AJR:186, March 2006 703

AJR 2006; 186:703717

0361803X/06/1863703

American Roentgen Ray Society

M E D I C A L I M A G I N G

A C E N T U R Y O F

M E D I C A L I M A G I N G

A C E N T U R Y O F

Martin et al.Radiographic and Clinical Findings of Hernias

G a s t ro i n t e s t i n a l I m ag i n g R ev i ew

Review of Internal Hernias: Radiographic and Clinical Findings

Lucie C. Martin1

Elmar M. MerkleWilliam M. Thompson

Martin LC, Merkle EM, Thompson WM

Keywords: colon, CT, gastrointestinal radiology, hernia, small bowel

DOI:10.2214/AJR.05.0644

Received April 14, 2005; accepted after revision August 9, 2005.

1All authors: Department of Radiology, Duke University Medical Center, Box 3808, Durham, NC 27710. Address correspondence to W. M. Thompson (thomp132@mc.duke.edu).

OBJECTIVE. Internal hernias, including paraduodenal (traditionally the most common),pericecal, foramen of Winslow, and intersigmoid hernias, account for approximately 0.55.8%of all cases of intestinal obstruction and are associated with a high mortality rate, exceeding50% in some series. To complicate matters, the incidence of internal hernias is increasing be-cause of a number of relatively new surgical procedures now being performed, including livertransplantation and gastric bypass surgery. A significant increase in hernias is occurring in pa-tients undergoing transmesenteric, transmesocolic, and retroanastomotic surgical procedures.It is important for radiologists to be familiar with and to understand the various types of internalhernias and their imaging features so that prompt and accurate diagnosis of these conditionscan be made.

CONCLUSION. This article illustrates the imaging findings of internal hernias, with em-phasis placed on the CT findings, especially in transmesenteric, transmesocolic, and retroanas-tomotic types of internal hernias.

lthough internal hernias have anoverall incidence of less than1%, they constitute up to 5.8% ofall small-bowel obstructions,

which, if left untreated, have been reportedto have an overall mortality exceeding 50%if strangulation is present [1, 2]. Over thepast decade, their incidence has been in-creasing because of the more frequent per-formance of liver transplantations and gas-tric bypass surgery for bariatric treatment.In this subset of patients, internal herniasaccount for just over half of all cases ofsmall-bowel obstruction, almost equal tothose caused by adhesions in one study [3,4]. Without a heightened awareness and un-derstanding of these hernias, they can oftenbe misdiagnosed, with subsequent signifi-cant morbidity and mortality. The purposeof this article is therefore not only to reviewthe definition and types of internal hernias,but also to describe the clinical and radio-graphic findings, with an emphasis placedon the CT features, because CT is rapidlybecoming the first-line imaging techniquein these patients.

DefinitionHernias are of two main types, external and

internal [1]. External hernias refer to prolapse

of intestinal loops through a defect in the wallof the abdomen or pelvis, and internal herniasare defined by the protrusion of a viscusthrough a normal or abnormal peritoneal ormesenteric aperture within the confines of theperitoneal cavity. The orifice can be either ac-quired, such as a postsurgical, traumatic, orpostinflammatory defect, or congenital, in-cluding both normal apertures, such as the fo-ramen of Winslow, and abnormal aperturesarising from anomalies of internal rotationand peritoneal attachment.

In the broad category of internal herniasare several main types, as traditionally de-scribed by Meyers [5], based on location.Specifically, using historical data, theseconsist of paraduodenal (53%), pericecal(13%), foramen of Winslow (8%), trans-mesenteric and transmesocolic (8%), inter-sigmoid (6%), and retroanastomotic (5%)(Fig. 1), with the overall incidence of inter-nal hernias being 0.20.9%. The other 7%described by Meyers included paravesicalhernias, which are not true internal herniasand thus are not described in this article. Ingeneral, internal hernias have no age or sexpredilection. With more new surgical proce-dures being performed using a Roux loop,the number of transmesenteric, transmeso-colic, and retroanastomotic internal hernias

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has been increasing. These are probablymore common than the traditional incidenceof the various types of internal hernias re-ported by Meyers (Table 1).

General Clinical FindingsClinically, internal hernias can be asymp-

tomatic or cause significant discomfortranging from constant vague epigastric painto intermittent colicky periumbilical pain[1, 5] (Table 1). Additional symptoms in-clude nausea, vomiting (especially after alarge meal), and recurrent intestinal ob-struction [1, 2, 57]. Symptom severity re-lates to the duration and reducibility of thehernia and the presence or absence of incar-ceration and strangulation [6]. These symp-toms may be altered or relieved by changesin patient position [5, 7]. Because of the pro-pensity of these hernias to spontaneously re-duce, patients are best imaged when they aresymptomatic [5, 7].

General Imaging Findings on Radiography and CT

Imaging studies often play an importantrole in the diagnosis of internal hernias be-cause they are often difficult to identify clini-

cally. In the past, these hernias were most fre-quently assessed with small-bowel oralcontrast studies. However, CT has become thefirst-line imaging technique in these patientsbecause of its availability, speed, and multi-planar reformatting capabilities. General ra-diographic features with barium studies in-clude apparent encapsulation of distendedbowel loops with an abnormal location, ar-rangement or crowding of small-bowel loopswithin the hernial sac, evidence of obstructionwith segmental dilatation and stasis, with ad-ditional features of apparent fixation and re-versed peristalsis during fluoroscopic evalua-tion [1, 5] (Table 1). On CT, additionalfindings include mesenteric vessel abnormal-ities, with engorgement, crowding, twisting,and stretching of these vessels commonlyfound and providing an important clue to theunderlying diagnosis [6].

Paraduodenal HerniasIn the classic older literature, paraduode-

nal hernias were the most common type ofinternal hernia, accounting for approxi-mately 53% of all cases [1]. Unlike mosttypes of internal hernias, this subtype doeshave a sex predilection, being found more

Fig. 1Diagrammatic illustration shows various types of internal hernias: A = paraduodenal, B = foramen of Winslow, C = intersigmoid, D = pericecal, E = transmesenteric, and F = retroanastomotic.

commonly in men by a ratio of 3:1 [1, 2, 6].There are two main types, left and right,with the former consisting of most (75%)cases [1, 68].

Left Paraduodenal HerniaLeft paraduodenal hernias have an overall

incidence of approximately 40% of all inter-nal hernias. They occur when bowel pro-lapses through Landzerts fossa, an aperturepresent in approximately 2% of the popula-tion (Fig. 1). These hernias therefore can beclassified as a congenital type, normal aper-ture subtype. Landzerts fossa is located be-hind the ascending or fourth part of theduodenum and is formed by the lifting up ofa peritoneal fold by the inferior mesentericvein and ascending left colic artery as theyrun along the lateral side of the fossa. Small-bowel loops prolapse posteroinferiorlythrough the fossa to the left of the fourthpart of the duodenum into the left portion ofthe transverse mesocolon and descendingmesocolon (Fig. 2).

Clinically, in addition to the aforemen-tioned symptomatology, these patients willalso often present with postprandial pain, typ-ically chronic in nature, with symptoms dat-ing back to childhood [5].

On radiography or oral contrast studies,these hernias will present as an encapsulatedcircumscribed mass of a few loops of smallbowel (usually jejunal) in the left upperquadrant, lateral to the ascending duodenum[1, 5, 9] (Fig. 3A). These loops may havemass effect, depressing the distal transversecolon and indenting the posterior wall of thestomach [1, 5] (Figs. 3B and 3C). Mildduodenal dilatation often occurs, and the ef-ferent loop often shows an abrupt caliberchange [5]. With CT, similar findings of en-capsulated bowel loops are noted, either atthe duodenojejunal junction between thestomach and pancreas to the left of the liga-ment of Treitz; behind the pancreatic tail it-self, displacing the inferior mesenteric veinto the left; or between the transverse colonand the left adrenal gland [57, 912](Figs. 4A4D). Evidence of small-bowel ob-struction with dilated loops and airfluidlevels is also commonly seen [5] (Fig. 4E).There is associated mass effect with dis-placement of the posterior stomach wall an-teriorly, the duodenojejunal junction infero-medially, and the transverse colon inferiorly[6, 7, 10]. Mesenteric vessel abnormalities,including enlargement, stretching, and ante-rior displacement of the main mesenteric

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TABLE 1: Clinical and Imaging Findings for Internal Hernias

Hernia Type Subtype IncidenceaCharacteristic

Clinical FindingsRadiography and Barium

Studies CT Findings Key Vessel

Left paraduodenal Congenital, normal aperture

40% of all hernias, 75% of paraduodenal hernias

Postprandial pain, may date back to childhood

Encapsulated cluster of jejunum in LUQ, lateral to the ascending duodenum; may have mass effect indenting posterior wall of stomach or displacing transverse colon inferiorly

Clustered dilated small-bowel loops between stomach and pancreas, behind pancreas itself, or between transverse colon and left adrenal gland

IMV in neck of hernial sac with anterior and upward displacement of IMV

Right paraduodenal Congenital, normal aperture

13% of all hernias, 25% of paraduodenal hernias

Postprandial pain, may date back to childhood

Encapsulated loops lateral and inferior to the descending duodenum; associated with small-bowel nonrotation

Encapsulated loops lateral and inferior to the descending duodenum; associated with small-bowel nonrotation

SMA displaced anteriorly

Pericecal Congenital or acquired, abnormal aperture

13% RLQ pain, differential diagnosis of appendicitis; high incidence of occlusive symptoms

Clustered small-bowel loops (usually distal) posterior and lateral to the cecum in right paracolic gutter

Clustered small-bowel loops (usually distal) posterior and lateral to the cecum in right paracolic gutter

None

Foramen of Winslow Congenital, normal aperture

8% Symptoms of proximal obstruction because of mass effect on stomach; symptom onset often preceded by changes in intraabdominal pressure (i.e., parturition, straining); relief of symptoms with forward bending

Circumscribed loops medial and posterior to the stomach; differential diagnosis of cecal volvulus

Loops in lesser sac between liver hilum and IVC

None; vessels stretched through foramen of Winslow

Intersigmoid Type 1: congenital, normal aperture; types 2 and 3: acquired, abnormal aperture

6% None U- or C-shaped cluster of small bowel posterior and lateral to the sigmoid colon

U- or C-shaped cluster of small bowel posterior and lateral to the sigmoid colon

None

Transmesentericb In children: congenital, abnormal aperture; in adults: usually acquired, abnormal aperture

8% Two typical patient populations: children and postsurgical adults; in adults, less vomiting because fewer secretions in proximal gastric pouch, onset more acute

Variable, air within gastric remnant; may simulate a left paraduodenal hernia

Small bowel lateral to colon; displaced omental fat with small bowel directly abutting abdominal wall

None

Retroanastomoticb Acquired, abnormal aperture

5% Usually within the first postoperative month; less vomiting because fewer secretions in the proximal gastric pouch

Variable Variable None

NoteLUQ = left upper quadrant, IMV = inferior mesenteric vein, SMA = superior mesenteric artery, RLQ = right lower quadrant, IVC = inferior vena cava.aIncidence for first six types from Meyers [5], which are historic data but only major source currently available. Incidence for these first six types of internal hernias totals only 93% because perivesical hernias reported [5] are not true internal hernias, so they were not included in this review.

bProbably more transmesenteric and retroanastomotic internal hernias currently because of number of liver transplant and gastric bypass operations being performed throughout the United States during past decade. The 5% refers to the incidence after Roux loops used during surgery for reasons other than liver transplantation or gastric bypass.

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Fig. 2Graphic illustration of a left paraduodenal hernia depicts loop of small bowel prolapsing (curved arrow) through Landzerts fossa, located behind inferior mesenteric vein and ascending left colic artery (straight arrow). Herniated bowel loops are therefore located lateral to fourth portion of duodenum.

trunks, especially the inferior mesentericvein, to the left, are also helpful findings [10,11]. If the vasculature is optimally visual-ized, one can often see additional findings ofengorged vessels grouped together at the en-trance of the hernia sac, with the proximaljejunal arteries showing an abrupt change ofdirection posteriorly behind the inferior me-senteric artery [6, 7] (Fig. 4F). The inferiormesenteric vein and ascending left colic ar-tery lie in the anterior and medial border ofthe left paraduodenal hernia and may be dis-placed laterally (Figs. 2 and 4C).

Right Paraduodenal HerniaRight paraduodenal hernias have an over-

all incidence of approximately 13% andoccur when bowel herniates throughWaldeyers fossa (representing a defect inthe first part of the jejunal mesentery), be-hind the superior mesenteric artery and in-ferior to the transverse or third portion of theduodenum (Fig. 5). This normal yet uncom-mon recess is found in less than 1% of thepopulation and, like left paraduodenal her-nias, the right paraduodenal hernia can beclassified as congenital type, normal aper-ture subtype [1, 9]. In these situations, theherniated contents are located in the right

half of the transverse mesocolon and behindthe ascending mesocolon. This type of her-nia occurs more frequently in the setting ofnonrotated small bowel [6, 8]. When com-pared with the left paraduodenal hernias,those on the right are usually larger and aremore often fixed [5].

Clinically, these hernias present in a simi-lar manner to the left paraduodenal herniaswith chronic postprandial pain [5].

On a standard barium gastrointestinal ex-amination, a larger and more fixed, encap-sulated, ovoid collection of bowel loops isnoted lateral and inferior to the descendingduodenum, in the right half of the transversemesocolon, or behind the ascending meso-colon [1, 5, 9] (Fig. 6A). As opposed to theleft paraduodenal hernias, both the afferentand efferent loops of bowel are closely op-posed and narrowed [1, 5]. With CT, an en-capsulated cluster of small-bowel loops isnoted in the right mid abdomen, with loop-ing of the small bowel around the superiormesenteric artery and vein at the root of thesmall-bowel mesentery being seen occa-sionally [5, 8] (Fig. 6B). Small-bowel ob-struction may be present with dilated loopscontaining airfluid levels. Because right-sided paraduodenal hernias are thought to

be congenital, related to abnormalities ofembryologic midgut rotation, there may beadditional clues such as small-bowel nonro-tation, as evidenced by the superior mesen-teric vein occupying a more ventral and left-ward position and the absence of a normalhorizontal duodenum [5, 8, 9]. The cecum,however, remains in its normal position.Vascular findings include jejunal branchesof the superior mesenteric artery and supe-rior mesenteric vein looping posteriorly andto the right of the parent vessel to supply theherniated loops [1, 5, 8, 9]. Additional vas-cular findings include the presence of thesuperior mesenteric artery, ileocolic artery,and right colic vein in the anterior margin ofthe neck of the hernial sac, displaced anteri-orly if there is sufficient mass effect by theencased small-bowel loops [8]. Again, ves-sel engorgement may also be present andprovide a clue to the diagnosis.

Pericecal HerniasHistorically, pericecal hernias account for

13% of all internal hernias. The pericecal fossais located behind the cecum and ascending co-lon and is limited by the parietocecal fold out-ward and the mesentericocecal fold inward [9].Although there are actually four subtypes (ile-ocolic, retrocecal, ileocecal, and paracecal) ofpericecal hernias, most commonly the herni-ated loop consists of an ileal segment protrud-ing through a defect in the cecal mesentery andextending into the right paracolic gutter [1, 9](Fig. 7). These hernias can therefore be subcat-egorized as either acquired or congenital de-fects in the cecal mesentery.

Clinically, patients with pericecal herniaspresent in a similar manner to those with allother types of internal hernias except for thelocation of symptoms, which tends to be inthe right lower quadrant, so that pericecalhernias are sometimes mistaken for appen-diceal abnormalities [1, 9]. A higher inci-dence of occlusive symptoms with rapid pro-gression to strangulation is also commonlyfound, with a mortality rate reported to be ashigh as 75% [9, 13].

Imaging studies, including both bariumand CT, show similar findings. These herniascan often be confidently diagnosed as a clus-ter of bowel loops (usually ileal) locatedposteriorly and laterally to the normalcecum, occasionally extending into the rightparacolic gutter [1, 9] (Fig. 8). Again, therewill be evidence of small-bowel obstructionand mass effect displacing the cecum anteri-orly and medially.

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A B

C

Fig. 3Left paraduodenal hernias shown on upper gastrointestinal series, and barium enema in one patient and lateral view of upper gastrointestinal series from different patients. A, 55-year-old man with gastrointestinal bleeding. Anteroposterior projection of oral contrast small-bowel study shows cluster of small-bowel loops in left upper quadrant, lateral to fourth portion of duodenum (arrow).B, Barium enema study (anteroposterior projection) from same patient as in A depicts inferior displacement of distal transverse colon and splenic flexure (arrow) caused by mass in left upper quadrant that was later revealed to be left paraduodenal hernia.C, Lateral radiograph from upper gastrointestinal series in 35-year-old woman with abdominal pain shows small-bowel loops (arrow) causing mass effect and indentation on posterior aspect of stomach (S), displacing it anteriorly.

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Foramen of WinslowThe foramen of Winslow is a normal com-

munication between the greater and lesserperitoneal cavities, located beneath the free

edge of the lesser omentum, the hepa-toduodenal ligament (Fig. 9). The posterior,superior, and inferior boundaries of this fo-ramen include the inferior vena cava, cau-

date lobe, and duodenum, respectively [14].This hernia can therefore be subcategorizedas a congenital type, normal aperture sub-type. It constitutes 8% of all internal hernias,

A B

C D

Fig. 4CT scans from six patients with left paraduodenal hernia.A, Axial contrast-enhanced CT scan in 11-year-old boy shows small-bowel loops (arrows) between stomach (S) and pancreas (P).B, Axial contrast-enhanced CT scan in 28-year-old man shows small-bowel loops (white arrow) behind pancreas (P) itself. Black arrow indicates stomach.C, Axial contrast-enhanced CT scan in 36-year-old man shows small-bowel loops (arrows) displaying inferior mesenteric vein (arrowhead) to left.D, Coronal reconstruction of contrast-enhanced CT data set in 28-year-old man shows small-bowel loops between transverse colon (T) and left adrenal gland (arrow).(Fig. 4 continues on next page)

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E F

Fig. 4 (continued)CT scans from six patients with left paraduodenal hernia.E, Unenhanced axial CT scan in 35-year-old man shows evidence of small-bowel obstruction of herniated contents as multiple loops of dilated small bowel (arrow) with fluidfluid levels noted.F, Axial contrast-enhanced CT scan in 23-year-old man shows multiple engorged and prominent vessels (arrow) in herniated sac caused by vascular congestion and obstruction.

with approximately two thirds containingsmall bowel alone, and the remaining onethird containing additional cecum and as-cending colon and occasionally gallbladder,transverse colon, and omentum [1, 5, 6, 9,

14]. Risk factors for this type of hernia in-clude an enlarged foramen of Winslow, anabnormally long small-bowel mesentery,persistence of the ascending mesocolon al-lowing marked mobility of bowel, and an

Fig. 5Graphic illustration of right paraduodenal hernia shows loop of small bowel prolapsing (curved arrow) through Waldeyers fossa, behind superior mesenteric artery (straight arrow) and inferior to third portion of duodenum (asterisk).

elongated right hepatic lobe (such as aRiedels lobe), which is thought to direct themobile intestinal loops toward the foramenof Winslow [1, 5, 6, 9, 14, 15].

The typical patient is middle-aged, withacute onset of severe, progressive pain andsigns of small-bowel obstruction [1]. Patientsalso often present with symptoms of a proxi-mal bowel obstruction, which are caused by apressure effect on the stomach by the herni-ated contents [14]. Symptom onset is oftenpreceded by a change in intraabdominal pres-sure, such as parturition or straining [1, 5].Occasionally, forward bending provides somerelief [1]. Rarely, patients will present withjaundice or a distended gallbladder, againfrom pressure or stretching of the commonbile duct by the herniated colon [5].

When the small bowel herniates, conven-tional radiographs may reveal a circum-scribed collection of gas-filled loops in theupper abdomen, medial and posterior to thestomach, which may progress to a locationanterior to the hepatic flexure [1, 5, 9]. Evi-dence of small-bowel obstruction will prob-ably be seen [1, 5]. With barium studies, ad-ditional mass effect will likely be shownbecause the stomach and the first and secondparts of the duodenum will shift anteriorlyand laterally [1, 5, 14] (Figs. 10A and 10B).On occasion, this type of hernia can have theappearance of a cecal volvulus if the herni-

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A B

Fig. 623-year-old man with abdominal pain.A, Anteroposterior projection from oral contrast small-bowel study reveals cluster of small-bowel loops (asterisk) posterior and lateral to second and third portions of duodenum (arrow).B, Contrast-enhanced CT scan shows abnormal loop of small bowel (arrow) in right upper quadrant and reveals right paraduodenal hernia.

Fig. 7Diagrammatic illustration of pericecal hernia shows loop of ileum prolapsing (arrow) through cecal mesenteric defect, behind and lateral to cecum, into right paracolic gutter.

ated sac contains cecum [1, 5, 14] (Fig. 11).The zone of transition of the obstruction isusually located near the hepatic flexure [1].On CT, multiple gas-filled loops are locatedin the lesser sac, posterior to the liver hilum,anterior to the inferior vena cava, and be-

tween the stomach and pancreas, with taper-ing of the herniation through the foramen ofWinslow [9] (Fig. 10C). There may be ante-rior and lateral displacement of the stomachand stretching of the mesenteric vesselsthrough the foramen of Winslow [6]. Com-

plications can arise if defects exist in the gas-trocolic or gastrohepatic omentum, allowingreentry of herniated loops into the greaterperitoneal cavity.

Foramen of Winslow hernias oftenpresent a similar radiographic appearance tothat of left paraduodenal hernias. One keyfeature that can be useful in distinguishingbetween these entities is the presence of anencapsulating membrane seen with leftparaduodenal hernias and not with those in-volving the foramen of Winslow. In addi-tion, if the entry point can be identified, itwill be slightly inferior and to the left of thespine, delineated anteriorly by the inferiormesenteric vein and the left colic artery withleft paraduodenal hernias, whereas with fo-ramen of Winslow hernias, the entry pointwill be relatively superior and to the right ofthe spine, delineated by the liver hilum an-teriorly. Along the same lines, mass effecton the transverse colon more commonly in-dicates a left paraduodenal hernia, again be-cause of its more inferior location. Occa-sionally, left paraduodenal hernias can besupracolic. Finally, prominent, congestedblood vessels have more commonly beendescribed with paraduodenal hernias, al-though not exclusively.

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Fig. 860-year-old man with right lower quadrant pain.A, Single anteroposterior radiograph from barium enema study shows retro-grade filling of herniated distal ileum (arrows) as loops of ileum pass poste-rior to cecum (C) through defect of ileocecal mesentery to reach right paracolic fossa. (Reprinted with permis-sion from [1])B, Contrast-enhanced axial CT scan shows loops of small bowel (arrow) posterior and lateral to cecum (asterisk) in right paracolic gutter, produc-ing small-bowel obstruc-tion. (Courtesy of Ghahre-mani GG, San Diego, CA)

A B

Fig. 9Graphic illustration of foramen of Winslow hernia shows bowel about to prolapse (arrow) into lesser sac, behind hepatoduodenal ligament, the free edge of the lesser omentum.

Sigmoid-Related HerniasTwo or three main types of sigmoid-related

hernias are seen, depending on the degree ofadherence to the definition of internal hernia[5, 16]. The most common and most disput-able, the intersigmoid type, develops whenherniated bowel, usually ileum, protrudes intothe intersigmoid fossa, formed between twoadjacent sigmoid segments and their respectivemesenteries [9]. Although this fossa is found at65% of autopsies [1], it is debatable whetherthe opening of this fossa truly is an aperture.These hernias are often easily reducible [1]. A

second type, the transmesosigmoid hernia, oc-curs when small bowel herniates through acomplete defect involving both layers of thesigmoid mesocolon to lie in a position poster-olateral to the sigmoid itself [1, 9, 16](Fig. 12). In this type of hernia, the orifice isusually a long slit with its edge bounded bybranches of the inferior mesenteric artery [5].The third and least common type, the intrame-sosigmoid hernia, is herniation of viscerathrough an incomplete defect involving onlyone of the layers (usually the left leaf) of themesosigmoid [1, 9, 16]. The third type there-

fore consists of a hernial sac that lies within thesigmoid mesocolon [16]. Both the second andthird types are acquired subtypes of internalhernia, whereas the first is a normal congenitalsubtype. However, these three types are radio-graphically difficult to distinguish, and differ-entiation is not so important because they aretreated surgically in a similar manner [9].

Clinically, these hernias present as de-scribed, with no distinctive or characteristicfindings on history or physical examination.

If the patient has no evidence of obstruction,these hernias can be diagnosed with postevac-uation barium enema radiographs, which willshow sacculated ileal loops occupying the leftlower quadrant and elevation and displacementof the sigmoid colon to the right [17]. If ob-struction is present, however, CT findings in-clude a cluster of Y- and X-shaped dilatedsmall-bowel loops entrapped behind the leftposterior and lateral aspect of the sigmoid co-lon, with the defect most commonly locatedbetween the sigmoid colon and the left psoasmuscle, or between sigmoid loops if it is an in-tersigmoid type [4, 17] (Fig. 13). These bowelloops often cause mass effect, displacing thesigmoid colon anteromedially [17]. Additionalfindings of mesenteric vessel congestion andstranding of the fat, suggesting strangulation,may be seen [17].

Transmesenteric HerniasAlthough previously an uncommon type of

hernia, transmesenteric hernias are increasingin incidence and surpassed the frequency of

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paraduodenal hernias in one study [4, 7, 10].These hernias have a bimodal distribution, oc-curring in both pediatric and adult patients. Inchildren, transmesenteric hernias are the mostcommon type of internal hernia, occurring in35% of this patient population [1, 5, 9, 18]. Inthis age group, they are thought to arise froma congenital defect in the small-bowel mesen-tery, near the ileocecal region or ligament ofTreitz [1, 5, 9]. One popular theory relates the

cause to prenatal intestinal ischemia and sub-sequent thinning of the mesenteric leavesbecause the prenatal intestinal ischemia is as-sociated with bowel atresia in 5.5% of the pe-diatric population [1, 2, 5, 18]. Other causespostulated include intraperitoneal inflamma-tion, trauma, partial development regression,and fenestration of the mesentery by the colonduring the embryologic displacement into theumbilical cord [6].

However, there is a second peak of occur-rence in the adult population, and in this sub-set of patients, the cause is iatrogenic, usuallyrelated to prior abdominal surgery, especiallywith Roux-en-Y anastomosis, trauma, or in-flammation [1, 5, 6, 9, 18]. Both liver trans-plantation and the most common type of gas-tric bypass surgery involve the formation of aRoux-en-Y loop at the choledochojejunos-tomy site, and these procedures are increasing

A B

C

Fig. 1054-year-old woman with abdominal pain.A, Anteroposterior radiograph from upper gastrointestinal series shows abnormal cluster of small-bowel loops located in lesser sac, representing foramen of Winslow internal hernia.B, Oblique lateral view from same gastrointestinal series shows abnormal cluster of small-bowel loops posterior to stomach (asterisk), indenting (arrows) and displacing stomach anteriorly.C, Contrast-enhanced axial CT scan shows cluster of small-bowel loops (arrow) located in lesser sac, posterior to stomach (arrowhead).

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A B

Fig. 1170-year-old man with severe epigastric pain.A, Anteroposterior projection of radiograph shows large collection of gas in left upper quadrant (arrows).B, Barium enema (anteroposterior view) shows large, air-filled structure in upper abdomen (arrows), originally thought to represent a distended stomach but surgically confirmed to be cecum involved in foramen of Winslow hernia.

Fig. 12Diagrammatic illustration of intersigmoid hernia shows bowel protruding (arrow) through defect in sigmoid mesocolon to lie posterolateral to sigmoid colon itself.

Fig. 1385-year-old man with abdominal pain. Axial CT scan of sigmoid-related hernia (type 2, transmesosigmoid) reveals small-bowel loops (arrow) protruding through defect in sigmoid mesocolon, which usually occurs between left psoas muscle (arrowhead) and sigmoid colon (S), to lie posterior and lateral to sigmoid colon itself.

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in frequency (Fig. 14). If the Roux loop isplaced anteriorly to the transverse colon, re-ferred to as antecolic, there will be no defectcreated in the transverse mesocolon; however,this procedure is less commonly performedbecause of the required long segment ofbowel needed to travel around the transversecolon to finally be anastomosed to the re-maining gastric pouch. A more direct routeinvolves creating a defect in the transversemesocolon, allowing a shorter Roux limblength. However, this second surgical proce-dure, also known as a retrocolic type, is moreassociated with the potential complication ofinternal hernia (Fig. 14). Interestingly, inter-nal hernias also appear to occur more com-monly after laparoscopic Roux-en-Y gastricbypass than after open Roux-en-Y gastric by-pass, for reasons unknown [15].

Three main types of transmesenteric inter-nal hernias are seen. The first and most com-mon is the transmesocolic, which has beendocumented to occur in 0.73.25% of patientsafter laparoscopic Roux-en-Y gastric bypass

surgery [15]. The second type of transmesen-teric internal hernia occurs when bowel pro-lapses through a defect in the small-bowelmesentery. Finally, the third type, known asthe Peterson type, has also been described andinvolves the herniation of small bowel behindthe Roux loop before the small bowel eventu-ally passes through the defect in the trans-verse mesocolon [4].

Several predisposing factors have beenpostulated. Although surgeons attempt toclose the defects created, they can be incom-pletely closed or can have a breakdown or apulling of the suture material through the me-socolic fat [3, 15]. Enlargement of the mesen-teric defect can occur with repeated hernia-tion. An additional possible predisposingfactor may be the rapid weight loss and de-creased intraperitoneal fat that occurs in bari-atric patients, causing enlargement of the de-fect [4]. Transmesenteric hernias are morelikely than other subtypes to develop volvulusand strangulation or ischemia, the incidenceof which is reported to be as high as 30% and

Fig. 14Diagrammatic illustration shows retrocolic Roux-en-Y procedure, with loop of small bowel about to herniate through transverse mesocolon (arrow) at surgically created defect, in keeping with transmesocolic internal hernia.

a 40%, respectively, with mortality rates of50% for the treated groups and 100% for thenontreated subgroups [57, 18]. Volvulus andstrangulation or ischemia may be partlycaused by the usual small aperture of the de-fect (25 cm) in addition to the lack of encap-sulation of the herniated loops, allowing alarge length of small bowel to herniatethrough the mesenteric defect [5, 18].

Clinically, in both the pediatric and adultpopulations, patients present with signs andsymptoms of small-bowel obstruction, withperiumbilical, crampy pain, nausea, and dis-tention [1, 7, 10]. Vomiting can be a lessprominent feature than with other types ofinternal hernias because of fewer gastric andenteral secretions from the proximal gastricpouch or Roux limb that can accumulateabove the level of obstruction [15]. Overall,however, symptom onset is often more acutethan with other types of hernias [6]. Most(93%) transmesenteric internal hernias inthe adult postoperative population occurmore than 1 month after surgery (mean, 235days), and the most common cause of ob-struction during the first postoperativemonth is adhesions [4]. On physical exami-nation, the Gordian knot of herniated intes-tine has been described, representing a ten-der abdominal mass [1].

Transmesenteric hernias are more difficult todiagnose on imaging studies because their ap-pearance and location are more variable. This ispartly because of the lack of a confining sac andtherefore their potential location anywhere inthe peritoneal cavity, although they tend to occurmore commonly in the right mid abdomen [6, 9,10]. Most commonly, it is the Roux loop itselfthat herniates with a cluster of a few loops of di-lated small bowel in the expected location of theRoux loop [4] (Fig. 15). On radiography, theremay be signs of small-bowel obstruction, occa-sionally with a closed loop appearance [18].

Although the transmesenteric hernia oftencauses obstruction of the limb proximal to theenteroenterostomy site, if the hernia is distal,an important clue may be the presence of sig-nificant air in the gastric remnant, which isonly a normal finding in the early postopera-tive course (Fig. 16). Otherwise, this findingis of concern for a distal obstruction at or be-yond the enteroenterostomy site.

Oral contrast material and cross-sectionalstudies will provide a variable appearance,depending on the type of transmesenteric her-nia and the segment and length of herniatedbowel. If the hernia is of the first type,through the mesocolon and consisting of only

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Fig. 1540-year-old woman with nausea and vomiting. Contrast-enhanced axial CT scan of transmesenteric internal hernia 19 months after Roux-en-Y procedure shows dilated loops of duodenum (large black asterisk) and jejunum (white asterisk) in expected location of Roux loop. Note that Roux limb (arrowhead) is compressed. Straight arrows, curved arrow, and small black asterisk represent colon. (Reprinted with permission from [19])

Fig. 1636-year-old man with sudden onset of abdominal pain. Radiograph (anteroposterior projection) shows distended air-filled gastric remnant, which is normal finding in recently postoperative patient. However, in this patient several months after surgery, this finding is most worrisome for obstruction at distal anastomosis of Roux-en-Y loop.

a few loops of small bowel, oral contrast stud-ies may show a beaked appearance of both theafferent and efferent loops and resultant masseffect on the stomach and transverse colon,simulating a left paraduodenal hernia [5].With CT as with barium studies, these herniascan often be mistaken for or occur concomi-tantly with a small-bowel volvulus and closedloop obstruction, including a beaklike appear-ance of the closely opposed afferent and effer-ent loops [9, 18]. Again, there may be masseffect on the stomach and transverse colon.

On the other hand, a slightly different ra-diographic picture will emerge if the herni-

ated segment is much longer or is of the sec-ond type, through the small-bowel mesentery.On CT, more typical findings have been de-scribed, in addition to the usual findings ofsmall-bowel obstruction with transition point[6]. As described by Blachar and Federle [6]and Blachar et al. [7], the presence of clus-tered bowel loops in the periphery of the peri-toneal cavity, lateral to the colon (a reversal ofthe normal pattern), with central, inferior, andposterior displacement of the transverse colonis one clue (Fig. 17A). Because, as previouslymentioned, the herniated bowel is locatedmore commonly on the right, it is often the

hepatic flexure that is displaced inferiorly andmedially [6].

The second described finding, again by thesame authors [6, 7], involves displacement ofthe overlying omental fat, with the obstructedloops compressed and directly abutting the ab-dominal wall (Fig. 17B). It has been suggestedthat these two findings of peripherally locatedsmall bowel and lack of omental fat betweenthe loops and the anterior abdominal wallmight be the most helpful CT signs, with anoverall sensitivity of 85% and 92% for each re-spective finding [7]. The mesenteric vascula-ture may also show an abrupt change in thecourse of the superior mesenteric artery, whichis often displaced to the right, with crowding,stretching, engorgement, and displacement ofits visceral branches [5, 6, 10, 18]. However,further studies concluded that the only statisti-cally significant signs were relatively nonspe-cific findings of small-bowel dilatation withtransition point, clustering of small-bowelloops, and mesenteric vessel abnormalities (in-cluding displacement of the main mesenterictrunk to the right), obtaining an overall averagesensitivity of 63%, specificity of 73%, and ac-curacy of 77% [10]. Again, a closed loopsign, twisting of the mesenteric vessels and thewhirl sign if volvulus is present, may also beseen [1, 6, 9]. Occasionally, there may even beevidence of ischemia with ascites and bowelwall thickening present [7].

With the third type of transmesenterichernia associated with Roux-en-Y surgery,known to surgeons as the Peterson type, littlehas been described in the imaging literatureother than nonspecific findings of partialsmall-bowel obstruction and crowding of themesenteric vessels.

As previously mentioned, because of theirvariable imaging appearances, transmesen-teric hernias are often confused with eitherright paraduodenal or pericecal hernias. Onehelpful distinguishing feature for the formerincludes the presence of an encapsulatingmembrane, which should be seen only withright paraduodenal hernias. Differentiating apericecal hernia from a transmesenteric her-nia can be more problematic. Location of her-niated bowel loops (in the right upper quad-rant for transmesenteric and right lowerquadrant for pericecal) can sometimes pro-vide a clue, although usually a history of sur-gery is the most helpful information.

Retroanastomotic HerniasRetroanastomotic hernias occur when small-

bowel loops herniate posteriorly through a de-

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A B

Fig. 17CT scans in two different patients with transmesenteric internal hernias.A, Contrast-enhanced axial CT scan of 84-year-old woman showing transmesenteric internal hernia after Roux-en-Y procedure shows dilated, fluid-filled loops of small bowel lateral to ascending colon (arrow) and displacing omental fat because loops of bowel lie directly beneath anterior abdominal wall (arrowheads).B, Axial contrast-enhanced CT scan at level of transverse mesocolon in a 40-year-old woman shows dilated loop of jejunum directly abutting anterior abdominal wall (white asterisk). In addition, note compression of pancreaticobiliary limb (straight arrows), whereas Roux limb (small arrowhead) is barely visible. Large arrowhead, black asterisk, and curved arrow indicate colon. (Reprinted with permission from [19])

Fig. 18Diagrammatic illustration shows retrocolic Roux-en-Y gastric bypass procedure. Arrow indicates loop of small bowel protruding posterior to enteroenterostomy, in keeping with a retroanastomotic internal hernia.

fect related to a surgical anastomosis; they aretherefore by definition considered an acquiredtype, abnormal aperture subtype of internal her-nia. Specifically, these hernias have been mostcommonly described with the Roux-en-Y for-mation and are ever increasing in incidence asliver transplantations and gastric bypasses forbariatric surgeries continue to become morefrequent and widespread. If the surgery is of theantecolic type, the borders of the aperture con-sist of the transverse mesocolon superiorly, the

ligament of Treitz inferiorly, and the gastroje-junostomy site and afferent limb of the jejunumanteriorly; hence the term retroanastomotic[5] (Fig. 18). The most common herniated loopconsists of the efferent jejunal segment, whichoccurs in approximately 75% of cases [1].However, controversy exists in the literature asto whether this occurs more commonly with theantecolic [5] or retrocolic [1] form of the sur-gery. Less commonly, a very long afferent limb,ileum, cecum, or omentum can herniate into the

retroanastomotic space. However, if antecolicsurgery is performed, the afferent loop will bethe most commonly involved segment.

As opposed to the transmesenteric type ofinternal hernia related to the Roux-en-Y sur-gery, the retroanastomotic type tends to occurmost commonly during the first postoperativemonth, with half of all cases presenting duringthis time [1, 5]. Of the remaining 50%, halfwill occur after the first year, and the other half(or 25% of the total) will occur betweenmonths 2 and 12 [5]. Also in contradistinctionto the transmesocolic type, the retroanasto-motic hernia is more common with the ante-colic form of the surgery. Symptoms again de-pend on whether the retrocolic or antecolicform of the surgery was performed. If retro-colic, symptoms may be nonspecific findingsrelated to a high small-bowel obstruction, suchas crampy abdominal pain and nausea [1, 5].Typically, there is less vomiting because of therelative lack of fluid and secretions in the gas-tric pouch or Roux limb [19]. On physical ex-amination, there may be a tender mass in theleft upper quadrant [1]. With the antecolicform of the surgery, patients more commonlypresent with persistent epigastric pain and ten-derness, nonbilious vomiting, and increasedamylase [1]. Compared with other types of in-ternal hernias, these hernias are less likely topresent with strangulation because of the largeaperture size. However, uncommonly, compli-cations can arise when the herniated loop reen-ters the greater peritoneal cavity through theforamen of Winslow or a gastrohepatic or gas-

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A B

Fig. 19CT scans from two different patients showing retroanastomatic hernias. A, Contrast-enhanced axial CT scan of retroanastomotic hernia in 35-year-old woman shows loops of dilated fluid-filled small bowel (arrow) in left upper quadrant.B, Axial CT scan in 58-year-old woman 2 months after Roux-en-Y gastric bypass shows herniated loop posterior to jejunojejunostomy site (straight arrow) and dilated proximal Roux limb (large arrowheads). Note decompressed distal ileal loops (small arrowheads) and colon (curved arrows). (Reprinted with permission from [19])

trocolic ligament [5]. Diagnostic consider-ations include more common postoperativecomplications such as gastric outlet obstruc-tion, dumping syndrome, and postoperativepancreatitis, with a delay in the diagnosis po-tentially leading to strangulation (with mortal-ity rates of 30% and 100% for the treated vsnontreated groups, respectively) [1, 5].

On radiographs, because the herniatedbowel most often occurs from right to left,there may be a collection of dilated loops inthe left upper quadrant. There also may besignificant dilation of the bypassed stomach(Fig. 16). Upper oral contrastenhanced gas-trointestinal studies may reveal the abnormalloop posterior and lateral to the gastrojejunos-tomy, often with some degree of stasis and di-latation [1, 5]. On CT, the herniated jejunalloops are often noted to be fixed in the left up-per quadrant, usually associated with somedegree of dilatation (Fig. 19). However, withthe antecolic form of surgery, findings on theoral contrastenhanced studies may be moredifficult to assess because of the difficulty inopacifying the afferent loop. On CT, a fluid-filled, markedly distended tubular structure isnoted. Nuclear medicine studies can often behelpful in terms of visualizing the afferentloop by using an agent excreted by the biliarysystem into the duodenum, thereby delineat-ing the location of the afferent loop.

SummaryAlthough internal hernias were previously

an uncommon cause of small-bowel obstruc-tion, the incidence appears to be increasing asmore Roux-en-Y surgeries are performed forvarious reasons, thereby creating a shift from

congenital to iatrogenic causes. It is importantfor the radiologist to be aware of the imagingfindings of these hernias, particularly on CT,because CT is rapidly becoming a mainstay inthe workup of acute abdominal pain.

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13. R. Gonzlez Conde, P. Pardo Rojas, E. Valeiras Domnguez, C. Prez Lpez, R. Santos LLoves, F. J. Gmez Lorenzo. 2013.Correct preoperative diagnosis of herniation through the Foramen of Winslow: two case reports. Hernia 17:3, 409-414. [CrossRef]

14. J. Cazejust, C. Lafont, M. Raynal, L. Azizi, A.C. Tourabi, Y. Menu. 2013. Internal hernia through the omental foramen. Answerto the e-quid Epigastric pain with sudden onset. Diagnostic and Interventional Imaging 94:6, 663-666. [CrossRef]

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26. Atsushi Okita, Masaki Fujimura, Yoshihide Chino, Tomotake Tabata, Makoto Mizutani, Reina Shimoshiro, Seiji Masuda, IsaoSato, Minoru Iida, Kimituka Kumano. 2013. An Internal Hernia Caused by an Abnormal Hole Located in the AscendingMesocolon. The Japanese Journal of Gastroenterological Surgery 46:3, 203-209. [CrossRef]

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29. Christiane S. Eberhardt, Laura Merlini, Valrie A. McLin, Barbara E. Wildhaber. 2012. Cholestasis as the leading sign of atransmesenteric hernia in a split-liver transplanted child - A case report and review of literature. Pediatric Transplantation 16:5,E172-E176. [CrossRef]

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31. Mayur Satish Bandawar, Praveen Nayak, Irfan Abubakar Shaikh, M. S. Sakthivel, Thakur Deen Yadav. 2012. Strangulated SmallBowel Obstruction Secondary to a Transmesosigmoid Hernia. Indian Journal of Surgery . [CrossRef]

32. Takako Sato, Shuntaro Abe, Kento Tsuboi, Misa Iwata, Akiyoshi Tamura, Hitoshi Tsuchihashi, Hajime Nishio, Koichi Suzuki.2012. Sudden death of a child because of an intestinal obstruction caused by a large congenital mesenteric defect. Legal Medicine14:3, 157-159. [CrossRef]

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43. Norio Hongo, Hiromu Mori, Shunro Matsumoto, Yuriko Okino, Ryo Takaji, Eiji Komatsu. 2011. Internal hernias after abdominalsurgeries: MDCT features. Abdominal Imaging 36:4, 349-362. [CrossRef]

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90. Antoine Mboyo, Ernest Goura, Richard Massicot, Vincent Flurin, Benedicte Legrand, Muriel Repetto-Germaine, Sigrid Caron-Bataille, Jean Ndi. 2008. An exceptional cause of intestinal obstruction in a 2-year-old boy: strangulated hernia of the ileumthrough Winslows foramen. Journal of Pediatric Surgery 43:1, e1-e3. [CrossRef]

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92. Shunsuke Hosono, Hiroshi Ohtani, Yuichi Arimoto, Yoshitetsu Kanamiya. 2007. Internal Hernia with Strangulation Through aMesenteric Defect After Laparoscopy-Assisted Transverse Colectomy: Report of a Case. Surgery Today 37:4, 330-334. [CrossRef]

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