacks such as the potential complications of stent migration, fracture, or occlusion [11].

Superior Mesenteric Artery Syndrome (Wilkie Syndrome)Background

First described comprehensively by Wilkie in 1927 [12], this syndrome is known more commonly as SMA syndrome as opposed to its eponymous namesake. SMA syndrome is caused by vascular com-pression of the third part of the duodenum between the aorta and SMA.

The duodenum is normally surrounded by mesenteric adipose tissue as it traverses the aortomesenteric plane (Fig. 2). This tis-sue functions as a natural fatty cushion and prevents extrinsic compression [13]. Hence, marked weight loss (e.g., weight loss caused by a chronic debilitating disease or an acute cata-bolic state) and a low body mass index (BMI [weight in kilograms divided by the square of height in meters]) are risk factors [14]. Relative lengthening of the spine after corrective scolio-sis surgery is also a known risk factor for SMA syndrome, probably because superior displace-ment of the SMA origin alters the aortomesen-teric angle between the aorta and SMA [15].

Demographics and Clinical FeaturesThe prevalence of SMA syndrome based

on fluoroscopy studies is estimated to be 0.013–0.3% [14], whereas other sources quote a prevalence of 0.0965% in a chronic hospital setting versus 0.00108–0.0052% in

Aorta

SMA

Adipose

Duodenum

Fig. 2—Illustration shows duodenum surrounded by adipose tissue. Loss of this fatty cushion results in narrowing of aortomesenteric angle. Angle measurement of less than 22° is diagnostic of superior mesenteric artery (SMA) syndrome. (Drawing by Fong JKK)

BAFig. 3—Asthenic 17-year-old girl with symptoms of abdominal distention and vomiting. A, Obliquely oriented sagittal reconstruction of superior mesenteric artery (SMA) obtained from contrast-enhanced CT scan shows aortomesenteric angle (lines) of 15°. B, Axial image from same study as A shows stomach is grossly dilated and duodenum abruptly narrows as it travels under SMA. Aortomesenteric distance measures 5 mm (arrowhead and line).

(Fig. 3 continues on next page)

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an acute general hospital setting [16]. This syndrome is rare. There is a slight female preponderance (64–66%), with 75% of cas-es occurring in individuals who range in age from 10 to 39 years [13, 17].

The typical symptoms of SMA syndrome are postprandial or intermittent abdominal pain and features of bowel obstruction such as vomiting, nausea, and anorexia [17]. Ma-neuvers such as lying prone or adopting the left lateral decubitus position are described as reducing the pain [18]. Patients are often underweight, with a reported median BMI at diagnosis of 17.4 [13].

Radiologic FindingsOn CT, the two key signs of SMA syn-

drome are an aortomesenteric angle of less than 22° (Fig. 3A) and an aortomesenter-ic distance of less than 8 mm. The former sign has a sensitivity of 42.8% and speci-ficity of 100% for SMA syndrome, whereas the sensitivity and specificity of the latter are 100% [19]. The normal aortomesenter-ic distance is typically between 10 and 28 mm and is measured at the level of the du-odenum as it travels between the aorta and SMA [19, 20] (Fig. 3B). Ancillary features include a dilated stomach and duodenum up to the aortomesenteric space followed by a

sharp narrowing as the duodenum travels underneath the SMA [21].

There is a wide variation in the aortomes-enteric angle values that have been reported as “normal”; originally, a normal aortomes-enteric angle was described as between 38° and 56° [22], whereas newer studies claim a 90° aortomesenteric angle in control sub-jects is a more accurate value for normal [23]. Nevertheless, it should be understood that simple sagittal reconstructions may be insufficient for assessing the aortomesenteric angle because of the normal slight anterolat-eral angulation of the SMA; an obliquely ori-ented sagittal reconstruction provides more accurate measurements [24].

Fluoroscopy was the previous mainstay of diagnosing SMA syndrome. The classic (but nonspecific) feature is a dilated prox-imal duodenum with an abrupt termina-tion at the third part of the duodenum with or without gastric distention (Figs. 3C and 3D). Other features include abrupt verti-cal compression of the mucosal folds at the third part of the duodenum, antiperistaltic flow of contrast material proximal to the ob-struction (resulting in a forward-backward “rocking” flow of barium), and relief of ob-struction when the patient is in a prone or left lateral decubitus position [14]. Howev-

er, a recent study using CT as the primary diagnostic modality found a false-negative rate of 18.6% for barium studies [13].

ManagementThe initial management of SMA syn-

drome is conservative, particularly in the acute setting of less than 1 month. Manage-ment includes insertion of a nasogastric tube to decompress the stomach and duodenum as well as to provide enteral feeding, hopefully restoring a normal aortomesenteric distance and relieving the obstruction [14].

If conservative management fails in a pa-tient with severe symptoms, surgery is indi-cated; the primary choice was traditionally open duodenojejunostomy, with good opera-tive results being reported in 79–100% of the cases [14]. However, a review of nine arti-cles found laparoscopic duodenojejunostomy to be a safe and effective treatment with a 100% success rate [25].

Median Arcuate Ligament Syndrome (“Dunbar Syndrome”)Background

In a report of dissection findings from a study of 83 cadavers, Lipshutz [26] noted that the celiac axis “is not infrequently partly cov-ered at its origin by the diaphragm.” The me-

C D

Fig. 3 (continued)—Asthenic 17-year-old girl with symptoms of abdominal distention and vomiting. C and D, Posteroanterior (C) and left lateral decubitus (D) views from barium meal study show abrupt vertical compression of mucosal folds and proximal dilatation of duodenum and stomach from vascular compression.

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Abdominal Vascular Compression Syndromes

dian arcuate ligament (MAL) is a fibrous arch that connects the diaphragmatic crura to form the anterior margin of the aortic hiatus. How-ever, the location of the MAL is “exceedingly variable” and the celiac artery origin was ei-ther at or superior to the MAL in 33% of a 75-case autopsy study [27]. The MAL may indent upon the celiac trunk and cause downward an-gulation, but this appearance may be a mere anatomic variant that is nonobstructive [28].

However, in certain individuals, compres-sion of the celiac trunk resulting in mesen-teric ischemia can occur. It was first Harjola [29] (1963) in a case study and then Dunbar et al. [30] (1965) in a larger case series of 15 subjects who linked the anatomic anomaly with clinical symptoms of intestinal angina. The syndrome is also known as celiac artery, axis, or trunk compression syndrome.

Demographics and Clinical FeaturesIn a retrospective study of aortograms of

1500 patients discovered compression of the celiac artery severe enough to cause symp-toms in approximately 1% of the patients [31]. However, the exact incidence of MAL syn-drome in the general population is unknown [31]. A retrospective study of 14 patients with MAL syndrome found a mean age of 28.4 years and 71% female preponderance [32].

The common complaint of patients with MAL syndrome is of vague intermittent ab-dominal pain, typically epigastric and usu-ally postprandial. Given the association with eating, weight loss is a common fea-ture. Associated symptoms include nausea

and diarrhea. Examination usually reveals mild epigastric tenderness, with an occa-sional finding of a midabdominal systolic bruit on auscultation [30].

Radiologic FindingsOn modern MDCT with its superior spa-

tial resolution, the MAL may be visible; a thickness of more than 4 mm is considered abnormal [33]. The definitive finding for the diagnosis of MAL syndrome is focal nar-rowing of the proximal celiac axis with a characteristic hooked appearance caused by the inferior displacement of the celiac ar-tery by the MAL (Fig. 4). Stenosis of the celiac artery is more obvious on its superior aspect where it indented upon by the MAL. On the other hand, atherosclerosis results in stenosis without the hooked appearance. The sagittal axis is optimal for visualizing the “hooking” of the proximal celiac trunk, and this finding may be extremely subtle and not visible on axial sections. Associ-ated findings include poststenotic dilata-tion or collateral vessel formation from the SMA branches [34].

CT and MRI studies for diagnosing MAL syndrome should ideally be performed in the end-inspiratory phase. Because the MAL attaches to the diaphragmatic crura, its position and, hence, compression of the celiac axis change during respiration [35]. True compression persists during end-inspi-ration, whereas transient compression may be seen only during end-expiration as the diaphragm ascends in healthy individuals.

Therefore, CT and MRI studies performed in the end-expiratory phase would show a greater degree of compression in individu-als with the syndrome but will also be pre-disposed to false-positive results [36]. If a patient in whom this syndrome is suspected is unable to follow breathing instructions, ancillary signs such as poststenotic dilata-tion and collateral vessel formation should be assessed.

Percutaneous angiography is the reference standard for the diagnosis of MAL syndrome and shows findings similar to CT such as su-perior indentation, hooking, and poststenotic dilatation of the celiac axis. A useful addi-tional finding on angiography is the ability to assess the stenosis in both end-inspiration and end-expiration [33]. Compression is con-sidered severe if it persists on end-inspiration and if poststenotic dilatation and retrograde filling of the celiac axis from the SMA and pancreaticoduodenal arcade are present [34]. However, CT has the advantages of being widely available, accurate, and noninvasive. Furthermore, postprocessing techniques that are useful for planning further intervention, such as multiplanar and 3D volume-rendered reconstructions, are available with CT [37].

Doppler ultrasound is a useful noninva-sive alternative for diagnosing MAL syn-drome. A peak systolic velocity of greater than 200 cm/s has a reported sensitivity and specificity of 75% and 89%, respectively, in detecting a stenosis of at least 70% [38]. Given the inherent dynamic nature of ultra-sound examination, flow turbulence, which is accentuated during the expiratory phase, can also be assessed [38].

ManagementThe existence of MAL syndrome is con-

troversial; the ideal treatment options are de-bated because imaging of 13–50% of healthy asymptomatic individuals may show com-pression of the celiac artery during expira-tion and the exact pathophysiology of MAL syndrome remains indeterminate [34, 39]. If surgical treatment is attempted, it will usu-ally be celiac decompression via ligation of the offending MAL, which can be performed laparoscopically [39]. A novel approach is laparoscopic release of the MAL using intra-operative duplex ultrasound for guidance. If the celiac artery flow abnormality on Dop-pler ultrasound persists after MAL release, angioplasty and stenting are advocated [40]. Surgical management is best reserved for patients in the 40- to 60-year-old age group

A

Fig. 4—63-year-old woman.A and B, Sagittal arterial phase CT image (A) and volume-rendered 3D reconstruction from abdominal aortogram (B). Hooked appearance of proximal celiac axis (arrows) with poststenotic dilatation is being caused by median arcuate ligament indenting upon superior aspect of artery.

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with clinical symptoms of postprandial pain and significant weight loss (20 lb [9 kg]), and with radiologic features of poststenotic dila-tation and collateral vessels [41].

May-Thurner SyndromeBackground

In 1957, May and Thurner [42] joint-ly first described this syndrome in which the left common iliac vein was compressed by the right common iliac artery against the lumbar vertebra in 22% of 430 cadav-ers. However, this syndrome was allud-ed to more than a century earlier by Vir-chow [43] in his observation that deep vein thrombosis (DVT) was five times more likely to occur in the left lower limb. Cock-ett, a British vascular surgeon, and Thomas [44] also reported this condition in 1965, so this syndrome is also known as “Cockett syndrome.” An alternative nomenclature is the more descriptive “iliac vein compres-sion syndrome” [44].

The inferior vena cava is located to the right of the spine, and the left common il-iac vein emerges at a sharp angle, crossing the midline where the natural convexity of the lumbar vertebrae is most prominent [45]. May and Thurner [42] proposed the pathol-ogy as intimal “spurs” developing from in-timal hypertrophy of the left iliac vein wall consequential to it suffering mechanical compression and arterial pulsation by the overlapping right common iliac artery [42]. These “spurs” are proposed to increase the risk of thrombosis.

Demographics and Clinical FeaturesMay-Thurner syndrome tends to occur

in women (72% of the cases) in the 2nd–4th decades [46]. Although the exact preva-lence is unknown [47], a study of 44 patients with isolated left lower limb DVT found that 27 patients had physiologic findings consis-tent with May-Thurner syndrome [48]. May-Thurner syndrome may be underdiagnosed because many patients initially diagnosed with DVT on ultrasound do not undergo fur-ther investigative workup [47].

The typical features of May-Thurner syn-drome overlap with those of DVT such as leg pain, swelling, venous claudication, and var-icose veins [45]; the complications of chron-ic disease include venous eczema, hyper-pigmentation, and ulcers [45]. Patients may alternatively present with persistent painless left lower limb edema [47].

Radiologic FindingsThere is no consensus about specific radi-

ologic signs that are diagnostic of May-Thurner syndrome. However, the most useful finding is of compression of the left common iliac vein by the right common iliac artery (Fig. 5A). In one study, the mean diameter of the left common iliac vein origin was 3.5 mm (range, 1.0–8.5 mm) in patients with May-Thurner syndrome versus 11.5 mm in a control group (range, 6.3–16.1 mm) [45]. Tortuous venous collaterals crossing the pel-vis to drain into the contralateral veins [47] and thrombus formation (Fig. 5B) are also suggestive features.

Conventional venography is considered the reference standard but is invasive and is also suboptimal for visualizing the cen-tral veins. Doppler ultrasound is most often used in screening for DVT because of the ease of availability and convenient bedside examination [45]. However, it is often diffi-cult to visualize the pelvic veins particularly if the patient is obese or if bowel gas inter-venes [49]. CT is widely available, is simple to perform, and is useful to rule out the pres-ence of an external mass compressing the left common iliac vein.

ManagementMay-Thurner physiology is found in as

many as 22–32% of cadavers, although the incidence of lower limb DVT approximates a mere 100 cases per 100,000 individuals (0.1%). Given the significant difference in indicence of May-Thurner physiology and that of lower limb DVT, the anatomic find-ing alone does not represent an increased risk of DVT [46]. One can have compres-sion of the left common iliac vein by the right common iliac artery and remain as-ymptomatic. Therefore, the term “May-Thurner physiology” should be used to de-scribe the anatomy, whereas “May-Thurner syndrome” should be reserved for cases of venous stasis and resultant thrombus forma-tion resulting from compression [50].

Treatment of DVT is typically antico-agulation therapy. However, it may be in-sufficient in patients with May-Thurner syndrome because it does not relieve the un-

A

Fig. 5—Axial CT images of 53-year-old woman with history of tense, swollen left lower limb. A, Left common iliac vein is compressed by overlying right common iliac artery (arrow). B, There is also thrombus of left common iliac vein (arrow) distal to vascular compression.

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Abdominal Vascular Compression Syndromes

derlying mechanical compression. The pa-tient may therefore be at risk for recurrent DVT or postthrombotic syndrome. Hence, a carefully selected small subset of lower limb DVT patients with diagnosed May-Thurn-er syndrome will benefit from endovascular stenting to achieve long-term patency [51].

SummaryThe four abdominal vascular compression

syndromes discussed here are uncommon and are potentially easily missed on a cur-sory review of radiologic examinations, par-ticularly in a nonspecific and vague clinical setting. Hence, knowledge of the typical im-aging findings and associated clinical symp-toms is essential so that they can be careful-ly sought and excluded. However, because these findings may also exist in healthy indi-viduals as anatomic variants, it is important to correlate radiologic findings with clinical symptoms to identify the subset of patients who will benefit from treatment.

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