Swirl Sign After Roux-en-Y Gastric Bypass: CT Recognition of Internal Hernia and Small-Bowel Volvulus

A radiologic approach to a potentially life-threatening postoperative complication

By Dr. SB Lee

Executive Answer

In a patient with previous Roux-en-Y gastric bypass who presents with acute abdominal pain, nausea, and bilious vomiting, a mesenteric swirl sign on CT should immediately raise concern for internal hernia with small-bowel obstruction and possible volvulus. In this case, dilated distal small-bowel loops, proximal decompression, and twisting of bowel and mesenteric vessels were associated with herniation through a mesenteric defect near the jejunojejunostomy. Exploratory laparoscopy confirmed an internal hernia, which was reduced and repaired. The key radiologic lesson is that altered postoperative anatomy should not obscure recognition of abnormal bowel configuration and mesenteric vascular rotation.

Why This Case Matters

Internal hernia is an important late complication of Roux-en-Y gastric bypass (RYGB). Unlike an external hernia, there may be no obvious abdominal wall abnormality. The obstructed bowel can instead migrate through an internal mesenteric defect, producing a complex combination of bowel obstruction, mesenteric crowding, vascular rotation, and potentially intestinal ischemia.

This makes the diagnosis particularly relevant to radiologists and emergency physicians. A patient with previous bariatric surgery may present with symptoms that initially appear nonspecific. However, the combination of a surgical history, obstructive symptoms, and an abnormal mesenteric configuration can provide a highly informative diagnostic pattern.

The case presented here illustrates an important principle:

The surgical anatomy is altered, but the radiologic signs of mechanical obstruction and mesenteric rotation remain recognizable.

The case describes a man in his 50s with approximately 1.5 days of midabdominal pain, nausea, and bilious vomiting. He had undergone Roux-en-Y gastric bypass seven years earlier. Examination demonstrated tachycardia, tachypnea, abdominal distention, diffuse tenderness, guarding, and rebound tenderness. casestudy

These clinical findings, combined with the imaging abnormalities, created a high-risk scenario for internal herniation and bowel compromise.

Clinical Scenario: Abdominal Pain Years After Gastric Bypass

The interval between bariatric surgery and presentation is clinically important.

A patient may undergo Roux-en-Y gastric bypass years before developing an internal hernia. Therefore, the absence of a recent operation does not exclude a postoperative mechanical complication.

In this case, the patient had undergone RYGB seven years earlier. He subsequently developed acute gastrointestinal symptoms characterized by:

Midabdominal pain
Nausea
Bilious vomiting
Abdominal distention
Tachycardia and tachypnea
Diffuse abdominal tenderness
Guarding
Rebound tenderness

The combination is concerning for mechanical small-bowel obstruction, particularly when accompanied by peritoneal signs.

The history of RYGB changes the diagnostic framework. The radiologist should not approach the examination simply as an ordinary case of small-bowel obstruction. Instead, the key question becomes:

Could the obstruction be caused by internal herniation through a postoperative mesenteric defect?

Imaging Findings

Plain Abdominal Radiography

The case includes a plain abdominal radiograph demonstrating markedly abnormal small-bowel configuration consistent with an obstructive process.

Figure 1. Simple abdominal radiograph demonstrating abnormal dilated small-bowel configuration in a patient with previous Roux-en-Y gastric bypass.

The radiograph provides evidence of the mechanical problem but does not by itself establish the precise cause.

That distinction is important.

Plain radiography may suggest bowel obstruction, but the altered anatomy after RYGB makes CT particularly valuable for identifying the mechanism of obstruction and the location of the internal hernia.

CT: The Swirl Sign

The decisive imaging clue in this case is the swirling of bowel and mesenteric vessels.

The CT demonstrated dilated distal small-bowel loops associated with proximal decompression and a characteristic rotation of the bowel and mesenteric vasculature. The case material describes herniation through a mesenteric defect at the jejunojejunostomy with associated vascular swirling. casestudy

Figure 2. Coronal CT image demonstrating dilated small-bowel loops and rotation of bowel and mesenteric vessels associated with internal herniation after Roux-en-Y gastric bypass.

The swirl sign refers to the characteristic spiral configuration created when mesenteric vessels and their surrounding fat rotate around a central axis.

In the post-RYGB patient, this finding should be interpreted in the context of the surgically altered bowel anatomy.

The important observation is not simply that the vessels appear curved.

The important observation is that:

bowel + mesentery + vessels have an abnormal rotational relationship.

That relationship can indicate internal herniation and volvulus.

Radiologist's Reasoning: From Obstruction to Mechanism

A useful diagnostic sequence is:

Symptoms → Small-bowel obstruction → Altered postoperative anatomy → Mesenteric swirl → Internal hernia

This sequence prevents the radiologist from stopping at the nonspecific diagnosis of "small-bowel obstruction."

Step 1: Is there mechanical obstruction?

Look for:

Dilated small-bowel loops
A transition point
Proximal and distal caliber differences
Abnormal clustering of bowel loops
Decompression of bowel proximal or distal to the obstructed segment
Abnormal bowel configuration

The case demonstrated dilated distal small-bowel loops with proximal decompression. casestudy

Step 2: Is the anatomy surgically altered?

The prior RYGB is a major diagnostic clue.

The radiologist should identify:

Gastric pouch
Roux limb
Jejunojejunostomy
Mesenteric relationships
Expected postoperative bowel orientation

The altered anatomy can make the CT appear unfamiliar, particularly to readers who do not frequently interpret post-bariatric imaging.

Step 3: Are the mesenteric vessels rotating?

This is where the swirl sign becomes critical.

A spiral or whirling configuration of mesenteric vessels and fat should prompt careful evaluation for internal herniation.

Step 4: Where is the internal hernia occurring?

The case specifically identifies herniation through a mesenteric defect associated with the jejunojejunostomy.

The exact defect location matters because it helps the surgeon understand the mechanism of obstruction and guides operative exploration.

Step 5: Is there evidence of bowel compromise?

Once internal hernia is suspected, the radiologist should actively search for signs suggesting strangulation or ischemia.

These may include:

Reduced or abnormal bowel-wall enhancement
Bowel-wall thickening
Mesenteric edema
Venous congestion
Mesenteric fluid
Pneumatosis
Portal venous gas
Free fluid
Closed-loop obstruction
Severe mesenteric vascular distortion

Not every patient will demonstrate these findings.

Their absence does not necessarily exclude internal herniation.

Why the Swirl Sign Is So Important

The swirl sign is one of the most recognized CT findings associated with internal hernia after RYGB.

Earlier CT studies demonstrated that mesenteric swirl is particularly useful for identifying internal hernia, although reported diagnostic performance varies among studies and depends on patient selection and interpretation criteria. A 2009 study reported sensitivity of 78–100% and specificity of 80–90% for mesenteric swirl in its study population.

The key clinical message is therefore not that every swirl represents an internal hernia.

Rather:

A new or convincing mesenteric swirl in a symptomatic post-RYGB patient should trigger a focused search for internal herniation and associated bowel obstruction.

This is particularly important because the anatomy can be difficult to interpret and the consequences of delayed recognition can be severe.

Why Internal Hernia Can Be Missed

Internal hernia after RYGB creates several diagnostic challenges.

1. The anatomy is unfamiliar

The normal relationships of the stomach and small bowel have been surgically rearranged.

A radiologist who is not familiar with RYGB anatomy may interpret an unusual bowel configuration as merely postoperative.

2. Symptoms may be nonspecific

Abdominal pain, nausea, and vomiting have many possible causes.

The diagnostic significance of these symptoms changes substantially when the patient has a history of RYGB.

3. The obstruction may be intermittent

Internal herniation can produce variable symptoms because bowel may move in and out of a mesenteric defect.

Therefore, a previous episode of transient abdominal pain does not necessarily exclude an internal hernia.

4. The primary abnormality may be vascular rather than dramatic bowel dilation

The mesenteric swirl can be an important clue even when the bowel obstruction is not immediately striking.

5. Satisfaction of search

Once bowel obstruction is identified, there is a risk of stopping the interpretation at:

"Small-bowel obstruction."

But the clinically important question is:

Why is the bowel obstructed?

In a patient with RYGB, that second question can change management.

Differential Diagnosis

The imaging differential for abdominal pain and small-bowel obstruction after RYGB includes several postoperative and non-postoperative processes.

Internal Hernia

This is the critical diagnosis in the present case.

A mesenteric swirl associated with abnormal bowel configuration and obstruction strongly supports internal herniation in the appropriate clinical context.

Adhesive Small-Bowel Obstruction

Adhesions are a common cause of postoperative bowel obstruction.

However, adhesive obstruction does not inherently explain a characteristic mesenteric vascular swirl.

Anastomotic Stricture

Anastomotic narrowing can cause obstruction, but the expected imaging pattern differs from internal herniation with mesenteric rotation.

Incisional Hernia

An incisional hernia can cause bowel obstruction but should usually be associated with a defect involving the abdominal wall rather than an internal mesenteric defect.

Intussusception

Intussusception produces a characteristic telescoping configuration of bowel and mesenteric structures. It should be distinguished from the spiral vascular configuration of a mesenteric swirl.

Volvulus

Volvulus represents twisting of bowel and its mesentery around a vascular pedicle or mesenteric axis. In the post-RYGB patient, internal herniation can provide the anatomic setting for this rotational process.

The case therefore illustrates the close relationship between:

Internal hernia → bowel obstruction → mesenteric rotation → volvulus → possible ischemia.

Diagnostic Algorithm

A practical radiologic approach can be summarized as follows:

This approach emphasizes mechanism rather than simply describing the presence of obstruction.

Structured CT Interpretation: Turning a Visual Clue Into a System

One of the broader lessons from this case is that internal hernia detection can benefit from structured CT assessment.

A structured approach can include:

CT Feature	What to Assess	Clinical Relevance
Swirl sign	Rotation of mesenteric vessels and fat	Major clue to internal herniation
Small-bowel obstruction	Dilated loops and transition	Demonstrates mechanical consequence
Clustered loops	Abnormal concentration of bowel loops	May support internal herniation
Mesenteric edema	Increased mesenteric attenuation	May indicate congestion
Venous congestion	Enlarged or crowded mesenteric veins	May indicate impaired venous drainage
Anastomotic position	Relationship of jejunojejunostomy to expected anatomy	May support altered bowel configuration
Bowel enhancement	Symmetric and preserved vs abnormal	Helps assess possible ischemia
Free fluid	Quantity and distribution	May accompany complicated obstruction
Pneumatosis	Intramural gas	Concerning for advanced bowel compromise
Portal venous gas	Gas within portal venous branches	Concerning for severe bowel injury

Structured CT reporting using multiple signs has been studied as a strategy for improving recognition of internal herniation after RYGB. Ederveen and colleagues reported improved specificity, positive predictive value, and overall accuracy with structured reporting compared with free-text reporting.

More recent work has continued to examine structured CT assessment and combinations of imaging signs, including swirl sign, venous congestion, and anastomotic configuration.

The broader lesson is important:

A structured checklist does not replace radiologist judgment; it helps protect the diagnostic process from incomplete review.

Imaging Physics and the Meaning of the Swirl

The swirl sign is fundamentally an anatomic phenomenon visualized by CT.

Contrast-enhanced CT allows simultaneous evaluation of:

Bowel lumen
Bowel wall
Mesenteric fat
Mesenteric arteries
Mesenteric veins
Anastomotic anatomy
Adjacent organs

When bowel and mesenteric structures rotate, the vessels become visible as curvilinear or spiral structures.

The diagnostic value comes from the spatial relationship among these structures rather than from a single isolated vessel.

This is why multiplanar CT reconstruction is particularly useful.

Axial images may demonstrate the rotational center.

Coronal images can clarify the direction and extent of the abnormal bowel configuration.

Sagittal reconstructions may help define the relationship between the herniated bowel and mesenteric defect.

For post-RYGB cases, interpretation should therefore be performed using the complete multiplanar dataset rather than relying on a single axial level.

AI Perspective: Could AI Help Detect Internal Hernia?

This case also illustrates a potential application for medical imaging AI.

The clinical problem is well suited to an AI-assisted detection framework because the radiologist must integrate several visual features:

An AI system could potentially function as a detection or second-reader tool, flagging examinations with patterns associated with internal herniation.

However, such a system should not be conceptualized as an autonomous diagnostic replacement.

The appropriate role is closer to:

CT → AI analysis → radiologist review → clinical correlation → surgical communication when appropriate

The radiologist remains responsible for determining whether the imaging pattern actually represents internal herniation and whether complications such as ischemia are present.

AI Workflow for Post-RYGB CT

The most useful AI implementation would not simply produce:

"Internal hernia detected."

A more clinically meaningful system could identify specific visual findings for review:

Suspected mesenteric swirl
Dilated small bowel
Abnormal bowel clustering
Mesenteric congestion
Possible ischemic bowel
Relevant prior postoperative anatomy

This would allow the radiologist to review the evidence rather than simply accept an unexplained binary alert.

AI Failure Modes

Any AI system applied to this problem would have important limitations.

False Negative

An AI model may fail to identify internal herniation when the swirl is subtle or when the anatomy is substantially distorted.

False Positive

Normal postoperative vascular configuration or other causes of mesenteric rotation could potentially be flagged incorrectly.

Domain Shift

An algorithm trained using one bariatric surgical technique, scanner population, or institutional imaging protocol may not generalize to all hospitals.

Anatomical Mislocalization

The model may detect abnormal bowel but incorrectly localize the transition point or mesenteric defect.

Image Quality

Motion, low contrast, incomplete coverage, or technical limitations may degrade model performance.

Automation Bias

A negative AI result should not override strong clinical and radiologic suspicion.

Conversely, a positive AI alert should not automatically establish the diagnosis.

The correct principle is:

AI may redistribute attention; it does not eliminate diagnostic responsibility.

Diagnostic Risk: The Cost of Missing the Mechanism

The most important risk in this case is not simply missing the word "internal hernia."

The greater risk is missing the mechanism of obstruction.

The case material specifically emphasizes that small-bowel obstruction associated with internal hernia after gastric bypass can progress to bowel necrosis and death and that early surgical management is important. casestudy

This is why the radiology report should communicate the level of concern clearly when the imaging findings and clinical presentation support internal herniation.

Healthcare Workflow Implications

Internal hernia is also a workflow problem.

A patient with severe abdominal symptoms may pass through several clinical stages:

The radiologist occupies a critical point in this pathway.

If the CT report simply states:

"Small-bowel obstruction."

the clinical team may have less information about the likely mechanism.

A more useful report should describe the relevant postoperative anatomy and explicitly communicate the imaging features that raise concern for internal hernia.

This does not mean every suspicious case should automatically be labeled as definite internal hernia.

It means that the degree of diagnostic concern should be communicated clearly enough to support timely clinical decision-making.

The Operative Correlation

The strength of this case is the correlation between imaging and surgery.

Exploratory laparoscopy demonstrated an internal hernia of the small bowel. The herniated bowel was reduced and the defect was closed. The postoperative course was uncomplicated, and the patient was discharged on postoperative day two. casestudy

This correlation demonstrates why postoperative imaging findings should be interpreted as part of a clinical-radiologic-surgical continuum.

The CT did not merely show an abnormal pattern.

It identified the mechanism that was subsequently confirmed surgically.

Practical Radiology Pearls

Pearl 1

A history of Roux-en-Y gastric bypass remains diagnostically important even many years after surgery.

Pearl 2

In a symptomatic post-RYGB patient, small-bowel obstruction should prompt a search for internal hernia.

Pearl 3

The mesenteric swirl sign is one of the most important CT clues.

Pearl 4

Do not stop at "small-bowel obstruction." Ask why the obstruction occurred.

Pearl 5

Review the jejunojejunostomy and surrounding mesenteric anatomy carefully.

Pearl 6

Use multiplanar CT images to understand the three-dimensional configuration of bowel and mesenteric vessels.

Pearl 7

Look actively for signs of venous congestion and bowel ischemia.

Pearl 8

A structured CT checklist can reduce the risk of overlooking relevant postoperative signs.

Pearl 9

AI may support detection and prioritization, but a negative AI output should not override convincing clinical or radiologic evidence.

Pearl 10

When internal hernia is strongly suspected, communication with the clinical and surgical team is part of the diagnostic process.

Common Diagnostic Pitfalls

Calling the examination simply "small-bowel obstruction" without considering internal hernia.
Assuming that a remote history of bariatric surgery is no longer relevant.
Failing to evaluate mesenteric vessel orientation.
Reviewing only axial images.
Mistaking abnormal postoperative anatomy for an incidental finding.
Failing to inspect the jejunojejunostomy.
Ignoring venous congestion or mesenteric edema.
Waiting for advanced ischemic findings before raising concern.
Treating an AI result as definitive rather than supportive.
Failing to communicate a potentially urgent diagnosis.

Frequently Asked Questions

What is the swirl sign after Roux-en-Y gastric bypass?

The swirl sign is a spiral configuration of mesenteric vessels and surrounding mesenteric structures. In a symptomatic patient after RYGB, it is an important CT clue for internal herniation and associated bowel obstruction.

Does the swirl sign always mean internal hernia?

No. The finding must be interpreted within the patient's surgical anatomy, symptoms, bowel configuration, and other CT findings. A convincing swirl in a symptomatic post-RYGB patient should, however, prompt focused evaluation for internal hernia.

Why does internal hernia occur after Roux-en-Y gastric bypass?

RYGB creates surgically altered bowel and mesenteric relationships, including potential mesenteric spaces or defects through which small bowel can migrate. The bowel may become trapped and rotate, producing obstruction and potentially vascular compromise.

What is the role of CT?

CT is particularly valuable because it can demonstrate bowel obstruction, mesenteric vessel rotation, abnormal bowel positioning, and other signs associated with internal herniation.

Can internal hernia cause bowel ischemia?

Yes. If herniated bowel becomes strangulated or its blood supply is compromised, ischemia and necrosis can develop. The case material specifically describes this potential progression. casestudy

Can AI detect internal hernia?

AI may potentially assist with detection of imaging patterns such as mesenteric swirl and bowel obstruction, but its role should be supportive. Human radiologist review and clinical correlation remain essential.

What is the most important imaging lesson from this case?

The most important lesson is to recognize the combination of post-RYGB anatomy + small-bowel obstruction + mesenteric swirl as a pattern that should raise concern for internal hernia.

Expert Insight: What This Case Teaches Beyond One Patient

The value of a case report is not limited to identifying the diagnosis.

This case teaches a broader radiologic principle:

Postoperative anatomy should change the diagnostic search pattern, not lower the diagnostic threshold.

For the radiologist, the prior operation becomes an imaging map.

The RYGB history tells us where bowel has been reconstructed.

The symptoms tell us that the reconstructed anatomy may now be mechanically compromised.

The CT shows the consequence.

The swirl sign provides the mechanism.

The surgical findings provide confirmation.

This creates a complete diagnostic chain:

History → Anatomy → Imaging → Mechanism → Surgical confirmation

That framework is transferable to many postoperative imaging problems.

Key Takeaways

Internal hernia is an important potential late complication after Roux-en-Y gastric bypass.
Acute abdominal pain, nausea, and bilious vomiting in a post-RYGB patient should prompt consideration of mechanical obstruction.
The mesenteric swirl sign is a key CT clue.
Dilated bowel loops and proximal decompression help establish the obstructive component.
The jejunojejunostomy and surrounding mesenteric anatomy deserve careful review.
CT interpretation should include assessment for vascular congestion and possible bowel ischemia.
Structured reporting can improve systematic evaluation of relevant CT signs.
AI may serve as a second-reader or prioritization tool but cannot replace radiologist judgment.
Clear radiology-clinical communication is particularly important when internal hernia is suspected.
The central lesson is simple: recognize the mechanism, not merely the obstruction.

References

Fernandez-Moure J, Sherman V. Swirl Sign — Intestinal Volvulus after Roux-en-Y Gastric Bypass. New England Journal of Medicine. 2017;376:e3. doi: 10.1056/NEJMicm1601775. New England Journal of Medicine
Lockhart ME, Tessler FN, Canon CL, et al. Internal hernia after gastric bypass: sensitivity and specificity of seven CT signs with surgical correlation and controls. AJR American Journal of Roentgenology. 2007;188(3):745–750. doi: 10.2214/AJR.06.0541. PubMed
Iannuccilli JD, Grand D, Murphy BL, Evangelista P, Roye GD, Mayo-Smith W. Sensitivity and specificity of eight CT signs in the preoperative diagnosis of internal mesenteric hernia following Roux-en-Y gastric bypass surgery. Clinical Radiology. 2009;64(4):373–380. doi: 10.1016/j.crad.2008.10.008. PubMed
Ederveen JC, van Berckel M, Jol S, Nienhuijs S, Nederend J. Diagnosing internal herniation after laparoscopic Roux-en-Y gastric bypass: usefulness of systematically reviewing CT scans using ten signs. European Radiology. 2018;28(9):3583–3590. doi: 10.1007/s00330-018-5332-3. PubMed Central (PMC)
Ederveen JC, Nienhuijs SW, Jol S, Robben SGF, Nederend J. Structured CT reporting improves accuracy in diagnosing internal herniation after laparoscopic Roux-en-Y gastric bypass. European Radiology. 2020;30(6):3448–3454. doi: 10.1007/s00330-020-06688-x. Springer
van Hogezand LL, Goense L, van der Hoeven EJRJ, et al. Optimal Use of Computed Tomography in Diagnosing Internal Herniation After Roux-en-Y Gastric Bypass: A Proposition for the Application of a Radiological Prediction Score. Obesity Surgery. 2025;35:5136–5144. doi: 10.1007/s11695-025-08323-4. Springer
van Hogezand LL, Tolenaars LM, van der Hoeven EJRJ, et al. Optimizing the diagnostic strategy for accurate detection of internal herniation following Roux-en-Y gastric bypass: a diagnostic accuracy study. Surgery for Obesity and Related Diseases. 2026;22(8):891–900. doi: 10.1016/j.soard.2026.04.006. ScienceDirect

Continue Learning

If you want to understand the imaging:

[INTERNAL LINK] CT Imaging of Small-Bowel Obstruction: Radiologic Approach to the Transition Point

If you want to understand the differential diagnosis:

[INTERNAL LINK] Differential Diagnosis of Mesenteric Swirl on CT

If you want to understand structured radiology:

[INTERNAL LINK ] Structured CT Reporting for Emergency Abdominal Imaging

If you want to understand AI:

[INTERNAL LINK] How AI Can Support Emergency Radiology Workflow