Internal hernia is the herniation of bowel loops through a peritoneal or mesenteric opening (congenital or acquired). Most commonly encountered as Petersen hernia or jejunojejunostomy mesenteric defect hernia after Roux-en-Y gastric bypass. Congenital forms include paraduodenal, transmesocolic, perisigmoid, and foramen of Winslow hernias. It constitutes a surgical emergency due to the risk of closed-loop small bowel obstruction (SBO) and mesenteric ischemia. CT findings of mesenteric swirl sign, clustered dilated bowel loops, and abnormally positioned bowel segments encircling the mesentery are diagnostic clues. When strangulation develops, decreased bowel wall enhancement, pneumatosis, and porto-mesenteric venous gas may be seen.
Age Range
30-70
Peak Age
50
Gender
Equal
Prevalence
Uncommon
In internal hernia, bowel loops traverse a peritoneal or mesenteric defect and displace into an abnormal compartment. In congenital forms, the defect arises from incomplete fusion of embryological peritoneal folds — in left paraduodenal hernia, the fossa behind the ligament of Treitz is a remnant of peritoneal rotation anomaly. In acquired forms, mesenteric defects created after Roux-en-Y gastric bypass are most commonly responsible: the Petersen space (between the Roux limb mesentery and transverse mesocolon) and the jejunojejunostomy defect. Herniated bowel loops form a closed loop — both afferent and efferent ends are trapped at the defect, occluding the lumen. Intraluminal pressure rises within this closed loop, venous return is obstructed, then arterial blood flow is cut off, and transmural ischemia-necrosis develops. The rotation of mesenteric vessels around the defect is seen as the swirl sign on CT — this finding indicates torsion of the mesenteric vascular pedicle and is a direct marker of strangulation risk. As ischemia progresses, bowel wall integrity is lost, leading to bacterial translocation and peritonitis.
Whirl-like rotation of mesenteric vessels and fat tissue on axial sections — the most reliable and pathognomonic CT finding for internal hernia. Indicates torsion of the mesenteric vascular pedicle around the herniation defect and directly signals strangulation risk.
Whirl-like rotation of mesenteric vessels and fat tissue in axial plane on portal venous phase (swirl sign). SMV and branches rotate around the SMA, with fat tissue wrapping along. The direction of the swirl has no prognostic value, but its presence predicts internal hernia with 80-90% accuracy. In intermittent forms, the swirl may resolve spontaneously.
Report Sentence
Swirl sign (whirl sign) is identified in mesenteric vessels and fat on axial sections, consistent with internal hernia.
Dilated, fluid-filled small bowel loops between two transition points form a closed loop. Loops show C or U-shaped configuration with mesenteric vessels arranged in a radial (spoke-wheel) pattern. Afferent and efferent segments show beak sign converging at the defect. Bowel wall thickness and enhancement within the closed loop are critical for strangulation assessment.
Report Sentence
Dilated small bowel loops in closed-loop configuration between two transition points are seen, consistent with closed-loop obstruction due to internal hernia.
Clustered small bowel loops are seen in a compartment where they should not normally be. In left paraduodenal hernia behind the Treitz, in Petersen hernia behind the transverse mesocolon, in transmesenteric hernia on the contralateral side. Hernia sac borders are surrounded by a thin peritoneal membrane giving a delicate capsule impression on CT.
Report Sentence
Clustered small bowel loops are identified in an abnormal compartment, consistent with internal herniation.
In strangulation, bowel wall enhancement within the closed loop decreases or disappears. Instead of normal homogeneous enhancement, non-enhancing or heterogeneously enhancing thin wall is seen. In advanced stages, bowel wall thickens (edema), with intramural gas (pneumatosis) and porto-mesenteric venous gas. Mesenteric fluid and free peritoneal fluid are late findings.
Report Sentence
Markedly decreased enhancement of bowel loops within the closed loop, consistent with strangulation; emergent surgical evaluation recommended.
Accumulation of fecal-like particulate material in small bowel proximal to the obstruction (small bowel feces sign). The mixture of gas bubbles and particulate debris mimics colonic contents. Aids in localizing the obstruction point — fecal material accumulates immediately proximal to the obstruction.
Report Sentence
Small bowel feces sign is noted proximal to the obstruction, indicating the level of obstruction.
Increased density in mesenteric fat at the herniation site due to edema and congestion (mesenteric haziness). Increased density between -20 and 0 HU instead of normal -80 to -100 HU is observed. An early sign of mesenteric venous obstruction, visible even in the pre-strangulation period.
Report Sentence
Increased density and haziness of mesenteric fat at the herniation site, consistent with mesenteric congestion.
Disruption of the SMA-SMV relationship — displacement or reversed rotation of the SMV relative to the SMA. Normally SMV lies to the right of SMA. In internal hernia, mesenteric rotation causes SMV to shift to the left or anterior of the SMA. Particularly suggests Petersen hernia in post-bariatric patients.
Report Sentence
Displacement and reversed rotation of the SMV relative to the SMA, suggestive of internal hernia with mesenteric rotation.
Criteria
Most common congenital type (53%). Herniation through fossa of Landzert behind ligament of Treitz. IMV and left colic artery course at hernia sac border.
Distinct Features
Clustered bowel loops posterior to Treitz in left upper quadrant. IMV and left colic artery define the herniation border.
Criteria
Herniation through Petersen space after Roux-en-Y. ~50% of post-bariatric internal hernias. Intra-abdominal fat loss widens the defect.
Distinct Features
Bowel loops passing behind transverse mesocolon. Swirl sign localized in Roux limb mesentery.
Criteria
Herniation through mesenteric defect at jejunojejunostomy after Roux-en-Y. Smaller defect but higher strangulation risk. ~30% of post-bariatric hernias.
Distinct Features
Swirl sign localized at jejunojejunostomy level. Defect usually smaller with tighter closed loop.
Criteria
Herniation through fossa of Waldeyer — 25% of congenital hernias. SMA courses at herniation border.
Distinct Features
SMA and branches course inferolateral to hernia sac. Bowel loops cluster behind SMA.
Distinguishing Feature
Mesenteric ischemia shows thrombus/embolus in SMA/SMV without swirl sign. Internal hernia involves mechanical rotation.
Distinguishing Feature
Adhesive SBO shows single transition point without swirl sign or clustering. Surgical scar history is a clue.
Distinguishing Feature
Volvulus involves rotation of entire small bowel mesentery. In internal hernia, rotation is focal and limited to defect.
Distinguishing Feature
External hernia: herniation outward through abdominal wall defect with external contour abnormality. Internal hernia: intraperitoneal with normal external contour.
Urgency
emergentManagement
surgicalBiopsy
Not NeededFollow-up
specialist-referralInternal hernia is a surgical emergency — emergent laparotomy or laparoscopy required due to closed-loop obstruction and strangulation risk. Without strangulation: reduction and defect repair; with strangulation: necrotic bowel resection. Surgical defect closure is preventive in post-bariatric patients. Mortality can reach 20-40% with strangulation.
Internal hernia requires emergency surgery. Early diagnosis before strangulation and necrosis is life-saving. Special attention needed in post-bariatric surgery patients. Awareness of swirl sign accelerates diagnosis.